Special form of internal gear pump, in which no sealing sickle is required, as the seal between the fixed gear ring and the rotor (planetary gear) is directly possible through the suitable design of the gears (e.g. cycloid teeth). This includes the fact that both are stored eccentrically to one another and that the rotor has one tooth less than the gear ring (e.g. 6 as opposed to 7 (Figure Z 2).
During the movement of a rotor teeth from one hole in the ring to another, each chamber fills and empties once. Due to the unfavourable (linear) seal, annular gear pumps/rotors are only suitable for moderate pressures (up to around 160 bar).
The best-known design of the annular gear motor is the orbit motor.
Figure Z 2: Principle of an annular gear pump/motor