One of our readers wrote to me recently with the following question:
"We have recently been involved in designing and building a hydraulic machine. The system has three, separate circuits each with an axial-piston pump and a common hydraulic reservoir. Case drain filtration was included to reduce the possibility of cross contamination if a failure occurs. After contacting the pump manufacturer I was led to believe that this is not the norm, but if the pressure drop across the filter is kept to less than 30 PSI it will be OK. This just forces filter maintenance. What filter beta and micron rating should be used?"
Fact is, installing filters on hydraulic piston pump and motor case drain lines can result in excessive case pressure, which causes seal failure and mechanical damage.
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 leakage past the seal. If case pressure exceeds the shaft seal's design limits, instantaneous failure can occur. The subsequent loss of oil from the case can result in damage through inadequate lubrication.
The effect of high case pressure on axial 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 inlet (see below). This can cause buckling of the piston retaining plate and/or separation of the slipper from the piston, resulting in catastrophic failure.
High case pressure can cause the pistons of radial piston motors to be lifted off the cam. This can occur in operation during the outlet cycle. The pistons are then hammered back onto the cam during inlet, destroying the motor. If residual case pressure remains high when the motor is stopped, loss of contact between the pistons and cam can allow the motor to freewheel, resulting in uncontrolled machine movement.
To avoid these problems, pump and motor case drain lines should be returned to the hydraulic reservoir through dedicated penetrations. These penetrations must be higher than the unit's case port and be connected to a drop-pipe inside the reservoir that extends below minimum oil level. For the reasons outlined above, filters are not recommended on case drain lines. While this does allow a small percentage of oil to return to the reservoir unfiltered, in most applications the contamination risk is low and can be effectively managed using oil analysis and other condition-based maintenance practices.
Filter with caution
If a filter is fitted to a hydraulic pump or motor drain line, I recommend a 125-micron screen, grossly oversized for the maximum expected flow rate. The filter housing must incorporate a bypass valve with an opening pressure lower than the maximum, allowable case pressure for the particular component (typically 5-15 PSIG). Installing a gauge or transducer upstream of the filter for monitoring case pressure is also advisable. To learn more about the failure modes of hydraulic components and how to avoid them, read Preventing Hydraulic Failures.
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