外文翻譯--齒輪和齒輪傳動(dòng).doc

GearsandgeardriveGearsarethemostdurableandruggedofallmechanicaldrives.Theycantransmithighpoweratefficienciesupto98%andwithlongservicelives.Forthisreason,gearsratherthanbeltsorchainsarefoundinautomotivetransmissionsandmostheavy-dutymachinedrives.Ontheotherhand,gearsaremoreexpensivethanotherdrives,especiallyiftheyaremachinedandnotmadefrompowermetalorplastic.Gearcostincreasessharplywithdemandsforhighprecisionandaccuracy.Soitisimportanttoestablishtolerancerequirementsappropriatefortheapplication.Gearsthattransmitheavyloadsorthanoperateathighspeedsarenotparticularlyexpensive,butgearsthatmustdobotharecostly.Silentgearsalsoareexpensive.Instrumentandcomputergearstendtobecostlybecausespeedordisplacementratiosmustbeexact.Attheotherextreme,gearsoperatingatlowspeedinexposedlocationsarenormallytermednocriticalandaremadetominimumqualitystandards.Fortoothforms,size,andquality,industrialpracticeistofollowstandardssetupbytheAmericanGearManufacturesAssociation(AGMA).ToothformStandardspublishedbyAGMAestablishgearproportionsandtoothprofiles.Toothgeometryisdeterminedprimarilybypitch,depth,andpressureangle.Pitch：StandardspitchesareusuallywholenumberswhenmeasuredasdiametralpitchP.Coarse-pitchgearinghasteethlargerthan20diametralpitchusually0.5to19.99.Fine-pitchgearingusuallyhasteethofdiametralpitch20to200.Depth:Standardizedintermsofpitch.Standardfull-depthhaveworkingdepthof2/p.Iftheteethhaveequaladdenda(asinstandardinterchangeablegears)theaddendumis1/p.Stubteethhaveaworkingdepthusually20%lessthanfull-depthteeth.Full-depthteethhavealargercontractratiothanstubteeth.Gearswithsmallnumbersofteethmayhaveundercutsothantheydonotinterferewithoneanotherduringengagement.Undercuttingreduceactiveprofileandweakensthetooth.Matinggearswithlongandshortaddendumhavelargerload-carryingcapacitythanstandardgears.Theaddendumofthesmallergear(pinion)isincreasedwhilethatoflargergearisdecreased,leavingthewholedepththesame.Thisformisknowasrecess-actiongearing.PressureAngle:Standardanglesare020and025.Earlierstandardsincludea14-02/1pressureanglethatisstillused.Pressureangleaffectstheforcethattendstoseparatematinggears.Highpressureangledecreasesthecontactratio(ratioofthenumberofteethincontact)butprovidesatoothofhighercapacityandallowsgearstohavefewerteethwithoutundercutting.Backlash:Shortestdistancesbetweenthenon-contactingsurfacesofadjacentteeth.GearsarecommonlyspecifiedaccordingtoAGMAClassNumber,whichisacodedenotingimportantqualitycharacteristics.Qualitynumberdenotetooth-elementtolerances.Thehigherthenumber,thecloserthetolerance.Number8to16applytofine-pitchgearing.Gearsareheat-treatedbycase-hardening,through-hardening,nitriding,orprecipitationhardening.Ingeneral,hardergearsarestrongerandlastlongerthansoftones.Thus,hardeningisadevicethatcutstheweightandsizeofgears.Someprocesses,suchasflame-hardening,improveservicelifebutdonotnecessarilyimprovestrength.DesignchecklistThelargerinapairiscalledthegear,thesmalleriscalledthepinion.GearRatio:Thenumberofteethinthegeardividebythenumberofteethinthepinion.Also,ratioofthespeedofthepiniontothespeedofthegear.Inreductiongears,theratioofinputtooutputspeeds.GearEfficiency:Ratioofoutputpowertoinputpower.(includesconsiderationofpowerlossesinthegears,inbearings,andfromwindageandchurningoflubricant.)Speed:Inagivengearnormallylimitedtosomespecificpitchlinevelocity.Speedcapabilitiescanbeincreasedbyimprovingaccuracyofthegearteethandbyimprovingbalanceoftherotatingparts.Power:Loadandspeedcapacityisdeterminedbygeardimensionsandbytypeofgear.Helicalandhelical-typegearshavethegreatestcapacity(toapproximately30,000hp).Spiralbevelgeararenormallylimitedto5,000hp,andwormgearsareusuallylimitedtoabout750hp.SpecialrequirementsMatched-SetGearing:Inapplicationsrequiringextremelyhighaccuracy,itmaybenecessarytomatchpinionandgearprofilesandleadssothatmismatchdoesnotexceedthetoleranceonprofileorleadfortheintendedapplication.ToothSpacing:Somegearsrequirehighaccuracyinthecircularofteeth.Thus,specificationofpitchmayberequiredinadditiontoanaccuracyclassspecification.Backlash:TheAMGAstandardsrecommendbacklashrangestoprovideproperrunningclearancesformatinggears.Anoverlytightmeshmayproduceoverload.However,zerobacklashisrequiredinsomeapplications.QuietGears:Tomakegearsasquitaspossible,specifythefinestpitchallowableforloadconditions.