Hydraulics At Work

How A Small Omission Can Result In A BIG Hydraulics Maintenance Disaster

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. 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.

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 center pin had seized in its socket 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 was a nasty failure.

It was crystal clear to me 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 completely 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 normally fill the case of a 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 vertically with the shaft up.

So the rest of my dream was about figuring out 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 it's yours or someone elses - you can never afford to take your eye off the ball. Doing so can be a costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get "Six Costly Mistakes Most Hydraulics Users Make... And How You Can Avoid Them!" available for FREE download here.

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