In its simplest form an oil pump consists of two gear wheels meshed together in a tight space so that oil cannot escape past the sides. One wheel is driven by the engine and, as the gears rotate in opposite directions, the dwell or recess between each tooth in each wheel traps a small quantity of oil from an inlet port. The trapped oil is carried round by each wheel towards an outlet port on the opposite side of the casing, where it is forced out by the meshing teeth.

The principle of squeezing oil from an ever-decreasing space is used in the rotor-type pump. Here an inner and outer rotor are mounted on different axes in the same cylinder.

The inner rotor, which commonly has four lobes, is driven by the engine, It meshes with an outer rotor which has five lobes. As they rotate, the spaces between them vary in size. The intake port is placed at a point where the space between the rotor lobes is increasing, drawing oil in. It is then carried round the pump, and as rotation continues, the space between the lobes gets smaller, compressing the oil which is then ejected through the outlet port.