Insane Hydraulics

Site theme image

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, and the endplate 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/endplate 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 on 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, which 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, the 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 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 its "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 rested 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 tests were done at the site, 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 to book an urgent flight to Bulgaria to "lay my hands" on the problem as soon as possible (production raise slots, remember?)

I wasn't upset at all, 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 wintertime, 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 a cylinder block lift, and 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-site.

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 knew 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 endplate against the valve-plate - and the surfaces seemed to match, I tested 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. Then we reassembled the motor and put it back in 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 how do you even say "lapping paste" in Bulgarian? Oh yes, it's that thing... Well, we're gonna have to go to the main shop at the surface and 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 out 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 endplate. 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 endplate 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 supplies container and brought down the "old" motor, and I created a "Frankenstein" unit with the "old" endplate 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 with 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 nonstop 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 in a row?

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 it hits me - 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 endplate from the "Bulgarian venture", and tested the motor once again restricting the return line, and, once again, the test registered a perfect 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 endplate!

My only regret was I didn't document that with pictures - that would have been one hell of a troubleshooting case. I did contact every supplier we knew for a replacement endplate, but since this was a discontinued series - I didn't manage to find a replacement, and the endplate 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: