Gear Pump

Written by Matt Milbury. Posted in Pump

A gear pump uses the meshing of gears to pump fluid by displacement. Due to their simplicity, versatility and their high reliability 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 wide range of viscosities. Internal gear pumps can pump fluids with viscosity ranges from 1cPs to over 1,000,000cP. The gear pump also has the ability to pump fluids at very high temperatures, in excess of 700o.

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.

The most common design of an external gear pump is where one of the gears is driven by a motor and the other runs free. A partial vacuum, created by the unmeshing of the rotating gears, draws fluid into the pump. This fluid is then transferred to the other side of the pump between the rotating gear teeth and the fixed casing. As the rotating gears mesh together, they generate an increase in pressure. Depending on the number of teeth, the "idler" gear might be driven directly by the "drive" gear. In other cases an extra gear external to the pump drives the secondary gear at the same rate.

External gear pumps can come in single or double (two sets of gears) pump configurations with different types of gear designs. These can be spur gears, helical gears, and herringbone gears. The outlet flow of gear pumps is very smooth when compared to a piston pump. However, among gear pumps, pumps with a delical or herringbone gears will have a smoother flow than spur gears. 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. Gear pumps can run at pressures beyond 3,000 PSI making them well suited for use in hydraulics applications.

This pump will pump in the reverse direction if you reverse the direction of rotation of the gears. Two pairs of valves can be added to make this a Reversing Gear Pump, which pumps in the same direction regardless of which direction the gears rotate.


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  • Hydraulic motors
  • Lubrication Pumps
  • Power steering units