in machinery, a device for providing a connection, readily broken and restored, between two adjacent rotating shafts. A coupling may provide either a rigid or a flexible connection; the flexibility may permit misalignment of the connected shafts or provide a torsionally flexible (yielding) connection, mitigating effects of shock.

A common type of rigid coupling consists of two mating radial flanges (disks) that are attached by key-driven hubs to the ends of the shafts and bolted together through the flanges. Alignment of the shafts is usually achieved by means of a short cylindrical projection (rabbet joint) on the face of one flange that fits snugly into a circular recess on the face of the other flange.

The chain coupling consists of two hardened-steel sprockets, one on each shaft, with a nylon or metal roller chain wrapped around the closely aligned sprockets and connected at the ends. Clearances between the sprocket teeth and the chain allow for a small amount of shaft misalignment.

For connecting shafts whose axes intersect but are inclined to one another at a larger angle than a flexible coupling can accommodate, universal joints are used. The most common of these is the Hooke, or Cardan, joint, which consists of two yokes attached to the shaft ends and a cross-shaped connecting member. See also hydraulic transmission.