What are Gear Pumps? How Do They Work?

What are Gear Pumps ?
Gear pumps, unlike centrifugal pumps, are a positive displacement pump and are a very good choice for transferring viscous or highly viscous fluids. Generally the gear pumps transfer the fluids from one part to another using gear mechanism. If the pressure of the system remains same then they will provide you with the fix flow rate also.

There are two basic types of gear pumps: 
  • External and 
  • Internal.

External Gear Pumps Overview :-
External gear pumps usually have two gears with an equal number of teeth on the outside of each gear. External gear pumps are a popular pumping principle and are often used as lubrication pumps in machine tools, in fluid power transfer units, and as oil pumps in engines.

External gear pumps  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 have close tolerances and shaft support on both sides of the gears. This allows them to run to pressures beyond 3,000 PSI / 200 BAR, making them well suited for use in hydraulics. With four bearings in the liquid and tight tolerances, they are not well suited to handling abrasive or extreme high temperature applications.

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 and metering applications involving polymers, fuels, and chemical additives.

Internal Gear Pumps Overview :-
Internal gear pumps have one larger gear with the teeth turned inward, meshing with a smaller gear with external teeth. 

If the larger gear has one tooth more than the inner gear, the two gears form a seal by themselves. If the larger gear has at least two teeth more than the smaller gear, then a crescent-shaped projection of the housing goes between the two gears to help form a seal. The operating principle is the same for all of these types of pumps, and they operate in similar fashion.

The useful viscosity range of an internal gear pump is from 1cPs to over 1,000,000 cP. While they are often used on thin liquids such as solvents and fuel oil, they excel at efficiently pumping thick liquids such as asphalt, chocolate, and adhesives.
In addition to their wide viscosity range, the pump has a wide temperature range as well, handling liquids up to 750 F / 400 C. This is due to the single point of end clearance (the distance between the ends of the rotor gear teeth and the head of the pump). This clearance is adjustable to accommodate high temperature, maximize efficiency for handling high viscosity liquids, and to accommodate for wear.

The internal gear pump is non-pulsing, self-priming, and can run dry for short periods. They're also bi-rotational, meaning that the same pump can be used to load and unload vessels. Because internal gear pumps have only two moving parts, they are reliable, simple to operate, and easy to maintain.

How Dose Gear Pumps Works ?

Gear pumps belong to a positive displacement rotary group, and are made by enclosing two or more gears in a close-fitting housing. A driver turns a shaft connected to one of the gears, causing it to rotate. This gear drives the other gear through the meshing of the teeth of the two gears, just as with power transmission gears.
As the gears rotate, on one side, the teeth are coming out of mesh with each other . As a tooth is pulled out of the space between two teeth of the other gear, it creates a vacuum. Since the housing forms a seal all around the set of gears, the liquid that rushes into this space to fill this void has to come in through the pump’s suction port. Once the spaces between gear teeth are filled with liquid, the liquid rides in these pockets, trapped in place by the housing, until it reaches the discharge side of the pump. The liquid stays in place between the teeth until it reaches the other side of the gear mesh, where the teeth are coming together. Then, when a tooth from the other gear comes into the space between the teeth, the liquid there is forced out. Since the housing still forms a seal around the gears, the only place for the displaced liquid to go is out the pump’s discharge port. The pump thus operates like a conveyor belt, with the pockets of liquid between the gear teeth being picked up at the gear mesh, carried to the other side, and dropped off at the other side of the mesh.

Applications :-
Common internal gear pump applications include, but are not limited to:
  • All varieties of fuel oil and lube oil
  • Resins and Polymers
  • Alcohols and solvents
  • Asphalt, Bitumen, and Tar
  • Polyurethane foam (Isocyanate and polyol)
  • Food products such as corn syrup, chocolate, and peanut butter
  • Paint, inks, and pigments
  • Soaps and surfactants
  • Glycol

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