第15-2次课-chapter 8-Other Mechanisms in Common Use

1、 Chapter 8 Other Mechanisms in Common Use Chapter 8 Other Mechanisms in Common Use In previous chapters, we have studied analysis and synthesis(综合综合) of planar linkage mechanisms, cam mechanisms, and gear mechanisms. Linkage mechanismGear mechanismCam mechanismIn addition to these, many machines and

2、 instruments(器械) contain other kinds of mechanism. In this chapter, some of them in common use will be introduced briefly. Hints of Introduction: (1) Working Principle (2) Classification (3) Applications8.1 Ratchet Mechanism8.1 Ratchet Mechanism A ratchet mechanism consists of a driving rocker 1, a

3、driving pawl(棘爪棘爪) 2, a driven ratchet 3, a holding pawl(止动爪止动爪 ) 4, and the frame 7. The holding pawl 4 and the driving pawl 2 are forced into contact with the ratchet 3 by springs (弹簧弹簧) 5 and 6, respectively.763421A5Working PrincipleWorking PrincipleA47123564573261A763421A5The rotation angle of t

4、he ratchet in one cycle can be adjusted (调节调节) by adjusting the rocking angle of the rocker. 2. Classification of Ratchet Mechanisms2. Classification of Ratchet Mechanisms According to the contour(轮廓轮廓) of ratchet: (1) Tooth(齿啮式齿啮式) ratchet mechanisms (2) Silent(无声的，摩擦式无声的，摩擦式) ratchet mechanisms763

5、421A512354(1) Tooth ratchet mechanisms(a) externally meshed(外啮合外啮合) (b) internally meshed(内啮合内啮合) A45674213123763421A5Characteristics: In tooth ratchet mechanisms, rigid impulse(刚性冲击刚性冲击) and noise(噪声噪声) are created when the driving pawl strikes the tooth root. 763421A5 (2) Silent(无声的，摩擦式无声的，摩擦式) ra

6、tchet mechanism In silent ratchet mechanism, the effect of friction produces the required motion. 12345Externally meshed 12Internally meshed 12345 Characteristics of the silent ratchet mechanisms are: (a) transmit motion smoothly and silently. (b) the rotation angle of the driven ratchet in one cycl

7、e can be changed freely. (c) the accuracy(精度精度) of motion is poor. (d) the transmitted torque(扭矩扭矩) is limited.The intermittent rotating direction of the ratchet may be fixed or changeable.A1245673123122 The oscillation of the driving rocker may be single The oscillation of the driving rocker may be

8、 single function or double functionfunction or double function(1) Single function ratchet mechanism(2) Double function ratchet mechanism 763421A5312233222112332221123. The Applications of Ratchet Mechanisms3. The Applications of Ratchet Mechanisms(1) Apart from its use for intermittent motion, the r

9、atchet mechanism can also be used as a safety device(保险装置保险装置) in lifting machines (起重机起重机).112Q(2) Ratchet mechanisms are often used in machines to allow overrun(超越超越)It should be pointed out here that overrun is the unique(独特的独特的) characteristic of the ratchet mechanism.1234(3) This overrun clutch

10、(离合器离合器) is used as connec-tion between the electric motor and the internal combustion engine in cars. 12 Design formulas can be found in design handbooks.8.2 Geneva Mechanism8.2 Geneva Mechanism( (槽轮机构槽轮机构) )1. Working principle of the Geneva mechanism O1nnmmGO2driving plate 1 with a roller G, driv

11、en Geneva wheel 2 with radial slots(槽槽),the frameO1nnmmGO2In order to start and stop the Geneva wheel smoothly, the Geneva mechanism should be so designed that O1G O2G when the roller G begins to enter into and leave the radial slot. No rigid impulse(刚性冲击刚性冲击) will exist。O1nnmmGO2O1O2GO1nnGO22. The

12、classification of Geneva mechanism (1) Planar Geneva mechanism ( (平面槽轮机构平面槽轮机构) ) (a) External Geneva mechanism ( (内槽轮机构内槽轮机构) ) (b) Internal Geneva mechanism ( (外槽轮机构外槽轮机构) ) (c) Geneva rack mechanism ( (槽轮齿条机构槽轮齿条机构) ) (2) Spherical Geneva mechanisms ( (球面槽轮机构球面槽轮机构) )(a) External Geneva mechanism

13、 (b) Internal Geneva mechanism (c) Geneva rack mechanism(d) Spherical Geneva mechanisms. 3. The Characteristics and Applications of Geneva Mechanisms(1) Geneva mechanisms have many advantages, e.g. simple structure, small dimensions, easy to manufacture, high mechanical efficiency(效率效率), no rigid im

14、pulse(刚性冲击刚性冲击), etc.(2) However, there is soft impulse(柔性冲击柔性冲击) when the Geneva wheel starts and stops. (3) The rotation angle of a Geneva wheel for each cycle can not be adjusted. Therefore, Geneva mechanisms are mostly used for index(分度分度) mechanisms in which it is not necessary to adjust the ro

