Four-bar linkage

A closed-chain mechanism of four rigid links connected by four joints. Its geometry constrains the links to move in a coordinated way, converting motion between input and output links.

Grashof's law: A criterion relating four link lengths to whether a planar four-bar linkage can achieve continuous rotation. It predicts which link arrangements permit a crank to make full revolutions.
Kinematic chain: An assembly of rigid links connected by joints that permits relative motion. A four-bar linkage is a closed kinematic chain with four links.
Windshield wiper: A vehicle device that sweeps a blade across a windshield to remove water or debris. Many wiper drives use a crank-rocker linkage to create oscillating blade motion.
Slider-crank mechanism: A mechanism that converts rotation and reciprocation using a crank, connecting rod, and sliding joint. It performs similar motion conversion but replaces one revolute joint with a slider.
Crank-rocker mechanism: A four-bar linkage in which one link rotates fully while another oscillates. It is a common motion pattern when only one link can rotate continuously.
Revolute joint: A joint that permits rotation about one fixed axis while restricting other relative motion. Four revolute joints commonly connect the links in a planar four-bar.
Bicycle suspension: A bicycle design that uses pivots and links to guide wheel motion relative to the frame. Four-bar arrangements tune the rear wheel's path and suspension response.
Five-bar linkage: A closed planar mechanism with five links and five joints, often used for parallel motion. Its extra link and joint alter mobility and workspace compared with a four-bar.
Double-crank mechanism: A four-bar linkage in which both links adjacent to the fixed link can rotate fully. It shows how link proportions can allow continuous rotation at both moving pivots.
Degree of freedom: The number of independent coordinates needed to specify a system's configuration. A typical planar four-bar has one independent motion coordinate.

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