外文翻译--故障的分析、尺寸的决定以及凸轮的分析和应用.doc

附录附录1FailureAnalysis，DimensionalDeterminationAndAnalysis，ApplicationsOfCamsINTRODUCTIONItisabsolutelyessentialthatadesignengineerknowhowandwhypartsfailsothatreliablemachinesthatrequireminimummaintenancecanbedesignedSometimesafailurecanbeserious，suchaswhenatireblowsoutonanautomobiletravelingathighspeedOntheotherhand，afailuremaybenomorethananuisanceAnexampleisthelooseningoftheradiatorhoseinanautomobilecoolingsystemTheconsequenceofthislatterfailureisusuallythelossofsomeradiatorcoolant，aconditionthatisreadilydetectedandcorrectedThetypeofloadapartabsorbsisjustassignificantasthemagnitudeGenerallyspeaking，dynamicloadswithdirectionreversalscausegreaterdifficultythanstaticloads，andtherefore，fatiguestrengthmustbeconsideredAnotherconcerniswhetherthematerialisductileorbrittleForexample，brittlematerialsareconsideredtobeunacceptablewherefatigueisinvolvedManypeoplemistakinglyinterpretthewordfailuretomeantheactualbreakageofapartHowever，adesignengineermustconsiderabroaderunderstandingofwhatappreciabledeformationoccursAductilematerial，howeverwilldeformalargeamountpriortoruptureExcessivedeformation，withoutfracture，maycauseamachinetofailbecausethedeformedpartinterfereswithamovingsecondpartTherefore，apartfails(evenifithasnotphysicallybroken)wheneveritnolongerfulfillsitsrequiredfunctionSometimesfailuremaybeduetoabnormalfrictionorvibrationbetweentwomatingpartsFailurealsomaybeduetoaphenomenoncalledcreep，whichistheplasticflowofamaterialunderloadatelevatedtemperaturesInaddition，theactualshapeofapartmayberesponsibleforfailureForexample，stressconcentrationsduetosuddenchangesincontourmustbetakenintoaccountEvaluationofstressconsiderationsisespeciallyimportantwhentherearedynamicloadswithdirectionreversalsandthematerialisnotveryductileIngeneral，thedesignengineermustconsiderallpossiblemodesoffailure，whichincludethefollowingStressDeformationWearCorrosionVibrationEnvironmentaldamageLooseningoffasteningdevicesThepartsizesandshapesselectedalsomusttakeintoaccountmanydimensionalfactorsthatproduceexternalloadeffects，suchasgeometricdiscontinuities，residualstressesduetoformingofdesiredcontours，andtheapplicationofinterferencefitjointsCamsareamongthemostversatilemechanismsavailableAcamisasimpletwo-memberdeviceTheinputmemberisthecamitself，whiletheoutputmemberiscalledthefollowerThroughtheuseofcams，asimpleinputmotioncanbemodifiedintoalmostanyconceivableoutputmotionthatisdesiredSomeofthecommonapplicationsofcamsareCamshaftanddistributorshaftofautomotiveengineProductionmachinetoolsAutomaticrecordplayersPrintingmachinesAutomaticwashingmachinesAutomaticdishwashersThecontourofhigh-speedcams(camspeedinexcessof1000rpm)mustbedeterminedmathematicallyHowever，thevastmajorityofcamsoperateatlowspeeds(lessthan500rpm)ormedium-speedcamscanbedeterminedgraphicallyusingalarge-scalelayoutIngeneral，thegreaterthecamspeedandoutputload，thegreatermustbetheprecisionwithwhichthecamcontourismachinedDESIGNPROPERTIESOFMATERIALSThefollowingdesignpropertiesofmaterialsaredefinedastheyrelatetothetensiletestFigure2.7StaticStrengthThestrengthofapartisthemaximumstressthatthepartcansustainwithoutlosingitsabilitytoperformitsrequiredfunctionThusthestaticstrengthmaybeconsideredtobeapproximatelyequaltotheproportionallimit，sincenoplasticdeformationtakesplaceandnodamagetheoreticallyisdonetothematerialStiffnessStiffnessisthedeformation-resistingpropertyofamaterialTheslopeofthemoduluslineand，hence，themodulusofelasticityaremeasuresofthestiffnessofamaterialResilienceResilienceisthepropertyofamaterialthatpermitsittoabsorbenergywithoutpermanentdeformationTheamountofenergyabsorbedisrepresentedbytheareaunderneaththestress-straindiagramwithintheelasticregionToughnessResilienceandtoughnessaresimilarpropertiesHowever，toughnessistheabilitytoabsorbenergywithoutruptureThustoughnessisrepresentedbythetotalareaunderneaththestress-straindiagram，asdepictedinFigure28bObviously，thetoughnessandresilienceofbrittlematerialsareverylowandareapproximatelyequalBrittlenessAbrittlematerialisonethatrupturesbeforeanyappreciableplasticdeformationtakesplaceBrittlematerialsaregenerallyconsideredundesirableformachinecomponentsbecausetheyareunabletoyieldlocallyatlocationsofhighstressbecauseofgeometricstressraiserssuchasshoulders，holes，notches，orkeywaysDuctilityAductilitymaterialexhibitsalargeamountofplasticdeformationpriortoruptureDuctilityismeasuredbythepercentofareaandpercentelongationofapartloadedtoruptureA5%elongationatruptureisconsideredtobethedividinglinebetweenductileandbrittlematerialsMalleabilityMalleabilityisessentiallyameasureofthecompressiveductilityofamaterialand，assuch，isanimportantcharacteristicofmetalsthataretoberolledintosheetsFigure2.8HardnessThehardnessofamaterialisitsabilitytoresistindentationorscratchingGenerallyspeaking，theharderamaterial，themorebrittleitisand，hence，thelessresilientAlso，theultimatestrengthofamaterialisroughlyproportionaltoitshardnessMachinabilityMachinabilityisameasureoftherelativeeasewithwhichamaterialcanbemachinedIngeneral，theharderthematerial，themoredifficultitistomachineCOMPRESSIONANDSHEARSTATICSTRENGTHInadditiontothetensiletests，thereareothertypesofstaticloadtestingthatprovidevaluableinformationCompressionTestingMostductilematerialshaveapproximatelythesamepropertiesincompressionasintensionTheultimatestrength，however，cannotbeevaluatedforcompressionAsaductilespecimenflowsplasticallyincompression，thematerialbulgesout，butthereisnophysicalruptureasisthecaseintensionTherefore，aductilematerialfailsincompressionasaresultofdeformation，notstressShearTestingShafts，bolts，rivets，andweldsarelocatedinsuchawaythatshearstressesareproducedAplotofthetensiletestTheultimateshearingstrengthisdefinedasthestressatwhichfailureoccursTheultimatestrengthinshear，however，doesnotequaltheultimatestrengthintensionForexample，inthecaseofsteel，theultimateshearstrengthisapproximately75%oftheultimatestrengthintensionThisdifferencemustbetakenintoaccountwhenshearstressesareencounteredinmachinecomponentsDYNAMICLOADSAnappliedforcethatdoesnotvaryinanymanneriscalledastaticorsteadyloadItisalsocommonpracticetoconsiderappliedforcesthatseldomvarytobestaticloadsTheforcethatisgraduallyappliedduringatensiletestisthereforeastaticloadOntheotherhand，forcesthatvaryfrequentlyinmagnitudeanddirectionarecalleddynamicloadsDynamicloadscanbesubdividedtothefollowingthreecategoriesVaryingLoadWithvaryingloads，themagnitudechanges，butthe