MET -105 POWER TRANSMISSIONDefinitionsA rotating machine is one in which the main working components rotate about a fixedcenter in a regular manner. Most such machines incorporate additional subsidiarymechanisms such as linkages, slides, gears and reciprocating components, and manyof the operating principles that apply to the rotating assembly also apply to these otherelements.Although there are many different types of rotating machines, they can all be classified intothree basic groups in terms of their function.Driving machines (engines or prime movers)This group includes all machines whose purpose is to drive other machines. Examplesinclude : Electric motors Steam turbines Diesel engines Petrol engines Air motorsThe common characteristic of these machines is that they convert an energy input ofvarying kinds into a mechanical output in the form of a rotating drive shaft.Transmission machinesThese are machines whose purpose is to transmit mechanical energy from a driving toa driven machine. Examples include : Gearboxes Differentials Variable speed drives Chain Belts
The mechanical energy transmitted often undergoes a speed transformation and thesemachines often incorporate some means of drive disengagement such as a clutch.Driven machinesThese machines cannot operate independently and need to be coupled to a drivingmachine. Examples include: Pumps Compressors Fans Generators Blenders Machine toolsThis group is by far the largest and includes a large number of different types ofmachines. The common characteristic is that the energy input is normally in the form ofa rotating drive shaft while output may be in a variety of forms including kinetic orpressure energy of a fluid, electrical energy, kinetic or potential energy of solidmaterials, etc.Power TransmissionThere are three major systems in use for transmission of rotary motion betweenadjacent shafts : belts, chains and gears.BELT DRIVEOne of the most common elements in power transmission systems, belt drives give dependableand cost effective power transmission with a minimum of maintenance.There are four basic types of belts used in power transmission1. Flat belts2. V-belts3. Toothed timing belts4. RibbedV-BELT TYPES
Most V-belt drives used in industrial applications fall into two categories: heavy duty(industrial) and light duty (fractional horsepower). There are primarily two types ofindustrial belts: the classic cross sections (A, B, C, and D) and the narrow cross sections(3V, 5V, and 8V) (Molded notch construction belts are usually designated with an X after thesection letter. A 3V molded notch belt would be designated 3VX.)Fractional-horsepower belts are used most often on drives transmitting less than 1 horsepower.Fractional-horsepower belts are available in the following sections: 2L, 3L, 4L, and 5L. A V-beltis specified by cross section and length.Principles of OperationV-belts are normally used to transfer power between two shafts whose axes are parelleland some distance apart. Figure 1. Typical V-belt arrangementsThe belt is mounted on pulleys that are attached to the driving and driven shafts and thedrive relies on friction between the belt and the pulleys for its operation. The belt sits inthe groove of the pulley and makes contact with the sides of the groove as shown inFigure2.
Figure2 Classic cross section. Narrow cross section.Molded notch belt. Joined belt cross section. Heavy problem solving joined
Light duty belt dimensions. Classic cross-sectional dimensions. Narrow cross-sectional dimensions.V-Belt LengthV-belt length can be measured in three ways: outside circumference (OC), datum length(DL), and effective length (EL). The outside circumference is measured by wrapping a tapemeasure around the outside surface of the belt.Datum Length. Datum length is a recent designation adopted by all belt manufacturers in orderto retain standard belt and sheave designations while more accurately reflecting the changes thathave occurred in belt pitch length and pitch-line location within the belt (pitch length is thelength of the neutral axis of the belt).Effective Length. The effective length is defined as the measured center distance plus the outsidecircumference of one of the inspection sheaves.
BELT DRIVES Total measuring forceSchematic of a V-belt measuring fixture.MisalignmentThere are three primary sources of misalignment in belt-drive systems:1. Driver and driven shafts are not parallel (both horizontal and vertical planes).2. Sheaves are not located in line axially with respect to one another on the shafts.3. Sheaves are tilted due to improper mounting (wobble while running).Sheave groove inspection.SYNCHRONOUS BELTSSynchronous belts are toothed belts in which power is transmitted through positiveengagement between belt teeth and pulley or sprocket grooves rather than by the wedgingfriction of V-belts. advantages of synchronous belts over other modes of power transmissioninclude a wider load/speed range, lower maintenance, increased wear resistance, and a smalleramount of required take up.
