External gear pumps/motors

In many applications, for operation at pressure up to 250 bar, external gear pumps/motors are used extensively because of their simplicity, low cost, good suction performance, low contamination sensitivity and relatively low weight. In applications 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 Figure 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 the trapped volume is discharged by the action of the gears 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 acts 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:

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