Gear pumps (with external teeth) (fixed displacement) are simple and economical pumps. The swept volume or displacement of gear pumps for hydraulics will be between about 1 and 200 millilitres. They have the lowest volumetric efficiency of all three basic pump types (gear, vane and piston pumps). These pumps create pressure through the meshing of the gear teeth, which forces fluid around the gears to pressurize the outlet side