54883《機(jī)械原理（英語）（第2版）》基本課件Chapter 9 Introduction of Screws,Hooks Couplings and Intermittent Mechanisms

Chapter9IntroductionofScrews,HooksCouplingsandIntermittentMechanisms

Fig9-1ashowsascrewmechanismwhichconsistsofscrew1,nut2andframe3,inwhichthepairsA,BandCarescrewpair,turningpairandprismaticpairrespectively.

Supposetheleadofscrewisl,andwhenthescrewrotatesanangle,thedistancemovedbythenutisasfollows:

Fig.9-1Screwmechanisms(螺旋機(jī)構(gòu))

1—Screw(螺桿)2—Nut(螺母)3—Frame(機(jī)架)A—Screwpair(螺旋副)

B—Turningpair(轉(zhuǎn)動(dòng)副)C—Slidingpair(移動(dòng)副)D—Screwpair(螺旋副)9.1ScrewMechanismsFig.9-2Rollerscrewmechanism(滾珠絲杠傳動(dòng)機(jī)構(gòu))

Accordingtothefrictionpropertybetweenthethreadandnut,thescrewscanbeclassifiedasslidingscrewsandrollingscrews.Squarethreadandacmethreadareusedtoscrewsbywhichpoweristobetransmitted.Fig9-2showsarollingscrewinwhichrollersarelocatedbetweenthethreadsofthescrewandnut.Itiscalledasarollerscrewmechanismwhichhashighefficiencyandlonglifeanditisusedwidelyasapowerscrew.

Fig.9-3Singleuniversaljoint(單萬向聯(lián)軸器)9.2UniversalJoints1.SingleUniversalJoint

Fig9-3ashowsasingleuniversaljointwhichconsistsofthedrivingyoke,drivenyokeandcrosspiecewhichconnectsthetwoyokes.TheaxesofthedrivingshaftanddrivenshaftintersectatpointOwhichistherotatingcenterofthecrosspiece,andtheacuteanglebetweenthetwoshaftsisdenotedasα.Althoughbothshaftsmustcompletearevolutioninthesamelengthoftime,theangularvelocityratiooftheshaftsisnotaconstantduringtherevolution,butvariesasafunctionoftheanglesαandφ1.AsshaftsIandⅡrotate,thepointAandBdescribetwocirclesshowninFig9-3b.

2.DoubleUniversalJoints

TohavethesameangularvelocitiesoftheinputshaftⅠandoutputshaftⅡ,thefollowingconditionsmustbesatisfied.

1)ThedrivingshaftⅠ,drivenshaftⅡandtheintermediateshaftⅢmustlieinthesameplane.

2)Thetwoyokesoftheintermediateshaftmustlieinthesameplane.3)Theanglebetweenthedrivingshaftandintermediateshaftmustbeequaltotheanglebetweenthedrivenshaftandtheintermediateshaft.Fig.9-4Doubleuniversaljoints(雙萬向聯(lián)軸器)9.3RatchetMechanisms

Fig9-5showsaratchetmechanismwhichconsistsofdrivingrock1,ratchetwheel3,pawls2、4andspring5.Asthedrivingrock1ismadetooscillatecounterclockwiseaboutthecenteroftheratchet,pawllevers,theratchet3willrotatecounterclockwisewithanintermediatemotion.Whenthedrivingrockoscillatesclockwise,thedrivingpawlwillslideoverthetoothbackoftheratchet,theholdingpawlwillmovethetoothspacetopreventtheratchetfromreversing.Fig.9-5Ratchetmechanism(棘輪機(jī)構(gòu))

1—drivingrocker(主動(dòng)搖桿)

2—drivingpawl(棘爪)3—ratchet(棘輪)

4—holdingpawl(止回棘爪)5—spring(彈簧)1.TypesandApplicationsofRatchetMechanisms(1)ToothratchetmechanismTheteethofratchetareoftenmadeintotheshapesofobliquetriangle,trapezoidandrectangle.Fig9-6ashowsanexternalratchetmechanism,Fig9-6bshowsaratchetrackmechanism,andFig9-6cshowsaninternalratchetmechanism.

Fig.9-6Toothratchetmechanisms(齒式棘輪機(jī)構(gòu))

InFig9-7a,thedrivingpawlswillpushtheratchettorotate,andinFig9-7b,thepawlswillpulltheratchettorotate.Fig.9-7Double-functionratchetmechanisms(雙動(dòng)式棘輪機(jī)構(gòu))Fig9-8showssomeratchetmechanismsinwhichtheratchetscanrotateindoubledirections.Theseratchetwheelsusuallyhaverectangularteeth,whilethepawlshouldbereversible.InFig9-8aandFig9-8b,thepawlcaninverseitselfaboutitscenterO,andinFig9-8candFig9-8d,thepawlmustbeliftfirst,thenrotate180°aboutitsaxisanddropdown.Fig.9-8Ratchetmechanismsinwhichtherotatingdirectioncanbechanged(雙向棘輪機(jī)構(gòu))Fig.9-9Table-feedmechanismofashaper

(牛頭刨床工作臺(tái)橫向進(jìn)給機(jī)構(gòu))Fig9-9showsatablefeedsystemofashaper.Thissystemconsistsofaratchetmechanism,agearmechanismandalinkage.

