Pistons are arranged radially to the eccentric shaft. The piston is guided in cylinder and pressed against the eccentric by the spring . During the downward movement of piston, the working chamber in the cylinder increases in size. The resulting negative pressure lifts the suction valve plate from the sealing edge. This opens the connection from the suction chamber to the working chamber. The working chamber fills with fluid. During the upward movement of piston, the suction valve closes and the pressure valve opens. Fluid can now flow to the system via pressure port.