Universal Joint

A universal joint 

is a joint in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges located close together, oriented at 90° to each other, connected by a cross shaft.

Universal joints are used to transmit motion from tractors to machines such as round balers etc. This allows the tractor to turn while transmitting rotary motion to the following machine.