1附錄二外文原文和翻譯shafts、couplingsandrollingcontactbearingskeywords:shafts、couplings、bearingsVirtuallyallmachinescontainshafts.Themostcommonshapeforshaftsiscircularandthecrosssectioncanbeeithersolidorhollow(hollowshaftscanresultinweightsavings).Rectangularshaftsaresometimesused,asinscrewdriverblades,socketwrenchesandcontrolknobstems.Ashaftmusthaveadequatetorsionalstrengthtotransmittorqueandnotbeoverstressed.Italsobetorsionallystiffenoughsothatonemountedcomponentdoesnotdeviateexcessivelyfromitsoriginalangularpositionrelativetoasecondcomponentmountedonthesameshaft.Generallyspeaking,theangleoftwistshouldnotexceedonedegreeinashaftlengthequalto20diameters.Shaftsaremountedinbearingsandtransmintpowerthroughsuchdevicesasgears,pullerys,camsandclutches.Thesedevicesintroduceforceswhichattempttobendtheshaft；hence,theshaftmustberigidenoughtopreventoverloadingofthesupportingbearings.Ingeneral,thebendingdeflectionofashaftshouldnotexceed0.01in.perftoflengthbetweenbearingsupports.Inaddition,theshaftmustbeabletosustainacombinationofbendingandtorsionalloads.Thusanequivalentloadmustbeconsidered2whichtakesintoaccountbothtorsionandbending.Also,theallowablestressmustcontainafactorofsafetywhichincludesfatigue,sincetorsionalandbendingstressreversalsoccur.Fordiameterslessthan3in.,theusualshaftmaterialiscold-rolledateelcontainingabout0.4percentcarbon.Shaftsareeithercold-rolledorforgedinsizesfrom3in.to5in.,shaftsareforgedandmachinedtosize.Pleasticshaftsarewidelyusedforlightloadapplications.Oneadvantageofusingplasticissafetyinelectricalapplications,sinceplasticisapoorconductorofelectricity.Componentssuchasgearsandpulleysaremountedonshaftsbymeansofkey.Thedesignofthekeyandthecorrespondingkeywayintheshaftmustbeprperlyevaluated.Forexample,stressconcentrationsoccurinshaftsduetokeyways,andthematerialremovedtoformthekeywayfurtherweakenstheshaft.Ifshaftsarerunatcirticalspeeds,severevibrationscanoccurwhichcanseriouslydamageamachine.Itisimportanttoknowthemagnitudeofthesecriticalspeedssothattheycanbeavoided.Asageneralruleofthumb,thedifferencebetweentheoperatingspeedandthecriticalspeedshouldbeatleast20percent.Anotherimportantaspectofshaftdesignisthemethodofdirectlyconnectingoneshafttoanother.Thisisaccomplishedbydevicessuchasrigidandflexiblecouplings.3Acouplingisadeviceforconnectingtheedsofadjacentshafts.Inmachineconstruction,ouplingsareusedtoeffectasemipermanentconnectionbetweenadjacentrotatingshafts.Theconnectionispermanentinthesensethatitisnotmeanttobebrokenduringtheusefullifeofthemachine,butitcanbebrokenandrestoredinanemergencyorwhenwornpartsarereplaced.Thereareseveraltypesofshaftcouplings,theircharacteristicsdependonthepurposeforwhichtheyareused.Ifanexceptionallylongshaftisrequiredinamanufacturingplantorapropellershaftonaship,itismadeinsectionsthatarecoupledtogetherwithrigidcouplings.Acommontypeofrigidcouplingconsistsoftwomatingradialflanges(disks)thatarecttachedbykey-drivenhubstotheedsofadjacentshaftsectionsandboltedtogetherthroughtheflangestoformarigidconnection.Alignmentoftheconnectedshaftsisusuallyeffetedbymeansofarabbetjointonthefaceoftheflanges.Inconnectingshaftsbelongingtoseparatedevices(suchasanelectricmotorandagearbox),precisealigningoftheshaftsisdifficultandaflexiblecouplingisused.Thiscouplingconnectstheshaftsinsuchawayastominimizetheharmfuleffectsofshaftmisalignment.Flexiblecouplingsalsopermittheshaftstodeflectundertheirseparatesystemsofwithoneanother.Flexiblecouplingscanalsoservetoreducetheintensityofshockloadsandvibrationstransmittedfromoneshafttoanother.4Virtuallyallshaftscontainrollingcontactbearings.Theconcernofamachinedesignerwithballandrollerbearingsisfivefoldasfollows:(a)lifeinrelationtoload；(b)stiffness,i.e.deflectionsunderload；(c)friction；(d)wear；(e)noise.Formoderateloadsandspeedsthecorrectselectionofastandardbearingonthebasisofloadratingwillusuallysecuresatisfactoryperformance.Thedeflectionofthebearingelementswillbecomeimportantwhereloadsarehigh,althoughthisisusuallyoflessmagnitudethanthatoftheshaftsorothercomponentsassociatedwiththebearing.Wherespeedsarehighspecialcoolingarrangementsbecomenecessarywhichmayincreasefrictionaldrag.Wearisprimarilyassociatedwiththeintroductionofcontaminants,andsealingarrangementsmustbechosenwithregardthehostilityoftheenvironment.Becausethehighqualityandlowpriceofballandrollerbearingsdependsonquantityproduction,thetaskofthemachinedesignerbecomesoneofselectionratherthandesign.Rolling-contactbearingsaregenerallymadewithateelwhichisthrough-hardenedtoabout900HV,althoughinmanymechanismsspecialracesarenotprovidedandtheintera