Common shafts can be classified into crankshafts, straight shafts, flexible shafts, solid shafts, hollow shafts, rigid shafts, and flexible shafts (soft shafts) according to the structural shape of the shaft. The straight shaft can be divided into: 1 shaft, which can bear both bending moment and torque when working. It is the most common shaft in the machine, such as shafts in various speed reducers. 2 The mandrel is used to support the rotating parts and only withstand the bending moment without transmitting the torque, and some of the mandrels rotate, such as the shaft of the railway vehicle, and some of the mandrels do not rotate, such as the shaft supporting the pulley. 3 Transmission shaft, mainly used to transmit torque without bending moment, such as the long optical axis in the crane moving mechanism, the drive shaft of the car and so on. The material of the shaft is mainly carbon steel or alloy steel, and ductile iron or alloy cast iron can also be used. The working ability of the shaft generally depends on the strength and rigidity, and when the rotational speed is high, it also depends on the vibration stability.