机械专业英语

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  • 1. Technical English Through Reading 专业英语阅读 王明赞 编 l Machine Design 机械设计 l Metalwork 金属加工 l Electrical Motor Control 电机控制 l Principles of Measurement system 测量系统原理
  • 2. Technical English Through Reading 1 Contents PART 1 MACHINE DESIGN .......................................................................................................... 1 1.1 INTRODUCTION TO MACHINE DESIGN...................................................................... 1 1.1.1 What Is Machine Design........................................................................................... 1 1.1.2 Fundamental Background for Machine Design....................................................... 1 1.1.3 Philosophy of Machine Design ................................................................................ 2 1.1.4 Communication of Design ........................................................................................ 4 1.1.5 Piston Engine Versus the Wankel Engine ................................................................ 5 1.1.6 The Four-stroke Automotive Piston Engine ............................................................ 5 1.1.7 Rotary Wankel Engine .............................................................................................. 6 1.1.8 Major of Machine Design ......................................................................................... 8 1.1.9 Initial Conception of Design .................................................................................... 9 1.1.10 Strength Analysis .................................................................................................... 9 1.1.11 Selection of Materials ........................................................................................... 10 1.1.12 Appearance ............................................................................................................ 10 1.1.13 Manufacturing ....................................................................................................... 11 1.1.14 Economy ................................................................................................................ 11 1.1.15 Safety ..................................................................................................................... 12 1.1.16 Environmental Effects .......................................................................................... 12 1.1.17 Reliability and Life ............................................................................................... 13 1.1.18 Legal Considerations ............................................................................................ 13 1.2 FAILURE ANALYSIS AND DIMENSIONAL DETERMINATION .............................. 14 1.2.1 Introduction ............................................................................................................. 14 1.2.2 Tensile Static Strength ............................................................................................ 15 1.2.3 Design Properties of Materials ............................................................................... 18 1.2.4 Compression and Shear Static Strength ................................................................. 19 1.2.5 Dynamic Loads........................................................................................................ 20 1.2.6 Dynamic Strength.................................................................................................... 20 1.2.7 Fatigue——The Endurance Limit Diagram .......................................................... 20 1.2.8 Stress Concentration ............................................................................................... 23 1.2.9 Allowable Stress and Factor of Safety ................................................................... 23 1.2.10 Creep: A Plastic Phenomenon .............................................................................. 25 1.3 LUBRICATION AND JOURNAL BEARINGS ............................................................... 25 1.3.1 Introduction ............................................................................................................. 25 1.3.2 Theory of Friction ................................................................................................... 25 1.3.3 Journal Bearings ...................................................................................................... 26
  • 3. 2 Contents 1.3.4 Lubricant Characteristics ........................................................................................ 28 1.4 ANTIFRICTION BEARINGS ........................................................................................... 29 1.4.1 Introduction ............................................................................................................. 29 1.4.2 Ball Bearings ........................................................................................................... 29 1.4.3 Life of Antifriction Bearings .................................................................................. 30 1.5 SHAFTS .............................................................................................................................. 31 1.5.1 Introduction ............................................................................................................. 31 1.5.2 Shaft Design ............................................................................................................ 32 1.5.3 Critical Speeds of Shafts......................................................................................... 34 1.6 FUNDAMENTALS OF GEARS ....................................................................................... 34 1.6.1 Introduction ............................................................................................................. 34 1.6.2 Gear Terminology.................................................................................................... 35 1.6.3 Gear Drive System .................................................................................................. 37 PART 2 METALWORK ................................................................................................................. 39 2.1 MARKING OUT AND MEASURING ............................................................................. 39 2.1.1 Tools in General Use ............................................................................................... 39 2.1.2 The Vernier Caliper Gauge ..................................................................................... 41 2.1.3 The Micrometer ....................................................................................................... 42 2.1.4 The Dial Indicator ................................................................................................... 44 2.1.5 Gauges...................................................................................................................... 44 2.2 DRILLING AND REAMING ............................................................................................ 45 2.2.1 Accident Prevention ................................................................................................ 45 2.2.2 Drilling..................................................................................................................... 46 2.2.3 Drilling Machines.................................................................................................... 49 2.2.4 Other Processes ....................................................................................................... 52 2.2.5 Reaming ................................................................................................................... 52 2.2.6 Cutting Fluids .......................................................................................................... 54 2.3 THE LATHE ....................................................................................................................... 55 2.3.1 The Center Lathe ..................................................................................................... 56 2.3.2 Lathe Sizes............................................................................................................... 59 2.3.3 Work Holding and Driving ..................................................................................... 59 2.3.4 Tool Posts................................................................................................................. 63 2.3.5 Lathe Tools .............................................................................................................. 63 2.4 LATHEWORK .................................................................................................................... 67 2.4.1 Turning on Centers .................................................................................................. 67 2.4.2 Screw-cutting in the Lathe...................................................................................... 69 PART 3 ELECTRIC MOTOR CONTROL ................................................................................. 72 3.1 GENERAL PRINCIPLES OF ELECTRIC MOTOR CONTROL ................................... 72 3.1.1 Motor Control Installation Considerations ............................................................ 72 3.1.2 Purpose of Controller .............................................................................................. 73
  • 4. Technical English Through Reading 3 3.1.3 Manual Control........................................................................................................ 75 3.1.4 Remote and Automatic Control .............................................................................. 75 3.1.5 Starting and Stopping.............................................................................................. 77 3.1.6 Speed Control of Motors......................................................................................... 78 3.1.7 Protective Features .................................................................................................. 79 3.1.8 Classification of Automatic Motor Starting Control Systems .............................. 81 3.2 PUSH BUTTONS AND CONTROL STATIONS ............................................................. 81 3.2.1 Push Buttons ............................................................................................................ 81 3.2.2 Selector Switches .................................................................................................... 83 3.3 RELAYS AND CONTACTORS ........................................................................................ 83 3.3.1 Control Relays ......................................................................................................... 83 3.3.2 Solid-state Relay ..................................................................................................... 85 3.3.3 The Transistor as A Switch ..................................................................................... 85 3.3.4 Surge Protection ...................................................................................................... 86 3.3.5 Contactors ................................................................................................................ 86 3.3.6 AC Mechanically Held Contactors and Relays ..................................................... 87 3.3.7 Thermostat Relay .................................................................................................... 89 3.4 TIMING RELAYS .............................................................................................................. 90 3.4.1 Introduction ............................................................................................................. 90 3.4.2 Fluid Dashpot Timing Relays ................................................................................. 90 3.4.3 Pneumatic Timers .................................................................................................... 91 3.4.4 Magnetic Time Limit Relay.................................................................................... 91 3.4.5 Capacitor Time limit Relay..................................................................................... 92 3.4.6 Electronic Timers .................................................................................................... 93 3.4.7 Selecting A Timing Relay ....................................................................................... 93 3.5 TWO-WIRE CONTROLS.................................................................................................. 94 3.6 THREE-WIRE AND SEPARATE CONTROLS ............................................................... 95 3.6.1 Three-Wire Controls ............................................................................................... 95 3.6.2 Push-to-test Pilot Light ........................................................................................... 96 3.6.3 Alarm Silencing Circuit .......................................................................................... 97 3.6.4 Separate Control ...................................................................................................... 97 PART 4 PRINCIPLES OF MEASUREMENT SYSTEM .......................................................... 99 4.1 THE FUNDAMENTALS OF TECHNICAL MEASUREMENT..................................... 99 4.1.1 The general measurement system........................................................................... 99 4.1.2 Calibration ............................................................................................................. 101 4.1.3 Types of Input Quantities...................................................................................... 101 4.1.4 Standards, Dimensions, and Units of Measurement ........................................... 102 4.1.5 Certainty/Uncertainty: Validity of Results .......................................................... 103 4.2 MEASURING SYSTEM RESPONSE ............................................................................ 104 4.2.1 Introduction ........................................................................................................... 104
  • 5. 4 Contents 4.2.2 Amplitude Response.............................................................................................. 104 4.2.3 Frequency Response.............................................................................................. 105 4.2.4 Phase Response ..................................................................................................... 105 4.2.5 Predicting Performance for Complex Waveforms............................................... 106 4.2.6 Delay, Rise Time, and Slew Rate ......................................................................... 107 4.3 SENSING ELEMENT...................................................................................................... 107 4.3.1 Resistive Sensing Elements .................................................................................. 108 4.3.2 Capacitive Sensing Elements................................................................................ 111 4.3.3 Inductive Sensing Element ................................................................................... 112 4.3.4 Electromagnetic Sensing Elements ...................................................................... 114 4.4 SIGNAL CONDITIONING ELEMENTS ....................................................................... 116 4.4.1 Deflection Bridges ................................................................................................ 116 4.4.2 Amplifiers .............................................................................................................. 118 4.5 SIGNAL PROCESSING ELEMENTS ............................................................................ 119 4.5.1 Analogue to Digital (A/D) Conversion ................................................................ 119 4.5.2 Typical Microcomputer System............................................................................ 125 4.5.3 Use of Microcomputer in A Speed Measurement System .................................. 128
  • 6. Technical English Through Reading 1 PART 1 MACHINE DESIGN 1.1 INTRODUCTION TO MACHINE DESIGN 1.1.1 What Is Machine Design Machine design is the application of science and technology to device new or improved products for the purpose of satisfying human needs. It is a vast field of engineering technology which not only concerns itself with the original conception of the conduct in terms of its size, shape and construction details, but also considers the various factors involved in the manufacture, marketing and use of the product. A product can be defined as any manufactured item, including machines, structures, tools, and instruments. People who perform the various functions of machine design are typically called designer, or design engineer. Machine design is a creative activity Basically. However, in addition to being innovative, a design engineer must also have a solid background in the fundamentals of engineering technology. 1.1.2 Fundamental Background for Machine Design A design engineer must have working knowledge in the areas of mechanical drawing, kinematics, material engineering, strength of materials and manufacturing processes. The following statements will indicate how each of these basic background subjects relates to machine design: (1) Mechanical drawing. Detailed drawings must be prepared noting the exact shape, size and material composition for each component, assembly drawings showing how the total product is put together by fastening each part in proper sequence are also needed. (2) Kinematics. Knowledge of this subject, for example, would permit analysis of the motion of the internal mechanism of "Smarty Bird" This analysis would include the attainment of the desired eye-rolling action. Normally, the very creation of the toy and its internal mechanism would occur during this initial phase of machine design called kinematics. (3) Mechanics. Use of this subject provides an analysis of the forces which, for example, act upon a lawn chair when a person is seated in it. Obviously, a person can damage the lawn chair by carelessly jumping on the seat. This motion, in effect, applies dynamic loading instead of the gradually applied loading taken into consideration when the lawn chair was designed. The result of this misuse is excessively large forces that can cause permanent damage. Therefore, using the laws of mechanics, a reasonable amount of dynamic loading should be taken into
  • 7. 2 Part 1 Machine Design account during the early design phase. (4) Materials of engineering. Because the lawn chair is commonly used in an outdoor environment, the tubing is made of aluminum to resist corrosion. The webbing is made of a plastic material that will not readily deteriorate with sustained exposure to sunlight and moisture. Obviously, the proper selection of materials is a vital area of machine design. (5) Strength of materials. The subject concerns itself with whether or not a part is strong enough to sustain the forces it will experience evaluated from mechanics. For example, the size and shape of the aluminum tubular sections of the lawn chair are determined in such a way, that failure will not occur (under normal use) due to excessive stresses and deflections. The magnitude of stresses and deflections depends on the size and shape of a given part as well as on its material, composition, and actual loads. (6) Manufacturing processes. "Smarty Bird" is no simple toy. How each component is produced and how the entire toy is assembled are established by using methods learned in manufacturing technology. It is here that a designer comes to grips with the reality of costs. The flexible shafts are used in "Smarty Bird" because they simplify of manufacturing by eliminating expensive parts and by cutting the labor costs of installing and aligning rigid shafting. In conjunction with the use of the processing fundamentals, there are many significant considerations, which must be detail with in the general field of machine design. Among these are safety, environmental effects, appearance, and economy. 1.1.3 Philosophy of Machine Design An unknown author wrote the following poem called “The designer.” It relates that a design engineer may enjoy making a design so complex that manufacturing of the product is virtually impossible. THE DESIGNER The designer bent across his board Wonderful things in his head wore stored. Said he as he rubbed his throbbing bean, “How can I make this tough machine? Now if I make this part just straight I know that it will work first rate, But that’s too easy to shape and bore It never would make the machinist sore. So I better put an angle there── Then watch those babies tear their hair. And there are the holes that hold the cap I’ll put them down where they’re hard to tap. Now this won’t work, I’ll bet a buck,
  • 8. Technical English Through Reading 3 It can’t be held in a shoe or chuck, In can’t be drilled and it can’t be ground, In fact, the design is exceedingly sound.” He looked again and cried: “At last! Success is mine──it can't even be cast.” Obviously, the foregoing poem is a satire. However, it clearly emphasizes the importance of a design engineer in establishing the manufacturability of a product. As stated previously, the purpose of machine design is to produce a product that will serve a need for man. Inventions, discoveries and scientific knowledge by themselves do not necessarily benefit people; only if they are incorporated into a designed product will a benefit be derived. It should be recognized, therefore, that a human need must be identified before a particular product is designed. Sometimes a human need may be recognized, but a decision is reached to do nothing about it. The reason could simply be that, at the moment, the rewards do not justify the time and effort that must be expended. If, however, the decision is reached to satisfy the human need by manufactured product, the entire project must be clearly defined. Machine design should be considered to be an opportunity to use innovative talents to envision a design of a product, to analyze the system and then make sound judgments on how the product is to be manufactured. It is important to understand the fundamentals of engineering rather than memorize mere facts and equations. There are no facts or equations, which alone can be used to provide all the correct decisions, required producing a good design. On the other hand, any calculations made must be done with the utmost care and precision. For example, if a decimal point is misplaced, an otherwise acceptable design may not function. Good designs require trying new ideas and being willing to take a certain amount of risk, knowing that if the new idea does not work the existing method can be reinstated. Thus a designer must have patience, since there is no assurance of success for the time and effort expended. Creating a completely new design generally requires that many old and will-established methods be thrust aside. This is not easy since many people cling to familiar ideas, techniques, and attitudes. A design engineer should constantly search for ways to improve an existing product and must decide what old, proven concepts should be used and what new, untried ideas should be incorporated. New designs generally have “bugs” or unforeseen problems which must be worked out before the superior characteristics of the new designs, can be enjoyed. Thus, there is a chance for a superior conduct, but only at higher risk. It should be emphasized that, if a design does not warrant radical new methods, such methods should not be applied merely for the sake of change. During the beginning stages of design, creativity should be allowed to flourish without a great number of constraints. Although many impractical ideas may arise, it is usually easy to eliminate them in the early stages of design before manufacturing requires firm details. In this
  • 9. 4 Part 1 Machine Design way, innovative ideas are not inhibited. Quite often, more than one design is developed, up to the point where they can be compared against each other. It is entirely possible that the design that is ultimately accepted, will use ideas existing in one of the rejected designs that did not show as overall promise. Psychologists frequently talk about trying to fit people to the machines they operate. It is essentially the responsibility of the design engineer to strive to fit machines to people. This is not an easy task, since there is really no average person for which certain operating dimensions and procedures are optimums. However, many human operator features must be considered including the following: (1) Size and locations of hand wheels, knobs, switches, and foot pedals; (2) Space allocations for working areas; (3) Ventilation; (4) Colors and lighting; (5) Strength of operator; (6) Safety features; (7) Monotonous operator motions; (8) Operator acceptance. 1.1.4 Communication of Design Another important point which should be recognized, is that a design engineer Must be able to communicate ideas to other people if they are to be incorporated. Initially, the designer must communicate a preliminary design to get management approval. This is usually done by verbal discussions in conjunction with drawing layouts and written material. To communicate effectively, the following questions must be answered: (1) Does the design really serve a human need? (2) Will it be competitive with existing products of rival companies? (3) Is it economical to produce? (4) Can it be readily maintained? (5) Will it sell and make a profit? Only time will provide the true answers to the preceding questions, but the Product should be designed, manufactured, and marketed only with initial affirmative answers. The design engineer also must communicate the finalized design to manufacturing through the use if detail and assembly drawings. Quite often, a problem will occur during the manufacturing cycle. It may be that a change is required in the dimensioning or tolerance of a part so that it can be more readily produced. This falls in the category of engineering changes that must be approved by the design engineer so that the product function will not be adversely affected. In other cases, a deficiency in the design may appear during assembly or testing just prior to shipping. These are always a better way that to do it and the designer should constantly strive towards finding that better way. One
  • 10. Technical English Through Reading 5 reality that needs to be kept in mind is that many of the products that will be in existence ten years from now have probably not yet even been conceived. 1.1.5 Piston Engine Versus the Wankel Engine The automobile, without a doubt, has had one of the most profound influences on people in the twentieth century. Powered predominantly by the conventional reciprocating piston engine during the first seven decades of the twentieth century, the automobile has been the basis for the largest industry in the world. Most people in the United States who are old enough to obtain a license own automobiles. This, however, has contributed to the new problem of "air pollution," which, stated simply, means that the atmosphere is gradually accumulating more and more chemical contaminants harmful to human life. One of the results of increasing air pollution has been a hard look at other types of engines which promise to provide fewer pollutants in exhaust emissions. One such engine recently receiving a great deal of development is the Wankel engine. It is a very possible replacement for the piston engine. A second, concurrent problem is a shortage of available crude oil from which gasoline is derived. Thus, there is also a need for a much more efficient engine as well as one which produces far fewer pollutants. Surprisingly enough, the first four stroke piston engine was built in 1866 by Nickolaas August Otto and Euger Langen of Germany. The present-day piston engine works on basically the same principles as the one built by Otto and Langen. In fact, the thermodynamic process operating in the modern piston is called the Otto cycle. On the other hand, the Wankel rotary engine was not invented until 1954, when Felix Wankel, also of Germany, discovered that he could reproduce the Otto cycle with a purely rotary-type engine. From an efficient point of view, the Wankel engine is superior because it is simpler, contains fewer parts and operates more quietly. It wasn’t, however, until the 1960s that much effort was put into the development of the Wankel engine. The main reason for this was that the piston engine was already a proven, reliably working power plant. It was the human needs for an engine with far less polluting exhaust emissions, which apparently spearheaded new engine developments in the early 1960s. In 1967, Toyo Kogyo of Japan was manufacturing Wankel-powered Mazda automobiles and the United States began to witness the impact by the early 1970s. Recognizing the great potential of the Wankel engine, General Motors in 1971 signed a $50 million licensing agreement with Wankel patent holders (Wankel Gmb H and Audi NSU and the U. S. licensee, Curtiss Wright). From 1971 to 1975, General Motors is to pay the $50 million and develop the Wankel engine for is own conduction. After 1975, General Motors can use its own designed Wankel engines without paying any additional licensing fees. Next, let us examine the design and operation of the piston and Wankel engines. 1.1.6 The Four-stroke Automotive Piston Engine
  • 11. 6 Part 1 Machine Design Figure 1-1 shows the conventional four-stroke piston engine, which contains a piston reciprocating in a fixed cylinder inside an engine block. A connecting rod is attached to the piston by a wrist pin and to the crank by a crankpin. As the piston reciprocates, the crank, and hence the crankshaft, is forced to rotate inside of bearings. The detailed operation is as follows: (a) Intake stroke (Figure 1-1a). The intake valve opens, allowing a mixture of fuel and air to enter the cylinder. The exhaust valve is closed during most of the stroke. The crankshaft rotates 180 degrees while the piston moves from top dead center (TDC) to bottom dead center (BDC). (b) Compression stroke (Figure 1-1b). Both valves are closed during this stroke. The fuel-air mixture is compressed as the piston rises. Near the end of the stroke, the spark plug fires. The piston moves from BDC to TDC as crankshaft rotates 180 degrees. (c) Power stroke (Figure 1-1c). Both valves are initially closed. The fuel-air mixture burns and increases the temperature. This causes the gas to expand and drive the piston down with power. The exhaust valve opens near the end of the stroke. The power stroke occurs while the crankshaft rotates through 180 degrees. (d) Exhaust stroke (Figure 1-1d). The exhaust valve opens fully as the products of combustion are removed from the cylinder. The intake valve opens near the end of the exhaust stroke. During this stroke, the crankshaft rotates 180 degrees. The following observations should be noted for the four-stroke piston engine: (1) There are four different strokes for one complete cycle of operation. (2) One complete cycle of operation (and thus each power stroke) requires two revolution of the crankshaft. (3) Timing is important. A camshaft is driven by the crankshaft through a gear system or timing chain. One rotation of the camshaft has a separate cam for each intake valve and a separate cam for each exhaust valve. For example, a six-cylinder engine will have twelve cams. (4) The rotary motion of the crankshaft is the desired engine output. However, it is obtained indirectly by a kinematics transformation from an up-and-down reciprocating piston whose motion must stop at all TDC and BDC position. 1.1.7 Rotary Wankel Engine Figure 1-2 shows the configuration of the new rotary Wankel engine, which is being considered as a possible replacement for the conventional piston engine. There is a triangular-shaped rotor which revolves inside a housing having an epitrochoid shape resembling the outline of a figure eight. The triangular-shaped rotor contains an internal gear, which is a planet gear meshing with a smaller sun gear attached to one side of the housing. An eccentric shaft goes through the center of the fixed gear and it is supposed by bearings in the front and rear ends of the housing. As the rotor revolves, it slides against the eccentric shaft. This sliding contact causes the eccentric shaft to rotate providing rotary-haft output. Because there are 1.5 times as many teeth on the planet gear as on the sun gear, the eccentric crankshaft
  • 12. Technical English Through Reading 7 makes three revolutions for each revolution of the triangular rotor. FIG.1-1 The four-stroke automotive piston engine The operation of the Wankel engine for one complete cycle, using face No. 1 as an example, is follows: (a) Intake (Figure 1-2a). Face no.1 of the rotor uncovers the intake port. As the space between the rotor and housing increases, the fuel air mixture is down into the engine. (b) Compression (Figure 1-2 b). Further rotation of the eccentric rotor seals the intake port and compresses the fuel-air mixture as the space between the rotor and the housing decreases. (c) Ignition (Figure 1-2c). When the space between the rotor and housing reaches a minimum, the spark plug fires and ignites the fuel-air mixture. (d) Expansion (Figure 1-2d). the sudden increase in pressure forces the rotor to revolve. (e) Exhaust (Figure 1-2e). Face no.1 of the rotor uncovers an exhaust port and the products of combustion are discharged.
