Insane Hydraulics

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A Common Malfunction of Negative Flow Control Systems

Today I want to talk about a common malfunction of Negative Flow Control (NFC) hydraulic systems - the obstructed pilot orifice. I've come across it many times, and it's fairly easy to diagnose with a couple of pressure gauges. That is - when you know how.

But before I begin I must apologize. Last week I happened to troubleshoot the hydraulics of a CAT345D excavator that had this very failure, and I realized that I should have filmed the pressure readings only after I had it fixed. In other words, I blew a great opportunity to show and tell. In my defense - when you are dealing with an urgent repair, and your hands (and clothes) are covered in oil, and all you want is to see the damned thing fixed, it's hard to think about filming stuff. At least I remembered to take pictures.

As you know, an NFC system uses an orifice placed at the end of the DCV by-pass gallery to supply pilot pressure to a pump equipped with a negative displacement control (the diagram of such a system can be seen in this post). A beautiful solution that gives stable control to large inertial loads.

One of the advantages of this system is the fact that there's only one "source of truth" - the orifice, which means that there's no need for a complex net of shuttle valves that can malfunction. Since the by-pass flow is relatively large, the orifice (or several orifices in parallel, which is the case of the system that we are considering today) is measured in millimeters (as opposed to fractions of mm), and therefore is less prone to clogging. However, less prone doesn't mean completely immune, and sometimes larger particles do find their way into the return flow, and occasionally lodge themselves in the pilot orifice, and cause the NFC system to malfunction. And when this happens - "all hell gets loose", and very often the main pump (or/and more stuff) gets an overhaul just for the heck of it.

A couple of weeks ago I overhauled the main pump of a CAT 345D excavator (after a catastrophic failure), went to the client's site, assisted the start-up, watched an operator try out and OK the machine (an important note - not the guy who was used to this machine, you'll see why this is important in a minute), and drove back home satisfied with another job that went well.

A day later I got a call that the excavator was "kind of slow and weak", and when I asked the client to inspect the return filters, I got the terrifying news that the filters contained "a certain amount" of bronze particles.

When you overhaul hydraulic pumps you never want to hear something like that. At least not about a system that employs the pump you just fixed. But hey, sh** happens, right?

So, I got the pump back, cracked it open - and found zero damage. All parts looked brand new! Hmm... I put it back in one piece, the pump got reinstalled, and after the commission, I began troubleshooting.

First of all - I tried the excavator out (the best I could, of course), and it kind of seemed OK to me, so I asked for the usual operator to step in, and the guy immediately told me the machine was slower than normal, and to top it off, it was especially slow to lift the tracks off the ground when you pushed the boom down. It even stopped at some point, not being able to lift the tracks from the ground up to a certain angle.

At least when you see a malfunction - it can be fixed, right? So, I installed my pressure gauges at the pump outlets (a Kawasaki double pump, K5V200DP), fired up the monitoring screen on the machine's control panel (very thoughtful of you, Mr. Caterpillar) - and watched how the pressures behaved. By the way, using wireless pressure gauges in this scenario was very convenient. Check out the gauges placed on top of the pump compartment:

As soon as I revved the engine up, I saw something that was not normal. While the "stand-by" pressure at idle speed was about 30 bar for both pumps (I actually saw a 7 bar discrepancy between the machine's sensors and my pressure gauges, which, I guess, once again proves that you should always double-check panel readings with a gauge that you trust), at higher RPM, the pressure of one of the pumps was 7-10 bar higher than the other, and I knew for a fact that the system didn't do that last time I was there. In my experience, this behavior - the abnormal pressure rise of the standby (i.e. NFC pilot) pressure when the engine revs up - most of the time means blockage of the NFC pilot orifice.

So, I removed the NFC pilot valves - and... Yep, just as I thought - one of them had three out of the 6 holes clogged:

When the main pump of an excavator suffers a major failure that injects a lot of particles into the system, you can never remove all of them. Fact of life. Note that not all catastrophic failures result in direct contamination of pressure lines. Very often the "ugliest" stuff ends up in the pump's case and the drain lines, and maybe the bottom of the tank, but in that particular case the bronze faces of the barrels got "ground down" to the steel in a matter of minutes, and so a lot of fine (and not so fine) bronze particles were injected directly into the pressure line.

Believe me, we did our best to clean and drain everything that was possible to clean and drain, and yet some of the large particles remained somewhere, and then caused the malfunction, as well as the false alarm of "bronze-like stuff in the return filter".

So, to the traditoinal bullet points: