The following industrial tale is an extremely good example of an
extremely bad example. This is a perfectly true story, personally
witnessed by me, and its main characters are a big O&K grader, kind
of like this one
(picture taken from the web), and a team of "prominent experts" from a
well established earth moving equipment reconditioning/reselling
business.
Malfunction description:
After approximately thirty minutes of work, when
hydraulic oil would reach its normal operating temperature, one of the
two cylinders operating the blade wouldn't lift. The other one was
functioning normally.
Troubleshooting process:
A pressure gauge is mounted on each cylinder and the
pressures are compared. The pressure on the "good" side is 120 bar,
while the "bad" side doesn't go higher than 50 bar - clearly the
problem is caused by insufficient pressure. To find out the root cause
a series of theories is tested (read "the fun begins now!").
Theory one - the cylinder has damaged
internal seals, therefore the excessive oil leak between chambers
doesn't allow the pressure to go higher than 50 bar.
Attempted solution - The cylinder is replaced with a different one, the malfunction continues...
Why is this wrong? - First of all, leaking piston seals do not cause
loss of force! If, however, the internal leak between the rod and
piston end were so big that it would indeed become a pressure limiting
factor (even with the relatively low oil flow this would equal removing completely
the piston seals), another symptom would appear - the piston wouldn't
be able to hold the blade in the air, since the load was being held by
the rod side of the cylinder (rod facing down), and this was never
detected.
Theory two - the cylinder is equipped
with an overcenter valve, and it is suspected that it has an internal
leak that is causing the loss of pressure.
Attempted solution - Since both
cylinders use the same type of valve, the valves are interchanged to
check if the problem changes sides as well - the malfunction continues
on the same side...
Why is this wrong? - First of all
- the internal design of the valve made it practically impossible for
such an internal leakage to exist (to be big enough to cause such a
loss of pressure), but even if it were possible for such a leak to
exist, it would have been much easier and much faster to plug the
connecting hoses and check the pressure instead of interchanging the
two valves. If the leak existed, other symptoms would be present, like
the characteristic noise and local overheating.
Theory three - someone remembers that
once before a similar problem occurred to another alike machine, and
the problem was solved by replacing the old worn-out pump with a new
one.
Attempted solution - Orders are - to
dismount the hydraulic pump and send it to a local hydraulic workshop
for overhaul. One small mistake is made during the process - the pump
that gets dismounted is actually the closed loop pump driving the front
axis of the grader...
Why is this wrong? - Well, this
is ALL wrong! Clearly the person responsible for removing the pump from
the machine was absolutely unaware of what closed loop and open loop
circuits are, in fact he didn't even bother to follow the pressure
hoses, and simply removed the first hydraulic pump he could find.
The closed loop pump was sent to our workshop, and it was
overhauled (minor lapping and new seals). When the client re-mounted
the pump on the machine the problem (of course) continued, so the
troubleshooting process went on...
Theory four - since there are no doubts now about the pump's condition, the distributor valve must be the problem.
Attempted solution - The distributor
valve is removed from the machine, and is replaced by one from an equal
machine - a tremendous amount of effort and time! The malfunction
continues...
Why is this wrong? - The distributor valve
was replaced on a "hunch" basis, without performing any tests
whatsoever to confirm its malfunction. It would have been much easier
and much faster to insert a flow-meter/tester into the P line and check
its reading, or (if no test gear was available) simply to disconnect
the T line to see if the oil was "escaping" through the distributor
valve (much flow from the T line) or "got lost on the way" to the
distributor valve (no or low flow from T port).
Theory five - it must be the damned pump then!
Attempted solution - Let's call the guys who
repaired the pump! Our repair shop is contacted and is informed that
the recently overhauled pump "is not working..." Since it is a warranty
claim (read matter of honor) yours truly is immediately dispatched to
see what is wrong. At that point the machine had already been stopped
for about a month - as you can imagine, removing components from the
machine that big involved certain amounts of time and manpower, and the
consecutive tests required dislocating the machine to a work site and
then bringing it back, which was not a simple task as well.
