linkage, in mechanical engineering, a system of solid, usually metallic, links (bars) connected to two or more other links by pin joints (hinges), sliding joints, or ball-and-socket joints so as to form a closed chain or a series of closed chains. When one of the links is fixed, the possible movements of the other links relative to the fixed link and to one another will depend on the number of links and the number and types of joints. With four pin-connected links, for example, the links all move in parallel planes, and regardless of which link is fixed, the other links have constrained motion; i.e., they move in a fixed and determinate way relative to the fixed link. By varying the relative lengths of the links, this four-bar linkage becomes a useful mechanism for converting uniform rotary to non-uniform rotary motion or continuous rotary to oscillatory motion; it is the most commonly used linkage mechanism in machine construction.

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mechanism, in mechanical construction, the means employed to transmit and modify motion in a machine or any assemblage of mechanical parts. The chief characteristic of the mechanism of a machine is that all members have constrained motion; i.e., the parts can move only in a determinate manner relative to one another. The nature of these relative motions is determined largely by the number of parts and the way in which they are connected.

Regardless of its complexity, the mechanism of a machine can always be analyzed as an assemblage of simple basic mechanisms, each of which contains members or links that transmit motion from one moving link to another with or without modification in degree or kind. In general, there are three ways in which this can be done: by a wrapping connector such as a chain (q.v.) or belt (see belt drive); by direct contact as in a cam or gear (qq.v.); or by a pin-connected link (see linkage).

This article was most recently revised and updated by Amy Tikkanen.