Making an informed decision can be so much easier when you understand the basics of what a gear pumps is and how it works. A gear pump uses gears to pump fluid by displacement. They are one of the most common types of pumps used for hydraulic applications. Gear pumps however are also used in chemical installations to pump fluid with a certain viscosity.

There are two main types: external gear pumps, which use two external spur gears; and internal gear pumps, which use an external and an internal spur gear.

Gear pumps are fixed displacement, meaning they pump a constant amount of fluid.

Some gear pumps can be made to use either as a pump or a motor.
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The gears inside your pump rotate and separate, on the intake side of the pump, creating a suction which is filled by oil.

The oil is carried by the gears to the discharge side of the pump, where the gears displace the oil.

The mechanical clearances are small, on the order of a thousandth of an inch.

The tight clearances, along with the speed of the rotation, effectively prevent the oil from leaking backwards.

The rigid design of the gears and housing allow for very large range of pressure and the ability to pump highly vicious fluids.

Although anything can be repaired, thanks to the high price of OEM spares and competitive pricing of pumps, gear pumps and motors are not repairable in a cost effective manner.

This can be the same for some obsolete pumps if a cheap replacement can be found.

Any reputable hydraulics company will advise you of this on first contact.

The information most companies will ask you for is as follows: make, model number and serial number.

However, if you do not know this information do not panic - there is a way you can spec up your own gear pump, here is a rough guide to what you will need to know: the shaft type and size (eg splined or keyed, how many splines); the mounting flange type and dimensions (bolt hole diameter and spigot dimensions); the port type and dimensions (eg pitch centre diameter of port fittings PCDs); the flow requirement at 1450rev/min; the working pressure; whether you need a multiple or single pump; and the port inlets and outlets.

If you are unable to measure these data yourself a reputable hydraulics company should be able to do this for you.