Four-Bar Linkage

a plane mechanism consisting of four links that form rotating kinematic pairs. A four-bar linkage (Figure 1) consists of a fixed link (0, the base), two rotating links (1 and 3), and a connecting link (2), which is also called a connecting rod or coupler. A rotating link is called a crank if it can undergo a complete rotation with respect to the axis of rotation; it is called a rocker if it can undergo only part of a rotation.

Figure 1

Depending on the type of rotating links present, four-bar linkages are subdivided into crank-and-rocker, draglink, and double-rocker types. A crank-and-rocker mechanism is used to convert the constant rotation of the crank into the reciprocal motion of the rocker. In manufacturing machinery, the crank is set in motion by a main shaft, and a working member that performs a given production operation is connected to the rocker. A drag-link mechanism converts uniform rotation into nonuniform rotation in a single direction; it is used in machines where the working member, while moving in a single direction, must have a velocity that significantly exceeds the average velocity on some segment. Double-rocker linkages are used chiefly in instruments to provide a mechanical reproduction of a given function, such as y = log x. Four-bar linkages are also used to reproduce the motion of a point along a given curve; for example, the Chebyshev parallel motion can be used to approximate a segment of a straight line.