《机械工程专业英语》课件-机械工程专业英语课件-PART3

PART

3

MechanicalElementsUnit11GearsUnit12ShaftsandCouplingsUnit13ClutchesUnit14RollingContactBearingsUnit15Belts,Clutches,Brakes,andChainsUnit11GearsNewwordsandexpressionsUnit11GearsAgearhavingtoothelementsthatarestraightandparalleltoitsaxisisknownasaspurgear.Aspurpaircanbeusedtoconnectparallelshaftsonly.Parallelshafts,however,canalsobeconnectedbygearsofanothertype,andaspurgearcanbematedwithagearofadifferenttype.（Figure11-1）Topreventjammingasaresultofthermalexpansion,toaidlubrication,andtocompensateforunavoidableinaccuraciesinmanufacture,allpower-transmittinggearsmusthavebacklash.Thismeansthatonthepitchcirclesofamatingpair,thespacewidthonthepinionmustbeslightinggreaterthanthetooththicknessonthegear,andviceversa.Oninstrumentgears,backlashcanbeeliminatedbyusingagearsplitdownitsmiddle;onehalfbeingrotatablerelativetotheother[1].Aspringforcesthesplitgearteethtooccupythefullwidthofthepinionspace.SpurandhelicalgearsHelicalgearshavecertainadvantages;forexample,whenconnectingparallelshaftstheyhaveahigherload-carryingcapacitythanspurgearswiththesametoothnumbersandcutwiththesamecutter.Becauseoftheoverlappingactionoftheteeth,theyaresmootherinactionandcanoperateathigherpitch-linevelocitiesthanspurgears.Thepitch-linevelocityisthevelocityofthepitchcircle.Sincetheteethareinclinedtotheaxisofrotation,helicalgearscreateanaxialthrust.Ifusedsingly,thisthrustmustbeabsorbedintheshaftbearings.Thethrustproblemcanbeovercomebycuttingtwosetsofopposedhelicalteethonthesameblank.Dependingonthemethodofmanufacture,thegearmaybeofthecontinuous-toothherringbonevarietyoradouble-helicalgearwithaspacebetweenthetwohalvestopermitthecuttingtooltorunout.Double-helicaliswellsuitedfortheefficienttransmissionofpowerathighspeeds.Helicalgearscanalsobeusedtoconnectnonparallel,non-intersectingshaftsatanyangletooneanother.Ninetydegreesisthecommonestangleatwhichsuchgearsareused.Inordertoachievelinecontactandimprovetheloadcarrying-capacityofthecrossed-axishelicalgears,thegearcanbemadetocurvepartiallyaroundthepinion,insomewhatthesamethatanutenvelopsascrew[2].Theresultwouldbeacylindricalwormandgear.Wormgearsprovidethesimplestmeansofobtaininglargeratiosinasinglepair.Theyareusuallylessefficientthanparallel-shaftgears,however,becauseofanadditionalslidingmovingalongtheteeth.Becauseoftheirsimilarity,theefficiencyofawormandgeardependsonthesamefactorsastheefficiencyofascrew.Single-threadwormsoflargediameterhavesmallleadanglesandlowefficiencies.Multiplied-threadwormshavelargerleadanglesandhigherefficiencies(Figure.11-2).WormandbevelgearsFortransmittingrotarymotionandtorquearoundcorners,bevelgearsarecommonlyused.Theconnectedshafts,whoseaxeswouldintersectifextended,areusuallybutnotnecessarilyatrightanglestooneanother.Whenadaptedforshaftsthatdonotintersect,spiralbevelgearsarecalledhypoidgears.Thepitchsurfacesofthesegearsarenotrollingcones,andtheratiooftheirmeandiametersisnotequaltothespeedratio.Consequently,thepinionmayhavefewteethandbemadeaslargeasnecessarytocarrytheload.Theprofilesoftheteethonbevelgearsarenotinvolutes;theyareofsuchashapethatthetoolsforcuttingtheteethareeasiertomakeandmaintainthaninvolutescuttingtools.Sincebevelgearscomeinpairs,aslongastheyareconjugatetooneanothertheyneednotbeconjugatetoothergearswithdifferenttoothnumber.Unit12ShaftsandCouplingsNewwordsandexpressionscoupling['kʌpliŋ]n.耦合；结合，联结,连接rectangular[rek'tæŋgjʊlə]adj.矩形的；成直角的crosssection截面，横断面，剖面screw[skruː]n.螺旋；螺丝钉screwdriver螺丝刀，螺丝起子，改锥blade[bleɪd]叶片，桨片socket['sɒkɪt]n.插座，插口，wrench[rentʃ]n.扳手，扳钳；猛扭socketwrench套筒扳手knob[nɒb]n.