凸輪設(shè)計外文翻譯參考文獻

1、凸輪設(shè)計外文翻譯參考文獻凸輪設(shè)計外文翻譯參考文獻(文檔含中英文對照即英文原文和中文翻譯)Failure Analysis，Dimensional Determination AndAnalysis，Applications Of CamsINTRODUCTIONIt is absolutely essential that a design engineer know how and why parts fail so that reliable machines that require minimum maintenance can be designedSometimes a failu

2、re can be serious，such as when a tire blows out on an automobile traveling at high speedOn the other hand，a failure may be no more than a nuisanceAn example is the loosening of the radiator hose in an automobile cooling systemThe consequence of this latter failure is usually the loss of some radiato

3、r coolant，a condition that is readily detected and corrected The type of load a part absorbs is just as significant as the magnitudeGenerally凸輪設(shè)計外文翻譯參考文獻speaking，dynamic loads with direction reversals cause greater difficulty than static loads，and therefore，fatigue strength must be consideredAnother

4、 concern is whether the material is ductile or brittleFor example，brittle materials are considered to be unacceptable where fatigue is involvedMany people mistakingly interpret the word failure to mean the actual breakage of a partHowever，a design engineer must consider a broader understanding of wh

5、at appreciable deformation occursA ductile material，however will deform a large amount prior to ruptureExcessive deformation，without fracture，may cause a machine to fail because the deformed part interferes with a moving second partTherefore，a part fails(even if it has not physically broken)whenever

6、 it no longer fulfills its required functionSometimes failure may be due to abnormal friction or vibration between two mating partsFailure also may be due to a phenomenon called creep，which is the plastic flow of a material under load at elevated temperaturesIn addition，the actual shape of a part ma

7、y be responsible for failureFor example，stress concentrations due to sudden changes in contour must be taken into accountEvaluation of stress considerations is especially important when there are dynamic loads with direction reversals and the material is not very ductileIn general，the design enginee

8、r must consider all possible modes of failure，which include the followingStressDeformationWearCorrosionVibrationEnvironmental damageLoosening of fastening devicesThe part sizes and shapes selected also must take into account many dimensional factors that produce external load effects，such as geometr

9、ic discontinuities，residual stresses due to forming of desired contours，and the application of interference fit jointsCams are among the most versatile mechanisms availableA cam is a simple two-member deviceThe input member is the cam itself，while the output member is called the followerThrough the

10、use of cams，a simple input motion can be modified into almost any conceivable output motion that is desiredSome of the common applications of cams areCamshaft and distributor shaft of automotive engine凸輪設(shè)計外文翻譯參考文獻Production machine toolsAutomatic record playersPrinting machinesAutomatic washing mach

11、inesAutomatic dishwashersThe contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematicallyHowever，the vast majority of cams operate at low speeds(less than 500 rpm) or medium-speed cams can be determined graphically using a large-scale layoutIn general，the greater th

12、e cam speed and output load，the greater must be the precision with which the cam contour is machinedDESIGN PROPERTIES OF MATERIALSThe following design properties of materials are defined as they relate to the tensile testFigure 2.7Static Strength The strength of a part is the maximum stress that the

13、 part can sustain without losing its ability to perform its required functionThus the static strength may be considered to be approximately equal to the proportional limit，since no plastic deformation takes place and no damage theoretically is done to the materialStiffness Stiffness is the deformati

14、on-resisting property of a materialThe slope of the modulus line and，hence，the modulus of elasticity are measures of the stiffness of a materialResilience Resilience is the property of a material that permits it to absorb energy without permanent deformationThe amount of energy absorbed is represent

15、ed by the area underneath the stress-strain diagram within the elastic regionToughness Resilience and toughness are similar propertiesHowever，toughness is the ability to absorb energy without ruptureThus toughness is represented by the total area underneath the stress-strain diagram， as depicted in

16、Figure 28bObviously，the toughness and resilience of brittle materials are very low and are approximately equal Brittleness A brittle material is one that ruptures before any appreciable plastic deformation takes placeBrittle materials are generally considered undesirable for machine凸輪設(shè)計外文翻譯參考文獻compo

17、nents because they are unable to yield locally at locations of high stress because of geometric stress raisers such as shoulders，holes，notches，or keywaysDuctility A ductility material exhibits a large amount of plastic deformation prior to ruptureDuctility is measured by the percent of area and perc

18、ent elongation of a part loaded to ruptureA 5%elongation at rupture is considered to be the dividing line between ductile and brittle materialsMalleability Malleability is essentially a measure of the compressive ductility of a material and，as such，is an important characteristic of metals that are t

19、o be rolled into sheetsHardness The hardness of a material is its ability to resist indentation or scratchingGenerally speaking，the harder a material，the more brittle it is and，hence，the less resilientAlso，the ultimate strength of a material is roughly proportional to its hardnessMachinability Machi

