A hydraulic repairer's worst nightmare...|
I had a strange dream last night. And at the risk of having you think I live, eat and sleep hydraulics, I'm going to tell you about it. For the record though, I normally dream about much more pleasant things.
Anyway, I was at a large industrial plant somewhere - don't ask me where, I don't know. And the plant had at least two, variable-displacement, bent-axis pumps. In the dream, these pumps were actually Rexroth A7V500 units. If that model code doesn't mean anything to you, don't worry, it's not important.
What is important is these pumps have a flooded housing. This means the inlet port is common to the case (housing). Or to say the same thing another way, the intake hose floods the case, and the pump draws its oil from the case. This arrangement also means a separate case-drain line is not required.
Anyway, these pumps were mounted with the shaft up. Which is kinda strange since I've never actually seen an installation of this type and size of pump mounted vertically with the shaft up. It's unusual to say the least.
To cut a long story short, both pumps had failed and I was representing the company who had rebuilt them -- a definite flashback to a past life. The ball of the center-pin had seized in its cup and one of the piston skirts had broken (see pages 7 and 32 of Preventing Hydraulic Failures for how this might look). Suffice to say it's a nasty failure.
Anyway, it was crystal clear to me that the pumps had failed as a result of inadequate lubrication. Further, this lubrication failure was a result of improper commissioning. In short, the pump housings hadn't been bled of air and therefore, were not full of oil prior to start-up. The worst thing about this from my point of view was the pumps had been installed by one of our technicians - who should have known better! And so all of a sudden my dream turned into a nightmare.
Here's the thing: when you install a pump with a flooded housing, it's a mistake to believe that because the pump case is common to the inlet, you don't need to fill the case with oil. Well, you don't in the same way that you'd fill the case of an axial piston pump through its uppermost case drain port.
With a flooded housing pump, you do this by opening the intake isolation valve (assuming one is fitted) and then cracking the uppermost plug in the housing to vent the case of air and ensure it is completely full of oil. If you don't do this, all the air in the intake line and housing simply gets compressed into the case, ensuring a 'dry start'. And this is especially true if the unit is mounted with the shaft up.
The rest of my dream, err... nightmare was about figuring why an experienced technician didn't do what he was suppose to, (he should have been given a start-up check list) and me sticking band-aids all over the situation to appease a desperately unhappy customer. No wonder I woke up tired this morning!
While this dream is instructive on its own, there is a deeper, less obvious message: When you're dealing with expensive hydraulic hardware - whether yours or someone else's - you can never afford to take your eye of the ball.
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...and it's gone bang, so now what?|
Related to my 'nightmare' scenario described above is this question from one of our members:
"I have always wondered about the guarantees of hydraulic repairs. It would seem they would be almost impossible to collect on. Let me illustrate what I mean. A pump fails. The user delivers it for repair. One of two things could now happen. The repairer asks what the pump is used for (lets say construction equipment) and whether the system will be flushed. Depending on the response it is conceivable that the repair could simply consist of rebuilding the pump with used parts knowing that if the pump did fail before its warranty runs out, an oil analysis could simply negate the warranty by indicating contamination in the system - a consequence of the previous failure. The other scenario would be an honest rebuild of the pump with the proviso and recommendation that the system be flushed. In either case, what constitutes a thorough flush -- without some type of certified oil analysis that indicates the system is truly clean? Given today's demands for on-time construction or production deadlines this latter requirement would probably fall in the wish list category rather than the 'must do' column."
This question opens up two conversations. What constitutes a repaired or rebuilt component? And when a repaired component fails prematurely, who determines the cause and how?
Let's deal with the first one first. Leaving the integrity of the repairer out of this for a moment, what is an "honest rebuild"? A repaired component is NOT a NEW component. So obviously, it will be rebuilt using used parts. Let's consider a hypothetical repair on an axial piston pump.
If the housing isn't cracked that can go again. Same with the head or rear cover. The valve plate has some light scoring, but that can be lapped out. Same goes for the swash plate. The piston retaining plate is like new. The servo piston and springs are in good nick, so it would be a waste of money to replace those. The splines on the drive shaft show light wear, but not enough to warrant its replacement. The pistons measure up within tolerance and their slippers are in good condition. The cylinder barrel needs replacing, as do all the bearings and seals. But that's it.
Is that an "honest rebuild"? Sure it is. Provided of course, the repairer doesn't charge for parts that were not replaced. And I can't speak for all repairers, but I think the idea that a repairer would tailor the quality of the rebuild depending on how a customer plans to treat the component before, during and after commissioning is a bit of a stretch. It's much more likely that if a repairer is dodgy, then ALL his repairs will be dodgy.
Now to the question of warranty. There are always a number of things that need to be done in order to ensure the life of a replacement hydraulic component, whether new or rebuilt, is not compromised during its installation and commissioning. Consequential damage caused by the debris from a previous failure is an obvious one, hence the emphasis in this member's question about flushing.
But whose responsibility is it to ensure what needs to be done, gets done? Well my short answer to this is: the party who installs the component on the machine. If the guy who installs the component on the machine doesn't know what he's doing is that the repairer's problem? Only if he works for the repairer.
Which is why in Insider Secrets to Hydraulics my advice to machine owners whose primary concern is not voiding the warranty on a replacement component, is to get the repairer to install and commission it - with a free hand, within reason, to carry out any work deemed necessary, i.e. flushing, etc.
And I'm sorry, but if time pressures won't allow the job to be done right - that's hardly the repairer's fault. In which case it's only fair that any risk of premature failure transfers to the machine owner
Similarly, if it is not practical for the repairer to install the component, the onus falls on the machine owner's technician or mechanic to know what he's doing and/or seek necessary advice. Good repairers take the lead on this issue by advising and warning their clients of the potential pitfalls - I know I used to.
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