1、MECHANISMS AND MACHINE THEORYCharles W. Beardsley, Mechanical Engineering, ASME, Regents Publishing Company, Inc. 2001Introduction to MechanismThe function of a mechanism is to transmit or transform motion from one rigid body to another as part of the action of a machine. There are three types of co
2、mmon mechanical devices that can be used as basic elements of a mechanism.1. Gear system, in which toothed members in contact transmit motion between rotating shafts.2. Cam system, where a uniform motion of an input member is converted into a nonuniform motion of the output member.3. Plane and spati
3、al linkages are also useful in creating mechanical motions for a point or rigid body.A kinematic chain is a system of links, that is, rigid bodies , which are either jointed together or are in contact with one another in a manner that permits them to move relative to one another. If one of the links
4、 is fixed and the movement of any other link to a new position will cause each of the other links to move to definite predictable position, the system is a constrained kinematic chain. If one of the links is held fixed and the movement of any other link to a new position will not cause each of the o
5、ther links to move to a definite predictable position then the system is an unconstrained kinematic chain,A mechanism or linkage is a constrained kinematic chain, and is a mechanical device that has the purpose of transferring motion and/or force from a source to an output. A linkage consists of lin
6、ks (or bars), generally considered rigid, which are connected by joints, such as pin Cor revolute) or prismatic joints, to form open or closed chains (or loops). Such kinematic chains, with at least one link fixed, become (1) mechanisms if at least two other links remain mobility, or (2) structures
7、if no mobility remains. In other words, a mechanism permits relative motion between its rigid links; a structure does not. Since linkages make simple mechanisms and can be designed to perform complex tasks, such as nonlinear motion and force transmission they will receive much attention in mechanism
8、 study.Mechanisms are used in a great variety of machines and devices. The simplest closed-loop linkage is the four-bar linkage, which has three moving links (plus one fixed link) and four pin joints. The link that is connected to the power source or prime mover and has one moving pivot and one grou
9、nd pivot is called the input link. The output link connects another moving pivot to another ground pivot. The coupler or floating link connected the two moving pivots, thereby coupling the input to the output link.The four-bar linkage has some special configurations created by making one or more lin
10、ks infinite in length. The slider-crank (or crank and slider) mechanism is a four-bar chain with a slider replacing an infinitely long output link. The internal combustion engine is built based on this mechanism. Other forms of four-link mechanisms exist in which a slider is guided on a moving link
11、rather than on a fixed link. These are called inversions of the slider-crank, produced when another link (the crank, coupler, or slider) is fixed link.Although the four-bar linkage and slider-crank mechanism are very useful and found in thousands of applications, we can see that these linkages have
12、limited performance level. Linkages with more members are often used in more demanding circumstances. However it is often difficult to visualize the movement of a multiloop linkage, especially when other components appear in the same diagram. The first step in the motion analyses of more complicated
13、 mechanisms is to sketch the equivalent kinematic or skeleton diagram. The skeleton diagram serves a purpose similar to that of the electrical schematic or circuit diagram in that it displays only the essential skeleton of the mechanism, which, however, embodies the key dimensions that affect its mo
14、tion. The kinematic diagram takes one of two forms: a sketch (proportional but not exactly lo scale), and the scaled kinematic diagram (usually used to further analysis: position, displacement, velocity, acceleration, force and torque transmission, etc. ). For convenient reference, the links arc num
15、bered (starting with ground link as number 1), while the joints are lettered.The next step in the kinematic analysis of mechanisms, following the drawing of the schematic diagram, is to determine the number of degree of freedom of the mechanism. By degree of freedom we mean the number of independent
16、 inputs required to determine the positions of all links of the mechanism with respect to ground. There are hundreds of thousands of different linkage types that one could invent. Envision a bag containing a large variety of linkage components; binary, ternary, quaternary, and so on, links: pin joints, slide joints; cams and cam followers; gears, chains, sprockets, belts, pulleys, and so on. (Spherical and helical joints as well as other connections that allow th