  • 13. 8 Part 1 Machine Design FIG.1-2 One complete cycle of the rotary Wankel engine It should be noted that the eccentric rotor and housing are equivalent to the camshaft, cams, pistons and cylinders of the piston engine. In effect, the triangular-shaped rotor replaces the three pistons, since face nos.2 and 3 of the rotor perform the same function as face no.1. Thus, with a rotor speed of one-third the eccentric crankshaft speed, one power pulse is produced for each revolution of the crankshaft. 1.1.8 Major of Machine Design As mentioned previously, machine design is a vast field of engineering technology. As such, it begins with the conception of an idea and follows through the various phases of design analysis, manufacturing, marketing and consumerism. The following is a list of the major areas of
  • 14. Technical English Through Reading 9 consideration in the general field of machine design: (1) Initial design conception; (2) Strength analysis; (3) Materials selection; (4) Appearance; (5) Manufacturing; (6) Economy; (7) Safety; (8) Environmental effects; (9) Reliability and life; (10) Legal considerations. Let us next examine each of the major areas of machine design (sections 1.9 through 1.18) with particular reference to the automotive industry. 1.1.9 Initial Conception of Design From a kinematics point of view, the Wankel engine is superior to the piston engine since the Wankel engine has only two moving parts─the rotor and the crankshaft. In addition, since each face of the revolving rotor forms a moving charmer, power delivered in a smooth, continuous form. There is no jolting up-and-down motion as experienced in the piston engine. One might also wonder why initial conception of the automatic engine was reciprocating piston design since the desired output is, in fact, a rotating shaft. The basic piston engine is an easier system to visualize from a geometrical point of view, although it does require additional parts, such as cams, a camshaft and intricate mechanisms, to facilitate the rapid opening and closing of the valves. Also, recall that there is one ignition pulse for every revolution of the output shaft of the Wankel engine, whereas there is only one power stroke for every two crankshafts revolutions in a four-stroke piston engine. Thus the ratio of horsepower to cubic inches of displacement for a Wankel engine is about twice that for a four-stroke piston engine. The conclusion here is that a Wankel rotary engine can produce the same amount of power in about one-half the space. After the preceding discussion, it should be readily apparent that the initial conception of a design is a vital area of machine design. It is during this process that a particular product is given its very character. However, before final judgment can be made on the choice of initial design, all other major areas of machine design should be considered. 1.1.10 Strength Analysis Strength is measure of the ability to resist, without failure, forces which cause stresses and strains. The forces may be (1) gradually applied;
  • 15. 10 Part 1 Machine Design (2) suddenly applied; (3) applied under impact; (4) applied with continuous direction reversals; (5) applied at low or elevated temperature. If a critical part of machine fails, the whole machine must be shut down until a repair is made. Thus, when designing a new machine, it is extremely important that critical parts be made strong enough to prevent failure. The designer should determine as precisely as possible the nature, magnitude, direction and point of application off all forces. Machine design is not, however, an exact science, and it is, therefore, rarely possible to determine exactly all the applied forces. In addition, different samples of a specified material will exhibit somewhat different abilities to resist loads, temperatures and other environmental conditions. In spite of this, design calculations based on appropriate assumptions are invaluable in the proper design of machines. 1.1.11 Selection of Materials The material composition of a part affects its ability to resist loads, corrosion and wear. Materials should be selected based on their particular properties, such as yield strength, resilience, toughness, ductility, hardness, fatigue strength, creep strength, corrosion resistance and machinability. Quite often, the most difficult problem a designer faces is the selection of a suitable material. First, the characteristics that are essential should be determined and separated from those that are merely desirable. Then the material which comes closest to providing the essential characteristics, should be selected. Of major concern in the Wankel engine is the life of the seals at each of the three edges. A seal material must be selected which will not break under operating conditions and will not dig into the housing due to vibration. The seals must withstand high temperatures and continuous sliding contact with the housing at high speeds. Temperatures exceeding 1000 F prevent the use of materials such as rubber. They require special materials such as ceramics and complex carbon-metal alloys. 1.1.12 Appearance Quite often, appearance can be the deciding factor in the buying of a product. This is especially true of the potential customer is the public, as is the case for automobiles, homes, boats, toys, electrical appliance and furniture. Generally speaking, the following characteristics tend to make the appearance of a product more pleasing to the human eye: (1) Symmetry; (2) Simplicity; (3) Durability; (4) Clean contour;
  • 16. Technical English Through Reading 11 (5) Comfort; (6) Distinction and richness; (7) Modern shape. 1.1.13 Manufacturing It is not the responsibility of the design engineer to select the manufacturing processes for a given product. However, the specific design configurations, tolerances and material compositions of each part directly affect the optimum manufacturing processes as well as the entire production cycle. For the contour of the part and the material properties of the part must be compatible with one or more of the following manufacturing processes: (1) Machining; (2) Casting; (3) Forging; (4) Bending; (5) Stamping; (6) Extruding; (7) Ejection molding. Each of the preceding processes has a general range of precision that can be readily attained. Thus, the specified part tolerance partially determines which process should be utilized. Another important manufacturing consideration is the fastening of various parts to produce an assembled product. Which one of the following fastening method to use depends on the design characteristics of the product? (1) bolting(permits easy disassembly); (2) Interlocking of flexible members(permits easy disassembly); (3) Riveting (disassembly possible, but not easy; (4) Welding (produces permanent joints); Desired surface finish also greatly determines the manufacturing process. Appearance and corrosion resistance requirements usually dictate the type of finishing process, such as painting, polishing, plating and plastic coating. 1.1.14 Economy No product will sell in appreciable quantities if the price is exorbitant. It is the responsibility of the design engineer to see that a product has the required quality at the lowest possible cost. The cost to the manufacturer consists largely of labor and materials, while the cost to the customer is not only the selling price of the product, but also includes operational and maintenance expenses. A low-priced product with poor quality may require continual maintenance, and such a product will have trouble staying on the market. Similarly, if the price is extremely high, the same difficulty in marketing will be experienced, even if the product quality is outstanding. It is
  • 17. 12 Part 1 Machine Design important to realize that lower cost does not necessarily mean lower quality. In considering a product, the designer should always ask the following questions: (1) What is the basic function of the product? (2) How can this function be best obtained? (3) Is a corrosion-resistant material necessary absolutely? (4) Can standard parts such as nut and bolt be used? (5) Can assembly and disassembly be readily accomplished? (6) Does the design provide for desired interchangeability? (7) Which component parts should be purchased from an outside vendor? (8) Can the tolerances be increased? (9) Is a very smooth surface really required? Relative to the last question, for example, in order to achieve a surface finish of 4 microinches or less, the following sequence of machining operations is generally required: rough machine, finish machine, rough grind, finish grind, bore and lap. 1.1.15 Safety A product should be designed so that it will not endanger the life of its user under proper operating conditions. Obviously, a design engineer has little control over misuse of a product. However, an attempt should be made to reduce the possibility of misuse by designing to prevent the user from accidentally circumventing a safety feature. In addition, the design should be such that it minimizes the skill required by the operator. This is especially important with the automobile, since it is operated by people who possess varying skills to drive under divers weather and road conditions. The following is a list of some of the many safety feature which being incorporated or developed for the automobile due to ever-increasing emphasis on safety: (1) Energy-absorbing bumpers; (2) Door buzzers; (3) Air bags which inflate upon heavy impact; (4) Dual hydraulic brake system with driver’s warning light; (5) Lap and shoulder belts with flashing light and buzzer sound when the driver and/or passenger have not buckled their lap belts; (6) Head restraints; (7) Steel guardrails inside doors; (8) Steel-belted, radial-ply tires. 1.1.16 Environmental Effects If a product is to be successful, it should not have a seriously damaging effect on the environment. One type of undesirable environmental effect, noise pollution, usually can be
  • 18. Technical English Through Reading 13 reduced with proper design. Since noise is generated by a vibrating, component vibration dampening is one possible solution. A second type of effect harmful to the environment is air pollution. From our previous discussion, one might expect that the Wankel engine produces less exhaust-emission pollutants than does the piston engine. This is generally not true. However, since the horsepower-to-weight ratio is twice as great for the Wankel engine, it is easier to attach additional antipollution devices, a fact which gives the Wankel engine greater potential for reducing exhaust-emission pollutants. 1.1.17 Reliability and Life Life is the period of time over which a product functions properly. In general, the price of a product is approximately proportional to its designed life. Therefore, it is not economical to provide an unnecessarily long life. On the other hand, a life that is considered too short by the user is generally thought of as being the result of poor quality. Thus, the design engineer must use good judgment, based on economy and customer attitudes, in selecting the optimal life for which to design a given product. The three most significant factors that affect life are corrosion, wear and fatigue. Having a close relationship with life is the factor reliability. A product is said to be reliable if it functions properly every time it is used. If a design is sensitive to moisture, heat, cold, manufacturing tolerances and slight variations in operator characteristics it is considered to have low reliability. Generally speaking, a simple machine is more reliable than a complex one, all other factors being equal. In 1952, Robert Lusser proposed a curve showing the failure rate for a hypothetical product. The curve depicts three phases of life which are called infant mortality, constant hazard and wear-out. The theory is that during the initial use of the product the failure rate is high due to "bugs" such as faulty parts, misassemble or slight design deficiencies. As the product is debugged, the failure rate decreases. Then comes the break-in period, during which the product operates reliably with a minimum failure rate under constant hazards of life. Failures, although infrequent, usually occur due to unexpected conditions, such as overloads, misuse or excessive environmental elements. After a given period of useful life, the product starts to wear out and the failure rate increases. This phase is usually attributed to fatigue, corrosion or simple wear of essential components. Preventive maintenance can usually prolong the useful life range by the use of planned systematic inspection and repair procedures. 1.1.18 Legal Considerations It is not reasonable to expect that a design engineer will be knowledgeable in all the various legal situations that can crop up due to the use of a product. However, it is the design engineer's responsibility to strive to design the product with safety as an utmost consideration. All safety
  • 19. 14 Part 1 Machine Design features required by low should be incorporated where applicable. In addition, the customer should be warned of any potential dangers that may arise, with misuse of the product. In a case where a lawsuit is brought against a manufacturer, it is very important to show that quality of the product is generally of the high level prevailing in the given industry. For example, a drawing of a product component or assembly is considered to be a legal document and as such, can be used in court. Another legal matter has to do with patents. Most technical employees, as a requirement for employment, must sign a patent agreement with their employer. If such an employer comes up with an invention, the patent agreement protects the employer from its competitors if a patent is awarded for the invention. The following is a list of facts dealing with patents: (1) The first step in applying for a patent is to make a patent search to see if the invention has previously been patented. (2) A patent cannot step be granted if the invention has appeared in a printed publication more than one year before the application was made or if the invention has been previously used by the public. (3) The term "patent pending" means that an application has been filed but the patent has not yet been issued or denied. (4) A patent is granted by the United States Government to a person (not a company) for a period of 17 years. (5) Devices that are not useful cannot be patented. (6) When a patent is granted, it prevents others from producing, using or selling the potential invention. (7) Patent can be sold since they are considered as personal property. (8) A legal suit can be made by the holder of a patent if another person infringes on the patent. (9) Any new product, process or material can be patented if it qualifies. 1.2 FAILURE ANALYSIS AND DIMENSIONAL DETERMINATION 1.2.1 Introduction The principal objective of this chapter is to answer the following two questions: (1) Why do mechanical parts fail? (2) How are the dimensional characteristic of a part determined? It is essential absolutely that a design engineer knows how and why parts fail so that reliable machines which require minimum maintenance can be designed. Sometimes a failure can be serious, such as when a tire blows out on an automobile traveling at high speeds. On the
  • 20. Technical English Through Reading 15 other hand, a failure may be no more than nuisance. An example is the loosening of the radiator hose in the automobile cooling system. The consequence of this latter failure is usually the loss of some radiator coolant, a condition that is readily detected and corrected. The type of load a part absorbs is just as significant as the magnitude. Generally speaking, dynamic loads with direction reversals cause greater difficulty than static loads and, therefore, fatigue strength must be considered. Another concern is whether the material is ductile or brittle. For example, brittle materials are considered to be unacceptable where fatigue is involved. Many people mistakenly interpret the word fatigue to mean the actual breakage of part. However, a design engineer must consider a broader understanding of what constitutes failure. For example, a brittle material will fail under tensile load before any appreciable deformation occurs. A ductile material, however, will deform a large amount prior to rupture. Excessive deformation, without fracture, may cause a machine to fail because the deformed part interferes with a moving second part. Therefore, a part fails (even if it has not physically broken) whenever it no fulfills its required function. Sometimes failure may be due to wear that can change the correct position of mating parts. The wear between two mating parts may be due to abnormal friction or vibration. Failure may also be due to a phenomenon called creep, which is the plastic flow of a material under load at elevated temperatures. In addition, the actual shape of a part may be responsible for failure. For example, stress concentrations due to sudden changes in contour must be taken into account. This is especially true when dynamic loads with direction reversals exist and the material is not very ductile. In general, the design engineer must consider all possible modes of failure, which include the following: (1) Stress; (2) Deformation; (3) Wear; (4) Corrosion; (5) Vibration; (6) Environmental damage; (7) Loosening of fastening devices. The part sizes and shapes selected must also take account many dimensional factors which produce external load effects, such as geometric discontinuities, residual, residual stresses due to forming of desired contours, and the application of interference fit joints. 1.2.2 Tensile Static Strength Lack of understanding of the behavior of materials under actual service conditions has been the cause of many serious failures. A great deal information can be revealed by performing laboratory tests. One such experimental evaluation of static strength is the tensile test, in which a standard test specimen with threaded ends to fit the screw grips of testing machine is used. Figure 1-3 shows the specimen, which has a circular cross section. The shank diameter of 0.505
  • 21. 16 Part 1 Machine Design 2 inch corresponds to a cross section area (A) of 0.2 in. . The testing machine gradually applies a tensile force (F) which is measured a dial in units of pounds. A device called extensometer is attached to the specimen at the two gauge marks that are separated initially by a distance l equal to 2 inches. The extansometer measurers the elongation ( Δl) of distance l between the gauge marks. FIG.1-3 Standard test specimen with threaded ends The stress (S) is calculated by equation 1-1. Since stress is defined as the force per unit area, the units for stress are pounds per squire inch (lb/in.2). S yield strength S allowable = (1-1) FS ductile material Strain is defined as the change in length divided by the original length. Hence, strain ( ε)can be found by using ∆l ε= (1-2) l A complete tensile test is obtained by continuing to gradually increase the load until the specimen breaks. A plot of the entire test is called a stress-strain diagram. A typical stress-strain diagram for a ductile material such as mild steel is shown in figure 1-4. FIG.1-4 Typical stress-strain diagram for mild steel (1) The following information should be clearly understood relative to a tensile test:
  • 22. Technical English Through Reading 17 The stress-strain diagram is linear up to point B, which is the proportional limit. The slope of the straight line of this relationship (called the modulus line) is defined as the modulus of elasticity (E) and is mathematically represented by Stress S E= = (1-3) Strain ε (2) If the load is released at any value below point B, the test specimen will contract exactly along the modulus line until point A, where stress and strain both equal zero, is reached. Thus, the proportional limit is defined as the highest stress a part can sustain and yet return to its original shape and size when the load is removed. The modulus line from A to B represents the elastic portion of the stress-strain diagram. (3) If the proportional limit is exceeded, the material begins to yield plastically. This means there is a small increase in stress while the strain increase appreciably. Assume that the load is increased until point D is reached, at which time the load is removed. The remarkable result is that the specimen returns along the dashed line (which is parallel to the modulus line) until point. G is reached. Obviously, the part has yielded plastically, because there is permanent strain although the load is zero. Since the permanent strain at point G is 0.002in. /in., point D is called the o.2 percent yield strength. Other percents of yield strength, such as 0.1 percent, could just as well have been chosen. These values simply indicate the stress levers required to produce corresponding amounts of permanent strain. Sometimes the term yield strength is used without a percent value. The yield strength (commonly called the yield point) is the stress level where the S-E diagram temporarily flattens out, as shown in Figure 1-4 at point C. (4) As loading progresses beyond point C, the specimen continues to elongate plastically. The load dial on the test machine and the extensometer output show that the load is increasing very slowly compared to the elongation. Maximum load and, thus, maximum calculated stress are reached at point E. The stress level corresponding to point E is called the ultimate strength. (5) When point E is reached, the specimen loses its ability to resist load. The load dial indicates an automatic drop in load until point F is reached and the specimen ruptures. The stress at point F is called the rupture stress, or rupture strength. (6) The two broken sections of the test specimen are removed from the machine grips and positioned with the two rupture ends together. Inspection reveals a thinning of the shank in the area if the rupture. This phenomenon is called necking occurs only with ductile materials. Notice that the shape of the necked-down section reveals a cup and cone configuration when the two broken sections are separated. (7) It should be recognized that all the stress calculations have been based in the original 2 cross-sectional area of 0.2 in. . This one value has been used for ease testing, since it would be very time-consuming to continuously monitor the actual values of area. Also, note that the area changes by a negligibly small amount up to the 0.2 percent yield strength. This fact is demonstrated in Figure 1-5, where the actual (using actual areas) and apparent (using original area) stress-strain diagrams are superimposed. Notice that in reality the stress continues to increase throughout the test although the dial on the test machine shows a decreasing load
  • 23. 18 Part 1 Machine Design beyond the ultimate strength. The actual S- ε diagram does not really provided any additional valuable information because most machine parts are designed to operated at stress levels below the 0.2 percent yield strength. The percent elongation and percent reduction in area, two very important parameters, can be obtained from examination of the specimen after rupture. FIG.1-5 Comparison of actual and apparent stress-strain diagrams (8) The modulus of the elasticity (at given temperature is the same for different types of steels. In reality, what differs is the value of the proportional limit. Specifically, all types of 2 steel possess a modulus of elasticity of approximately 30×106lb/in. at room temperature. (9) A brittle material does not exhibit large plastic deformations prior to rupture. This is shown in Figure 1-6 for cast iron and concrete. Also, there is no necking down of a brittle material loaded to failure, because there is no significant amount of plastic deformation. Recall that for a ductile steel part loaded to failure, over 95 percent of the deformation is plastic as compared to elastic. FIG.1-6 S-εdiagram comparison between ductile and brittle materials 1.2.3 Design Properties of Materials The following design properties of materials are defined as they relate to the tensile test: (1) Static strength. The strength of a part is the maximum stress that the part can sustain without losing its ability to perform its required function. Thus, the static strength may be considered to be approximately equal to the proportional limit since no plastic deformation takes place and damage theoretically is done to the material.
  • 24. Technical English Through Reading 19 (2) Stiffness. Stiffness is the deformation-resisting property of a material. The slope of the modulus line and, hence, the modulus of the elasticity are measures of the stiffness of a material. (3) Resilience. Resilience is the property of a material which permits it to absorb energy without permanent deformation. Therefore, it is represented by the area underneath the S- ε diagram within the elastic region. (4) Toughness. Resilience and toughness are similar properties. However, toughness is the ability to absorb energy without rupture. Thus toughness is represented by the total area underneath the S- ε diagram. Obviously, the toughness and resilience of brittle materials are very low and are approximately equal. (5) Brittleness. A brittle material is one which ruptures before any appreciable plastic deformation takes place. Brittle materials are generally considered undesirable for machine components because they are unable to yield locally at location of high stress due to geometrical stress raisers such as shoulders, holes, notches or keyways. (6) Ductility. A ductile material exhibits a large amount of plastic deformation prior to rupture. Ductility is measured by present reduction of area and percent elongation of a part loaded to rupture. A 5 percent elongation at rupture is considered to be the dividing line between ductile and brittle materials. (7) Malleability. Malleability is essentially a measure of the compressive ductility of a material and, as such, is an important characteristic of metals that are to be rolled into sheets. (8) Hardness. The hardness of a material is its ability to resist indentation or scratching. Generally speaking, the harder a material, the more brittle it is and, hence, the less resilient. Also, the ultimate strength of a material is roughly proportional to its hardness. (9) Machinability. Machinability is a measure of the relative ease with which a material can be machined. In general, the harder the material, the more difficult it is to machine. 1.2.4 Compression and Shear Static Strength In addition to the tensile tests, there are other types of static load testing which provide valuable information. Two of these are as follows: (1) Compression testing. Most ductile materials have approximately the same properties in compression as in tension. The ultimate strength, however, cannot be evaluated for compression. As a ductile specimen flows plastically in compression, the material bulges out, but there is no physical rupture as is the case in tension. Therefore, a ductile material fails in compression due to deformation, not stress. (2) Shear testing. Shafts, bolts, reverts and welds are loaded in such a way that shear stresses are produced. A plot of the shearing stress-shearing strain diagram reveals a similar pattern as compared to the tensile test. The ultimate shearing strength is defined as the stress at which failure occurs. The ultimate strength in shear, however, does not equal the ultimate strength in tension. For example, in the case of steel, the ultimate shear strength is
  • 25. 20 Part 1 Machine Design approximately 75 percent of the ultimate strength tension. This difference must be taken into account when shear stresses are encountered in machine components. 1.2.5 Dynamic Loads An applied force that does not vary in any manner is called static, or steady load. It is also common practice to consider applied forces that seldom vary to be static load. On the other hand, which vary frequently in magnitude and direction are called dynamic loads. Dynamic loads can be subdivided to the following three categories: (1) Varying load. With varying loads, the magnitude changes but the direction does not. For example, the load may produce high and low tensile stresses but no compression stresses. (2) Reversing load. In this case, both the magnitude and direction change. These load reversals produce alternately varying tensile and compressive stresses which commonly referred to as stress reversals. (3) Shock load. This type of load is due to impact. One example is an elevator dropping on a nest of springs at the bottom of chute. The resulting maximum spring force can be many times greater than the weight elevator. The same type of shock load occurs in automobile springs when a tire hits a bump or hole in the road. 1.2.6 Dynamic Strength Parts designed based on the proportional limit are acceptable for static loads. However, these same parts will not function properly if they are subjected to a large number of load reversals. A different material parameter called fatigue strength must be considered in machines with moving parts. For example, it is possible for a part to fail even though it has never been stressed up to its proportional limit. This due to a phenomenon called fatigue. To understand the mechanism of fatigue, refer to Figure 1-7 that shows a fatigue-testing machine. The test specimen, which has a polished surface, is mounted in bearings as shown. A motor rotates the test specimen, which is actually a circular shaft of known size and material composition. A second set of bearings supports a trunnion which permits the application of specified amounts of weight (W) at the midpoint position of length of the test specimen. By adding or removing weights, the desired magnitude of the load can be selected. We have, in effect, a rotating shaft which is experiencing a banding load. 1.2.7 Fatigue——The Endurance Limit Diagram The test specimen of Figure 1-3, after a given number of stress reversals, will experience a crack at the outer surface where the stress is greatest. The initial crack starts where the stress exceeds the strength of the grain on which it acts. This is usually where there is a surface defect such as a material flaw or a tiny scratch. As the number of cycles increases, the initial crack begins to propagate into a continuous series of cracks that all around the periphery of the shaft.
  • 26. Technical English Through Reading 21 The conception of the initial crack is in itself a stress concentration which accelerates the crack propagation phenomenon. Once the entire periphery becomes cracks, the cracks start to move toward the center of the shaft. Finally, when the remaining solid inner area becomes small enough, the stress exceeds the ultimate strength and the shaft suddenly breaks. Inspection of the break reveals a very interesting pattern. The outer annular area is relatively smooth because mating cracked surfaces had rubbed against each other. However, the center portion is rough, indicating a sudden rupture similar to that experienced with the fracture of brittle materials. FIG.1-7 Fatigue testing machine This brings out an interesting fact. When actual machine parts fail due to static loads, they normally deform appreciably because of the ductility of the material. Thus, many static failures can be avoided by making frequent visual observations and replacing all deformed parts. However, fatigue failures give no warning. They fail suddenly without deformation and, thus, are more serious. It has been estimated that over 90 percent of broken automobile parts have failed through fatigue. The fatigue strength of a material is its ability to resist the propagation of cracks under stress reversals. Endurance limit is a parameter used to measure the fatigue strength of a material. By definition, the endurance limit is the stress value below which an infinite number of cycles will not cause failure. Let us return our attention to the fatigue testing machine of Figure 1- 7. The test is run as follows: A small weight is inserted and the motor is turned on. At failure of test specimen, the counter registers the number of cycles, N, and the corresponding maximum bending stress is calculated. The broken specimen is then replaced by an identical one and an additional weight is inserted to increase the load .A new value of stress is calculated and the procedure is repeated until failure requires only one complete cycle. A plot is then made of stress versus number of cycles. Figure 1-8a shows the plot, which is called the endurance limit, or S-N curve. Since it would take forever to achieve an infinite number of cycles, 1 million cycles is used as a reference. Hence, the endurance limit can be found from Figure 1-8a by noting that it is the
  • 27. 22 Part 1 Machine Design stress level, below which the material can sustain 1 million cycles without failure. The relationship depicted in Figure 1-8 is typical for steel because the curve becomes horizontal as N approaches a very large number. Thus, the endurance limit equals the stress level where the curve approaches a horizontal tangent. Due to the large number of cycles involved, N is usually plotted on a logarithmic scale as shown Figure 1-8b. When this is done, the endurance limit value can be readily detected by the horizontal straight line. For steel, the endurance limit equals approximately 50 percent of the ultimate strength However, if the surface finish is not of a polished quality, the value of the endurance limit will be lower. For example, for steel parts with a machined surface finish of 63 microinches, the percentage drops to about 40 percent. For rough surfaces (300 microinches or greater), the percentage may be as low as 25 percent. FIG.1-8 Typical endurance limit diagram The most common type of fatigue failure is that due to bending. The next most frequent is torsion failure, whereas fatigue failure due to axial loads occurs very seldom. Spring materials are usually tested by applying variable shear stresses that alternate from zero to a maximum value, simulating the actual stress patterns. In the case of same nonferrous metals, the fatigue curve does not become horizontal as the number of cycles becomes very large. This means that a large number of stress reversals will cause failure regardless of how small the value of stress is. Such a material is said to have no endurance limit. For most nonferrous metals having an endurance limit, the value is about 25
  • 28. Technical English Through Reading 23 percent of the ultimate strength. 1.2.8 Stress Concentration Basic formulas for stress assume that there are no irregularities in the shape of the parts undergoing load. However, it is impossible to design a machine without allowing discontinuities in the contour of parts. The following are a few examples where changes in contour are necessary: (1) Shafts with shoulders to accommodate the seating of bearings; (2) Keyways in shafts which use keys to secure pulleys, cams and gears; (3) Threads on one end of a bolt and a head on the other end; (4) Fillets at the base of gear teeth. Geometrical irregularities cause the stress in the area of the discontinuity to exceed that predicted by equations. The physical discontinuity is called a stress raiser or a region of stress concentration. A stress concentration factor, K, is defined as the maximum actual stress divided by the average calculated stress based on the minimum cross-sectional area. Stress concentration factors have been experimentally determined for various geometrical and load configurations; several of the popular experimental methods used are photoelastic study, strain gauge application and the Moire technique. You will find that graphs provide detailed stress concentration factors in reference books such as Stress Concentration Design Factors by R. E. Peterson. It is important for the design engineer to have an intuitive grasp of the relative effects of various geometrical configurations on stress concentration. This can be done by assuming that forces and, hence, stresses flow through loaded parts. 1.2.9 Allowable Stress and Factor of Safety The stress at which a part is designed to operate based on calculation is called the allowable, or design, stress. It would be risky to design machine parts to operate at the yield strength where some permanent deformation takes place. In deciding on an allowable stress value, the design engineer must take into account the consequences of failure, such as danger to humans and high costs of repair. The types of loads and the number of unknowns which affect the life and reliability of a machine must also be considered. As result, a factor of safety (FS) is applied to the yield strength for ductile materials. A different factor safety is applied to the ultimate strength when brittle materials are used. The reason for using the ultimate strength in he latter case is that brittle materials do not have a pronounced yield strength. Therefore, We can define the allowable stress mathematically by Equations 1-3 and 1-4 for ductile and brittle materials respectively: S yield strength S allowable = (1-4) FS ductile material
  • 29. 24 Part 1 Machine Design S ultimate strength S allowable = (1-5) FS brittle material Factor of safety is, in a large measure, an ignorance factor which compensates for various possible unknowns, including the following: (1) Exact type and magnitude of all loads; (2) Material property variations; (3) precise stress concentration effects; (4) Extremes of environmental condition such as heat and moisture; (5) Approximate stresses analysis formulas; (6) Residual stresses produced during manufacturing. Consequently, the determination of a realistic factor of safety is difficult task and is usually handled by a senior design engineer. If the fact of safety is excessive, the cost and sizes of parts increase. On the other hand, too small a factor of safety results in premature failure which can be very costly in terms of human safety as well as in dollars. The following equation defines factor of safety as a function of four main parameters: FS=a×b×c×d (1-6) The significance of each parameter is explained as follows (not that the values presented are approximate and can vary somewhat depending on the judgment of the design engineer): a=1 for constant magnitude and direction loads. a=1 for complete load reversals in order to take fatigue into account. b=1 for gradually applied loads. b=2 for suddenly applied loads. b=3 for more (depending on the severity)for impact loads. Factor c considers the consequences of failure as they relate to human safety, cost, etc.: c normally varies between 1.2 and 2,depending on the seriousness of a failure. Factor d differentiates FS ductile material from FS brittle material: ultimate strength d= yield strength Factor d is used only when basing the factor of safety on the ultimate strength, which would normally be done for brittle materials. Otherwise, d equals unity. The value of allowable stress obtained from Equation 1-4 or 1-5 is the value which the design engineer attempts to match during the design and stress analysis process. It should be noted that all stress concentrations are included as part of the stress analysis procedure. Therefore, the design should be such that a thorough stress analysis predicts that the maximum actual operating stress will equal the allowable stress. Also note that the strength of some parts, such as pressure vessels, are dictated by government codes which include the use of specific equations. Caution must be exercised in the use of these equations since the desired factor of safety is usually built in.
  • 30. Technical English Through Reading 25 1.2.10 Creep: A Plastic Phenomenon Temperature effects bring us to a phenomenon called creep, which is the increasing plastic deformation of a part under constant load as a function of time. Creep also occurs at room temperature, but the process is so slow that it rarely becomes significant during the expected life of the part involved. If the temperature is raised to 300°F or more, the increasing plastic deformation can become significant within a relatively short period of time. The creep strength of a material is its ability to resist creep, and creep strength data can be obtained by conducting long-time creep tests simulating actual part operating conditions. During the test, the plastic strain is monitored for given materials at specified temperatures. 1.3 LUBRICATION AND JOURNAL BEARINGS 1.3.1 Introduction A bearing can be defined as a member specifically designed to support moving machine components. The most common bearing application is the support of a rotating shaft which is transmitting power from one location to another; one example is the crankshaft bearings of automatic engine; another example is the shaft bearings used all types of electric motors. Since there is always relative motion between a bearing and its mating surface, friction is involved. In many instances, such as the design of pulleys, brakes and clutches, friction is desirable. However, in the case of bearings, the reduction of friction is one of prime considerations: friction results in loss of power, generation of heat and wear of mating surfaces. Journal and antifriction bearings are the two general types of bearings existence. Journal bearings operate with sliding contact, whereas antifriction bearings experience predominantly rolling contact. The amount of sliding friction in journal bearings depends on the surface finishes, materials, sliding velocities and the type of lubricant used. The principle motion-retarding effect in antifriction bearings is called rolling resistance rather than rolling friction. This is so because the resistance of motion is essentially due to the deformation of the rolling elements and, hence, it is not a sliding phenomenon. Antifriction bearings will be in chapter 1.4. To reduce the problems associated with sliding friction in journal bearings, a lubricant is used in conjunction with compatible mating materials. When selecting the lubricant and mating materials, one must take into account bearing pressures, temperatures and rubbing velocities. The principle function of the lubricant in sliding contact bearings is to prevent physical contact between the rubbing surfaces. Thus the maintenance of an oil film under varying loads, speeds and temperature is the prime consideration in sliding contact bearings. 1.3.2 Theory of Friction
  • 31. 26 Part 1 Machine Design Friction is the resistance one part exerts on a second part when relative sliding motion occurs or is attempted. Thus friction takes place whenever two surfaces rub together. The cause of friction is the inevitable interlocking of the tiny irregularities of the two mating surfaces. A force is required to deform the tiny peaks and valleys to permit motion. When a block of weight W rests on a horizontal fixed surface, a force P is applied to the block. Initially, P equals zero, but its value constantly increases as a function of time. Due to friction, a force F is created between block and fixed surface. The direction of the frictional force, F, is opposite that of P, because friction always opposes motion or attempted motion. Also note that the normal force, N, acting perpendicular to the mating surface is equal and opposite to the weight, W, of the block. As P increases, the frictional force, F, increases because P and F are equal in magnitude until the block starts to move. Finally, a sufficiently large value of P is reached so that the block is on the verge of moving. This situation is called impending motion. When sliding motion starts, the frictional force reduces somewhat. Hence, P and F are equal until impending motion is reached. Then, as P continues to increase, there is an immediate reduction in the value of F. Due to the unbalanced force (P-F), the block accelerates while the frictional forces remains essentially constant. The reason for the reduction in friction at the onset of motion is that the hills and valleys of mating surfaces interlock with less frequency during sliding motion. The force required to sustain motion (F kinetic), assuming no acceleration, is somewhat less than the force required to impend motion (F static). Test have shown that there is a definite relationship between the normal force N and the frictional force F, which is expressed by the following two equations: μstatic=Fstatic/N (1-7) μkinetic=Fkinetic/N (1-8) where μstatic=static coefficient of friction. μkinetic=kinetic coefficient of friction. Fstatic=maximum frictional force which occurs when motion is impending. Thus the frictional force can be found if μ and N are known. The normal force, N, is determined by applying the law statics. It has been shown by tests that the coefficients of friction( μstatic andμ kinetic ) depend on the following parameters: (1) Surface roughness; (2) Dry or lubricated surfaces; (3) Clean or oxidized surfaces; (4) Compatibility of the two mating materials. For a given set of conditions, μstatic andμkinetic are each essentially constant. In general, μ is known design value based on available test data. 1.3.3 Journal Bearings
  • 32. Technical English Through Reading 27 A journal bearings, in its simplest form, is a cylindrical bushing made of a suitable material and containing properly machined inside and outside diameters. It is also commonly called a sleeve bearing. Notice the oil hole which leads to an axial oil groove. Such a bearing is designed to provide lubrication to reduce friction. The bore (inside diameter) of a journal bearing is machined to accept a rotating shaft as shown in Figure 1-9a. The bearing outside diameter is supported in properly machined bore of a fixed housing. An oil cup is shown attached to one side of the housing where a passageway leads to the oil hole of bushing. Notice that the axial oil groove in the bushing does not extend all the way to the end of the bearing. This prevents undue loss of oil from the bearing. In Figure 1-9b, the diametral clearance, C, between the bushing and the shaft is greatly exaggerated for illustration purposes. Generally, the diametral clearance is between 0.001 and 0.002 inch per inch of shaft diameter. Also, the thickness of e bushing is often made approximately equal to 1/8 inch per inch of shaft diameter. Moreover, notice the annular groove in the bushing of Figure 1-9;such grooves insure more complete distribution of the lubricant. Since there will normally be a load R coming into the bearing, a normal force, N, and friction force, F, will exist between the shaft and bushing as shown in Figure 1-9b. Interlocking of the hills and valleys at the resulting deformation result in the wearing down of the bushing bore, a situation which makes the proper selection of bearing materials an important consideration. The following material properties are considered desirable for journal bearings: FIG.1-9 Journal bearing assembly (1) Compatibility. Certain combinations of materials do not rub well together. For example, some materials may have a tendency to fuse where there is metal-to-metal contact. This results in damaged surfaces and the materials are said to be incompatible. In general, dissimilar materials are more compatible than similar materials. (2) Embeddability. Dirt and other foreign particles will always work their way inside of bearing. A bearing material with outstanding embeddability allows these foreign particles to
  • 33. 28 Part 1 Machine Design become embedded into the bushing, which prevents scratching and other damage from occurring on the surface of the journal or shaft. (3) Conformability. A good bearing material should have a tendency to compensate for small amounts of misalignment and shaft deflection. A material with a low modulus of elasticity has this property of conformability. (4) Corrosion resistance. A bearing material must not corrode due to its contact with the lubricant which may have lubricity improvement additives. These additives sometimes tend to be corrosive with certain materials. (5) High thermal conductivity. A high thermal conductivity is desirable to remove heat rapidly from the bearing. (6) High fatigue strength. Since bearing loads continuously vary in magnitude, a high fatigue strength is desirable. 1.3.4 Lubricant Characteristics A lubricant is a special substance whose principal function is to form a film between mating bearing surfaces, preventing metal-to-metal contact. Such lubrication reduces the abrasive action which would cause wear and surface damage. A lubricant film separates the surface irregularities of a moving shaft from those of the stationary journal bearing. This complete separation, of course, assumes a stable and adequately thick lubricant film. The most common type of lubricant is oil, although the types of fluids are used, such as gasoline, water and air. Grease is also considered as a lubricant and is commonly used where adequate sealing of liquid is difficult to achieve and on parts which are not readily accessible. Also in common use are solid lubricants such as graphite, which comes in either powdered or flake form. When applied to a part, graphite tends to strongly adhere to the surface and, thus, becomes a very stable lubricant. Quite often, graphite is used in applications where high temperatures are encountered because it is stable up to 600°F, a temperature at which most oils and greases burn. Since oil is most common lubricant, let us now discuss some of these are defined as follows: (1) Pour point: lowest temperature at which oil will pour. (2) Flash point: temperature at which the vapor above the oil surface will ignite if exposed to a flame. (3) Fire point: temperature at which oil releases enough vapor to support combustion. (4) Oiliness: ability to adhere to a surface. (5) Detergency: ability to clean surfaces of deposits. (6) Stability: ability to resist oxidation, which can produce acids and sludge. (7) Foaming: formation of bubbles in the oil that speeds up oxidation. (8) Viscosity: measure if the sluggishness with which a fluid flows. (9) Viscosity index: measure of the sensitivity of an oil's change in viscosity with a change
  • 34. Technical English Through Reading 29 in temperature. 1.4 ANTIFRICTION BEARINGS 1.4.1 Introduction Antifriction bearings operate with rolling elements (either balls or rollers) and, hence, rolling resistance, rather than sliding friction, predominates. The cause of rolling resistance is the deformation of the mating surfaces of the rolling elements and raceways. The three basic types of antifriction bearing are: (1) ball bearings; (2) roller bearings; (3) needle bearings. Ball and roller bearings can be designed to absorb thrust loads in addition to radial loads, whereas, needle bearings are limited to radial load application only. One of the principal advantages of antifriction(rolling contact) bearings is the almost complete elimination of friction. Therefore, the main function of antifriction-bearing lubricant is to help prevent corrosion and dirt contamination. Since some friction does exist, especially between the rolling elements and their separator, the lubricant must also remove the heat caused by this sliding action. As result, although there is minimal friction present, a lubricant is still essential absolutely. Fatigue, which is the cause of failure of a properly lubricated antifriction bearing, is a stress reversal phenomenon which takes place on the contacting surfaces of the raceways and rolling elements. Hence,the life of an antifriction bearing is measured by the number of shaft revolutions which occur prior to fatigue failure. The greater the bearing load, the shorter will be its life in revolutions; also, the greater the shaft speed, the shorter will be the bearing life. Since fatigue is a statistical occurrence, the percent probability of failure must also be considered. It is essential to know the advantages and disadvantages of the various types of bearings, as well as their salient features. For example, ball and roller bearings can be of the self-aligning type to allow for shaft misalignment and deflections. Also, shields and seals are available to protect the bearing from foreign contaminants. Some antifriction bearings contain a permanently sealed lubricant which is administered by the manufacturer and lasts for the life of the bearing. Manufacturers of antifriction bearings produce a multitude of types and sizes. Handbooks are available which tabulate the various types and sizes along with recommended speeds and loads. Thus the principle problem is not the design of an antifriction bearing; rather, it is the selection of the proper bearing for a given application. 1.4.2 Ball Bearings
  • 35. 30 Part 1 Machine Design The design and terminology of typical single-row ball bearing is illustrated in Figure 1-10. Observe that there are essentially four different components: (1) Outer ring, which contains the outer raceway; (2) Inner ring, which contains the inner raceway; (3) Complement of balls; (4) Two-piece separator. FIG.1-10 Terminology of typical single-raw ball bearing.(Courtesy of New Departure Hyatt Bearings, Division of General Motors Corporation, Sandusky, Ohio) This design is called a Conrad bearing and is assembled by initially moving the inner ring into an eccentric position. Then as many balls as possible are inserted into the space between the inner and outer rings. Next, the inner ring is returned to its concentric position and the balls are equally spaced. Finally, the two halves of separator are positioned from each side and fastened using rivets. The Conrad-type bearing is primarily a radial capacity is somewhat limited because only slightly more than half the annular space between the inner and outer can be filled with a Conrad design. It should be noted that the terms ring and race are used interchangeably. 1.4.3 Life of Antifriction Bearings Antifriction bearings are subjected to repeated stress cycles which can ultimately lead fatigue failure. Contact surfaces on the races and rolling elements are loaded in compressive stress cycles which vary from zero to some maximum value. Fatigue failure is said to occur when a
  • 36. Technical English Through Reading 31 tiny flake of metal is removed from one of the races or the rolling element and is usually proceeded by very tiny surface cracks which are developed during the repeated stress applications. The life of a bearing is the number of revolutions, or the number of hours at a fixed speed, that a bearing will run before fatigue begins. If a large number of identical bearings are tested to failure under given operating conditions, a life probability curve can be established. Here, the B-10 life is defined as the number of hours which 90 percent of the bearings tested will exceed. Thus, for a large number of bearings tested to failure, the probability of failure is 10 percent at the B-10 life. The average life is defined as number of hours which 50 percent of the bearings will exceed. Since the bearing contact stresses increase with load, the fatigue life decreases as the load increases. Specifically, the life in revolutions or cycles is proportional to the reciprocal of load raised to exponents 3 and 10/3 for ball and roller bearings respectively. Thus, doubling the load reduces the life by eight times for ball bearings and by ten times for roll bearings. Another significant parameter is the basic dynamic capacity (C), which is defined as the radial load a bearing can sustain for 1 million revolutions before failure occurs. The reason why the basic dynamic capacity is based on revolutions rather than hours is that many machines such as the automobile are variable-speed systems. The phenomenon of fatigue relates uniquely with the number of stress cycles (not with the time) because of speed variability. 1.5 SHAFTS 1.5.1 Introduction Virtually all machines contain shafts. The most common shape for shafts is circular and the cross section can be either solid or hollow (hollow shafts can result in weight savings). Rectangular shafts are sometimes used, as in screwdriver blades, socket wenches and control knob stems. A shaft must have adequate torsional strength to transmit torque and not be overstressed. It must also be torsionally stiff enough so that one mounted component does not deviate excessively from its original angular position relative to a second component mounted on the same shaft. Generally speaking, the angle of twist should not exceed one degree in a shaft length equal to 20 diameters. Shafts are mounted on bearings and transmit power through such devices as gears, pulleys, cams and clutches. These devices introduce forces which attempt to bend the shaft; hence, the shaft must be rigid enough to prevent overloading of the supporting bearings. In general, the bending deflection of shaft should not exceed 0.01in. per ft of length between bearing supports, but, if the bearings are self-aligning, a greater deflection may be acceptable. In addition, the shaft must be able to sustain a combination of bending and torsional loads.
