鹅鹅鹅饿饿.docx

鹅鹅鹅饿饿.docx

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鹅鹅鹅饿饿

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1英文文献翻译

1.1Gearmechanisms

Gearmechanismsareusedfortransmittingmotionandpowerfromoneshafttoanotherbymeansofthepositivecontactofsuccessivelyengagingteeth.Inabout2,600B.C.,ChineseareknowntohaveusedachariotincorporatingacomplexseriesofgearslikethoseillustratedinFig.2.7.Aristotle,inthefourthcenturyB.C.wroteofgearsasiftheywerecommonplace.InthefifteenthcenturyA.D.,LeonardodaVincidesignedamultitudeofdevicesincorporatingmanykindsofgears.Incomparisonwithbeltandchaindrives,geardrivesaremorecompact,canoperateathighspeeds,andcanbeusedwhereprecisetimingisdesired.Thetransmissionefficiencyofgearsisashighas98percent.Ontheotherhand,gearsareusuallymorecostlyandrequiremoreattentiontolubrication,cleanliness,shaftalignment,etc.,andusuallyoperateinaclosedcasewithprovisionforproperlubrication.

Gearmechanismscanbedividedintoplanargearmechanismsandspatialgearmechanisms.Planargearmechanismsareusedtotransmitmotionandspatialgearmechanisms.Planargearmechanismsareusedtotransmitmotionandpowerbetweenparallelshafts,andspatialgearmechanismsbetweennonparallelshafts.

Typesofgears

（1）Spurgears.ThespurgearhasacylindricalpitchsurfaceandhasstraightteethparalleltoitsaxisasshowninFig.2.8.Theyareusedtotransmitmotionandpowerbetweenparallelshafts.Thetoothsurfacesofspurgearscontactonastraightlineparalleltotheaxesofgears.Thisimpliesthattoothprofilesgointoandoutofcontactalongthewholefacewidthatthesametime.Thiswillthereforeresultinthesuddenloadingandsuddenunloadingonteethasprofilesgointoandoutofcontact.Asaresult,vibrationandnoiseareproduced.

（2）Helicalgears.Thesegearshavetheirtoothelementsatanangleorhelixtotheaxisofthegear（Fig.2.9）.Thetoothsurfacesoftwoengaginghelicalgearsinnplanargearmechanismscontactonastraightlineinclinedtotheaxesofthegears.Thelengthofthecontactlinechangesgraduallyfromzerotomaximumandthenfrommaximumtozero.Theloadingandunloadingoftheteethbecomegradualandsmooth.Helicalgearsmaybeusedtotransmitmotionandpowerbetweenparallelshafts[Fig.2.9（a）]orshaftsatanangletoeachother[Fig.2.9（d）].Aherringbonegear[Fig.2.9（c）]isequivalenttoaright-handandaleft-handhelicalgearplacedsidebyside.Becauseoftheangleofthetooth,helicalgearscreateconsiderablesidethrustontheshaft.Aherringbonegearcorrectsthisthrustbyneutralizingit,allowingtheuseofasmallthrustbearinginsteadofalargeoneandperhapseliminatingonealtogether.Oftenacentralgrooveismadearoundthegearforeaseinmachining.

（3）Bevelgars.Theteethofabevelgeararedistributedonthefrustumofacone.Thecorrespondingpitchcylinderincylindricalgearsbecomespitchcone.Thedimensionsofteethondifferenttransverseplanesaredifferent.Forconvenience,parametersanddimensionsatthelargeendaretakentobestandardvalues.Bevelgearsareusedtoconnectshaftswhicharenotparalleltoeachother.Usuallytheshaftsare90deg.toeachother,butmaybemoreorlessthan90deg.Thetwomatinggearsmayhavethesamenumberofteethforthepurposeofchangingdirectionofmotiononly,ortheymayhaveadifferentnumberofteethforthepurposeofchangingbothspeedanddirection.Thetoothelementsmaybestraightorspiral,sothatwehaveplainandspiralbevelgears.Hypoidcomesfromthewordhyperboloidandindicatesthesurfaceonwhichthetoothfacelies.Hypoidgearsaresimilartobevelgears,butthetwoshaftsdonotintersect.Theteetharecurved,andbecauseofthenonintersectionoftheshafts,bearingscanbeplacedoneachsideofeachgear.Theprincipaluseofthidtypeofgearisinautomobilerearendsforthepurposeofloweringthedriveshaft,andthusthecarfloor.

（4）Wormandwormgears.Wormgeardrivesareusedtotransmitmotionandowerbetweennon-intersectingandnon-parallelshafts,usuallycrossingatarightangle,especiallywhereitisdesiredtoobtainhighgearreductioninalimitedspace.Wormsareakindofscrew,usuallyrighthandedforconvenienceofcutting,orlefthandeditnecessary.Accordingtotheenvelopingtype,wormscanbedividedintosingleanddoubleenveloping.Wormsareusuallydriverstoreducethespeed.Ifnotself-locking,awormgearcanalsobethedriverinasocalledback-drivingmechanismtoincreasethespeed.Twothingscharacterizewormgearing（a）largevelocityratios,and（b）highslidingvelocities.Thelattermeansthatheatgenerationandpowertransmissionefficiencyareofgreaterconcernthanwithothertypesofgears.

