A basic gar pump features a rotary housing containing 2 or more gears; that is toothed wheels, helical gears or lobed cams. Tight tolerances are required between the casing and gears, bore and gears and also between the gears. Typical housing will have an inlet and outlet, for suction and discharge respectively. The 2 main types are; external gear pumps, which use two external gears and internal gear pumps which use internal and external gears. The term positive displacement for gear pumps describes the fixed amount of fluid they move for each revolution.
The rotation of the gears causes suction at the inlet and a subsequent discharge at the outlet. The liquid is carried around the casing to the outlet by the teeth where they eventually mesh, causing the fluid to discharge via the outlet.
External gear pump
This gear pump uses 2 external gears that displace non-lubricating fluids (gears are oil lubricated). The mechanism is usually driven by one of the toothed gears, which in turn drives the other. 3 factors are involved in the regulation of flow: volume of cavity between the teeth, speed of gears, and the amount of fluid that slips back to the inlet via the mechanism. There are 3 main types of gears; spur, helical and herringbone. Helical and herringbone deliver more flow at higher pressure while also being quieter, but may require a greater inlet pressure than spur.
Internal gear pump
An internal gear pump utilizes internal and external gears. The gears themselves are lubricated by the fluid, which is of a lubricating nature. The internal design is seen as being reliable, easy to operate and maintain; due to only two moving parts being present. Only one drive gear is required for the mechanism to function but it is possible to use two. The pump will usually contain at least one bushing. The design can also be modified to include a crescent shaped portion that enhances performance when pumping high viscosity fluids. Internal gear pumps have relatively low speed and inlet pressure requirements.
Gear pumps are capable of moving small suspended solids but due to the meshing of gears they can be damaged by pumping large solids. When solids are present in the fluid they may act as abrasives, causing damage to the gears and increasing downtime. In terms of construction materials, gear pumps can be made from a wide variety of materials, ranging from bronze, iron and stainless steel to cast iron, depending on the application and fluid properties.