Why Filters are Dangerous in Hydraulic Pump and Motor Drain-Lines

Cracked pump housing -- installer FORGOT to connect the case drain line!

My previous blog-post featured this picture of a cracked, piston-pump housing. One of my FaceBook Friends made the comment that for this to happen, the pump’s rotating group would have to have failed and come apart in a spectacular way. Not so. And based on my long experience in the hydraulics repair biz, this being the cause of a cracked housing is a rare occurrence indeed.

Further, a cracked pump or motor housing due to a restricted (or blocked) case-drain line is far more common than you might think. Of course, if you forget to connect the drain line in the first place, you effectively turn the pump or motor housing into a pressure vessel–something it’s not intended to be. Bang!

And if you connect a flowmeter into the pump or motor case drain line, to measure internal leakage, but forget to open the loading valve, this has the same effect: the housing fails spectacularly shortly after start-up. Don’t laugh, I know of instances where this HAS happened.

But there’s another all-to-common restriction to the drain line that can result in a cracked housing: filters. If there’s a surge of leakage into the pump or motor case, and it can’t escape due to the restriction of the filter, the resulting pressure spike CAN crack the housing. Or blow out its seal. Even if it doesn’t have this dramatic effect, the presence of a filter in a piston pump or motor case-drain line often results in excessive case pressure. And high case pressure causes seal failure and mechanical damage.

Seal failure

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. And as already mentioned, if case pressure exceeds the shaft seal’s design limits, complete failure can occur. The subsequent loss of oil from the case can result in damage through inadequate lubrication.

Mechanical damage

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.fig3

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.

For the reasons outlined above, depth filters are not recommended on case drain lines. While this does allow a small percentage of fluid 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.

If a filter must be fitted to a pump or motor case-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. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.

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