TYPES OF SYNCHRONOUS BELTSynchronous belt profiles.Modified Curvilinear BeltsThe modified curvilinear belt tooth form is a refinement of the curvilinear system. The belttooth and sprocket groove forms were optimized for smoother belt tooth entry/exit propertiesand improved belt tooth support in the sprocket grooves.Curvilinear BeltsThe curvilinear belt tooth form was developed to provide increased load capacity andperformance over trapezoidal belts. The curvilinear belt consequently has a higher horsepowercapacity than does a comparably sized trapezoidal beltTiming BeltsTiming belts were the first family of synchronous belts introduced to the market and weredesigned with trapezoidal teeth. The belt horsepower ratings are relatively low compared tocurvilinear or modified curvilinear belts introduced later, but the synchronization qualities areexcellent for accurate positioning or registration sensitive applications
PitchThe word „pitch‟ is commonly used in connection with many kinds of machinery andtypes of mechanical operations and calculations. Its definition, as applied to mechanicalpower transmission, is simple yet very important: the distance from a point to acorresponding point. In figure 1 are several examples of this measurement of distancefrom point to corresponding point.Pitch CircleAlthough the “pitch circle” is not visible, its dimension can be stated specifically as thepitch diameter of a gear, sheave, sprocket, etc. These dimensions are a necessary partof all rotary power-transmission calculations. These calculations are based on theconcept of disc or cylinders in contact, as illustrated in Figure 2.The rotation of one disc causes the disc with which it is in contact to rotate. Thisconcept assumes that no slippage occurs between the surfaces of the discs. Thesurfaces then travel equal distances at equal surface speeds.
. One revolution of a 2-inch circle will result in a ½ revolution of a 4-inch circle 4” PITCH DIA. 2” PITCH DIA. One revolution of a 4-inch circle will result in two revolutions of a 2-inch circle Shaft speeds are inversely proportional to pitch diameters.Using the term “driver” to indicate the driving gear, sprocket, or sheave, and the term“driven” to indicate the gear, sprocket, or sheave that is being driven, the ratio can bestated or expressed in the form of an equation, as the follows : Driver Rotational Speed Driven Pitch Diameter = Driven Rotational Speed Driver Pitch DiameterThe equation can be simplified by substituting letters and numbers for the words. Usethe letter (N) to signify speed, the letter (D) to indicate pitch, the number (1) to indicatethe “driver”, and the number (2) to indicate the “driven”. Driver rotational speed = N1
Driven rotational speed = N2 Driver pitch diameter = D1 Driven pitch diameter = D2The equation then becomes N2 D1 N1 D2These equations may be used to find unknown values by simple substitution of knownvalues in the appropriate equation. Following is an example of the use of each of one ofthese equations.Example 1The pitch diameter of the driver unit that is turning at 100 rpm is 2”. What will be thespeed of the driven unit of its pitch diameter is 4”? 4” PITCH DIA. DRIVER 100 RPM 2” PITCH DIA. Figure 6.Known values : N1 = 100 D1 = 2 D2 = 4 D1 x N 1 2 x100 200Unknown N2 = N2 or or 50 D2 4 4Speed of driven unit is 50 rpm.
