Gear pumps uses the meshing of gears to pump fluid by displacement. They are one of the most common types of pumps for hêydraulic fluid power applications. It was invented around 1600 by Johannes Kepler.
Gear pumps are also widely used in chemical installations to pump high viscosity fluids. There are two main variations: external gear pump which use two external spur gears, and internal gear pump which use an external and an internal spur gears (internal spur gear teeth face inwards, see below). Gear pump are positive displacement (or fixed displacement), meaning they pump a constant amount of fluid for each revolution. Some are designed to function as either a motor or a pump.