InsaneHydraulics - Sergiy Sydorenko © 2009-2012 All Ridghts Reserved
The best way to get "real" knowledge on hydraulic pumps (and I mean usable knowledge
- like control function, adjustments, troubleshooting techniques and
such) is to run them in test benches - nothing can beat this. Test
benches are workshop machines, yes, but they also are by far the
best practical learning tools you will ever find. Toying around
functional parameters, like the input speed, outlet pressure, control
signals, and also fiddling with the adjustments with the objective to
see how they affect operation of a pump gives invaluable practical
knowledge - something you will never find in manuals and catalogues.
And don't forget that benches offer the benefit of being able to do all
of the above in the comfort of your workshop. Take my word for it
- working a unit on a test bench offers plenty of opportunities for
learning, as opposed to field conditions, when you adjust an 80 C hot
hydraulic pump hidden under the belly of some industrial vehicle while
lying on your back on a pile of sh-sh-sh-sh and your "learning attempts"
are constantly being disturbed by the feeling of warm hydraulic oil
streaming down your elbows on the way to your armpits as you unscrew the
lock-nut of an adjustment screw... educational experience? - yes, but
"far less memorable"... Anyhow - I like using our test bench for
didactic purposes - to teach about them pumps, and sometimes Lady Luck
throws in a unit "with a twist" bearing a particularly uncommon
malfunction, and it is these units that are the best choice both for
teaching/learning, as well as for putting one's troubleshooting skills
to a test - and it is one of such cases that I would like to describe
A couple of days ago I was introducing a new
colleague of ours to the world of hydraulic pump troubleshooting. The
man had some (but not much) experience in this field, and a
malfunctioning Sauer Danfoss series 90 closed loop pump seemed like a
perfect opportunity to show the "recruit" how the things are done... The
pump had been working flawlessly till the day a scheduled regular
maintenance was performed, after which it stopped responding to the
control signal (standard electric displacement control). The maintenance included replacement of hydraulic oil
and filters. Despite of the fact that the same procedure had been
performed by the same crew countless times before, something went
wrong... The problem was to find out what. Several mechanics
troubleshot the transmission, but to no avail - all standard (let's
replace it and see what happens) techniques failed, so the unit was
dismounted from the vehicle and sent over to us.
The whole troubleshooting process took about an hour, and
the machine owner was quite surprised when he found out the reason why
his pump had stopped pumping so abruptly, but I will address this later -
for now, since I already touched the topic of didactics, I would like
to transform the troubleshooting episode into a series of brief generic
troubleshooting instructions, with short reports regarding this
particular case after each step - and, now that I mentioned it, this is
how any troubleshooting should be done - one step at a time! Note, please, that the key word here is brief, in other words simplified.
OK, so here I go:
IH Guide to Troubleshooting Sauer Danfoss Series 90 Pumps
Part 1 - Information Gathering and Preparations.
Listen to the client tell the pump's story
(patiently, and with a polite smile on your face... don't overdo it
though, the smile I mean - this all smiling business can get pretty
awkward if you overdo it) and ask questions about the circumstances of
the failure. Squeeze out as much information as you can, talk to the
operator or the person directly involved with the machine in question.
Remeber - you are the Sherlock Holmes of the hydraulic crime world...
(don't overdo this one either...).
The complaint is - the pump is not responsive to the
input signal, the background story is - a regular maintenance was
performed (oil and filter change), after which the pump never pumped
Even if the client reassures you that the pump
should be "all good inside", still - before mounting the pump on your
(quite expensive) test stand inspect it (e.g. three simple checks) just to make sure that you won't blow the stand up or contaminate it with all sorts of debris as soon as you turn it on.
Done and all looks OK.
Connect the pump to the stand and run it - you have
to run the pump to see how it runs and confirm the existence of the
problem, as well as confirm if the coils have nominal resistance (not
Done - the existence of the malfunction is confirmed - the pump is "blind and deaf" - which means that now it's time for
Part 2 - Troubleshooting itself.
Essentially - troubleshooting of a closed loop
pump is a simple process of measuring pressures in key points and
drawing logical conclusions from the readings. Tools required - test
fittings and gauges plus a sound head and background knowledge.