15、tation angle.Design formulas can be found in design handbooks.8.3 Cam-Type Index(分度) MechanismsThey are often used to transform the continuous even (均匀的均匀的) rotation into an intermittent (间歇间歇) rotation in one direction between two crossed shafts.1. Cylindrical Cam Index Mechanisms(圆柱凸轮分度机构）圆柱凸轮分度机构

16、） There is a portion of the driving ridge(背脊背脊) where the pitch(周节周节) is zero and the driven plate is held stationary.2. Hourglass (沙漏沙漏) Cam Indexing Mechanisms(蜗杆分度机构）蜗杆分度机构） There is a portion of the driving ridge where the pitch(周节周节) is zero and the driven plate is held stationary.3. Characteri

17、stics and ApplicationsSince the motion curve of the mechanism can be chosen according to the actual work conditions of the driven plate, both rigid and soft impulse(冲击冲击) can be avoided when the driven plate starts and stops. Therefore, these kinds of mechanisms can transmit motion smoothly and thei

18、r dynamical characteristics are very good. Noise and vibration is very low. Any ratio between motion time and dwell time can be designed. Applications: These two mechanisms are used widely in high-speed, high-accuracy index mechanisms, e.g. high-speed punch presses(冲压冲压机机), multi-colour printers, pa

19、ckers, etc.8.4 Universal Joints8.4 Universal Joints( (万向联轴节万向联轴节) )1. The Single Universal JointThe single universal joint is a spatial lower-pair mechanism which can transmit rotation between two intersecting ( (交叉交叉) ) shaftsProblem: When the driving shaft 1 rotates at constant angular velocity 1,

20、 then the angular velocity 3 of the driven shaft 3 will fluctuate(波动波动) periodically within 1cos13. 13 is the angle between the two shafts of the single universal joint. In practice, 13 does not exceed 3545. i0.70.80.91.01.11.21.31.41310306090120150oooooo180o=1513o=3013o=4513o2. The Double Universal

21、 Joint In order to make 13, a double universal joint should be used.1232 31 21231 22 3In order to make the ratio i13 1 (13), the following two conditions must be: (1) The angle 12 must be equal to the angle 23.(2) The two yokes of the intermediate shaft 2 must lie on the same plane.1232 31 21231 22

22、312The Double Universal Joint used in a car 3. Application The universal joint is easy to assemble and can be used in poor working conditions. It is suitable for applications where the angle and the distance between the two shafts change continuously. Therefore, it is used widely in automobiles, mac

23、hine tools, and textile(纺织纺织) machinery, etc. 8.5 Screw Mechanisms8.5 Screw Mechanisms( (螺旋传动机构）螺旋传动机构）1. Working Principle and Types of Screw Mechanisms1. Working Principle and Types of Screw MechanismsScrew mechanisms can be divided into single-thread (螺纹螺纹) screw mechanisms, double-thread screw m

24、echanisms, etcsingle-thread(螺纹螺纹) screw mechanismsDouble-thread(螺纹) screw mechanisms2C31AB32BCALB1AALB)(LSuppose that the lead(导程) of the threads A and B are LA and LB, respectively, and both are right-hand threads. When the screw bar 1 rotates once in the direction, the screw bar 1 will translate a

25、 distance LA to the left relative to the stationary nut (or frame) 3 along its centre line. Similarly, the screw bar 1 will translate a distance LB to the left relative to the nut 2 since the nut 2 can not rotate. In other words, the nut 2 will translate a distance LB to the right relative to the sc

26、rew bar 1. 32BCALB1AALB)(L If LALB, then the nut 2 will translate (LA-LB) to the left. If LALB, then it will translate (LB-LA) to the right.32BCALB1AALB)(LIf the leads LA and LB are quite closely similar, then a fine(细微) feed(进给) will results. This kind of mechanism is often called a differential(差动

27、) screw mechanism 32BCALB1AALB)(L When the leads of the threads A and B in the differential screw mechanism are closely similar, not only can the nut 2 have a fine translation, but also the leads of the two threads A and B do not need to be too small(so that the screws have enough strength). Therefo

28、re, differential screw mechanisms are often used as fine measuring instruments, index(分度) mechanisms, and many fine adjustment devices. 345AB1 276fine adjustment mechanism for adjusting the boring(镗) tool in a boring machine.If LA=2 .25mm and LB=2mm, then the boring tool 1 translates only 0.25mm for

29、 one revolution of the adjusting screw 7. Thus, the feed of the boring tool 1 can be adjusted accurately The translation of the nut 2 will be (LA+LB) if the spiral(螺旋) directions of the threads A and B are opposite. This gives a quick translation of the nut 2. This gives a quick translation of the nut 2. 123ABCLALBLBAL)( Shown in Fig. is a self-centering bench vice(老虎钳). 123452. The Characteristics and Applications of Screw Mechanisms (1) The structure of screw mechanisms is quite simple. (2) The screw is easy to manufacture. (3) The motion accuracy is quite high. (4) The speed ratio is qu