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Pump Overview - Theory of Operation

By: Kal Banev

Gear pump
A Gear pump uses the meshing of gears to pump fluid by displacement. They are one of the most common types of pumps for hydraulic fluid power applications. Gear pumps however are also widely used in chemical installations to pump fluid with a certain viscosity. There are two main variations; 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 for each revolution. Some gear pumps are designed to function as either a motor or pump.

Theory of operation
As the gears rotate they separate on the intake side of the pump, creating a void 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 displace the fluid. The mechanical clearances are small—on the order of a thousandth of an inch (micrometres). The tight clearances, along with the speed of rotation, effectively prevent the fluid from leaking backwards.
The rigid design of the gears and housing allow for very high pressures and the ability to pump highly viscous fluids.
Many variations exist, including; helical and herringbone gear sets (instead of spur gears), lobe shaped rotors similar to Roots Blowers (commonly used as superchargers), and mechanical designs that allow the stacking of pumps. The most common variations are shown below

External Gear Pump Overview
External gear pumps are a popular pumping principle and are often found in lubrication pumps in machine tools, in fluid power transfer units, and as oil pumps in engines.
External gear pumps can come in single or double (shown, left) pump configurations with spur (shown), helical, and herringbone gears. Helical and herringbone gears typically offer a smoother flow than spur gears, although all gear types are relatively smooth. Large-capacity external gear pumps typically use helical or herringbone gears. Small external gear pumps usually operate at 1750 or 3450 rpm and larger models operate at speeds up to 640 rpm. External gear pumps handle viscous and watery-type liquids, but speed must be properly set for thick liquids. Reduced speeds with high-viscosity liquids results in greater efficiency.

The design of external gear pumps allows them to be made to close tolerances. Tighter internal clearances provide for a more reliable measure of liquid passing through a pump and for greater flow control. Because of this, external gear pumps are popular for precise transfer applications involving polymers, fuels, and expensive liquids.

External gear pumps are similar in pumping action to internal gear pumps in that two gears come into and out of mesh to produce flow. However, the external gear pump uses two identical gears rotating against each other -- one gear is driven by a motor and it in turn drives the other gear. Each gear is supported by a shaft with bearings on both sides of the gear.

1. As the gears come out of mesh, they create expanding volume on the inlet side of the pump. Liquid flows into the cavity and is trapped by the gear teeth as they rotate.

2. Liquid travels around the interior of the casing in the pockets between the teeth and the casing -- it does not pass between the gears.

3. Finally, the meshing of the gears forces liquid through the outlet port under pressure.

Because the gears are supported on both sides, external gear pumps are quiet-running and are routinely used for high-pressure applications such as hydraulic applications. With no overhung bearing loads, the rotor shaft can't deflect and cause premature wear.

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