外文翻译.docx

1、外文翻译Failure Analysis，Dimensional Determination And Analysis，Applications Of Cams故障的分析、尺寸的决定以及凸轮的分析和应用It 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 failure can be serious，such as when

2、 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 radiator coolant，a condition that is

3、readily detected and correctedThe type of load a part absorbs is just as significant as the magnitudeGenerally speaking，dynamic loads with direction reversals cause greater difficulty than static loads，and therefore，fatigue strength must be consideredAnother concern is whether the material is ductil

4、e 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 what appreciable deformation occursA ductile

5、 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 it no longer fulfills its required functi

6、onSometimes 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 may be responsible for failureFor example，st

7、ress 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 engineer must consider all possible modes of fail

8、ure，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 geometric discontinuities，residual stresses due t

9、o 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 use of cams，a simple input motion can be m

10、odified into almost any conceivable output motion that is desiredSome of the common applications of cams areCamshaft and distributor shaft of automotive engineProduction machine toolsAutomatic record playersAutomatic washing machinesAutomatic dishwashersThe contour of high-speed cams (cam speed in e

11、xcess 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 the cam speed and output load，the greater must be the precision with whic

12、h the cam contour is machinedDESIGN PROPERTIES OF MATERIALSThe following design properties of materials are defined as they relate to the tensile testFigure 2.7Static StrengthThe strength of a part is the maximum stress that the part can sustain without losing its ability to perform its required fun

13、ctionThus 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 materialStiffnessStiffness is the deformation-resisting property of a materialThe slope of the modulus line and，henc

14、e，the modulus of elasticity are measures of the stiffness of a materialResilienceResilience is the property of a material that permits it to absorb energy without permanent deformationThe amount of energy absorbed is represented by the area underneath the stress-strain diagram within the elastic reg

15、ionToughnessResilience 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 Figure 28bObviously，the toughness and resilience of brittle materials are v

16、ery low and are approximately equalBrittlenessA brittle material is one that ruptures before any appreciable plastic deformation takes placeBrittle materials are generally considered undesirable for machine components because they are unable to yield locally at locations of high stress because of ge

17、ometric stress raisers such as shoulders，holes，notches，or keywaysDuctilityA ductility material exhibits a large amount of plastic deformation prior to ruptureDuctility is measured by the percent of area and percent elongation of a part loaded to ruptureA 5%elongation at rupture is considered to be t

18、he dividing line between ductile and brittle materialsMalleabilityMalleability 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 sheetsFigure 2.8HardnessThe hardness of a material is its ability to resi

19、st 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 hardnessMachinabilityMachinability is a measure of the relative ease with which a material can be machinedIn

20、 general，the harder the material，the more difficult it is to machineCOMPRESSION AND SHEAR STATIC STRENGTHIn addition to the tensile tests，there are other types of static load testing that provide valuable informationCompression TestingMost ductile materials have approximately the same properties in

21、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 is the case in tensionTherefore，a ductile material fails in compression as a result of deforma

22、tion，not stressShear TestingShafts，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 failure occursThe ultimate strength in shear，however，does not equal the ultimate strength in

23、tensionFor example，in the case of steel，the ultimate shear strength is approximately 75% of the ultimate strength in tensionThis difference must be taken into account when shear stresses are encountered in machine componentsDYNAMIC LOADSAn applied force that does not vary in any manner is called a s

24、tatic 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 the other hand，forces that vary frequently in magnitude and direction are called dynamic loadsDynamic lo

25、ads can be subdivided to the following three categoriesVarying LoadWith varying loads，the magnitude changes，but the direction does notFor example，the load may produce high and low tensile stresses but no compressive stressesReversing LoadIn this case，both the magnitude and direction changeThese load

26、 reversals produce alternately varying tensile and compressive stresses that are commonly referred to as stress reversalsShock LoadThis type of load is due to impactOne example is an elevator dropping on a nest of springs at the bottom of a chuteThe resulting maximum spring force can be many times g

27、reater 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 specimen in Figure 2.10a，after a given number of stress reversals will experience a crack at the outer surfac

28、e 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 material flaw or a tiny scratchAs the number of cycles increases，the initial crack begins to propagate into a cont

29、inuous 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 entire periphery becomes cracked，the cracks start to move toward the center of the shaftFinally，when the remaini

30、ng 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 2.13The outer annular area is relatively smooth because mating cracked surfaces had rubbed against each otherHo

31、wever，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 fail as a result of static loads，they normally deform appreciably because of the ductility of the materialFigur

32、e 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 that 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，th