Gerotor Pump (Gear+Rotor )

The gerotor pump is a combination internal external gear pump. these pumps are having six tooth and Four tooth. The spur type drive gear is turned  by an accessory drive from the engine.  As it turns, it rotates the seven-tooth internal gear. 

In diagram the two marked teeth are meshed and the tooth of the spur gear almost completely fills the cavity in the rotor(Fig A).  As the drive gear rotates and pulls the driven gear round, the volume of the cavity increases until at (fig C) it is at its maximum.  During the rotation from (fig A) to (fig C) the expanding cavity is under the inlet port and fluid is drawn into the pump.

As the gears continue to rotate, the cavity formed by the marked teeth moves under the outlet port.  As the drive gear meshes with the cavity next to the marked cavity in the rotor, its volume decreases.  The fluid in this cavity is forced out of the pump through the outlet port.this process continues and provide a positive flow at the outlet.