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附录FailureAnalysis，DimensionalDeterminationAndAnalysis，ApplicationsOfCamsJohnPoliReceived:January5,2004;revised:September20,2004Keywords:,cams,failure,dimensionandsoon.INTRODUCTIONItisabsolutelyessentialthatagraduateknowhowandwhypartsfailsothatreliablemachinesthatrequireminimummaintenancecanbedesigned．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——LooseningoffasteningdevicesThepartsizesandshapesselectedalsomusttakeintoaccountmanydimensionalfactorsthatproduceexternalloadeffects，suchasgeometricdiscontinuities，residualstressesduetoformingofdesiredcontours，andtheapplicationofinterferencefitjoints．Camsareamongthemostversatilemechanismsavailable．Acamisasimpletwo-memberdevice．Theinputmemberisthecamitself，whiletheoutputmemberiscalledthefollower．Throughtheuseofcams，asimpleinputmotioncanbemodifiedintoalmostanyconceivableoutputmotionthatisdesired．Someofthecommonapplicationsofcamsare——Camshaftanddistributorshaftofautomotiveengine——Productionmachinetools——Automaticrecordplayers——Printingmachines——Automaticwashingmachines——AutomaticdishwashersThecontourofhigh-speedcams(camspeedinexcessof1000rpm)mustbedeterminedmathematically．However，thevastmajorityofcamsoperateatlowspeeds(lessthan500rpm)ormedium-speedcamscanbedeterminedgraphicallyusingalarge-scalelayout．Ingeneral，thegreaterthecamspeedandoutputload，thegreatermustbetheprecisionwithwhichthecamcontourismachined．DESIGNPROPERTIESOFMATERIALSThefollowingdesignpropertiesofmaterialsaredefinedastheyrelatetothetensiletest．StaticStrength．Thestrengthofapartisthemaximumstressthatthepartcansustainwithoutlosingitsabilitytoperformitsrequiredfunction．Thusthestaticstrengthmaybeconsideredtobeapproximatelyequaltotheproportionallimit，sincenoplasticdeformationtakesplaceandnodamagetheoreticallyisdonetothematerial．Stiffness．Stiffnessisthedeformation-resistingpropertyofamaterial．Theslopeofthemoduluslineand，hence，themodulusofelasticityaremeasuresofthestiffnessofamaterial．Resilience．Resilienceisthepropertyofamaterialthatpermitsittoabsorbnergywithoutpermanentdeformation．TheamountofenergyabsorbedisrepresentedbytheareaFigure2.7underneaththestress-straindiagramwithintheelasticregion．Toughness．Resilienceandtoughnessaresimilarproperties．However，toughnessistheabilitytoabsorbenergywithoutrupture．Thustoughnessisrepresentedbythetotalareaunderneaththestress-straindiagram，asdepictedinFigure2．7Obviously，thetoughnessandresilienceofbrittlematerialsareverylowandareapproximatelyequal．Brittleness．Abrittlematerialisonethatrupturesbeforeanyappreciableplasticdeformationtakesplace．Brittlematerialsaregenerallyconsideredundesirableformachinecomponentsbecausetheyareunabletoyieldlocallyatlocationsofhighstressbecauseofgeometricstressraiserssuchasshoulders，holes，notches，orkeyways．Ductility．Aductilitymaterialexhibitsalargeamountofplasticdeformationpriortorupture．Ductilityismeasuredbythepercentofareaandpercentelongationofapartloadedtorupture．A5%elongationatruptureisconsideredtobethedividinglinebetweenductileandbrittlematerials．Malleability．Malleabilityisessentiallyameasureofthecompressiveductilityofamaterialand，assuch，isanimportantcharacteristicofmetalsthataretoberolledintosheets．Hardness．Thehardnessofamaterialisitsabilitytoresistindentationorscratching．Generallyspeaking，theharderamaterial，themorebrittleitisand，hence，thelessresilient．Also，theultimatestrengthofamaterialisroughlyproportionaltoitshardness．Figure2.