Correct evaluation of the condition of a hydraulic pump or motor parts
is an important stage of an overhauling process. This is when you
define which components can be re-used and which should be replaced.
However, not all types of wear are evident and easily detectable,
making it possible for some damaged parts to skip detection, and lead
to a premature failure of an overhauled pump/motor.
These pictures are a good example of what I call a
"sneaky damage". These parts don't present any apparent, visible at
first glance damage (with the exception of normal wear), but under a
closer inspection appear to be damaged beyond repair. Take this cylinder block,
for example, which is from a Komatsu open circuit pump from a PC-340
excavator. Upon inspecting the respective swashplate, pistons, the
valve plate, and the sealing surface of the block, you will find
nothing but normal particle wear, which may trick you into assuming
that with a little bit of grinding and lapping the pump will be good
for an extra couple of thousand hours... But if you take a flashlight
and peek inside the bores, you will see that there are chunks of material missing in every single bore due to cavitation erosion, which makes the part unserviceable.
By the way, check out the unequal pitch
of the pressure ports. This is a patented Komatsu's noise reduction
solution. According to Komatsuneers, it disperses the noise frequency
phase, thus making the noise less harsh. I am not sure that it makes
that much of a difference, but suppose it can't hurt...
Another example. This spool
is from a motion control valve mounted on a propel motor of a small
excavator. You will have to look real hard to find that cavitation
erosion "scooped out" a narrow but very deep (around 4 mm) pit. The
spool appears to be in a good condition, but should be replaced, as
pretty soon the thin wall will come to an end, and "kaboom!" - the
spool is gone along with the respective housing...
It is in human nature to assume things. It is just
the way we are built. That is why it is not uncommon for a mechanic,
after finding a series of parts in good condition, to assume that
the rest of the parts of an assembly are in good condition too.
In 95 percent of cases this assumption will be true, leaving the five
percent for nasty making-you-look-stupid surprises.
Some types of damage, which is especially the case
with the cavitation erosion, can be concealed, and require additional
attention to be found. That is why it is important to carefully
evaluate every part of an assembly, without jumping into rushed assumptions and taking the parts condition for granted.