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    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 below.

    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.
    Step 1:
    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...).
   
    Result:
    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 again...

    Step 2:
   
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.

    Result:
    Done and all looks OK.

    Step 3:
    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 burned out).

    Result:
   
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.

    Step 1:
   
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

    Result:
    The charge pressure is OK.

    Step 2:
   
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...)?

    Result:
   
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.

    Step 3:
 
  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.

    Result:
    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

    Step 4:
    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!

    Step 5:
   
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.

    Result:
    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.