高职机械类专业英语

书名：机械类专业英语ISBN：9787111429326出版社：机械工业出版社本书配有电子课件Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientPro.Copyright2004-2011AsposePtyLtd.1Unit3TransmissionofGears3.1Introduction3.2

AdvantagesandDisadvantages3.3TypeofGears

3.4GearFailuresandDesignCriteria

Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientPro.Copyright2004-2011AsposePtyLtd.2Gearsarethemostwidelyusedmethodoftransmittingpowerfromoneshafttoanotherortoarackwhichtranslatesinastraightline.Gearshavethecapacitiesoftransmittingseveraltenthousandkilowatts(kW)ofpowersandrotatingat150meterspersecond(m/s)ofpitch-linespeed.Themaximumpitch-linespeedthatgearscanachieveisapproximately300m/s.Themaximumoutsidediametersmaybemadeabove10meters(m)andthemaximumsingletransmissionratiomaybeabove8.Normally,thesmallgearofthetwomatinggearsiscalledthepinionandthelargeoneisreferredtoasthegear,regardlessofwhichisthedriverandwhichisthefollower.1Whenthepinionisonthedrivingshaft,thegearpairactsasaspeedreducer;whenthegeardrives,thegearpairisaspeedincreaser.3.1IntroductionBack→Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientPro.Copyright2004-2011AsposePtyLtd.33.2

AdvantagesandDisadvantagesof

Gears

3.2.1Advantages

highefficiencyhighreliabilityhighservicelifeconstanttransmissionratiocompactnesshavingalargerangeofpowerandspeedtransmitted.3.2.2Disadvantages

specialmachinetoolsareneededtomanufacturethegearsgreatqualityclassofgearsisrequiredtheycostmuchthevibrationandnoisewilloccurwhengearshavelowprecisionsgearscannotbeusedinthecaseoflargecenterdistance.Back→Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientPro.Copyright2004-2011AsposePtyLtd.43.3TypeofGears

●Therearefourmaintypesofgears:spur,helical,worm,andbevel

Spur:

Agearhavingtoothelementsthatarestraightandparalleltoitsaxisisknownasaspurgear.Aspurpaircanbeusedtoconnectparallelshaftsonly.Parallelshafts,however,canalsobeconnectedbygearsofanothertype.Spurgearsarethesimplestandleastexpensivetypeofgear;also,spur-geardefinitionsareusuallyapplicabletoothertypes.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientPro.Copyright2004-2011AsposePtyLtd.5(b)

Helical:Helicalgearsofferarefinementoverspurgears.Theleadingedgesoftheteetharenotparalleltotheaxisofrotation,butaresetatanangle.Sincethegeariscurved,thisanglingcausesthetoothshapetobeasegmentofahelix.Helicalgearshavecertainadvantages;forexample,whenconnectingparallelshaftstheyhaveahigherload-carryingcapacitythanspurgearswiththesametoothnumbersandcutwiththesamecutter.Becauseoftheoverlappingactionoftheteeth,theyaresmootherinactionandcanoperateathigherpitch-linevelocitiesthanspurgears.Helicalgearscanalsobeusedtoconnectnonparallel,non-intersectingshaftsatanyangletooneanother.Ninetydegreesisthecommonestangleatwhichsuchgearsareused.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientPro.Copyright2004-2011AsposePtyLtd.6(c)

Worm:Wormgearsprovidethesimplestmeansofobtaininglargeratiosinasinglepair.Theyareusuallylessefficientthanparallel-shaftgears,however,becauseofanadditionalslidingmovementalongtheteeth.Becauseoftheirsimilarity,theefficiencyofawormandgeardependsonthesamefactorsastheefficiencyofascrew.Single-threadwormsoflargediameterhavesmallleadanglesandlowefficiencies.Multiple-threadwormshavelargerleadanglesandhigherefficiencies.Forleadanglesofabout15degreesandacoefficientoffrictionlessthan0.15,theefficiencyrangesfromabout55percentto95percent,andthegearcandrivetheworm.Suchunitsmakecompactspeedincreasers;theyhavebeenusedfordrivingsuperchargersonaircraftengines.Inself-lockingworms,thegearcannotdrivetheworm,andtheefficiencyislessthan50percent.Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientPro.Copyright2004-2011AsposePtyLtd.7(d)

Bevel:

Fortransmittingrotarymotionandtorquearoundcorners,bevelgearsarecommonlyused.Theconnectedshafts,whoseaxeswouldintersectifextended,areusuallybutnotnecessarilyatrightanglestooneanother.Thepitchsurfacesofbevelgearsarerolling,truncatedcones,andtheteeth,whichmustbetaperedinboththicknessandheight,areeitherstraightorcurved.Althoughcurved-toothbevelgearsarecalledspiralbevelgears,thecurveoftheteethisusuallyacirculararc.Thecurvatureoftheteethresultsinoverlappingtoothactionandasmoothertransmissionofpowerthanwithstraightteeth.Forhighspeedsandtorques,spiralbevelgearsaresuperiortostraightbevelgearsinmuchthesamewaythathelicalgearsaresuperiortospurgearsforconnectingparallelshafts.3Back→Evaluationonly.CreatedwithAspose.Slidesfor.NET3.5ClientPro.Copyright2004-2011AsposePtyLtd.83.4GearFailuresandDesignCriteria

Thecommongearfailuresofgearteetharebreakage,pitting,abrasivewearing,gluingandridging.

Breakage

ofgeartoothgenerallyoccursatitsbase,becauseageartoothissomewhatsimilartoacantileverbeam.●twotypesofbreakageingearing:fatiguebreakageoverloadbreakage.

Pitting

isaphenomenonthatsmalllocalfracturesarelostfromthesurfaceofthetooth.Itisactuallythefatiguefailureofthetoothsurfacebecauseofmanyrepetitionsofhighcontactstresses.

Prolongedoperationonapitsurfac