Four-Bar Linkage

Four bars joined end to end by four revolute joints into a closed loop, with just 1 degree of freedom.

A four-bar linkage connects four bars — one of which is fixed, called the frame — end to end with four revolute joints into a closed loop; it's the simplest movable closed-chain mechanism, and in the planar case it has only 1 degree of freedom: rotate any one bar and the positions of all the others are fully determined. The bar attached to the frame that can rotate all the way around is called the crank; one that can only rock back and forth is a rocker; the bar floating in between is the coupler. Whether a bar can rotate fully is decided by the Grashof condition: if the sum of the shortest and longest bar lengths is no greater than the sum of the other two, the shortest bar can rotate fully relative to its neighbors, which is how linkages get classified into crank-rocker, double-crank, and double-rocker types. Its job is to turn a motor's simple rotation into a desired output trajectory or angular relationship — windshield wipers and car suspensions both use it, and robots often use it to place a motor closer to the torso and drive a knee or ankle through a linkage, while a parallelogram four-bar can keep a gripper's fingers always parallel.

ExampleA car windshield wiper: the motor spins a crank continuously, which drives a rocker back and forth through a coupler bar, and the wiper blade sweeps left and right across the windshield.

Also called

Four-Bar Mechanism, Planar Four-Bar Linkage

Related

Grübler's Formula · Linkage Transmission · Parallel Mechanism · Parallel Ankle Mechanism · Degrees of Freedom (DoF) · Link

Sources

Wikipedia: Four-bar linkage
Wikipedia: Grashof condition
Lynch & Park, Modern Robotics（预印本 PDF，第 2 章 Example 2.3） (Chinese)