I sincerely believe that I could disassemble and then reassemble a Rexroth A10VO100 pump blindfolded, with my right hand tied behind the back, and guarantee it would work afterwards! It is me saying how very common this model is around here. Same old wear, same old parts, same old cookie-cutter repair estimate...
However this overhaul some strange circular marks on the piston surface caught my eye. I am accustomed to seeing scored pistons, but common sense tells me that with reciprocating axial movement the scores and wear marks should also be axially aligned, right? And in this case the wear lines were exactly perpendicular to the piston axis, wrapping around the piston cylindrical surface. How could it be?
A closer inspection revealed the cause - the circular bands turned out to be thousands of very closely located axial scores, caused by abrasive particles that got caught in the barrel-piston gap, and lodged themselves in the barrel wall.
This curious pattern can tell a lot of things. First of all, it tells us the amplitude of the axial movement when the scoring was happening. Since most of the wear patterns have more or less the same (and very narrow) width - I would venture to say that most of the scores were "engraved" when the pump was in stand-by mode. Then, the fact that these marks got "wrapped around" the piston tells us that when the pump is working, the piston rotates in its bore, but apparently at a very slow rate, because the "scratch marks" are so close to each other.
In fact - in some places the scores are so thin and closely spaced together, that they turned into tiny diffraction gratings, splitting the reflected light into a spectrum of colors. And by the way, a laboratory diffraction grating can have hundreds of scores per mm! You have to really zoom into the pictures to see the tiny rainbows clearly - but they are definitely there! How cool is that!
And this particular image is even more special (again - you will have to really zoom in to be able to see it). Apart from showing the spectrum, it also "caught" the moment the pump was transitioning, most likely from stand-by, and the process got registered on the surface of the piston like an electrocardiogram line. If you look closely - you will see that with bigger amplitude the spacing between the score-marks becomes larger too, which means that at higher displacements the pistons rotation inside the barrel becomes faster.
And there you have it - it takes a damaged classic swash-plate type variable displacement axial piston pump and some grains of sand to:
a) measure the piston travel when the pump is in stand-by mode
b) visually demonstrate how pistons rotate inside the barrel, and prove that the rate of this rotation is displacement-dependent, and
c) prove once and for all that you can make pretty, albeit tiny, rainbows from hydraulic pump parts!