The structure and function of a universal joint are somewhat similar to joints in the human limbs, allowing the included angle between connected parts to vary within a certain range. To meet the requirements of power transmission and accommodate the angular changes caused by steering and the vertical movement of the vehicle during operation, universal joints are commonly used to connect the drive axle, half-shafts, and wheel axles in front-wheel drive vehicles.
However, due to limitations in axial dimensions and the requirement for relatively large offset angles, a single universal joint cannot ensure that the instantaneous angular velocities of the output and input shafts are equal, easily causing vibration, accelerating component damage, and generating significant noise. Therefore, various types of constant velocity universal joints are widely used. In front-wheel drive vehicles, each half-shaft uses two constant velocity universal joints; the universal joint closer to the drive axle is the inner half-shaft universal joint, and the one closer to the wheel axle is the outer half-shaft universal joint. In rear-wheel drive vehicles, the engine, clutch, and transmission are mounted as a single unit on the frame, while the drive axle is connected to the frame via a flexible suspension. There is a distance between the two, requiring a connection. Uneven road surfaces causing vibrations during vehicle operation, load changes, or misalignment between the two assemblies can all alter the angle and distance between the transmission output shaft and the drive axle final drive input shaft. Therefore, rear-wheel drive vehicles typically employ double universal joints in their universal joint transmission system, meaning there is one universal joint at each end of the drive shaft. Their function is to ensure that the included angles at both ends of the drive shaft are equal, thereby guaranteeing that the instantaneous angular velocities of the output and input shafts remain constant.
