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，ApplicationsOfCamsINTRODUCTIONItisabsolutelyessentialthatadesignengineerknowhowandwhypartsfailsothatreliablemachinesthatrequireminimummaintenancecanbedesigned．Sometimesafailurecanbeserious，suchaswhenatireblowsoutonanautomobiletravelingathighspeed．Ontheotherhand，afailuremaybenomorethananuisance．Anexampleisthelooseningoftheradiatorhoseinanautomobilecoolingsystem．Theconsequenceofthislatterfailureisusuallythelossofsomeradiatorcoolant，aconditionthatisreadilydetectedandcorrected．Thetypeofloadapartabsorbsisjustassignificantasthemagnitude．Generallyspeaking，dynamicloadswithdirectionreversalscausegreaterdifficultythanstaticloads，andtherefore，fatiguestrengthmustbeconsidered．Anotherconcerniswhetherthematerialisductileorbrittle．Forexample，brittlematerialsareconsideredtobeunacceptablewherefatigueisinvolved．Manypeoplemistakinglyinterpretthewordfailuretomeantheactualbreakageofapart．However，adesignengineermustconsiderabroaderunderstandingofwhatappreciabledeformationoccurs．Aductilematerial，howeverwilldeformalargeamountpriortorupture．Excessivedeformation，withoutfracture，maycauseamachinetofailbecausethedeformedpartinterfereswithamovingsecondpart．Therefore，apartfailsevenifithasnotphysicallybrokenwheneveritnolongerfulfillsitsrequiredfunction．Sometimesfailuremaybeduetoabnormalfrictionorvibrationbetweentwomatingparts．Failurealsomaybeduetoaphenomenoncalledcreep，whichistheplasticflowofamaterialunderloadatelevatedtemperatures．Inaddition，theactualshapeofapartmayberesponsibleforfailure．Forexample，stressconcentrationsduetosuddenchangesincontourmustbetakenintoaccount．Evaluationofstressconsiderationsisespeciallyimportantwhentherearedynamicloadswithdirectionreversalsandthematerialisnotveryductile．Ingeneral，thedesignengineermustconsiderallpossiblemodesoffailure，whichincludethefollowing．StressDeformationWearCorrosionVibrationEnvironmentaldamageLooseningoffasteningdevicesThepartsizesandshapesselectedalsomusttakeintoaccountmanydimensionalfactorsthatproduceexternalloadeffects，suchasgeometricdiscontinuities，residualstressesduetoformingofdesiredcontours，andtheapplicationofinterferencefitjoints．Camsareamongthemostversatilemechanismsavailable．Acamisasimpletwomemberdevice．Theinputmemberisthecamitself，whiletheoutputmemberiscalledthefollower．Throughtheuseofcams，asimpleinputmotioncanbemodifiedintoalmostanyconceivableoutputmotionthatisdesired．SomeofthecommonapplicationsofcamsareCamshaftanddistributorshaftofautomotiveengineProductionmachinetoolsAutomaticrecordplayersPrintingmachinesAutomaticwashingmachinesAutomaticdishwashersThecontourofhighspeedcamscamspeedinexcessof1000rpmmustbedeterminedmathematically．However，thevastmajorityofcamsoperateatlowspeedslessthan500rpmormediumspeedcamscanbedeterminedgraphicallyusingalargescalelayout．Ingeneral，thegreaterthecamspeedandoutputload，thegreatermustbetheprecisionwithwhichthecamcontourismachined．DESIGNPROPERTIESOFMATERIALSThefollowingdesignpropertiesofmaterialsaredefinedastheyrelatetothetensiletest．Figure2.7StaticStrength．Thestrengthofapartisthemaximumstressthatthepartcansustainwithoutlosingitsabilitytoperformitsrequiredfunction．Thusthestaticstrengthmaybeconsideredtobeapproximatelyequaltotheproportionallimit，sincenoplasticdeformationtakesplaceandnodamagetheoreticallyisdonetothematerial．Stiffness．Stiffnessisthedeformationresistingpropertyofamaterial．Theslopeofthemoduluslineand，hence，themodulusofelasticityaremeasuresofthestiffnessofamaterial．Resilience．Resilienceisthepropertyofamaterialthatpermitsittoabsorbenergywithoutpermanentdeformation．Theamountofenergyabsorbedisrepresentedbytheareaunderneaththestressstraindiagramwithintheelasticregion．Toughness．Resilienceandtoughnessaresimilarproperties．However，toughnessistheabilitytoabsorbenergywithoutrupture．Thustoughnessisrepresentedbythetotalareaunderneaththestressstraindiagram，asdepictedinFigure2．8b．Obviously，thetoughnessandresilienceofbrittlematerialsareverylowandareapproximatelyequal．Brittleness．Abrittlematerialisonethatrupturesbeforeanyappreciableplasticdeformationtakesplace．Brittlematerialsaregenerallyconsideredundesirableformachinecomponentsbecausetheyareunabletoyieldlocallyatlocationsofhighstressbecauseofgeometricstressraiserssuchasshoulders，holes，notches，orkeyways．Ductility．Aductilitymaterialexhibitsalargeamountofplasticdeformationpriortorupture．Ductilityismeasuredbythepercentofareaandpercentelongationofapartloadedtorupture．A5elongationatruptureisconsideredtobethedividinglinebetweenductileandbrittlematerials．Malleability．Malleabilityisessentiallyameasureofthecompressiveductilityofamaterialand，assuch，isanimportantcharacteristicofmetalsthataretoberolledintosheets．Figure2.8Hardness．Thehardnessofamaterialisitsabilitytoresistindentationorscratching．Generallyspeaking，theharderamaterial，themorebrittleitisand，hence，thelessresilient．Also，theultimatestrengthofamaterialisroughlyproportionaltoitshardness．Machinability．Machinabilityisameasureoftherelativeeasewithwhichamaterialcanbemachined．Ingeneral，theharderthematerial，themoredifficultitistomachine．COMPRESSIONANDSHEARSTATICSTRENGTHInadditiontothetensiletests，thereareothertypesofstaticloadtestingthatprovidevaluableinformation．CompressionTesting．Mostductilematerialshaveapproximatelythesamepropertiesincompressionasintension．Theultimatestrength，however，cannotbeevaluatedforcompression．Asaductilespecimenflowsplasticallyincompression，thematerialbulgesout，butthereisnophysicalruptureasisthecaseintension．Therefore，aductilematerialfailsincompressionasaresultofdeformation，notstress．ShearTesting．Shafts，bolts，rivets，andweldsarelocatedinsuchawaythatshearstressesareproduced．Aplotofthetensiletest．Theultimateshearingstrengthisdefinedasthestressatwhichfailureoccurs．Theultimatestrengthinshear，however，doesnotequaltheultimatestrengthintension．Forexample，inthecaseofsteel，theultimateshearstrengthisapproximately75oftheultimatestrengthintension．Thisdifferencemustbetakenintoaccountwhenshearstressesareencounteredinmachinecomponents．DYNAMICLOADSAnappliedforcethatdoesnotvaryinanymanneriscalledastaticorsteadyload．Itisalsocommonpracticetoconsiderappliedforcesthatseldomvarytobestaticloads．Theforcethatisgraduallyappliedduringatensiletestisthereforeastaticload．Ontheotherhand，forcesthatvaryfrequentlyinmagnitudeanddirectionarecalleddynamicloads．Dynamicloadscanbesubdividedtothefollowingthreecategories．VaryingLoad．Withvaryingloads，themagnitudechanges，butthe