  • 37. 32 Part 1 Machine Design Thus, an equivalent load must be considered which takes into account both torsion and bearing. In addition, the allowable stress must contain a factor of safety which includes fatigue, since torsional and bending stress reversals occur. For diameters less than 3 in, the usual shaft material is cold-rolled steel containing about 0.4 percent carbon. Shafts are either cold-rolled or forged in size from 3 in. to 5 in. For sizes above 5 in., shafts are forged and machined to size. Plastic shafts are widely used for light load applications. One advantage of using plastic is safety inn electrical applications, since plastic is a poor conductor of electricity. In selecting a shaft diameter, the calculated size is considered the minimum value. A standard size which is the smallest standard size exceeding the calculated value should be selected. Components such as gears and pulleys are mounted on shafts by means of keys. The design of the key and the corresponding keyway in the shaft must be properly evaluated. For example, stress concentrations occur in shafts due to keyway further weakens the shaft. Another important aspect of shaft design is the method of directly connecting one shaft to another. This is accomplished by devices such as rigid and flexible couplings. If shafts are run at critical speeds, severe vibrations can occur which can seriously damage a machine. It is important to know the magnitude of these critical speeds so that they can be avoided. As a general rule of thumb, the difference between the operating speed and the critical speed should be at least 20 percent. 1.5.2 Shaft Design Torsion of Circular Shafts A shaft experiences torsion when it transmits torque. When a shaft is subjected to a torque, torsional stresses and deflections are produced. To evaluate these stresses and deflections, let us first refer to Figure 1-11. In Figure 1-11a, we see an unloaded shaft, whereas Figure 1-11b shows the same shaft loaded with a torque T. In Figure 1-11a, rectangle ABCD represents an imaginary cutting plane penetrating the shaft up to its centerline. Line AB is a scribe line which is actually marked on the outside surface of the untorqued shaft. When a torque T is applied (Figure 1-11b), the shaft twists through an angle of twist, which is the torsion deflection. The original flat area, ABCD, becomes warped as the scribe line assumes a helical path. The concert of the angle of twist can be clarified by referring to a side view of the shaft (Figure 1-12). Imagine the shaft to consist of a number of thin circular discs (a total of ten is chosen here for discussion purposes). Notice that the untorqued scribe line is horizontal and exists if the applied, each disc slides rotationally a small amount relative to its neighbor. Disc no.1 is fixed and, hence, is the zero-rotation position. The total angle of twist is the summation of the relative rotations of individual disc. This means that a long shaft produces a greater angle of twist than does a short shaft. Observe that the series of short line segments represents an approximation to the actual torqued scribe line. If we let the number of discs become very large,
  • 38. Technical English Through Reading 33 each disc thickness become very small and each short line segment approaches a point. The curve through all these points gives the torqued scribe line depicted in Figure 1-11b. FIG.1-11 Untorqued versus torqued shaft configurations. FIG.1-12 Shaft angle of twist Bending of Circular Shafts Shafts transmit power with devices such as gears and pulleys. In addition to torque loads, these devices introduce forces which attempt to bend the shaft as it rotates. The bending stresses and deflections must be evaluated to assure adequate bending strength and rigidly. A bending stress and deflection analysis proceeds as follows: (1) Shear diagram; (2) Moment diagram; (3) Bending stress; (4) Shaft bending deflection. Combined Torsion and Bending It should be apparent that a shaft is transmitting power will undergo a combination of torsion and bending. The stress due to torsion act parallel to the across-sectional area of the shaft. On the other hand, bending stresses act in a perpendicular or normal direction. Stresses are victors since they are essentially unit forces. Therefore, we cannot simply add algebraically the torsion and bending stresses to arrive at a total value. In addition, the phenomenon of torsion is different from that of bending. Hence, the effect on failure is not same. There are several proposed methods for properly combining the effects of torsion and bending. Two of these are the equivalent torque method and the equivalent bending moment method.
  • 39. 34 Part 1 Machine Design The allowable stress (shear or tensile) must be based on a factor of safety which considers endurance, because shafts are subjected to stress reversals which can lead failure. Frequently, keys are used to connect components such as gears and pulleys to shafts. The resulting keyway in the shaft lowers the strength of the shaft. In this cases, the allowable stress is reduced by 25 percent (as a rule of thumb) to compensate for the concentration effect and loss of material. In general, a shaft is analyzed by both methods, and the greater calculated shaft size is used. 1.5.3 Critical Speeds of Shafts A shaft supported between bearings deflects under its own weight. Hence, the center of gravity of a shaft does not coincide with the axis of rotation which is the centerline of the bearings. The eccentricity becomes even greater when the shaft carries components such as gears, pulleys, cams and flywheels. Since the centrifugal force rotates with the shaft, it is an unbalanced force which causes forced vibrations. In high-speed machinery, it is necessary to balance the shaft system because the centrifugal force is proportional to the square of the speed,ω.However, it is impossible to obtain a perfectly balanced system. Thus the centrifugal force will always exist and, in most cases, its effect on vibration is small enough not to cause problems. However, at certain speeds (called critical speeds) the shaft system can become dynamically unstable. This occurs when the shaft speed equals the natural frequency of the system. The natural frequency is the frequency of oscillation at which the system wants to operate. For example, if a bending force is applied and suddenly removed, the shaft will oscillate at the natural frequency until all the dynamic energy is removed due to friction. Therefore, the shaft deflections become so large that severe damage to the entire machine can occur. This phenomenon is commonly called shaft whirl. It is, therefore, necessary to calculate these critical speeds so that they can be avoided. A general rule of thumb is that the value of operating speed be at least 20 percent away from the value of the critical speed. 1.6 FUNDAMENTALS OF GEARS 1.6.1 Introduction Throughout the historical evolution of machinery, gears have proven to be the most widely used method of transmitting power from one shaft to another. In most applications, the shaft required to rotate at varying speeds, but the speed ratio (output shaft speed divided by input shaft speed) must remain a constant. The kinematics of gear systems has been developed to the point where the following
  • 40. Technical English Through Reading 35 orientations of any two shafts centerlines can be adequately handled: (1) Parallel; (2) Collinear; (3) Perpendicular and intersecting; (4) Perpendicular and nonintersecting; (5) Inclined at any arbitrary angle. Modern gears are made to high precision standards. As a result, they normally are purchased from gear manufacturers rather than designed and machined at the user's plant. However, a person cannot arbitrarily order any gear from a manufacture's catalog for a particular application. One must have a working knowledge of gear design, including its limitations, in order to produce a satisfactory gear drive system. If the loads are low, with resulted low torque transmission, friction drive roller system can satisfactorily provide a constant speed ratio. In many applications, however, large torque are involved. Of course, the frictional resistance could be increased by roughening the surface of the two rollers. Historically, this was practiced, leading to the addition of cogs on one of the rollers and pockets in the other roller. Records show that these wooden cogwheels, or so-called toothed wheels, have been in existence since 2600 B.C. This did produce a positive drive, but the cogs and pockets were very crudely made, and thus, did not properly mesh with one another. It is not enough merely to have positive drive via arbitrarily shaped toothed wheels; the teeth must have a proper form and be precisely positioned so that smooth rolling contact is assured at all speeds. 1.6.2 Gear Terminology Before we can properly learn haw to design gears, it is necessary to learn some of the important terms. The following definitions should be studied in conjunction with Figure 1-13, which depicts a portion of a spur gear. Spur gears will be considered first because they are the simplest and, in general, the definitions used for them apply to other types of gears. One distinguishable feature of spur gears is that they are formed on a cylindrical surface with the teeth running parallel to the axis of the gear. As a result, spur gears are used for transmitting power between parallel shafts. (1) Pitch circle. The imaginary circle on which most gear calculations are based. When two gears are meshing, the two pitch circles are tangent to each other. If two friction rollers (with circles equal to the two pitch of a pair of meshing spur gears) were rolling together without sliding, they would have the same speed ratio as the gears. (2) Pitch diameter (Dp). The diameter of the pitch circle. (3) Pitch point. Point on the line joining the centers of two meshing gears where the pitch circles touch.
  • 41. 36 Part 1 Machine Design FIG.1-13 Gear nomenclature (4) Addendum circle. The circle which bounds the outer ends of the teeth and whose center is the center of the gear. (5) Dedendum circle. The circle which bounds the bottoms ends of the teeth and whose center is the center of the gear. (6) Addendum (a). The radial distance from the pitch circle to the outer end of the tooth. (7) Dedendum (b). The radial distance from the pitch circle to the bottom of the tooth. (8) Circular pitch (Pc). The distance between corresponding points on two adjacent teeth, measured along the pitch circle. For two meshing gears, the circular pitches must be equal if they are to operate properly. (9) Diametral pitch (PD). The number of teeth per in. of pitch diameter. A small diametral pitch implies a large tooth size. Diametral pitches have been standardized. When the word pitch is used by itself, it implies the diametral pitch (not circular pitch). (10) Tooth space. The space between adjacent teeth measured along the pitch circle. (11) Tooth thickness. The thickness of the tooth measured along the pitch circle. The pitch
  • 42. Technical English Through Reading 37 circle cut the teeth at such a location that the tooth thickness equals the tooth space, assuming no backlash (see Definition 18 for backlash). (12) Face width. Length of the tooth measured parallel to the axis of the gear. (13) Face. The surface between the pitch circle and the top of the tooth. (14) Flank. The surface between the pitch circle and the bottom of the tooth. (15) Pressure angle ( α). The angle between the line of action and a line tangent to the two pitch circles at the pitch point. (16) Line of action. The locus of all the points of contact of two meshing teeth from the time the teeth go into contact until they lose contact. Thus the load is transmitted from one gear to another along the line of action. (17) Pinion. The smaller of two meshing gears. The larger is called the gear. (18) Backlash. The difference between the tooth thickness of one gear and the tooth space of the meshing gear measured along the pitch circle. With backlash, there is looseness between meshing teeth, which becomes apparent when the direction of rotation of the driver is reversed by hand. To be precise, the tooth thickness along the pitch circle equals one-half the circular pitch minus the backlash. The values of backlash are somewhat standardized in following amounts: 0.030 in./PD, 0.040 in./PD and 0. 050 in. /PD. Hence, a typical range of values for backlash would be 0.005 in. to 0.020 in., depending on the teeth size. The value selected should be the largest one acceptable, since the manufacture of gears with small values of backlash is very expensive. Also, too small a value of backlash can cause binding. (19) Clearance (CL). The addendum minus the dedendum. Clearance and backlash are both required to prevent binding. (20) Working depth. The distance that one tooth of meshing gear penetrates into the tooth space. (21) Bash circle. The imaginary circle about which the tooth involute profile is developed. Most spur gear teeth have an involute shape which runs from the base circle to the top of the tooth. (22) Fillet. The radius where the flank of the tooth blends into the dedendum circle. 1.6.3 Gear Drive System In addition to spur gears, there are many other types of gears which have been developed for various applications. Some of the types which we will consider are helical, herringbone, bevel and worn-and-wheel. It is important to know the significant features as well as limitations of these different types so that the appropriate gears will be selected for the particular application. One of the first considerations in gear selection is the geometrical relationship between the centerlines of the shafts. The size limitations, speed ratios, and horsepower requirements should then be introduced.
  • 43. 38 Part 1 Machine Design Quite often, a gear train of more than two meshing gears is required to satisfy all the design requirements. For example, a large speed ratio may be needed in addition to a compact size arrangement. If only two gears are used, the size of the large gear may exceed the space limitation. Cost considerations are also involved, since a very large gear is often very expensive. The following types of gear trains will be analyzed: (1) Simple; (2) Compound; (3) Reverted; (4) Epicycle (sometimes referred to as planetary). A gear train is a system which contains two or more pairs of meshing gears designed to provide a desired speed ratio within specified space limitations, shaft centerline orientations and horsepower requirements. When contemplating the design of a gear train, the following should be taken into account: (1) Repetition of tooth contact. A gear pair may consist of 40 teeth and 30 teeth for the gear and pinion, respectively, in order to produce a speed ratio of 2 to 1. However, with this ratio, the same pairs of teeth will make contact once for every other revolution of the pinion. It would be preferable to use 41 and 21 teeth even though the speed ratio of 2 to 1 will not be obtained exactly, because, in this case, the same pair of teeth will make contact only once in every 41× 21, or 861, revolutions. It is desirable to have contact that varies from tooth to tooth, because a more uniform pattern of wear will occur. This actually slows wear down and helps compensate for minor discrepancies in manufacturing tolerances. (2) Number of gears in train. It is often possible to obtain a specified speed ratio with any number of pairs of gears. For the case where the speed ratio is accomplished in one step, the gear size and, hence, space requirements can be quite large. If many steps are used, a large number of shafts and smaller gears are needed, which usually means added cost. Thus a proper compromise between space requirements, economy and efficiency of operation must be made to arrive at the best overall system. (3) Strength of tooth. When a large amount of horsepower is being transmitted at low speeds, a huge value of torque will exist. This torque exerts a high load on the gear teeth, necessitating large-size teeth. This means that at the low-speed end of gear train the teeth need to be larger than at the high-speed end. The amount of load determines the maximum permissible diametral pitch which may be used. This, in turn, affects the pitch diameter and number of teeth. (4) Integer number of teeth. A gear must have an integer number of teeth. This requirement makes it frequently difficult to arrive at an exact speed ratio or center distance between shafts. In many cases, an exact speed ratio or center distance is not mandatory and deviations from design values are acceptable.
  • 44. Technical English Through Reading 39 PART 2 METALWORK 2.1 MARKING OUT AND MEASURING Marking out is done by scribing fine lines on the metal and in a few cases, some preparation is needed so that they will show clearly. On many of the non-ferrous metals and alloys, scribed lines show clearly and there is no need to prepare surfaces which are smooth and coated with a film of oxide. The bright surfaces of steel, when de-greased are prepared by brushing or swabbing on a solution of copper sulphate which immediately deposits a film of copper. The surplus solution is then washed off and when the metal is dry, scribed lines will show very clearly. With iron castings, a thin coat of flat oil paint will suffice. 2.1.1 Tools in General Use Engineer's Square Squares with 100 mm blades and of ‘workshop’ grade are suitable for use in small fitting work. Made in bright steel and with hardened and tempered blades, these tools will give good service but must be looked after and not left lying about where they might be knocked on to the floor. A few larger squares will be needed for the occasional large job and a combination set, with adjustable blade, miter square, protractor, level and the center square is undoubtedly a useful piece of equipment. A small slot is cut in the stock of the engineer’s square to avoid errors caused by filings or dirt which might be picked up, or from, burred edges which have inadvertently been left on the work. In testing surfaces for squareness, the stock is held firmly against the work face and with the blade held normal (at right angles) to the edge under test, the square is slid down so that the blade comes into contact. With the work held towards a light source, inaccuracies are clearly revealed. Scriber The scriber is used for marking out by scratching very fine lines and is used in conjunction with the square, straight-edge or bevel in the same way that one would use pencil and drawing instruments. It is made of tool steel, with hardened points. In squaring lines across the work, the scriber point is first placed exactly on location and the square is slid up to the scriber with the stock held firmly against the work.
  • 45. 40 Part 2 Metalwork Bevel and Bevel Protractor The bevel is used in transferring angles and in marking them out but cannot be used for measuring them. For setting a bevel to a desired angle, a bevel protractor can be used and for very accurate settings, a bevel protractor with a Vernier attachment is needed. Straight-edges For most purposes in the small workshop, a good steel rule of 300 mm will serve as a straight-edge for testing and marking out on small work, but a few straight-edge of `workshop' grade will occasionally be needed. These are available from 300 mm to 1800 mm in length, but it is doubtful if anything longer than 750 mm will ever be needed. Dividers With a fine adjustment, spring dividers are useful in the division of lengths into equal parts. The knurled finger grip enables one to ‘walk’ the dividers along a line until the correct setting is obtained. The divisions can then be marked by striking an arc at each step. Dividers are also used in transferring lengths and in locating points with intersecting arcs. The wing compass is a more robust tool but with limited applications. Calipers Outside calipers are for taking off sizes in all situations on round or other sections, for comparing objects in conjunction with the feeler gauge, and for testing parallelism, all with moderate precision. Inside calipers are for use on interior work of the same nature. In taking off sizes with the firm-joint calipers, it is better not to close the points on to the work, but to set the calipers so that in drawing the points across the work, they are automatically adjusted to its size and can be felt to be in contact with the surfaces. Odd Legs Also are called ‘Jenny’ calipers. These are used for scribing lines parallel to finished edges and are useful in finding the centers of round bars by striking arcs. Dot and Center Punches These two punches look alike, but the dot punch is often of lighter stock than the center punch and is ground with a point at 60 degree whilst center punches are usually ground at 90 degree. The dot punch is useful in locating hole centers accurately with a very small dot which can easily be drawn over with a second attempt if the first is not correctly positioned. This can then be followed with a heavier indent from the center punch before drilling. The dot punch can also be used for `confirming' a scribed line with a series of dots as a guide when filling. A scribed line always seems to disappear before the filed edge reaches it, but the dots become visible at the crucial moment and give a positive guide. Work should be solidly supported for dot or center punching.
