When Is It OK For a Piston Pump NOT to Have a Case Drain?

IM000260A repairer rebuilt a hydraulic piston pump. A week later it came back like a boomerang, the subject of a warranty claim. Upon tear down the piston retaining plate was found to be broken–see inset pic. Fearing the aftermarket parts they used were not up to scratch, the repair shop quietly accepted warranty and rebuilt the pump for a second time-using genuine parts.

About a week later the boomerang came back (again!). Similar mode of failure: cracked/buckled piston retaining plate. Can’t blame the gray parts this time. Since this type of damage is consistent with high case pressure–see page 18 of Preventing Hydraulic Failures, the repairer now points the finger at the machine owner, suspecting a restricted case drain line. To which the machine owner replies, “There is no case drain line”. “What theā€¦? This explains why two pump rebuilds have disintegrated!” Trouble is, the machine owner is adamant he has other similar pumps, happily working away with no case drain line connected.

The repairer can’t believe his ears, so he drives down the machine’s location whereupon the penny drops: these pumps don’t have a case drain line, because they’re not intended to have a shaft seal! On piston pumps which are ganged or ‘piggy-backed’ on the back of a pump with a case drain line, it is common for the rear pump not to have a shaft-seal installed. And this makes a case drain line on the rear pump optional. This situation can also apply if the pump is driven from a gearbox or transfer case that is lubricated with hydraulic fluid.

With the riddle solved, you might be wondering: if the pump came to the repairer without a shaft seal installed, why ship it back to the customer with a shaft seal installed? Well, if the pump has failed catastrophically, it’s not unusual for the shaft seal to be blown, although for it to be missing completely is less common. The other reason, but not the issue here, is it is necessary to install a shaft seal (and a case drain line!) to bench-test the pump before it leaves the shop. After which, the shaft seal would need to be removed in this instance.

So if in response to the question: when is it OK for a piston pump NOT to have a case drain line? You answered: “when it has a flooded housing” (inlet port on the pump is common with the case) you are correct. And although it’s probably not the first answer that comes to mind: “when it’s not intended to have a shaft seal” is the other correct answer.

The other lesson here is: not clarifying the as-built configuration of a hydraulic installation before carrying out repairs or adjustments can 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.

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