Two main architectures are used: external gear pumps (two identical gears on parallel shafts) and internal gear pumps (a smaller rotor inside a larger ring gear with a crescent partition). External gear designs are compact and widely used in hydraulic systems, while internal gear units offer superior suction capability and better tolerance for high viscosity fluids; they are often selected for filling and emptying duties and can provide bi‑directional flow. For very high viscosities, internal gear pumps are typically the first choice.