机械工程导论翻译第三章

13.1OVERVIEWAsintroducedinChapter1,theprimaryobjectiveofmechanicalengineersistomakemachinesthatwork,thatareuseful,andthatimprovesocietyinsomeway.Machinesingeneral,andpowertransmissionequipmentinparticular,areoftenconstructedofstandardizedcomponentsorbuildingblocks.Justasanelectricalengineermightselectoff-the-shelfresistors,capacitors,andtransistorsastheelementsofacircuit。mechanicalengineershavegoodintuitionforspecifyingthedifferentavailabletypesofbearing,shafts,gears,beltsandothercomponentsintheirdesigns.Oncethedetailsofamachinehavebeenworkedoutandthecomponentsforithavebeenselected,theproductthenneedstobebuilt.Mechanicalengineersalsodevisemanufacturingandassemblyprocesses,andattheconclusionofthischapterwewillintroduceseveralcommonmachinetools.Asolidappreciationofmachinecomponentsandtoolsisvaluableatthreelevels.First,andfromourprimarystandpointinthistextbook,aworkingknowledgeofthishardwareisimportantforyoutodevelopatechnicalvocabulary!Mechanicalengineeringhasitsownpreciselanguage,andspecializedtermsareusedtodescribetheconstructionandoperationofmachines.Tocommunicateeffectively2withotherengineers,youwillneedtolearn,adopt,andsharethatlanguage.Second,havingsuchabackgroundisnecessarytoselectthepopercomponentforanapplicationandtospecifytheappropriatemachinetoolforaproductionprocess.Ofcourse,itisnotpossibletolistanddescribeeverymachineandcomponentthatembodiesmechanicalengineeringprinciples,andthatisnotourintentioninthischapter.However,byexaminingjustafewmachinecomponentsandtools,youwilldevelopagrowingappreciationforpracticalmachineryissues.Finally,thischapterisagoodplaceforyoutobeginthejourneyofunderstandingtheinnerworkingsofmechanicalhardware.It'sintellectuallyhealthyforyoutobecuriousaboutmachines,wonderhowtheyweremade,dissectthem,andthinkabouttheycouldhavebeenmadedifferentlyorbetter.3.1概述正如第1章介绍，机械工程师的首要目标是制造机器，制造能够工作的、有用的并以某种方式，改善社会的机器。普通机器，以及像电力传输设备这样的特殊机器设备，往往是由各种标准化的部件或称为“模块”组成的。就像一个电气工程师可能会选择现成的电阻，电容和晶体管做为电路元件，机械工程师在他的设计中可以得心应手的选择合适类型的轴承、轴、齿轮、皮带和其它机械零件。一旦一种机器的设计方案制订出来，它的组件被选定，然后需要制造产品。机械工程师也设计制造和装配工艺过程，在本章结尾，我们将介绍几种常见的机床。3牢固全面地了解各种机器零件和机床，在三方面是有价值的。首先，也是本教科书的最初目的，了解硬件工作方面的知识，对扩展你的技术词汇是非常重要的。机械工程有它自己独特的语言，需要使用一些专业术语来描述机械的结构和操作。为了有效地与其他工程师沟通交流，你需要学习，采纳，并分享这种语言。其次，为某种实际应用场合选择合适的部件，为某种生产工艺选择合适的机床，都需要你具有各种机械零件和各种机床的知识。当然，我们不可能列出并描述机械工程学科所包括的所有机器和零件，这也不是本章的目的。然而，仅仅通过学习几种机械零件和机床的知识，将增进你对实际机械问题的了解和掌握。最后，通过本章的学习，能让你很好地了解机械零件的内部结构和工作原理。激发起你对机器的好奇心，想想他们是如何制成的，剖析他们，研究他们如何能够作出不同或更好。