Insane Hydraulics

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An Impossible Shaft Seal Leak

Today's case is especially interesting because it is clearly impossible. Would you believe that a brand new BABSL shaft seal (classic medium pressure shaft seal with a spring-loaded sealing lip and a dust lip) properly installed in an axial piston hydraulic motor in good working condition with but minor wear of the shaft sealing surface and no play or damage in the shaft bearings, can leak liters of oil during an 8-hour shift without any signs of damage whatsoever?!!

When a mechanic hears about a hydraulic motor that is dumping large amounts of hydraulic oil into the respective gearbox, he expects to find something like this:

But this is not the case here. I am talking about a new seal that magically drains liters of oil without any damage at all! Who doesn't like magic?

So, in this industrial tale, I will relate the story of this failure, and I will tell you the solution. This case is very unique, but I still think that it's nothing short of amazing to know that you can create conditions under which a perfect shaft seal will let large quantities of oil pass through it "in the wrong direction" without breaking. I promise you one thing - if I personally didn't participate in this case, I would say that this is impossible if someone told me about it.

To the story now.

The "machine under investigation" was a medium-sized raise boring rig that employed two Danfoss 51V110 hydraulic motors, driven by a classic closed-loop HPU. Nothing new or special here. The hydraulic motors were mounted vertically on top of the gearbox, with shafts facing down. For a long time, this rig worked problem-free punching "normal" bores, but when it was "promoted" to drilling production slots (smaller bores) the leaking seal problem appeared.

The operators noticed that the oil level in the gearbox was rising. On some days they would have to drain 5-10 liter per shift. Since the hydraulic motors were the only point in the system where the hydraulic oil could enter the gearbox - and since both motors had already done significant hours - they were sent to our shop for an overhaul.

I knew the HPU pretty well, I knew the maintenance crew, and I knew how the client's company operated, so I wasn't surprised to find both of the motors with nothing but a normal minor wear one would expect from an 8000-hour old unit that ran in perfect conditions. The filtering system on these rigs is top-notch, the temperature is always in check - so the hydraulic components are always happy. Still - the motors got new seal kits and some polishing of the shafts. I also put a small spacer under one of the shaft seals to relocate the lip to an undamaged area, since the shaft did develop a small groove in it, but again - nothing too special. Normal stuff I see and deal with every day.

And yet, despite all that - the problem continued. Sometimes the rig would last a complete shift problem-free, and sometimes it would add a bucket of oil into the gearbox during a single shift!

So, after some time, the motors were removed from the machine, and thoroughly re-inspected. And once again I couldn't find any damage. No damage at all! The seals looked brand new from all angles. But the fact remained that they were leaking! The shaft seals were the only place where the hydraulic oil could physically enter the gearbox.

Moving further - the seals got replaced with new ones, again, the motors went back, the leak remained! One of those WTF moments, I guess. So some weeks later, the client decided to replace the motors altogether - better safe than sorry, right! Two brand new motors go in and... what do you know! They started leaking too! Brand new motors. Leaking liters of oil through the shaft seals into the gearbox! No visible damage to the seals! Insane hydraulics, right?

Now - for the tests and the simple thing that solved the problem.

Naturally, simple stuff was checked and re-checked by all. Things like excessive case pressure, size of drain hoses, a failure in the flushing system, a fissure in the motor flange, etc... All pressures were within specs, all the "mechanicals" were in check. The seals were intact, free of cuts or puncture holes, the motors would pass all the bench tests with flying colors, and then return to the rig to continue dumping shameless amounts of oil into the gearbox!

Then the rig mechanic had an idea - he tried it, and it worked! I will tell you what he did in a second, but let me tell you how he got to it first.

The company ran several other rigs powered by tandem Hagglunds CA motors with a closed-looped system that we helped to design, and so all of the motors had a low pressure (1.5 bar) bladder accumulator connected to the drain ports to reduce the load on the shaft seal when the free-wheeling was turning off - a standard and highly recommended practice for such systems.

Anyhow - the lad installed a digital pressure gauge in the drain line of the motors and monitored the readings for a while - and he saw that even though the highest pressure spikes never surpassed the maximum allowable rating for the shaft seal, the pressure shifts were very fast, violent even, and so he figured that installing an accumulator would help to smoothen the pressure curve out - just like they did with the Hagglunds motors, and since he already had the accumulators around, he went ahead and installed two accumulators - one for each motor, for good measure.

And you know what? It nailed it! The leak was gone for good!

It would be so great to study this case "scientifically". But there's no way I can do it, unfortunately. I can only come up with theories now.

In my head here's what was happening. When the rig switched to production slots, the drilling operation went to a higher speed and a more shock-prone load. Even though the HPU was equipped with a charge pressure accumulator near the pumps, the load spikes, both positive and negative would create a situation when the flushing spool would shift back and forth, cutting momentarily the flushing flow into the motor's case, but when this was happening - the large column of oil was still moving down the large and long drain line (and on these machines the motors can raise 4-5 meters above the HPU when the drilling head is up) which would create a moment when it would lower the pressure in the motor case enough for the seal lip to "open in" like a check valve of sorts and fill the opened gap with a certain amount of oil, which when it would spring back to its normal state - would then be dumped into the gearbox through the dust seal. Like a small vacuum/pressure cycles driven pump of sorts. Repeat this situation enough times - and there you have it - an undamaged yet leaking shaft seal! Adding the low-pressure accumulator helped to keep the case pressure in the "always positive" range and solved the problem. I could kill to test the system with a high-speed logger!

Maybe one day I will devise a rig to test this theory. I envision an oil-filled box with a shaft and a seal and a piston to create the vacuum cycles. If I ever do it - I'll be sure to report it here.

Anyways - there you have it, folks! If you try hard, you can make good shaft seals leak large amounts of oil without breaking!