InsaneHydraulics - Sergiy Sydorenko © 2009-2011 All Ridghts Reserved
As I was re-reading my post on the Atlas-Copco case, I remembered another curious episode, that shows the importance of taking oil flow in the LS line into account.
From what I've been seeing, a lot of "recently
advanced" hydraulic techs are convinced that in static (please, note
that in this article I am referring only to static, or as I like calling it, classic load sensing, not dynamic)
closed center load sensing systems that use variable displacement
pumps, the LS signal line transports nothing but pressure signal, or, in
other words, the flow inside the LS line is minimal, and is only
defined by the control spool leakage factor. This is not always true.
Depending on the pump control design a flow in the LS line can exist,
and it should be considered when projecting or troubleshooting such
circuits. As I already described in the above mentioned troubleshooting episode, flow in the LS line causes a pressure drop that adds
to the delta P introduced by the metering element (distributor valve,
restrictor, calibrated orifice, etc...), which means that the size and
the length of the line affects performance. Another very important thing
to consider is the simple fact that oil flow always brings along
particles, which, when big enough, can cause troubles - especially in
load sensing lines that incorporate small orifices. The following
troubleshooting episode is a good example of such "particle related
...A hydraulic bone-crushing press broke down,
and a hydraulic technician was called in. When I say bone-crushing press
I mean literally - used for crushing bones to squeeze out the attached
meat leftovers and juices - a part of the production process of a big
sausage plant. The symptom was the complete loss of pressing force. In
fact, the piston could barely move, even with no bones in the press.
Turning the machine off and on would make it work fine for a short
period of time, but after a couple of minutes the press would slow down
again. The circuit was powered by a Parker PV140 variable displacement
pump, equipped with load sensing, pressure compensator, and torque
limiter control (schematics).
The LS port was connected directly to the pump outlet via a short 8 mm
metal pipe. The rest of the system consisted of a couple of custom built
manifolds incorporating logic elements and electric pilots - a pretty
standard solution for the purpose.
Pressure readings confirmed that during the "slow
stretches" the system was at stand-by pressure. Suspecting pump control
malfunction the technician tried to adjust the torque limiter setting to
see how the control would respond. The main reason why he went directly
for the torque limiter was the fact that the previous alike failure had
been caused by the torque limiter malfunction. Of course, he tried to
adjust the torque limiter during the "fast stretch" - after the machine
had been turned off and on. As soon as he barely touched the torque
limiter adjustment screw, turning it out around an eighth of a turn -
the system went slow. Apparently, the torque limiter was faulty, and
indeed, when it was disassembled, a distinct wear marks and a deep
groove were discovered on the limiter needle. Since the pump had already
had enough hours on its shoulders to justify an overhaul, it was
dismounted and brought over to the shop.
After the consequent (and expensive) overhaul
the pump, naturally, ended up on the test bench. Since no replacement
for the torque limier cartridge was available immediately, and the press
had to be put back in service as fast as possible, a decision was made
to rectify the needle of the torque limiter relief valve and "see what
happens"... The rest of the pump controls were disassembled, inspected
and thoroughly cleaned. The pump worked very decently for a couple of
minutes, and then - the malfunction reemerged when an attempt to
lower the torque limiter setting was undertaken - the pump all of a
sudden went into the stand-by mode. Since the problem would occur every
time a torque limiter adjustment was fiddled - a logical assumption was
made that the torque limiter relief valve was warn out beyond repair and
needed replacement. By chance, during one of the tests the technician
decided to adjust the pressure compensator before adjusting the torque
limiter - and exactly the same thing happened - as soon as the
adjustment screw was turned out - the pump "went stand-by" and stayed
there. In both cases, when the electric motor of the test bench was
stopped and restarted - the symptom would apparently disappear for some
time. Everybody was puzzled...
Despite seemingly mysterious, the case has a very
simple explanation, and if you carefully look into the operation
of the pump control, you will see that it is very easy to come up with a
theory that can explain all of the symptoms. This pump's control
operation is based on a single spool (schematics)
- namely the LS spool, which along with the natural load sensing
function (keeping the constant delta P between the LS port and the pump
outlet) also assures the pressure compensator and torque limiting
function by means of venting the signal line through the two relief
valves - one with fixed setting (pressure compensator function), and one
with displacement dependent setting (torque limiter).
Let us ask ourselves - what is common for both
situations - when the torque limiter and the pressure compensator are
turned out? What is that common thing that happens inside the control
when any of the two adjustments is lowered? - That's right - whenever
any of the two relieves kicks in - the oil in the LS line starts to flow.
It doesn't take much to imagine a condition when the LS signal gets
blocked whenever a flow builds up - and the condition is - loose
particle over an orifice, that functions like a sort of a load anti-drop
valve, closing the orifice every time the oil flow is strong enough to
lift it up.
Take a look at this drawing
- despite having been cleaned several times, the fitting mounted above
the orifice had never been unscrewed, and since the person responsible
for cleaning the parts could clearly see the light through the
apparently unobstructed orifice, he assumed that it was not necessary to
disassemble it. The hard particle, that got caught in the cavity above
the orifice was plugging the orifice whenever a flow strong enough to
lift it built in the line. The fitting was taken out - and a small
piece of hardened rubber was discovered under it, and, of course,
removing it solved the problem.
Troubleshooting is all about coming up with theories and
testing them. And I must tell you, that it was extremely rewarding for
me to hear about the symptoms, and then to be able to tell what was happening and indicate exactly where to look for the problem, and to be right about it! I'm not bragging, I merely show that
a) complete understanding of a component's operation, and
b) sufficient test data
are the two main conditions for coming up with a valid theory that has the best chance to prove right.
So, in static closed center load sensing systems,
depending on the type of the variable displacement pump control, oil
flow can build in the LS signal line - a fact not to be forgotten to
assure efficient troubleshooting of such hydraulic circuits.