Insane Hydraulics

Theme Image

Small Symptoms - Major Failures

Here's a phrase that I hear almost every day:

Our ___ (insert something hydraulic)___ pump was running just fine and then all of a sudden it broke...

There seems to exist a general and persistent misconception that if a hydraulic pump is operating properly - it must be "all good" inside.

The fact that a pump is capable of sustaining its nominal flow at its nominal pressure means only that, and nothing else.

When a piece of hydraulic equipment has been running a medium duty hydraulic pump for 15000-ish hours without an overhaul, but with clean and cool oil, there's a good chance that the sealing surfaces are still in decent shape, but there's also a good chance that there's a bearing inside that is about to break and cause catastrophic failure, destroying the pump without any major warning.

The key word here is "major" - because sometimes (although not always) there are "minor" warnings that get overlooked precisely due to their seemingly insignificant nature.

Whenever I get a pump that suffered a major failure, I try to investigate and ask if anyone saw anything "strange" before the failure happened, and usually I get a three part answer that

begins with:

then transitions to:

and finally ends with:

So, here are two of these "small and negligible" warnings that I hear most often:

Leaking shaft seal.

The pump starts leaking oil in the shaft seal area, maybe a drop a minute when hot, no biggie, and you tell yourself - It's just a tiny oil leak that can be fixed with a 10 dollar seal I can get online any time I want, I will run the machine for another week and then take care of it. Then comes the next week, and an urgent job comes up which you can't refuse, and so the seemingly minor repair is postponed for another couple of months and then... Bam! You open the pump to find out that the front bearing died and took the rest of the rotary group with him... And you wonder - how can this be? The machine was operating just fine - normal speed, normal oil temperature, normal noise...

A shaft seal leak in a hydraulic pump is never a good sign because

"Floating zero" in a closed loop transmission.

Your closed loop transmission begins to creep when in neutral. Very very slowly. And often the creep can be stopped by operating the transmission in the opposite direction or by applying the parking brake, and you tell yourself - it must be a matter of simple adjustment, I'll get someone to have a look at that next week... The rest follows just as described above - only this time it's the swash-plate roller bearing race that broke and took the rest of the pump with it.

As always, an example. This Rexroth A4VG125 closed loop pump is a perfect illustration for this post. The transmission was running absolutely fine - no overheating, sustainable pressure of 450 bar in both legs the loop, stable charge pressure - good as new. It did develop a leak in the shaft seal area, and some time later would creep when in neutral. That's all.

The owner of the machine did call a tech eventually, in the hope that he could re-adjust the neutral position - and the tech was experienced enough to tell him to send the pump to a pump shop, which he did.

Unfortunately it was already too late for this one. As you can see - the front bearing is severely deteriorated. Hard steel particles, originated from this wear, caused irreparable damage to the piston slippers, the retainer plate and the ball guide. Also - the saddle bearing races and the swash-plate are heavily grooved, which means that they have some good 10,000 hours of operation or more on them. Had this unit ran for another day or two - it would fail abruptly in the most spectacular manner. It's getting a complete and expensive overhaul now.

Note that the valve plate and the barrel, although already showing signs of wear (but without deep scratches) allowed the pump to perform nominally even with the piston slippers in such a bad shape.

To resume - minor symptoms like a shaft seal leak or a transmission that suddenly decided to "float" a little, can be a sign of an impending catastrophic failure, and therefore should be investigated as soon as they are detected. When there's no service history, and you can't tell for sure how many hours of operation passed since the last overhaul and what parts were replaced, such minor symptoms should definitely be treated as major ones.