Amin Almasi is a senior rotating machinery consultant in Sydney, Australia. He holds a bachelor’s degree and master’s degree in mechanical engineering. He specializes in rotating machinery including pumps, compressors, gas turbines, steam turbines, engines, condition monitoring and reliability. Almasi is an active member of Engineers Australia, IMechE, ASME and SPE. He has authored more than 150 papers and articles dealing with pumps, rotating equipment, condition monitoring and reliability.
In a gear pump, the friction torque and consequent pump operation and required power can be affected by liquid temperature as well as operating pressure and pump speed. When the pressure differential is large, the friction torque decreases first and then increases with an increase in pump speed. For a large pressure differential, the friction torque could become higher with an increase in liquid temperature in a low pump speed region, but it could have the opposite tendency in a high pump speed region.
Transient Operation & Cavitation
When a gear pump operates with a relatively low suction pressure (for instance, when liquid is from a tank at a lower level), pressures in the suction piping and chamber get closer to vapor pressure, and cavitation can take place upstream from the gear meshing region.
Another common operational problem is cavitation in the case of transient operations. One frequent cause of cavitation is insufficient flow into the expanding inter-tooth volumes. In many theoretical or operational studies on these topics, the inter-tooth volumes that are formed at the roots of the driver and driven gears should be considered. Compressible flow into and out of these volumes plays important roles in cavitation and transient operation.
To study the effects of operation parameters such as suction pressure on pump operation, in a case study a gear pump has been operated at 1,200 rpm and 3,400 rpm speeds with around 20 Barg discharge pressure. The pump suction is from an atmospheric tank. An 0.8 bar pressure drop in the suction was observed when pump was operated at 3,400 rpm. In other words, at around 3,400 rpm, the gear pump should be operated with a mean suction absolute pressure of 0.2 bar absolute (Bara), which is relatively close to the pump limit, and cavitation should be expected. At 1,500 rpm, this same situation represented a smaller suction pressure drop of only about 0.5 bar; this resulted in a mean suction absolute pressure of approximately 0.5 Bara with some good margin against cavitation.
Manufacturing & Performance
Gear pumps can usually come in single or double (two sets of gears) pump configurations with different types of gear such as spur, helical, herringbone gears. Helical and herringbone gears typically offer a smoother flow compared to spur gears, although all gear types are relatively smooth. Straight spur gears are easiest to cut and are the most widely used. Helical and herringbone gears run more quietly but cost more. They are typically used in large capacity gear pumps.
Displacement volumes of a gear pump are directly affected by the gear tooth profile. Since the involute gear tooth profile is easily manufactured and the technology for the power transmission gear can be applied, this profile is usually adopted for a low cost gear pump. In an involute gear, the profiles of the teeth are involutes of a circle.
The pressure angle is the acute angle between the line of action and a normal to the line connecting the gear centers. Theoretically, gear manufacturers can produce any pressure angle. However, the most common gears have a 20 degree pressure angle, with 14.5 degree and 25 degree pressure angle gears as other common options. Increasing the pressure angle increases the width of the base of the gear tooth, leading to greater strength and load carrying capacity. Decreasing the pressure angle provides lower backlash, smoother operation and less sensitivity to manufacturing errors. Only used in limited situations are helical involute gears, where the spirals of the two involutes are of different “hand” and the “line of action” is the external tangents to the base circles.
Many gear pumps use helical gears. The teeth on helical gears are cut at an angle to the face of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in full engagement. This gradual engagement makes helical gears operate more smoothly and quietly than spur gears. Because of the angle of the teeth on helical gears, a thrust load (axial load) is created on the gear when they mesh.
This load should be properly addressed, for example, by using thrust (axial) bearings. The use of helical gears is indicated when the application involves relatively high speeds, relatively high power pumps or where noise abatement is important.
As an indication, the speed might be considered to be high when the pitch line velocity exceeds 20 meters per second.
A herringbone gear is a specific type of double helical gear that is a side-to-side combination of two helical gears of opposite hands. From above, the helical grooves of this gear looks like the letter “V.” Unlike helical gears, herringbone gears do not produce an additional axial load. Like helical gears, herringbone gears have the advantage of operating smoothly because more than two teeth will be in mesh at any moment in time. Their advantage over the helical gears is that the side-thrust of one half is balanced by that of the other half. This means that herringbone gears can be used without requiring a substantial thrust bearing.
Precision herringbone gears are more difficult to manufacture than equivalent spur or single helical gears and consequently are more expensive. A disadvantage of the herringbone gear is that it cannot be cut by simple gear hobbing machines, as the cutter would run into the other half of the gear. Therefore, advanced, expensive manufacturing machineries such as modern CNCs are needed.
External vs. Internal Gear Pumps
EXTERNAL GEAR PUMPS are similar in pumping action to internal gear pumps in that two gears come into and out of mesh to produce flow. However, an external gear pump uses two identical gears rotating against each other. One gear is driven by a driver (electric motor), and it in turn drives the other gear. Each gear is supported by a shaft with bearings. External gear pumps are typically quiet running and are routinely used for high-pressure applications.
INTERNAL GEAR PUMPS have one or two fewer teeth in the inner gear than the outer one. Relative speeds of the gears in these designs are low. For example, in a pump design, the number of gear teeth in the inner and outer gears is 10 and 11, respectively. The inner gear would turn 11 revolutions, while the outer would turn 10. Internal gear pumps are compact units. Generally external gear pumps are more popular. With no overhung bearing loads, the rotor shaft cannot deflect and cause premature wear.