Insane Hydraulics

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Danfoss H1B160 Hydraulic Motor Gets Locked But Not Shattered

I just assisted in an unbelievable breakdown of a Danfoss H1B160 hydraulic motor from a drilling head of a core drilling rig, and I feel I must share what I saw!

If you don't know core drilling rigs - their hydraulic motors are bound to suffer, not only due to the high speeds and pressures, but also to the oil quality that often "leaves much to be desired", to say the least, mostly because of the heat exchanges, that occasionally blow out, and contaminate the hydraulic oil with water. In most of such mishaps, nobody has time to take care of the contamination in a proper manner, because these rigs can't stop for long due to production deadlines, and so the poor pumps and motors have to, pretty much, grind through (both metaphorically and physically). That's general mining industry maintenance in a nutshell for you, I guess...

Traditionally, all of our core-drilling clients used to employ Rexroth A6VMs, but we converted several rigs to Danfoss series 51 over the years, and these motors proved to be more than OK, and about five years ago or so, we began to slowly introduce the Danfoss H1 motors as well. With similar results. For a rig that runs with occasionally crappy oil - if the head motor manages to survive 10000-ish hours - it's a good motor in my book. Judging from this, perhaps a slightly red-neck-ish approach, Danfoss does make good bent-axis motors...

Anyhow, I was asked to have a look at this very strange malfunction this week. It was an H1B160 motor that we sold some five years ago - and the info I got was this it was completely locked in both directions. Dead in the water, so to speak, with but a tiny play.

The mechanic went to the rig, disengaged the motor from the drilling head, and even then the shaft would not turn in any direction. He could see the A and B lines pressurize alternatively as he was switching the forward/reverse rotation, but the motor shaft would not turn for more than a tiny bit. He even tried to "boost" it with a pretty big pipe wrench - to no avail. Now, if you have any experience with variable displacement hydraulic motors, in your honest opinion, what do you think could be causing this?

I know your answer - and it's the same that I gave to the client when he related the "symptoms" - the motor is, obviously, shot. Most likely beyond any viable repair. How many hours does it have? 8000? Not too bad, given the water contamination history of the rig. Let us thank it for its service, give it a rest-in-peace salute, and get a replacement.

However... when the motor was opened - the mechanics saw no apparent damage. And when they tried - the shaft could be rotated by hand. Say what? Yes - no damage, free rotating shaft! Obviously, the parts were worn out, quite significantly actually - which is normal for an 8000-hour-old moor running on a water-in-oil emulsion, but there was nothing apparent that would suggest a locked shaft scenario. Luckily this is our next-door client (quite literally), so I got to their shop a couple of minutes after I received the call asking me to take a look at something "bizarre and possibly cool".

So - I get to their shop and behold the H1B160 motor already disassembled over the bench:

Danfoss H1B160 hydraulic motor disassembled
Danfoss H1B160 hydraulic motor - the pistons are worn, but definitely not bent or broken
Danfoss H1B160 hydraulic motor - the valve plate has marks but is reusable
Danfoss H1B160 hydraulic motor - the barrel valve plate is also pretty worn out, but this would not cause the locking of the shaft

As promised - obviously worn-out parts, and as promised nothing catastrophically broken. No bent or shattered pistons, no broken bearings, no debris of any kind, and definitely no hydraulic problems on the rig. Everybody was puzzled. Myself included, and I already told them that we should probably put the motor back together and take it for a spin on our test bench - when one small detail caught my eye - and then it immediately became apparent to me what was going on! If you haven't caught it - check out the pictures again - you can actually see it there!

Did you see it? The locking was caused by the synchronizing shaft! If you look at it - you'll see that it is twisted pretty badly! And if you look at the pistons carefully, you'll see that they are marked because they were working at such an angle that the rods were rubbing on the barrels. They even left small marks!

Danfoss H1B160 hydraulic motor - the synchronizing shaft is twisted!
Danfoss H1B160 hydraulic motor - the synchronizing shaft is twisted!
Danfoss H1B160 hydraulic motor - marks on the piston rods
Danfoss H1B160 hydraulic motor - marks where the piston rods were rubbing on the barrels

What I find unbelievable is that each and every time I say a synchronizing shaft of a similarly built motor fail - it would always snap and "cause havoc" inside the motor. But not this one! Look - it's twisted pretty bad and there's even a hairline crack in it - which means that it is about to snap in half, but it did not!

Check ou the hairline crack in the synchronizing shaft!
Check ou the hairline crack in the synchronizing shaft!

Incredible! Especially given the amounts of torque that were applied to the shaft both hydraulically and mechanically to try and make it turn. The poor shaft stayed in one piece and prevented the rest of the motor from exploding. I guess, we should be saluting the poor synchronizing shaft then!

I've already ordered a new sync shaft, and I'm sure that even with the wear the motor should run fine for another thousand hours, mybe even two, which should give enough time for his younger brother to arrive and "take over".

Sill - this is the first time I am seeing something like this. Coming so freaking close to a catastrophic failure and yet managing to somehow avoid it!

So - now you know, a twisted synchronizing shaft of a bent-axis hydraulic motor can lock it without breaking anything else. If someone ever tells you this is impossible - please give them a link to this page!