Fourbar

A fourbar, or four-bar linkage, is a basic planar mechanical device used to convert and transmit motion and force. It consists of four rigid links connected in a closed loop by pin joints. One of the links is fixed to the frame or ground, while the remaining three are movable and typically called the input crank, the coupler, and the output crank. All joints are revolute (pin) joints, allowing rotation while maintaining the loop constraint.

Kinematics and mobility: The four-bar enforces a closed-loop relationship among the link angles, so the motion of the entire mechanism is determined by the motion of a single actuator. In planar form, a four-bar generally has one degree of freedom, assuming ideal joints and no singular configurations. The input crank’s rotation drives the coupler, which in turn moves the output crank.

Configurations and variants: The behavior of a four-bar depends on the relative lengths of the four links. Common types include crank-rocker (one link functions as a crank and the opposite as a rocker), double-crank (two rotating links), and parallelogram (opposite sides remain parallel, yielding a more uniform motion). Some designs aim to approximate straight-line motion, notably the Watt straight-line linkage and the Chebyshev linkage, which are specific four-bar configurations optimized for path accuracy.

Applications: Four-bar linkages are widely used in engineering, robotics, and automation to convert rotary input into a desired motion profile or to guide a follower along a prescribed path. Design involves selecting link lengths and pivot positions to meet performance targets while avoiding singular configurations.