54883《机械原理（英语）（第2版）》（基本课件）Chapter 1 Introduction

TheoryofMachinesandMechanismsChapter1Introduction1.

TheConceptofMachinery1.1GeneralInformation(1)MechanismAmechanismisadevice,whichproducesspecificmechanicalmotions.Thus,thefunctionofamechanismistotransmitandmodifyamotion.Weoftenuseadrawingwithsomesimplelinesandsymbolstodescribethemechanism,anditiscalledkinematicaldiagram.

(2)MachineAmachineisadevicewhichproducesspecificmechanicalmotions,anditcantransmitormodifymechanicalenergy,materialsandinformation.Amachinemaybeamechanismoracombinationofmechanismscapableoftransmittingormodifyingmotionandmechanicalenergy.

(3)MachineryIntheviewpointofkinematics,mechanismandmachinehavenodifference，somechanismsandmachinesaregenerallycalledmachineryinmechanicalengineering.Thereisnoenergytransformingandmodifyinginamechanism,forexample,mechanicalwatchisamechanismbutnotamachine,becauseitcannottransformenergy.Fig.1-1Internalcombustionengineandscheme(内燃机及其机构简图)

1—piston(活塞)2—coupler(连杆)3—crankshaft(曲轴)4、5、6—gear(齿轮)7—cam(凸轮)

8—follower(推杆)9—lever(杠杆)10—valve(气门)11—frame(缸体)2.CompositionofMachinesFig.1-2Compositionofmachine(机器组成示意图)

Amachineconsistsofprimepower,transmission

systemandworkingsystem.Amodernmachinealsocontainsacontrolsystem.Fig.1-2showstheblockdiagramofcompositionofamachine.Fig.1-3Automaticgate(电动大门示意图)

1—driver(驱动器)2—gate(大门)3—electromotor(电动机)4—reducer(减速器)

5—chaindriver(链传动)6—rollers(滚轮)

Fig.1-3bshowsadiagramofanautomaticgate.Fig.1-3ashowsthecompositionofthedriver.Theprimepoweriselectromotor3whichhasahighspeed.Butgate2needsalowspeedtowork,sowehavetoinstallgearmechanismsandchainmechanismstomodifythespeedofelectromotor3.Thereducer4andchaindriver5aretransmissionsystem.Gate2consistingoftheparallellinkagesiscalledworkingsystem.Fig.1-4Constituteofthecoupler(连杆的组成)

1—bodyofthecoupler(连杆体)2—bearing(小端轴承)3—bearingbush(轴瓦)

4—headofthecoupler(连杆头)5—bolt(螺栓)6—nut(螺母)7—washer(垫圈)1.2TeachingContentandObjectoftheCourse1.ResearchObjectoftheCourse

Theresearchobjectofthecourseisthetheoryofmachinery,andmachinerycontainsmechanismsandmachines.Therefore,thetheoryofmechanismsandmachinesistheimportantcontentofthiscourse.

Thepiston,coupler,crank,cam,gearandsoonshowninFig.1-1aresmallestmotionunits,andtheyarecalledlinks.Themotionoflinksisanimportantcontentforus.

2.ContentofThisCourse(1)StructuralanalysisofmechanismsItinvolvesthecompositionofmechanism,kinematicaldiagram,calculationofdegreeoffreedomandanalysisofmechanism.

(2)KinematicsofmechanismsItdealswiththerelativemotionsoflinksandforcesactingonthelinks,thusitinvolvesthevelocityanalysis,accelerationanalysis,forceanalysisandsynthesisofmechanism.

(3)DynamicsofmachineryItinvolvestheinertiaforcesbalancingandthefluctuationinspeedandregulation.1.Purpose

Themachineryiscloselyrelatedtohumanlife,economicconstruction,andnationaldefenseconstruction,somechanicalindustrycanreflectaindustrylevelofacountryandalevelofscienceandtechnology.Althoughtherearealotofmachines,themechanismsconsistingmachinesarefinite.Soonthebasisoffinitemechanismstodesignsomenewmachineisacreativework.Itisalsothepurposeofthiscourse.Thiscourseisthebasisofsomeothersubsequentcourses,suchasMachineryDesign,AdvancedKinematicsandDynamicsofMechanisms,MechanicalManufacture,etc.Sothepurposeofthiscourseisalsotolaythefoundationforthefurthermachinedesign.1.3PurposeofThisCourse2.Conclusion1)Machineryisveryimportanttodeveloptheeconomy.2)TheoryofMechanismandMachinesisveryusefultodesignmachines.3)Mechanismisadevicewhichcantransmitandmodifymotion.4)Thekinematicaldiagramisasimpleformtodescribethemechanisms.5)Amachinemaybeamechanismorcombinationofmechanismscapableoftransmittingormodifyingmotionandmechanicalenergy.6)Machineryisthegenerictermofmechanismsandmachines.7)Alinkisasma