Internal gear pumps use internal and external gears. The gear itself is lubricated by a fluid with lubricating properties. The internal design is considered reliable and easy to operate and maintain as there are only two moving parts. The mechanism requires only one drive gear to work, but two can also be used. The pump will usually contain at least one bushing (EPW, 2012). The design can also be modified to include a crescent-shaped cross section to improve performance when pumping highly viscous fluids. Internal gear pumps require lower speeds and inlet pressures.

The working principle of the internal gear pumps is divided into three stages of filling, transmission and conveying. As the gears come out of the inlet side of the pump, they increase their volume. As the gear teeth continue to rotate against the pump diaphragm and pump casing, liquid flows into the cavity and is captured by the gear. The captured fluid moves around the casing from the inlet to the outlet. Because the teeth of the gears are interlocked on the discharge side of the pump, the volume is reduced and the fluid is forced to drain. The tight tolerances between the housing and the gear cause the pump to create suction at the inlet and prevent fluid from leaking back from the discharge side.

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