A recent arrival on ‘Plant Brendan’ who works for Bosch Rexoth’s ‘Large Hydraulic Drives’ division in Sweden (a.k.a. Hagglunds division) wrote me this after reading my “6 Hydraulic Mistakes….” report:
“I would have liked more information on why you shouldn’t have filters on the drain line on piston pumps. We actually use that solution in our drive units and we have had very few problems — even if in some applications we have problems with the pump shaft seal leaking. The pump manufacturer doesn’t want us to use this solution. The reason we have filters on the drain line is that we want to have a clean tank.”
Traditionally, a Hagglunds drive unit, if supplied complete from Hagglunds, featured a Denison Gold Cup (closed circuit) pump powering the Hagglunds motor. And on the units I encountered at least, case drain line filtration was a standard feature.
The concept of installing filters on case drain lines for contamination control is ideal. But the problem with depth filters installed on piston pump and motor drain lines, is the effect it has on (increasing) case pressure. A filter element is a variable resistance that is neither pressure nor temperature compensated, In other words, case pressure will fluctuate with pressure drop across the element, which changes in flowrate, oil viscosity and degree of clogging.
Chronically high case pressure can result in premature leaking of the shaft seal-often with ‘grooving’ of the shaft’s seal area. This is not desirable. But it’s not as bad as the potential for catastrophic failure if there is a surge of flow into the case, which when combined with the restriction of a filter, results in a damaging spike in case pressure. In a piston pump, such a flow surge can occur if a system pressure peak causes the cylinder barrel to momentarily separate from the valve plate.
Susceptibility to damage when a spike in case pressure occurs does depend on the construction of the particular component. A Hagglunds Viking motor has an allowable, maximum continuous case pressure of 3 bar (45 psi) and 8 bar (120 psi) intermittent-5 seconds duration. A Denison Gold Cup pump can tolerate a case pressure of 5 bar (75 psi) continuous and 8.6 bar (125 psi) intermittent. On paper at least, these are two components that should be able to withstand the potential negative effects caused by a depth filter installed on their case drain lines.
But when Bosch Rexroth bought Hagglunds several years ago, I’m sure the Hagglunds/Dension relationship ended abruptly. And the Dension Gold Cup pump was replaced with Rexroth’s A4VSO version. The maximum continuous case pressure for this pump is 3 bar (45 psi), and above 4 bar (60 psi), the integrity of the case is at risk. This explains our new member’s comment: The pump manufacturer doesn’t want us to use this solution (case drain filtration). Certainly, Bosch Rexroth’s position on case drain line and suction filters is pretty clear: don’t use them. Or more correctly, if you use them, don’t expect warranty.
A compromise solution I sometimes use in situations where control of contamination is a significant concern is to install a grossly oversized, coarse screen (125 microns). NOT a depth filter. However, there is always the risk that a well intentioned technician or maintenance guy will at some point substitute the 125 micron screen with a 10-micron absolute depth element. This can turn out to be a costly mistake. 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.