A few days ago I had an interesting conversation with a man, who'd
brought in a "broken" Sauer Danfoss series 51 motor, which was a part
of hydrostatic transmission of a street sweeper (one pump/two motors,
one motor for each side, hydraulic proportional controls). Apparently,
the mechanic, who had labeled the motor as problematic, had years of
experience and even had some 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
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 an overhauled motor should.
The "troubleshooter" was contacted on the subject, which lead me to writing
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 the 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, 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 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-deserving! "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 a classic closed loop
arrangement, with charge pump and charge pressure relief valves
incorporated in the main pump, and loop-flushing incorporated in the
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 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
from the case.
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 bath. 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 by 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 technical troubleshooting approaches.
Bucket techniques may be suitable for extreme
no-other-alternative situations, but 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
situation, (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 no-feedback training
sessions, even when provided by brand officials, are useless and an
apparent waste of time and money (all but coffee breaks, coffee breaks
Just another very good reason to invest in self-education...