（5）Racks.Arackisagearwithaninfiniteradius,oragearwithitsperimeterstretchedoutintoastraightline.Itisusedtochangereciprocatingmotiontorotarymotionorviceversa.Alatherackandpinionisgoodexampleofthismechanism.

Geometryofgeartooth

Thebasicrequirementofgear-toothgeometryistheprovisionofangularvelocityrationsthatareexactlyconstant.Ofcourse,manufacturinginaccuraciesandtoothdeflectionswellcauseslightdeviationsinvelocityratio;butacceptabletoothprofilesarebasedontheoreticalcurvesthatmeetthiscriterion.

Theactionofapairofgearteethsatisfyingthisrequirementistermedconjugategear-toothaction,andisillustratedinFig.2.12.Thebasiclawofconjugategear-toothactionstatesthatasthegearsrotate,thecommonnormaltothesurfacesatthepointofcontactmustalwaysintersectthelineofcentersatthesamepointPcalledthepitchpoint.

Thelawofconjugategear-toothcanbesatisfiedbyvarioustoothshapes,buttheonlyoneofcurrentimportanceistheinvolute,or,moreprecisely,theinvoluteofthecircle.（Itslastimportantcompetitorwasthecycloidalshape,usedinthegearsofModelTFordtransmissions.）Aninvolute（ofthecircle）isthecurvegeneratedbyanypointonatautthreadasitunwindsfromacircle,calledthebasecircle.ThegenerationoftwoinvolutesisshowninFig.2.13.Thedottedlinesshowhowthesecouldcorrespondtotheouterportionoftherightsidesofadjacentgearteeth.Correspondingly,involutesgeneratedbyunwindingathreadwrappedcounterclockwisearoundthebasecirclewouldfortheouterportionsoftheleftsidesoftheteeth.Notethatateverypoint,theinvoluteisperpendiculartothetautthread,sincetheinvoluteisacirculararcwitheverincreasingradius,andaradiusisalwaysperpendiculartoitscirculararc.Itisimportanttonotethataninvolutecanbedevelopedasfarasdesiredoutsidethebasecircle,butaninvolutecannotexistinsideitsbasecircle.

Letusnowdevelopamatingpairofinvolutegearteethinthreesteps:

frictiondrive,beltdrive,andfinally,involutegear-toothdrive.Figure2.14showstwopitchcircles.Imaginethattheyrepresenttwocylinderspressedtogether.Ifslippagedoesnotoccur,rotationofonecylinder（pitchcircle）willcauserotationoftheotheratanangularvelocityratioinverselyproportionaltotheirdiameters.Inanypairofmatinggears,thesmallerofthetwoiscalledthepinionandthelargeronethegear.（Theterm“gear”isusedinageneralsensetoindicateeitherofthemembers,andalsoinaspecificsensetoindicatethelargerofthetwo.）Usingsubscriptspandgtodenotepinionandgear,respectively.

Inordertotransmitmoretorquethanispossiblewithfrictiondrivealone,wenowaddabeltdriverunningbetweenpulleysrepresentingthebasecircles,asinFig2.15.Ifthepinionisturnedcounterclockwiseafewdegrees,thebeltwillcausethegeartorotateinaccordancewithcorrectvelocityratio.Ingearparlance,angleΦiscalledthepressureangle.Fromsimilartriangles,thebasecircleshavethesameratioasthepitch;thus,thevelocityratioprovidedbythefrictionandbeltdrivesarethesame.

InFig.2.16thebeltiscutatpointc,andthetwoendsareusedtogenerateinvoluteprofilesdeandfgforthepinionandgear,respectively.ItshouldnowbeclearwhyΦiscalledthepressureangle:

neglectingslidingfriction,theforceofoneinvolutetoothpushingagainsttheotherisalwaysatanangleequaltothepressureangle.AcomparisonofFig.2.16andFig.2.12showsthattheinvoluteprofilesdoindeedsatisfythefundamentallawofconjugategear-toothaction.Incidentally,theinvoluteistheonlygeometricprofilesatisfyingthislawthatmaintainsaconstantpressureangleasthegearsrotate.Noteespeciallythatconjugateinvoluteactioncantakeplaceonlyoutsideofbothbasecircles.

Nomenclatureofspurgear

Thenomenclatureofspurgear（Fig.2.17）ismostlyapplicabletoallothertypeofgears.

Thediameterofeachoftheoriginalrollingcylindersoftwomatinggearsiscalledthepitchdiameter,andthecylinder’ssectionaloutlineiscalledthepitchcircle.Thepitchcirclesaretangenttoeachotheratpitchpoint.Thecirclefromwhichtheinvoluteisgeneratediscalledthebasecircle.Thecirclewherethetopsoftheteethlieiscalledthededendumcircle.Similarly,thecirclewheretherootsoftheteethlieiscalledthededendumcircle.Betweentheaddendumcircleandthededendumcircle,thereisanimportantcirclewhichiscalledthereferencecircle.Parametersonthereferencecirclearestandardized.Themodulemofagearisintroducedonthereferencecircleasabasicparameter,whichisdefinedasm=p/π.Sizesoftheteethandgearareproportionaltothemodulem.

The