Insane Hydraulics
Bold and audacious blog about fluid power
Note: In this article I am referring to the orifice that's hidden inside of the swash-plate of this series of Rexroth pumps. If you haven't read the article about this recurring failure, I strongly suggest that you do so before reading this post.
When I got the pallet form Epiroc with these three little babies, I rubbed my hands together and laughed diabolically, because although I knew that I would spend the day opening pumps that I wouldn't be fixing, it would still be a great opportunity to check yet another batch of 5x series Rexroth A10VO pumps for "standard" failures - like the "hidden orifice" for one.
I knew I wouldn't be fixing them because although I'd already opened dozens of pumps for Atlas (I keep forgetting it's Epiroc now), I only serviced but a couple, and it was something like a new seal kit deals, nothing too serious. This is, of course, due to the fact that they buy pumps in large quantities directly from Rexroth and, therefore, have access to ridiculously low prices, making any serious overhaul a non viable option.
It must be nice to be able to treat a hydraulic pump like you would treat, say, a fuse - and be able to replace it "just because you can". I can imagine the in-house maintenance:
What's that, no system pressure? - Don't worry about it, replace the pump with a new one...
We have a warning message that says "Clogged air filter"! - Replace the pump with a new one...
The rig ran out of diesel! - Dude, haven't you learned this already? Go to the pump pile and fetch a new pump!..
Anyways, I am combining the three units in one post, because each one of them has something interesting to "tell".
So, numero uno here - a beautiful sample of sheer destruction. Marvelous, breath taking... You always know that a pump's condition is going to be fun to look at when you see metal chips pouring out of a drain line...
Of course, the failure is already fantastic as it is, but what I find the most interesting is the fact that even among this total obliteration you can trace a string of events. Have a good look at this debris pile - see anything interesting here? Take a good look at what's left from the rotary group and then look at the scrap pile again... Still nothing? Ok, I'll tell you.
This small round thing in the bottom is a piston head (or at least what's left of it). It's the only one that's separated, and you can see that it's actually grinded/mashed down to half of its size, and the respective "donor" piston is literally nailed into the cylinder block like a rivet of sorts. (How awesome are these axial cracks?)
This tells me that the pump bashed the single piston on its slipper-less head for some time before severing it and then turning to its brethren! Which means that most likely the machine was in operation even after the normal "z-z-z-z" transformed into "pa-pa-ta-ta".
This a good thing. If someone stopped the rig at first signs of the noise change, most likely the repair would boil down to the pump replacement. But since the machine was in operation till the pump exploded, the complete hydraulic system is now full of metallic particles, which is great for the hydraulic industry in general! Just imagine the amount of pumps and valves that will have to be serviced or replaced and the amount of man-hours this will require! If everybody estimated their hydraulic systems we'd be long gone out of business. Such operation practices keep pump factories running and create a permanent demand for hydraulic service professionals. My hat is off to you, my good sirs, keep up the good work!
I did remove the swash plate blind plug - and I found the orifice intact, but absolutely loose inside the threaded hole - this tells me that the way Rexroth fixes these orifices doesn't work too well, apparently. It was plugged with something malleable and black, almost rubber like, by the way.
Now let us move on to the pump number two. It has a lot of signs of being very new. Almost no wear to the barrel pins, very slight marking of the ball guide, no marks on the shaft spline, no marks on the shaft seal area - this pump is busted, but it looks like it's busted new! I called the mechanic, and he did confirm that it has only several weeks of operation! Now that's a shame.
In any case - lets investigate, shall we? Apart from heavy scratch marks on the valve plate and the barrel, the piston heads were absolutely stiff, I could hardly move them at all. This could mean only one thing - hard particle contamination, and an ugly one. I wonder if this pump served as the replacement to the first one after the catastrophic failure?
What about our orifice? Well - it was indeed in place, and fixed this time - but it was completely plugged with bronze and steel particles - from the pressure side. This tells a lot, actually. This tells me that small particles can easily find their way through the piston slipper and the swash plate lubrication orifice. In fact, some of the smaller particles did pass even such a tiny hole and caused typical particle wear to the cradle bearing surface.
Since the pump is going to scrap anyway, I may as well cut open a cylinder slipper just to "peek inside". Se here? Clear evidence of particle contamination - now we know why the slippers aren't moving.
Now to the pump three. The lucky three! I look inside the hole in the swash-plate balancing pocket on the suction side - and I see a classic case of the lost orifice syndrome! Here's the orifice after I managed to take it out - there's about a half of it left. Also - have a look at the respective cradle bearing. As you can see, this time the hard particle damage is notorious. I'm sure the particles that caused these groves were originated, at least in part, by the disintegrated orifice itself.
This type of particle damage is very interesting. Note that the grooves on the bronze surface aren't at all aligned with the movement of the swash plate. This is so cool! First of all it shows that the particles aren't dislocated by the mechanical movement of the swash plate, but rather by the pressure gradient in the oil film, secondly - the grooves serve as an almost perfect visualization of the pressure gradient vector - just look how nicely they all point to the "outside" from the balancing pocket. And, thirdly - they really look great! Almost like rivers feeding into an ocean in a satellite image of the Earth's surface! Also, if you zoom in the pictures, you will be able to see small steel particles still lodged in the bronze - apparently my camera can pick them really well.
I am sorry, Rexroth, but putting a tiny and badly fixed orifice in a hidden threaded gallery was not the best design choice, you could do better on this one.