Abstract: In an external or internal gear pump or gear motor with one pair of helical spur gears, the trapping of fluid in the enclosed tooth cavity and the risk for cavitation in the said enclosed tooth cavity is considerably reduced by the shift of the pressure port connected and the suction port connected relief grooves at the leading lateral face of the helical gears in the direction of the suction port and the shift of the pressure port connected and suction port connected relief grooves at the lagging lateral face of the helical gears in the direction of the pressure port.