InsaneHydraulics - Sergiy Sydorenko © 2009-2011 All Ridghts Reserved
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
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
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
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
Just another very good reason to invest in self-education...