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
External Gear Pumps
These pumps have been around the longest and are also the simplest. They employ two parallel gears mounted on shafts and can have single or double rows of teeth. The simplest version has straight cut ‘spur’ gears of involute form but ‘herring bone’ and also ‘helical form’ teeth are also available. These latter designs are quieter and slightly more efficient than simple spur teeth but also more difficult to manufacture. All gear pumps rely in fine clearances especially when handling thin fluids to prevent internal leakage around the gears also known as ‘slip’ For this reason gear pumps become less efficient with thin fluids especially when differential pressures are high. Spur gear pumps have the advantage of being well balanced and can run at high speeds often making them a better choice for thin fluids. They can also be manufactured in very small sizes making them useful for dosing, sampling and metering.
Internal Gear Pumps
There are several types of internal gear pump but all are fundamentally the same. The ‘gear within a gear’ design is a relative recent development. The advantage is that mechanically the shafts have less supporting bearings and the teeth are bigger but fewer making them especially useful for very viscous fluids. Although they use an overhung design and are less able to handle very high pressures they have become very popular for chemicals, bitumen, resin etc principally due to their simplicity of maintenance and versatility. One great advantage is that the pump end-clearance can be adjusted thus extending the life and versatility of the pump versus an external gear pump which have factory pre-set clearances. The design shown below has simple straight cut teeth. These have the benefit of being very open which makes them excellent for very viscous fluids, however they are not a ‘true’ involute form and suffer mechanically as a result of this.
These are similar to internal gear pumps but employ a different mechanical arrangement. They are compact and very cheap to produce especially in small sizes. Their simplicity makes them exceptional for OEM applications such as lubrication and circulation. This makes them the automatic choice for automobile gearbox lubrication and aviation fuel applications. Mechanically they have a true rolling, tooth form and can operate at very high speeds. At Applied Pumps Ltd we offer you ALL types of gear pumps very often from stock. Gear pumps have certain distinct properties but they must be applied correctly! As independent specialists, we are here to help you.
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