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Pressure

Fig. 2

External Gear Pumps

External gear pumps are not pumps with their gears on the outside of the pump body. They have gear shafts with the teeth on the outside (external) part of the shaft, but inside the pump housing as in figure 1. Gear pumps are used to generate a flow of fluid and to impart the necessary forces on this fluid flow to create pressure against a resistance such as a cylinder piston or hydraulic motor gears. Figure 2 shows the functional operation of an external gear pump. As the gears rotate and the teeth come clear of the meshing point, a vacuum is created on the left hand side. This vacuum allows atmospheric pressure acting on the oil supply in the hydraulic tank to push the fluid into the spaces between the teeth. The gears then carry the fluid, from the suction (left hand) side, in the spaces between the teeth and the pump housings, in the direction of the arrows to the pressure (right hand) side. As the gears mesh together on the pressure (right hand) side, the fluid is forced out of the spaces between the teeth into the output port of the pump. The meshing of the gears and the small clearance between the teeth and the pump housing, prevent the oil from returning to the suction (left hand) side. The displacement is equal to the size of each space between teeth multiplied by the number of spaces which pass in a single input shaft revolution. The number of spaces is equal to the number of teeth on each gear multiplied by two, since there are two gears. The flow output of the pump is equal to the rotational speed (rpm) times the displacement. Remember, at Central Qld Hydraulics, we take any old pumps and motors that are preserved well, replace their damaged parts to make them go on doing their outstanding work as new pumps and motors. Figure 3 shows some examples of external gear pumps. Go here for internal gear pumps...

Fig. 1

Fig. 3

A description of how a gear pump works

CENTRAL QUEENSLAND HYDRAULICS

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