  • 46. Technical English Through Reading 41 The automatic dot or center punch is very useful tool with its adjustable control over the weight of the blow delivered when the body is depressed. Surface Plate and Surface Table The surface plate provides a true ‘plane of reference’ for the testing of other surfaces and for measuring and marking out of all kinds. The plate is made from fine grade cast iron, is ribbed on the underside to prevent warping and is usually arranged to stand on three small feet to give stability and to ensure that no stresses are imposed when the plate stands on an uneven surface. A great range of sizes is made from 100 mm x 150 mm upwards. The very largest sizes are often fitted with legs to stand on the floor and they are then known as surface table. The finishing of the surfaces is commonly done to two specified grades of precision, viz.: grades ‘A’ and ‘B’. Grade ‘A’ plates are brought to a high precision by hand scraping and in grade ‘B’ plates, the surface is finished by accurate machining. Grade ‘A’ plates are naturally very costly items, but for general use and in the school workshop, grade ‘B’ plates are sufficiently accurate. Surface plates and tables should be kept scrupulously clean and protected with covers when not in use. Surface Gauge Sometimes is referred to as the ‘scribing block’. The gauge can be set to scribe lines at any height above and parallel to the surface plate, its heavy base keeping it steady and requiring only a little hand pressure. If the scriber points are kept nicely sharpened only the lightest stroking action will be needed to scribe a clean line. Used in conjunction with the vee blocks the surface gauge can be used to locate accurately the centers of round bars. Both the scriber and the pillar are held in adjustable clamps on the universal surface gauge, giving the tool an increased reach. Fine adjustments are made by turning the knurled screw which moves the rocker arm up or down. With the scriber pointing downwards, the surface gauge can be used to check work for parallelism by `feeling' the surfaces and with the aid of the feeler gauge, discrepancies can be measured, the feelers being inserted singly or in groups until a combination is found which just drags slightly on being drawn through the gap. 2.1.2 The Vernier Caliper Gauge Named after its inventor, the Vernier scale is incorporated in many measuring instruments, the most commonly used of which is the sliding caliper gauge. It can be used for taking internal and external measurements. These gauges are made from fine quality alloy steels and are very accurately finished by grinding and lapping. In measuring an object externally, the locking-screws A and B are both slackened off and the sliding-jaw assembly is moved along, almost on to the object. Locking-screw A is tightened down and then, by turning the knurled screw C, the jaws are gently closed on to the surfaces without putting any pressure on them. Locking-screw B is then tightened down and the calipers
  • 47. 42 Part 2 Metalwork removed from the work for reading. The jaw tips are stopped and rounded on the outsides so that internal measurements can be taken and to whatever reading is obtained, the widths of the jaws must be added. This measurement is found engraved n the jaw face for reference. Reading the Vernier In Figure 2-1, we see part of the caliper main scale which is divided into centimeters and millimeters. The actual length of the Vernier scale is 49 mm and this length is divided into 50 parts. Each division of the Vernier will therefore be equal to 0.980 mm, i.e. each will be 1/50th mm shorter than each division on the main scale. The reading is taken as follows: reading along the main scale up to the Vernier zero, we can see the number of whole mm in the measurement and the line on the Vernier which coincides with a line on the main scale indicates the number of 1/50th mm to be added on. FIG.2-1 Metric Vernier scale In the illustration, the reading is 37.66mm,i.e.37 whole mm on the main scale, plus 33/50ths mm on the Vernier which equals 0.66 mm, giving a total of 37.66 mm. Tools incorporating the Vernier are often provided also with scales in Imperial measure, the main scale inches sometimes divided into 1/40ths. With a Vernier scale of 24/40ths ins in length and divided into 25 parts, readings of 1/1000ths ins can be taken. 2.1.3 The Micrometer The micrometer is a precision-made measuring instrument in which a threaded spindle, passing through a ‘nut’ in the form of a tube, can be made to pass through very precise distances from a fixed point when it is rotated. The principle is very simple, single- start threads being used so that for one complete turn of the spindle, its axial movement will be equal to the thread pitch. Micrometers measuring in millimeters are usually made with a thread of 0.5 mm pitch, thus, the spindle will travel 1 mm if given two complete turns, 0.5mm for one turn and correspondingly smaller distances for fractions of a turn. Along the sleeve is scribed a datum line which is divided into whole and half mm. To the outer end of the spindle is attached a thin walled tube or ‘thimble’ which fits closely over the sleeve. As the spindle is rotated, the thimble moves along the sleeve, its beveled edge
  • 48. Technical English Through Reading 43 registering the movement along the datum, viz.: 1 mm for two turns, 0.5 mm for one turn and so on. With the beveled edge of the thimble divided into 50 equal parts around its periphery, it is a simple matter to turn the spindle 1/50th of a turn, producing a movement equal to 1/50th of 0.5 mm which is 0.01 mm. Objects are measured between the spindle and the anvil faces, the spindle being rotated by means of the ratchet knob. This device ensures that an even pressure is applied under all conditions. The micrometer, with a ’C’ shaped frame, reads from zero to 25 mm (or 1 inch in Imperial sizes), the next size up reads from 25 to 50 mm, the next from 50 to 75 mm and so on up to 300 mm. Before attempting to adjust the setting of a micrometer, the locknut, which prevents accidental movement of the thimble, must be released, and having set the micrometer for measuring or perhaps for reading practice, the locknut should be tightened. Reading The Metric Micrometer Figure 2-2 shows a close-up of a sleeve and thimble reading in hundredths of a millimeter (0.01 mm). The datum line is graduated in millimeters on top and to avoid any confusion, half millimeters are marked below. The thimble edge registers the number of whole and half millimeters over which it has traveled from zero along the datum and the fifty divisions around the thimble indicate the hundredths of a millimeter to be added on, the reading being taken at the datum. Figure 2-3 shows a sleeve and thimble graduated to read to thousandths of a millimeter (0.01 mm) and this is accomplished by the addition of a Vernier scale on the sleeve, reading in conjunction with the thimble. On the sleeve and parallel to the datum are marked 5 equal divisions occupying the same space as 9 divisions on the thimble, each division on the Vernier scale representing 2 thousandths of a millimeter. FIG.2-2 Close-up of a sleeve and thimble FIG.2-3 Sleeve and thimble graduated to reading in hundredths of a millimeter read thousandths of a millimeter In taking a reading from this micrometer, a reading is first taken as previously described, in hundredths of a millimeter and then a note is made of which Vernier line coincides with a graduated line on the thimble and this gives the number of thousandths of a millimeter to be added. When there is no coincidence of lines when reading the Vernier, the intermediate
  • 49. 44 Part 2 Metalwork thousandths can be estimated, i.e. if the reading lies between 4 and 6 (for example), then the additional thousandths reading would be 0.005 mm. Care of Micrometers Micrometers should never be left laying around on benches or machines where they may pick up dirt or get knocked on to the floor and damaged. They should be clean and returned to their cases immediately after use. They should not be held in the hand too long as warmth will cause expansion and possibly faulty readings. It is important that anvil faces should be kept clean by wiping lightly with absorbent paper and in the smallest sizes, where the two faces meet, a piece of paper can be lightly gripped and then withdrawn. Zero reading should be regularly checked after closing the freshly cleaned anvil face, not forgetting to use the ratchet. For this purpose, micrometers larger than 0 to 25 mm (or 1 inch) are provided with accurately made setting gauges on which the micrometer is closed. Any adjustment needed must be made in accordance with the maker's instructions. 2.1.4 The Dial Indicator This is a delicate measuring instrument for checking on parallelism, on the flatness of surfaces and on the concentricity of round objects. It is also used extensively in setting up work for machining. The dial or ‘clock’ is provided with several mountings for adaptability and in use, is traversed across the surface under test, contact being made via a spring-loaded stud or plunger protruding from the body. The slightest movement of the stud, in or out, as it traverses the surface is magnified through a gear train, causing a needle to move to right or left of zero on the dial face, thus indicating high or low spots. The dial face can be rotated independently of the body so that the zero mark can be brought round to the needle point wherever it stops after adjustment against the work face. Mountings with powerful magnetic bases provide a ready means of mounting the dial gauge on machines in almost any situation. 2.1.5 Gauges Many kinds of gauges are used in engineering for the checking of sizes by comparing them with accurate dimensions between surfaces on the gauge. For checking external sizes, gauges incorporating holes or gaps are used whilst for internal sizes, they are in the form of plugs or distance pieces. The measuring faces are always made with extreme accuracy and although the principle of checking by comparison is elementary, some care must be taken over the handling and using of gauges. Only the very lightest of pressure should be used to ‘feel’ a gauge over or into the work,
  • 50. Technical English Through Reading 45 taking care to offer it in line with the work to avoid jamming. Plug Gauges Circular plug gauges are made in two forms, viz., standard and limit gauges. The standard gauge is made precisely to the nominal diameter and will be used when boring exactly to a given size, but quit often, small variations above and below nominal sizes are acceptable. Where this occurs in repetition work, limit gauges are used because they give immediate checks on sizes without having to spend time with measuring instruments. The comparison of the work with the taper gauge calls for a fine sense of touch in judging whether the two are mating accurately. Whilst the angle of the taper is compared with the gauge, its size is gauged by the depth to which the plug enters and this is often indicated by a line scribed around the plug. Ring Gauges These are made for testing parallel or taper turning. They give a positive check of size, and are usually made from mild steel and case hardened. Parallels Made of hardened steel and ground on all faces, parallels range from 100 mm to 300 mm in length and in a variety of square and rectangular sections between about 6 mm and 75 mm. They can be used in the setting up of work for machining, in marking out and in are inspection. Parallels should not be confused with slip gauges, which various combinations as references for standard sizes in the workshop. Feeler Gauges These are comprised of tempered steel blades whose thickness range from 0.03 to 1.00 mm and from 0.001 to 0.025 inches, the thickness being marked on each blade. Feelers are used singly or in combination to measure clearances between parts by touch and can be judged to fit a gap when a slight pull is felt on withdrawing them. Trammels For striking arcs beyond the capacity of the dividers, trammels are used. They can also be used in transferring measurements from one part to another or as inside or outside calipers for large diameters. 2.2 DRILLING AND REAMING 2.2.1 Accident Prevention
  • 51. 46 Part 2 Metalwork Any special safety precautions are mentioned as the need arises. but at this point, a few general precautions are noted. It must always be borne in mind that all power tools can inflict serious injury if handled carelessly and they must be treated with profound respect at all times. Familiarity with a machine does not bring immunity from accidents, a momentary distraction or inattention bringing disaster to the experienced machinist as readily as to the novice. The beginner should seek instruction before attempting to use any machine with which he is unfamiliar, not only for his personal safety but to avoid damaging costly plant. He should be quite sure of the position of the ‘stop’ button on each machine and should never start one until he is sure that the guards are in position, that no one is touching or adjusting any part of the machine, that the cutter, drill or work is free to turn and that no rags or other obstructions are anywhere near rotating parts. Revolving parts should never be touched, however, slowly they may be turning or however smooth they appear. Rags not be used to clean moving parts. Before adjusting working parts of machines or to cutters or gear trains, the machine should be isolated by throwing its main switch. Depressing the ‘stop’ button is not sufficient safeguard. In all machine shops there will be emergency ‘stop’ buttons painted red and placed prominently at strategic points so that anyone can cut off the power in an emergency. The human failings which lead to trouble in machine shops include: (1) Untidiness, for example, long flowing neckties which can get caught up in revolving parts, especially when leaning over a lathe, also long unkempt hair, loose clothing and unbuttoned shirt cuffs; (2) Over-confidence, which leads people to use machines they do not understand without first seeking instruction; (3) Inquisitiveness, which makes people do silly things like taking the headstock guard off to watch the gears running, over-confidence perhaps prompting them to do some oiling at same time; (4) Thoughtlessness, which lets people touch rotating work in the lathe to see if it is smooth, or perhaps to clean the oil or swarf away with a rag or the fingers; (5) Forgetfulness, which lets people do silly things like leaving the key in a chuck, with the risk of its being thrown out violently when the machine starts; (6) Inattentiveness, which enables one to do two things at the same time for a very short time, e.g. screw cutting and gossiping, followed by trouble; (7) The adventurous and inquiring nature, which urges people on to dismantle and repair the electric motor or to investigate the switch gear without first turning off the main switch and so on. Then, of course, there is the fool, who treats the workshop as his playground in a light-hearted fashion. He should be barred from entering. 2.2.2 Drilling In the course of his work the fitter uses the drilling machine whenever possible, but on the odd
  • 52. Technical English Through Reading 47 occasions when the job cannot be taken to the machine, he uses the hand-drill, breast-drill or the electric hand-drill. The hand-drill has a capacity of up to about 8 mm, whilst the breast drill, with a choice of two speeds has a capacity of up to about 12 mm. Electric hand-drills are a great boon but as many of them still have only one speed, care must betaken if drills are not to be spoiled by over-heating them. This can be avoided by easing the drill back at regular intervals when working and, of course, the use of coolant would help but this is not always possible in awkward spots. The work must be properly supported and should never be steadied with one hand whilst the other holds the drill. This practice is dangerous, for the electric drill would run out of control if the drill itself should break whilst under pressure. Twist Drills Drills are classified by several features, the first considerations being: (1) Drill diameter, from 0.30 to 100 mm; (2) The steel in the drill, either carbon or high-speed steel; (3) The type of shank, either parallel or tapered; (4) The length of the drill. For general purposes, the jobber's drill is used in sizes up to 16 mm diameter. The long series and stub drills would be used for special jobs and on production work. Twist drills are also made with different spirals and unless specially requested when ordering, it is always assumed that the standard spiral is required. The slow spiral is for use on brass, gunmetal, phosphor bronze and on plastics, whilst the quick spiral is for use on copper, aluminium and other ‘soft’ metals. A quick-spiral drill should never be used on brass or it will almost certainly dig-in and the job may be thrown if it is not secured firmly to the drilling table. In the absence of a slow-spiral drill, a standard-spiral can be converted by altering the helix angle at the cutting edges. This is done by grinding on a round-edged stone, but it must be done very expertly if the drill is not to be spoiled. Parallel-shank drills can be used only in chucks and are dependent for alignment on the condition of both chuck and drill sank. Taper sank drills are more satisfactory in this respect. At the end of the taper shank is a small flat tang which engages with a slot in the spindle. The taper is the Standard Morse Taper, in sizes ranging from No.1, the smallest, to No.6. Where the drill taper shank is smaller than the taper in the spindle, the two can be matched by fitting a sleeve to the drill shank so that a drill with No.1 Morse taper, for example, could be fitted into a spindle made with a No.2 Morse taper. Sockets with taper shanks can also be used for this purpose and these are made also for reducing the taper so that a drill with a No.2 taper shank, for example, could be adapted to a spindle with a No.1 taper. The socket with an unturned end is of a type which would be used in holding taper shank drill and other tools in turret and capstan lathes. Sleeves, sockets and taper shanks are all very accurately ground and if damaged, they will not run truly and may become jammed together. In assembling taper shank tools and sleeves or sockets, it is permissible to tap them home with a soft hammer and for separating them, a tapered key (drift) is used, being used to eject a
  • 53. 48 Part 2 Metalwork drill from the machine spindle. Core drills are made with either three or four flutes and are for use in opening out holes already made by coring (as in casting), by punching or by smaller drillings. Opening out a hole with the normal double-fluted drill often results in digging-in by the drill lips or it may lead to inaccurate drilling. The modified lips and extra lands in the core drill obviate these troubles. Drill maintenance Figure 2-4 gives details of drill nomenclature and this should be studied carefully. The web thickness increases from the drill tip to the run-out of the flutes to give extra strength, but this has the effect of increasing the length of the chisel edge as the drill is worn away by re-grinding. This longer edge calls for extra feed pressure to overcome the resistance offered by this part of the drill, but this trouble can be rectified by reducing the web thickness by grinding on a thin, round-edged wheel. FIG.2-4 Drill nomenclature Twist drills should be re-ground immediately there is any sign of inefficient working which will be revealed by: (1) The need for excessive feed pressure to make the drill cut; (2) The ejection of scored cuttings which indicates chipped cutting-edges; (3) Chattering or screaming from the drill when pressure is applied. This is caused by the drill rubbing instead of cutting and will quickly cause overheating. Faulty grinding is indicated by: (1) Two cuttings of unequal thickness, or only one cutting being ejected; (2) Over-size holes which are caused by unequal lip length or unequal lip-angles; (3) Broken lips or digging-in that may be caused by excessive clearance angles. Standard drill tip-angles must be ground accurately on a fine grade wheel. This is best done with the aid of a grinding jig. Drill grinding can be done ‘off hand’ (free hand) but this requires a lot of practice before it can be done satisfactorily. Drills should always be kept in the drill
  • 54. Technical English Through Reading 49 stand, taking care not to mix different types together. A drill gauges is useful in identifying sizes especially where markings have been obliterated by chuck slipping on parallel shanks. High-speed drills can be identified by the letters ‘HS’ stamped on the shank. Chip-breaking Drills These drills give increased efficiency by chip control. The swarf is broken into small chippings and this brings several advantages: (1) There is no danger to the operator from long coils of swarf as with the ordinary drill; (2) Faster drilling speeds are possible because there is no swarf clogging in the flutes; (3) The drill can be sharpened by standard practice and equipment; (4) The stiffer web gives greater rigidity; (5) The drill is suited equally well to precision and heavy constructional work; (6) It costs no more than other drills. 2.2.3 Drilling Machines Beach Drill A modern bench drilling machine fitted with a tapping attachment where the chuck would normally be. This device is used in production work, the tap automatically reversing when it is withdrawn. The machine spindle is usually bored with a No.1 or No.2 Morse taper and these machines are commonly fitted with four-step vee pulleys which give a speed range from about 500 to 3000 rev./min. As with all motorized machines, they are equipped with a ‘no volts’ release switch and this, in the event of a power failure or switching off at the main, automatically breaks the circuit. There is then no danger of the machine re-starting on its own when the power is restored. The drill table should be firmly supported before the locking bolt is slackened when adjusting the table height. It is a heavy component and could cause serious injury if allowed to drop, apart from the damage it might cause to the machine. When adjusting the height of the machine head, the tool tray with which most machines are fitted, should be first adjusted to serve as a stop before the locking bolt is slackened just enough to permit of the head being moved. The machine illustrated is a ‘sensitive’ drill, it is sensitive inasmuch as the human touch is used in controlling the feed pressure. Feed is controlled through a rack and pinion and an automatic return is effected by means of a strong coil spring enclosed in a case and which operates on the pinion spindle. An adjustable depth-stop is provided so that drillings to any predetermined depth can be made. The table can be tilted for angular drilling or can be swung aside so that large work can be accommodated on the machine base. Pillar Drill The pillar drill, in a range of larger sizes, resembles the bench drill but has a longer column
  • 55. 50 Part 2 Metalwork and stands upon the floor. When fitted with a gear reduction which gives a range of low speeds, it is a very useful machine. As with the bench drill, the spindle is bored to a Morse taper. The same precautions are necessary when adjusting the head or the table. In another kind of machine——the radial drill─—the head can be moved along an arm which pivots sideways on the column enabling any point on the table to be covered without moving the table or the work. Drilling machines are also made with geared heads, speed changes being made by operating gear levers. These machines are of larger capacities and are usually provided with power feeds. Drill Speeds To ensure good work, without damaging or breaking a drill, it is important that the correct speed is used and this will depend on the metal being drilled and on the type of drill (carbon or high speed steel). Suitable cutting speeds for various materials are always quoted in meters or feet per minute and these must be related to the drill periphery (circumference). Therefore, to maintain a constant cutting speed for different drill sizes, it will be obvious that the rev/min must increase as the drill size decreases and vice versa. Speeds quoted in the tables are for high-speed drills and all must be reduced by about 50 per cent for carbon steel drills. From the table it will be noticed that the brasses and aluminium are among those metals drilled at the highest speeds, followed by copper and certain alloys. Carbon steels and alloy steels are among those drilled at the lowest speeds. Speeds for cast iron depend on the type of iron. The Location of Holes The precise location of a hole can be fixed by scribing intersecting lines after which the center is very lightly dot-punched. A small dot can be drawn over quite easily if not accurately placed, and this is followed by center punching. The spring-loaded dot-unch is very useful as it can be placed accurately and set to give a small indent. With small holes it is quite satisfactory if the dill is run straight in on the center dot, but with larger holes it is good practice to drill first of all a small ‘pilot’ hole which not only helps to keep the drill on course, but also removes the metal which would offer resistance to the drill web. Holding Work for Drilling It is very important that work should be held securely by clamping it to the table to avoid inaccurate work and to avoid accidents. When the drill is breaking through on the underside, the feed pressure may cause the point to push through before cutting is quite complete and two small ‘ears’ of metal are left in the hole. These catch up in the drill flutes but if the work is fastened properly, they will be taken off by the drill. If the work is not secured properly, it may run up the drill and be thrown aside or spin until the machine is stopped. All work should be bolted down unless it is so large and heavy that it could not possibly give trouble. It is always a good thing to ease off feed pressure as the drill nears the point of breaking
  • 56. Technical English Through Reading 51 through. Vee Blocks, Angle Plate and Clamps The centerline is taken along the bar whilst still in the vee blocks and the holes are located by dot and center punches. A pilot hole is a great help in starting holes on curved surfaces. Work may be held in clamps adapted to suit the job or it can be bolted straight down if suitable holes or openings can be maneuvered over the tee-slots in the drilling table. Clamps should always be arranged to grip as near as possible to the drilling and the free end of each clamp should be packed so that it is level. The work also, may require packing to leave clearance when the drill breaks through or to bring it up level if the bottom surface is shaped in any way. Drilling in Sheet Metal It is not easy to drill through thin metal without the drill snatching or the hole being distorted and this is more noticeable with larger holes. One way of overcoming this trouble is to sandwich the thin metal between two stouter pieces and to drill through all three or the drill pointed can be ground in the form commonly used by sheet metalworkers. The point is ground to leave a pointed center and the lands in form of scribers which cut the hole by removing a disc. Another way of reducing the danger of snatch is to grind a more obtuse point-angle so that the drill dose not break through before it is cutting to its full diameter. The drill point should also be thinned. Location of Large Holes When large holes are to be accurately located, it is a good plan to scribe a circle of the hole size, from the center dot. As the drill begins to cut, it is lifted two or three-times before cutting to its full diameter to check that the conical hole is running true. If it is off-center, it can be drawn over by cutting a small groove with a diamond-point or a half-round chisel. This is, perhaps, a ‘chancy’ business, but it can be avoided by using properly ground drills and by drilling accurately placed pilot holes. Drilling for Bolts and Screws When parts are to be held together by bolts or screws with nuts and washers, clearance holes are required through the components and this is best done with the parts located and held securely together whilst drilling. When parts are to be screwed together, the top one will require clearance holes with tapping holes in the other. With the parts held together, this can be done effectively. The clearance drill point enters the lower part just far enough to leave a shallow counter-bore whilst providing a lead for the tapping drill. The counter-bore will leave a clean and neat finish after tapping. Where components must be accurately located together, reamed holes can be provided for the bolt shanks. Alternatively, the parts can be located with steel pins (dowels) in reamed holes
  • 57. 52 Part 2 Metalwork but provision must be made for their removal, i.e. they should not be fitted into blind holes. Components can then be separated and replaced exactly. They would be held by screws or bolts in normal clearance holes. 2.2.4 Other Processes Counter-boring This process is quite often used in order that the heads of bolts or screws can lay flush with the face of a job, the hole being opened out for part of its depth with a counter-bore. The tool shown has an interchangeable pilot for adapting to various sizes of holes and a solid type is made in one with the shank. Spot-facing In spot-facing, the surface around a hole is trued up so that bolt heads or other components can bed squarely. This is done with tools similar to those used in counter-boring but with teeth formed on the end face only. Spot-facing is commonly used on castings where it is desired to bed bolt heads squarely without machining a whole surface, small raised portions being left for facing. Countersinking To accommodate the countersunk (conical) heads of screws, chamfers are formed around the edges of the holes with the countersinking tool. These are made with included angles of either 60 or 90 degree. They should always be used at slow spindle speeds to get good results. 2.2.5 Reaming The reamer is a cylindrical tool cutting along its sides and is used in opening out holes smoothly and accurately to size. Taper reamers are also available. The cutting edges are formed by either straight or spiral flutes, the latter being more favoured as there is generally less tendency to chatter which results from the long axial cut with straight flutes. Reamers are usually made with unequal spacing of cutting edges and this helps to obviate chatter whilst working. Providing a quick and positive means for the repeated finishing of holes to fine limits, the reamer is used extensively in production work as well as in the small workshop producing `one off' jobs. Reamers are made with square ended shanks for hand use and with Morse taper shanks for use in machines. For general purposes and in the smaller sizes, the reaming of drilled holes is an acceptable practice, but in larger sizes and for more accurate work it is better if a drilled hole can be opened out by boring prior to finishing by reaming, all three operations being carried out at the same setting in the machine. This ensures true alignment in the hole and also makes it possible
  • 58. Technical English Through Reading 53 to leave only the lightest of cuts for the reamer. These are both very desirable conditions since the reamer will not necessarily correct any misalignment in the hole whilst leaving only a light cut for the reamer is a great help in securing a good finish. The reamer has to remove all the metal in one cut and a copious supply of the appropriate cutting fluid is an important feature when reaming─as in all machining operations. Reamers are precision tools and should be stored in racks or in cases with compartments and not left about on benches or machines. Hand Reamers Reamers for hand operation are provided with squire-ended shanks, for holding in a tap wrench. To facilitate entry of a parallel reamer into the hole, the end is formed with a short bevel lead and, in addition, a short taper lead which extends over about a quarter of the flute length. This taper lead helps with the initial lining-up in the hole and gives a progressive cutting action. Taper lead reamers are satisfactory when the taper lead can pass clear through the work, but for deeper passes, reamers with bevel lead only are used. Machine Reamers For adaption in lathe spindles, tailstocks and turrets, machine reamers are provided with taper shanks, but they are also made with parallel shanks for holding in chucks. All are normally made with bevel lead only, but they can be obtained with taper lead as well. For bottoming in blind holes, cutting edges are made with square corners. To prevent reamers form ‘chattering’ and producing poorly finished holes, the spacing of the cutting edges is varied slightly around the reamer during manufacture. The machine jig reamer has a parallel shank accurately ground to size and this runs in a bush fitted in a jig which is designed to guide the reamer at work. Reamers for making standard tapers are commonly used in pairs, the roughing reamer being used in opening out a parallel hole and this is followed by the finishing reamer. Note the notches cut in the roughing teeth which serve to break the chips and to help in rapid cutting. The right-hand spiral helps in feeding-in which is not an undesirable feature for course cutting in tapered holes. The finishing reamer has a left-hand spiral. Drill Sizes and Reaming Speeds The following suggested allowances for machine reamers of standard design are reproduced by kind permission of Firth Brown Tools Ltd., Sheffield. Reamer size range Allowance for reaming ─────────────────────────── 1.5 to 3.0 mm 0.13 to 0.20 mm 3.0 to 6.0 mm 0.15 to 0.28 mm
  • 59. 54 Part 2 Metalwork 6.0 to 12.5 mm 0.25 to 0.38 mm 12.5 to 25.0 mm 0.25 to 0.50 mm 25.0 to 38.0 mm 0.38 to 0.65 mm ─────────────────────────── Material allowance left in the hole for hand reaming is usually 0.05 to 0.10mm It must be remembered that faulty drill point grinding will affect the allowance left in the hole. The drill point grinding should be checked before use. Feeds and speeds for reaming will be largely dependent on the material being machined, on the type of reamer and on the condition of the machine. As a guide for using standard reamers of high-speed steel, the feed can be taken as two to three times that for a drill of the same diameter and the speed as two thirds to three quarters of that for a drill of the same diameter. An appropriate coolant should always be used freely in reaming. Adjustable Reamers These tools are made with 4, 5 or 6 blades and they are very useful because it is possible to ream to any size within the range of adjustment of each reamer. This range is not large, and it is necessary to hold these reamers in sets, one or two on their own being of little use. The tapered blades slide in identically- tapered slots cut in the body of the tool and adjustment is effected by turning the two end nuts in unison, thus moving the blades in either direction along the slots. The nuts also serve to retain the blades in their slots, an internal bevel engaging over the beveled ends of the blades. Taper Reamers and Taper Pins The taper pin provides an excellent means of securing thing like pulleys, gears and collars to shafts. Taper pins are made to a standard taper of 1 in 48 and are bought ready-made, being specified by the larger diameter. Taper-reamers are made specially for these pins. With the component on the shaft, the initial hole is drilled to the smaller diameter and the hole is then reamed out until the pin can be pressed home ‘finger tight’ after which it is tapped firmly into place. 2.2.6 Cutting Fluids Whilst it is possible to carry out some machining operations dry, wet machining with the appropriate cutting fluid (coolant) offers many advantages, giving extended tool life, a better finish and allowing higher cutting speeds. Today, even the traditional `no coolant' cast iron can be machined wet and not only does this give better results, but it solves the problem of fine swarf penetration into bearings, slides and electric motors. The use of coolant in the drilling of cast iron is not recommended, however, as this tends to cause clogging in the drill flutes. In the school workshop, it is often sufficient to feed cutting fluids on with a small brush, but this is better if done with a drip-can mounted above the machine. The ideal way is, of course,
  • 60. Technical English Through Reading 55 by means of a coolant pump circulating filtered fluid which can be directed in a continuous flow over the work and tool point. Many kinds of cutting oil are available nowadays, covering the needs of every machining operation on different metals. Those cutting oils most likely to be found in the school workshop fall in to two groups, viz:(1)the soluble oils, and (2)the neat oils. Soluble Oils These are mineral oils containing emulsifying agents which enable them to be mixed with water and to remain stable. The oil is always added to the water and the ratio of the mix will depend on the work in hand. Soluble oils are inexpensive, they find many applications in the machining of metals and although often employed more for their coolant properties, they do provide some measure of lubrication. Heavy duty soluble oils, containing EP (extreme pressure) additives are available and with much-improved lubricant properties, the range of applications of these fluids is extended considerably. Neat Cutting Oils. These are marketed in many forms and except for screw threading and tapping, they are not likely to be in general use in the school workshop. Neat mineral oils. These low viscosity oils are suitable only for light machining, mainly in free-cutting steel and brass as the lubricant film will not stand up to any heavy tool loading. They are used more for their coolant properties and a good flow should be maintained. Mineral and fatty oil mixtures. These mixtures of neat mineral oils and fatty oils(e.g. lard oil) have a wider range of applications than the neat oils. With better lubricant properties, they are used in medium-heavy machining in lathes, thread milling, screw-cutting and other such work in which heavier tool loading takes place. Sulphurised oils. With the level of sulphur and its manner of incorporation arranged to suit any particular working conditions, these neat cutting oils form a very useful group of fluids. they are used mainly for their lubricant properties in heavy lathe work, gear cutting and screw-cutting and with pressured coolant systems in deep drilling, boring and trepanning in nickel and copper alloys. Some of these oils cause staining of these two alloys, but this can be avoided by the proper selection of the cutting oil. Sulphured oils. These are a variation of the sulphur-bearing neat oils and have similar applications. They cause staining of the high nickel and copper alloys. Neat oils containing sulpho-chlorinated additives in the form of chemical compounds form another group and these have EP (extreme pressure) properties for sever working conditions in machining stainless steels and high duty nickel alloys. 2.3 THE LATHE Although the lathe is basically a machine for generating cylindrical forms, it is in fact much
  • 61. 56 Part 2 Metalwork more than this, being a readily adaptable piece of mechanism which can be used to perform numerous other machining operations in addition to its basic functions. The work, normally rotating towards the operator, can be set up between two centers which engage in countersunk holes at either end, or it can be gripped in a chuck or bolted to a face-plate. The cutting tool, mounted on top of the carriage, can be moved along the machine or square across it and these two motions perform the basic functions in the generation of a true cylinder. The lengthwise traverse of the tool is commonly referred to as ‘sliding’ which produces a round face and the cross-traverse as ‘surfacing’ (or ‘acing’) which produces a flat surface. In addition to sliding and surfacing, the lathe can be used to produce tapered work, to cut screw threads, for boring and recessing, for profiling (shaping to contours), Whilst the chucks and face-plates can be used in machining a variety of flat, cylindrical or irregular forms. A further range of operations can be undertaken by reversing locations of tool and work, the tool rotating whilst the work is held on the carriage and brought up to the tool. Many types of lathes are made, some being designed for repetition work, and these are the automatic, capstan and turret lathes. Some are designed for special purpose, for example, the brass finishing lathe——used exclusively for that one metal whilst the spinning lathe is used in producing bowl forms from sheet metal The lathe which is of direct concern here is the center lathe, designed primarily for turning work held between centers or when held in chuck or on the face-plate. The center lathe appears in a great many forms and whilst generally made as a screw-cutting lathe, it is also made in simple form, without lead-screw or automatic feeds for the very young pupil who naturally finds it easier to learn the basic principles on an uncomplicated machine. Such machines are known as basic training lathes, not to be confused with the training lathes used by engineering apprentices. These lathes often embody many of the features found in the production models. 2.3.1 The Center Lathe The Bed This is the foundation of the lathe, and made in cast iron, it is usually of a very robust box-like form, robbed on the inside and ported so that coolant and swarf can pass through easily. The top surfaces of the bed, known as the ‘ways’ are accurately machined and often hardened, the satisfactory working of the lathe being very largely dependent on the alignment of these surfaces which are usually finished by precision grinding. Gap Bed Many lathes are made with a short gap in the bed in front of the headstock and this increases the capacity of the lathe for turning large wheels and pulleys. In smaller machines, the gap is usually left permanently open but in larger machines, a gap-piece is provided to afford maximum support for the carriage when normal work is being machined close to the headstock.
