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ID:241129
大小:52.00 KB
页数:11页
时间:2017-07-13
《轴承的摩擦与润滑 毕业设计论文外文文献翻译》由会员上传分享,免费在线阅读,更多相关内容在学术论文-天天文库。
1、Friction,LubricationofBearingInmanyoftheproblemthusfar,thestudenthasbeenaskedtodisregardorneglectfriction.Actually,frictionispresenttosomedegreewhenevertwopartsareincontactandmoveoneachother.Thetermfrictionreferstotheresistanceoftwoormorepartstomovement.Frictionisharmfulorvaluabledependi
2、nguponwhereitoccurs.frictionisnecessaryforfasteningdevicessuchasscrewsandrivetswhichdependuponfrictiontoholdthefastenerandthepartstogether.Beltdrivers,brakes,andtiresareadditionalapplicationswherefrictionisnecessary.Thefrictionofmovingpartsinamachineisharmfulbecauseitreducesthemechanical
3、advantageofthedevice.Theheatproducedbyfrictionislostenergybecausenoworktakesplace.Also,greaterpowerisrequiredtoovercometheincreasedfriction.Heatisdestructiveinthatitcausesexpansion.Expansionmaycauseabearingorslidingsurfacetofittighter.Ifagreatenoughpressurebuildsupbecausemadefromlowtempe
4、raturematerialsmaymelt.Therearethreetypesoffrictionwhichmustbeovercomeinmovingparts:(1)starting,(2)sliding,and(3)rolling.Startingfrictionisthefrictionbetweentwosolidsthattendtoresistmovement.Whentwopartsareatastateofrest,thesurfaceirregularitiesofbothpartstendtointerlockandformawedgingac
5、tion.Toproducemotionintheseparts,thewedge-shapedpeaksandvalleysofthestationarysurfacesmustbemadetoslideoutandovereachother.Therougherthetwosurfaces,thegreaterisstartingfrictionresultingfromtheirmovement.Sincethereisusuallynofixedpatternbetweenthepeaksandvalleysoftwomatingparts,theirregul
6、aritiesdonotinterlockoncethepartsareinmotionbutslideovereachother.Thefrictionofthetwosurfacesisknownasslidingfriction.Asshowninfigure,startingfrictionisalwaysgreaterthanslidingfriction.Rollingfrictionoccurswhenrollerdevcesaresubjectedtotremendousstresswhichcausethepartstochangeshapeordef
7、orm.Undertheseconditions,thematerialinfrontofarollertendstopileupandforcestheobjecttorollslightlyuphill.Thischangingofshape,knownasdeformation,causesamovementofmolecules.Asaresult,heatisproducedfromtheaddedenergyrequiredtokeepthepartsturningandovercomefriction.Thefriction
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