Universal Coupling

A universal coupling—also called a universal joint or U‑joint—is a mechanical device used to connect two shafts whose axes are inclined to each other, allowing the transmission of torque and rotary motion even when the shafts are not in perfect alignment. It consists of two yokes, one on each shaft, connected by a cross-shaped intermediate member (often called a spider or cross shaft) with four bearings. The yokes are oriented at 90° to each other, and the cross allows each shaft to pivot in two perpendicular planes. This arrangement accommodates angular misalignment between the shafts while still transmitting power. Universal couplings are widely used in applications such as automobile drive shafts, industrial machinery, and marine propulsion systems, where the driving and driven shafts may change their relative angle during operation. However, a single U‑joint does not transmit motion at constant velocity—its output speed varies cyclically—so in high‑precision or high‑speed systems, two U‑joints are often used in series with their yokes phased to cancel out this speed variation.