Technical English
     Through Reading
                   专业英语阅读
                                    王明赞 编




l Machine D...
Technical English Through Reading                                                          1




                         ...
2                                                                 Contents
        1.3.4 Lubricant Characteristics ..........
Technical English Through Reading                                                            3

            3.1.3 Manual C...
4                                                             Contents
         4.2.2 Amplitude Response.....................
Technical English Through Reading                             1




PART 1 MACHINE DESIGN

1.1 INTRODUCTION TO MACHINE DES...
2                                       Part 1 Machine Design
account during the early design phase.
      (4) Materials o...
Technical English Through Reading                             3

    It can’t be held in a shoe or chuck,
    In can’t be ...
4                                      Part 1 Machine Design
way, innovative ideas are not inhibited. Quite often, more th...
Technical English Through Reading                              5

reality that needs to be kept in mind is that many of th...
6                                       Part 1 Machine Design
Figure 1-1 shows the conventional four-stroke piston engine,...
Technical English Through Reading                            7

makes three revolutions for each revolution of the triangu...
8                                       Part 1 Machine Design




FIG.1-2 One complete cycle of the rotary Wankel engine
 ...
Technical English Through Reading                                9

consideration in the general field of machine design:
...
10                                     Part 1 Machine Design
      (2) suddenly applied;
      (3) applied under impact;
 ...
Technical English Through Reading                             11

     (5) Comfort;
     (6) Distinction and richness;
   ...
12                                    Part 1 Machine Design
important to realize that lower cost does not necessarily mean...
Technical English Through Reading                                13

reduced with proper design. Since noise is generated ...
14                                     Part 1 Machine Design
features required by low should be incorporated where applica...
Technical English Through Reading                             15

other hand, a failure may be no more than nuisance. An e...
16                                          Part 1 Machine Design
                                                        ...
Technical English Through Reading                              17

The stress-strain diagram is linear up to point B, whic...
18                                       Part 1 Machine Design
beyond the ultimate strength. The actual S- ε diagram does ...
Technical English Through Reading                               19

     (2) Stiffness. Stiffness is the deformation-resis...
20                                     Part 1 Machine Design
approximately 75 percent of the ultimate strength tension. Th...
Technical English Through Reading                             21

The conception of the initial crack is in itself a stres...
22                                        Part 1 Machine Design
stress level, below which the material can sustain 1 milli...
Technical English Through Reading                          23

percent of the ultimate strength.

1.2.8 Stress Concentrati...
24                                      Part 1 Machine Design

                                                      S ult...
Technical English Through Reading                             25

1.2.10 Creep: A Plastic Phenomenon

Temperature effects ...
26                                       Part 1 Machine Design
Friction is the resistance one part exerts on a second part...
Technical English Through Reading                           27

A journal bearings, in its simplest form, is a cylindrical...
28                                     Part 1 Machine Design
become embedded into the bushing, which prevents scratching a...
Technical English Through Reading                                29

in temperature.


1.4 ANTIFRICTION BEARINGS

1.4.1 In...
30                                        Part 1 Machine Design
The design and terminology of typical single-row ball bear...
Technical English Through Reading                              31

tiny flake of metal is removed from one of the races or...
32                                      Part 1 Machine Design
Thus, an equivalent load must be considered which takes into...
机械专业英语
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机械专业英语

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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,

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