Internal Gear Pump

Figure shows an internal gear pump. The teeth of one gear project outward, while the teeth of the other gear project inward toward the center of the pump. One gear wheel stands inside the other. This type of gear can rotate, or be rotated by, a suitably constructed companion gear. An external gear is directly attached to the drive shaft of a pump and is placed off-center in relation to an internal gear. The two gears mesh on one side of a pump chamber, between an inlet and the discharge. On the opposite side of the chamber, a crescentshaped form stands in the space between the two gears to provide a close tolerance. The rotation of the internal gear by a shaft causes the external gear to rotate, since the two are in mesh. Everything in the chamber rotates except the crescent, causing a liquid to be trapped in the gear spaces as they pass the crescent. Liquid is carried from an inlet to the discharge, where it is forced out of a pump by the gears meshing. As liquid is carried away from an inlet side of a pump, the pressure is diminished, and liquid is forced in from the supply source. The size of the crescent that separates the internal and external gears determines the volume delivery of this pump. A small crescent allows more volume of a liquid per revolution than a larger crescent.