Failure AnalysisDimensional Determination And AnalysisApplications Of Cams.docx

Failure AnalysisDimensional Determination And AnalysisApplications Of Cams.docx

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FailureAnalysisDimensionalDeterminationAndAnalysisApplicationsOfCams

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，holes，notches，orkeyways．

Ductility．Aductilitymaterialexhibitsalargeamountofplasticdeformationpriortorupture．Ductilityismeasuredbythepercentofareaandpercentelongationofapartloadedtorupture．A5%elongationatruptureisconsideredtobethedividinglinebetweenductileandbrittlematerials．

Malleability．Malleabilityisessentiallyameasureofthecompressiveductilityofamaterialand，assuch，isanimportantcharacteristicofmetalsthataretoberolledintosheets．

Figure2.8

Hardness．Thehardnessofamaterialisitsabilitytoresistindentationorscratching．Generallyspeaking，theharderamaterial，themorebrittleitisand，hence，thelessresilient．Also，theultimatestrengthofamaterialisroughlyproportionaltoitshardness．

Machinability．Machinabilityisameasureoftherelativeeasewithwhichamaterialcanbemachined．Ingeneral，theharderthematerial，themoredifficultitistomachine．

COMPRESSIONANDSHEARSTATICSTRENGTH

Inadditiontothetensiletests，thereareothertypesofstaticloadtestingthatprovidevaluableinformation．

CompressionTesting．Mostductilematerialshaveapproximatelythesamepropertiesincompressionasintension．Theultimatestrength，however，cannotbeevaluatedforcompression．Asaductilespecimenflowsplasticallyincompression，thematerialbulgesout，butthereisnophysicalruptureasisthecaseintension．Therefore，aductilematerialfailsincompressionasaresultofdeformation，notstress．

ShearTesting．Shafts，bolts，rivets，andweldsarelocatedinsuchawaythatshearstressesareproduced．Aplotofthetensiletest．Theultimateshearingstrengthisdefinedasthestressatwhichfailureoccurs．Theultimatestrengthinshear，however，doesnotequaltheultimatestrengthintension．Forexample，inthecaseofsteel，theultimateshearstrengthisapproximately75%oftheultimatestrengthintension．Thisdifferencemustbetakenintoaccountwhenshearstressesareencounteredinmachinecomponents．

DYNAMICLOADS

Anappliedforcethatdoesnotvaryinanymanneriscalledastaticorsteadyload．Itisalsocommonpracticetoconsiderappliedforcesthatseldomvarytobestaticloads．Theforcethatisgraduallyappliedduringatensiletestisthereforeastaticload．

Ontheotherhand，forcesthatvaryfrequentlyinmagnitudeanddirectionarecalleddynamicloads．Dynamicloadscanbesubdividedtothefollowingthreecategories．

VaryingLoad．Withvaryingloads，themagnitudechanges，butthedirectiondoesnot．Forexample，theloadmayproducehighandlowtensilestressesbutnocompressivestresses．

ReversingLoad．Inthiscase，boththemagnitudeanddirectionchange．Theseloadreversalsproducealternatelyvaryingtensileandcompressivestressesthatarecommonlyreferredtoasstressreversals．

ShockLoad．Thistypeofloadisduetoimpact．Oneexampleisanelevatordroppingonanestofspringsatthebottomofachute．Theresultingmaximumspringforcecanbemanytimesgreaterthantheweightoftheelevator，Thesametypeofshockloadoccursinautomobilespringswhenatirehitsabumporholeintheroad．

FATIGUEFAILURE-THEENDURANCELIMITDIAGRAM

ThetestspecimeninFigure2.10a．，afteragivennumberofstressreversalswillexperienceacrackattheoutersurfacewherethestressisgreatest．Theinitialcrackstartswherethestressexceedsthestrengthofthegrainonwhichitacts．Thisisusuallywherethereisasmallsurfacedefect，suchasamaterialflaworatinyscratch．Asthenumberofcyclesincreases，theinitialcrackbeginstopropagateintoacontinuousseriesofcracksallaroundtheperipheryoftheshaft．Theconceptionoftheinitialcrackisitselfastressconcentrationthatacceleratesthecrackpropagationphenomenon．Oncetheentireperipherybecomescracked，thecracksstarttomovetowardthecenteroftheshaft．Finally，whentheremainingsolidinnerareabecomessmallenough，thestressexceedstheultimatestrengthandtheshaftsuddenlybreaks．Inspectionofthebreakrevealsaveryinterestingpattern，asshowninFigure2.13．Theouterannularareaisrelativelysmoothbecausematingcrackedsurfaceshadrubbedagainsteachother．However，thecenterportionisrough，indicatingasuddenrupturesimilartothatexperiencedwiththefractureofbrittlematerials．

Thisbringsoutaninterestingfact．Whenactualmachinepartsfailasaresultofstaticloads，theynormallydeformappreciablybecauseoftheductilityofthematerial．

Figure2.13

Thusmanystaticfailurescanbeavoidedbymakingfrequentvisualobservationsandreplacingalldeformedparts．However，fatiguefailuresgivetowarning．Fatiguefailmatedthatover90%ofbrokenautomobilepartshavefailedthroughfatigue．

Thefatiguestrengthofamaterialisitsabilitytoresistthepropagationofcracksunderstressreversals．Endurancelim