Example 2The driver unit is turning at a speed of 600 rpm. The driven unit is turning at 2000 rpmand its pitch diameter is 3”. What is the pitch diameter of the driver unit? DRIVER 3” PITCH DIA. 600 RPM DRIVEN 2000 RPM Figure 7.N1 = 600 RPMN2 = 2000 RPMD2 = 3“ N 1x D 2 2000 x 3 6000Unknown D1 = or D 1 or or 10 D1 600 600The pitch diameter of the driver unit is 10”Speed, Torque and PowerThe power generated by driver equipment and transmitted to the driven is used atdifferent rotational speeds.If the power is assumed to be transmitted without any mechanical losses, so the totalvalue of the power will be the same for both driver and driven.As P = T. 2 N P = T. 60Where P = Mechanical Power
= Angular speed T = TorqueThen, between driver and driven shafts and assuming no mechanical losses P1 = P2 (Power is completely transmitted) 2 N1 2 N2 T1 T2 60 60 T1.N1 = T2.N2Note : The homogeneity of units in the application of these formulae is required.In S.I. units : P = watts (w) kW or 1 HP = 746 Watts T = N.m = radians / sec output powerefficiency ( ) = input powerIn order to be able to transmit power, the belt must be under tension so that it is forceddown into the groove. The depth of the groove is always greater than the thickness ofthe belt, however, and the belt should never bottom in the groove. The operation of thebelt and its ability to transmit power depend on the size of the friction force and the arcof contact of the belt. The greater the arc of contact the more power the belt cantransmit. (Figure 3). LARGER ARC OF CONTACT ARC OF CAN TRANSMIT MORE CONTACT POWER Figure 3. Relationship between power and arc of contact
As well as performing its primary function of transmitting power, a V-belt can be used tochange the speed of the driver output and hence the torque transmitted to the drivenunit. There are three basic alternatives as shown on Figure 4. SPEED RATIO 1:1 SPEED INCREASE SPEED DECREASE DRIVER DRIVEN Figure 4. Alternative arrangements for V-belt drivesThe speed ratio between the two pulleys of a belt drive can be calculated from a simpleformula. driver pulley diameter ( mm )Driven speed (RPM) = x driver speed ( RPM ) driven pulley diameter ( mm )It is generally accepted that V-belt drives are limited to belt speeds between 300 and 3000meters per minute (1000-10,000 feet per minute). If required to operate at higher speeds thendynamic balancing of the pulleys becomes increasingly importantTYPES OF BELTSThere are many types of belts, some of them are commonly used and some other arerarely used. In the following the common types of belts:
1 round belts2. flat belts3. single V-belt4. banded V-belt5. linked V-belt6. timing belt7. V-ribbed belts The common types of beltsBELT TENSION TECHNIQUES1. Belt tension by using slotted holes of the bolts of the motor base2. Motor with slide rail for the whole base3. Pivoted motor base4. Belt tension by using idler pulley
CHECK OF BELT TENSION1 By using human sense and experience ( by hand sensitivity )2 by depress the belt and measuring the deflection3 by using the mechanical belt tension tool4 by measuring the belt elongation after applying tension5 by measuring the belt vibration frequency ( advanced method )
CHAINS FOR POWERTRANSMISSIONChain drives consist of an endless series of chain links which mesh with toothed wheels, calledsprockets. The sprockets are keyed to the shafts of the driving and driven mechanisms.A roller chain has two kinds of links—roller links and pin links—alternately assembledthroughout the chain lengthDimensions for roller-chain identificationChordal action is a serious limiting factor in roller-chain performance. It may be described asthe vibratory motion caused by the rise and fall of the chain as it goes over a small sprocket
Chordal action.Figure shows schematically a roller chain entering a sprocket (A); the line of approach is nottangent to the pitch circle. The chain makes contact below the tangency line, is then lifted tothe tangent line (B),and then is dropped again (C) as sprocket rotation continues. Because of itsfixed-pitch length, the pitch line of the link cuts across the chord between two pitch points onthe sprocket and remains in this position relative to the sprocket until the chain disengages.Principles of OperationChains and sprockets fulfil the same basic function as belts and pulleys in transferringpower between two parallel shafts. Instead of relying on friction, a chain drive is apositive drive in which the links of the chain engage with specially formed teeth on thesprocket.Standard roller chain is made up of alternate roller links and pin links.
The pitch of the chain is determined by the length of the side plates, and the bushingsand pins are press-fitted into the side plates. The pins of a special joining link may belonger and grooved to take spring clips as shown in Figure 11. Figure 9. Chain and Sprocket . Standard roller chain
. Special joining link Pitch, width and roller diameter are the critical dimensions of roller chain.Types and ArrangementsStandard roller chain is available in single and multi-strands form, and the number ofstrands required will depend on then power to be transmitted. Double pitch chains arealso available. They are cheaper, and are suitable for light loads and low speeds.Chain drives are used most commonly as horizontal drives and any slack in the chainresulting from wear, should accumulate on the lower strands as shown in Figure 15.Vertical drives should be arranged so that accumulated slack falls into the drivensprocket rather than away from it, to prevent misengagement.
