《機械工程專業(yè)英語》課件-機械工程專業(yè)英語課件-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['k?pli?]n.耦合；結(jié)合，聯(lián)結(jié),連接rectangular[rek't??gj?l?]adj.矩形的；成直角的crosssection截面，橫斷面，剖面screw[skru?]n.螺旋；螺絲釘screwdriver螺絲刀，螺絲起子，改錐blade[ble?d]葉片，槳片socket['s?k?t]n.插座，插口，wrench[rent?]n.扳手，扳鉗；猛扭socketwrench套筒扳手knob[n?b]n.節(jié)，球塊，旋鈕，圓形把手stem[stem]n.桿，棒，柱，軸torsional['t????n?l]adj.扭轉(zhuǎn)的；扭力的mounted['ma?nt?d]adj.安裝好的，固定好的deviate['di?v?e?t]v.脫離；使偏離deviatefrom背離，偏離，與……有偏差twist[tw?st]v.使扭轉(zhuǎn),扭,使轉(zhuǎn)動clutch[kl?t?]n.離合器；vt.抓住bending['bend??]n.彎曲度v.彎曲deflection[d?'flek?(?)n]n.彎曲,彎曲度,撓度reversal[r?'v??s(?)l]n.顛倒,相反,反向,改變方向,倒轉(zhuǎn)cold-rolln.;v.冷軋,冷軋機forge[f??d?]n.;v.鍛造,打制,鍛工車間key[ki?]n.鍵keyway['ki?we?]n.[機]鍵槽,鎖槽,銷座indeterminate[,?nd?'t??m?n?t]adj.不確定的；模糊的；含混的staticallyindeterminate靜不定；超靜定conservative[k?n's??v?t?v]n.保守派,有裕量adjacent[?'d?e?s(?)nt]adj.鄰近的semipermanent[,sem?'p??m?n?nt]adj.非永久性的；暫時的propeller[pr?'pel?]n.[航][船]螺旋槳；推進器flange[fl?n(d)?]n.凸緣、法蘭hub[h?b]n.中心部分,襯套,輪轂bolt[b??lt]n.螺栓，螺釘alignment[?'la?nm(?)nt]n.直線對準,調(diào)準,對中心rabbet['r?b?t]n.插孔,塞孔,缺口gearbox['g??b?ks]n.變速箱；齒輪箱flexible['fleks?b(?)l]adj.靈活的；柔韌的；易彎曲的shock[??k]n.沖擊,沖撞,打擊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