Hydraulic transmission is a kind of energy transmission mode emerging in modern times. It has the characteristics of easy control, high automation and easy realization of stepless speed change. There is no strict restriction on space layout with tubing connection between components. In recent years, it is widely used in engineering machinery. Excavator main engine driven pump, high pressure hydraulic oil transfer to the main control valve, control valve is controlled by the operating handle, each executive element, the distribution of high pressure oil to avoid action of actuators is hydraulic motor, because of the excavator on rotary motion, walk from the main control valve to the motor line must be set between the rotary joint, or line will twist together. The hydraulic motor turns the sprocket and the sprocket drives the track. When the oil supply from both sides of the motor is equal, the motor rotation speed is equal, and the excavator runs in a straight line, while the oil supply is unequal, it can turn to walk. Change the oil supply, can change the speed.