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外文翻译--齿轮和齿轮传动.doc

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外文翻译--齿轮和齿轮传动.doc

GearsandgeardriveGearsarethemostdurableandruggedofallmechanicaldrives.Theycantransmithighpoweratefficienciesupto98andwithlongservicelives.Forthisreason,gearsratherthanbeltsorchainsarefoundinautomotivetransmissionsandmostheavydutymachinedrives.Ontheotherhand,gearsaremoreexpensivethanotherdrives,especiallyiftheyaremachinedandnotmadefrompowermetalorplastic.Gearcostincreasessharplywithdemandsforhighprecisionandaccuracy.Soitisimportanttoestablishtolerancerequirementsappropriatefortheapplication.Gearsthattransmitheavyloadsorthanoperateathighspeedsarenotparticularlyexpensive,butgearsthatmustdobotharecostly.Silentgearsalsoareexpensive.Instrumentandcomputergearstendtobecostlybecausespeedordisplacementratiosmustbeexact.Attheotherextreme,gearsoperatingatlowspeedinexposedlocationsarenormallytermednocriticalandaremadetominimumqualitystandards.Fortoothforms,size,andquality,industrialpracticeistofollowstandardssetupbytheAmericanGearManufacturesAssociationAGMA.ToothformStandardspublishedbyAGMAestablishgearproportionsandtoothprofiles.Toothgeometryisdeterminedprimarilybypitch,depth,andpressureangle.PitchStandardspitchesareusuallywholenumberswhenmeasuredasdiametralpitchP.Coarsepitchgearinghasteethlargerthan20diametralpitch–usually0.5to19.99.Finepitchgearingusuallyhasteethofdiametralpitch20to200.DepthStandardizedintermsofpitch.Standardfulldepthhaveworkingdepthof2/p.Iftheteethhaveequaladdendaasinstandardinterchangeablegearstheaddendumis1/p.Stubteethhaveaworkingdepthusually20lessthanfulldepthteeth.Fulldepthteethhavealargercontractratiothanstubteeth.Gearswithsmallnumbersofteethmayhaveundercutsothantheydonotinterferewithoneanotherduringengagement.Undercuttingreduceactiveprofileandweakensthetooth.Matinggearswithlongandshortaddendumhavelargerloadcarryingcapacitythanstandardgears.Theaddendumofthesmallergearpinionisincreasedwhilethatoflargergearisdecreased,leavingthewholedepththesame.Thisformisknowasrecessactiongearing.PressureAngleStandardanglesare020and025.Earlierstandardsincludea1402/1pressureanglethatisstillused.Pressureangleaffectstheforcethattendstoseparatematinggears.Highpressureangledecreasesthecontactratioratioofthenumberofteethincontactbutprovidesatoothofhighercapacityandallowsgearstohavefewerteethwithoutundercutting.BacklashShortestdistancesbetweenthenoncontactingsurfacesofadjacentteeth.GearsarecommonlyspecifiedaccordingtoAGMAClassNumber,whichisacodedenotingimportantqualitycharacteristics.Qualitynumberdenotetoothelementtolerances.Thehigherthenumber,thecloserthetolerance.Number8to16applytofinepitchgearing.Gearsareheattreatedbycasehardening,throughhardening,nitriding,orprecipitationhardening.Ingeneral,hardergearsarestrongerandlastlongerthansoftones.Thus,hardeningisadevicethatcutstheweightandsizeofgears.Someprocesses,suchasflamehardening,improveservicelifebutdonotnecessarilyimprovestrength.DesignchecklistThelargerinapairiscalledthegear,thesmalleriscalledthepinion.GearRatioThenumberofteethinthegeardividebythenumberofteethinthepinion.Also,ratioofthespeedofthepiniontothespeedofthegear.Inreductiongears,theratioofinputtooutputspeeds.GearEfficiencyRatioofoutputpowertoinputpower.includesconsiderationofpowerlossesinthegears,inbearings,andfromwindageandchurningoflubricant.SpeedInagivengearnormallylimitedtosomespecificpitchlinevelocity.Speedcapabilitiescanbeincreasedbyimprovingaccuracyofthegearteethandbyimprovingbalanceoftherotatingparts.PowerLoadandspeedcapacityisdeterminedbygeardimensionsandbytypeofgear.Helicalandhelicaltypegearshavethegreatestcapacitytoapproximately30,000hp.Spiralbevelgeararenormallylimitedto5,000hp,andwormgearsareusuallylimitedtoabout750hp.SpecialrequirementsMatchedSetGearingInapplicationsrequiringextremelyhighaccuracy,itmaybenecessarytomatchpinionandgearprofilesandleadssothatmismatchdoesnotexceedthetoleranceonprofileorleadfortheintendedapplication.ToothSpacingSomegearsrequirehighaccuracyinthecircularofteeth.Thus,specificationofpitchmayberequiredinadditiontoanaccuracyclassspecification.BacklashTheAMGAstandardsrecommendbacklashrangestoprovideproperrunningclearancesformatinggears.Anoverlytightmeshmayproduceoverload.However,zerobacklashisrequiredinsomeapplications.QuietGearsTomakegearsasquitaspossible,specifythefinestpitchallowableforloadconditions.Insomeinstances,however,pitchiscoarsenedtochangemeshfrequencytoproduceamorepleasant,lowerpitchsound.Usealowpressureangle.Useamodifiedprofiletoincluderootandtiprelief.Allowenoughbacklash.Usehighqualitynumbers.Specifyasurfacefinishof20in.orbetter.Balancethegearset.Useanonintegralratiosothatthesameteethdonotrepeatedlyengageifbothgearandpinionarehardenedsteel.Ifthegearismadeofasoftmaterial,anintegralratioallowsthegeartocoldworkandconformtothepinion,therebypromotingquietoperation.Makesurecriticalareatleast20apartfromoperatingspeedingorspeedmultiplesandfromfrequencyoftoothmesh.MultiplemeshgearMultiplemeshreferstomovethanonepairofgearoperatinginatrain.Canbeonparallelornonparallelaxesandonintersectionornonintersectingshafts.Theypermithigerspeedratiosthanarefeasiblewithasinglepairofgears.SeriestrainsOverallratioisinputshaftspeeddividedbyoutputspeed,alsotheproductofindividualratiosateachmesh,exceptinplanetarygears.Ratioismosteasilyfoundbydividingtheproductofnumbersofteethofdrivengearsbytheproductofnumbersofteethofdrivinggears.Speedincreaserswithstepupratherthanstepdownratiosmayrequirespecialcareinmanufacturinganddesign.Theyofteninvolvehighspeedsandmaycresteproblemsingeardynamics.Also,frictionalanddragforcesaremagnifiedwhich,inextremecases,mayleadtooperationalproblems.EpicyclicGearingNormally,agearaxisremainsfixedandonlythegearsrotates.Butinanepicyclicgeartrain,variousgearsaxesrotateaboutoneanthertoprovidespecializedoutputmotions.Withsuitableclutchseandbrakes,anepicyclictrainservesastheplanetarygearcommonlyfoundinautomatictransmissions.Epicyclictrainsmayusespurorhelicalgears,externalorinternal,orbevelgears.Intransmissions,theepicyclicorplanetarygearsusuallyhavemultipleplanetstoincreaseloadcapacity.Inmostcases,improvedkinematicaccuracyinagearsetdecreasesgearmeshexcitationandresultsinlowerdrivenoise.Gearsetaccuracycanbeincreasedbymodifyingthetoothinvoluteprofile,bysubstitutinghigherqualitygearingwithtightermanufacturingtolerances,andbyimprovingtoothsurfacefinish.However,ifgearmeshexcitationgeneratersresonancesomewhereinthedrivesystem,nothingshortofaperfectgearsetwillsubstantiallyreducevibrationandnoise.Toothprofilesaremodifiedtoavoidinterferenceswhichcanresultfromdeflectionsinthegears,shafts,andhousingasteethengageanddisendgage.Ifthesetoothinterferencesarenotcompensatedforbyprofilemodifications,gearsloadcapacitycanbeseriouslyreduced.Inaddition,thedrivewillbenoisierbecausetoothinterferencesgeneratehighdynamicloads.Interferencestypicallyareeliminatedbyrelivingthetoothtip,thetoothflank,orboth.Suchprofilemodificationsareespeciallyimportantforhighload,highspeeddrives.Thegraphofsoundpressurelevelvstipreliefillustrateshowtoothprofilemodificationscanaffectoveralldrivenoise.Ifthetipreliefislessthanthisoptimumvalue,drivenoiseincreasesbecauseofgreatertoothinterferenceagreateramountoftipreliefalsoincreasenoisebecausethecontactratioisdecreased.Tightermanufacturingtolerancesalsoproducequietiergears.Tolerancesforsuchparametersasprofileerror,pitchAGMAqualitylevel.Forinstance,thegraphdepictingSPLvsbothspeedandgearqualityshowshownoisedecreasesexample,noiseisreducedsignificantlybyanincreaseinaccuracyfromanAGMAQn11qualitytoanAGNAQn15quality.However,formostcommercialdriveapplications,itisdoubtfulthattheresultingsubstantialcostincreaseforsuchanaccuracyimprovementcanbejustifiedsimplyonthebasisofreduceddrivenoise.Previously,itwasmentionedthatgearsmusthaveadequateclearancewhenloadedtopreventtoothinterferenceduringthecourseofmeshing.Tipandflankreliefarecommonprofilemodificationsthatcontrolsuchinterference.Gearsalsorequireadequatebacklashandrootclearance.Noiseconsiderationsmakebacklashanimportantparametertoevaluateduringdrivedesign.Sufficientbacklashmustbeprovidedunderallloadandtemperatureconditionstoavoidatightmesh,whichcreatesexcessivelyhighnoiselevel.Atightmeshduetoinsufficientbacklashoccurswhenthedriveandcoastsideofatoothareincontactsimultaneously.Ontheotherhand,gearswithexcessivebacklashalsoarenoisybecauseofimpactingteethduringperiodsofnoloadorreversingload.Adequatebacklashshouldbeprovidedbytooththinningratherthanbyincreaseincenterdistance.Tooththinningdosenotdecreasethecontactratio,whereasanincreaseincenterdistancedoes.However,tooththinningdoesreducethebendingfatigue,areductionwhichissmallformostgearingsystems.

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