What is a Gear Pump?

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  • Written By: James Back
  • Edited By: Jenn Walker
  • Last Modified Date: 07 November 2019
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A gear pump is a mechanical pump that moves liquids by the use of two rotating gears. Liquid flows under pressure from the pump intake to the discharge in the space formed by the gear teeth. The liquid also serves to lubricate the gears. The small clearances between the walls of the pump chamber and gear teeth create a tight seal, thereby preventing liquid from flowing back through the intake. Unlike other types of pumps, gear pumps do not need to be primed and can be ran dry for short periods without damaging the pump. They are typically used to pump water, oil and other liquids.

Gear pumps are capable of producing very high internal pressures and are often used to pump thick liquids such as pitch and crude oil. Pump speeds must be reduced when viscosity increases to allow enough time for the liquid to fill the voids between the gear teeth. A gear pump is capable of pumping very precise amounts of liquid at high pressure and is often used to pump fuel and heating oil, diesel fuel and gasoline. Gear pumps are commonly used in automobile oil pumps and household sump pumps as well.


A gear pump works on the principle of displacement. As the gears rotate inside the gear pump chamber, they create areas of low and high pressure. An area of low pressure, or vacuum, is created between the teeth of the gears when they un-mesh. Liquid flows into this area of lower pressure. As the gear continues to rotate, the liquid becomes trapped in the pocket formed by the gear teeth and the wall of the pump chamber. Finally, an area of higher pressure is created once the gears begin to mesh, forcing the liquid from the gear teeth and into the discharge outlet. Extremely small clearances between the gear teeth and the wall are required in order for the pump to work effectively. Over time, the gear pump will gradually lose efficiency as the clearances increase due to normal wear and tear.

There are two main types of gear pumps, internal and external. The internal gear pump, also known as a gerotor, has two gears. A smaller internal gear fits inside a larger one and both rotate in the same direction. The smaller gear always has one less tooth than the larger one and is mounted off-center in relation to the shaft of the larger gear. This allows the internal gear to rotate freely inside the external gear, while at the same time providing the space needed to pump the liquid from the intake to the discharge of the gear pump. Power is applied to the shaft of the internal gear, which then drives the external gear.

External gear pumps have two identical gears that rotate in opposite directions. Liquid is transported in the space created between the teeth of the gear and the sides of the pump chamber. Power is applied to the shaft of the driving gear, which in turn drives the other gear. Spur gears are usually used in external gear pumps, although helical and herringbone gears are sometimes used when excessive pump noise is a concern.


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