Almost any catastrophic failure is a good example of a bad example. This is why this blog hosts the Kaboom section - a collection of graphic reminders that learning from others' mistakes is still a thing and is much cheaper than making similar mistakes on your own.
But in this article, I would like to switch gears and document a good example of a good example, with the help of a Sandvik LH621 mining loader, that suffered an unfortunate accident and "donated" both of its hydraulic hearts for our appreciation.
The accident had nothing to do with hydraulics. The loader was working via remote control in one of the production tunnels when the ceiling collapsed, and one of the heavier boulders crushed the cabin and hit the engine so hard that it sheared the mounts and broke the hydraulic pumps' flanges off as the gearbox fell onto the chassis.
Spectacular yes, but not very interesting. What is interesting, however - is the condition of the pumps, because the machine had clocked exactly 7500 hours of intense operation in mining conditions when the accident happened, and 7500 hours is a nice round number that for most medium-duty open-loop pumps means "retirement approaching" - i.e. the period when you're still kicking, but notice that more and more people address you as "Mister" or "Sir" rather than "Dude", and when the aches and pains start making you wonder if you should schedule a check-up.
This case is an "ideal" example and I am glad I caught it. Let me explain why.
First the pump model - which is the Rexroth A10VO series 31 open-loop unit in its "beefed up" 110 cm3 version. This may be a protected model, but internally it is still the ubiquitous A10VO***/31 that everyone knows and loves. So, it's nice to document its wear at "productive age". Normally these pumps arrive at our shop in the "it's already too late" condition...
Second - the oil. This machine used a standard mineral HLP hydraulic oil, grade 68 - nothing special about it, which, again, makes the wear a "textbook" case. The system ran at about 65 degrees Cº if you must now.
Third - operating conditions, which are the best (or worst) you can find to battle-test a hydraulic system. Relatively small oil tank, lots of dust and moisture, 2000 plus rpm and constant shock loads, with pressures reaching 300 bar. Make a design mistake - and you will be lucky if your A10VO lasts for 5000 hours. So - did the Sandvik engineers do a good job designing the hydraulic system?
Let me tell you right away - they did! A darn good job! This can be clearly seen by the wear (or better - the absence of wear) that both of the pumps had when I opened them. Since their condition is so equal - I am only putting the pics of the unit tagged as R902488271 (in case you wonder what the part number is).
Let us start with the shaft and the bearings. As you can see, both the front and the tail tapered bearings have signs of normal wear, but nothing too serious. This tells me that the oil was right and clean. If I were in a tight spot - I would reuse them, but since I am not and the bearings are cheap - I will replace them for good measure.
The shaft wear in the shaft seal area is minor, which is surprising because this system does use a pressurized oil tank. Abrasive contamination (on both sides of the shaft seal) contributes a lot to shaft wear, so I am guessing the oil was very clean.
Moving on - the swash-plate mechanism. The swash-plate looks almost new. I can tell you from experience that these scratch marks on the sliding surface are nothing - I wouldn't bother lapping them out. (Although I know that the perfectionist in me will most likely lap them out. I don't care - it's his problem!) The cradle bearing surfaces and even the bronze liners are only slightly marked - which is impressive for 7500 hours. The large servo piston rod has some minor scratches, and the small one is in perfect condition. Absolutely re-usable.
And now the moment you've all been waiting for - the rotary group. First - the valve plate. Yep - the Teflon coating wears out pretty quickly on these plates, so I am not surprised, but the overall wear is very acceptable for a unit that has 7500 hours. You'll get these small scratches even with the cleanest of oils eventually, especially on mobile. Now, the cavitation erosion is interesting. These machines have very wide suction lines and pressurized tanks, and still, some erosion happened. I haven't checked the control valve for the T-orifice, but A10VOs are fast enough to cavitate when they come on stroke, so my guess is it's not there, and the dynamic conditions are the primary cause for the erosion. I'll put a new shiny valve plate in and be done with it.
The pistons look brand new, with minor scratches and zero play - so, again, the conditions were just right for the rotary group not to pull the shoes off of the pistons during operation. I'll lap the slippers, shouldn't take long.
Obviously, the needles that support the ball guide have cracks at the top, but that's expected. They're cheap to replace, too. The ball guide has marks in the respective spots, too, so I'll make sure I turn it when I assemble the unit.
Now for the control - the spools are in mint condition, so, once again - very respectable.
This example does two things:
First - it serves as a documented reference of what a medium-duty classic design open-loop pump should look like after 7500 hours of operation in a properly designed hydraulic system.
And second - it proves that if you keep your pump happy (i.e. the oil clean and cool, and no funny pressures in the suction or the drain) - you'll get a pump that looks almost new, even if it's a medium duty series that's done "two years of mining".