Internal gear pumps , like external gear pumps, move and pressurize liquid by the meshing and unmeshing of gear teeth. With an internal gear pump, a rotor having internally cut teeth meshes with and drives an idler gear having externally cut teeth. Pumps of this type usually have a crescent-shaped partition that moves the liquid through the pump with minimal slip.

Internal gear pumps’ advantages include their few moving parts, relatively low cost, and the fact that they have only one seal. They can usually operate well in either direction, and reversing rotation causes a reversal in the direction of flow. Disadvantages include the fact that there is one bearing in the pumped liquid, that the one bearing must support an overhung load, and these pumps generally will not work with abrasives or solids