In many applications, for operating pressure up to 250 bar, external gear pump/motor are used extensively because of their simplicity, low cost, good suction performance, low contamination sensitivity and relatively low weight. In application requiring low noise, vane or internal gear pumps are often used.

Essentially the unit consists of two meshing gear pinions, mounted in bearings and contained in a housing or body as shown in Figures 1. As the pinions are rotated, oil is trapped in the spaces between the gear teeth and the housing and carried round from the pump inlet to its outlet port when trapped volume is discharge by the action of gear meshing together.

Torque is required at the input shaft at a level that is dependent on the outlet pressure acting on the gear teeth. When supplied with high-pressure flow the unit act as a motor by providing torque to drive the load on the output shaft.

Some of the outlet fluid is transferred back to the low pressure side by way of small leakage flows through the:

• Clearance space between the teeth and the housing.
• Shaft bearing clearances.
• Clearance between the gear faces and the side plates in the housing. Most gear units have pressure loaded side plates to minimise this leakage.

The design of the unit is such as to minimise flow losses as they reduce its efficiency, particularly when using fluids of low viscosity such as with some wa­ter based fluids. The geometric capacity, or displacement, cannot be varied so their displacement is fixed. For a given gear form the manufacturer can produce pumps of different displacements by using different gear widths.

Standard types operate at speeds of 1000 to 3000 RPM and at pressures up to 250 bar but higher speeds and pressures are available. Powers range from 1 to over 100 kW. The efficiency of gear units has been raised during recent years, with peak overall efficiencies of 90% or above.