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外文翻译---轴、联轴器和滚动轴承.doc

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外文翻译---轴、联轴器和滚动轴承.doc

1附录二外文原文和翻译shafts、couplingsandrollingcontactbearingskeywordsshafts、couplings、bearingsVirtuallyallmachinescontainshafts.Themostcommonshapeforshaftsiscircularandthecrosssectioncanbeeithersolidorhollowhollowshaftscanresultinweightsavings.Rectangularshaftsaresometimesused,asinscrewdriverblades,socketwrenchesandcontrolknobstems.Ashaftmusthaveadequatetorsionalstrengthtotransmittorqueandnotbeoverstressed.Italsobetorsionallystiffenoughsothatonemountedcomponentdoesnotdeviateexcessivelyfromitsoriginalangularpositionrelativetoasecondcomponentmountedonthesameshaft.Generallyspeaking,theangleoftwistshouldnotexceedonedegreeinashaftlengthequalto20diameters.Shaftsaremountedinbearingsandtransmintpowerthroughsuchdevicesasgears,pullerys,camsandclutches.Thesedevicesintroduceforceswhichattempttobendtheshafthence,theshaftmustberigidenoughtopreventoverloadingofthesupportingbearings.Ingeneral,thebendingdeflectionofashaftshouldnotexceed0.01in.perftoflengthbetweenbearingsupports.Inaddition,theshaftmustbeabletosustainacombinationofbendingandtorsionalloads.Thusanequivalentloadmustbeconsidered2whichtakesintoaccountbothtorsionandbending.Also,theallowablestressmustcontainafactorofsafetywhichincludesfatigue,sincetorsionalandbendingstressreversalsoccur.Fordiameterslessthan3in.,theusualshaftmaterialiscoldrolledateelcontainingabout0.4percentcarbon.Shaftsareeithercoldrolledorforgedinsizesfrom3in.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.Acommontypeofrigidcouplingconsistsoftwomatingradialflangesdisksthatarecttachedbykeydrivenhubstotheedsofadjacentshaftsectionsandboltedtogetherthroughtheflangestoformarigidconnection.Alignmentoftheconnectedshaftsisusuallyeffetedbymeansofarabbetjointonthefaceoftheflanges.Inconnectingshaftsbelongingtoseparatedevicessuchasanelectricmotorandagearbox,precisealigningoftheshaftsisdifficultandaflexiblecouplingisused.Thiscouplingconnectstheshaftsinsuchawayastominimizetheharmfuleffectsofshaftmisalignment.Flexiblecouplingsalsopermittheshaftstodeflectundertheirseparatesystemsofwithoneanother.Flexiblecouplingscanalsoservetoreducetheintensityofshockloadsandvibrationstransmittedfromoneshafttoanother.4Virtuallyallshaftscontainrollingcontactbearings.Theconcernofamachinedesignerwithballandrollerbearingsisfivefoldasfollowsalifeinrelationtoloadbstiffness,i.e.deflectionsunderloadcfrictiondwearenoise.Formoderateloadsandspeedsthecorrectselectionofastandardbearingonthebasisofloadratingwillusuallysecuresatisfactoryperformance.Thedeflectionofthebearingelementswillbecomeimportantwhereloadsarehigh,althoughthisisusuallyoflessmagnitudethanthatoftheshaftsorothercomponentsassociatedwiththebearing.Wherespeedsarehighspecialcoolingarrangementsbecomenecessarywhichmayincreasefrictionaldrag.Wearisprimarilyassociatedwiththeintroductionofcontaminants,andsealingarrangementsmustbechosenwithregardthehostilityoftheenvironment.Becausethehighqualityandlowpriceofballandrollerbearingsdependsonquantityproduction,thetaskofthemachinedesignerbecomesoneofselectionratherthandesign.Rollingcontactbearingsaregenerallymadewithateelwhichisthroughhardenedtoabout900HV,althoughinmanymechanismsspecialracesarenotprovidedandtheinteractingsurfacesarehardenedtoabout600HV.Itisnotsurprisingthat,owingtothehighstressesinvolved,apredominantformoffailureshouldbemetalfatigue,andagooddealofworkiscurrentlyinprogressintendedtoimprovethereliabilityofthistypeofbearing.Designcanbe

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