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Hydraulic external gear motor pressure drop

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Old   December 20, 2018, 09:38
Default Hydraulic external gear motor pressure drop
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I have several experimental datasets for external gear motors which indicate a linear relationship between the pressure drop and the rotational speed of the machine in the unloaded state. Datasheet values from major gear motor manufacturers seem to further support this data.

I am struggling to understand the fluids dynamics behind this linear trend. As far as I can see the pressure across a gear motor in the unloaded state should only be a function of the machine's losses. I can only see two loss mechanisms in operation 1) Internal friction which I would expect to require an approximately constant torque to overcome regardless of speed (once you are above the critical speed for hydrodynamic lubrication) and 2) viscous losses which I would expect to be proportional to the square of the mean fluid velocity and hence the motor's rotational speed. Summed together this should give me a quadratic relationship between motor unloading speed and pressure drop but there is a clear linear trend.

Can anyone enlighten me?

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