Today I want to clarifiy 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, filters are

NOT recommended on case drain lines.

For more on these and many other hydraulics tips,
point your browser to:

Yours for better hydraulics knowledge,

Brendan Casey
Author of 'Insider Secrets to Hydraulics'; and
'Preventing Hydraulic Failures'.