A gear-within-gear pump that consists of an inner gear with one less tooth than the outer gear.

A gerotor is a positive displacement pump, its name derived from “generated rotor”.

The axis of the inner rotor is offset from the axis of the outer rotor and both rotors rotate on their respective axes. The geometry of the two rotors partitions the volume between them into different dynamically-changing volumes.

During the assembly’s rotation cycle, each of these volumes changes continuously, so any given volume first increases, and then decreases. An increase creates a vacuum. This vacuum creates suction, and hence, this part of the cycle is where the intake is located. As a volume decreases compression occurs. During this compression period, fluids can be pumped, or, if they are gaseous fluids, compressed.