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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.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.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

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