  • 62. Technical English Through Reading 57 Headstock At the left, and in the form of a stout box-casting, the headstock is precisely located and bolted to the bed. It is occasionally cast in one piece with the lathe bed. The headstock carries the spindle in precision bearings which must take both radial and end loads. They are usually of the tapered-roller type and on assembly, are pre-loaded to eliminate end-float and side-play in the spindle. The spindle is hollow, to accommodate long bars in the chuck and the inner end of the bore is machined to a standard taper to receive the live center or other accessories as required. The Morse standard taper is used on English lathes. The ‘live’ center is so called because it is the one which always rotates with the work and is associated with the driving. The hollow spindle facilitates the ejection of the live center with a length of rod passed through the bore. The traditional English spindle nose is threaded so that chucks, face- and catch-plates can be screwed on against a shoulder and over a plain portion (register) which aligns the component. The driving mechanism is inside the headstock and in a basic training lathe for young pupils, this is often simply a four- or five-step cone pulley on the spindle, driven by an identical pulley mounted the opposite way round on a lay shaft which is driven by the motor. This range of four or five speeds is doubled in the screw cutting lathe by engaging a back gear mechanism which reduces all speeds in a set ratio. The back gear is used in screw cutting and the low speeds it gives are useful in turning large diameters and machining hard materials. The moving of the bell drive on stepped pulleys is not necessary in the all-geared headstock in which one belt drive brings power into the assembly, after which, speed changes are made through gears, operated from external hand controls. In conjunction with a foot operated spindle brake, the all-geared head makes speed changing a very fast operation, but brakes can only be fitted where chucks (etc.) are directly mounted on the spindle. See under ‘Chuck mountings’. Another method of speed changing which avoids stopping to change a belt over employs two variable-width vee pulleys. Both split at the bottom of the vee and as one pulley opens out along the axis, the other closes, the vee belt sinking in one pulley as it opens, whilst rising in the other pulley as it closes. This gives an infinitely variable speed range which can be adjusted with the machine running. Tailstock The tailstock supports the ‘free’ end of the work and s used also in the drilling and reaming of work held in chuck or on face-plate. It slides on and guided by the bed-ways and in most lathes is made in two parts which permit of a lateral adjustment. This is used in off-center taper- turning. The casting is bored to receive the barrel (or ‘sleeve’) whose axis is precisely in line with that of the spindle. The inner end of the barrel is machined to receive the tapered center which can be of the stationary or rotating kind. The taper-socket in the barrel is used for holding
  • 63. 58 Part 2 Metalwork taper-shank drills, reamers or other accessories. At the outer end, the barrel is threaded to take the adjusting screw which is operated by a hand-wheel. Major adjustments to the location of the tailstock are made by sliding it along the bed and clamping it by operating a lever, after which, fine adjustments to bring the center up to the work can be made with the hand-wheel. The barrel also, can be clamped after setting, so that it cannot slack off during working. Carriage or Saddle This forms the base of the unit which supports the cutting tool and it can be traversed along the whole length of the bed by hand control or by power feed. It can be clamped at any point along the bed. A cross slide is provided for cross traversing or ‘surfacing’ and on this slide is mounted the compound slide (top slide) which can be pivoted and locked at any angle for use in turning short tapers. To the front of the carriage is fixed the apron which extends well sown over the front of the bed and here are found the controls for hand- or power- feeding when surfacing, sliding or screw-cutting. Hand-traversing of the carriage is by rack and pinion, the handwheel turning the pinion and the rack being fitted under the over hand of the bed-way. Lead-screw The lead-screw, which transmits feed motion for screw cutting, extends the whole length of the bed, passing behind the apron. It can be engaged with, or freed from the carriage by a clutch mechanism which can be operated whilst the lead-screw is turning. This clutch is quite simple, consisting of a large split nut (‘half nuts’) which can be opened or closed over the lead-screw by the movement of the lever on the apron. This mechanism is only used when screw-cutting. Feed Shaft In addition to the lead-screw, a feed shaft is employed in operating the carriage or the cross slide in automatic turning. The lead-screw is not used for this purpose to avoid wearing it on work for which it is not needed (it is a costly item) and also because the feed it gives would often be too fast. The feed shaft, with a key-way (a lengthwise slot), runs alongside the lead-screw and passes behind the apron where a keyed worm wheel, mounted on the shaft, turns with it and is free to slide along it. The worm wheel drives a gear wheel and from this, the feed can be directed either to the cross slide or to the carriage by operating a control on the apron. On some lathes, the lead-screw itself is made with a keyway cut through the threads. The keyway drives the worm wheel and the screw is used for screw-cutting. Automatic turning is very useful in long traverses, the steady movement of the carriage giving a superior finish to that usually obtained by hand feeding. One point to not is that in end facing with automatic feed it is impossible to maintain a constant cutting speed since the speed at the work periphery will be at its maximum and this will diminish towards the center. A
  • 64. Technical English Through Reading 59 compromise must be sought in these situations. The motion for the lead-screw and the feed shaft is taken from the spindle and because the revolutions of the lead-screw must be positively related to the spindle for screw-cutting, the drive is always taken through a gear train. This subject is dealt with more fully under ‘Screw-cutting’ and for the moment, it will be sufficient to know that the velocity ratio of spindle to lead-screw can be changed to cut various thread pitches, that the rotation of lead-screw and feed shaft can be reversed and that either or both can be taken out of drive when not required. Most lathes are fitted with some kind of safety device which either prevents the accidental engagement of more than one feed at the same time, or in the event of two feeds being engaged together, prevents damage being done, sometimes with a slipping clutch mechanism or with a shear pin which breaks under any abnormal load. 2.3.2 Lathe Sizes The size of a lathe is commonly expressed in the UK by the height of the centers over the lathe bed, indicating the radius of a cylinder which will clear the bed, but quite often the ‘swing’ of a lathe is quoted and this refers to the diameter of such a cylinder. Whichever way the size is expressed, it must be remembered that the work has to clear the lathe carriage when turning between centers. The bed length of a lathe does not indicate its capacity which is quoted as length between centers. 2.3.3 Work Holding and Driving Chucks Every lathe should be equipped with two chucks, one a self-centring (SC) 3-jaw chuck and the other a 4-jaw independent chuck, both of the size recommended by the lathe manufacturer. Both types are used in many different machining operations on short work pieces which can be held in the chuck, whilst longer items can be supported on the tailstock center or a steady at the outer end. Long lengths of bar, can, of course, be passed right through the hollow spindle whilst griped in the chuck. Self-centring chucks The 3-jaw SC chuck will automatically center rounds or hexagons, all jaws opening or closing together as the scroll is turned with the key. The jaws are matched to the scroll during manufacture and each is numbered so that it can be returned to its correct slot after removal. For holding large diameter work, scroll chucks are supplied with spare sets of jaws with reversed steps. The jaws themselves cannot be reversed. To remove the jaws, the chuck is opened out until all three jaws can be withdrawn as they disengage from the scroll. Re-assembly with either set of jaws is done in the following manner: the scroll is turned as for opening and when the screw end as has disappeared from No. 1 slot, No.1 jaw is pressed home and engaged
  • 65. 60 Part 2 Metalwork by closing the chuck. When the scroll end appears in No.2 slot, it is would back a little and No.2 jaw is pressed home and engaged with the scroll. This is repeated for No.3 jaw. SC chucks are also made with two, four and six jaws, and of these, only the 4-jaw chuck is likely to find uses in the school workshop as it will center square bar stock. Jaws with external steps can be used to hold large rings or tubes by opening them on to the inside of the work. Scroll (SC) chucks will center work with reasonable accuracy, but unless great care is taken, they will quickly be thrown out of truth, mainly by wear or straining of the scroll from over-tightening of from digging-in which throws heavy loads on jaws and scroll, especially if short work pieces are wrenched from the chuck. Chuck jaws should be removed at regular intervals for the cleaning of the scroll, the jaw teeth and the slides. Over-lubrication is not a good thing, since excess oil either will be thrown out or will form a destructive grinding compound with any fine swarf. The effect of inaccurate centring can be nullified by planning chuck jobs so that all of the important operations are carried out at one setting of the work. Once the job is removed, it is difficult to get it running true again. Independent Jaw Chucks Whilst the 4-jaw independent chuck is indispensable for holding work of irregular shape and for off-centre turning, it can also be used for holding squares or rounds. Centring takes a little longer but it can be done very accurately using each individual jaw adjustment. The procedure for centring in the 4-jaw chuck is described under ‘Chuck-work’. Independent jaw chucks are supplied with only one set of jaws since these are reversible. Draw-in Collet Chuck. The draw-in collet provides a quick and accurate means of holding small parts for models, instrument- and clock making. Made of heat treated steel, the collet is in the form of a sleeve, bored to receive round, hexagon and square sections closely approximating to the bore size. The collet, split into three or more segments, is drawn into the lathe spindle with a threaded tube (draw-bar), the tapered end bears against a matching taper inside an adaptor or ‘closer’ on the lathe spindle and the collet closes on to the work. Because of its restricted range of adjustment, it becomes necessary to hold one collet for each size and section of bar likely to be machined and this could mean a large number. ‘Multiblade’ collet chuck. The ‘Multiblade’ collet consists of a steel body carrying six spring loaded blades arranged radially. When the collet is pushed into a conical housing,the blades move in with a parallel grip with a range of movement of more than 3 mm. On releasing the pressure, the springs retract the blades, allowing them to move forwards, releasing the work. With this large range of adjustment, fewer collets will be required and a total of twenty will hold any size of bar between 1.5 mm and 62 mm, round of hexagon. These collets are part of an integrated system of work holders in which any one collet can be used in a range of closing devices which include: (1) Key operated chucks for lathe work;
  • 66. Technical English Through Reading 61 (2) Lever or power operated chucks which can be opened or closed whilst the machine is running. There are also vertical mounting chucks for use in milling machines and drill or tool holders for use on capstan and automatic lathes. In a key-operated chuck the key rotates the tightening sleeve on its ball-race. The thread on the inside of this ring draws back the closing ring which is prevented from turning by means of the guide key. This closing ring draws the collet back into the conical housing, closing the blades. Chuck Mountings For many years, the normal method of mounting a chuck on the spindle-nose has been by means of thread and register on the spindle, on to which screws a back-plate. The chuck, in its turn is bolted to the back- plate. In its initial assembly, the back-plate is screwed on to the spindle-nose and is then faced and finished dead to size to fit into a recess formed in the back of the chuck. The two are then screwed together and from then on, that chuck and backplate ‘belong’ to that particular lathe. Attention is drawn to the plain-shouldered portion (register) which centers the chuck. This mounting exhibits several undesirable features and there is an increasing trend for chucks to be mounted directly on to spindles made to various international Standards Specifications. The advantages which these spindle-nose forms offer are: (1) the register is quite positive and is not subject to the wear which takes place when chucks are screwed on to the spindle; (2) the chuck is held very securely with no danger of its ‘running off’ when the lathe is stopped (even when the spindle is fitted with a brake); (3) the overhang is reduced and with the chuck much closer to the bearings, the whole assembly is much more rigid, making for better work; (4) the chuck is quickly mounted and removed, except with the type where the chuck is bolted to a flange in a semi-permanent mounting. Whatever form of mounting is used, it is very important that both spindle and chuck registers and threads are quite clean before assembly. When removing chucks, the operator should always be ready to take the weight which comes suddenly, to avoid damage to lathe bed or fingers. A softwood block, resting on the lathe bed is good ‘accident prevention’ here. Face-plate This accessory is used for mounting work of awkward shapes which cannot readily be chucked. Catch-plate The catch-plate, mounted on the lathe spindle is commonly used to drive work between centers, a driving pin engaging with a ‘carrier’ (or ‘dog’) which screws on to the end of the
  • 67. 62 Part 2 Metalwork work. Catch-plates are often made with a radial slot across the face so that the driving pin removed, a bent-tail carrier can then be used for driving the tail engaging in the slot. Lathe Centers These are commonly made from high-speed steel and are hardened, but in fact, it is not essential for the live center (in the spindle) to be hardened as there is no rubbing. Tailstock centers are also made with cemented carbide tips which give long and arduous service. Centers are accurately ground to standard tapers with the points usually finished to an included angle of 60 degree. The live center is adapted to the spindle socket by means of a taper sleeve. It is of the utmost importance that the centers fit perfectly in the spindle and tailstock barrel and before insertion they should be wiped clean and inspected for damage, the slightest burring of the tapers causing them to run out of truth. The spindle and tailstock tapers also require attention and cleaning should be done with a rag on a stick (not the finger) and never with the spindle revolving. The tailstock center is removed by retracting the barrel and when it is almost home, the end of the screw will eject the center. The tailstock center-point will require lubrication because the work is turning and its adjustment up to the work must be made carefully so that all end-float is eliminated without any undue pressure which will heat and expand under heavy cutting and will bind on the center and damage it if the pressure is not relieved by a re-adjustment. Long bars will almost certainly show enough expansion to cause binding with only a slight rise in temperature and the adjustment and lubrication of the center must be watched carefully. Live Tailstock Center The use of live (revolving) tailstock center eliminates friction (but not work expansion). Other kinds are made for different conditions of service and with various point forms, including conical adaptors for centring tubes. The center rotates on precision bearings which take radial and thrust loads and elaborate precautions are taken to prevent the ingress of swarf or coolant. The bearings are lubricated for life and the unit should never require dismantling. Half center The use of a half center facilitates the end-facing of work between centers by allowing the tool to be fed right in. For end-facing with a normal center, the tool tip must be ground back at a little less than 60 degree. The truing of centers At the first signs of wear, center points must be retrued and this can be done with a tool-post grinder (a small, self contained grinding machine), mounted on the top slide which is set over at 30°. With the center rotating slowly in the headstock, the grindstone is traversed, slowly, back and forth along the cone, taking only the very lightest of cuts. When the best possible finish has been achieved with the grinder, the cone can be given a high finish by honing with a perfectly flat stone. Lathe bed and slides are kept covered during grinding to keep all abrasive grit away.
  • 68. Technical English Through Reading 63 Carriage Stop This is a very useful device for halting the carriage at predetermined points along the bed and can be used for the repeated finishing of parts of identical. Graduated Dials The cross- and top-slide hand wheels are fitted with graduated dials giving accurate indications of slide movement in setting the depth of cuts. The dial is in the form of a collar which rotates with the hand wheel, usually through a friction drive which allows of its independent rotation. This enables one to set the depths of cuts from a zero reading by turning the dial and without altering the tool position. Dial graduations are read against a zero line scribed on the slide body. It must always be remembered that any adjustment of the cross slide will be doubled off the work diameter unless the lathe is fitted with a direct reading dial which shows the amount taken off the diameter. Because of market requirements, UK manufactures now supply lathes which operate in (1) metric terms only, i.e. with metric pitch lead screw and with dials reading in millimeters (to 0.02 mm), (2) English terms only, i. e. With English pitch lead screw and dials reading to 0.001 ins, or (3) combinations of both, e.g. English pitch lead screw with dials reading in millimeters or with dual reading dials which give inch or metric reading as required. Dual reading dials have been developed to cope with metrication in the UK. These dials can be fitted as standard equipment on new machines or can be fitted to machines already in use. They are designed for single reading, i.e. when set to read metric, only inch graduations are visible and when set to read metric, only metric graduations are visible. The conversion, which is instantaneous, is brought about by sliding the thimble in or out, a system of sun and planet wheels giving precise conversions from inches to millimeters. 2.3.4 Tool Posts The American `ring and rocker' post is shown holding a tipped tool but it can be used for tool holders. Tools are quickly adjusted at center height by moving the rocker which beds on the loose ring. The four-way post is more suited to repetition work where a cycle of operations is to be performed, but nevertheless, it will be of occasional use in the school workshop. a modern slotted-block tool post dispenses with the need for packings to set the tool-height, this adjustment being made by means of a knurled screw. The tool, still in its holder, can be quickly removed for sharpening and no adjustment is needed when it is replaced in the post. 2.3.5 Lathe Tools However good a lathe may be, its efficient operation and the quality of the finish on its products will be dependent on the tools used. Apart from the actual shape of the tool point, the material
  • 69. 64 Part 2 Metalwork of which it is made is of prime importance, the essential qualities being toughness which enables it to withstand shock and heavy pressure, and hardness which enables it to hold a cutting-edge. Although carbon tool steel has these properties when properly heat-treated, its applications are limited and it has been largely replaced by the high-speed steels and other alloys which retain a cutting-edge for much longer periods and under much more rigorous conditions, viz. : heavy cuts at elevated temperatures without losing their hardness. The air hardening property of high-speed steel which can be used on hard materials. The other alloys referred to are non-ferrous, they are extremely hard and brittle (and expensive) and are used in the form of tips which are brazed or butt welded on to shanks, often of high tensile steel. Tipped tools are also made with expendable tips which are held in place with a small clamp and when dull, the tip is discarded and replaced with a new one. Stellite (an alloy of cobalt, chromium and tungsten) is used in making tips and the hardest are made from the cemented carbides (for example, tungsten carbide). All permit of cutting-speeds well in excess of those possible with high-speed steels. Lathe tools can be divided into four groups: (1) Solid (one-piece) tools of carbon─or high-speed steel, gripped directly in the tool post; (2) Tool holder bits of high speed steel and of square or round section. Held in tool holders; (3) Tipped tools of various kinds; (4) Special tools, e.g. boring tools, boring bars which hold tool bits, dee bits, form tools of various kinds and knurling tools. Tool Angles The wedge form of the cutting part of a lathe tool involves two components, viz, clearance below the cutting edge, without which the tool could only rub on the work, and rake which is the slope on the face, away from the cutting edge. Whilst clearance varies only slightly for different purposes, rake shows marked variations, being adjusted to suit the metal being turned. The direction of rake is determined mainly by feed direction and the shape of the cutting edge. Clearance Clearance, formed on both the major and minor flanks of the tool is kept as small as possible, consistent with the proper working of the tool. Excessive clearance reduces support for the cutting edge and increases the tendency to chatter. In addition to the slight variations in clearance for different metals, there are several cases where further adjustments are required, viz: (1) Because of the convex face of the work in turning, front-cutting tools will operate with slightly reduced clearance, but not for surfacing cuts; (2) For traversing cuts, the correct clearance may need to be increased slightly because the helical form of the transient surface becomes more pronounced, unless the feed speed is reduced;
  • 70. Technical English Through Reading 65 (3) Boring tools will require increased clearance to avoid rubbing on the concave work surface. A second flank will provide relief at the heel and with very small bores, this secondary angle may have to be quite large. Rake and Cutting Action tools for brittle metals like hard brass and cast iron, are formed with little or no rake, the near-flat face presented to the work causing the waste metal to shear and break away in small chips. Where the metal forms a flowing chip (and many metals do this), the tool must be given rake because the waste metal cannot be forced off by shearing, but must be lifted off by the wedge action of the tool. Chip pressure is very considerable when heavy cuts are taken and tool cutting parts weakened by excessive rake or clearance will be prone to chatter and to digging in caused by the ‘feeding in’ effect of pressure on sloping face. Rake should produce the smallest possible chip deflection whilst leaving the cutting part stiff enough to withstand the load without springing. For very hard metals rake is quite small, e.g. , about 10 for tool steel, increasing to 30 or more for soft and ductile metals like copper and aluminum. In Figure 2-5 are seen examples showing how rake direction influences the line of chip flow from the transient surface. Whilst this is an important feature in tool designing. There are also other factors to be considered. Tool Geometry The geometry of the parting off tool is probably the least complicated and though shown in Figure 2-6 with the major cutting edge perpendicular to the tool axis, it is usually formed at a small angle to give a clean break through. For the tool shown in Figure 2-5, the direction of feed motion aligns with the plane of normal rake and with the tool set horizontally in the lathe, there is a simple and obvious relationship FIG. 2-5 Rake feed direction and between tool angles and surfaces both in its making Chip-flow
  • 71. 66 Part 2 Metalwork and in its operation. FIG.2-6 Lathe and tool nomenclature and angles Tools, however, are produced with the cutting edges in a variety of orientations and when in use, the tool shank may be set at any angle in relation to the work surface. the angles thus formed by tool cutting edges, faces and flanks relative to the resultant cutting direction and to the work surfaces are known as ‘working angles’ which are not always readily oriented or measured in the making or the re-grinding of the tool. The recommendations of the British Standards Institution in resolving these problems involve the use of two systems of reference planes through a ‘selected point’ on the cutting edge by means of which ‘tool angles’ and ‘working angles’ can be defined. The first, known as the ‘tool-in-hand’ system defines the geometry of a tool to facilitate its manufacture whilst the other, known as the ‘tool-in-use’ system, defines the effective tool geometry when it is performing a cutting operation. The nomenclature for defining rake, clearance and other cutting tool angles is known as the ‘normal rake system’ and it is the preferred system replacing the former ‘maximum rake system’.
  • 72. Technical English Through Reading 67 It is not possible to go further into this subject within the context of this book and the reader who seeks further information is advised to refer to the British Standards publication BS 1296 Part 2, ‘Glossary of Terms for Single Point Tools’. Cutting Speeds Cutting speeds are usually expressed in terms of meters per minute and the speeds at which the tool should pass over the work to produce the best results at suitable speeds. Cutting speeds are governed by two factors: (1) The hardness of the metal being cut; (2) The type of tool being used, HSS and tipped tools operating at much higher speeds than carbon steel tools. To maintain these speeds, small diameter work will have to turn much faster than larger diameters. Steadies Two kinds of steady are used for supporting long work against the pressure of the tool. The fixed steady is secured to the lathe bed whilst the travelling steady is mounted on the carriage and moves along the work behind the tool, as each cut is taken. The fixed steady can be used to support the end of a bar, gripped in the chuck, for end-facing, for center drilling, for boring or for threading. 2.4 LATHEWORK 2.4.1 Turning on Centers The Center Drill The first operation in the turning of work between centers is the drilling of the holes for mounting the work, and for this, the combined drill and countersink (center drill) is used. Three types of drill are made (A, B and R), but only the double-ended type A is considered here. The hole at the bottom of the countersinking leaves clearance for the lathe center point and provides a reservoir for lubricant. When center drilling in the lathe, the bar can be passed through the spindle if not too large and held in the 3-jaw chuck. The appropriate cutting fluid should be used and the drill must not be bumped against the work or forced as this might cause the point to split and break off in the work. If the bar is too large for the spindle bore, it can be gripped in a chuck with the free end supported in a fixed steady. When a bar has to be finished to length between centers, the allowance for machining should not be excessive, otherwise the bearing surfaces on the centers will become too slight when the facing is done.
  • 73. 68 Part 2 Metalwork Turning Example A piece of work for turning on centers is shown in Figure 2-7 with suggestions for the operations involved. The bar should be cut off long enough to allow for a carrier and some clearance for the turning tool. FIG.2-7 A turning example (1) Both ends should be faced and center drilled with a No. 3 or 4 center drill. In the end facing, care should be taken to avoid leaving a center ‘pip’ which may cause the center drill to start off center. (2) A work piece on centers is fitted with a straight carrier. The back center adjustment must be made carefully to avoid undue pressure and friction. Many turners fill the center hole with tallow initially and in any case, the center must be lubricated during working, with oil. During working, slackness of the centers is often indicated by a rattling from driving pin striking the carrier tail (or from the catch plate slot) as cuts are started. The back center should be checked at intervals for heating. (3). If the end facing tool is left in the tool post, it can be used, with the work stationary, to scribe a short line as a guide for setting up the next tool accurately to center height. (4) Beginners will probably feel safer using a cranked tool, since they will be working towards a carrier. (5) The reduction of the bar to size is started off with hand traversing and when the work is approaching finished size, a fine automatic feed can be engaged, using a nicely honed finishing tool. The bar should be marked clearly to indicate a limit for turning to avoid striking the carrier. Cutting oil should be used freely to promote a good finish. (6) Note that the locating spigot does not pass right through the hole and this ensures that the nut grips tightly against the framework. (7) On reversing the work to repeat the operations, it may be more convenient to drive by the spigot, using a smaller carrier with a brass pad to prevent damage to the finished surface. Length can be checked over the shoulders with sliding calipers or a rule measurement may be considered satisfactory, in which case, a stop of some kind should be held against the left-hand shoulder from which the measurement is made. (8) Since this job is an exercise in turning, the threads should be cut in the lathe and this subject is dealt with under a separate heating.
  • 74. Technical English Through Reading 69 Taper Turning Tapers can be turned in several ways: (1) The compound slide rest can be swiveled round for turning short internal tapers; (2) The tailstock can be offset for longer external taping; (3) The taper turning attachment can be used for external tapers and for boring short internal tapers; Tapers can be expressed in three ways: (1) as a given amount on the diameter per unit length, e.g.10 mm per 250 mm, (2) as an incline, e.g.1 in 20 on the diameter, and (3) as an included angle. Before adjusting the tailstock, it is first clamped lightly and after releasing one of the adjusting screws, both are used to move the tailstock towards the tool so that the small end of the taper is at the tailstock. The screws are finally tightened up against each other. A rough measurement of the offset is found by reading the datum line against the scale on the tailstock base, but the offset can be fact, be measured quite accurately from a short piece of bar held in the tool post. The gap between the bar and tailstock barrel is measured with the vernier calipers before the tailstock is offset and form this measurement is deducted the required offset. The tailstock can then be moved over this distance. Measurement of the offset can also be done with the work set up on centers and with a dial gauge held in the tool post, the offset over a known distance along the work can be accurately measured. Of course, the dial gauge plunger must be at center height. One very important point which must never be overlooked in taper turning is that the tool point must be set exactly at center height otherwise an incorrect taper will be generated. One effect of taper turning on centers which should be considered is that whilst the work axis is aslant to the lathe axis, the centers remain parallel to it. This uneven bearing on the centers is probably of little consequence in slight tapers but with steep tapers, this is not a good thing. The use of a live tailstock center gets over the problem of the uneven rubbing, to a limited extent. 2.4.2 Screw-cutting in the Lathe The means by which the lathe carriage is traversed in screw-cutting has already been touched on briefly and this is amplified in the diagram in Figure 2-8 which shows how the motion is taken from spindle to lead screw and carriage in the ‘traditional’ lathe. In a geared headstock, the drive from spindle to first driver, with reverse mechanism, is enclosed. To cut a thread of any particular pitch, it is necessary to relate the carriage-traverse precisely to the rotation of the work and this is done by relating the sizes of the change wheels in the gear train. In all case, the pitch of the lead-screw must be taken into account since this will effect the ratio when rotation is converted into lateral movement.
  • 75. 70 Part 2 Metalwork The Lead-screw Drive In Figure 2-9 is shown a ‘standard’ drive mechanism but before studying this further, two simple facts concerning speed ratios must be understood, viz.: FIG.2-8 Diagram showing how movement of carriage is related to the rotation of work FIG.2-9 A ‘standard’ change gear mechanism with tumbler reverse gear (1) A small wheel driving a larger one will bring about a speed reduction and vice versa, and the ratio of the speeds will inversely as the ratio of the wheel sizes, or, with gear wheels, as the ratio between the number of teeth on each wheel. Thus, a 20-tooth wheel driving a 60-tooth wheel will effect a speed reduction of 3:1 and so on. (2) A third wheel (or ‘idler’) meshing in between to others will not affect the ratio but will cause the ‘driven’ wheel to turn in the same direction as the ‘driver’.
  • 76. Technical English Through Reading 71 (3) From Figure 2-9 it will be seen that the drive from the spindle gear is taken to a driver or stud gear which turns at the same speed. The drive is taken through a ‘tumbler’ gear mechanism which is used to reverse the rotation so that the carriage can be traversed in either direction. Placed in the neutral position, the tumbler disconnects the gear train altogether when the feed-motion is not required. From the stud gear, either a simple or a compound gear train can be taken to the lead-screw. Simple Train Where the speed change can be brought about with one pair of change wheels, a simple train is used, the two wheels connect stud to lead screw through a ‘idler’ whose size is immaterial and determined mainly by the gap between driver and driven wheels. Compound Train Where the speed change requires two pairs of change wheels a compound train is used and this must occupy the same space. The first pair of wheels connects stud to intermediate pin and the first wheel of the second pair, also mounted on the intermediate pin and keyed to the other wheel, drives the lead-screw wheel. The wheels on the intermediate pin are not idlers, one is a driven wheel and the other a driving wheel. For each different pitch required, the gear train must be dismantled and re-assembled with other wheels and all brought into mesh by adjusting the swing plate. However well practiced one may be, this takes time which can be saved if a quick-change gearbox is fitted as indicated in Figure 2-9. These gearboxes, often of the Norton type, are driven from a simple train which, with one wheel change, provides a wide range of drive speeds. Setting up A Gear Train Before opening the end cover to expose the tumbler reverse and the gear train, the lathe must be isolated by turning off its main switch; simply pressing the stop button on the machine does not provide sufficient safeguard. All screw cutting lathes are supplied with screw cutting charts showing the selection and arrangement of the gears for any desired thread and where a metric thread is to be cut from an English lead screw, this can be done by the inclusion of a transposing gear with 127 teeth in the gear train. The facility for cutting metric threads is now an important design feature in modern, geared head lathes, the cutting of English or metric threads being quickly arranged by selecting the required pitch at the gear box, regardless of whether the lead screw is in English or matric pitch.
  • 77. 72 Part 3 Electrical Motor Control PART 3 ELECTRIC MOTOR CONTROL 3.1 GENERAL PRINCIPLES OF ELECTRIC MOTOR CONTROL There are certain conditions that must be considered when selecting, designing, installing, or maintaining electric motor control equipment. The general principles are discussed to help understanding and to motivate students by simplifying the subject of electric motor control. Motor control was simple problem when motor was used to drive a common line shaft to which several machines were connected. It was simply necessary to start and stop the few times a day. However, with individual drive, the motor is now almost an integral part of the machine and it is necessary to design the motor controller to fit the needs of the machine to which it is connected. Motor control is a broad term that means anything for a simple toggle switch to a complex system with components such as relays, timers, and switches, the common function of all controls, however, is to control the operation of an electric motor. As a result, when motor control equipment is selected and installed, many factors must be considered to insure that the control will function properly for the motor and the machine for which it is selected. 3.1.1 Motor Control Installation Considerations When choosing a specific device for a particular application, it is important to remember that the motor, machine, and motor controller are interrelated and need to be considered as a package. In general, five basic factors influence the selection and installation of a controller. 1. Electrical Service Establish whether the service is direct (dc) or alternating current (ac). If ac, determine the frequency (hertz) number of phases in addition to voltage. 2. Motor The motor should be matched to the electrical service, and correctly sized for the machine load in horsepower rating (hp). Other considerations include motor speed and torque. To select proper protection for the motor, its full load current rating (FLC), service factor (SF), time rating (duty), and other pertinent data——as shown on the motor nameplate─must be used. 3. Operating Characteristics of Controller The fundamental tasks of a motor controller are to start and stop the motor, and to protect the motor, machine, product and operator. The controller may also be called upon to provide supplementary functions such as reversing, jogging or inching, plugging, operating at several speeds or at reduced levels of current and motor torque.
  • 78. Technical English Through Reading 73 4. Environment Controller enclosures serve to provide safety protection for operating personnel by preventing accidental contact with live parts. In certain applications, the controller itself must be protected from a variety of environmental conditions which might include: ■ Water, rain, snow or sleet ■ Dirt or noncombustible dust ■ Cutting oils, coolant or lubricants Both personnel and property require protection in environments made hazardous by the presence of explosive gases or combustible dusts. 5. Electrical Codes and Standards Motor control equipment is designed to meet the provisions of the National Electrical Code (NEC). Also, local code requirements must be considered and met when installing motors and control devices. Presently, code sections applying to motors, motor circuits, and controllers, Article 440 on air conditioning and refrigeration equipment, and Article 500 on hazardous locations of the National Code. The 1970 Occupational Safety and Health Act (OSHA) as amended, requires that each employer furnish employment in an environment free from recognized hazards likely to cause serious harm. Standards established by the National Electrical Manufacturers Association (NEMA) assist users in the proper selection of control equipment. NEMA standards provide information concerning the construction, testing, performance, and manufacture of motor control devices such as starters, relays and contactors. One of the organizations which actually test for conformity to national codes and standards is Underwriter’s Laboratories (UL). Equipment that is tested and approved by UL is listing in an annual publication, which is kept current by means of bimonthly supplements to reflect the latest additions and deletions. A UL listing does not mean that a product is approved by the NEC. It must be acceptable to the local authority having jurisdiction. 3.1.2 Purpose of Controller Factors to be considered when selecting and installing motor control components for use with particular machines or systems are described in the following paragraphs. Starting The motor may be started by connecting it directly across the source of voltage. Slow and gradual starting may be required, not only to protect the machine, but also to insure that the line current inrush on starting is not too great for the power company's system. Some driven machines may be damaged if they are started with a sudden turning effort. The frequency of starting a motor is another factor affecting the controller.
  • 79. 74 Part 3 Electrical Motor Control Stopping Most controllers allow motors to coast to a standstill. Some impose braking action when the machine must stop quickly. Quick stopping is a vital function of the controller for emergency stops. Controllers assist the stopping action by retarding centrifugal motion of machines and lowering operations of crane hoists. Reversing Controllers are required to change the direction of rotation of machine automatically or at the command of an operator at a control station. The reversing action of a controller is a continual process in many industrial applications. Running The maintaining of desired operational speeds and characteristics is a prime purpose and function of controllers. They protect motors, operators, machines and materials while running. There are many different types of safety circuits and devices to protect people, equipment and industrial production and processes against possible injury that may occur while the machines are running. Speed Control Some controllers can maintain very precise speeds for industrial processes. Other controllers can change the speeds of motors either in steps or gradually through a continuous range of speeds. Safety of Operator Many mechanical safeguards have been replaced or aided by electrical means of protection. Electrical pilot devices in controllers provide a direct means of protecting machine operators from unsafe conditions. Protection from Damage Part of the operation of an automatic machine is to protect the machine itself and the manufactured or processed materials it handles. For example, a certain machine control can reverse, stop, slow, or do whatever is necessary to protect the machine or processed materials. Maintenance of Starting Requirements Once properly installed and adjusted, motor starters will provide reliable operation of starting time, voltages, current, and torque for the benefit of the driven machine and the power system. The National Electrical Code, supplemented by local codes, governs the selection of the proper sizes of conductors, starting fuses, circuit breakers, and disconnect switches for specific system requirements.
