Troubleshooting Hydraulic Circuits with Buckets

       A few days ago I had an interesting conversation with a man, who brought in a "broken" Sauer Danfoss series 51 motor, which made part of hydrostatic transmission of a street sweeper (one pump/two motors, one motor for each side, hydraulic proportional displacement controls). Apparently, the mechanic, who had labeled the motor as problematic, had years of experience and even had some industrial hydraulics training, provided by the brand they represented. It was a warranty claim - the motor had been rebuilt days before, and of course the case demanded urgent attention.

    When the motor was taken apart for inspection, no damage at all was detected. The motor was reassembled and tested, and presented good efficiency and controllability, just like any recently overhauled unit. The "troubleshooter" was contacted on the subject, and the consequent dialogue lead me to writing this article.

    The motor evaluation technique the mechanic had been using for, apparently, a long time, was the good old "check the drain" drill, which is OK for many motors, but not for closed circuit motors with a built-in loop flushing function. When he was confronted with the flushing theory, he, I must give him credit now, said that if it was normal for the motor to present high case flow, then all the machines they had were malfunctioning, as this one was very different from all the others (had way more case flow!). According to him, the other machines, equipped with identical transmissions, presented high flow discharge when the machine was starting to move, and then the case flow would almost disappear. Anything different from this pattern was considered a malfunction. He even said that it was the official brand mechanic from abroad, who had taught them the technique.

   No need to say that I went speechless for some time, fascinated by the conviction and the ardent speech. In the end, the man insisted on retesting the motor, and it was retested, and once again presented no flaws. When I wondered what the machine was doing wrong, it turned out that the problem was unstable oscillating movement, rather than lack of torque. When I asked what the charge pressure reading was when the machine was malfunctioning, he didn't know. Servo pressures in the pump? No? Motor? Also no? Ok, suppose you do have excessive leakage from the motor case, but how much exactly, twenty liters per minute, thirty?. The answer was prize-winning! "the oil from the hose HIT THE WALL, that's how much the case flow was!.." I mean come on, man, seriously, the wall?!!!

    I hope you understand, that at this point, there was absolutely no way I could defend my point. How can you possibly dispute THE WALL argument? Anyhow, he's the one insisting on extra tests and his company is the one paying for it, no point in denying the client what he wants, right?

    However sad this case might be, this is still a perfect example of how wrong assumptions lead to wrong conclusions. Let's analyze. (Note that I am talking about the classic closed loop arrangement, with charge pump and charge pressure relief valves incorporated in the main pump, and loop-flushing incorporated in the motor).

     First and most important: a closed circuit can NOT leak more than the charge pump flow, no matter how damaged it might be. The only place the oil enters the circuit is through the fixed displacement charge pump, which is relatively small, often around 20 percent of the main pump's displacement. So you can have high flow rates inside the loop, but will never be able to get more than the charge pump flow to the outside of the loop, because closed loop circuits do NOT make oil, they just push it around, it is oil companies that make oil.

   Theoretically, it is possible for case flow of a closed circuit pump or motor to be higher than that of the charge pump, but only for a very short period of time, and is caused by the "accumulator effect" of hoses during fast pressure changes or oil discharges from servo-piston system (when you have different servo-piston areas), we are talking about tenths of seconds here.

    In average, when loop flushing kicks in, the motor will have case drain almost equal to the charge pump flow (minus pump's internal leakage) in systems that use limiter as flushing valve, or will correspond to the valve's flow setting, when a flow regulator is used to limit flushing flow. Whichever the system is used, case drain evaluation is a piss-poor technique to evaluate the motor's condition, at least not until you eliminate the loop flushing function. Even if you take out the motor's cylinder block (don't take out the cylinder block, though) you will still have the same charge pump flow coming out of the drain port.

    But why this motor's behavior is so different from the other machines? To answer this question with full certainty, I would need to see the machine itself, which I didn't. But my imagination draws the following picture:

The hose from the motor case is disconnected and pointed to a bucket. Of course, everybody knows that normally only about thirty percent of oil will be caught in the bucket and everybody is prepared for an oil shower. The operator commands the machine to move slowly. (I imagine the mechanic holding the hose in his hand and staring inside the hose, as he waits for the oil....) As the pressure differential between the motor lines rises during the acceleration, the loop flushing spool dislocates to one side, cracking the flushing valve, and the mechanic sees the "initial" high flow discharge, thirty percent inside the bucket, seventy percent all over himself and the machine. Although this seems like a lot of fun, the driver understands that it's probably not a good idea to go full throttle when he has a colleague and a bucket hanging from the side of his vehicle, and the windshield all covered with oil from the sudden discharge, reducing the so needed visibility, so he stops accelerating, and lets the machine go at steady slow speed. At this point there's practically no pressure differential between the pressure lines, as the machine is not accelerating, and is moving mainly due to inertia, so the flushing spool goes to the middle position, and the above mentioned "slow" case drain is seen. By this point the bucket falls on the ground, and the test is stopped. As the bucket is being lifted, the loose hose end is resting on the ground...

   Let us think for a moment about the main symptom - the unstable movement. It can be caused by one of the two: either unstable pump flow, or unstable motor displacement, so my first guess would be displacement control issues. Maybe the pump's control is faulty. Maybe the motor is receiving a faulty signal to go to a smaller displacement, thus increasing the lines delta P, and not allowing the machine to enter the "inertia" mode, loop flushing always working, shooting oil to the motor's casing and over the technician... Anyhow, the signal pressure and the motor's servo-pressures should have been checked. And yes, this situation is different from other machines they have, but further investigation should have been made to pinpoint the exact origin of the problem.

   In fact, in the worst case scenario, the problem might be motor related, but might be not detectable on a test stand. For example, excessive wear of the motor's displacement control spool could be causing self-oscillation of the motor's servo-system. A situation, which is very difficult to simulate on a test rig, as many factors, like pump flow, dynamic torque demands, oil type, you name it, are not repeatable and unique for this particular machine. So it should have been confirmed BEFORE dismounting the motor from the machine. Self-oscillation phenomenae in hydrostatic transmissions are a very interesting topic, which I am going to discuss in a separate article.

    The main point here is, you CAN NOT troubleshoot closed circuits with a bucket. You just can't. You need AT LEAST a pressure gauge, a set of test fittings and, most importantly, you must UNDERSTAND how a closed circuit works to know WHERE to take pressure reading and what conclusions to draw, otherwise unjustified expenses are guaranteed. I do realize that some time ago a friend of a friend of a friend of yours did something like this and it worked, but if you want to succeed on a regular basis, you should rely on more scientific troubleshooting approaches.

   Bucket techniques may be OK for extreme no-other-alternative situations, but they are not suitable not for modern workshop environments. For example, if you were in Africa, 500 kilometers away from any civilization (been there and did this...), and the only tool you had were a monkey wrench, then you could check the motor drain for, say, foaming, if you suspected cavitation, (something, which could very easily be seen by measuring charge pressure in a civilized world), or indeed try to evaluate flow with a bucket, but come on, we're talking about official brand representatives in Europe!

   These situations also prove that training sessions without feedback, even when provided by brand officials, are useless and an apparent waste of time and money (all but coffee breaks, coffee breaks are good).

     Just another very good reason to invest in self-education...