Insane Hydraulics

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Rexroth A6VM355 - Bulgarian Ventures and Hydraulic Motor Tests

If you're serious about your repairs, every repair should be backed by a warranty. I want to tell you a story about a warranty episode that I participated in some years ago. You know how it is with assistance calls - some you forget immediately after you've cleaned your tools, and some... well, let me just say that I won't be forgetting this one anytime soon..

The adventure began when I received an old A6VM355 to overhaul. It was an early series 6.0 motor, which, I guess, could be called an industrial relic even then, and it came from an Idau raise-boring rig. As usual, I got a green light to replace whatever I deemed necessary.

Raise-boring rigs are supposed to run 24/7, and you don't want to see one stop over a broken hydraulic motor for more time than it takes to replace one, therefore having spare components is a must, and that particular motor was destined to serve as exactly that - an "emergency" replacement in case the currently operating motor broke down.

I inspected the cylinder block, the valve plate, the end plate and saw that the cylinder block was more or less OK, or in other words "lap-able", however the valve plate had a couple of deeper scratches, and in the end I thought I didn't care much for lapping these grooves out, and replaced the valve plate with a new one.

Everything fit well, and the new valve plate seemed to be a perfect copy of the "old" one. Naturally I lapped and mated the valve plate to the cylinder block , did the normal "marker test" on the barrel/valve plate/end plate to confirm the surfaces - and the tests came out OK. (The "marker test" is when you paint one of the mating surfaces with a felt-tip marker and then rub one against the other to identify high/low spots.)

Then I put the motor back together, and tested it in our test stand. We do have a "proper" motor test stand in our Lisbon shop, but the "braking" hydraulic motor used there is all too small for a 355cc beast, so the only viable way for me to test it was the good old "return line restriction" test.

For the very few who might not know it - "return line restriction" is a "cheat" way to test a hydraulic motor, and basically boils down to running it with a restricted outlet to raise the pressure inside the rotary group while monitoring the case drain. This test allows you to detect internal leakage issues and, obviously, check for stuff like displacement control operation, presence of strange noises and external oil leaks. Not ideal, but better than no test at all.

That motor passed the test with flying colors! I raised the pressure to about 200 bar and saw but a tiny trickle of oil coming out of the drain line - which is supposed to be the sure sign of an exceptional volumetric efficiency, especially for a hydraulic motor that big. So, I billed the repair, packed the motor and sent it out to Bulgaria, where the "mother ship" was punching production slots in the Chelopech mine.

A raise boring machine that does slot raises can't stop! It simply can't. Because no slots means no blasts, and no blasts means no production, so having that spare motor around was very important for our client.

The motor laid quietly in the storage container for about a year before something broke down in the running one, and the fun began!.. The drillers (a very professional crew, by the way) quickly swapped the motors, started the rig - and immediately saw the lack of torque. Say wha-a-a-t?

Some test were done at sight, and all pointed to the hydraulic motor, which meant that the over-hauler (myself) was to be contacted immediately to see if anything could be done about it. We tried a couple of "tricks" over the phone, but very quickly it became clear that I had book an urgent flight to Bulgaria in order to "lay my hands" on the problem as soon as possible (production raise slots, remember?)

I wasn't upset at all, actually, I love Bulgarian food! And I was positive I would find and fix the problem because, well, I didn't have much choice.

It was winter time, by the way, and it felt refreshing getting out of Portuguese +10 C to Bulgarian -15. Especially taking into account the fact that it was -15 on the surface and above 30 down in the mine.

A good thing about failures that put mine production at risk is the fact that paperwork and safety inductions get done much faster than usual. So, a couple of hours after I got there I was already "hugging" the motor.

I ran the machine, heard the hissing noise inside the motor and the typical boom-bang sound - and clearly saw all telltale signs of the cylinder block lift, as I also confirmed that indeed almost all of the flow went down the drain line.

OK - so it was the motor, it was the motor that I repaired and tested! Screw me! There was nothing else I could do but remove it from the rig, crack it open and see what happened and hope that I would be able to repair it on sight.

As you can imagine, removing a 355cc hydraulic motor inside a mine without the luxury of your familiar tools and with the help of a very precariously positioned telehandler is definitely not an easy task - but hey, I already know it was going to be fun, and when you're in a mine you get done what needs to get done with whatever you have at hand.

So, we removed the motor, took it to the underground shop, where I opened it on top of a wooden pallet applying the usual "sledgehammer plus an old Allen key" technique, and... I saw nothing strange or alarming inside. At first sight - nothing! So - I tested the end-plate against the valve plate - and the surfaces seemed to match, I tested the the valve plate against the cylinder block - and again - it all seemed to be just fine...

All right, maybe it's 90% fine but not 100% fine, so I got some lapping paste and re-lapped (by hand) all the parts the best I could - still in the mine. The we reassembled the motor and put it back in the the rig.

This is very easy to write about - but when you're working in a foreign mine, or in any mine for that matter, logistics becomes a serious problem. Moving from a drilling site to a workshop can take an hour, transportation and loading is hard, getting a lapping paste is tricky, and in any case - how do yo even say "lapping paste" in Bulgarian ? Oh yes, it's that thing... Well, we're gonna have to go the the main shop at the surface an see if we can get it. That's another hour for you.

Never mind that. Two shifts later the motor's back in the rig. So, I fire it up, turn the motor, everything's fine so far, try building some torque and - PSSSSS - all the flow goes down the drain hose, AGAIN! Screw me double time!

All right, lads, time to pull the motor back our again. We're getting pretty good at that, aren't we? That's another shift for you and three in a row so far. How energizing!

So, I crack the motor open, look at it and say:

"Guys, clearly there's something wrong with this motor that I can't seem to figure it out, but pretty sure that it's either the cylinder block lifting from the valve plate or the valve plate lifting from the end-plate. If it's the latter - I won't be able to lap the sense back into it, not with this lapping compound and not in the time frame that we've got - unless, of course, you want to watch me lap it till Summer."

The lads around me didn't respond. Clearly their silence meant they weren't interested at all.

"I tell you what, let us bring over the old motor and see if I can reuse the end-plate and the valve plate from it."

The lads around me didn't respond. Clearly their silence meant they were extremely interested.

So, we went back up to the container and brought down the "old" motor, and I created a "Frankenstein" unit with the "old" end plate and valve plate. I did have to re-lap the cylinder block once again to match it to yet another valve plate.

Since people in the mine shop were seeing me struggle with the same components for a day and a half now non stop, some of them began offering help. And by help I mean all sorts of advice, which, coupled to the language barrier, was all but productive. Try explaining with gestures "I believe we're witnessing a block lift here..." to a person who is not familiar with hydraulic motors.

Never mind that. Two shifts later the motor's back in one piece... that's four in a row now, isn't it? The drillers that helped me were doing shifts, but myself and their foreman (Nuno, I salute your bravery, my friend!) had to work non stop all this time.

Anyhow, we're back in the mine, we put the freaking motor back into the rig - try running it, loading it - and YES SIR!! This time it worked!!! What a relief... By the way - what's that now, five shifts without stopping?

So, the rig seemed to run just fine, and we left the drillers to drill, and went back up to the office. Picture the following: we're sitting down, I look at Nuno across the desk and he's looking back at me, and I tell him - "Now's that an all-nighter one can be proud of! - and I hear back - "Chr-chr-chr-chr..." - Say, what? "Chr-chr-chr-chr..."

And then I realize - the poor guy is sleeping and snoring with his eyes wide open! That was the first time I saw someone do that. Then his gaze gains focus, the snoring stops and he goes: "I'm sorry, man, did you say something?.."

In the end I got the parts to our shop and put the motor back into one piece using the valve plate and the end plate from the "Bulgarian venture", and tested the motor once again restricting the return line, and, once again, the test registered a prefect pass. But I knew better now, so, with the help of the client's welder, we rigged up a contraption that allowed us to block the shaft from rotation - and this time I did manage to observe the lift.

I got lucky - this motor had huge drain ports, and I managed to visually confirm that when the lift was happening it was not between the barrel and the valve plate but rather between the valve plate and the end plate!

My only regret was I didn't document that with pictures - that would have been one hell of a troubleshooting case. I did contact each and every supplier we knew for a replacement end cap, but since this was a discontinued series - I didn't manage to find a replacement, and the end plate for the newer series couldn't be used on that body. I did try to rectify it, but then another similar motor broke down and we had to urgently re-use the main shaft from this one, and in the end the client opted for buying a new motor to replace this one, so, eventually, the parts ended up in scrap.

Three lessons that I learned the hardest way:

1) Testing a hydraulic motor by restricting return line flow should be used responsibly. The fact that both kidney ports of the valve plate are subjected to high pressure during such a test means that the compressing force from the hydraulically balanced cylinder block is twice as high in comparison to normal operating conditions, and also applied more symmetrically, which can be just enough to impede the valve plate from lifting and mask a malfunction.

Of course, if you have to test a large displacement motor and don't have a way to induce shaft torque, especially dynamically, performing a restricted return line test is better that no test at all, but it's important to know that this test has its limitations and puts the rotary group in a "more advantageous" conditions (at least for axial piston units).

2) When you overhaul a bent axis type variable displacement hydraulic motor, it is very important to verify if a new valve plate matches the old end plate. Very important!

3) Five consecutive shifts in a mine can make a human being sleep and snore while sitting up at a desk with the eyes wide open. Now that you know it's possible you don't need to test this for yourself!