  • 80. Technical English Through Reading 75 3.1.3 Manual Control A manual control is one whose operation is accomplished by mechanical means. The effort required to actuate the mechanism is usually provided by a human operator. The motor may be controlled manually using any one of the following devices. Toggle Switch A toggle switch is a manually operated electric switch. Many small motors are started with toggle switches. This means the motor may be started directly without the use of magnetic switches or auxiliary equipment. Motor started with toggle switches is protected by the branch circuit fuse or circuit breaker. These motors generally drive fans, blowers, or other light loads. Safety Switch In some cases it is permissible to start a motor directly across the full line voltage if an externally- operated safety switch is used. The motor receives starting and running protection from dual-element, time-delay fuses. The use of a safety switch requires manual operation. A safety switch, therefore, has the same limitations common to most manual starters. Drum Controller Drum controllers are rotary, manual switching devices which are often used to reverse motors and to control the speeds of ac and dc motors, They are used particularly where frequent start, stop or reverse operation is required. These controllers may be used without other control components in small motors, generally those with fractional horsepower ratings. Drum controllers are used with magnetic starters in large motors. Faceplate Control Face controllers have been in use for many years to start do motors. They are also used for ac induction motor speed control. The faceplate control has multiple switching contacts mounted near selector arm on the front of an insulated plate. Additional resistors are mounted on the rear to form a complete unit. The use of faceplate starters offers advantages and features not abstained with other manual controllers. They are also cheaper to purchase than automatic controls. 3.1.4 Remote and Automatic Control The motor may be controlled by remote control using push buttons, when push button remote control is used or when automatic devices do not have the electrical capacity to carry the motor starting and running currents, magnetic switches must by included. Magnetic switch control is one whose operation is accomplished by electromagnetic means. The effort required to actuate the electromagnet is supplied by electrical energy rather then by the human operator. If the
  • 81. 76 Part 3 Electrical Motor Control motor is to be automatically controlled, the following two wire pilot devices may be used. Float switch The raising or lowering of a float which is mechanically attached to electrical contacts may start motor-driven pumps to empty or fill tanks. Float switches are also used to open or close piping solenoid valves to control fluids. Pressure Switch Pressure switches are used to control the pressure of liquids and gases (including air) within desired range. Air compressors, for example, are started directly or indirectly on a call for more air by a pressure switch. Time Clock Time clock can be used when a definite “on and off” period is required and adjustments are not necessary for long periods of time. A typical requirement is a motor that must start every morning at the same time and shut off every night at the same time, or switches the floodlights on and off. Thermostat In addition to pilot devices sensitive to liquid levels, gas pressures, and time of day, thermostats sensitive to temperature changes are widely used. Thermostats indirectly control large motors in air conditioning systems and in many industrial applications to maintain the desired temperature range of air, gases, liquids, or solids. There are many types of thermostats and temperature-actuated switches. Limit Switch Limit switches designed to pass an electrical signal only when a predetermined limit is reached. The limit may be a specific position for a machine part or a piece of work, or a certain rotating speed. These devices take the place of a human operator to be present or to efficiently direct the machine. Limit switches are used most frequently as overtravel stops for machines, equipment, and products in process. These devices are used in the control circuits of magnetic starters to govern the starting, stopping, or reversal of electric motors. Electrical or Mechanical Interlock and Sequence Control Many electrical control devices described in this unit can be connected in an interlocking system so that the final operation of one or more motors depends upon the electrical position of each individual control device. For example, a float switch may call for more liquid but will not be satisfied until the prior approval of a pressure switch or time clock is obtained. To design, install, and maintain electrical controls in any electrical or mechanical interlocking system, the
  • 82. Technical English Through Reading 77 electrical technician must understand the total operational system and the function of the individual components. With practice, it is possible to transfer knowledge of circuits and descriptions for an understanding of additional similar controls. It is impossible — in instructional materials—to show all possible combinations of an interlocking control system. However, by understanding the basic functions of control components and their basic circuitry, and by taking the time to trace and draw circuit diagrams, difficult interlocking control systems can become easier to understand. 3.1.5 Starting and Stopping In starting and stopping a motor and its associated machinery, there are a number of conditions which may affect the motor. A few of them are discussed here. Frequency of Stating and Stopping The stating duty of a controller is an important factor in determining how satisfactorily the controller will perform in a particular application. Magnetic switches, such as motor starters, relays, and contactors, actually beat themselves apart from repeated opening and closing thousands of times. A experienced electrician soon learns to look for this type of component failure when trouble——shooting any inoperative control panels. NEMA standards require that the starter size be derated if the frequency of start-stop, jogging, or plugging is more than 5 times per minute. Therefore, when the frequency of starting the controller is great, the use of heavy duty controllers and accessories should be considered. For standard duty controllers, more frequent inspection and maintenance schedules should be followed. Light or Heavy Duty Starting Some motors may be started with no loads and other must be started with heavy loads. When motors are started, large feeder line disturbances may be created which can affect the electrical distribution system of the entire industrial plant. The disturbances may even affect the power company's system. As a result, the power companies and electrical inspection agencies place certain limitations on “across-the-line” motor starting. Fast or Heavy Starting To obtain the maximum twisting effort (torque)of the rotor of an ac motor, the best starting condition is apply full voltage to the motor terminals. The driven machinery, however, may be damaged by the sudden surge of motion. To prevent this type of damage to machines, equipment, and processed materials, some controllers are designed to start slowly and then increase the power speed gradually in definite steps. This type is often used by power companies and inspection agencies to avoid electrical line surges.
  • 83. 78 Part 3 Electrical Motor Control Smooth Starting Although reduced electrical and mechanical surges can be obtained with a step-by-step motor starting method, very smooth and gradual starting will require different controlling methods. These are discussed in detail later in the text. Manual or Automatic Starting and Stopping While the manual starting and stopping of machines by an operator is still a common practice many machines and industrial processes are started and restarted automatically. These automatic devices result in tremendous savings of time and materials. Automatic stopping devices are used in motor control systems for the same reasons. Automatic stopping devices greatly reduce the safety hazards of operating some types of machinery, both for the operator and the materials being processed. An electrically operated, mechanical brake may be required to stop a machine's motion in a heavy to protect materials being processed or people in the area. Quick Stop or Slow Stop Many motors are allowed to coast to a standstill. However, manufacturing requirements and safety considerations often make it necessary to bring machines to as rapid a stop as possible. Automatic controls can retard and brake the speed of a motor and also apply a torque in the opposite direction of rotation to bring about a rapid stop. This is referred to as “plugging”. Plugging can only be used if the driven machine and its load will not be damaged by the reversal of motor torque. The control of deceleration is one of the important functions of a motor controller. Accurate Stop An elevator must stop at precisely the right location so that it is aligned with the floor level. Such accurate stops are possible with the use of automatic devices interlocked with control systems. Frequency of Reversals Required Frequent reversals of the direction of the motor impose large demands on the controller and the electrical distribution system. Special motors and special starting and running protective devices may be required to meet the condition of frequent reversals. A heavy duty drum switch-controller is often used for this purpose. 3.1.6 Speed Control of Motors The speed control is connected not only with starting the motor but also with maintaining or controlling the motor speed while it is running. There are a number of conditions to be considered for speed control.
  • 84. Technical English Through Reading 79 Constant Speed constant speed motors are used on water pumps. Maintenance of constant speed is essential for motor generator sets under all load conditions. Constant speed motors with ratings as low as 80 rpm and horsepower ratings up to 5000 hp are used in direct drive units. The simplest method of changing speeds is by gearing. Using gears, almost any “pre-determined” speed may be developed by coupling the input gear to the shaft of a squirrel cage induction motor. Varying Speed A varying speed is usually preferred for cranes and hoists. In this type of application, the motor speed slows as the load increases and speeds up as the load decreases. Adjustable Speed With adjustable speed controls, an operator can gradually adjust the speed of motor over a wide range while the motor is running. The speed may be preset, but once it is adjusted it remains essentially constant at any load within the rating of the motor. Multispeed For multispeed motors, such as the type used on turret lathes in machine shop, the speed can be set at two or more definite rates. Once the motor is set at a definite speed, the speed will remain practically constant regardless of load changes. 3.1.7 Protective Features The particular application of each motor and control installation must be considered to determine what protective features are required to be installed and maintained. Overload Protection Running protection and overload protection refer to the same thing. This protection may be an integral part of the motor or be separate. A controller with electrical overload protection will protect a motor from burning up while allowing the motor achieve its maximum available power under a range of overload and temperature conditions. An electrical overload on the motor may be caused by mechanical overload on the driven machinery, a low line voltage, an open electrical line in a polyphase system resulting in single-phase operation, motor problem such as too badly worn bearings, loose terminal connections, or poor ventilation within the motor. Open Field Protection Dc shunt and compound-wound motors can be protected against the loss of field excitation by field loss relays. Other protective arrangements are used with starting equipment for dc and ac synchronous motors. Some sizes of dc motors may race dangerously with the loss of field excitation while other motors may not race due to friction and the fact that they are small.
  • 85. 80 Part 3 Electrical Motor Control Open-phase Protection Phase failure in a three-phase circuit may be caused by a blown fuse, an open connection, a broken line or other reasons. If phase failure occurs when the motor is at a standstill during attempts to start, the stator current will rise to a very high value and will remain there, but the motor will remain stationary (not turn). Since the windings are not properly ventilated while the motor is stationary, the heating produced by the high currents may damage them. Dangerous conditions also are possible while the motor is running. When the motor is running and an open-phase condition occurs, the motor may continue to run. The torque will decrease, possibly to the point of motor “stall”; this condition is called breakdown torque. Reversed Phase Protection If two phase of this supply of a three-phase induction motor are interchanged (phase reversal), the motor will reverse its direction of rotation. In elevator operation and industrial applications, this reversal results in serious damage. Phase failure and phase reversal relays are safety devices used to protect motors, machines, and personnel from the hazards of open-phase or reversed-phase conditions. Overtravel Protection Control devices are used in magnetic starter circuits to govern the starting, stopping, and reversal of electric motors. These devices can be used to control regular machine operation or they can be used as safety emergency switches to prevent the improper functioning machinery. Overspeed Protection Excessive motor speeds can damage a driven machine, materials in the industrial process, or the motor. Overspeed safety protection is provided in control equipment for paper and printing plants, still mills, processing plants, and the textile industry. Reversed Current Protection Accidental reversal of currents in direct-current controllers can have serious effects. Direct-current controllers used three- wire phase alternating-current systems which experience phase failures and phase reversals are also subject to damage. Reverse current protection is an important provision for battery charging and electroplating equipment. Mechanical Protection An enclosure may increase the life span and contribute to the trouble-free operation of a motor and controller. Enclosures with particular ratings such as general purpose, watertight, dustproof, explosionproof, and corrosion resistant are used for specific application. All enclosures must meet the requirements of national and local electrical codes and building codes.
  • 86. Technical English Through Reading 81 Short Circuit Protection For large motors with greater than fractional horsepower ratings, short circuit and ground fault protection generally is installed in the same enclosure as the motor-disconnecting means. Overcurrent devices (such as fuse and circuit breakers) are used to protect the motor branch circuit conductors, the motor control apparatus, and the motor itself against sustained overcurrent due to short circuit and grounds, and prolonged and excessive starting currents. 3.1.8 Classification of Automatic Motor Starting Control Systems The numerous types of automatic starting and control systems are grouped into the following classifications: current limiting acceleration and time delay acceleration. Current Limiting Acceleration This is also called Compensating Time. It refers to the amount of current or voltage drop required to open and close magnetic switches when used in a motor accelerating controller. The rise and fall of the current or voltage determines a timing period which is used mainly for dc motor control. Examples of types of current limiting acceleration are: ■ Counter emf or voltage drop acceleration ■ Lockout contactor or series relay acceleration Time Delay Acceleration For this classification, definite time relays are used to obtain a present timing period. Once the period is present, it does not vary regardless of current or voltage changes occurring during motor acceleration. The following timers and timing systems are used in interlocking circuits for automatic control systems. ■ Individual dashpot relays ■ Multicircuit dashpot relays ■ Pneumatic timing ■ Inductive time limit acceleration ■ Motor-driven timers ■ Capacitor timing ■ electronic timers 3.2 PUSH BUTTONS AND CONTROL STATIONS 3.2.1 Push Buttons A push-button station is a device that provides control of a motor through a motor starter by pressing a button which opens or closes contacts. It is possible to control a motor from as many
  • 87. 82 Part 3 Electrical Motor Control places as there are stations——through the same magnetic controller. This can be done by using more than one push-button station. A single circuit push-button station. Two sets momentary contacts are usually provided with push buttons so that when the button is pressed, one the set of contact is opened and the other set is closed. The use of this combination push button is simply illustrated in the wiring diagram of Figure 3-1. When the push button is in its normal position as shown in Figure 3-1(A), current flows from L1 through the normally closed contacts, through the red pilot light (on top) to L2 to form a complete circuit; this lights the lamp. Since the power push-button circuit is open by the normally open contact, the green pilot lamp does not glow. Note that this situation is reversed when the button is pushed, see Figure 3-1(B). Current now flows from L1 through the pushed closed contact and lights the green pilot lamp when completing the circuit to L2.The red lamp is now out of the circuit and does not glow. However, because the push buttons are momentary contact (spring loaded), we return to the position shown in Figure 3-1(A) when pressure on the button is released. Thus, by connecting to the proper set of contacts, either a normally open and or a normally closed situation is obtained. Normally open and normally closed mean that the contacts are in a rest position, held there by spring tension, and are not subject to either mechanical or electrical external forces. FIG.3-1 Combination push button: (A) Red pilot light is lit through normally closed push-button contact. (B) Green pilot light is lit when the momentary contact button is pushed. Push-button stations are made for two types of service: standard duty stations for normal applications safely passing coil currents of motor starters up to size 4, and heavy duty stations, when the push buttons are to be used frequently subjected to hard or rough usage. Heavy duty push- button stations have higher contact ratings. The push-button station enclosure containing the contacts is usually made of molded plastic or sheet metal. Some double-break contacts are made of copper. However, in most push-buttons, silver-to-silver contact surfaces are provided for better electrical conductivity and longer life. The push-button terminals in Figure 3-1 represent one-half of the terminals shown in Figure 3-2, for a double-pole, double-throw push button. Since control push buttons are subject to high momentary voltages caused by the inductive effect of the coils to which they are connected. Good clearance between the contacts and insulation to ground and operator is provided.
  • 88. Technical English Through Reading 83 The push-button station may be mounted adjacent to the controller or at a distance from it. The amount of current broken by a push button is usually small. As a result, operation of the controller is hardly affected by the length of the wires leading from the controller to a remote push-button station. FIG.3-2 Terminal configurations used by Push buttons can be used to control any different manufacturers. Both or all of the many operating conditions of a configurations represent the same push motor, such as start, stop, forward, reverse, button. fast, and slow. Push buttons also may be used as remote stop buttons with manual controllers equipped with potential trip or low-voltage protection. 3.2.2 Selector Switches Selector switches are usually “maintained” contact position, with three and sometimes two Selector positions. Selector switch position are made by turning the operator knob ─ not pushing it. These switches may also have a spring return to give momentary contact operation. Figure 3-3(A) shows a single-break contact selector switch connected to two lights. The red light may be selected to glow by turning the switch to the red position, Figure 3-4(B). Current now flows from line 1 through the selected red position of the switch, through the red lamp to line 2, completing the circuit. Note this switch has an off position in the center. There are two position selector switches available, with no off position. Here, whichever light is selected would burn continuously. Figure 3-4(A) is an elementary diagram of the heavy duty. Note that the red light will glow with the two-position switch in this position, the green indicating lamp is deenergized, and there is no “off” position. Figure 3-4(B) illustrates a three-position selector switch containing an “off” position. Both lamps may be turned off using this switch, but not in (A). 3.3 RELAYS AND CONTACTORS 3.3.1 Control Relays Control magnetic relays are used as auxiliary devices to switch control circuits and large motor starter and contactor coils, and to control small loads such as small motors, solenoids, electric heaters, pilot lights, audible signal devices and other relays. A magnetically held relay is operated by an electromagnet which opens or closes electrical
  • 89. 84 Part 3 Electrical Motor Control contactors when the electromagnet is deenergized. FIG.3-3 Elementary diagram using (A) FIG.3-4 Heavy duty, double selector A single-break, three-position selector switch for (A)Two position switch and (B) Two-position, single- and (B) Three position switches break switch Relays are generally used to enlarger or amplify the contact capability, or multiply the switching function of a pilot device by adding more contacts to circuit. Most relays are used in control circuits; therefore, their lower ratings (0-15 amperes maximum to 800 volts) show the reduced current levels at which they operate. Magnetic relays do not provide motor overload protection. This type of relay ordinarily is used in a two wire control system (any electrical contact-making device with two wires). Whenever it is designed to use momentary contact pilot devices, such as push buttons. any available normally open contact can be wired as a holding circuit in a three-wire system. Starters, contactors, and relays are similar in construction and operation but are not identical. Control relays are available in single- or double-throw arrangements with various combinations of normally open (NO) and normally closed (NC) contact circuits. While there are some single-break contacts used in industrial relays, most of the relays used in machine tool control have double-break contacts. It may be of particular interest to an electrician to know about changing contacts that are normally open to normally closed, or the other way around NC to NO. Most machine tool relays have some means to make this change. It ranges from simple flip-over contact to removing the contacts and relocating with spring location changes. Also, by overlapping contactors in this case, one contact can be arranged to operate at a different time relative to another contact on the same relay. For example, the normally open contact closes (makes) before the normally closed contact opens (breaks). Relays differ in voltage ratings, number of contacts, contact rearrangement, physical size, and in attachments to provide accessory functions such as mechanical latching and timing. In using a relay for a particular application, one of the first steps should be determine the control (coil)voltage at which the relay will operate The necessary contact rating must be made, as well as the number of contacts and other characteristics needed. Because of the variety of styles of relays available, it is possible to select the correct relay for almost any application. Relays are used more often to open and close control circuits than to operate power circuits.
  • 90. Technical English Through Reading 85 Typical applications include the control of motor starter and contactor coils, the switching of solenoids, and the control of other relays. A relay is a small but vital switching component of many complex control systems. Low-voltage relay systems are used extensively in switching residential and commercial lighting circuits and individual lighting fixtures. While control relays from various manufacturers differ in appearance and construction, they are interchangeable in control wiring system if their specifications are matched to the requirements of the system. Control relays are available in many shapes and configurations. There is dustproof, transparent enclosure of a control relay. The terminals plug in, like an electron tube. Another type of relay is the very small reed relay, with the contacts enclosed in glass. It is operated with a magnetic field. 3.3.2 Solid-state Relay In comparison to an electromagnetic relay, the solid-state relay has no coil or contacts and requires only minimum values of voltage and current to turn it on and off. The solid-state relay depends on electronic devices, such as transistors and silicon controlled rectifiers (SCR) for switching. 3.3.3 The Transistor as A Switch Figure 3-5 shows a basic solid-state switching device used in a logic component (relay). The transistor is the heart of the element. The base of the transistor controls the current flow between the emitter and the collector. In this type of the transistor, a negative voltage on the base allows emitter- base current to flow. This is due to the properties of the material at the junction of the emitter and the base. The emitter-base current causes the transistor to conduct a current flow from the emitter to the collector. A positive voltage on the base prevents emitter-base current from flowing, and the transistor stops FIG.3-5 Transistor solid-state conducting. Therefore, it behaves as a closed contact switching device in the first state and as an open contact in the second. For this reason, the action is called solid-state switching, that is, no moving contacts are required. There is only an electrical signal to open or close the circuit. As a result, the solid-state device is very reliable and has an exceptionally long life. Solid-state devices are not subject to arcing, wear, or deterioration, as are magnetic relays.
  • 91. 86 Part 3 Electrical Motor Control 3.3.4 Surge Protection When solid-state devices are used with magnetic switches, a voltage transient suppressor may be necessary to prevent some of the more harmful electrical “noise”. Much more sophisticated protection is required for microcomputers that control robots on assembly production lines. 3.3.5 Contactors Magnetic contactors are eletromagnetically operated switches that provide a safe and convenient means for connecting and interrupting branch circuits. The principal diference between a contactor and a motor starter is that the contactor does not contain overload relays. Contactors are used in combination with pilot control devices to switch lighting and heating loads and to control ac motors in those cases where overload protection is provided separately. The larger contactor sizes are used to provide remote control of relatively high-current circuits where it is too expensive to run the power leads to the remote controlling location, FIG.3-6.This flexibility is one of main advantages of electromagnetic control over manual control. Pilot devices such as push buttons, float switches, pressure switches, limit switches, and thermostats are provided to operate the contactors. FIG.3-6 An advantage of a remote control load Magnetic Blowout Heavy-duty contact arc-chutes are provided on most of larger contactors. The chutes contain heavy copper coils called blowout coils, mounted above the contacts in series with the load to provide better arc suppression. These magnetic blowout coils help to extinguish an electric arc at contacts opening under alternating current and direct-current loads. The arc may be similar in intensity as the electric arc welding process. An arc-quenching device is used to assure longer contact life. Since the hot arc is transferred from the contact tips very rapidly, the contacts remain cool and so they last longer. Contact and motor starter contacts that frequently break heavy currents are subject to a destructive burning effect if the arc is not quickly extinguished. The arc that is formed when the contacts open can be lengthened, and extinguished by motor action if it is in a magnetic field. This magnetic field is provided by the magnetic blowout coil. Since the coil of the magnet is usually in series with the line, the field strength and extinguishing action are in proportion to the
  • 92. Technical English Through Reading 87 size of the arc. Figure 3-7 is a sketch of a blowout magnet with a straight conductor (ab) located in the field and in series with the magnet. This Figure can represent either dc polarity or instantaneous ac. With ac current, the blowout coil magnetic field and conductor (arc) magnetic field will reverse simultaneously. According to Fleming’s left-hand rule, motor action will tend to force the conductor in an upward direction. The application of the right-hand rule for signal conductor shows that the magnetic field around the conductor aids the main field on the bottom and opposes it on the top, thus producing an upward force on the conductor. Figure 3-8 shows s section of Figure 3-7 with the wire (ab) replaced by a set of contacts. The contacts have started to open and there is an arc between them. Figure 3-9 shows what happens because of the magnetic action. Part A shows the beginning deflection of the arc because of the effect the motor action. Part B shows that the contacts are separated more then in A and the arc is beginning to climb up the horns because of the motor action and the effect of increased temperature. Part C of Figure 3-9 shows the arc near the tips of the horns. At this point, the arc is so lengthened that it will be extinguished. FIG.3-7 Illustration of the magnetic blowout FIG.3-8 Section of blowout magnet with principle. Straight conductor simulates arc. straight conductor replaced by a set contacts. An arc is conducting between the contacts. The function of the blowout magnet is to move the arc upward at the same time that the contacts arc opening. As a result, the arc is lengthened at a faster rate than will normally occur because of the opening of the contacts alone. It is evident that the shorter the time the arc is allowed to exist, the less damage it will do to the contacts. Most arc quenching action is based upon this principle. 3.3.6 AC Mechanically Held Contactors and Relays A mechanically held relay, or contactor, is operated by electromagnets but the electromagnets are automatically disconnected by contacts within the relay. Accordingly, these relays are mechanically held in position and no current flows through the operating coils of these
  • 93. 88 Part 3 Electrical Motor Control electromagnets after switching. It is apparent, therefore, that near continuous operation of multiple units of substantial size will lower the electrical energy requirements. Also, the magnetically held relay, in comparison, will change contact position upon loss of voltage to the electromagnet, whereas the mechanically held relay will respond only to the action of the control device. Sequence of Operation Referring to Figure 3-10, when the “off” push button is pressed momentarily, current flows from L1 through the “on” push button contact energizing the M coil through the now closed clearing contact, to L2. The relay now closes and latches mechanically. At the same time it closes M contacts (in Figure 3-11), lighting a bank of lamps when the circuit breaker is closed. To unlatch the relay, thereby turning the lamps off, the off button is pressed momentarily, unlatching the relay and opening the contacts M, turning off the lamps. Most operating coils are not designed for continuous duty. Therefore, they are disconnected automatically by contacts to prevent an accidental coil burnout. These coil clearing contacts change position alternately with a change in contactor latching position. FIG.3-9 Arc deflection between contacts FIG.3-10 Mechanically held relay control circuit Figure 3-12 shows a three-phase power load application using one main contactor to disconnect distribution panel. Selective single, or three-phase, branch circuits may be switched independently by other mechanically held contactors or relays. These mechanically held contactors and relays are electromechanical devices, Figure 3-13. They provide a safe and convenient means of switching circuits where quiet operation, energy efficiency, and continuity of circuit connection are requirements of the installation. For example, circuit continuity during power failures is often important in automatic processing equipment, where a sequence of operations must continue from the point of interruption after power is resumed——rather then return to beginning of the sequence. Quiet operation of contactors and relays is required in many control systems used in hospitals, schools, and office buildings. Mechanically held contactors and relays are generally used in locations where the slight hum, characteristic of alternating-current magnetic devices, is objectionable. In addition, mechanically held relays are often used in machine tool control circuits. These relays can be latched and unlatched through the operation of the limit switches, timing relays, starter interlocks, timeclocks, photoelectric cells, other control relays, or push buttons.
  • 94. Technical English Through Reading 89 Generally, mechanically held relays are available in 10- and 15-ampere sizes; mechanically held contactors are also available in sizes ranging from 30 amperes up to 1200 amperes. FIG.3-11 Load connections for a 115/ FIG.3-12 Mechanically held contactor 230-volt, three-phase load loads for three-phase power FIG.3-13 Two types latched-in or mechanically held relays in service. The upper coil is energized momentarily to close contacts, and the lower coil is energized momentarily to open the contact circuit. The momentary energizing of the coil is an energy-saving feature. (Courtesy Square D Co.) 3.3.7 Thermostat Relay Thermostat-type are used with three-wire, gauge-type thermostat controls or other pilot controls having a slowly moving element which makes a contact for both the closed and open positions of the relay. The contacts of the thermostat control devices usually cannot handle the current to a starter coil; therefore, a thermostat relay must be used between the thermostat control and the starter, Figure 3-14. When the moving element of the thermostat control touches the closed contact, the relay closes and is held in this position by a containing contact. When the moving element touches the
  • 95. 90 Part 3 Electrical Motor Control open contact, the current flow bypasses the operating coil through a small resistor and causes the relay open. The resistor is usually built into the relay and serves to prevent a short circuit. FIG.3-14 Starter coil(M) is controlled by thermostat relay The thermostat contacts must not overlap or be adjusted too closely to one another as this may result in the resistance unit being burned out. It is also advisable to compare the inrush current of relay with the current rating of the thermostat. 3.4 TIMING RELAYS 3.4.1 Introduction A timing relay is similar to a control relay, except that certain of its contacts are designed to operate at a present time interval, or time lag, after the coil is energized, or de-energized. Many industrial control applications require timing relays that can provide dependable service and are easily adjustable over the timing ranges. The proper election of timing relays for a particular application can be made after a study of the service requirements and with the knowledge of the operating characteristics inherent in each available device. A number of timing devices are manufactured with features suitable for a wide variety of applications. 3.4.2 Fluid Dashpot Timing Relays Magnetically operated, oil dashpot timing relay may be used on voltages up to 800 volts ac or dc. The contacts are operated by the movement of the iron core. The magnetic field of a solenoid coil lefts the iron core against retarding force of a piston moving in an oil-filled dashpot. This type of relay is not very accurate. The piston must be allowed to settle back down to the bottom of the dashpot between successive timing periods. If the piston is not allowed to make a full return, the timing is erratic. The dashpot timing relay provides time delay after the magnet is energized. The contact may be normally open or normally closed for different applications. Unlike the magnetic overload relay, the dashpot timing relay operates with a potential coil connected across the line through contacts or switches. The overload relay operates with a current coil that is affected by the motor current load.
  • 96. Technical English Through Reading 91 Fluid dashpot timing relays are used for a number of applications: ■ to control accelerating contactors of motor starters ■ to time the closing or opening of valves on refrigeration equipment ■ for any application where the operating sequence a delay. It is necessary that the elapsed time of the delay be extremely accurate. These relays use a silicone dashpot fluid which is not an oil. The fluid helps to eliminate the effect of varying viscosity on the timing due to changes in ambient temperature. The silicone fluid operates successfully in an ambient temperature range of +48.9℃ to —34.4℃. The timing range can be adjusted easily from two seconds to 30 seconds. Multicontact dashpot timing relays are used for dc motor starting. When the coil is energized on this type of timing relay, the contacts close in succession with a time lag between each closing. 3.4.3 Pneumatic Timers The construction and performance features of the pneumatic (air) timer make it suitable for the majority of industrial applications. Pneumatic timers have the following characteristics: ■ unaffected by normal variations in ambient temperature or atmospheric pressure ■ adjustable over a wide range of timing periods ■ good repeat accuracy ■ available with a variety of contact and timing arrangements. This type of relay has a pneumatic time-delay unit that is mechanically operated by a magnet structure. The time-delay function depends upon the transfer of air through a restricted orifice by the use of reinforced synthetic rubber bellows or diaphragm. The timing range is adjusted by positioning a needle valve to vary amount of orifice or vent restriction. The process of energizing or deenergizing pneumatic timing relays can be controlled by pilot devices such as push buttons, limit switches, or thermostatic relays. Since the power drawn by a timing relay coil is small, sensitive control devices may be used to control the operating sequence. Pneumatic timing relays are used for motor acceleration and in automatic control circuits. Automatic control is necessary in applications where repetitive accuracy is required, such as controls for machine tools and control of sequence operations, industrial process operation, and conveyor lines. 3.4.4 Magnetic Time Limit Relay If current is increasing in a coil, then the emf due to self-induction acts in a direction to oppose the increase of current in that coil. If a current is decreasing in a coil, the emf due to self-induction in the coil acts in a direction to oppose the decrease of current in the coil. One type of magnetic time limit relay has a single coil wound on a hollow copper cylinder
  • 97. 92 Part 3 Electrical Motor Control containing an iron core. Other time limit relays have copper -jacketed coils. For either type of relay, time delay is provided when the relay drops out. When the electric circuit in the coil is disconnected, the current quickly falls to zero. Therefore, as the flux in the coil decreases, it cuts the short-circuited copper cylinder and induces a voltage in the cylinder. This induced voltage sends a current through the copper cylinder. As a result of this current, a flux is produced that holds the magnet armature up for a period of time after the coil circuit is broken. The delay time is limited by the number of turns on the coil and the amount of iron in the circuit. Figure3-15 is a sketch of a magnetic time limit relay. When the relay is energized, the armature (M) is drawn against the core (N). At the same time, the tension of the spring tends to draw the armature away from the core. As the flux maintained by the current in the copper sleeve dies away, the time at which the armature is released depends to some degree on the tension of the spring. The release time also can be varied by inserting a bronze shim in the gap between M and N, the thicker the shim, FIG.3-15 Dc magnetic time limit relay the lower the flux, and the sooner the armature is released. The magnetic time limit relay is used to short out resistance steps in the start-up of dc motors. With this type of relay armature, pickup is instantaneous. The time delay at dropout is obtained by the use of a nonmagnetic armature shim and the adjustment spring. 3.4.5 Capacitor Time limit Relay Assume that a capacitor is charged by connecting it momentarily across a dc line and then the capacitor dc is discharged through a relay coil. The current induced in the coil will decay slowly, depending on the relative values of capacitance, inductance, and resistance in the discharge circuit. If a relay coil and a capacitor are connected in parallel to a dc line, the capacitor is charged to the value of the line voltage and a current appears in the coil. If the coil and capacitor combination is now removed from the line, the current in the coil will start to decrease along the curve shown in Figure 3-16. If the relay is adjusted so that the armature is released at current i1, a time delay of t1 is obtained. The time delay can be increased to a value of t 2 by adjusting the relay so that the armature will not be released until the current is reduced to a value of i2. A potentiometer is used as an adjustable resistor to vary the time. This resistance-capacitance (RC) theory is used in industrial electronic and solid-state controls also. This timer is highly accurate and is used in motor acceleration control and in many industrial
  • 98. Technical English Through Reading 93 processes. FIG.3-16 Charged capacitor discharging through a relay coil. The graph at the right illustrates the current decrease in the coil 3.4.6 Electronic Timers Electronic timers use solid-state components to provide the desired time delays. A elementary diagram for this timer is shown in Figure 3-17. The timer has a light-emitting diode (LED) that is off when the timing relay is deenergized, flashes while timing, and is on while energized. The unit fits standard industrial control relay mounting. 3.4.7 Selecting A Timing Relay In selecting a timing relay for a specific application, the following factors should be carefully considered. ■ Length of time delay required ■ Timing range required ■ Allowable error ■ Cycle or operation and reset time ■ Cost FIG.3-17 Typical elementary diagram ■ Additional requirements for a solid-state timer Length of Time Delay Required The length of time delay required is determined by the type of machine or process that the timer will control. The time delay will range from a fraction a second to as several minutes. Timing Range Required The phrase timing range means the various time intervals over which the timer can be adjusted. Timers are available which can be set for a time delay of 1 second, and 100 seconds, or any value of delay between 1 and 100 seconds. When selecting a timer for use a machine or process, the range should be wide enough to handle the various time-delay periods that may be
  • 99. 94 Part 3 Electrical Motor Control required by the machine or process. The exact timing value for any position within the timing the range must be found by trial and error. A scale provided with a timer is intended primarily to permit a quick reset of the timer to the timing position previously determined to be correct for a given operation. Allowable Error All timers are subject to some error, that is, there may be a plus or minus time variation between successive timing operations for the same setting. The amount of error varies with the type of timer and the operating conditions. The error is usually stated as some percentage of the time setting. The percentage of error for any timer depends on the type of timer, the ambient temperature (especially low temperature), coil temperature, line voltage, and the length of time between operations. Cycle of Operation Required and Reset Time For one type of timer, the timer becomes operative when a electrical circuit opens or closes. A Time delay then occurs before the application process begins. As soon as the particular process action is complete, the timer circuit resets itself. The circuit must be energized or deenergized each time the timing action is desired. A Second type of timer is called a process timer. When connected into a circuit, the time provides control for a sequence of events, one after another. The cycle is repeated continuously until the circuit is deenergized. An important consideration in the selection of a timer is the speed at which the timer resets. Reset time is the time required for the relay mechanism to return to its original position. Some industrial processes require that the relay reset instantaneously. Other processes require a slow reset time. The reset time varies with the type of timing relay and the length of the time delay. Cost When there are several electromagnetic timers that meet the requirements of a given application. It is advisable to select the timer with the smallest number of operating parts. In other words, select the simplest timer. In most cases, this timer will probably be the lowest in cost. 3.5 TWO-WIRE CONTROLS A two-wire control may be toggle switch, pressure switch, float switch, limit switch, thermostat, or any other type of switch having definite on and off positions. As indicated, devices of this type generally are designed to handle small currents. Two-wire control devices will not carry sufficient current to operate large motors. In addition, 230-volt motors and three-phase motors require more contacts than one contact usually provided on two-wire devices. Two-wire controls may be connected to operating coils of magnetic switches, as shown in
  • 100. Technical English Through Reading 95 Figure 3-18. When the switch is closed, the control circuit is completed through the coil (M). When the coil is energized, it closes the contacts at M and runs the motor. When the switch is opened, the coil is deenergized and the contacts open to stop the motor. In the case of an overload contact in the thermal heaters open the overload contacts in the control circuit and deenergized the coil, thus stopping the motor. Two-wire control provides no voltage (or law voltage) release. When the start is wired. as shown in Figure 3-18, it will operate automatically in FIG.3-18(A) Basic two-wire control response to the control devices. A human circuit——elementary diagram operator is not required. the control maintaining contact 2-3(show in the wiring diagram) is furnished with the starter. However, this contact is not used in two-wire control. For simplicity, this contact is omitted from two-wire elementary diagram. The motor starter in Figure 3-18 is a line voltage, or across-the-line, starter. FIG.3-18(B) Basic two-wire control circuit——wiring diagram 3.6 THREE-WIRE AND SEPARATE CONTROLS 3.6.1 Three-Wire Controls A three-wire control circuit uses momentary contact, start-stop stations and holding circuit interlock connected in parallel with the start button, to maintain the circuit. In general, three-wire devices are connected, as shown in Figure 3-19. Although the arrangement of the various parts may vary from one manufacturer’s switch to another, the basic circuit remains the same.
