机械英文短章Gears Shafts and Bearings.docx

机械英文短章Gears Shafts and Bearings.docx

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机械英文短章GearsShaftsandBearings

CHAPTER19

Gears,Shafts,andBearings

LearningObjectives

Uponcompletionofthischapteryouwillbeabletoaccomplishthefollowing:

1.Recognizemechanicaldevicesthataredesignedtotransmitmotionfromonemachineelementtoanother.

2.Analyzegearblankstockmaterial,typesofhubs,andmethodsofattachinggearstoshafts.

3.Definethevariousgearcategoriesandbasicspecificationsformanufactureandinspection.

4.Explaincommongeartermsandsymbols.

5.Demonstrateanunderstandingofgear,shaft,andbearingdrawingpractices.

6.Developabilitytocommunicategear,shaft,andbearingdatabymeansofANSIstandarddimensioningandnotation.

7.Definethetypesandpurposesofbearingsandtheirrespectivehousingsandmountings.

8.ExploretheuseofCADingeardesign.

19.1Introduction

Thischaptercoversmechanicaldevicesdesignedtotransmitmotionfromonemachineelementtoanother.Mostgears（Fig.19.1）aremountedonshafts,which,inturn,aresecuredbybearingsinstalledinavarietyofhousings.Gearsaredesignedtotransferrotarymotionfromoneshafttoanother.Thespeedofthemotionisincreasedordecreasedbychangingthesizeofthedrivegearandthedrivengear.

Theselectionofgeartypesisbasedontherelativepositionoftheshafts.Theshaftswillbeeitherintersecting,nonintersecting,orparallel.Shaftscanbepositionedperpendicular,parallel,oratanygivenangletoeachother,dependingonthedesignapplication.Figure19.2showstwospurgearsmountedonparallelshafts.ShaftAholdsthedrivegear;shaftBholdsthedrivengear.Spurgearsarecommonlyusedtotransfermotionfromoneparallelshafttoanother.Inthisexample,thedrivegearissmallerthanthedrivengear;therefore,thedrivengearwilltakelongertocompleteonerevolutionanditsspeedwillbelessthanthatofthedrivegear.

Somegearsaredesignedtochangerotarymotionintoreciprocating（linear--backandforth）motion.Thesemachineelementsarecalledpinionsandgearracks.Anothermethodoftransmittingrotarymotionintoreciprocating（linear--upanddown）motionutilizesacamandfollowerassembly.ThismethodisdiscussedinChapter20,CAMS.

Whetherthetransferofmotionisrotarytorotaryorrotarytolinear,therotaryelementmustbemountedonsomekindofashaftandthatshaftmustrotatefreelyTherefore,shaftsrideonbearings[Fig.19.3（a）].Thegearitselfmayhaveaintegralkeyseattoprovideapositiveconnectiontotheshaft（Figure19.3（b）].

19.2Gears

Thischapterintroducesthemostcommontypesofgears,showshowtheyarerepresentedondrawings,andprovidesmethodsforcallingouttheirspecifications.Gearspecificationsarethemostimportantinformationthatissuppliedtothegearmanufacturer.Gearscomeinmanystyles,includingspurgears,pinions,ringgears,wormgears,bevelgears,mitergear,hypoidbevelgears,racks,andothers.Gearsaremadeofmetalsandnonmetals.Spurgearsarethemostcommongeartypemanufactured.

19.2.1GearTeeth

Gearteethareprojectionsdesignedtofitintothetoothspacesofmatinggearsandcontactmatingteethalongacommonlineknownasthepressureline.Thepressurelineisalsocalledthelineofaction（Fig.19.4）.Themostcommonformforthetoothflankisinvolute.Thepressurelinedeterminestheparticularinvoluteshape.TheAmericanNationalStandardsInstitute（ANSl）hasstandardizedtwopressureangles141/2°（nowrarelyused）and20°（Fig.19.5）.

19.2.2Gears,SplinesandSerrations,andRacks

Thefollowingtermsanddescriptionsdefinethegeneralgearcategoriescoveredinthechapter:

SpurgearsGearsconnectingparallelshaftsandhavingstraightteethelements（paralleltotheaxisoftheshafts）.Thesmallergearofapairofgearsiscalledapinion.

HelicalgearsGearsconnectingshaftswithprojectednonintersectingcenterlines.Helicalgearteethelementsarespiralorhelicalinshape.

BevelandmitergearsGearsthatareconicalinformandoperateonshaftshavingprojectedintersectingcenterlines.Whenthegearsaredifferentsizes,theyarecalledbevelgears.Bevelgearsofthesamesize（onetooneratio）withshaftsintersectingatrightangles,arecalledmitergears.

InternalgearsGearsconnectingparallelshafts,withteethelementsthatareeitherstraightorhelicalandwithapitchcirclethatistangentinternallytothematingspurorspiralgear.

WormgearsGearsconnectingnonparallel,nonintersectingshafts.Theyhaveteethelementsthatarehelical.Wommgearinggenerallyiscomposedofaworm（screw）andwommwheel（gear）inmatchedsets.

RackgearArackmaybeconsideredtobeagearofaninfinitelylongpitchradius.Thepitchlineofarackisastraightline;thepitchisdescribedasthelinearpitch.Arackgearisaflatspurgear.

Splinesands