Universal Joint

A universal joint, almost always shortened to U-joint, is a mechanical coupling that transmits rotary motion between two shafts whose axes meet at an angle. It allows drive to continue around a bend in the driveline, accommodating changes in that angle as components move. The classic form, the Cardan or Hooke's joint, consists of two yokes set at right angles and connected by a central cross-shaped piece known as a spider, whose four arms ride in needle-roller bearings.

In operation, one yoke is fixed to the input shaft and the other to the output shaft, with the spider linking them. As the input rotates, the spider pivots within its bearings, letting the output shaft follow even though it points in a slightly different direction. This is most familiar at the ends of a vehicle's propeller shaft, where the joint allows the gearbox output and the rear axle to be at different heights and angles, and to move relative to one another as the suspension travels over bumps.

The characteristic that defines the joint, and its principal drawback, is that it does not transmit rotation at a perfectly constant velocity when working through an angle. Within each revolution the output shaft speeds up and slows down twice relative to the input, a fluctuation that grows more pronounced as the operating angle increases. This non-uniform motion induces a torsional vibration that can be felt and heard, and it places cyclic stress on the driveline.

Engineers manage this in a well-known way: by using two universal joints in a propeller shaft and arranging their yokes in phase with equal operating angles at each end, so that the speed-up of the first joint is cancelled by the slow-down of the second, leaving the wheels turning at a steady velocity. This is why a typical rear-wheel-drive driveshaft has a joint at each end. Where a smooth output is needed through large and varying angles, particularly at the front driven wheels of a car that also steer, the constant-velocity joint is used instead, as it transmits torque at uniform speed regardless of angle.

Universal joints are simple, robust, and capable of handling high torque, which keeps them in widespread use on driveshafts and half-shafts of trucks and rear-drive cars. They do require attention: many have grease nipples needing periodic lubrication, and worn needle bearings announce themselves with a clunk on taking up drive or a vibration that worsens with speed. The U-joint works in concert with the driveshaft it connects, the differential it feeds, and the constant-velocity joint that complements it at the wheels.