A Gear pump works by the gears rotating, and creating a separate on the intake side of the pump, creating a void and suction which is filled by fluid. The fluid is carried by the gears to the discharge side of the pump, where the meshing of the gears displaces the fluid. The mechanical clearances are small—on the order of a thousandth of an inch (micrometers). The tight clearances, along with the speed of rotation, effectively prevent the fluid from leaking backwards.
The rigid design of the gears and housing allow for very high pressures and the ability to pump highly viscous fluids. Many variations exist, including; helical and herringbone gear sets (instead of spur gears), lobe-shaped rotors similar to Roots Blowers (commonly used as superchargers), and mechanical designs that allow the stacking of pumps.