A call for help came from a local scrap yard, an O&K rotary grabber
(you know, one of them rotary excavators with a rotary grapple
attached instead of a bucket) was "loosing force" as the client
explained on the phone. The main pump had been overhauled recently by "yours truly", so I
"grabbed" my tool box and rushed "to rescue".
I must confess here, that I always get over-excited
every time I go to a scrap yard. It's like a little boy's dream come
true. You just wouldn't believe how many "almost new" good things can
be found at a scrap yard, suitable both for using or smashing... One
could never get tired of watching one of them grabbers "deal" with used
cars, throwing them around like toys. Oh, and then there's the PRESS,
turning scrap into cubes, and lot's of other potentially cool stuff to
do, like the possibility of instantaneous stress relief by smashing an
"about to be cubed", but still operating car into another one, or
conducting an extreme scientific experiment to find how long will a car
engine run if the oil has been replaced with water (no oil, coolant,
etc.). The possibilities are limited by your imagination only....
Anyway, it appeared that the machine did seem
to "lack force" as the main hydraulic double pump was stalling the
engine whenever a rapid movement was made. Clearly it had
something to do with the power management. No hydraulics diagram was
available on site (as usual). After an hour of "hose pulling" it became
clear that the main pump, a classic Hydromatik A8VO double pump,
was equipped with a torque limiter regulator with threshold setting hydraulic
override. The possibility of overriding the main pump's torque limiter
setting is a standard feature for such machines, driven by diesel
engines, to allow the hydraulics torque demand follow the engine's torque
delivery at various rpms. The pilot signal for the regulator was coming
from an electric proportional pressure reducing valve,
hidden under the belly of the beast. The solenoid was OK and
the valve itself presented no excessive wear, however no current was
reaching the solenoid and that's why the pumps were running at their
maximum torque demand, which was more than enough to stall the diesel.
When I told the client the problem was electrical he didn't seem to be
hundred percent convinced, so I had to find a fast way to convince him.
But where could I get a proportional electric
signal on the spot to make the valve work? Wait a minute, it's a scrap
yard, you can build anything from scrap! It was some time ago, so I do
not remember exactly what the coil's resistance was, I think it rounded
25 ohms, which suggested around 1 ampere of full voltage current, so I
knew I didn't need a precise low current regulator, which made
the challenge easy. What I needed was a current source (car battery),
a potentiometer (dash board light regulator from a volkswagen), current
limiter to protect the circuit (a bulb from the same passat's tail
light, which also served as a perfect visual current indicator), some
wires (lots and lots of them around) and that's about it. To reach the
coil operating current band and nice proportionality I had to
experiment a little changing the bulb size and input voltage (one/two
batteries).
It worked like a charm! As the current through
the solenoid was increasing, which was clearly being indicated by the
glowing of the bulb, the pilot pressure was increasing and the torque
limiter setting of the main pump was decreasing, the machine was no
longer stalling! Not only the client was convinced, but he also wanted
to install the new "power management system" (which you can see on the picture)
onboard to avoid calling the electrician. Later on it turned out the
main computer was malfunctioning and replacing it solved the problem
(anything but cheap it was!). As a temporary solution the pump's
torque limiter setting was lowered to the point where the machine would
work normally with the engine at normal operating speed, although it
would still stall the engine at low rpm.
Seeing is believing, my friends, seeing is believing....
