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1、 Introduction to mechanismThe function of 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 common mechanical devices that can be used as basic elements of a mechanism.(1) Gear system, in which toothed members in
2、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 spatial linkages are also useful in creating mechanical motions for a point or rigid body.A kinematic chain is a system o
3、f links, 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 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,
4、the system is a constrained kinematic chain. Otherwise, the system is an unconstrained kinematic chain.A mechanism or linkage is a constrained kinematic chain, and is a mechanical divice that has the purpose of transferring motion and force from a source to an output. A linkage consists of links (or
5、 bar),generally considered rigid,which are connected by joints, such as pin (or revolute) or prismatic joints, to form open or closed chains (or loops). Such kinematic chains,with at least one link fixed, become (i) mechanisms if at least two other links remain mobility, or (ii) stuctures if no mobi
6、lity 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 ransmission, they will receive much attention in mechanism study.M
7、echanisms 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 ground pivot
8、 is called the input link. The output link connects another moving povit to another ground povit. 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 links inf
9、inite in length。The slider-crank （or crank and slider）mechanism is a four-bar chain with a slinder replacing an infinitely long output link。The internal combustion engine is built based on this mechanism。Other forms of four- bar mechanisms exist in which a slider is guided on a moving link rather th
10、an 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 limited perform
11、ance 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 fist step in the motion analysis of more complicated mechanisms is to sk
12、etch 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 the key dimensions that affect its motion。The kinematic d
13、iagram takes one of two forms：a sketch（proportional but not exactly to 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 are numbered（starting with ground link as
14、 number 1），while the joints are lettered。The next step in the kinematic analysis of mechanisms is to determine the number of degree of freedom of the mechanism。By degree of freedom we mean the number of independent inputs required to determine the positions of all links of the mechanism with respect
15、 toground。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。（spheric
16、al and helical joints as well as other connections that allow three-dimensional relative motion are not included，as only planar motion in parallel planes are discussed here）。Furthermore，imagine the possibility of forming all sorts of linkage types by putting these components together。Are there any rules that help govern how these mechanisms are formed？ Actually most mechanism tasks require a single input to be transferred to a
