Gear pumps come in several forms. They are positive displacement pumps, which are usually selected when applications call for constant flow, high pressure, high viscosities, or specialist requirements for certain fluids.
The four common types of gear pump are external gear, rotary lobe, vane pumps and internal gear pumps. Depending on the application, one type of gear pump may offer superior features over the others. All are self priming pumps and deliver smooth pulse-less flows.
External Gear Pumps
External gear pumps have two identical gears. A driven gear, “driving” another gear. The action of two rotating gears coming out of mesh at the suction side of the pump develops the vacuum that pulls liquid into the casing. Fluid on the inlet side flows into the chamber and becomes “trapped” between the rotating gear teeth and the chamber housing. The spaces between the gear teeth transport the fluid along the outer perimeter of the housing to the discharge side. Here the gears “re-mesh”, forcing fluid out the discharge.
The gears are supported on both sides, making them ideal for high pressure applications like those found in the fluid power industry.
They are however not suitable for abrasive applications [like paint and ink] because there is no end clearance adjustment, meaning a worn pump needs to be re-built rather than re-adjusted. They are also not as good on highly viscous applications [as speed must be slowed considerably].
A vane pump has a slotted rotor with vanes or blades fitting within these slots. The rotor is off-set inside its housing to form a crescent-shaped cavity. As the rotor rotates and fluid enters the pump, centrifugal force, hydraulic pressure, and/or pushrods push the vanes to the walls of the housing. The tight seal between the vanes and the pump housing is the key to the good suction characteristics common to the vane pumping principle. This characteristic makes the vane pump excellent at priming and handling low viscosity liquids.
Vane pumps are also not suited to abrasive applications because [as with external gear pumps] they have fixed end clearances, and once wear occurs, cannot be adjusted. They are also not as well suited to pumping viscous fluids, as speeds need to be slowed dramatically.
Vanes fit into a slotted rotor Rotor in housing – 90o ports A typical vane pump
Lobe pumps usually consist of two, two, three or four lobe rotors that rotate inside a casing. External timing gears found within a casing external to the pump chamber prevent the lobes coming in contact with each other. Pump shaft support bearings are located in the timing gear case.
The motion developed by the rotating lobes, expands the volume on the suction inlet side of the chamber by trapping the liquid in the pockets between the lobes and the casing of the chamber. The re-meshing of the rotors on the discharge side, forces liquid out of the pump.
Lobe pumps are frequently used in the food industry because of their capacity to handle large solid particles and because some can be supplied as “sanitary” pumps.
If not being used to transport solid particles, a lobe pump can be an expensive option, as the requirement for timing gears, two shafts and two mechanical seals make them an expensive option. They also have pressure limitations because of the over-hung load created by the bearings not being external to the pump chamber.
Flow path through an internal gear pump. Internal gear pump with 180o ports A typical heavy duty rotary [internal] gear pump
Internal [Rotary] Gear Pumps
Rotary gear pumps use the “gear-within-a-gear” principle. A rotor gear drives an inner or idler gear which rotates on a stationary pin and operates inside the larger rotor. A stationary crescent is fitted between the outer edge of the idler gear and inner edge of the rotor.
The gears create voids as they come out of mesh and liquid flows into the gear pump. The liquid travels between the rotor teeth “below” the crescent and between the idler gear teeth “above” the crescent. When the gears “re-mesh” the fluid is forced out of the discharge of the pump.
Internal gear pumps are probably the most versatile of the gear pumps. They are self priming pumps, can handle thin liquids such as ammonia, solvents, alcohol and fuel through to very viscous products such as molasses, food syrups and even grease. They can also cope with abrasive and high temperature liquids.
Although able to handle relatively high pressures [to 300 psi or 20.7 BAR], which gives them an advantage over lobe pumps and vane pumps, they can’t reach the pressures of an external gear pump. They also cannot handle the solids of a lobe pump. They do however have end clearance adjustment, which allows clearances to be adjusted as the gear pump wears.
Flow path through an internal gear pump Internal gear pump with 180o port A typical heavy duty rotary [internal] gear pump