(Insomeinstances,however,pitchiscoarsenedtochangemeshfrequencytoproduceamorepleasant,lower-pitchsound.)Usealowpressureangle.Useamodifiedprofiletoincluderootandtiprelief.Allowenoughbacklash.Usehighqualitynumbers.Specifyasurfacefinishof20in.orbetter.Balancethegearset.Useanonintegralratiosothatthesameteethdonotrepeatedlyengageifbothgearandpinionarehardenedsteel.(Ifthegearismadeofasoftmaterial,anintegralratioallowsthegeartocold-workandconformtothepinion,therebypromotingquietoperation.)Makesurecriticalareatleast20%apartfromoperatingspeedingorspeedmultiplesandfromfrequencyoftoothmesh.MultiplemeshgearMultiplemeshreferstomovethanonepairofgearoperatinginatrain.Canbeonparallelornonparallelaxesandonintersectionornonintersectingshafts.Theypermithigerspeedratiosthanarefeasiblewithasinglepairofgears.Seriestrains:Overallratioisinputshaftspeeddividedbyoutputspeed,alsotheproductofindividualratiosateachmesh,exceptinplanetarygears.Ratioismosteasilyfoundbydividingtheproductofnumbersofteethofdrivengearsbytheproductofnumbersofteethofdrivinggears.Speedincreasers(withstep-upratherthanstep-downratios)mayrequirespecialcareinmanufacturinganddesign.Theyofteninvolvehighspeedsandmaycresteproblemsingeardynamics.Also,frictionalanddragforcesaremagnifiedwhich,inextremecases,mayleadtooperationalproblems.EpicyclicGearing:Normally,agearaxisremainsfixedandonlythegearsrotates.Butinanepicyclicgeartrain,variousgearsaxesrotateaboutoneanthertoprovidespecializedoutputmotions.Withsuitableclutchseandbrakes,anepicyclictrainservesastheplanetarygearcommonlyfoundinautomatictransmissions.Epicyclictrainsmayusespurorhelicalgears,externalorinternal,orbevelgears.Intransmissions,theepicyclic(orplanetary)gearsusuallyhavemultipleplanetstoincreaseloadcapacity.Inmostcases,improvedkinematicaccuracyinagearsetdecreasesgearmeshexcitationandresultsinlowerdrivenoise.Gearsetaccuracycanbeincreasedbymodifyingthetoothinvoluteprofile,bysubstitutinghigherqualitygearingwithtightermanufacturingtolerances,andbyimprovingtoothsurfacefinish.However,ifgearmeshexcitationgeneratersresonancesomewhereinthedrivesystem,nothingshortofa“perfect”gearsetwillsubstantiallyreducevibrationandnoise.Toothprofilesaremodifiedtoavoidinterferenceswhichcanresultfromdeflectionsinthegears,shafts,andhousingasteethengageanddisendgage.Ifthesetoothinterferencesarenotcompensatedforbyprofilemodifications,gearsloadcapacitycanbeseriouslyreduced.Inaddition,thedrivewillbenoisierbecausetoothinterferencesgeneratehighdynamicloads.Interferencestypicallyareeliminatedbyrelivingthetoothtip,thetoothflank,orboth.Suchprofilemodificationsareespeciallyimportantforhigh-load,high-speeddrives.Thegraphofsoundpressurelevelvstipreliefillustrateshowtoothprofilemodificationscanaffectoveralldrivenoise.Ifthetipreliefislessthanthisoptimumvalue,drivenoiseincreasesbecauseofgreatertoothinterference；agreateramountoftipreliefalsoincreasenoisebecausethecontactratioisdecreased.Tightermanufacturingtolerancesalsoproducequietiergears.Tolerancesforsuchparametersasprofileerror,pitchAGMAqualitylevel.Forinstance,thegraphdepictingSPLvsbothspeedandgearqualityshowshownoisedecreasesexample,noiseisreducedsignificantlybyanincreaseinaccuracyfromanAGMAQn11qualitytoanAGNAQn15quality.However,formostcommercialdriveapplications,itisdoubtfulthattheresultingsubstantialcostincreaseforsuchanaccuracyimprovementcanbejustifiedsimplyonthebasisofreduceddrivenoise.Previously,itwasmentionedthatgearsmusthaveadequateclearancewhenlo