  • 101. 96 Part 3 Electrical Motor Control FIG.3-19 Basic three-wire control circuit The sequence of operation for this circuit is follows: when the start button is pushed, the circuit is completed through the coil (shown as M) and the contacts at M close. The power circuit contacts to the motor also close (not shown). When the start button is released, the holding contact at M keeps this auxiliary contact on. When the starter is used in this manner, it is said to be “maintaining” or “sealing”. With the holding contact closed, the circuit is still complete through the coil. If the stop button is pushed, the circuit is broken, the coil loses its energy, and the contacts at M open. When the stop button is released, the circuit remains open because both the holding contact and the start button must be pushed again to complete the circuit. The operation of the overload protection opens the control circuit, resulting in the same effect. If the supply voltage fails, the circuit is deenergized. When the supply voltage returns, the circuit remains open until the start button is pushed again. This arrangement is called no-voltage protection and protects both operator and equipment. The push button station wiring diagram (B of Figure 3-19) represent the physical station. It shows the relative positions of the units, the internal wiring, and the connections to the starter. The wires to the terminals are labeled 1,2, and 3(giving rise to the name “three-wire control”). Normally closed auxiliary contacts are used to switch the pilot light on and off. When the motor is not running, these contacts are open; when motor is stopped, the contacts are closed and the pilot light is on. A pilot light may be installed to indicate when the motor is running. For this case, the pilot light is connected between control terminal 3 and line 2. Except for this modification, the circuit is a basic three-wire, push-button control circuit. 3.6.2 Push-to-test Pilot Light It is necessary to restart a motor after it has been stopped by a three-wire control circuit with low voltage protection. An indicating lamp often is to signal this when a motor stops so that it can be restarted after the problem causing the stoppage is cleared. Because pilot lights are an important component in such cases, they are tested frequently to insure operation. Push-to-test pilot lights show immediately if a circuit is off or if the lamp is burned out. Part of Figure 3-20 shows the schematic wiring for such a circuit. The three-wire motor starter control circuit is wired as usual. Note that the pilot lamp is energized from terminal 3 down to C, through a normally closed push button to L2. To test, the lamp is pushed opening the circuit at C and closing it across L1. With a push-button arrangement, the lamp is tested directly across lines 1
  • 102. Technical English Through Reading 97 and 2. Fig.3-20 Push-to-test push button (Courtesy Square D Co.) 3.6.3 Alarm Silencing Circuit Sirens, horns, and loud buzzers are also used in production systems to call attention to malfunctions. The problem is acknowledged by attempts to silence this “noise pollution”. A typical circuit is shown in Figure 3-21. Assume a high pressure in an industrial system is dangerous to continue. Such a condition will close a pressure switch. When this switch closes, the alarm is sounded through normally closed contact S. In addition, the red indicating lamplight. When alerted, maintenance personnel can silence the alarm by depressing the “off” push button. The red light continues to silently announce the problem until it is cleared. After the pressure switch opens, the alarm system can be reactivated by pressing the on button. There are virtually unlimited control circuits using the three-wire system. FIG.3-21 Alarm silencing circuit 3.6.4 Separate Control It is sometimes desirable to operate push buttons or other control devices at some voltage lower than the motor voltage. In the control system for such a case, a separate source-such as an isolating transformer or an independent voltage supply─provides the power to the control circuit. This independent voltage is separate from the main power supply for the motor. One
  • 103. 98 Part 3 Electrical Motor Control form of separate control is shown in Figure 3-22. This is an elementary diagram of cooling circuit for a commercial air-conditioning installation. When the thermostat calls for cooling, the compressor motor starter coil (shown as M) is energized through the step-down isolating transformer. When coil M is energized, power contacts in the 240-volt circuit close to start the refrigeration compressor motor. Since the control circuit is separated from power circuit by the isolating control transformer, there is no electrical connection between the two circuits. For this reason, the wire jumper attached to L2 on a starter should be removed for different voltages. However, the overload relay control contact must be included in the separate control wiring. The maintenance technician must also insure that control transformer voltages match the voltages used and that the proper connections are made. FIG.3-22 Separate control used in conditioning cooling circuit
  • 104. Technical English Through Reading 99 PART 4 PRINCIPLES OF MEASUREMENT SYSTEM 4.1 THE FUNDAMENTALS OF TECHNICAL MEASUREMENT 4.1.1 The general measurement system The purpose of a measurement system is to present an observer with a numerical value corresponding to the variable being measured. In general this numerical value or measured value does not equal the true value of the variable. Thus the measured value of the flow rate down a pipe as presented on a pointer-scale indicator may be 11.0 m3/hr, whereas the true flow may be 11.2 m3/hr. The measured speed of an engine as indicated on a digital display may be 3140 r.p.m, whereas the true speed may be 3133 r.p.m. The problems involved in trying to establish the true value of a variable will be discussed in the next chapter. For the present, it is sufficient to realize that the input to the measurement system is the true value of the variable and the output is the measured value (see Fig.4-1). FIG.4-1 Purpose of measurement system The measurement system consists of several elements or blocks. It is possible to identify four types of elements, although in a given system one type of element may be missing or may occur more than once. The four types are shown in Fig.4-2 and can be defined as shown in Fig.4-2. FIG.4-2 General structure of measurement system Sensing Element This is in contact with the process and gives an output which depends in some way on the
  • 105. 100 Part 4 Principles of Measurement System variable to be measured. Examples are thermocouple where millivolt e.m.f. depends on temperature; strain gauge where resistance depends on mechanical strain; orifice plate where pressure drop depends on flow rate. If there is more than one sensing element in a system, the element in contact with the process is termed the primary sensing element, the others secondary sensing elements. Signal Conditioning Element This takes the output of the sensing element and converts it into a form more suitable for further processing, usually a d.c. voltage, d.c. current or frequency signal. Examples are: deflection bridge which converts an impedance change into a voltage change; amplifier which amplifies millivolts to volts; oscillator which converts an impedance change into a variable frequency voltage. Signal Processing Element This takes the output of the conditioning element and converts it into a form more suitable for presentation. Examples are analogue to digital converter which converts a voltage into a digital form for input to a computer; a microcomputer which calculates the measured value of the variable from the incoming digital data. Typical calculations are the computation of total mass of product gas from flow rate and density data, the integration of chromatograph peaks to give the composition of a gas stream, correction for sensing element nonlinearity. Data Presentation Element This presents the measured value in a form which can be easily recognized by the observer. Examples are a simple pointer-scale indicator, chart recorder, alphanumeric display, and visual display unit. Figure 4-3 shows a system for weight measurement which incorporates all the elements mentioned above. FIG.4-3 Weight measurement system The word ‘transducer’ is commonly used in connection with measurement and instrumentation. This is a manufactured package which gives an output voltage (usually) corresponding to an input variable such as pressure or acceleration. We see therefore that such a transducer may incorporate both sensing and signal conditioning elements; for example a weight
  • 106. Technical English Through Reading 101 transducer would incorporate the first four elements shown in Fig.4-3. It is also important to note that each element in the measurement system may itself be a system made up of simpler components. A block diagram approach is very useful in discussing the properties of elements and systems. 4.1.2 Calibration Every measuring system must be provable, that is, it must prove its ability to measure reliably. The procedure for this is called calibration. It consists of determining the system’s scale. At some point during the preparation of the system for measurement, known magnitudes of the basic input quantity must be fed into the detector-transducer, and the system’s behavior must be observed. If the system has been proved linear, perhaps single-point calibration will suffice, wherein the effect of only a single value of the input is used. If the system is not linear, or if it has not been so proved, a number of values must be used and there results observed. The input may be static or dynamic, depending on the application; however, quite often dynamic response must be based on static calibration, simply because a practical dynamic source cannot be had. Naturally this is not optimum procedure; the more nearly the calibration standard corresponds to the unknown in all of its characteristics, the better the situation. Occasionally the nature of the system or one of its components makes the introduction of sample of the basic input quantity difficult or impossible. One of the important characteristics of the bonded resistance-type strain gage factor applied by the manufacturer. Instead of attempting to apply a known unit strain to gage installed on the test structure, which if possible would often result in an ambiguous situation, a resistance change is substituted. Through the predetermined gage factor, the system's strain response may thereby be obtained. 4.1.3 Types of Input Quantities Time Relationship Mechanical quantities, in addition to their inherent defining characteristics, also have distinctive time-amplitude properties, which may be classified as follows: 1. Static 2. Dynamic (1) Steady-state periodic (2) Nonrepetitive or transient A. Single pulse or aperiodic B. Continuing or random Of course, the static, nonchanging measurand is the most easily measured. If the system is terminated by some form of meter-type indicator, the meter pointer has no difficulty in
  • 107. 102 Part 4 Principles of Measurement System eventually reaching a definite indication. It is the rapidly changing measurand that presents the real measurement challenge. There are two general forms of dynamic input: steady-state periodic and transient. The steady-state periodic quantity is one whose magnitude has a definite repeating time cycle, whereas the time variation of a transient magnitude does not repeat. “Sixty-cycle” line voltage is an example of steady-state periodic signal. So also are many mechanical vibrations, after a balance has been reached between a constant input exiting energy and energy dissipated by damping. An example of pulsed transient quantity is the acceleration-time relationship accompanying an isolated mechanical impact. Occasionally, the magnitude is temporary, being completed in a matter of milliseconds, with the portions of interest existing perhaps for only a few microseconds. Extremely high rates of change, or wavefronts, exist, placing severe demands on the measuring system. Analog or Digital Most measurands of interest vary with time in an analog fashion. That is, they vary in a continuous manner over a range of magnitude. For instance, the speed of automobile, as it starts from rest, has same magnitude at every instant during its motion. no matter how finely the time intervals between measurements are taken. The speed varies in an analog manner as a function of time. The voltage in utility power lines varies sinusoidally with time. This is an analog quantity also. Certain quantities, however, may vary digitally, changing in a stepwise manner between two distinct magnitudes: a high and a low voltage, for instance. The revolutions of a shaft could be counted with a cam-actuated electrical switch that is closed or open, depending on the position of the cam. If the switch controls current from a battery, current either flows with a given magnitude or does not flow. The current flow would behave digitally. Analog-originating mechanical quantities, such as temperatures, fluid flow, stress and strain, and pressure, normally all behave timewise in an analog manner. There may be distinct advantages, however, in converting an analog-type input to an equivalent digital signal for the purposes of signal conditioning and/or read-out. Noise problems are reduced or sometimes eliminated altogether, and data transmission is simpler. Most computers are designed to process digital information and direct numerical display or recording is more easily accomplished by manipulating digital quantities. 4.1.4 Standards, Dimensions, and Units of Measurement The term dimension connotes the defining characteristics of an entity, and a unit is a basis for quantification of the entity. For example, length is a dimension whereas centimeter is a dimension and the second is a unit of time. A dimension is unique, however, a particular dimension, say length, may be measured in feet, meters, inches, miles, etc.
  • 108. Technical English Through Reading 103 Newton’s second law may be expressed in various ways, one of which is An article acted upon by an external force will be accelerated in proportion to the force magnitude and inverse proportion to the mass of the particle; the direction of the acceleration will coincide with the line-of-action of the force. Algebraically, F=ma (4-1) where F=the magnitude of the applied force, m=the mass of particle, and a=the resulting acceleration. From experiment we know that near the earth's surface a particle (body) acted on solely by gravitational attraction accelerates at the rate of about 32.2ft/s2 (9.81 m/s2). In this situation the acting force is weight, which may be expressed in pounds-force (lbf), dynes, etc., depending on the particular system of units that is used, and magnitude of mass may be expressed variously as slugs, pounds-mass (ibm), kilograms, etc. In any case whichever system is used, a consistent, compatible balance of units must be maintained. The inertial law (Newton’s) is of particular interest in this regard because it demands a careful distinction between the units of force and mass. In the United States it has long been the habit to use the abbreviation “lb” as the unit for mass and force except when a distinction is absolutely required. Then the abbreviation “lbm” and “lbf” are used. The movement toward the use the metric system in the United States with promotion of the SI system of units should help to correct this confusion. 4.1.5 Certainty/Uncertainty: Validity of Results Error may by defined as the difference between the measured result and the true value. We do not know that true value, hence we do not know the error, We can discuss an error and can estimate (guess) an error, but we can never know its actual magnitude. If we wish to assign a value to our estimate of error, then we commonly refer to that number as uncertainty. Uncertainty, then, is our best estimate of error. There are two basic types of error (remember we can discuss it without ever knowing its magnitude); systematic error and random error. Should an unscrupulous butcher place a ball of putty under the scale pan, the scale read-outs would be consistently in error. The scale would indicate a weigh of product too great by the weight of the putty. This represents one type of systematic error, zero offset. Shrink rules are used to make patterns for the casting of metals. Cast steel shrinks in cooling by about two percent, hence the patterns used for preparing the molds are oversized by the proper percentage amounts. The pattern maker uses a shrink rule on which the dimensional units are increased by that amount. Should a pattern maker's shrink rule for cast steel be inadvertently used for ordinary length measurements, the read-outs would be consistently undersized by 1/50th in one. This is an example of scale error. In each of the foregoing examples the errors are constant and of a systematic nature. Such errors are not susceptible to statistical analysis; however, they may be estimate.
  • 109. 104 Part 4 Principles of Measurement System An inexpensive frequency counter may use the 60Hz power-line frequency as a comparison standard. Power-line frequency is held very close to the 60Hz standard. Although it does slowly wander above and below the average value, over a period of time, say a day, the average is very close to 60Hz. The wandering is random and the moment-to-moment error in the frequency meter read-out (from this source) is called random error. Randomness may also be introduced by lack of definition of the measurand. If a number of hardness readings are made on a given sample of steel, a range of readings will be obtained. An average hardness may be calculated and presented as the actual hardness. Single readings will deviate from the average, some higher and lower. Of course, a primary reason for the variation in this case is the nonhomogeneity of the crystalline structure of the test specimen. The deviations will be random and are due to a lack of definition on the measurand. Random uncertainty (error) is susceptible to statistical treatment. 4.2 MEASURING SYSTEM RESPONSE 4.2.1 Introduction Quite simply, response is a measure of system’s fidelity to purpose. It may be defined as an evaluation of system’s ability to faithfully sense, transmit, and present all the pertinent information included in the measurand and to exclude all else. We would like to know if the output information truly represents the input. If the input information is in the form of a sine wave, a square wave, or a sawtoothed wave, does the output appear as a sine wave, a square wave, or a sawtoothed wave, as the case may be? Is each of the harmonic components in a complex wave treated equally, or are some attenuated, completely ignored, or perhaps shifted timewise relative to the others? These questions are answered by the response, characteristics of the particular system, that is (a) amplitude response, (b) frequency response, and (d) slew rate. What basic physical factors govern response? In terms of practical system formation we are confronted with two fundamental segments of construction: mechanical and electrical. The basic mechanical elements are mass, some form of equilibrium-restoring element, and damping. Corresponding electrical elements are resistance, inductance, and capacitance. 4.2.2 Amplitude Response Amplitude response is governed by the system’s ability to treat all input amplitude uniformly. If an input of 5 units is fed into a system and an output of 25 indicator divisions is obtained, we can generally expect that an input of 10 units will result in an output of 50 divisions. Although this is the most common case, there are other special nonlinear responses that are occasionally required. Whatever the arrangement, whether it be linear, exponential, or some other amplitude
  • 110. Technical English Through Reading 105 function, discrepancy between design expectations in this respect and actual performance results in poor amplitude response. Of course no system exists that is capable of faithfully responding over an unlimited range of amplitudes. All systems can be overdriven. Figure 4-4 shows the amplitude response of a voltage amplifier suitable for connecting a strain-gage bridge to an oscilloscope. The usable range of the amplifier is restricted to the horizontal portion of the curve. The plot shows that for input above 0.01 volts the amplifier becomes overloaded and the amplification ceases to be linear. 4.2.3 Frequency Response Good frequency response is obtained when system treats all frequency components with equal faithfulness. If a 100 Hz sine wave with an input amplitude of 5 units is fed into a system, and peak-to-peak output of 2.5 in. result on an oscilloscope screen, we can expect that a 500 Hz sine wave input of the same amplitude would also result in a 2.5 in. peak-to-peak output. Changing the frequency of the input signal should not alter the system’s output magnitude so long as the input amplitude remains unchanged. Yet here again there must be a limit to the range over which good frequency response may be expected. This is true for any dynamic system, regardless of its quality. Figure 4-5 illustrates the frequency-response relations for the same voltage amplifier used in Fig.4-4. Frequencies above about 10 kHz are attenuated and the input below this limit only is amplified in the correct relative proportion. FIG.4-4 Gain vs. input voltage for FIG.4-5 Frequency response curve for amplifier section of commercially amplifier section of a commercially available strain measuring system. available strain measuring system. (For frequency = 1kHz.) ei=10mV. 4.2.4 Phase Response
  • 111. 106 Part 4 Principles of Measurement System Amplitude and frequency responses are important for all types of input waveforms, simple or complex. Phase response, however, is of importance primarily for the complex wave only. Time is required for the transmission of signal through any measuring system. Often when a simple sine-wave voltage is amplified by a single stage of amplification, the output trails the input by approximately 180 degrees, or one-half cycle. For two stages, the shift may be about 360 degrees, and so on. Actually the shift will not be exact multiples of half wavelengths but will depend on the equipment and also on the frequency. It is the frequency-dependent in determining phase response. For the single sine-wave input, any shift would normally be unimportant. The output produced on the oscilloscope screen could show true waveform, and the proper parameters could be determined. The fact that the shape being shown was actually formed a few microseconds or a few milliseconds after being generated is of no consequence. Let us consider, however, the complex wave made up of numerous harmonics, Suppose that each component is delayed by a different amount. The harmonic components would then emerge from the system on phase relations different from when they entered. The whole waveform and its amplitudes would be changed, a result of poor phase response. Figure 4-6 illustrates typical phase-response characteristics for a voltage amplifier. Fig.4-6 Phase lag vs. frequency for the same amplifier used for FIG.4-4 and 4-5 4.2.5 Predicting Performance for Complex Waveforms Response characteristics of an existing system or a component of a system may be determined experimentally by injecting as input a signal of known form, then determining the output, and finally comparing the results. Of course, the most basic waveform is the sine wave. If we know the sine-wave response of a device, can we use this information to predict how it will respond to a complex input, such as a square wave or one of the various sawtooth waveforms? The answer is yes. It should be clear that we could make calculations for any waveform for which a harmonic series can be written. For example, we can apply a Fourier analysis to almost any random
  • 112. Technical English Through Reading 107 waveform and use the result to investigate the effects of various response characteristics. 4.2.6 Delay, Rise Time, and Slew Rate Finally, a fourth type of response, is delay, or rise time. When a stepped or relatively instantaneous input is applied to a system, the output may lag as shown in Fig.4-7.The time delay,Δt, after the step is applied but before proper output magnitude is reached is known as delay, or rise time. It is a measure of the system’s ability to handle transients. Slew rate is the maximum rate of change that the system can handle. In electrical terms, it is de/dt, or volts per unit of time (e. g., 25 volts/microsecond). The term slew rate or slew speed is also used in other ways; for example, the slew speed of the stylus of an x-y plotter would be expressed in terms of cm/s, a reference to the maximum speed with which the stylus can traverse its range of motion. FIG.4-7 Response of a typical system to a pulse-type input;Δt is the rise-time 4.3 SENSING ELEMENT In Section 4.1.1 we saw that, in general, a measurement system consists of four types of elements: sensing, signal conditioning, signal processing, and data presentation elements. The sensing element is the first element in the measurement system; it is contact with, and draws energy from, the process or system being measured. The input to this element is the true value of the measured variable; the output of the element depends on this value. The element classified according to whether the output signal is electrical or mechanical. Elements with an electrical output are further divided into passive and active. Passive devices such as resistive, capacitive and inductive elements require an external power supply in order to give a voltage or current output signal; active devices, e.g. electromagnetic elements, need no external power supply. Resistive element can sense temperature, heat flux, flow velocity, displacement, strain and gas composition. Elastic elements can sense force, pressure, torque, level and density. Sensors with a mechanical output are commonly used as the primary sensing element in measurement
  • 113. 108 Part 4 Principles of Measurement System systems for mechanical variables such as force or flow rate. In order to obtain an electrical signal, this primary element is followed by secondary sensing element with an electrical output signal. Examples are a resistive strain gauge sensing the strain in an elastic cantilever in a force measurement system, and an electromagnetic tachogenerator sensing the angular velocity of a turbine in a flow measurement system. 4.3.1 Resistive Sensing Elements Potentiometers for Linear Displacement Measurement The potentiometric displacement sensor consists of cylindrical stator with either a wire-wound track or film of conductive plastic deposited on it. The resistance per unit length is constant so that the ratio output voltage to supply voltage is proportional to the fractional displacement x of the slider. The resolution error of a wire-wound potentiometer is 100/n percent, where n is the number of turns, and is thus determined by the diameter of the wire. A typical family of wire-wound potentiometers covers displacement spans from 0.5 to 100 inches, with non-linearity ±0.2 percent, resolution from 0.008 per cent and resistance values of 1kΩ /inch. Conducting plastic film elements have zero resolution error but have higher temperature coefficients of resistance. A family of conductive plastic potentiometers covers displacement spans from 25 to 250 mm, with non-linearity up to±0.04 percent and resistance values from 500 Ω to 800 kΩ. The most modern development is the hybrid track potentiometer which is manufactured by depositing a conductive plastic coating on a precision wire-wound resistance track and incorporates the best features of wire-wound and film types. The resistance of any load (recorder or indicator) used with the potentiometer must be several times greater than the potentiometer resistance otherwise non-linear effects occur. Resistance Thermometers and Thermistors for Temperature Measurement The resistance of most metals increases reasonably linearly with temperature in the range -100 to +800°C. The general relationship between the resistance RT Ω of a metal element and temperature T ℃ is a power series of the form RT=R0(1+αT+βT2+γT3+...) (4-1) where R0 Ω is the resistance at 0℃ and α, β, γ are temperature coefficients of resistance The magnitude of the non-linear terms is usually small. Figure 4-8 (a) shows the variation in the ratio RT /R0 with temperature for the metals platinum, copper and nickel. Although relatively expensive, platinum is usually chosen for industrial resistance thermometers, cheaper metals, notably nickel and copper, are used for less demanding applications. Platinum is preferred because it is chemically inert, has linear and repeatable resistance-temperature characteristics, can be used over a wide temperature range (-200 to +800 ℃) and in many types of environments. It can be refined to a high degree of purity which ensures that statistical variations in resistance, between similar elements at the same temperature, are small. The change in resistance between the ice point and the steam point is called the fundamental interval,
  • 114. Technical English Through Reading 109 in the above element this is 38.5 Ω. The maximum non-linearity as a percentage of full-scale deflection (f.s.d.), between 0 and 200 ℃, is +0.76 percent. The British Standard BS 1904 lays down tolerance limits on the maximum variation in resistance between platinum elements at a given temperature. For Grade 1 elements the tolerance limits are ±0.075 Ω at 0 ℃,±0.13 Ω at 200 ℃,and for Grade 2 elements the tolerance limits are ±0.1 Ω at 0 ℃, ±0.35 Ω at 200 ℃. The amount of electrical power dissipated in the element should be limited in order to avoid self-heating effects; in a typical element 10mW of power causes a temperature rise of 0.3 ℃. Another potential source of error is temperature variation in the resistance RC of the leads connecting the sensor to the bridge circuit. This is overcome using the four lead system shown in Fig.4-8(b); any variations in RC affect two arms of the bridge almost equally and have little effect on output voltage. FIG.4-8 Resistance-temperature characteristics of metals and four-lead connection Resistive temperature elements made from semiconductor materials are known as thermistors. The most commonly used type is prepared from oxides of the iron group of
  • 115. 110 Part 4 Principles of Measurement System transition metal elements such as chromium, manganese, iron, cobalt and nickel. The resistance of these elements decreases with temperature─in other words there is a negative temperature coefficient (N.T.C) ─ in a highly non-linear way. Figure 4-9 shows typical thermistor resistance-temperature characteristics. Thermistors are usually in the form of either beads, rods or discs (FIG.4-9).Bead thermistors are enclosed in glass envelopes. The manufacturer’s tolerance limits on the above figures are ±7 percent, which is far wider than for metal elements. The element time constant is 19 sec in air, 3 sec in oil and the self heating effect is 1 ℃ rise for every 7 mW of electrical power. Thermistors with positive temperature coefficients (P.T.C) are also available; the resistance of a typical element increases from 100 Ω at –55 ℃ to 10 kΩ at 120 ℃. FIG.4-9 Thermistor resistance-temperature characteristics and types Metal and semiconductor resistance strain gauges A strain gauge is a metal or semiconductor element whose resistance changes when under strain. We can define the gauge factor G of a strain gauge by the ratio (fractional change in resistance) / (strain). Hence, Resistance-strain relationship for a strain gauge is (4-2) y 0 = R tan α where ΔR=change in resistance of the gauge R0=unstrained resistance of the gauge e=longitudinal strain in the element For most metals the gauge factor G is around 2.0. A popular metal for strain gauges is the alloy ‘Advance’; this is 54 per cent copper, 44 percent Nickel and 1 percent Manganese. This alloy has a low temperature coefficient of resistance (2×10-5 /℃) and a low temperature coefficient of linear expansion. Temperature is both an interfering and a modifying input and the above properties ensure that temperature effects on zero and sensitivity are small. The most common strain gauges are of the bounded type, where the gauge consists of metal foil, cut into grid structure by a photo-etching process, and mounted on a resin film base. The film backing is then
  • 116. Technical English Through Reading 111 attached to the structure to be measured with a suitable adhesive. The gauge should be positioned so that its active axis is alone the direction of the measured strain: the change in resistance, due a given strain, along the passive axis is very small compared with that produced by the same strain along the active axis. A typical gauge has a gauge factor of 2.0 to 2.2, unstrained resistance of 120±1 Ω, linearity within ±0.3 percent, maximum tensile strain 2× 10-2, maximum compressive strain -1×10-2, maximum operating temperature 150℃. The change in resistance at maximum tensile strain is, and ΔR=-2.4Ω at maximum compressive strain. A maximum gauge current between 15 mA and 100 mA, depending on area, is specified in order to avoid self-heating effects. Unbounded strain gauges consisting of fine metal wire stretched over pillars are used in some applications. In semiconductor gauges the gauge factor can be large. The most common material is silicon doped with small amounts of ‘P’ type or ‘N’ type material. Gauge factors of between 100 and 175 are common for P type silicon, and between -100 and -140 for N type silicon. A negative gauge factor means a decrease in resistance for a tensile strain. Thus semiconductor gauges have the advantage of greater sensitivity to strain than metal ones, but have the disadvantage of greater sensitivity of temperature changes. Typically a rise in ambient temperature from 0 to 40 ℃ causes a fall in gauge factor from 135 to 120. Also the temperature coefficient of resistance is larger, so that the resistance of a typical unstrained gauge will increase from 120 Ω at 20 ℃ to 125 Ω at 60 ℃. Strain gauge elements are incorporated in deflection bridge circuits. (Section 4.4.1). 4.3.2 Capacitive Sensing Elements The simplest capacitor or condenser consists of two parallel metal plates separated by a dielectric or insulating material (Fig.4-10). The capacitance of ε 0εA C= (4-3) d whereε 0 is the permittivity of free space (vacuum) of magnitude 8.85 pF/m,ε is the relative permittivity or dielectric constant of the insulating material, A m2 is the area of overlap of the plates and d m their separation. From (4-3) we see that C can be changed by changing either d, A orε; Fig.4-10 shows capacitive displacement sensors using each of these methods. A commonly used capacitive pressure sensor is shown in Fig.4-10.Here one plate is a fixed metal disc, the other is a flexible flat circular diaphragm, clamped around its circumference; the dielectric material is air ( ε≈1). The diaphragm is an elastic sensing element which is bent into a curve by the applied pressure P. The deformation of the diaphragm means that the average separation of the plate is reduced, the resulting increase in capacitance is given. The variable separation displacement sensor has the disadvantage of being non-linear. This problem is overcome by using the three-plate differential or push-pull displacement sensor shown in Fig.4-10. This consists of a plate M moving between two fixed plates F1 and F2; if x is the displacement of M from he center line AB, then the capacitances C1 and C2 are formed by
  • 117. 112 Part 4 Principles of Measurement System MF1 and MF2 respectively. The relations between C1, C2 and x are non-linear, but when C1 and C2 are incorporated into the a.c. deflection bridge, the overall relationship between bridge output voltage and x is linear. The final sensing element shown in Fig.4-10 is a level sensor consisting of two concentric Metal cylinders. The space between the cylinders contains liquid to the height h of the liquid in the vessel. If the liquid is non-conducting, it forms a suitable dielectric and total capacitance of the sensor is the sum of liquid and air capacitance. The capacitance/unit length of two coaxial cylinders, radii b and a (b>a), separated by a dielectricεis 2πε0ε/ loge(b/a). FiG.4-10 Capacitive sensing elements Capacitance sensing elements are incorporated into the a.c. Deflection bridge circuits or electrical oscillator circuit. In designing these circuits allowance must be made for the following: (1) Capacitive sensors are not pure capacitances but have a resistance in parallel to represent dielectric losses. (2) The capacitance of the cable connecting the sensor to the measurement circuit. Variable Inductance (Variable Reluctance) Displacement Sensors The reluctance R of a magnetic circuit is given by l R= (4-4) µ0 µA where l is the total length of the flux path, μ is the relative permeability of the circuit material, μ0 is the permeability of free space =4π×10-7 H/m and A is the cross sectional area of the flux path. Figure 4-11(a) shows the core separated into two parts by an air gap of variable width. The total reluctance of the circuit is now the reluctance of both parts of the core together with
  • 118. Technical English Through Reading 113 the reluctance of the air gap. Since the relative permeability of air is close to unity and that of the core material many thousands, the presence of the air gap cause a large increase in circuit reluctance and a corresponding decrease in flux and inductance. Thus a small variation in air gap causes a measurable change in inductance so that we have the basis of an inductive displacement sensor. Figure 4-11(b) shows a typical variable reluctance displacement sensor, consisting of three elements: a ferromagnetic core in the shape of a semitoroid (semicircular ring), a variable air gap and a ferromagnetic plate or armature. The total reluctance of the magnetic circuit is the sum of the individual reluctance, i.e. RTOTAL=RCORE+RGAP+RARMATURE (4-5) FIG.4-11 Variable reluctance elements (a) Basic principle of reluctance sensing elements (b) Reluctance calculation for typical element Linear Variable Differential Transformer (LVDT) Displacement Sensor This sensor is a transformer with a single primary winding and two identical secondary windings wound on a tubular ferromagnetic former (Fig.4-12). The primary winding is energized by a.c. voltage of amplitude VS, frequency fS Hz; the two secondaries are connected in ˆ series opposition so that the output voltage Vout sin( 2πf S t + ϕ ) is the difference V1-V2 of the voltages induced in the secondaries. A ferromagnetic core or plunger moves inside the former; this alters the mutual inductance between the primary and secondaries. With the core removed ˆ the secondary voltages are ideally equal so that Vout = 0 . With the core in the former. V1 and V2 ˆ change with core position x, causing amplitude Vout and phase ϕ to change; the relationships
  • 119. 114 Part 4 Principles of Measurement System ˆ between Vout , ϕ and x are shown in Fig.4-12. We see that there is a null point C at the center ˆ of the sensor(x=l/2) where Vout = 0 (ideally); here there is equal coupling between the primary and secondaries so that V1=V2. At the points A and B, equally spaced either side of the ˆ null points, Vout has the same value V0. However, at A the output voltage is 180°out of phase with the primary voltage, i.e. ϕ =-180°(V2>V1), whereas at B the output voltage is in phase with the primary voltage i.e. ϕ =0°(V1>V2). Non-linear effects occur at either end (D and E) as the core moves to the edge of the former. FIG.4-12 Linear variable differential transformer (L.V.D.T.) and characteristics The a.c. output signal is converted into d.c. in a way which distinguishes between the situations at A and B, where the amplitude is the same but there is a phase difference of 180°. A phase sensitive demodulator is used which senses this phase difference and gives a positive voltage at position B. The corresponding d.c. characteristics are shown in Fig.4-12; these show that the non-linearity will increase as the displacement range is increased. L.V.D.T. displacement sensors are available to cover range from ±0.25mm to ±2.5cm. 4.3.4 Electromagnetic Sensing Elements These elements are used for the measurement of linear and angular velocity and are based on Faraday's law of electromagnetic induction. This states that if the flux N linked by a conductor
  • 120. Technical English Through Reading 115 is changing with time, then a back e.m.f. is induced in the conductor with magnitude equal to the rate of change of flux, i.e. E=-dN/dt (4-6) In an electromagnetic element the change in flux is produced by the motion being investigated; this means that the induced e.m.f. depends on the linear or angular velocity of the motion. A common example of an electromagnetic sensor is the variable reluctance tachogenerator for measuring angular velocity (Fig.4-13). It consists of a toothed wheel of ferromagnetic material (attached to the rotating shaft) and a coil wound onto a permanent magnet, extended by a soft iron pole piece. The wheel moves in close proximity to the pole piece, causing the flux linked by the coil to change with time, thereby inducing an e.m.f. in the coil. FIG.4-13 Variable reluctance tachogenerator, angular variations in reluctance and flux The magnitude of the e.m.f. can be calculated by considering the magnetic circuit formed by the permanent magnet, air gap and wheel. The m.m.f. is constant with time and depends on the field strength of the permanent magnet. The reluctance of the circuit will depend on the width of the air gap between the wheel and pole piece. When a tooth is close to the pole piece the reluctance is minimum but will increase as the tooth moves away. The reluctance is maximum when a ‘gap’ is adjacent to the pole piece but falls again as the next tooth approaches the pole piece. Figure 4-13 shows the resulting cyclic variation in reluctance R with angular rotation θ. The output signal for variable reluctance tachogenerator is given by E = bmω sin mωt (4-7) where m is the number of teeth. This is a sinusoidal signal of amplitude EMAX=bmω and frequency f=mω/(2π);i.e. both amplitude and frequency are proportional to the angular velocity of the wheel.
