Applied Pumps Ltd

Gear Pumps

Gear Pumps are a type of Rotary Positive Displacement Pump. They consist of at least two separate and rotating gears with intermeshing teeth. As these meshed teeth separate, they create a partial vacuum which is filled by the fluid being pumped. As the gears then continue to rotate the fluid becomes trapped and is carried around the casing to the discharge side of the pump.

Here as the gear teeth begin to re-mesh the fluid is ejected creating a pumping action. There are a number of different Gear Pump designs but ultimately they all employ this same pumping principle. Note that contrary to common belief, pumps do not suck! Instead they create a partial vacuum within them which becomes filled or partially by the fluid which must be at a higher pressure for it to work.

The initial fluid pressure might be atmospheric or it maybe be greater or less than atmospheric depending upon the system. In effect the fluid is pushed into the pump as a result of this pressure difference between the outside and the inside of the pump.

Gear Pump Advantages

As with all positive displacement pumps, the mechanical movement of fluid by gears versus by say centrifugal action has certain advantages:

• Viscous fluids that a centrifugal pump would be unable to handle are easily moved with a gear pump
• As the pumping action is quite finite, the output is very controllable. Doubling the size or speed, in theory simply doubles the output
• Unlike with a centrifugal pump, a gear pump’s output is loosely independent of discharge pressure altogether
• Not having to generate centrifugal forces to operate means that pump speeds can be low making them controllable, smooth and less disruptive
• The action makes gear pumps self-priming
• Subject to physical constraints such as motor power, casing strength, shaft sealing and to hydraulic factors such as volumetric efficiency the discharge pressure of a gear pump can be very high
• Centrifugal pumps have a Best Efficiency Point (BEP) which can be critical. This does not apply to gear pumps
• NPSHr for gear pumps is low versus centrifugal pump making them very useful where there is little net positive suction head available (NPSHa)
• Gear pumps are birotational

Gear Pump Disadvantages

As with all engineered equipment, gear pumps have a design envelope and when used outside of this will give a disappointing performance and/or a reduced life.

• Having meshing parts makes this pump wholly unsuitable for solids or abrasive media except in some very specific situations
• Gear pumps rely on precision clearances and have several rotating elements. This makes them more expensive than a centrifugal pump
• Their ability to generate high pressure can also be a problem! If there is any risk of a ‘closed valve’ situation then a pressurerelief valve and/or a pressure switch is essential to prevent a potentially hazardous situation from the pump causing excessive discharge pressures
• The mechanical design of gear pumps means that compared to centrifugal pumps less material options exist
• Meshing gears can also be noisy. Thin fluids and high speeds can result in a noisy pump that might not be suitable for some working environments
• Ultimately gear pumps are limited by size. When large bulk flow rates are required they become physically unsuitable versus other pump technology