Figure 14. Double pitch chain RIGHT WRONG Figure 15. In a horizontal drive, slack should accumulate on the lower strandWhere chain tensioners are used they should be used on the side of the chain wherethe slack is expected to accumulate (Figure 17). RIGHT WRONG
Figure shows Accumulated slack in a vertical drive should fall into, rather than away from the driven sprocket Figure 17. Using a chain tensioner DRIVE TYPE Belt Drives Chain Drives 1. Belt drives rely on friction between 1. Chain drive is a positive drive in the belt and pulleys for their which the links of the chain engage operation. with specially formed teeth on the sprocket. 2. needs pulley alignment 2. needs sprockets alignment (more sensitive for misalignment ) 3. needs belt tension adjustment 3. needs chain tension adjustment 4. sensitive to temperature 4. needs lubrication 5. Sensitive to any liquid, oil, dust and 5. Sensitive to dust and environmental environmental conditions. conditions 6. Considerable speeds 6. Limited in the transferred speed ( up to 1350 meters per minute ) 7. Slippage always exists so that it is 7. Accurate speed transfer. inaccurate in speed transfer. 8. usually smooth running 8. usually noisy running 9. cheap in the cost 9. expensive in the cost
GEARS AND GEAR BOXESGear DrivesGear drives are used to transmit power from one machine to another where changes ofspeed, torque, direction of rotation or shaft orientation are required. They may consist ofone or more sets of gears depending on the requirements. In most cases the gears aremounted on shafts supported by an enclosed casing which also contains a lubricant.Principles of OperationA gear is a form of wheel with teeth machined around the outer edge which allow it engagewith similar wheel or rack. The most important features of a gear are the tooth profile orcross-sectional shape, and the number of teeth. In order to understand the geometry ofgears. However, there is a limit to the torque that can be transmitted by friction and so teethare cut into the outer edges of the discs to provide a means of positive engagement as shownin figure 1.The imaginary circles on which the gears are cut are called the pitch circles, and the pitchcircle diameter is the major dimension on which gear geometry is based. The otherimportant dimension is the pressure angle. This is the angel between a tangent to the pitchcircle and the line of contact of two mating teeth as shown in figure 2. The Teeth provide a means of positive engagement
If two gears are to mesh properly they must have the same pressure angle. Standardpressure angles of 14.5 and 20 are used with 20 being the most common. PRESSURE ANGLE LINE OF ACTION PRESSURE ANGLE Fig.2 The pressure angleIn practice, gears are cut to provide running clearance between mating teeth. Thisknown as backlash. Figure 3. Terms used in circular gear geometry
In practice, gears are cut to provide running clearance between mating teeth. This isknown as backlash (figure 4). Figure 4. Gears are cut to provide backlashThe characteristics of mating gears are often described by the term, diametral pitch.This term refers to the ratio of the number of teeth to the pitch circle diameter of thegear and reflects the size and shape of the teeth. Hence two mating gears must alsohave the same diametral pitch as well as the same pressure angle.There are several ways in which diametral pitch can be calculated. Diametral pitch = circular pitch number of teeth Diametral pitch = pitch circle diameter number of teeth 2 Diametral pitch = outside diameter D PZ P : Pitch Circle Z : Number of teeth D : DiameterThe speed relationship between two mating gears depends on the number of teeth oneach gear and can be determined as follows : Speed of driven gear = spead of driver x no. of teeth on driver (RPM) (RPM) no. of teeth on driven
Types and arrangements of GearA gear train consists of one or more gear sets intended to give a specific velocity ratio,or change direction of motion. Gear and gear train types can be grouped based on theirapplication and tooth geometry. Table 1. Gear Types Grouped According to Shaft Arrangement arallel Axes Non-Intersecting (Non- Rotary to Translation Intersecting Axes parallel) Axes Spur Gears Bevel gears: Hypoid gears Rack and Pinion Helical Gears Straight bevel Crossed helical gearsHerringbone or double Zerol bevel Worm gears helical gears Spiral bevelSpur gears (Fig. 5): Spur gears connect parallel shafts, have involute teeth that areparallel to the shafts, and can have either internal or external teeth. Notes: 1. Spur gears are inexpensive to manufacture. 2. They cause no axial thrust between gears. 3. They give lower performance, but may be satisfactory in low speed or simple applications 4. Simple overall design and assembly.