20、nability is a measure of the relative ease with which a material can be machinedIn general，the harder the material，the more difficult it is to machineFigure 2.8COMPRESSION AND SHEAR STATIC STRENGTHIn addition to the tensile tests，there are other types of static load testing that provide valuable inf

21、ormationCompression Testing Most ductile materials have approximately the same properties in compression as in tensionThe ultimate strength，however，can not be evaluated for compressionAs a ductile specimen flows plastically in compression，the material bulges out，but there is no physical rupture as i

22、s the case in tensionTherefore，a ductile material fails in compression as a result of deformation，not stressShear Testing Shafts，bolts，rivets，and welds are located in such a way that shear stresses are producedA plot of the tensile testThe ultimate shearing strength is defined as the stress at which

23、 failure occursThe ultimate strength in shear，however，does not equal the ultimate strength in tensionFor example，in the case of steel，the ultimate shear凸輪設(shè)計外文翻譯參考文獻strength is approximately 75% of the ultimate strength in tensionThis difference must be taken into account when shear stresses are enco

24、untered in machine componentsDYNAMIC LOADSAn applied force that does not vary in any manner is called a static or steady loadIt is also common practice to consider applied forces that seldom vary to be static loadsThe force that is gradually applied during a tensile test is therefore a static loadOn

25、 the other hand，forces that vary frequently in magnitude and direction are called dynamic loadsDynamic loads can be subdivided to the following three categoriesVarying Load With varying loads，the magnitude changes，but the direction does notFor example，the load may produce high and low tensile stress

26、es but no compressive stressesReversing Load In this case，both the magnitude and direction changeThese load reversals produce alternately varying tensile and compressive stresses that are commonly referred to as stress reversalsShock Load This type of load is due to impactOne example is an elevator

27、dropping on a nest of springs at the bottom of a chuteThe resulting maximum spring force can be many times greater than the weight of the elevator，The same type of shock load occurs in automobile springs when a tire hits a bump or hole in the roadFATIGUE FAILURE-THE ENDURANCE LIMIT DIAGRAMThe test s

28、pecimen in Figure 2.10a，after a given number of stress reversals will experience a crack at the outer surface where the stress is greatestThe initial crack starts where the stress exceeds the strength of the grain on which it actsThis is usually where there is a small surface defect，such as a materi

29、al flaw or a tiny scratchAs the number of cycles increases，the initial crack begins to propagate into a continuous series of cracks all around the periphery of the shaftThe conception of the initial crack is itself a stress concentration that accelerates the crack propagation phenomenonOnce the enti

30、re periphery becomes cracked，the cracks start to move toward the center of the shaftFinally，when the remaining solid inner area becomes small enough，the stress exceeds the ultimate strength and the shaft suddenly breaksInspection of the break reveals a very interesting pattern，as shown in Figure 2.1

31、3The outer annular area is relatively smooth because mating cracked surfaces had rubbed against each otherHowever，the center portion is rough，indicating a sudden rupture similar to that experienced with the fracture of brittle materialsThis brings out an interesting factWhen actual machine parts fai

32、l as a result of static loads，they normally deform appreciably because of the ductility of the material凸輪設(shè)計外文翻譯參考文獻Figure 2.13Thus many static failures can be avoided by making frequent visual observations and replacing all deformed partsHowever，fatigue failures give to warningFatigue fail mated tha

33、t over 90% of broken automobile parts have failed through fatigueThe fatigue strength of a material is its ability to resist the propagation of cracks under stress reversalsEndurance limit is a parameter used to measure the fatigue strength of a materialBy definition，the endurance limit is the stres

34、s value below which an infinite number of cycles will not cause failureLet us return our attention to the fatigue testing machine in Figure 2.9The test is run as follows：A small weight is inserted and the motor is turned onAt failure of the test specimen，the counter registers the number of cycles N，

35、and the corresponding maximum bending stress is calculated from Equation 2.5The broken specimen is then replaced by an identical one，and an additional weight is inserted to increase the loadA new value of stress is calculated，and the procedure is repeated until failure requires only one complete cyc

36、leA plot is then made of stress versus number of cycles to failureFigure 2.14a shows the plot，which is called the endurance limit or S-N curveSince it would take forever to achieve an infinite number of cycles，1 million cycles is used as a referenceHence the endurance limit can be found from Figure

37、2.14a by noting that it is the stress level below which the material can sustain 1 million cycles without failureThe relationship depicted in Figure 2.14 is typical for steel，because the curve becomes horizontal as N approaches a very large numberThus the endurance limit equals the stress level wher

38、e the curve approaches a horizontal tangentOwing to the large number of cycles involved，N is usually plotted on a logarithmic scale，as shown in Figure 2.14bWhen this is done，the endurance limit value can be readily detected by the horizontal straight lineFor steel，the endurance limit equals approxim

39、ately 50% of the ultimate strengthHowever，if the surface finish is not of polished equality，the value of the endurance limit will be lowerFor example，for steel parts with a machined surface finish of 63 microinches ( in)，the percentage drops to about 40%For rough surfaces (300inor greater)，the perce

