Axial Piston Pumps and Field Failures

Pumping against a "dead end" will certainly result in damage, though the failure mode will only be determined by field trials....

What evidence is there for field failures, and what can be drawn from the data?

It's been my experience that the main faillure mode for Axial Piston Pumps running Hydraulic oil is contamination from poor filtration practices.

Contamination plugs the Orifice in the Piston that feeds oil to te Shoe and allows the Shoe to ride against the Swash Plate instead of floating as designed. You can see my understanding of the process on Page 8-12 Fig. 8-22 and the text in my basic Fluid Power book here, http://www.hydraulicspneumatics.com/200/eBooks/ At least that was how it was explained to me by a knowledgeable person.

I always use a 3 Micron Off-Line filter circuit that is around 10% higher flow than the Axial Piston Pump in the circuit and send this filtered oil to tank through a Back Pressure Check Valve with a 10 PSI Spring. This Off-Line filter circuit feeds the Piston Pump with a 10 PSI positive Inlet Pressure so it always/only receives ample filtered Oil.

I have had excellent service from Axial and Bent Axis Hydraulic pumps with this setup. Many have run 24/7 at 3,000 PSI for years.

Not cheap to clean the oil at startup, may use 10-20 3 Micron filters on a 200 Gallon tank, but it's a lot cheaper than buying pumps and all the lost production caused by frequent failures.

Your E-book is intereting. I will sometimes refer to it. Thanks.

If it is a hydraulic axial piston pump the failures depend-in the field- mostly of the filtering quality i.e. how fine is the oil filtered and how much dirt remains. Hydraulic pumps do not come to a dead end but because they work with very fine gaps (in the range of less 10 µm) and need oil as lubricant in those gaps if dirt comes in less oil flows and scorring can occur. Dirt has also an increasing effect on wear and increases gaps and related leaks. If you obtained piston shoes wear and destruction there are several reasons but one of the most probable is the low viscosity of oil in the gap reducing the oil film portance. Either the oil did not have the right viscosity or you run it too hot.

An other wear is the mirror wear in the zones whare the barrel goes from high pressure to low pressure. The high pressure jets destroy the plate and again increase leaks. Presence of dirt will accelerate the process.

I appreciate that all answers helped. Take care.

Pad rims are broken ! This only can happen when the pad cannot tilt as it should and it cannot tilt when it gets not enough oil to build up a porting oil layer. This can happen when the orifice is clogged by dirt. It appears that your oil is not as clean as Linde asks for and may be you even have a too thin oil. Any way the working conditions do NOT correspond to at least normal not to speak about optimal.

dirt... that's what we thought last time, so the hydraulic system oil and filters were changed, and filters were checked and changed about 3 weeks afterwards, with no dirt found. It has been working with no problems for years and i 1 year we have had to change it twice. Oil follows Caterpillar indications.

Dirt is not always "dirt". In hydraulics the dangerous dirt is the very very fine one in the µm and subµm range. When dirt is fine it cannot be even seen if the quantity is not big.

There is at the distribution windows a jet wear to notice which is in general generated by cavitation and erosion due to a jet + dirt particles.

In general when a sliding pair fails the main reason is that the lubricator (oil) layer is not thick enough and roughnesses do touch. This can occur when the oil layer is interrupted, oil is too hot and viscosity too low, when oil carries hard particles which harm the surface (mirror) so that the oil layer can break. Do you make maintenance with original parts ?

Can you make pictures of sliding surface with more details ? and the windows at their ends where the erosion occurs ?

What kind of filter do you have ? How fine and which is its beta factor ?

I added more pics at

Swing pump CAT-245-1sj

:( i'm afraid my new camera is not so good taking very close pictures!

the sliding surface doesn't show wear, but scratches caused by the steel of the pistons after the brass piston shoes were destroyed.

I don't see any erosion at the pistons oriffices.

Oil is as caterpillar recommendations.

I'm not sure about the quality of the filters.. they were not original, but next time i'll try caterpillar high eficiency filters.

This machine has more axial piston pumps but we only have problem with this one. we have a similar excavator using same oil & filters and didn't have such problem.

The first time (april 2011) we bought a whole "original" pump.. but now i start thinking it wasn't original and i'm getting very angry with my dealer.

1st remark: the different colors of the sliding pads. 2nd the wear is characteristic for a loss of oil portance which leads to local contact of sliding surfaces and to the generation of wear particles which contribute further to the oil layer destruction. This leads further to local temperature rise and viscosity reduction correlated to loss of portance. What I am surprised is that you came to such destruction since as soon as the pad does not work properly any more you get a loss of flow ans thus performance. I am pretty sure that 1- filtering was NOT correct and 2- the pump was a geometric copy without any respect of material quality. If you have the possibility it would be good to let the different pads be analysed and compare with original parts. If it is the only pump suffering then the whole circuit has to be analysed and sealing of cylinders checked or motors or piping. Clean oil is the ONLY way to get long service.