Insane Hydraulics
Bold and audacious blog about fluid power
Here's one phrase I hear a lot:
"Our 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.
Let me tell you something - the fact that a pump is capable of sustaining its nominal flow at its nominal pressure means only that and nothing else. Acceptable volumetric efficiency is never a guarantee of longevity!
When a piece of hydraulic equipment runs a medium-duty hydraulic pump for 15000-ish hours 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 warnings.
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 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 get the most:
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 that 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 it! And you wonder - how can this be? The machine was operating just fine - normal speed, normal oil temperature, normal noise...
Yes Sir! A shaft seal leak in a hydraulic pump is never a good sign because
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 look at it 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.
Floating zero may seem like a small problem, especially if the creep is slow and intermittent, and the transmission is as strong as it was the day you bought the machine, but if you don't know the service history of this pump and its internal construction, you can never tell for sure if it's a matter of re-adjusting the control, or the sign of serious wear in the swash-plate mechanism.
If the floating zero condition developed itself due to the old age of the control, the pump should definitely be checked in a shop.
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....
Unfortunately, it was already too late for this one. As you can see - the front bearing is severely deteriorated. Hard steel particles, originating 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 run 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 be treated as major ones.