外文翻译--故障的分析、尺寸的决定以及凸轮的分析和应用-精品.doc
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外文翻译--故障的分析、尺寸的决定以及凸轮的分析和应用-精品.doc
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附录1
FailureAnalysis,DimensionalDeterminationAndAnalysis,ApplicationsOfCams
INTRODUCTION
Itisabsolutelyessentialthatadesignengineerknowhowandwhypartsfailsothatreliablemachinesthatrequireminimummaintenancecanbedesigned.Sometimesafailurecanbeserious,suchaswhenatireblowsoutonanautomobiletravelingathighspeed.Ontheotherhand,afailuremaybenomorethananuisance.Anexampleisthelooseningoftheradiatorhoseinanautomobilecoolingsystem.Theconsequenceofthislatterfailureisusuallythelossofsomeradiatorcoolant,aconditionthatisreadilydetectedandcorrected.
Thetypeofloadapartabsorbsisjustassignificantasthemagnitude.Generallyspeaking,dynamicloadswithdirectionreversalscausegreaterdifficultythanstaticloads,andtherefore,fatiguestrengthmustbeconsidered.Anotherconcerniswhetherthematerialisductileorbrittle.Forexample,brittlematerialsareconsideredtobeunacceptablewherefatigueisinvolved.
Manypeoplemistakinglyinterpretthewordfailuretomeantheactualbreakageofapart.However,adesignengineermustconsiderabroaderunderstandingofwhatappreciabledeformationoccurs.Aductilematerial,howeverwilldeformalargeamountpriortorupture.Excessivedeformation,withoutfracture,maycauseamachinetofailbecausethedeformedpartinterfereswithamovingsecondpart.Therefore,apartfails(evenifithasnotphysicallybroken)wheneveritnolongerfulfillsitsrequiredfunction.Sometimesfailuremaybeduetoabnormalfrictionorvibrationbetweentwomatingparts.Failurealsomaybeduetoaphenomenoncalledcreep,whichistheplasticflowofamaterialunderloadatelevatedtemperatures.Inaddition,theactualshapeofapartmayberesponsibleforfailure.Forexample,stressconcentrationsduetosuddenchangesincontourmustbetakenintoaccount.Evaluationofstressconsiderationsisespeciallyimportantwhentherearedynamicloadswithdirectionreversalsandthematerialisnotveryductile.
Ingeneral,thedesignengineermustconsiderallpossiblemodesoffailure,whichincludethefollowing.
——Stress
——Deformation
——Wear
——Corrosion
——Vibration
——Environmentaldamage
——Looseningoffasteningdevices
Thepartsizesandshapesselectedalsomusttakeintoaccountmanydimensionalfactorsthatproduceexternalloadeffects,suchasgeometricdiscontinuities,residualstressesduetoformingofdesiredcontours,andtheapplicationofinterferencefitjoints.
Camsareamongthemostversatilemechanismsavailable.Acamisasimpletwo-memberdevice.Theinputmemberisthecamitself,whiletheoutputmemberiscalledthefollower.Throughtheuseofcams,asimpleinputmotioncanbemodifiedintoalmostanyconceivableoutputmotionthatisdesired.Someofthecommonapplicationsofcamsare
——Camshaftanddistributorshaftofautomotiveengine
——Productionmachinetools
——Automaticrecordplayers
——Printingmachines
——Automaticwashingmachines
——Automaticdishwashers
Thecontourofhigh-speedcams(camspeedinexcessof1000rpm)mustbedeterminedmathematically.However,thevastmajorityofcamsoperateatlowspeeds(lessthan500rpm)ormedium-speedcamscanbedeterminedgraphicallyusingalarge-scalelayout.Ingeneral,thegreaterthecamspeedandoutputload,thegreatermustbetheprecisionwithwhichthecamcontourismachined.
DESIGNPROPERTIESOFMATERIALS
Thefollowingdesignpropertiesofmaterialsaredefinedastheyrelatetothetensiletest.
Figure2.7
StaticStrength.Thestrengthofapartisthemaximumstressthatthepartcansustainwithoutlosingitsabilitytoperformitsrequiredfunction.Thusthestaticstrengthmaybeconsideredtobeapproximatelyequaltotheproportionallimit,sincenoplasticdeformationtakesplaceandnodamagetheoreticallyisdonetothematerial.
Stiffness.Stiffnessisthedeformation-resistingpropertyofamaterial.Theslopeofthemoduluslineand,hence,themodulusofelasticityaremeasuresofthestiffnessofamaterial.
Resilience.Resilienceisthepropertyofamaterialthatpermitsittoabsorbenergywithoutpermanentdeformation.Theamountofenergyabsorbedisrepresentedbytheareaunderneaththestress-straindiagramwithintheelasticregion.
Toughness.Resilienceandtoughnessaresimilarproperties.However,toughnessistheabilitytoabsorbenergywithoutrupture.Thustoughnessisrepresentedbythetotalareaunderneaththestress-straindiagram,asdepictedinFigure2.8b.Obviously,thetoughnessandresilienceofbrittlematerialsareverylowandareapproximatelyequal.
Brittleness.Abrittlematerialisonethatrupturesbeforeanyappreciableplasticdeformationtakesplace.Brittlematerialsaregenerallyconsideredundesirableformachinecomponentsbecausetheyareunabletoyieldlocallyatlocationsofhighstressbecauseofgeometricstressraiserssuchasshoulders,hole
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