A gear pump is a type of positive displacement (PD) pump. Gear pumps use the actions of rotating cogs or gears to transfer fluids. The rotating gears develop a liquid seal with the pump casing and create a vacuum at the pump inlet. Fluid, drawn into the pump, is enclosed within the cavities of the rotating gears and transferred to the discharge. A gear pump delivers a smooth pulse-free flow proportional to the rotational speed of its gears.
There are two basic designs of gear pump: internal and external (Figure 1). An internal gear pump has two interlocking gears of different sizes with one rotating inside the other. An external gear pump consists of two identical, interlocking gears supported by separate shafts. Generally, one gear is driven by a motor and this drives the other gear (the idler). In some cases, both shafts may be driven by motors. The shafts are supported by bearings on each side of the casing.
This article describes plastic gear pump in more detail.
There are three stages in an internal gear pump’s working cycle: filling, transfer and delivery (Figure 2).
The close tolerances between the gears and casing mean that these types of pump are susceptible to wear particularly when used with abrasive fluids or feeds containing entrained solids. External gear pumps have four bearings in the pumped medium, and tight tolerances, so are less suited to handling abrasive fluids. For these applications, universal gear pump are more robust having only one bearing (sometimes two) running in the fluid. A gear pump should always have a strainer installed on the suction side to protect it from large, potentially damaging, solids.
Helical gear pumps
Similar to the spur gear pump, the helical gear pump uses a pair of single- or double-helical (herringbone) gears. Helical gears run quieter than spur gears but develop thrust loads which herringbone gears are intended to counteract. These designs are often used to move larger volumes than spur gear pumps. Helical gears produce fewer pulsations than stainless gear pump as the meshing of teeth is more gradual compared with spur-gear designs. Helix angles run between 15 and 30°.
Both the helical and herringbone gear pumps eliminate the problem of trapping fluid in the mesh. These designs can introduce leakage losses where the teeth mesh, however, unless very tight tooth clearances are maintained. The higher manufacturing costs associated with herringbone gear pumps must be balanced against their improved performance.
External gear pumps are the least costly of the various positive-displacement pumps but also the least efficient. Pressure imbalances between suction and discharge sides can promote early bearing wear, giving them somewhat short life expectancies.
One general disadvantage that all heat preservation gear pump share over some other positive-displacement pump styles – vane pumps, for instance – is their inability to provide a variable flow rate at a given input speed. Where this is a requirement, a work-around is to use drives capable of speed control, though this is not always a practical solution.
Finally, while rotary, positive-displacement pumps are capable of pumping water, their primary application is in oils and viscous liquids because of the need to keep rubbing surfaces lubricated and the difficulty in sealing very thin fluids. For most applications where water is the media, the centrifugal, or dynamic-displacement pump, has been the clearer choice.