On an internal gear pump, should the inside gear have the same number of teeth as the outside gear or should it have fewer teeth?

In an internal gear pump, the inside gear must have fewer teeth than the outside gear. This design allows for efficient operation and the characteristic pumping action of the internal gear pump. When the outside gear has more teeth, it engages with the fewer-teethed inside gear, creating a continuous cycle where the outside gear drives the inside gear.

This configuration enables the creation of cavities that form as the teeth of the gears mesh and pull fluid into the pump. As the inside gear rotates, it creates a space where the fluid is drawn in, while the outside gear, rotating in the opposite direction, allows for the expulsion of the fluid. If the inside gear had the same number of teeth as the outside gear, they would not have the necessary differential motion to create this functional pumping action, and would effectively lock up. Thus, fewer teeth on the inside gear is essential for the internal gear pump's operational efficacy.