Once upon a time a client from far far away sent over an A2FM32/61W
Rexroth hydraulic motor to repair. The motor was used in a simple open
circuit to run a big wood cutting saw. Local mechanics had already
opened the motor and verified that the unit was in bad shape, so they
joined all the pieces and reached for "professional" help.
Quick verification showed that, in fact, the motor
was pretty much destroyed, and so the complete rotary group had to be
replaced - the cylinder block, the pistons, the central pin, the
retainer plate and the valve plate, topped up with a seal kit. The
motor was assembled, tested and sent to the client.
Not a day passed when the man called claming
the motor was not working properly - it would run fine for about a
minute, sometimes less, and then would stop. To set it again in
motion the machine had to be stopped and restarted. Suspecting
bad start-up, contamination, mounting error, component failure, etc...
he was advised to dismount the motor and send it over (the guy opted
for a more than 350 km long drive and brought the motor himself,
because he really needed the saw running!) for verification.
When the motor was disassembled, the rotary group
appeared to be in perfect condition, apart from some minor scoring of
the valve plate and the cylinder block, all the parts looked as if
they'd just come from the shelf. Even the client got puzzled, because
he was expecting to see a major failure. He was claiming that the
motor had stopped rotating completely, and was also positive that the
rest of the hydraulic circuit was working fine. As no damage was
detected, the motor was reassembled, retested in front of the client,
showed perfect efficiency and the client was advised to look for a
problem elsewhere in the circuit of the saw.
The man took the motor back (another 350 km stretch)
prepared to look for a malfunction. He was calling the next day to say
the motor was stopping the same way as before. A couple of minutes of
service - that's all he could get from the saw, then the motor would
stop. The man had a happy idea to check the motor drain line, and when
the motor stopped the line presented HUGE increase in flow, so it had
to be the damned motor!
Although the poor fellow was not happy about the
hours he'd spent behind the wheel, he had no choice but to bring the
motor over once again. The motor went through the same procedure as
before - it was disassembled, no wear was detected, it was assembled,
tested - and worked OK. At this point yours truly was asked to give it
a glance. It would be really ugly for the man to have to go through the
same trouble again...
There's nothing I like more than "puzzle solving",
especially when it has something to do with hydraulics. Not only the
problem had to be nailed, a way to convince the client that the problem
was detected and solved had to be found, which could only mean
one thing - creative testing! (Hurray!!!...)
A few words about how the previous tests were
made. It was a fixed displacement motor with no loop flushing block, so
the first time only a very basic test was performed - the motor was
connected to a test bench distributor, and then a needle valve was used
to restrict the flow coming out of the motor to raise the pressure
inside the rotary group, then the case drain flow was evaluated to come
to a conclusion if the rotary group had excessive leakage (simplified schematics).
Although this test is not perfect, it is a fast way to check for rotary
group excessive case leakage and, also, verify the existence of
external leaks. The second time the motor was mounted on a test bench,
which used another hydraulic motor to create braking torque, again the case
drain, entering flow and the RPMs were being monitored, different
speeds and pressures were applied and yet again the motor was performing
perfectly - as any decent motor that'd just gotten a rotary group
replaced would. Obviously the workshop test was not recreating
the real life working conditions.
At the machine, the motor was spinning a massive
wood saw, which had tons of inertia, there even was a system of
anti-cavitation check valves installed to compensate for the
blade over-run when the distributor would decrease oil supply. To
simulate that condition, I coupled the motor to a 30 KW electric motor,
to use the electric motor massive rotor to simulate the heavy blade
overrun conditions. The hydraulic motor was still being run by the
first test bench.
During the previous tests the case drain had always
been insignificant, very low, so it had to be evaluated in a very
scientific "by eye" manner. This time was no different, the case drain
hose was hanging freely from side of the motor. I started the pump, and
slowly moved the distributor handle to set the hydraulic motor/electric
motor assembly in motion. As I was increasing and decreasing the speed
of the motor, there was a moment when it suddenly lost all the torque
and the drain line, instead of giving out a match thick trickle was
pouring out a serious jet, having immediately set the standers-by
fleeing. There it was, the malfunction, staring us in the face.
I like to say that successful remake of a problem in
workshop equals solving the problem. There it was - all the oil
entering the motor and ending up in the casing. However, stopping the
flow and the motor, and then starting it over again slowly made
it run fine. And then when I decelerated it abruptly, and then
accelerated - the case drain boosted up once again... The first and
most obvious explanation would be the cylinder block lift, but how?
With new original parts? Well, let's tear it down once again!
All the parts were checked, the valve plate was a
perfect match to the cylinder block, the disc springs (or belleville
washers) were there, wait a minute, what's this? It became clear at
once what the problem was! The mechanic who'd mounted the motor had
mounted only four bellville washers (the nasty A6VM habit), while this motor required six! As you can see from the picture, four washers, even when stacked in an alternating direction, are lower than
the guide washer. Even with new parts, the cylinder block was not
sufficiently pre-loaded against the valve plate, It was merely touching
it, allowing it to pass all the tests, as the steady system pressure
would hold the hydrostatically compensated cylinder block compressed
against the valve plate.
When the massive saw blade was over-running the oil
supply, low pressure was being created in the motor line ( the motor
working as a pump), and as the cylinder block was not sufficiently
pre-loaded, the case pressure was enough to cause the cylinder block
lift. Once lifted, the block would stay separated due to the
passing oil flow and wouldn't settle until the the oil supply stop.
The test was a classic cylinder block lift
situation. It was so classic that I even made a video of it (for future
didactic purposes), which was then successfully erased from the
camera by a colleague of mine (thanks a lot!). The problem was solved
by adding the two missing washers. By the way, there are different ways
you can stack the washers. Rexroth advises to stack them in the following manner.
I know from experience that these motors run fine when all the six
washers are stacked in an alternating direction, so if you mounted one
like this, don't beat yourself about it. Stacking the washers in
parallel does create a stiffer spring, though. Belleville washers are
the old design, all newer series use coil springs, which are better.
What does this prove? It proves that
pre-loading of a cylinder block is not a mandatory condition for high
volumetric efficiency. A well designed cylinder block is held
compressed against the valve plate by the system pressure, so the whole
pre-load spring system is only necessary to guarantee that it
stays compressed when there is no hydraulic force condition, which
happens at start-up, or, as we've just seen, during the overrun, when a
lower than the case pressure develops in working lines. Of
course, there are also vibrations, pressure surges and peaks, extreme
velocities, temporary low oil viscosity, and many other things
contributing to block lift and making the pre-load system necessary.
But at a steady speed and pressure you can completely remove the
pre-load springs, and the cylinder block will remain adequately
compressed against the valve plate by means of the system pressure only!
Understanding this phenomenon as well as
understanding what a cylinder block lift is and how it can manifest
itself is very important for correct troubleshooting. It is also
critical to be aware of the fact that standard workshop hydraulic tests
are nothing but a good approximation
to real life conditions, therefore some malfunctions are very difficult
to recreate in a workshop. In these situations a good technician should
be able to think outside the box and be ready to perform "creative
testing" to nail the problem.