节，球块，旋钮，圆形把手stem[stem]n.杆，棒，柱，轴torsional['tɔːʃənəl]adj.扭转的；扭力的mounted['maʊntɪd]adj.安装好的，固定好的deviate['diːvɪeɪt]v.脱离；使偏离deviatefrom背离，偏离，与……有偏差twist[twɪst]v.使扭转,扭,使转动clutch[klʌtʃ]n.离合器；vt.抓住bending['bendɪŋ]n.弯曲度v.弯曲deflection[dɪ'flekʃ(ə)n]n.弯曲,弯曲度,挠度reversal[rɪ'vɜːs(ə)l]n.颠倒,相反,反向,改变方向,倒转cold-rolln.;v.冷轧,冷轧机forge[fɔːdʒ]n.;v.锻造,打制,锻工车间key[kiː]n.键keyway['kiːweɪ]n.[机]键槽,锁槽,销座indeterminate[,ɪndɪ'tɜːmɪnət]adj.不确定的；模糊的；含混的staticallyindeterminate静不定；超静定conservative[kən'sɜːvətɪv]n.保守派,有裕量adjacent[ə'dʒeɪs(ə)nt]adj.邻近的semipermanent[,semɪ'pɜːmənənt]adj.非永久性的；暂时的propeller[prə'pelə]n.[航][船]螺旋桨；推进器flange[flæn(d)ʒ]n.凸缘、法兰hub[hʌb]n.中心部分,衬套,轮毂bolt[bəʊlt]n.螺栓，螺钉alignment[ə'laɪnm(ə)nt]n.直线对准,调准,对中心rabbet['ræbɪt]n.插孔,塞孔,缺口gearbox['gɪəbɒks]n.变速箱；齿轮箱flexible['fleksɪb(ə)l]adj.灵活的；柔韧的；易弯曲的shock[ʃɒk]n.冲击,冲撞,打击Unit12ShaftsandCouplingsVirtuallyallmachinescontainshafts.Themostcommonshapeforshaftsiscircularandthecrosssectioncanbeeithersolidorhollow(hollowshaftscanresultinweightsavings).Rectangularshaftsaresometimesused,asinscrewdriverblades,socketwrenchesandcontrolknobstems.Ashaftmusthaveadequatetorsionalstrengthtotransmittorqueandnotbeoverstressed.Itmustalsobetorsionallystiffenoughsothatonemountedcomponentdoesnotdeviateexcessivelyfromitsoriginalangularpositionrelativetoasecondcomponentmountedonthesameshaft.Generallyspeaking,theangleoftwistshouldnotexceedonedegreeinashaftlengthequalto20diameters.Shaftsaremountedinbearingsandtransmitpowerthroughsuchdevicesasgears,pulleys,camsandclutches.Thesedevicesintroduceforceswhichattempttobendtheshaft;hencetheshaftmustberigidenoughtopreventoverloadingofthesupportingbearings.Ingeneral,thebendingdeflectionofashaftshouldnotexceed0.01in.perftoflengthbetweenbearingsupports.Inaddition,theshaftmustbeabletosustainacombinationofbendingandtorsionalloads.Thusanequivalentloadmustbeconsideredwhichtakesintoaccountbothtorsionandbending,Also,theallowablestressmustcontainafactorofsafetywhichincludesfatigue,sincetorsionalandbendingstressreversalsoccur.Fordiameterslessthan3in.,theusualshaftmaterialiscold-rolledsteelcontainingabout0.4percentcarbon.Shaftsareeithercold-rolledorforgedinsizesfrom3in.to5in.Forsizesabove5in.,shaftsareforgedandmachinedtosize[1].Plasticshaftsarewidelyusedforlightloadapplications.Oneadvantageofusingplasticissafetyinelectricalapplications,sinceplasticisapoorconductorofelectricity.Componentssuchasgearsandpulleysaremountedonshaftsbymeansofkey.Thedesignofthekeyandthecorrespondingkeywayintheshaftsmustbeproperlyevaluated.Forexample,stressconcentrationsoccurinshaftsduetokeyways,andthematerialremovedtoformthekeywayfurtherweakenstheshaft.Ifshaftarerunatcriticalspeeds,severevibrationscanoccurwhichcanseriouslydamageamachine.Itisimportanttoknowthemagnitudeofthesecriticalspeedssothattheycanbeavoided.Asageneralruleofthumb[2],thedifferencebetweentheoperatingspeedandthecriticalspeedshouldbeatleast20percent.Manyshaftsaresupportedbythreeormorebearings,whichmeansthattheproblemisstaticallyindeterminate.Textsonstrengthofmaterialsgivemethodsofsolvingsuchproblems.Thedesigneffortshouldbeinkeepingwiththeeconomicsofagivensituation.Forexample,ifonelineshaftsupportedbythreeormorebearingsisneeded,itprobablywouldbecheapertomakeconservativeassumptionsastomomentsanddesignitasthoughitweredeterminate.Theextracostofanoversizeshaftmaybelessthantheextracostofanelaboratedesignanalysis.Anotherimportantaspectofshaftdesignisthemethodofdirectlyconnectingoneshafttoanother.Thisisaccomplishedbydevicessuchasrigidandflexiblecouplings.Acouplingisadeviceforconnectingtheendsofadjacentshafts.Inmachineconstruction,couplingsareusedtoeffectasemipermanentconnectionbetweenadjacentrotatingshafts.Theconnectionispermanentinthesensethatitisnotmeanttobebrokenduringtheusefullifeofthemachine,butitcanbebrokenandrestoredinanemergencyorwhenwornpartsarereplaced.Thereareseveraltypesofshaftcouplings,theircharacteristicsdependonthepurposeforwhichtheyareused.Ifanexceptionallylongshaftisrequiredinamanufacturingplantorapropellershaftonaship,itismadeinsectionsthatarecoupledtogetherwithrigidcouplings.Acommontypeofrigidcouplingconsistsoftwomatingradialflanges(disks)thatareattachedbykey-drivenhubstotheendsofadjacentshaftsectionsandboltedtogetherthroughtheflangestoformarigidconnection.Alignmentoftheconnectedshaftsisusuallyeffectedbymeansofarabbetjointonthefaceoftheflanges.Inconnectingshaftsbelongingtoseparatedevices(suchasanelectricmotorandagearbox),precisealigningoftheshaftsisdifficultandaflexiblecouplingisused.Thiscouplingconnectstheshaftsinsuchawayastominimizetheharmfuleffectsofshaftmisalignment[4].Flexiblecouplingsalsopermittheshaftstodeflectundertheirseparatesystemsofloadsandtomovefreely(float)intheaxialdirectionwithoutinterferingwithoneanother.Flexiblecouplingscanalsoservetoreducetheintensityofshockloadsandvibrationstransmittedfromoneshafttoanother.Unit13ClutchesNewwordsandexpressionsUnit13ClutchesAclutchisadeviceforquicklyandeasilyconnectingordisconnectingarotatableshaftandrotatingcoaxialshaft.WormandbevelgearsFrictionclutcheshavepairsofmatingconical,disk,orring-shapedsurfacesandmeansforpressingthesurfacestogether.Thepressuremaybecreatedbyaspringorbyaseriesofleverslockedinpositionbythewedgingactionofaconicalspool.Onaspring-loadedclutchtheoperator,bycontrollingtherateatwhichthespringpressureisappliedtotheclutch,canregulatethespeedofclutchengagementandthetorqueappliedtothedrivenshaft.Thereisalwayssomeslippage,however,andtheefficiencyofafrictionclutchcanneverexceed50percent;i.e.,duringaclutchingoperationatleastone-halfoftheinputenergyislostbyfrictionintheclutchandproducesheat.Thefrictionsurfacesonclutchesshouldhaveahighcoefficientoffrictionbeabletoconducttheheatawayrapidly.Thesepropertiesaredifficulttoobtaininasinglematerialandforthisreason,oneofeachpairofmatingsurfaceisusuallymetallic,whiletheotheriseitherleather,cork,oranasbestos-basedfacingrivetedtoametalplatet.Somefrictionclutchesarerundry,whileothersoperateinoil.Dryclutcheshaveahighercoefficientoffrictionthanwetclutches,buttheoilhelpstocarryofftheheat.Figure13-1showsahalf-sectionofamultiple-platediskclutchinwhichinputmember2iskeyedtothedrivingshaft1andoutputmember3iskeyedtothedrivenshaft4.Thefrictionplatebhasexternalgearteethorsplinesthatmatewithteethontheinsideofmember2,whilefrictionplatechasinternalteeththatmatewithexternalteethonmember3.Platebcanslideaxiallyin2,whileplateccanslideaxiallyon3.Theclutchisengagedbymovingthespooltotheleft,which,byawedgingaction,rotatestheleveraboutthepivotpandcreatesaforcethatsqueezestheplatestogether.MagneticClutchesMagnetic-particleclutcheshaveanannular(ring-shaped)gapbetweenthedrivinganddrivenmembersthatisfilledwithpowderedironandgraphite.Whenamagneticfieldisinducedacrossthegapbyadirect-currentcontrolcoil,theironparticleformchainsacrossthegapandtransmitatorquethatdependsonthestrengthofthefield[2].Controlledbyvaryingthecurrent,theloadcanbeengagedsmoothlyandthereisnoslippagewhentheclutchistransmittingthetorqueforwhichitwasdesigned.Unit14RollingContactBearingsNewwordsandexpressionsUnit14RollingContactBearingsTheconcernofamachinedesignerwithballandrollerbearingsisfivefoldasfollows:(a)lifeinrelationtoload;(b)stiffness,i.e.deflectionsunderload;(c)friction;d)wear;e)noise.Formoderateloadsandspeedsthecorrectselectionofastandardbearingonthebasisofloadratingwillusuallysecuresatisfactoryperformance.Thedeflectionofbearingelementswillbecomeimportantwhereloadsarehigh,althoughthisisusuallyoflessmagnitudethanthatoftheshaftsorothercomponentsassociatedwiththebearing.Wherespeedsarehighspecialcoolingarrangementsbecomenecessarywhichmayincreasefrictionaldrag.Wearisprimarilyassociatedwiththeintroductionofcontaminants,andsealingarrangementsmustbechosenwithregardtothehostilityoftheenvironment.Thefrictionsurfacesonclutchesshouldhaveahighcoefficientoffrictionbeabletoconducttheheatawayrapidly.Thesepropertiesaredifficulttoobtaininasinglematerialandforthisreason,oneofeachpairofmatingsurfaceisusuallymetallic,whiletheotheriseitherleather,cork,oranasbestos-basedfacingrivetedtoametalplatet.Somefrictionclutchesarerundry,whileothersoperateinoil.Dryclutcheshaveahighercoefficientoffrictionthanwetclutches,buttheoilhelpstocarryofftheheat.Figure13-1showsahalf-sectionofamultiple-platediskclutchinwhichinputmember2iskeyedtothedrivingshaft1andoutputmember3iskeyedtothedrivenshaft4.Thefrictionplatebhasexternalgearteethorsplinesthatmatewithteethontheinsideofmember2,whilefrictionplatechasinternalteeththatmatewithexternalteethonmember3.Platebcanslideaxiallyin2,whileplateccanslideaxiallyon3.Theclutchisengagedbymovingthespooltotheleft,which,byawedgingaction,rotatestheleveraboutthepivotpandcreatesaforcethatsqueezestheplatestogether.Becausethehighqualityandlowpriceofballandrollerbearingsdependsonquantityproduction,thetaskofthemachinedesignerbecomesoneofselectionratherthandesign.Rolling-contactbearingsaregenerallymadewithsteelwhichisthrough-hardened[1].toabout900HV,althoughinmanymechanismsspecialracesarenotprovidedandtheinteractingsurfacesarehardenedtoabout600HV.Itisnotsurprisingthat,owingtothehighstressesinvolved,apredominantformoffailureshouldbemetalfatigue,andagooddealofworkiscurrentlyinprogressintendedtoimprovethereliabilityofthistypeofbearing.Designcanbebasedonacceptedvaluesoflifeanditisgeneralpracticeinthebearingindustrytodefinetheloadcapacityofthebearingasthatvaluebelowwhich90percentofabatchwillexceedalifeofonemillionrevolutions.Notwithstandingthefactthatresponsibilityforthebasicdesignofballandrollerbearingsrestswiththebearingmanufacturer,themachinedesignermustformacorrectappreciationofthedutytobeperformedbythebearingandbeconcernednotonlywithbearingselectionbutwiththeconditionsforcorrectinstallation.Thefitofthebearingracesontotheshaftorontothehousingsisofcriticalimportancebecauseoftheircombinedeffectontheinternalclearanceofthebearingaswellaspreservingthedesireddegreeofinterferencefit.Inadequateinterferencecaninduceserioustroublefromfrettingcorrosion[2].Theinnerraceisfrequentlylocatedaxiallybyabuttingagainstashoulder.Aradiusatthispointisessentialfortheavoidanceofstressconcentrationandballracesareprovidedwitharadiusorchamfertoallowspaceforthis.Wherelifeisnotthedeterminingfactorindesign,itisusualtodeterminemaximumloadingbytheamounttowhichabearingwilldeflectunderload.Thustheconceptof“staticload-carryingcapacity”[3]isunderstoodtomeantheloadthatcanbeappliedtoabearing,whichiseitherstationaryorsubjecttoslightswivelingmotions,withoutimpairingitsrunningqualitiesforsubsequentrotationalmotion.Thishasbeendeterminedbypracticalexperienceastheloadwhichwhenappliedtobearingresultsinatotaldeformationoftherollingelementandracewayatanypointofcontactnotexceeding0.01percentoftherolling-elementdiameter.Thiswouldcorrespondtoapermanentdeformationof0.0025mmforaball25mmindiameter.Thesuccessfulfunctioningofmanybearingsdependsuponprovidingthemwithadequateprotectionagainsttheirenvironment,andinsomecircumstancestheenvironmentmustbeprotectedfromlubricantsorproductsofdeteriorationofthebearingsurfaces.Achievementofthecorrectfunctioningofsealsisanessentialpartofbearingdesign.Moreover,sealswhichareappliedtomovingpartsforanypurposeareofinteresttotribologistsbecausetheyarecomponentsofbearingsystemandcanonlybedesignedsatisfactorilyonthebasisoftheappropriatebearingtheory.Notwithstandingtheirimportance,theamountofresearcheffortthathasbeendevotedtotheunderstandingofthebehaviorofsealshasbeensmallwhencomparedwiththatdevotedtootheraspectsofbearingtechnology.Unit15Belts,Clutches,Brakes,andChainsNewwordsandexpressionsUnit15Belts,Clutches,Brakes,andChainsBelts,clutches,brakes,andchainsareexamplesofmachineelementsthatemployfrictionasausefulagent.Abeltprovidesaconvenientmeansfortransferringpowerfromoneshafttoanother.Beltsarefrequentlynecessarytoreducethehigherrotativespeedsofelectricmotorstothelowervaluesrequiredbymechanicalequipment.Clutchesarerequiredwhenshaftsmustbefrequentlyconnectedanddisconnected.Thefunctionofthebrakeistoturnmechanicalenergyintoheat.Thedesignoffrictionaldevicesissubjecttouncertaintiesinthevalueofthecoefficientoffrictionthatmustnecessarilybeused.Chainsprovideaconvenientandeffectivemeansfortransferringpowerbetweenparallelshafts.TherayonandrubberV-beltiswidelyusedforpowertransmission[1].Suchbeltsaremadeintwoseries:thestandardV-beltandthehighcapacityV-belt.Othertypesofbeltsareavailableforpowertransmissionpurposes.Theteethofso-calledtimingbeltwillkeepshaftscompletelysynchronized.WhendesigningaV-beltdriveitisagoodplantocalculatethecostoftwoorthreedifferentlayoutsofbeltsandpulleystodeterminewhichhasthesmallestoverallcost.ThecatalogsofthevariousmanufacturersofV-beltscontainmuchpracticalinformation.TheV-beltisanimportantelementinthefieldofpowertransmission.Itiscontinuallybeingimprovedbythevariousmanufacturersandloadingvaluesarerevisedfromtimetotime.Thedesignergenerallyisguidedbythecurrentliteraturefortheparticularbrandofbeltheexpectstouse.Theforgoingdevelopmentshouldbeconsideredamethodthatshowshowthefatiguelifeofabeltisinfluencedbyfactorssuchasbending,centrifugaleffects,andpowertransmitted.Aclutchisadeviceforquicklyandeasilyconnectingordisconnectingarotatableshaftandarotatingcoaxialshaft[3].Clutchesareusuallyplacedbetweentheinputshafttoamachineandtheoutputshaftfromthedrivingmotor,andprovideaconvenientmeansforstartingandstoppingthemachineandpermittingthedrivermotororenginetobestartedinanunloadedstate.Therotor(rotatingmember)inanelectricmotorhasrotationalinertia,andatorqueisrequiredtobringituptospeedwhenthemotorisstarted.Ifthemotorshaftisrigidlyconnectedtoaloadwithalargerotationalinertia,andthemotorisstartedsuddenlybyclosingaswitch,themotormaynothavesufficienttorquecapacitytobringthemotorshaftuptospeedbeforethewindingsinthemotorareburnedoutbytheexcessivecurrentdemands[4].Aclutchbetweenthemotorandtheloadshaftswillrestrictthestartingtorqueonthemotortothatrequiredtoacceleratetherotorandpartsoftheclutchonly.Onsomemachinetoolsitisconvenienttoletthedrivingmotorruncontinuouslyandtostartandstopthemachinebyoperatingaclutch.Abrakeissimilartoaclutchexceptthatoneoftheshaftsisreplacedbyafixedmember.Thebasicfunctionofabrakeistoabsorbenergy(i.e.,toconvertkineticandpotentialenergyintofrictionheat)andtodissipatetheresultingheatwithoutdevelopingdestructi