Common shafts can be divided into crankshaft, straight shaft, flexible shaft, solid shaft, hollow shaft, rigid shaft and flexible shaft (flexible shaft) according to the structural shape of the shaft. Straight shaft can be divided into: ① rotating shaft, which bears both bending moment and torque when working, is the most common shaft in machinery, such as the shaft in various reducers. ② Spindle is used to support rotating parts to bear bending moment without transmitting torque. Some spindles rotate, such as the axle of railway vehicles, while others do not, such as the axle supporting pulleys.