When I got to the shop, and confronted the mechanics with
the idea that it was very unlikely for a closed loop pump that had been
repaired at our shop to drive such a circuit, one of the mechanics
remembered that there were "some other pumps" in the machine. Following
the pressure hoses revealed a triple gear pump. Very happy with the
"discovery" the foreman was already giving orders to replace the pump
with a new one, and got notably disappointed when I told him that it
would be better to first check if the pump indeed was faulty before
dismounting it.
The next thing I did was the good old touch-it test (read my post about this simple technique here)
- I shoved myself into the small square opening in the frame under the
operator seat, put my hand over the gear pump, and told the operator to
pull the "troublesome" lever - as expected the pressure rose to the
50-60 bar and stayed there and I was feeling no heating of the
pump's body whatsoever - the pump remained cold. I then suggested the
man to look for malfunction somewhere else, as the gear pump was
missing the main symptom of a warn-out gear pump - local overheating.
Still very suspicious about my test and conclusion, the man told me
that I "could give it a try"...
It took me another three or four hours to check the whole
circuit. Most of the time was spent doing "hose pulling" because, as
always, no hydraulic schematics was available, and in the end the
malfunction was narrowed down to a manifold, mounted between the
distributor valves and the pumps. The shop foreman got even more
suspicious, when I told him that all of the hard "troubleshooting" work
he did for the last month was re-checked by me during the last four hours,
and that the pressure loss was being caused by internal leakage inside
the manifold, which could be caused by a number of reasons, and since
no hydraulic schematics was available, the manifold should be removed
from the machine and inspected (read back-engineered).
When I opened the manifold, I discovered that it was
housing two flow limiters, and two piloted logic elements (no need to
go in much detail here), but I didn't discover any "apparent" damage,
like broken seals, worn out seats, or clogged orifices. At this point I
told the man that the manifold didn't present apparent failures, and to
reach any definite conclusions I'd need to take it to our workshop to
analyze it better through "wet tests" on a test bench. The man
disagreed - he was sure that since no visible damage was present - the
manifold was OK. He insisted on remounting it on the machine to "see
what happens", and do the necessary tests with the manifold mounted on
the machine, since he couldn't afford "wasting time" studying the
components. All my attempts to reason the man were futile...
Why is this wrong? First - since no changes
were made to the manifold, most probably it would remain in the
"malfunctioning" condition, so simply remounting it on a machine to see
what happens was a stupid and illogical waste of time. The manifold was
heavy like hell, and it took good two hours and two mechanics to put it
in place and connect all the hoses. Second - back-engineering of an
unknown component is not a waste of time!!! You can't fix something you
don't know! Third - it was virtually impossible to conduct any sort of
productive test with the manifold mounted on the machine due to the
very difficult access and lack of space.
Despite all this, the manifold was re-mounted and (I
wonder why!!!) the problem remained. After seeing that it was virtually
impossible to do anything with the manifold in place, I told the man
that there was nothing else I could do with it in these conditions.
Since there were two equal machines in the workshop, I advised the man
that the only fast way to put this particular machine back in service
would be through replacing the manifold with the one from the other
machine.
When the manifold was replaced - the problem was
solved. I took the malfunctioning manifold with me, and it took me six
workshop hours to nail and fix the problem, again no need to go into
technical detail here (in two words - it turned out that one of
the logic elements had an excessive internal leakage in the dampening
section, which caused the area of the section to be excluded from the
hydraulic balance, a simple slotted shim increased the flow to the
section compensating for the leakage and solving the problem).
Resuming all of the above in one sentence - a simple
and relatively easily identifiable hydraulic problem, that could have
been diagnosed and solved in two eight hour days tops (or one
sixteen-hour day....), took over a month to be resolved, with tons of
unnecessary expenses included, due to incompetent and illogical troubleshooting and lack of very basic industrial hydraulics knowledge.
Conclusions? Basic oil-hydraulics knowledge plus common logics is GOLD! Absence of those two is ... Well... Draw your own conclusions!
