The present invention is a high-balancing gear pump (motor) with the top surface of gear as a support. It is provided with a control device of variable-displacement. Its side plate is pieced together by use of two plates which are different in the size of outside diameter. There is a radial-movable stator between a driven gear and case of pump. The variable mechanism moves the stator and the stator drives the gear to move together with it, so that it changes the overlap coefficient of gear engagement and the central distance. In the range of overlap coefficient from maximum to equal to one, the smaller the central distance, the more the displacement, and otherwise the less. The variable-displacement principle of inside engagement is identical to this. |