Written by Camilo H Rueda
This type of hydraulic motor is one of the most used and applied in the whole world. Also known as internal gear motor,
it can be high speed-low torque when it does not contain commutator and is a direct drive, but the one that does is an indirect drive or orbital motor.
The rotating group is composed by the inner, outer rotor and rollers. The inner rotor has one less teeth than the outer and its center is not concentric, it has an excentricity. The outer rotor and rollers are stationary, the inner rotor while is forced to rotate by the flow it orbits around the center of the outer.
As you can see, the inner rotor in the graphic has 6 teeth and 7 rollers. The spaces among them are the pressure chambers where the oil is going to do its job of turning the inner rotor. The commutator receives the flow from the ports and send it to the manifold to feed the chambers.
Now, let's se how this motor works:
As shown in view 1, there is only one tooth that is making contact straight with a roller, above. At this point the tooth is under high pressure in the left side and low pressure or return in the right side creating an imbalance of forces that is needed to generate the rotation to the right (clockwise), noting that the commutator is shifted down (as opposed to the tooth touching the roll). When the shaft tuns (view 2), and the internal rotor spins 1 / 7 of a turn the next tooth on the left is now touching the center of the other roller. As the commutator has also turned 1 / 7 of a turn (counterclockwise) the distribution of the fluid also changed so that the tooth that is now facing the roller is under high pressure on the left and lower on the right.
If we continue with this process we will see that if the shaft is turning clockwise, the pressure switching process is turning counterclockwise and for each complete turn of the shaft there will be 6 made by the commutator.
In short, each chamber receives flow at high pressure 6 times per shaft revolution generating 6 times more torque capacity but requiring 6 times more flow. It is for this reason that this motor is capable of withstanding high torques for the comparatively small size.
For a better understanding, let's take a look at the following continued animation:
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