Insane Hydraulics

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Hydraulic Motor Case Pressure

This myth is related to axial piston motors.

Some mechanics believe that hydraulic motors should have the lowest case pressure possible, and therefore the widest and shortest drain line that runs directly to tank. The logic is - the lower the pressure - the longer the shaft seal (and the shaft) will last, which is a good thing.

This may have been true in the old days - when hydraulic motors ran much slower and shaft seals weren't that advanced, and may still be true for some units that run at low speeds (1500-1600 rpm or lower), but with modern axial piston hydraulic motors that often operate at high speeds (above 4000 rpm) - having a small positive pressure (0,3 - 2 bar) inside the casing is necessary to ensure proper lubrication of passively lubricated parts.

Modern shaft seals can easily take such case pressures. Of course, maximum allowable pressure level should be respected as well, to make sure the seal gets its nominal seal life or doesn't "cut the shaft in half".

In fact, sometimes it may even be necessary to install a check valve in the drain line to guarantee minimum case pressure.

So, if you panic when you read 2 bar of case pressure in a high speed axial piston motor - don't - it's all good! Identify the motor model, working rpm - and check out the data sheet for recommended minimum and maximum case pressure levels.

Just an example - Parker recommends a minimum case pressure of 1 bar for the V14-110 motor at 3000 rpm and 2 bar at 5000 rpm.