The universal joint is a mechanism to achieve variable-angle power transmission, and is used to change the position of the drive shaft direction. In front-engined front-wheel drive vehicles, the drive shaft is omitted and the universal joint is mounted between the front axle half-shaft, which is responsible for both driving and steering, and the wheels.

The structure and role of the universal joint is somewhat like the joints on the human limbs, it allows the angle between the connected parts to vary within a certain range, in order to meet the power transmission, adapt to the steering and the car running up and down jump caused by the angle change, the front drive car drive axle, half shaft and wheel axle are often connected to the universal joint. However, due to the limitations of the axial size, the required deflection angle is relatively large, a single universal joint can not make the output shaft and the input shaft of the instantaneous angular velocity equal, easy to cause vibration, aggravate the damage to the components, and produce a lot of noise, so a wide variety of equal-speed universal joints.

In front drive vehicles, two equal speed universal joints are used for each half shaft, the one near the variable speed drive axle is the inner half shaft universal joint and the one near the axle is the outer half shaft universal joint. On rear drive cars, the engine, clutch and transmission are mounted as a unit on the frame, while the drive axle is connected to the frame by means of a flexible suspension, with a distance between them that needs to be connected. The unevenness of the road surface in the operation of the car produces jumping, load changes or the bit difference between the two assemblies installed, etc., will make the angle and distance between the transmission output shaft and the drive axle main reducer input shaft change, so in the rear drive car universal joint transmission form are used double universal joint, is the drive shaft ends have a universal joint, its role is to make the drive shaft ends of the angle equal, so as to ensure that the output shaft and input shaft The function is to make the angle between the two ends of the drive shaft equal, thus ensuring that the instantaneous angular velocity of the output and input shafts are always equal.