Automobile transmission shaft is composed of shaft tube, telescopic sleeve and universal joint. The telescopic sleeve can automatically adjust the change of the distance between the transmission and the drive axle. The universal joint ensures the change of the angle between the output shaft of the transmission and the input shaft of the drive axle, and realizes the constant angular speed transmission of the two shafts.

Universal joint is the key component of automobile transmission shaft. On rear wheel drive vehicles, the universal joint transmission shaft is installed between the transmission output shaft and the drive axle input shaft; For front wheel drive vehicles, the transmission shaft universal joint is omitted and installed between the front axle and the wheel, which is responsible for both driving and steering. A car is a moving object. On the rear drive vehicle, it is installed on the frame as a whole, and the drive axle is connected to the frame through an elastic suspension. There is a distance between the two, which needs to be connected.

When the vehicle is running, the uneven road surface will produce runout, the load change or the installation position difference of the two assemblies will change the included angle and distance between the transmission output shaft and the drive input shaft. Therefore, a "variable strain" device should be used to solve this problem, so there is a universal joint.