From the viewpoint of energy conversion, hydraulic pump and hydraulic motor are reversible hydraulic components. Inputting working fluid to any kind of hydraulic pump can turn it into the working condition of hydraulic motor; Conversely, when the main shaft of the hydraulic motor rotates driven by external torque, it can also be changed to the working condition of the hydraulic pump. Because they have the same basic structural elements - closed and periodically variable volume and corresponding oil distribution mechanism. However, due to the different working conditions of hydraulic motor and hydraulic pump, there are still many differences between the same type of hydraulic motor and hydraulic pump. Firstly, the hydraulic motor should be able to forward and reverse, so its internal structure is required to be symmetrical; The speed range of hydraulic motor needs to be large enough, especially for its minimum stable speed. Therefore, it usually adopts rolling bearing or hydrostatic sliding bearing; Secondly, because the hydraulic motor works under the condition of input pressure oil, it does not have to have self-priming capacity, but it needs a certain initial sealing to provide the necessary starting torque. Due to these differences, the hydraulic motor and hydraulic pump are similar in structure, but they can not work reversibly.