In a gerotor pump, liquid enters the suction port between the teeth of the elements. There is a smaller, inner, driver element…and a larger, outer, driven element. While tooth counts vary, the outer element has one more tooth than the inner element, creating cavities between the teeth. As the pump rotates, these cavities expand on the inlet side and contract on the outlet side.

As the teeth come out of mesh, a low-pressure zone is created at the suction port, pulling liquid into the spaces. The liquid continues to travel through the pump, between the teeth of the gear-within-a-gear principle. Liquid is prevented from returning to the suction side due to the close tolerance of the gears, acting as a seal between the suction and discharge ports. When the teeth mesh completely, liquid is forced out of the spaces and out of the discharge port.