In your last hydraulics bulletin, I wrote about the importance of following proper procedures when re-starting a hydraulic system after carrying out maintenance work – to avoid premature failures.

Today I want to clarify another myth, that causes a lot of hydraulic problems – and even failures.

Myth #5. All oil returning to the hydraulic reservoir should be filtered.

True. With one VERY important exception: the case drains of hydraulic piston pumps and motors. Connecting case drain lines to return filters can cause excessive case pressure, which has a number of damaging effects.

High case pressure results in excessive load on the lip of the shaft seal. This causes the seal lip to wear a groove in the shaft, which eventually results in a leaking shaft seal.

The effect of high case pressure on in-line piston pumps is the same as excessive vacuum at the pump inlet. Both conditions put the piston ball and slipper-pad socket in tension during intake.

In severe cases this can result in buckling of the piston retaining plate and/or separation of the bronze slipper from the piston, causing major failure.

Under certain conditions, high case pressure can cause the pistons of radial piston motors to be lifted off the cam during outlet. When this happens, the pistons are hammered back onto the cam during inlet, destroying the motor.

For the reasons described above, conventional depth filters are generally NOT recommended on case drain lines.

Yours for better hydraulics knowledge,

Brendan Casey
Author of Insider Secrets to Hydraulics