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How a hydraulic gear motor works.

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The hydraulic gear motor is probably one of the most inexpensive if you compare it against vane or piston motors. Because of its internal construction it is a high speed motor (from 600 rpm to 6.000 rpm).

Its internal components are basically, 2 gears engaged externally, one of them is the output shaft. Both gears are located inside the gear housing fitting with a very close tolerance, 2 thrust plates and covers with the ports.

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As can be seen in the following picture, when the oil flows into the inlet port (red colored) pressure generates forces over the gear teeth that are closer to the inlet, forcing them to rotate in opposite directions (white arrows). The oil is transported from the inlet to the outlet in the space formed between 2 consecutive teeth, the housing and the thrust plates until they are expelled in the outlet (blue colored).

The shaft speed (rpm) is going to depend on the size (volume) of these sealed chambers (internal displacement) and the flow coming into the motor.

The pressure that is needed at the inlet depends on the torque developed (load) and the internal displacement.