Universal Joint

The universal joint, U joint, Cardan joint or Hardy-Spicer joint is a rigid divide that allows the rod to 'bend' in any direction. It consists of a pair of ordinary hinges located close together, but oriented at 90° relative to each other.

To prevent the jerky rotation of the axle or drive shaft there usually are two U-joints in a three part shaft assembly. The second U-joint will convert the jerky movement back to an even, uniform speed of the third shaft - if both the driving and the driven shaft are parallel and the two universal joints are correctly aligned with each other.