Figure. 5 Spur GearsHelical gears (Fig. 6): Helical gears also connect parallel shafts, but the involute teethare cut at an angle (called the helix angle) to the axis of rotation. Note that two matinghelical gears must have equal helix angle but opposite hand. These are found inautomotive transmissions, and any application requiring high speed rotation and goodperformance. Notes: 1. Helical gears run smoother and more quietly than spurs (due to continuous tooth mating). 2. They have a higher load capacity (teeth have a greater cross section). 3. They are more expensive to manufacture. 4. Helical gears create axial thrust. Figure 6. Helical gearsHerringbone gears (Fig. 7): To avoid axial thrust, two helical gears of opposite handcan be mounted side by side, to cancel resulting thrust forces. These are called doublehelical or herringbone gears
Figure. 7 Herringbone gearsBevel gears (Fig. 8): Bevel gears connect intersecting axes, and come in several types(listed below). For bevel gears, the pitch surface is a cone, (it was a cylinder in spur andhelical gears) and mating spiral gears can be modeled as two cones in rolling contact.Types of bevel gears: 1. Straight bevel: These are like spur gears, the teeth have no helix angle. Straight bevel gears can be a. Miter gears, equal size gears with a 90 degree shaft angle, b. Angular bevel gears, shaft angle other than 90 degrees, or c. Crown gears, one gear is flat, has a pitch angle of 90 degree. 2. Spiral bevel gears(Fig. 8a): Teeth have a spiral angle which gives performance improvements much like helical gears 3. Zerol bevel gears (Fig. 8b): Teeth are crowned, so that tooth contact takes place first at the tooth center. Zerol bevel gears offer performance that is equivalent to that of straight bevel gears and are spiral bevel gears with a spiral angle of 0°. They offer the advantage of low axial thrust over spiral bevel gears.
Figure 8b. Zerol bevel gearsHypoid gears (Fig. 9): Similar to spiral bevel gears, but connect non-parallel shafts thatdo not intersect. The pitch surface of a hypoid gear is a hyperboloid of revolution (ratherthan a cone, the pitch surface in bevel gears), hence the name. Hypoid pinions (thesmaller driving gear) are stronger than spiral bevel pinions because the helix angle ofthe pinion is larger than that of the gear. Hypoid gears are found in auto differentials. Ialso know that a hypoid gear set is used in my NH baler, connecting the flywheel to therear driveshaft. Figure 9. Hypoid gears
Crossed helical gears (Fig. 10): Helical gears that connect skew shafts. The teethhave sliding motion and therefore lower efficiency. One application is connectingdistributer to cam shaft in pre-electronic ignition vehicles. Figure 10. Crossed helical gearsWorm Gears (Fig. 11): The driving gear is called a worm, and typically has 1, 2, or fourteeth. The low number of teeth on the worm can result in a very large velocity ratio.These can also be designed to be non-backdriveable, and can carry high loads.Because of sliding action, efficiency is low. Figure 11. Worm Gears
Rack and Pinion (Fig. 12): These transmit rotary motion (from the pinion) totranslational motion (of the rack). The rack is a gear with infinite radius; its teeth,although flat sided, are involute. The rack and pinion is commonly used in steering unitsand jacks. Figure 12. Rack and PinionGear reduction arrangementsWhatever type of gear is employed, the arrangement may involve one or more pairs ofgears depending on the degree of speed reduction requiredGears are generally made from steel or cast iron and are surface hardened in order toincrease the wear resistance. Figure 13. Gear Reduction
GEAR DRIVES AND SPEED REDUCERSCommon Gear TypesCommon types of gears used in industrial gear drives include spur, helical, double-helical,bevel,spiral bevel, hypoid, zerol, worm, and internal gears
Double helical drivesEpicyclic Gear DrivesIn an epicyclic gear drive, power is transmitted between prime mover and driven machinerythrough multiple paths. The term epicyclic designates a family of designs in which one or moregears move around the circumference of meshing, coaxial gears, which may be fixed or rotatingabout their own axis. Individual gears within an epicyclic drive may be spur, helical, or double-helical.Because of the multiple power paths, an epicyclic gear drive will normally provide thesmallest drive for a given load-carrying capacity. Other advantages include high efficiency, lowinertia for a given duty, high stiffness, and a high torque/power capabilityThe basic elements of an epicyclic drive are a central sunwheel, an internally toothed annulusring,a planet or star carrier, and planet or star wheels. Depending on which of the first three
