The liquid pressure produces torque relative to the axis O1 and O2. Under the action of the torque, the gear motor rotates in the direction of the diagram arrow and drags the external load to work. With the rotation of the gears, teeth 1 and 1 ' swept volume than teeth 3 and 4 ' swept the volume of small, so with the constant change of meshing teeth, the volume of oil into the cavity increased, high pressure oil will continue to enter, at the same time is constantly in the belt back into the oil cavity discharge.