Universal coupling

A universal coupling, or universal joint, transmits torque between two shafts whose axes meet at an angle, using a cross-piece and yokes that let the angle change while the shaft turns. Onboard it connects drives that cannot be held in line: steering-gear linkages, valve-operating extension spindles through decks, and the input drives of some azimuth and Z-drive units where the propulsion power turns a corner. A single joint runs the output at a fluctuating speed through each revolution, so paired joints in a double-Cardan or constant-velocity arrangement cancel the fluctuation. It is distinct from a flexible coupling, which corrects small misalignment and damps torsional vibration rather than working through a large angle.