A gear pump uses the meshing of gears to pump fluid by displacement. They are one of the most common types of pumps for pumping oil. Gear pumps are also widely used in chemical installations to pump fluid with greater viscosity than water. There are two main variations; external gear pumps which use two external spur or helical gears, and internal gear pumps which use an external and an internal spur gear. The Oberdorfer Pumps are of the external helical gear design. Gear pumps are positive displacement, meaning they pump a constant amount of fluid for each revolution.