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Excessive case drain flow

Excessive case drain flow

Excessive case drain flow


I need advise on the problem we are experiencing with pumps on a scrap shear. There are 8 variable displacement axial piston pumps (A4VSO 250) installed in parallel on the machine. During operation depending on the size of the driven cylinders different number of pumps activated. So pump number 1 is used for all movements and has the hardest life, while pump 8 is activated only for around 20% of operation time.
A few month ago we noticed that during some of the movements the maximum pressure in the system would be lower than 350 bar set at the compensator. Having deadheaded each pump individually i found that pump 1 could develop only 140 bar, pump 2 - 170 bar, pump 3 - 200 and so on. Only pump 8 would work correctly and go into compensation at 350 bar.
The measured case drain flow of pump 1 was 40 l/min at standby compensator pressure of 15 bar. So it looks that at 140 bar all 370 l/min are going to the case and then tank.
I cannot afford to overhaul the pumps yet and actually still satisfied with the machine performance. It does not overheat and productivity is still acceptable. What would be the consequences of running the pumps in this condition? I would expect all this flow to raise the case pressure with leakage at the shaft seals but so far none are leaking. Is there risk of the pistons pulled out of the slippers or other catastrophic failure?


RE: Excessive case drain flow

If the pistons had been separated from the slippers, the pump would be very noisy.

It could be that the port plate is damaged and oil is leaking across it and into the case.

It could be that the pressure cut off compensator spool is stuck, venting outlet flow to the case and also keeping the pump awash angle off maximum.

How did you dead head the pumps to test the flow?

RE: Excessive case drain flow

I would suspect full flow is not going through the case at 140bar. You would have a high heat load.


RE: Excessive case drain flow

Q: Is there risk of the pistons pulled out of the slippers or other catastrophic failure?

A: It depends on what the failure is.

Since the failure seems well related to usage rather than age and a number of data points show no catastrophic trend, then I'd expect it to gradually get worse until the pumps are ineffective and unable to produce any pressure.

You could make a graph of estimated in-use hours vs peak pressure for the 8 pumps you have and try a curve fit to see where it will drop below being useful as a gage to how many hours you have left.

"Condition Monitoring of Axial Piston Pump" by Zeliang Li http://www.collectionscanada.gc.ca/obj/s4/f2/dsk3/... has a nice table on p33 of the pdf, and the paper includes thoughts about diagnostics based on output pressure wave form.

"A Guide to Analyzing Axial Piston Pump Failures" by Oilgear doesn't list excess leakage as a symptom associated with their catastrophic failures - http://www.alliedsystems.com/pdf/Wagner/Forms/80/8... but does look at case pressure as a way to diagnose other failure modes.

Google found others - perhaps others will have more luck, but they mostly are re-formulations of the Oilgear presentation which may have originated elsewhere.

If I was more metrically inclined (and less lazy) I would calculate the wattage from the suspected leak rates. It seems like it would be high and, as hydtools points out, result in a lot of heat. (looked at data sheet - up to 300hp capacity for the normal pump; so maybe a quarter of that if it's losses from described leakage? https://dc-us.resource.bosch.com/media/us/products...)

If the pumps are identical, would re-ordering them be possible to extend the life of the weaker pumps?

One might look at deciding whether wear-leveling would be useful or if staggered replacement is a better use of funding. They will all eventually fail and some plan should be in place to decide how to handle it.

Is there a check valve that will prevent back-flow from the other pumps? I would expect the case drain to handle significant damage to a pump, but not be able to handle the output from up to 7 other pumps.

Is the case drain line sized for significant flow? A system I was peripherally involved with had motors blowing shaft seals because the drain line was about 70 feet long and developed too much back-pressure from the leakage of all the motors that shared it.

RE: Excessive case drain flow

On a new pump rexroth A11VO260cm3/rot with oil at 46.4°C and 36cSt we make this measurement of the drain flow between 50 to 340bar 1500RPM:
At full flow: Q(l/mn) = 1.82 + 0.068 x P(bar)
Cut off position: Q=4.1 + 0.0763 x P

If you have a drain flow 40l/mn at 15bar on the pump 1: you have big wear and the shear don't work correctly. Because if the pump 1 has too much leakeage you must have defect on the automatic cycle.

How is the filtration and pollution???

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