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     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!
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