What Are Gear Pumps and How Do They Work

Educational

Dec 29th, 2021

What Are Gear Pumps and How Do They Work

The first gear pump was invented by Johannes Kepler back in 1600. That means that gear pumps have been around for over 400 years. Even though gear pumps have taken various designs since their invention, they didn’t change much in how they work. 

Today, you can easily spot a gear pump in any of these machines: 

  • Car's oil pump
  • Lawn care equipment
  • Log splitters
  • Power units 

Gear pumps control the flow rate of liquids, and for that reason, they’re commonly used in industries for pumping high-viscosity fluids. They can also be modified to work as motors. 

So, What Exactly Is a Gear Pump?

A gear pump is an encased system of gears responsible for moving fluid mechanically through cyclic pumping motions. It is referred to as a positive displacement (aka fixed displacement) pump because it draws a constant amount of fluid during each revolution, accomplishing this with the use of the gears’ meshing.

How Does a Gear Pump Work?

Gear pumps operate by enclosing a specific volume of a fluid using interlocking cogs before transferring it through cyclic pumping actions. The rotating element creates a liquid seal with the pump casing and develops suction at the pump’s inlet.

The drawn fluid is confined inside the spaces of the rotating gears, which is then transferred to the outlet. The gear pump delivers a smooth flow proportional to the speed of the rotating gears.

Two Types of Gear Pump

Gear pumps come in two types, namely:

  • External gear pumps which contain two external spur gears inside a pump housing
  • Internal gear pumps which consist of one external and one internal spur gear located inside the external gear (the internal spur gear teeth face inwards)

External Gear Pump

An external gear pump consists of two interlocking external gears supported by separate shafts. A motor drives one of the cogs or gears, called a drive gear, which in turn drives the other gear, called the idler gear.

In some designs, both shafts are driven by motors. Bearings on the side of the pump casing support the shafts.

As the rotating gears come out of the mesh, they create a volume on the inlet of the pump. Liquid or fluids then move into this space. The gear teeth then girdle this volume of fluid as the gears continue in their cyclic motion against the pump casing.

The fluid is smoothly transferred from the inlet to the discharge or outlet of the casing. The gear teeth interlock at the outlet side, reducing the volume of liquid and forcing it out under high pressure. 

The interlocking of the gears prevents the fluid from flowing back through the centre, while its close tolerances develop suction that directs the liquid from the discharge side to the suction side.

External gear pumps can take various designs using different shapes of gears, like spur, helical, or herringbone.

Features of External Gear Pump

An external gear pump has the following features:

  • Compact in size with a simple design
  • Can manage low, medium, or high pressure
  • Has close tolerance as well as shaft support on both ends of the gears 
  • Has sizable outlet ports that enable them to deliver high volume of fluids

Internal Gear Pump

Unlike in external gear pumps, the two interlocking gears in an internal gear pump are not identical. Not only are the gears differently sized, but one gear also rotates inside the other while still using the same principle of operation. Cool, right?

The larger of the two gears is the internal gear, sometimes referred to as a rotor. One way to identify an internal gear is by its teeth, which face inwards. Within the rotor is a relatively smaller gear that serves as the external gear. The off-centre mounted external cog is the idler.

The idler is designed to interlock with the rotor in a way that at one point, the gear teeth engage. A resultant crescent partition occupies the void developed by the off-centre mounting location of the external gear. This spacer additionally acts as a seal between the inlet and outlet ports.

A volume is created in the inlet end of the pump when the gears come out of mesh. The fluid then moves into the crescent partition, where the gear teeth trap the liquid. Meanwhile, the gears keep rotating against the casing.

The trapped fluid flows to the discharge around the casing. On the outlet end, the interlocking of the gears results in volume reduction. Consequently, the liquid is forced out because of the pressure.

Unlike the other type, internal gear pump designs make use of spur gears only.

Features of Internal Gear Pumps

Internal gear pumps have the following features: 

  • Can be run dry for a short while
  • Have a low net positive suction head (NPSH) requirement

The Pros & Cons of Gear Pumps

Gear pumps come in small sizes and provide pulseless and stable fluid flows, making them superior to other pumps, like peristaltic and diaphragm pumps. Let’s list down their advantages and disadvantages.

Advantages of Gear Pumps

  • Gear pumps are compact in size and simple. They also have few moving parts, making them less prone to problems. 
  • They offer high pressures and throughput compared to either vane or lobe pumps, making them ideal for pumping viscous fluids.
  • External gear pumps can withstand enormous pressures and flow rates compared to internal gear pumps because they pack rigid shaft support as well as closer tolerances. 
  • Internal gear pumps have better suction, which makes it suited for highly-viscous fluids. 

Disadvantages of Gear Pumps

  • Gears and bearings in the gear pumps are prone to wear after a long time of operation. This wear, in turn, reduces the overall efficiency of the gear pump.
  • Most gear pumps are limited to a specific range of thermal conditions outside which they won’t work effectively. High temperatures, for instance, may lead to the expansion of gears and pump casing which reduces clearance.

Other Concepts Related to Gear Pumps

Tolerance, Flowrate, and Speed

  • Gear pumps are not a good choice for pumping liquids with solids in them due to the close tolerance. This type of liquid may also damage the gears and other components. 
  • Gear pumps generally do not pump fluids of high flowrate and lower discharge pressure applications. 
  • Most gear pumps need to be run at much lower speeds to work effectively. The higher the viscosity of the liquid being pumped, the lower the speed of gear pump required.
  • They provide minimal variation in the flowrate even with an increase or decrease in the discharge pressure in the system. 
  • The horsepower required for a given gear pump will increase as the pressure or the viscosity increases. 

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By Sam O.