  • 121. 116 Part 4 Principles of Measurement System 4.4 SIGNAL CONDITIONING ELEMENTS As stated in section 4.1.1, signal conditioning elements convert the output of sensing elements into a form suitable for further processing. This form is usually a d.c. voltage, d.c. current or a variable frequency a.c. voltage. 4.4.1 Deflection Bridges Deflection bridges are used to convert the output of resistive, capacitive and inductive sensors into a voltage signal. Thevenin Equivalent Circuit for A Deflection Bridge Any network can be represented by a Thevenin equivalent circuit consisting of a voltage source ETh together with a series impedance ZTh . Figure 4-14 shows a general deflection bridge network. ZTh is the open circuit output voltage of the bridge, i.e. when circuit i in BD=0. Using Kirchoff's laws, we have Thevenin voltage Z1 Z2 ETh = VS ( + ) (4-8) Z1 + Z 4 Z 2 + Z 3 ZTh is the impedance, looking back into the circuit, between the output terminals BD, when the supply voltage VS is replaced by its internal impedance. Assuming the internal impedance of the supply is zero, then this is equivalent to a short circuit across AC (see Fig.4-14).We see that Thevenin impedance for general deflection bridge ZTh is equal to the parallel combination of Z2 and Z3 in series with the parallel combination of Z1 and Z4, i.e. FIG.4-14 Calculation of Thevenin equivalent circuit for a deflection bridge Z1 Z 4 Z Z Z Th = + 2 3 (4-9) Z1 + Z 4 Z 2 + Z 3 If a load, e.g. a voltmeter or amplifier, of impedance ZL is connected across the output terminals BD, then the current through the load is i=ETh /( ZTh +ZL ). The corresponding voltage across the load is VL=ETh ZL/( ZTh +ZL ).Thus in the limit that|ZL |>>|ZTh |,VL→ETh .
  • 122. Technical English Through Reading 117 Design of Resistive Deflection Bridge In a resistive or Wheatstone bridge all four impedances Z1 to Z 4 are pure resistances R1 to R4. From (4-8) we have R1 R2 ETh = VS ( + ) (4-10) R1 + R4 R2 + R3 We first consider the case when only one of the resistance is a sensing element. Here R1 depends on the input measured variable I, i.e. R1=RI and R2, R3, R4 are fixed resistors. This gives 1 1 ETh = VS ( + ) (4-11) 1 + R 4 / R I 1 + R3 / R I from which we see that to design a single element bridge we need to specify the three parameters VS , R4, and R3/R2. The individual values of R2 and R3 are not critical, it is their ratio which is crucial to the design. The three parameters can be specified by considering the range and linearity of the output voltage and electrical power limitations for the sensor. Thus if I MIN, I MAX are minimum and maximum values of the measured variable, and RMIN, RMAX, are the corresponding sensor resistances, then in order for the bridge output voltage to have a range from VMIN, to VMAX the following conditions must by obeyed: 1 1 VMIN = VS ( + ) (4-12) 1 + R4 / RIMIN 1 + R3 / R2 1 1 VMAX = VS ( + ) (4-13) 1 + R4 / RIMAX 1 + R3 / R2 Often we require VMIN=0, i.e. the bridge to be balanced when I=I MIN; in this case (4-12) reduce to relationship between resistances in a balanced Wheatstone bridge R4 R = 3 (4-14) RIMIN R2 A third condition is required to complete the design. One important considerations is the need to limit the electrical power i22RI in the sensor to a level which enables it to be dissipated as heat flow to the surrounding fluid; otherwise the temperature of the sensor rises above that of the surrounding fluid, thereby affecting the sensor resistance. The value of the ratio R3/R4 varies according to the type of resistive sensor used; it is useful to have a graph which gives some insight into this. From (4-14) we have R4 =(R3/R2) RIMIM; substituting for R4 in (4-11) gives ETh 1 1 = − (4-15) VS 1 + ( R3 / R2 )( RIMIN / RI ) 1 + R3 / R2 Thus for a bridge with a single strain gauge, we require R2, R3,R4 to all equal the unstained
  • 123. 118 Part 4 Principles of Measurement System gauge resistance R0. The VS is determined by the power condition. We have VS ∆R VS ETh = = Ge (4-16) 4 R0 4 i.e. the relationship between ETh and e is linear. 4.4.2 Amplifiers Amplifiers are necessary in order to amplify low level signals, e.g. thermocouple or strain gauge bridge output voltages, to a level which enables them to be further processed. The Ideal Operational Amplifier The operational amplifier can be regarded as the basic building block for modern amplifiers. It is a high gain, integrated circuit amplifier designed to amplify signals from d.c. up to many kHz. It is not normally used by itself but with external feedback networks to produce precise transfer characteristics which depend almost entirety on the feedback network. Usually there are two input terminals and one output terminal, the voltage at the output terminal being proportional to the difference between the voltages at the input terminals. Figure 4-15 shows the circuit symbol and a simplified equivalent circuit for an operational amplifier. If we assume ideal behavior, then the calculations of transfer characteristics of operational amplifier feedback networks are considerably simplified.These calculations can then be modified if it is necessary to take into account the non-ideal behavior of practical amplifiers. FIG. 4-15 Circuit symbol and simplified equivalent circuit for operational amplifier Instrumentation Amplifiers An instrumentation amplifier is a high performance differential amplifier system consisting of several closed loop operational amplifiers. An ideal instrumentation amplifier gives an output voltage which depends only on the difference of two voltages V1 and V2 i.e. VOUT = K (V2 − V1 ) (4-16) where the gain K is precisely known and can be varied over a wide range. A practical instrumentation amplifier should have a gain which can be set by a signal external resistor and should combine high input impedance, high common mode rejection ratio, low input offset voltage and a low temperature coefficient of offset voltage. The differential amplifier which uses a single operational amplifier is inadequate; in order to obtain high gain R1 must be low,
  • 124. Technical English Through Reading 119 this means low input impedance and low common rejection. Figure 4-16 shows a typical instrumentation amplifier system consisting of three operational amplifiers A1, A2 and A3.The two input non-inverting amplifiers A1 and A2 provide an overall differential gain of (1+2R1/RG) and a common mode gain of unity. The output amplifier A3 is a unity gain differential amplifier. An amplifier of this type has ZIN≈300 to 5000 MΩ, (C.M.M.R.) dB≈74 to 110 dB, offset voltage V OL≈0.2 mV, temperature coefficient of offset voltage γ≈0.25 to 10 μV/℃. FIG.4-16 Typical instrumentation amplifier 4.5 SIGNAL PROCESSING ELEMENTS The output signal from conditioning elements is usually in the form of a d.c. voltage, d.c. current or variable frequency a.c. voltage. In simple systems the signal conditioning element can be connected directly to a data presentation element; if an amplifier output voltage is proportional to temperature, the amplifier can be directly connected to a pointer-scale indicator with a scale marked in degrees Centigrade. Often, however, calculations must be performed on the conditioning element output signal, in order to establish the value of the variable being measured. These calculations are referred to as signal processing and in the modern measurement systems are performed using a digital microcomputer. The first of this chapter discusses the principles of analogue to digital conversion and the operation of typical analogue to digital converters. In the following section the layout, operation and programming of a typical microcomputer are explained. The final section explains the use of the microcomputer as a processing element in a speed measurement system. Other signal processing application of microcomputers are error reduction, mass flow measurement, hot wire anemometer linearisation and gas chromatography. 4.5.1 Analogue to Digital (A/D) Conversion This section commences by discussing the three operations involved in A/D conversion; these are sampling, quantisation and encoding. The first operation is performed by a sample-and-hold device; the second and third are combined in an analogue to digital converter.
  • 125. 120 Part 4 Principles of Measurement System Sampling We saw that a continuous signal y(t) could be represented by a set of samples y i, i=1,..., N, taken at discrete intervals of time ΔT (sample interval). The operation is shown in Fig.4-17; the switch is closed fS times per second, where sampling frequency fS =1/(ΔT). In order for the sampled signal yS (t) to be an adequate representation of y(t). fS should satisfy the conditions of the Nyquist sampling theorem, which can be stated as follows: A continuous signal can be represented by , and reconstituted from, a set of sample values providing that the number of samples per second is at least twice the highest frequency present in the signal. Mathematically we require f S ≥ 2 f MAX (4-17) where fMAX is the frequency beyond which the continuous signal power spectral density Φ(f) becomes negligible. The above result can be explained by examining the power spectral density ΦS(f) of the sample signal. Because the sampled signal is a series of sharp pulses, ΦS(f) contains additional frequency components which are centred on multiples of the sampling frequency. Fig.4-17 shows ΦS (f) in the three situations fS >2 fMAX , fS =2fMAX and fS <2fMAX . If fS >2 fMAX , then the additional frequency components can easily be filtered out with an ideal low pass filter of bandwidth 0 to fMAX and the original signal reconstituted. If fS =2 fMAX , it is just possible to filter out the sampling components and reconstitute the signal. If fS <2 fMAX , the sampling components occupy the same frequency range as the original signal and it is impossible to filter them out and reconstitute the signal. The effect of sampling at too low a frequency is shown in Fig.4-18. Here a sine wave of period 1 second, i.e. frequency 1Hz, is being sampled approximately once every second, i.e. the sampling frequency is below the Nequist minimum of 2 samples/second. The diagram shows that it is possible to reconstruct an entirely different sine wave of far lower frequency from the sample values. This is referred to as the `alias' of the original signal; it is impossible to decide whether the sample values are derived from the original signal or its alias. The phenomenon of two different signals being constructed from a given set of sample values is referred to as aliasing. The operation of analogue to digital conversion can take up to a few milliseconds; it is necessary therefore to hold the output of the sampler constant at the sampled value while the conversion takes place. This is done using a sample-and-hold device as shown in Fig.4-19. In the sample state the output signal follows the input signal; in the hold state the output signal is held constant at the value of the input signal at the instant of time the hold command is sent. The sample-and-hold waveform shown is ideal; in practice errors can occur due to the finite time for the transition between sample-and-hold states (aperture time) and reduction in the hold signal (droop). Quantisation Although the above sample values are taken at discrete intervals of time, the values y i can
  • 126. Technical English Through Reading 121 take any value in the signal range y MIN to y MAX (Fig.4-20). In quantisation the sample voltage are rounded either up or down to one of Q quantisation values or levers Vq , where q=0, 1, 2,..., Q-1. These quantum levels correspond to the Q decimal numbers 0, 1, 2,..., Q-1. If V0=VMIN and VQ-1 =y MAX, then there are (Q-1) spacings occupying a span of y MAX-y MIN. The spacing width or quantisation interval ΔV is therefore FIG.4-17 Time waveform and frequency spectrum of sample signal FIG.4-18 Aliasing
  • 127. 122 Part 4 Principles of Measurement System FIG.4-19 Sample and hold FIG.4-20 Quantisation y MAX − y MIN ∆V = (4-18) Q −1 The operation of quantisation produces an error eq =Vq -y i termed the quantisation error. Normally if y i is above the halfway point between two levels q, q+1 it is rounded up to Vq+1; if y i is below halfway it is rounded down to Vq (Fig.4-20). The maximum quantisation error e qMAX is
  • 128. Technical English Through Reading 123 therefore ±ΔV/2 or expressed as a percentage of span y MAX-y MIN. We see that the relationship between Vq and y i is characterized by a series of discrete steps or jumps. Obviously the greater the number of levels Q, the lower the quantisation errors. Encoding The encoder converts the quantisation values Vq into a parallel digital signal corresponding to a binary coded version of the decimal numbers 0, 1, 2,..., Q-1.The commonly used decimal or binary number system uses a base or radix of 10, so that any positive integer is expressed as a series of power of 10 (decades): d n × 10 n + d n −1 × 10 n −1 + ... + d i × 10 i + d1 × 101 + d 0 × 10 0 (4-19) where the di are the respective weights or digits which take the values 0 to 9. In digital computers the binary number system is used. This has a base of 2 that any positive integer can be expressed as a series of powers of 2: d n × 2 n + d n−1 × 2 n−1 + ... + d i × 2 i + d1 × 21 + d 0 × 2 0 (4-20) ↑ ↑ MSB (Most significant bit) LSB(Least significant bit) where the weightings bi are referred to as bits or digits. A bit can take only the values 0 or 1 so that calculations on binary numbers are easily performed by logic circuits which distinguish between two states─on or off, true or false. In order to convert the decimal number 183 to binary, we express it in the form 1×27+0×26+1×25+1×24+0×23+1×22+1×21+1×20 giving the 8-bit binary number 10110111. To produce an electrical signal corresponding to this number we require 8 wires in parallel, the voltage on each wire being typically 5V for a ‘1’ and 0 V for a ‘0’; this is a 8-bit parallel digital signal. The number of binary digits n required to ncode Q decimal number is given by n = log 2 Q = log 10 Q / log 10 Q (4-21) Thus if Q=200, n=log10200/log102=7.64 since, however, n must be an integer, we require 8 bits which corresponds to Q=28=256. The corresponding maximum quantisation error is ± 100/(2×255) percent=±0.196 percent. Other commonly used codes are as follows: Binary Coded Decimal (b.c.d.) Here each decade of the decimal number is separately coded into binary. Since 23=8 and 24=16, four binary digits DCBA are required to encode the 10 numbers 0 to 9 in each decade. In 8:4:2:1 b.c.d.; A≡20=1, B≡21 =2, C≡22=4, D≡23=8; thus the decimal number 369 becomes DCBA DCBA DCBA 0011 0110 1001 3 6 9
  • 129. 124 Part 4 Principles of Measurement System The number of decades p of b.c.d. required to encode Q decimal numbers is given by: Q= 10p,i.e. p=log10Q and the corresponding total number of binary digits is 4 p = 4 log 10 Q (4-22) The input signal to alphanumeric displays is normally in b.c.d form; since the signal is already separated into decades the conversion into seven segment or hexadecimal dot code is easier than with pure binary. Octal Code Here a base of 8 is used so that the weights take the values 0 to 7. In order to convert 183 decimal, i.e. (183)10, into octal we express it in the form 2(64)+6(8)+7(1) i.e. 2×82+6×81+7×80 i.e. 267 octal or (267)8 A binary number is easily coded into octal by arranging the digits in groups of 3,where for each group A≡20=1, B≡21=2, C≡22=4. Thus 010110111 (binary code for (183)10) becomes CBA CBA CBA 010 110 111 2 6 7 i.e. 267 octal Hexadecimal Code (hex) Here a base of 16 is used and the digits are the 10 numbers 0 to 9 and 6 letters ABCDEF. Since 24=15, each hexadecimal digit corresponding to 4 binary digits: Binary 0000 0001 0010 0011 0100 0101 0110 0111 Hex 0 1 2 3 4 5 6 7 Binary 1000 1001 1010 1011 1100 1101 1110 1111 Hex 8 9 A B C D E F Some corresponding decimal, binary and hexadecimal numbers are Decimal Binary Hexadecimal 94 01011110 5E 167 10100111 A7 238 11101110 EE Analogue to Digital Converters (A.D.C.s) Figure 4-21 shows a schematic diagram of a successive approximation analogue to digital converter. This method involves making successive guesses at the binary code corresponding to the input voltage y i. The trial code is converted into an analogue voltage using a D.A.C. and a comparator is used to decide whether the guess is too high or too low. On the basis of this result another guess is made, and the process repeated until V q is within half a quantisation interval of yi.
  • 130. Technical English Through Reading 125 FIG.4-21 Successive approximation converter 4.5.2 Typical Microcomputer System Figure 4-22 shows the structure of a typical microcomputer system, an 8-bit system is shown but 12 and 16 bit systems are also available. The system consists of several elements: microprocessor, read only memory (R.O.M.), random access memory (R.A.M.), input/output interface which are interconnected by a data bus, address bus and control lines. Input data from a counter or A/D converter enters the computer at the input parallel interface as an 8-bit word or byte. The data is then carried to the different elements using a set of 8 parallel digital signal. A string of 8 bits is referred to as an 8-bit word or byte. The data is then carried to the different elements using a set of 8 parallel wires called an 8-bit data bus. The data bus will carry, for example, data from the input interface or R.A.M. to the microprocessor, data from the microprocessor to the R.A.M., data from the microprocessor or R.A.M. to the output interface. Date leaves the computer at the parallel or serial output interface to pass to a date presentation element such as an alphanumeric display, C.R.T. display or digital recorder. The computer system is sequential, i.e. it operates by executing a sequence of steps. Each step is sequence of instructions is called a program. Instructions are normally stored in the R.O.M. and both instructions and data in the R.A.M. The data bus is used to carry instructions from these memory devices to the processor. Both types of memory devices have many storage elements, each with an associated address. This is a binary or hexadecimal number which specifies the location of the storage elements, each with an associated address. This is a binary or hexadecimal number which specifies the location of the storage element within the memory. Reading is an operation whereby a copy of a data word or instruction is transferred from a given storage location to another device without changing the contents of the store. Writing is an operation whereby a data word or instruction is placed in a given storage location. Information can be written into or read out of any part of the R.A. M; it is however volatile memory; i.e. the stored information is lost when the power supply is switched off. The R.O.M. is a permanent
  • 131. 126 Part 4 Principles of Measurement System memory, but information can only be read out of the R.O.M. In order to carry out these read-write operations the storage location must be addressed; here the microprocessor generates a parallel digital signal which corresponds to the address of the location and presents it to the memory device. The system of Fig.4-22 has a 16-bit address bus which is capable of addressing up to 216 different locations. In this system the R. O. M. has a storage capacity of 1024 8-bit bytes and requires a 10-bit address signal. The R.A.M. has a storage capacity of 256 8-bit and requires an 8-bit address signal. The address bus will carry, for example, the address of the next instruction to be executed to the R.O.M., the address of a data word to the R.A.M., the address or number of an input or output device to the input/output (I/O) interface. FIG.4-22 Simplified schematic of typical 8-bit microcomputer system The control lines carry clock signals and control signals to each element in the computer to ensure the coordination and synchronization necessary for the information transfers mentioned above. Input and output data can be in both parallel and serial form. Figure 4-23 is a simplified diagram of a typical microprocessor. The program counter is a register which contains the address of the instruction to be executed next. This instruction address passes to the address registers (index register and stack pointer) which hold the address prior to passing it to the address bus. The corresponding instruction is received from the data bus and is passed to the instruction register. Here it is held while it is decoded in the instruction decoder and executed. The address registers also pass data addresses to the address bus. The accumulators are data registers also these receive data words from the data bus (prior to the execution of an execution of an instruction). The arithmetic-logic unit (A.L.U.) is a logic circuit which can perform arithmetic (addition, subtraction) and logical (AND, OR) operations on one or two data words held in accumulators. The result of the operation is held in an accumulator prior to passing to the data bus. Advanced microprocessors have circuits which also multiply and divide.
  • 132. Technical English Through Reading 127 FIG.4-23 Simplified diagram of typical microprocessor Typical Microcomputer Instructions Each instruction in a microcomputer program is of the basic form operator operand i.e. an operator operating on an operand. The operand can be either a data word or an address. In a 8-bit computer the basic data word is an 8-bit byte corresponding to 0 to 255 decimal or 00 to FF hexadecimal. However if two numbers are added or multiplied together the result may lie outside this range, e.g .if 157 is added to 201 i.e. the result is 358 decimal or 166 hexadecimal. To accommodate this a 2-byte operand is used corresponding 0 to 65535 decimal or 0000 to FFFF hexadecimal. A computer with a 16-bit address bus can address up to 216 =65536 different locations. This means that a single- byte operand is used to address locations 0 to 255 (00 to FF) and a 2-byte operand to address locations 256 to 65535 (0100 to FFFF). The microprocessor has to send address signals to the R.O.M., R.A.M., I/O interface and any other device in the system. It is necessary therefore to have an address map which specifies the address of each device and the address of each storage location or register within the device. Table 4-1 shows a possible address map for the system of Fig.4-22 which has a 256-byte R.A.M., 1024-byte R.O.M. and an I/O interface. Table 4-1 Possible address map for microcomputer Device Address bits Address range 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 R.A.M. * * * *0 0**xxxxxxx 0 to 255,0000 to 00FF R.O.M. * * * *1 1xxxxxxxxx 3072 to 4095,0C00 to 0FFF I/O interface * * ** 0 1******xxx 1024 to 1029,0400 to 0405 x≡address bit; 0 or 1≡device select bit; *≡irrelevant
  • 133. 128 Part 4 Principles of Measurement System The table shows that bits 10 and 11 are used for selecting which device is to be addressed; bits 12 to 15 are not used. The I/O interface is allocated an address range of 0400 to 0405 to enable different interfaces to be addressed. The system can be expanded by adding extra units of 1024 byte (1k byte) R. O. M. and 256 byte R.A.M.. The operator specifies the operation to be carried out on the operand e.g. add, subtract, clear, increment, load and store. In assembly language each operator is represented by a 3- or 4-letter mnemonic statement which suggests as closely as possible the operation to be carried out. A typical microprocessor is capable of executing between 50 and 100 different instructions or operations; examples are given in Table 4-2. Each mnemonic statement has a hexadecimal code associated with it, this is referred to as op-code or machine code. The program must be translated from mnemonic to machine code versions of a simple program for adding two numbers. The program begins by clearing accumulator A and B, then loads the contents of storage location 2C into accumulator A and the contents of location 7B into accumulator B. The contents of accumulator B are added to those of accumulator A. The result is placed in accumulator A and then transferred to storage location BF. In the machine code version the computer is told the address of every operator and operand. Thus for the instruction LDB7B, the operator code 1B is stored in location 9B and the operand code 7B is stored in the next location 9C. Table 4-2 Typical microcomputer instructions and program Operator Mnemonic Op-code Description ADAB 01 Adds contents of accumulator B to contents of accumulator A and places result in accumulator A SBAB 03 Subtracts contents of accumulator B from contents of accumulator A and places result in accumulator A LDB 1B Loads the contents of memory into accumulator B STRA 29 Stores the contents of accumulator A in memory INCA 33 increments contents of accumulator by 1 DIX 3F Decrements contents of index register by 1 SHRA 4D Shifts contents of accumulator A one place to right CLRB 5A Clears accumulator B LDX 5E Loads contents of memory into index register JMP 63 Jump to instruction stored in operand address 4.5.3 Use of Microcomputer in A Speed Measurement System In this system the angular velocity of a motor is sensed by a variable reluctance tachogenerator and the measured speed in r.p.m. displayed on a cathode ray tube display (visual display unit). The tachogenerator (Section 4.3.4) gives an a.c. output voltage whose frequency is proportional to motor angular velocity. This signal is input to the frequency to digital converter i.e. a Schmitt trigger followed by an 8-bit binary counter which gives an 8-bit parallel digital output signal. This signal is connected to the parallel input interface of the microcomputer as shown in Fig.4-24. The input digital signal to the C.R.T. display must be serial form and in ASCII code;
  • 134. Technical English Through Reading 129 this is provided by the serial output interface. The parallel output interface provides the STOP COUNT (LSB) and RESET logic control signals for the counter. FIG.4-24 Speed measurement and input/output interface