Ujoints

A universal joint, commonly called a U-joint, is a mechanical connection between two rotating shafts that are not in a straight line. It transmits torque and rotational motion while allowing angular misalignment between the shafts. The joint consists of two yokes connected by a cross-shaped component (often called a spider or cross) with bearings at its ends. The cross sits in trunnions mounted in each yoke, and lubrication is provided by grease or through sealed bearings in modern designs.

When the input shaft rotates, the output shaft follows, but the angular velocity is not constant if the shafts are at an angle. The output speed oscillates during each revolution, typically at twice the input frequency, even though the average speed matches the input. This velocity fluctuation can induce vibrations and stress unless mitigated. To minimize or cancel these fluctuations, engineering solutions include using a double Cardan joint (two U-joints with a compensating shaft) or employing a constant-velocity (CV) joint, which is designed to maintain nearly constant output speed over a range of angles.

Commonly found in automotive drive shafts, U-joints are also used in industrial machinery, robotics, and other mechanical systems requiring flexible shaft connections. Maintenance considerations include proper lubrication, attention to misalignment, and limits on operating angle to reduce wear and noise. Modern sealed U-joints reduce maintenance, while some applications replace single U-joints with CV joints or double Cardan configurations to improve performance at larger angles.