A shaft is a cylindrical object worn in the middle of a bearing or between the middle of a wheel or in the middle of a gear, but a few parts are square. A shaft is a mechanical part that supports a rotating part and rotates together to transmit motion, torque, or bending moment. Generally, it is a rod like rod with different diameters in each section. The revolving parts in the machine are mounted on the shaft. The common shaft can be divided into crankshaft, straight axis, soft axis, solid axis, hollow axis, rigid shaft, flexible axis (soft axis). Straight axis can be divided into: (1) rotating shaft, working both bending moment and torque when working, it is the most common shaft in the machinery, such as the shaft in all kinds of reducer. (2) the mandrel is used to support the rotating parts. It only bears the bending moment and does not transmit the torque. Some spindles are rotating, such as the axle of a railway vehicle, etc. Some spindles do not rotate, such as the shaft of the supporting pulley, etc. The drive shaft is mainly used to transfer torque and bearing moment, such as the long axis, the automobile crane moving mechanism in the drive shaft. The main materials of the shaft are carbon steel or alloy steel, as well as spheroidal graphite cast iron or alloy cast iron. The working ability of the shaft generally depends on the strength and stiffness, and the vibration stability is also determined by the high speed.