《凸轮机构讲义》PPT课件.ppt
Chapter 4.Cams and Its Design,Sun Zhihong67792580,Todays topics,Characteristics of cam mechanisms凸轮机构的特点 Classification and application of cam mechanisms凸轮机构的分类及应用The followers motion type从动件的运动形式,What is a cam mechanism?A system/mechanism that uses a cam(凸轮)and a follower(从动件)to create a specific motion.In most cases,the cam is merely a flat piece of metal that has an unusual shape or profile(轮廓).The cam is attached to a shaft(轴)which enable it to be turned by applying a turning action to the shaft.As the cam rotates,it is the profile or shape of the cam that causes the follower to move in a particular way.The movement of the follower is then transmitted(传递)to another mechanism or another part of the mechanism.,Advantages of cam mechanismsSimple and compact(简单、紧凑)there only three links,cam(凸轮),follower(从动件),frame(机架).Sometimes,there is a roller(滚子).The follower can precisely realize any type motion.Example Disadvantages of cam mechanismsHigher pair(高副),line contact or point contact(线或点接触)Cant transfer(传递)large power.FunctionContinuous turning motion translating(往复移动)or oscillating(摆动)motion.Example,Application of cam mechanism,Camshaft（凸轮轴）of an Engine incorporating a Rocker Arm（摆臂）:The diagram shown right is a typical cam and follower system that could be used in an engine.This system incorporates a rocker arm.In this case the motion that the cam imparts on the follower is translated to the valve through a push rod and the rocker arm.,Dobby 多臂提花机,作者：潘存云教授,绕线机构,Yarn winding machine(绕线机),作者：潘存云教授,送料机构,Feeding mechanism 送料机构,Feeder of Automatic Machine Tools自动机床的进刀机构,园柱凸轮回转滚子带动推杆往复摆动刀架的进刀和退刀,机床进给机构,录音机卷带机构,织机开口机构,Classification of cam mechanism凸轮机构的分类,By the cam shape(按凸轮形状分)By the follower shape(按从动件类型分)By the followers motion type(按从动件运动形式分)By the manner of keeping the cam and the follower in contact(按从动件与凸轮的锁合方式分),Edge,plate or face cams 盘状凸轮,Translating cams 移动凸轮,Cylindrical or drum cams 圆柱凸轮,By the follower ends shapeKnife-edge follower 尖顶从动件,The end of the follower is very sharp(nearly a point)The contact between the cam and the follower is a pointThe follower is easy to be worn.Its rarely used.,作者：潘存云教授,By the follower ends shapeDisk cam with roller follower 滚子从动件,The end of the follower is a roller)Line contactThe friction coefficient is very small(sliding friction).This type of roller is widely used.,作者：潘存云教授,By the follower ends shapeDisk cam with flat follower 平底从动件,The follower end shape is a flat planThe pressure angle of the mechanism always is 0,so it can transmit large power.But it is difficult to design and manufacture.,By the followers motion typeOscillating follower 摆动从动件,By the followers motion typeTranslating follower 移动从动件,4.By the manner of keeping the cam and follower in contactForce-closed cam mechanism(力锁合),With an external downward force Vs,Without an external downward force,弹簧力锁合(spring force closed),优点：只需要设计适当的轮廓曲线，从动件便可获得任意的运动规律，且结构简单、紧凑、设计方便。,缺点：线接触，容易磨损。,槽道凸轮 Grooved cam,等径凸轮Constant radius cam,r1+r2=const,优点：只需要设计适当的轮廓曲线，从动件便可获得任意的运动规律，且结构简单、紧凑、设计方便。,缺点：线接触，容易磨损。,等宽凸轮(constant width cam),等宽凸轮,优点：只需要设计适当的轮廓曲线，从动件便可获得任意的运动规律，且结构简单、紧凑、设计方便。,缺点：线接触，容易磨损。,共轭凸轮(Conjugate cam),主回凸轮,优点：只需要设计适当的轮廓曲线，从动件便可获得任意的运动规律，且结构简单、紧凑、设计方便。,缺点：线接触，容易磨损。,设计凸轮机构时的主要任务:1)Selection of the cam type 选择凸轮类型,而根据工作要求选定推杆运动规律，是设计凸轮轮廓曲线的前提。,2)Selection of the followers motion type选择从动件形式,3)Decision of the structure dimension设计结构尺寸,4)Designing the cam profile设计凸轮轮廓,The followers motion type 从动件的运动规律,凸轮机构设计的基本任务:1)根据工作要求选定凸轮机构的形式;,名词术语：,一、推杆的常用运动规律,基圆、,推程运动角、,基圆半径、,推程、,远休止角、,回程运动角、,回程、,近休止角、,行程。一个循环,而根据工作要求选定推杆运动规律，是设计凸轮轮廓曲线的前提。,2)推杆运动规律;,3)合理确定结构尺寸;,4)设计轮廓曲线。,The followers motion type 从动件的运动规律,Flowers motion:the flowers displacement(位移)S,velocity v and acceleration a during rise(升程)and return(回程):,S=S()v=v()a=a(),The followers motion type 从动件的运动规律,Uniform velocity(等速运动规律)Constant Acceleration Motion(等加速等减速运动规律)Simple Harmonic Motion(简谐运动规律/余弦加速度运动规律)Combined motion(组合运动规律),The followers motion type 从动件的运动规律,Uniform velocity(等速运动规律),Rigid shock刚性冲击,推程运动方程：s h/0,v h/0,v-h/0,a0,a 0,回程运动方程：sh(1-/0),从动件作等速运动时，其位移线图为一过原点的倾斜直线。速度线图为一条水平直线，V0 的大小为位移线图中倾斜直线的斜率。加速度为零。,此种运动规律只适用于低速运动的场合.,Constant acceleration/deceleration motion(等加速等减速运动),减速段推程运动方程为：,s h-2h(0)2/20,v-4h(0-)/20,a-4h2/20,加速段推程运动方程为：,s 2h2/20,v 4h/20,a 4h2/20,这种运动规律只适用于中速运动场合.,Simple Harmonic Motion(余弦加速度),推程：sh1-cos(/0)/2,v hsin(/0)/20,a 2h2 cos(/0)/220,回程：sh1cos(/0)/2,v-hsin(/0)/20,a-2h2 cos(/0)/220,在起始和终止处理论上a为有限值，产生柔性冲击。,Selection of follower motion,1.机器的工作过程只要求凸轮转过一角度0时，推杆完成一行程h(直动推杆)或(摆动推杆)，对运动规律并无严格要求。则应选择直线或圆弧等易加工曲线作为凸轮的轮廓曲线。如夹紧凸轮。,2.机器的工作过程对推杆运动有要求，则应严格按工作要求的运动规律来设计凸轮廓线。如刀架进给凸轮。,Selection of follower motion,理由：,amax,动量mv,若机构突然被卡住，则冲击力将很大,(F=mv/t)。,对重载凸轮，则适合选用vmax较小的运动规律。,惯性力F=-ma,对强度和耐磨性要求。,对高速凸轮，希望amax 愈小愈好。,vmax,Pn,Selection of follower motion,3.对高速凸轮，要求有较好的动力特性，除了避免出现刚性或柔性冲击外，还应当考虑vmax和 amax。,从动件常用运动规律特性比较,Have You Finished Your Homework?Page 171:9-1，9-3,
