When I was writing the article on the new pump control invention
for a John Deere forester, I remembered the following troubleshooting
episode, which happened a long time ago, but still has a good point.
A client appeared in the workshop with an open
circuit pump from a forest crane. I do not remember what model of crane
it was, all I remember that it was Timberjack (at the time), and the
pump was Hydromatik A10VO something, maybe 140, good old series 3. The
client was convinced that the pump was damaged. The main symptom was
the crane movements becoming slow as the oil heated up. The man had
worked with hydraulics for a long time, and was pretty much sure that
the problem was being caused by the pump's low efficiency, with the
internal leakage aggravating with hotter and, therefore, more fluid oil.
The pump was opened and no serious damage or wear
was detected, so it was reassembled and tested, presenting good
efficiency and controllability on the test stand. However, when the
pump was installed on the machine the next day, the crane displayed the
same symptom - movements slowing down as the oil was heating up,
becoming very slow eventually and making work with the crane impossible.
The pump was dismounted, brought to our workshop, striped
down, and yet again nothing wrong beyond normal wear was detected. Once
again, the pump was reassembled, retested and showed good efficiency
and correct control function (pressure limiter and load sensing). When
the pump was remounted on the machine nothing changed, the crane
movements kept on slowing down as the temperature rose. This time the
man asked if I could "take a look", so I grabbed my gear and went to
see what was going on.
The hydraulic circuit of the crane was of a closed
center load sensing type. The main distributor valve was electric
proportional. Comparing the pressures at the pump's outlet and the load
sensing line showed steady delta P of 30 bars when the movements were
getting slow, even when the joystick was "pedal to the metal". Clearly,
something was throttling the oil between the pump's outlet and the load
sensing signal port. The most obvious explanation for this would be
partial stroke of the distributor spools. Further digging into showed
that the pilot pressure reducing valve, supplying oil for the
proportional solenoids, was malfunctioning, causing the pilot pressure
to drop significantly with the rise of the oil temperature. There was
simply not enough pilot pressure to make the spools go full stroke, and
the pump's load sensing system was sensing the pressure drop through
the partially opened spool, as it should, and compensating for it with
the reduced flow. To tell the truth, I don't remember what exactly was
causing the valve to malfunction, but I do remember that the problem
was solved "on the spot" and the crane's normal operation was restored.
If you look more carefully into this case,
you'll see that there is a major flaw in the troubleshooting conclusion
that the problem was caused by the pump's low efficiency. Indeed,
it is absolutely plausible for an worn or damaged open circuit pump to
be able to perform much better with cold and more viscous oil, and fail
to produce sufficient outlet flow with hot and more fluid oil, BUT, in
such situations another major symptom would have been detected - SEVERE
OIL OVERHEATING! When we loose significant part of flow to leakage, all
the energy, spent to produce it, will be transformed into heat. So, if
the pump indeed were worn or damaged, and the SIGNIFICANT loss of speed
were connected to leakage losses, the oil temperature would have been
much higher than usual.
In this case the problem was starting to occur when the oil temperature was only getting close
to the normal operating temperature, without further overheating, thus
putting the low efficiency theory under question. I know it because the
machine was equipped with an oil temperature gauge and an alarm, going
off at 70 C, which is common for Timberjack/John Deere foresters, and
is something I consider a "must" for any piece of hydraulic equipment.
This is just another example of how important it is
to, at least, know what a normal operating oil temperature of your
hydraulics is. It is, by far, the most important troubleshooting reference point.
This is also a good example of how precipitated
troubleshooting conclusions can easily cause unnecessary downtime. It
is always a good practice to double-check your theories whenever it is
possible.
