External gear pumps have two meshing gears, which may be of the spur, helical, or herringbone type. . Liquid is carried between the gear teeth and displaced as the teeth mesh. Close clearances between the gear teeth and between the teeth and the casing walls minimize slippage of liquid from the high-pressure side to the low-pressure side.
Spur gears are simple, and generally cost less to manufacture than helical or herringbone gears. They have good characteristics but can be noisy and inefficient. The trapped liquid between the teeth where the teeth mesh has no place to exit. As the trapped liquid squeezes by the tight clearances, it can make a loud screaming sound. Some manufactures provide a relief slot in this area to give the fluid a place to escape. This minimizes noise and increases efficiency, especially with viscous liquids. Spur gears have the added advantage of minimal axial thrust.
Helical gears are another means of giving the trapped liquid a path to escape. The helix shape gives the liquid a place to exit. Helical gear pumps are generally efficient and quiet. They do have one disadvantage: they are forced axially against the pump thrust bushing. Axial wear in a gear pump decreases performance much faster than radial wear, so it is important to maintain tight clearances in the axial direction.