Constant velocity universal joint is a device that connects two shafts which have included angle or mutual position change, and makes the two shafts transmit power at the same angular speed. It can overcome the problem of unequal velocity existing in common cross universal joint. At present, the widely used constant velocity universal joint mainly includes ball fork type universal joint and ball cage type universal joint.

The function of constant velocity universal joint is to connect two rotating shafts which have included angle or mutual position change, and make the two shafts transmit power at the same angular speed. It can overcome the problem of unequal velocity existing in common cross shaft universal joint, and is especially suitable for the use of steering drive axle.

In steering drive axle, the front wheel is not only the driving wheel, but also the steering wheel. When steering, the deflection angle is very large, and the maximum deflection angle can be more than 40 degrees. At this time, the traditional universal joint with very small deflection angle can not be used. When the deflection angle of common universal joint is large, the speed and torque will fluctuate greatly. It is difficult for the power of automobile engine to be transmitted to the wheels smoothly and reliably. At the same time, it will also cause the vibration, impact and noise of automobile. Therefore, the constant velocity universal joint with large deflection angle, stable power transmission and uniform angular velocity must be used to meet the requirements.

In mechanical transmission, to solve the problem of constant speed power transmission with angle between shafts, a typical example is bevel gear transmission. As shown in the figure below, for a pair of bevel gears with the same size, the included angle between the axes of the two gears is a, the meshing point P of the two gears is located on the bisector of the intersection angle, and the vertical distances from point P to the two axes are equal to R. At point P, the circumferential speeds of the two gears are equal, so the angular speeds of the two gears are also equal. Based on this principle, if the force transfer point of the universal joint is always in the angular bisector when the included angle changes, the equal angular speed transmission relationship between the two universal joint forks can be maintained. The basic principle of constant velocity universal joint is to ensure that the force transfer point of the joint is always on the bisector of the angle between the driving shaft and the driven shaft.