elements is fixed, three types of epicyclic drives are possible: a planetary gear drive, a stargear drive, or a solar gear drive. In a planetary gear drive the annulus ring is fixed.(A) High-speed gearbox, enclosed. (B) High-speed gearbox, showing internal parts.GEAR-TOOTH WEAR AND FAILUREExperience indicates that the vast majority of gear-tooth wear and failure types may besummed up under nine basic headings in two classifications:Classification A: Surface deterioration1. Wear2. Plastic flow3. Scoring4. Surface fatigue5. Miscellaneous tooth-surface deteriorationsClassification B: Tooth breakage6. Fatigue7. Heavy wear8. Overload9. Cracking
Rippling Slight scoring. COUPLINGSCouplings are the devices used to connect two shafts with a common axis ofrotation. Couplings, no matter what type, all have one thing in common: they needproper alignment. Any coupling that isn‟t aligned won‟t perform properly. No matterhow flexible its center member is, it‟ll wear out. This is the primary point in maintainingcouplings. There are two main types of couplings : 1. Rigid 2. FlexibleRigid couplingsRigid couplings usually require no further maintenance than correct alignment. Oncethey are aligned, corrected sized coupling bolts should be installed and then tightenedto the correct torque for that bolt. Since there are no moving parts, no wear shouldoccur. On inspection, the bolts should be checked to insure that they haven‟t loosened.Flexible couplingsFlexible couplings require more maintenance than do rigid couplings. They should bealigned to the same standards, as misalignment causes rapid wear. Flexible couplingscan be divided into two classes: Mechanical - Flexing Material - Flexing
Mechanical flexible couplings depend on some form of a mechanically flexible element.In this class falls the gear, chain, grid, spindle, and universal joint .These all require theuse of lubrication to prevent wear. If they aren‟t lubricated, they „ll wear excessivelyfast. The lubricant should be clean. If sufficient lubrication is not applied, metal-to-metal contact will occur between contacting metal parts under load and rapid wear willoccur.The following guidelines should be used : 1 Gear – half full of clean lubricant 2 Chain – packed with clean oil 3 Grid – packed with clean grease 4 Spindle – same as gear 5 Universal joint – dependant on applicationMaterial flexible couplings use some form of flexible material between the two couplinghalves to absorb some limited misalignment but this isn‟t a cure-all. The inspectorshould give attention to the flexible member during his inspection. If the member hasbeen hot or is cracked and showing wear, it should be replaced and the alignmentchecked. When the alignment is bad, it flexes the material, heats up, and wears morerapidly than it should.
FLEXIBLE COUPLINGSFOR POWER TRANSMISSIONA flexible coupling is a mechanical device used to connect two axially oriented shafts. Itspurpose is to transmit torque or rotary motion without slip and at the same time compensatefor angular, parallel,and axial misalignment.CAUSES OF COUPLING FAILUREMost failures due to internal faults are the result of improper or poor machining. Anothermajor cause of failure due to internal faults is improper product design. On mechanical-flexingcouplings, the major problem is to provide adequate lubrication between the sliding contactfaces, since lack of a lubricating film between these high pressure surfaces will result in rapidwear. On material-flexing couplings, improper design of the flexing-element section andmethod of attachment to the hubs are the main causes of premature fatigue.Most common causes of failure due to external conditions have to do with improperselection, improper assembly, and excessive misalignmentCoupling SelectionProper selection as to the type of coupling is the first step of good maintenance. A well-chosencoupling will operate with low cross-loading of the connected shafts, have low powerabsorption, induce no harmful vibrations or resonances into the system, and have negligiblemaintenance costs. The primary considerations in selecting the correct type of flexiblecouplings, as well as its size and style, are1. Type of driving and driven equipment2. Torsional characteristics3. Minimum and maximum torque4. Normal and maximum rotating speeds5. Shaft sizes6. Span or distance between shaft ends7. Changes in span due to thermal growth, racking of the bases, or axial movement of theconnected shafts during operation8. Equipment position (horizontal, inclined, or vertical)9. Ambient conditions (dry, wet, corrossion, dust, or grit)10. Bearing locations11. Cost (initial coupling price, installation, maintenance, and replacement).