First test point - the charge pressure. Why? If
the charge pressure isn't stable, or too low and you can't adjust it -
then you'll have to troubleshoot the charge system (the charge pump,
charge relief, filter, etc...) or the rotary group (excessive internal
leakage), and most likely disassemble the unit
The charge pressure is OK.
Test point two - servo-pressures. Measure
pressures in the servo-cylinder (both sides at the same time). Why?
Reading servo-pressures gives you an answer to the question why the
pump isn't stroking - whether it's a hydraulic problem (bad "P", read low
servo-pressure, or bad "T", read oil can't come out of the
servo-cylinder) or a mechanic problem (the servo-pressure
differential is present, but the swashplate isn't moving, which can be
caused by a damaged swashplate bearing, a broken servo-link, etc...)?
Measured - when the input signal is "on" the
servo-pressure doesn't change. Now we are getting somewhere - the
problem is definitely hydraulic and not mechanic.
Electric displacement control check - control module
ports X1 and X2 (both at the same time) - to check if the PCP (pressure
control pilot valve) is working properly. Why? The series ninety
displacement control is a two stage device - the PCP pilots the spool of
the displacement control, and the spool distributes oil from the charge
pressure gallery to the servo-cylinder chambers to position it
according to the input signal. If the PCP hydraulic signal is OK - the
displacement control spool is stuck (or the feed orifice is blocked), if
the PCP hydraulic signal is bad - the problem is the PCP.
Measured - very low pressure was recorded, less tan
one bar for both sides (normal value would be around 8 bar in both ports
at null), no changes with the input signal change: Great! We're on the
right track! Now we further narrowed down our search - the PCP is not
supplying any oil to move the spool of the four port proportional valve
that controls the servo-piston. At this point, by the way, I was pretty
sure that I would find the PCP inlet screen blocked with contamination -
something I'd seen many times before. In any case these pressure
readings meant - PCP out!... So we took it out - and the filter looked
pretty darn clean, the small orifices also turned out to be unobstructed
- bummer! Passing on to
Well, since we just found out the PCP was all
good and unobstructed, the only explanation for the oil not coming out of it had to be no oil coming in,
in other words something was blocking the connection between the charge
pressure gallery and the PCP. Let's have a look, shall we? -
displacement control valve out! Maybe there's a blocked orifice...
So we took this one out, too - and, what do you know, no blocked
orifices were found - bummer!
Ok, so we have no blocked orifices in the control
module, is it really possible that the charge oil isn't reaching the
control? I wish there were a way to test this, wait a minute - there is!
This is a test stand - you can do whatever you please here, so if you
want to check if the oil is really reaching the control, simply
have a look! Although such a procedure may seem somewhat redneck-ish, it
is definitely the "take away all the doubts" test, and it boils down to
running the pump (ve-e-ery slowly) without the displacement control valve in place - just to see if oil is coming out of the charge pressure connecting channel under the control. Don't laugh now, visual confirmation
of flow (if you know what to expect) is a valid "poor boy's"
troubleshooting technique (relatively low flows, of course), and also
this means that a well built pump test stand should be able to catch all
possible oil spills.
Done - and no oil is coming out of the charge pressure connecting port!
Hurray! It's not he control, not the PCP, not mechanical, not
electrical - the problem is the obstructed oil passage between the
charge pressure line and the displacement control. Now, to find the
obstruction (before throwing in the towel and passing on to the
disassembly stage) let's take out the small screen in the passage to
inspect it more thoroughly.
Done and - found it!!!!
- a small piece of plastic film got inside the body channel and blocked
it completely against the screen. After removing it the pump was as
good as new again!
So, once again the day is saved thanks to the powerful logical approach to troubleshooting!
Where did the piece of plastic came from? - might you ask -
Quite simple, lads, the piece of plastic is a small piece of the
wrapping of the new charge filter, which fell inside the filter when the
wrapping was removed, and eventually found its way to the small screen
behind the control. Way to do a scheduled maintenance!
Morals of the story:
a) Logical "one step at a time" approach to troubleshooting is the best way to go.
b) When changing charge pressure filters be very careful when you remove the plastic wrapping.