In a previous article on hydraulic filters, I pointed out that all fluid returning to the reservoir should be filtered.
The one exception to this rule is 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 the outlet cycle. When this happens the pistons are hammered back onto the cam during inlet, destroying the motor.
What does this mean?
The case drain line of piston pumps and motors should be returned to the reservoir through a dedicated penetration below minimum fluid level. For the reasons described above, hydraulic filters are not recommended on case drain lines. However, if a filter is fitted it should be generously oversized to minimize back pressure in the pump or motor case. If in doubt, consult the hydraulic pump or motor manufacturer.