A shaft is a cylindrical object that runs through the middle of a bearing, a wheel or a gear, but a small number of them are square. A shaft is a mechanical part that supports rotating parts and rotates together with them to transmit motion, torque or bending moment. It is generally in the shape of a metal round rod, and each section can have different diameters. The parts that make rotary motion in the machine are installed on the shaft.

Common shafts can be classified into straight shafts, flexible shafts, solid shafts, hollow shafts, rigid shafts and flexible shafts (flexible shafts) based on their structural shapes. Straight shafts can be further classified as:

 ① rotating shafts, which bear both bending moment and torque during operation. They are the most common types of shafts in machinery, such as those found in various reducers.

② Mandrels are used to support rotating parts and only bear bending moments without transmitting torque. Some mandrels rotate, such as the shafts of railway vehicles, while others do not rotate, such as the shafts supporting pulleys.

③ Drive shafts are mainly used to transmit torque without bearing bending moments, such as the long smooth shafts in the moving mechanisms of cranes and the drive shafts of automobiles.