A universal joint is a joint or coupling 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 capable of transmitting torque and rotational motion from one shaft to another when their axes are inclined to each other by some angle, which may constantly vary under working conditions. Universal joints are incorporated in the of vehicle’s transmission system to perform three basic applications :
(a) Propeller shaft end joints between longitudinally front mounted gearbox and rear final drive axle.
(b) Rear axle drive shaft end joints between the sprung final drive and the unsprung rear wheel stub axle.
(c) Front axle drive shaft end joints between the sprung front mounted final drive and the unsprung front wheel steered stub axle.
Universal joints have movement only in the vertical plane when they are used for longitudinally mounted propeller shafts and transverse rear mounted drive shafts. When these joints have been used for front outer drive shaft they have to move in both the vertical and horizontal plane to accommodate both vertical suspension deflection and the swivel pin angular movement to steer the front road wheels. The compounding of angular working movement of the outer drive shaft steering joint in two planes imposes large and varying working angles even when the torque is being transmitted to the stub axle. Due to the severe working conditions, special universal joints known as constant velocity joints are employed.