机械原理(双语)试卷1答案

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1、机械原理（双语）试卷答案一、Checkpoint(markyouranswerontheanswersheetbyTrueorFalse)(20points2points/each)1.ThegradeofamechanismisdefinedasthehighestgradeofAssurgroupinthemechanism.(T)2.AnymechanismwhichhasdeterminedmotionscanbeassembledfromabasicmechanismbyconnectingAssurgrouptothedeterminedl

2、inksusingouterpairs,groupbygroup.Thisisthecompositionprincipleofmechanism(T)3.Inversionofamechanismcanchangetherelativemotionbetweenitslinks(F)4.Ifamechanismhasquickreturncharacteristics,thenweshouldsetthefasterstrokeasworkingstroke.(F)5.Inacammechanism,thesineaccelerationmotion

3、curvewouldproducesoftimpulse(F)6.Arotorisdynamicallybalancedifithasbeenstaticallybalanced.(F)7.Ifcrank-rockermechanism(rockerisdriver)reachit’sdeadpoint,thentransmissionangleγisequaltozero.(T)8.Iftheefficiencyofmechanismη≤0,thenthemechanismisself-lockmechanism.(T)9.Thepressurean

4、gleremainsthesameatallpointsonthetoothprofilesoftherack(ak=a),whichisalsocalledthenormalpressureangle.(T)10.Thepropermeshingconditionsforexternalhelicalgearismn1=mn2=mn,an1=an2=anandβ1=－β2(T)二．CalculatethedegreeoffreedomofthemechanismasshowninFig(10points)Solution:A：passiveDOFn=

5、8,pl=11,ph=1F=3n-2Pl-Ph(3points)=3x8-2x11-1=1(7points)三.InthecammechanismwithinlinetranslatingFlat-facedfollowershowninFig,thecamisacirculardisk(Theradiusofthecirculardiskr=50mm).lA0=202mmSupposingthattheangularvelocityω1ofthecamisknown,ω1=10rad/s,thevelocityV2ofthefollower2isto

6、befoundwhenθ=45°byinstantcentermethod.（15points）Solution:BecausepointcisinstantcentrebetweenCam1andfollower2,(6points)2then:BVC2=VC1=ω1.LAC=10x202x2/2=200mm/S(6points)Becausefollower2isatranslatingfollowerThen:1OV2=VC2=200mm/S(3points)θcA四.Anoffsetslider-crankmechanismisasshowni

7、nFig.Supposingthatthelengtha=120mm，b=600mm，offsete=120mm.findoutthecoefficientKoftravelspeedvariationandTheworkingstrokeHbygraphicmethod(15points)Solution:Selectul=1/10,drawouttwolimitingpositions(AC1,AC2)ofslider-crankmechanismAeC2C1AC1=AB+BCAC2=BC-BC(6points)0THENθ=∠C1AC2=4.9(

8、3points)K=（180+θ）/（180-θ）=1.06(3points)H=C1C2/u