If a positive displacement pump uses interconnecting of gears to pump the fluid then it is known as the gear pump. Gear Pumps are mostly used for hydraulic fluid power applications.


A basic gear pump will have a rotary housing containing two or more gears, helical gears. Tight tolerances are required between the casing & gears, bore & gears, and between the gears. A typical housing will have an inlet and outlet, for suction and discharge respectively.

There are two main variations

1.       External Gear

An external gear pump uses two external gears that displace non-lubricating fluids. The mechanism is usually driven by one of the toothed gears, which in turn drives the other. Three factors are involved in the regulation of flow: volume of the cavity between the teeth, speed of gears, and the amount of fluid that slips back to the inlet via the mechanism.

2.       Internal Gear

 An internal gear pump uses internal and external gears. The gears themselves are lubricated by the fluid, which is of a lubricating nature. The internal design is seen as being reliable, easy to operate, and maintain – due to only two moving parts being present. Only one drive gear is required for the mechanism to function but it is possible to use two. The pump will usually contain at least one bushing. Internal gear pumps have relatively low speed and inlet pressure requirements.


As the gears rotate they separate on the intake side of the pump, creating a vacuum and suction which is filled by fluid. The fluid is carried by the gears to the discharge side of the pump, where the meshing of the gears transfers the fluid. The rigid design of the gears and houses allows for very high pressures and the ability to pump highly viscous fluids.

Many variations exist, including; helical gear sets, lobe-shaped rotors, and mechanical designs that allow the stacking of pumps.


An external precision gear pump is usually limited to a maximum working pressure of 210 bars (21,000 kPa) and a maximum speed of 3,000 rpm. Further increase in pressure and speed pump tend to be noisy.


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