(2)SilentratchetmechanismSilentratchetmechanismsareshowninFig9-10.Therearenoteethontheratchet,andthedevicesdependuponthewedgingofsmoothsurfacebetweenthepawlandratchet.InFig9-10a,theintermittentrotationoftheratchetdependsuponthefrictionalforcebetweenthepawlandtheratchetsurface,soithasnonoisetotransmitmotion.Fig9-10bandFig9-10careinternalratchetmechanisms.InFig9-10c,thepawlsarerollerslocatedbetweenthestarwheel1andtoroid3.Fig.9-10Silentratchetmechanisms(摩擦式棘輪機(jī)構(gòu))2.DesignOutlinesofRatchetMechanisms(1)ModuleandnumberofteethThedimensionsofratchetwheelaredependeduponthemodulelikegears,andthenumbersofteethoftheratchetaredependedupontheminimumrotatingangleθmin.

Fig.9-11Designofratchetmechanism

(棘輪機(jī)構(gòu)的設(shè)計(jì))(2)ObliqueangleofthetoothsurfaceandactionangleofthepawlFig9-11showsatoothratchetmechanism,inwhichthelineofactionn—nofthedrivingpawlandtoothmustpassthroughthelineO1O2,sothatthepawlremainsincontactwiththetooth.Inthisfigure,theangleαbetweenthetoothsurfaceABandtheradiallineO2Aiscalledtheobliqueangleofthetooth;theangleδbetweenthelineconnectedtheaddendumAandthepawlcenterO1andthelineofn—npassingthroughpointAiscalledtheactionangleofthepawl.9.4GenevaMechanismsGenevamechanismsareveryusefulinproducingintermittentmotion.AGenevamechanismconsistsofaGenevawheel,acrank,alockingplateandaframe,asshowninFig9-12a.Fig9-12ashowsaGenevamechanisminwhichtherotatingdirectionofthewheelisoppositewiththatofcrank;andintheinternalGenevamechanism,thewheelandcrankhavethesamedirectionofrotation,asshowninFig9-12b.

Fig.9-12Genevawheelmechanisms1(槽輪機(jī)構(gòu)1)1.TypesoftheGenevaMechanisms

Fig9-13ashowsaspatialGenevamechanisminwhichtheaxesoftherollerandcrankareintersectingatthespherecenter.Fig9-13bshowsareciprocatingGenevamechanism.ThecontinuousrotationofthecrankcanbechangedintointermittenttranslationoftheGenevarack.

Fig.9-13Genevawheelmechanisms2(槽輪機(jī)構(gòu)2)Fig.9-14Genevawheelmechanismshavingspecialfunctions

(有特殊功能的槽輪機(jī)構(gòu))Sometimes,whenthecrankrotatesonerevolution,andtheintermittentperiodofthewheelisnotthesame,wecanusethemechanismsshowninFig9-14.

Iftherollersaredistributedunevenly,thetimesforeachdwelloftheGenevawillbedifferent,asshowninFig9-14a.Ifwewanttoincreasethesmoothnessofthewheel,wecanusetheGenevawheelinwhichtheslotismadetobeacurveshape,asshowninFig9-14b.

2.DesignOutlinesofGenevaMechanisms(1)ActioncoefficientofthewheelInFig9-15,whenthecrankrotatesonerevolutionwithatimet1,thetimewithwhichthewheelhaveaintermittentmotionist2,theratiobetweenthetimet2andt1isdefinedastheactionratio.

(2)ThenumberofteethofthewheelSincetheratioisalwaysgreaterthanzero,thenumberofteethmustbegreaterthanthree.(3)DimensionsThedimensionoftheGenevamechanismcanbeconsultedfromsomehandbook..Fig.9-15DesignofGenevawheel(槽輪機(jī)構(gòu)設(shè)計(jì))Fig.9-16ApplicationofGenevamechanisms(槽輪機(jī)構(gòu)的應(yīng)用)1.CylindricalCamIndexingMechanisms9.5IndexingCamMechanisms

Fig9-17ashowsacylindricalcamindexingmechanisminwhichthecamisacylinderwithagroovecurve,andtherollersontheturningplatematewiththegroove.Fig9-17bshowsacylindricalcamindexingmechanisminwhichthecamisacylinderwitharidgecurve,andtherollersontheturningplatematewiththespacesoftheridge.Fig.9-17Cylindricalcamindexmechanisms(圓柱凸輪分度機(jī)構(gòu))

1—drivingcam(主動(dòng)輪)2—drivenplate(從動(dòng)輪)2.WormcamIndexingMechanismsFig.9-18Worm-camindexmechanisms(蝸桿式凸輪分度機(jī)構(gòu))

1—drivingcam(主動(dòng)輪)2—drivenplate(從動(dòng)輪)

Thiscammechanismissimilartoasinglethreadwrappedonacylinder,asshowninFig9-18,andtheplatedistributedrollersradicallyissimilartoawormgear.Thedifferencebetweenthecylindricalcamandthiscamisthatthepitchoftheconvexorconcavecurvesurfacesonthecylindricalcamarezero,andthepitchonthiscamisnotzero.Therotationofthedrivenplatedependsonthepitchofthethread.9.6IntermittentGearMechanismsThismechanismisaintermittentdeviceinwhichthedrivinggea