Tips for choosing a gear pump
Gear pumps belong to the group of positive displacement pumps. In the pump world, positive displacement pumps are a minority with a share of around 33%, and in this subgroup, gear pumps once again represent a specialized niche. In the case of gear pumps, a distinction is made between externally and internally geared pumps. The pumps differed as follows:
- External gear pumps have two equally sized gears.
- Internally toothed pumps have an externally toothed wheel that runs in a toothed ring.
The choice depends on the application.
Gear pumps are generally used for applications where uniform delivery of the medium under comparatively high pressure is required. Gear pumps are particularly suitable for highly viscous media - allowing them to be conveyed gently and efficiently.
Differences between internally and externally geared pumps
External gear pumps and internal gear pumps are commonly found in industrial plants. There are a variety of applications in which the internally geared pumps do an excellent job. Due to their design, the fields of application of these types of gear pumps are different. The limitation of the application depends in each case on the viscosity and the required pressures at the pump.
External gear pumps
- 4 Bearings
- Large feeding range of the gears
- High pressures
- High viscosities
- Both gears have the same number of teeth
Internally geared pumps
- 2 Bearing, overhung
- Fluid must pass through the outer toothed ring so that the conveying process can be realized
- Low differential pressures
- Low viscosities
- Lower suction side filling
- Number of teeth of the outer ring is greater than the number of teeth of the inner ring gear.
Criteria for selecting the right pump technology
The selection of the right pump technology and the right pump size depends significantly on various influencing factors. In order to select the right technology and determine whether the application can be served by a gear pump, you have to take a close look at the process parameters. The more detailed and the more parameters are available, the more precisely the pump can be designed.
The most important factors are the suction pressure at the pump, the flow rate, the viscosity and the temperature of the medium. But the medium itself also plays a decisive role. If it contains particles or solids, for example, the use of a gear pump can become difficult. The lubricating properties of the fluid are also important. Gear pumps are equipped with product-lubricated friction bearings. This means that there is no external lubrication of the bearings. Therefore, in order to prevent the shaft journals from running into the bearing bores, it must be ensured that a sufficiently strong hydrodynamic lubricating film is formed, which allows the shafts to float in the bearing.
The individual characteristics of the pump are determined by the following criteria.
- Selection of the correct pump type: application and suction pressure
- Selection of the right pump size: viscosity and flow rate
- Selection of a suitable shaft seal: viscosity, chemical properties of the fluid, suction pressure
- Materials & Matches: viscosity, chemical properties of the fluid, differential pressure, temperature.
Where are gear pumps used?
The principle of operation of gear pumps
In special cases requiring very high torque, both gears can also be driven. This reduces the load on the gears and the wear on the teeth. The pumped medium is drawn into the pump by the rotation of the shafts. As the gears rotate, a chamber is formed between two teeth and the housing which is filled with the fluid to be pumped.
As the gears rotate, the liquid is pumped along the outside of the housing from the suction side to the discharge side. The sucked-in fluid in the center of the pump seals the two sides of the pumps from each other. Where the two gears meet again, the pumped medium is squeezed out of the gap between the teeth. The gear pump is assigned to the category of rotary positive displacement pumps.
Components of a WITTE gear pump
Suction and pressure side
The pump openings are at the two points where the gears touch. One opening is on the suction side and the other opening is on the discharge side of the pump. Normally, one gear is driven by the shaft coming out of the pump housing.
Three important factors are achieved by the interlocking teeth:
- Transport of the liquid
- Sealing of the suction and discharge sides of the pump
- Transmission of the torque
Why choose a gear pump?
The gear pump belongs to the group of rotary displacement pumps. Compared to the oscillating displacement pumps, the rotating displacement pumps convey with a significantly reduced pulsation in the product flow. This is a major advantage, especially in sensitive processes - such as extrusion. The use of a rotary displacement pump also means that pulsation dampeners, for example, can be dispensed with. This not only makes the process setup smaller, but also more cost-effective. The adjacent diagram shows the pulsation behavior of a gear pump compared to a 1-cylinder pump. With additional stages (cylinders) or pulsation dampeners, a similar pulsation behavior can be achieved with cylinder or piston pumps as with a gear pump. Due to the additional units, the setup requires significantly more space in the plant and is also much more cost-intensive.
Due to its design, however, the gear pump is also subject to limitations in its area of application. For example, multi-stage pump arrangements with several pumps connected in series are rather unusual. Gas- or solids-laden fluids pose a high risk for the pumps. Particles can only be pumped up to a certain size and concentration. Solids in themselves, such as nuts in the food industry or mineral pigments in plastic melts, can cause severe damage up to and including total pump failure.
Particles can present a variety of challenges and problems if they get between components that are moving relative to each other.
The critical locations in the pump for pigment-laden fluids are:
- radially between the shaft journal and the sleeve bearing,
- axially between the face of the shaft and the plain bearings,
- between the gear head and the housing, and
- at the loaded tooth flanks when the teeth mesh ne.