40、ntage may be as low as 25%The most common type of fatigue is that due to bendingThe next most frequent is torsion failure，whereas fatigue due to axial loads occurs very seldomSpring materials are凸輪設(shè)計外文翻譯參考文獻usually tested by applying variable shear stresses that alternate from zero to a maximum valu

41、e，simulating the actual stress patternsIn the case of some nonferrous metals，the fatigue curve does not level off as the number of cycles becomes very largeThis continuing toward zero stress means that a large number of stress reversals will cause failure regardless of how small the value of stress

42、isSuch a material is said to have no endurance limitFor most nonferrous metals having an endurance limit，the value is about 25% of the ultimate strengthEFFECTS OF TEMPERATURE ON YIELD STRENGTH AND MODULUS OF ELASTICITYGenerally speaking，when stating that a material possesses specified values of prop

43、erties such as modulus of elasticity and yield strength，it is implied that these values exist at room temperatureAt low or elevated temperatures，the properties of materials may be drastically differentFor example，many metals are more brittle at low temperaturesIn addition，the modulus of elasticity a

44、nd yield strength deteriorate as the temperature increasesFigure 2.23 shows that the yield strength for mild steel is reduced by about 70% in going from room temperature to 1000oFFigure 2.24 shows the reduction in the modulus of elasticity E for mild steel as the temperature increasesAs can be seen

45、from the graph，a 30% reduction in modulus of elasticity occurs in going from room temperature to 1000oFIn this figure，we also can see that a part loaded below the proportional limit at room temperature can be permanently deformed under the same load at elevated temperaturesFigure 2.24CREEP: A PLASTI

46、C PHENOMENONTemperature effects bring us to a phenomenon called creep，which is the increasing plastic deformation of a part under constant load as a function of timeCreep also occurs at room temperature，but the process is so slow that it rarely becomes significant during the expected life of the tem

47、perature is raised to 300oC or more，the increasing plastic凸輪設(shè)計外文翻譯參考文獻deformation can become significant within a relatively short period of timeThe 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 actu

48、al part operating conditionsDuring the test，the plastic strain is monitored for given material at specified temperaturesSince creep is a plastic deformation phenomenon，the dimensions of a part experiencing creep are permanently alteredThus，if a part operates with tight clearances，the design engineer

49、 must accurately predict the amount of creep that will occur during the life of the machineOtherwise，problems such binding or interference can occurCreep also can be a problem in the case where bolts are used to clamp tow parts together at elevated temperaturesThe bolts，under tension，will creep as a

50、 function of timeSince the deformation is plastic，loss of clamping force will result in an undesirable loosening of the bolted jointThe extent of this particular phenomenon，called relaxation，can be determined by running appropriate creep strength testsFigure 2.25 shows typical creep curves for three

51、 samples of a mild steel part under a constant tensile loadNotice that for the high-temperature case the creep tends to accelerate until the part failsThe time line in the graph (the x-axis) may represent a period of 10 years，the anticipated life of the productFigure 2.25SUMMARYThe machine designer

52、must understand the purpose of the static tensile strength testThis test determines a number of mechanical properties of metals that are used in design equationsSuch terms as modulus of elasticity，proportional limit，yield strength，ultimate strength，resilience，and ductility define properties that can

53、 be determined from the tensile testDynamic loads are those which vary in magnitude and direction and may require an investigation of the machine parts resistance to failureStress reversals may require that the allowable design stress be based on the endurance limit of the material rather than on th

54、e yield strength or ultimate strengthStress concentration occurs at locations where a machine part changes size，such as a hole in a flat plate or a sudden change in width of a flat plate or a groove or fillet on a circular shaftNote that for the case of a hole in a flat or bar，the value of the maxim

55、um凸輪設(shè)計外文翻譯參考文獻stress becomes much larger in relation to the average stress as the size of the hole decreasesMethods of reducing the effect of stress concentration usually involve making the shape change more gradualMachine parts are designed to operate at some allowable stress below the yield streng

56、th or ultimate strengthThis approach is used to take care of such unknown factors as material property variations and residual stresses produced during manufacture and the fact that the equations used may be approximate rather that exactThe factor of safety is applied to the yield strength or the ul

57、timate strength to determine the allowable stressTemperature can affect the mechanical properties of metalsIncreases in temperature may cause a metal to expand and creep and may reduce its yield strength and its modulus of elasticityIf most metals are not allowed to expand or contract with a change

58、in temperature，then stresses are set up that may be added to the stresses from the loadThis phenomenon is useful in assembling parts by means of interference fitsA hub or ring has an inside diameter slightly smaller than the mating shaft or postThe hub is then heated so that it expands enough to slip over the shaftWhen it cools，it exerts a pressure on the shaft resulting in a strong frictional force that prevents looseningTYPES OF CAM CONFIGURATIONSPlate CamsThis type of cam is the most popular type