3.2ROLLINGELEMENTBEARINGSBearingareusedtoholdshaftsthatmustrotaterelativetosupportsthatarefixedforinstance,thehousingofamotor,gearbox,ortransmission.Awidevarietyofbearingexist,andeachparticulartypeisappropriateforadifferentsetofinstallationandoperatingconditions.Althoughaballbearingmightbethebestchoiceforoneapplication,itmightnothaveenoughload-carryingcapabilityforanother.Thecommonfeatureofallbearingisthattheyenableshaftstorotatewithlittleresistancewhilebeingwellsupportedagainstforcesactinginotherdirections.4Bearingareclassifiedintotwobroadgroups,calledrollingcontactandjournal.Inthischapter,wediscussonlyrollingcontactbearings;thesetypicallycompriseaninnerrace;anouterrace;rollingelementsintheformofballs,cylinders,orcones,andaseparatorthatpreventstherollingelementsintheformrubbingagainstoneanother,Rollingcontactbearingareubiquitousinmachinedesign,andtheyarefoundinapplicationsasdiverseasbicyclewheels,roboticjoint,andautomobiletransmissions.Journalbearing,ontheotherhand,havenorollingelements.Instead,theshaftsimplyrotateswithinapolishedsleevethatislubricatedbyoiloranotherfluid.Justasthepuckonanairhockeytableslidessmoothlyoverathinfilmofoil.Alsoubiquitous,journalbearingsupportcrankshaftsininternalcombustionenginesandshaftsinsidepumpsandcompressors.3.2滚动轴承轴承是用来支撑轴的，而轴又必须相对于固定的支撑物旋转，例如，一个电机、变速箱的壳体（轴承座）就是这样固定的支撑物。目前，有各种类型的轴承存在，每一种特定的类型都适合不同的安装和工作条件。尽管球轴承可能是某种应用场合的最佳选择，但它对另一种场合却没有足够的承载能力。所有球轴承的共同特点是，它们使轴在转动时阻力最小，同时能承受其它方向的载荷。轴承可分为两大类，称为滚动轴承和滑动轴承。在这一章中，我们5只讨论滚动接触的轴承。滚动轴承通常包括内圈、外圈、滚动体和分离架，其中滚动体常见的有：球，圆柱体，圆锥体，而分离架的作用是防止滚动体相互接触而产生摩擦。滚动接触轴承在机械设计中无处不在，在自行车的车轮，机器人关节，汽车零件等场合随处可以看到它们的身影。另一方面对于滑动轴承来讲，它没有滚动体。取而代之的是，轴只简单地在抛光的套筒内旋转，由油或空气形成的薄膜来润滑。Justasthepuckonanairhockeytableslidessmoothlyoverathinfilmofoil.就像在冰球桌上的冰球光滑地在油膜上滑动一样。滑动轴承的应用也是无处不在，如内燃机曲轴、泵和压缩机的轴都是由滑动轴承支撑的。AsampleinstallationofarollingcontactbearingisillustratedinFigure3.2.Theshaftandthebearing’sinnerracerotatetogether,whiletheouterraceandthetransmission’shousingarestationary.Whenashaftissupportedinthisway,thebearing’souterracewillfittightlyintoamatchingcircularrecessthatisformedinthehousing.AstheshaftinFigure3.2turnsandtransmitspower,thebearingcouldbesubjectedtoforcesdirectedeitheralongtheshaft(calledathrustforce)orperpendiculartoit(aradialforce).Anengineerwillmakeadecisiononthetypeofbearingtobeusedinamachinedependingonwhetherthrustforces,radialforces,orsomecombinationofthetwoactonthebearing.Thus,engineersapplythepropertiesofforcesystems(asdescribedinChapter4)whentheyanalyzeandselectbearingsandothermachinecomponents.6滚动轴承安装示意图如图3.2所示。轴和轴承的内圈一起旋转,而外圈和传输箱是静止的。当轴以这种方式安装时,轴承的外圈与传动箱紧密配合。当图3.2的轴旋转并传输功率时,轴承可能