The oil pumps fitted in a recirculatory system are normally gear-type or Gerotor type pumps. The pumps are usually mounted in a pack containing one pressure pump and several scavenge pumps. They are driven by a common shaft through the engine gear train. Gear type pumps require suitable machining of the gear teeth, or the provision of a milled slot in the casting (adjacent to the delivery side of each pump), to prevent pressure locking of the gears.
Gerotor type pumps use an inner and outer rotor, where the inner rotor is driven by the engine, and the outer rotor which has an extra gear tooth rotates with it. The inner rotor is eccentric to the outer and it is the stepping of the teeth that pumps the oil. The pump also requires kidney shaped slots as inlet and outlet ports.
The scavenge pumps have a greater capacity than the pressure pump to ensure complete scavenging of the bearings in a dry sump system. Furthermore, air tends to leak into the bearing housings from the air pressurised seals and this aeration of the oil means that the scavenge pumps have to pump an increased oil/air volume. As we saw in the previous paragraph the air is subsequently removed by the de-aerator.