This short post is a mere reminder
to all that hydraulic machines can have more
than one hydraulic oil tank, and that not all double gear pumps are
internally connected at the suction side.
The first time I came across one of these was in
the field, naturally. It was an agricultural tree
planting trailer, equipped with sophisticated hydraulic leveling system that
ensured that all trees were planted vertically regardless of the
terrain, and also a combination of jacks to move the whole planting assembly
up and sideways. The hydraulics was powered by a small double gear pump
and the detailed schematics was available, showing the two pumps, two
independent oil distributors and one oil tank.
In the course of troubleshooitng process I required to
test some priority valves I suspected were
malfunctioning. During the test I connected a needle
valve to the outlets to simulate loaded condition. The tank port of the
needle valve had to be connected to the tank, of course, and the
easiest and the fastest solution was to stick a hose into the
filling cap. Don't laugh now, the expected flow rates were of around
2-5 liters per minute ( a gallon), so the oil shower risks were minimum,
and, as the flow was so low, I actually had to SEE the oil stream to
evaluate flow rate.
I turned the clutch on, and concentrated my attention on the
pressure gauge as I was raising the pressure. When I looked up a
couple of seconds later, I saw, to my GREAT SURPISE, the oil poring out
quietly (sneaky bastard!) from the completely topped up oil cap! I
rushed to the controls and disengaged the clutch to stop the circus, but
all too late - the frame, valves, hoses and the
ground underneath the tank got all covered with oil! And I
HATE cleaning-up oil spills!
When I calmed down and looked at the machine with the "seeing" eyes, I saw that it was equipped with another oil
tank, and the double gear pump had a separating oil
retainer in the intermediate plate. The two separate circuits were
working with two separate oil tanks, and during my test I was using the wrong oil
tank to dump the oil... I thanked out loud the good engineer who'd
made the schematics, clearly showing ONE oil tank, and then slapped
myself in the face (not literally) for not paying attention.
This happens a lot, I mean people assuming
that double gear pumps always work from the same oil tank. The reason for it
is simple - most of the times, 99 out of 100 maybe, they do, and the
standard, most common double pump assembly has suction sides of the
pumps connected internally. But then there's that one case when they
don't, and if you miss it and, say,
supply a "normal" double pump instead of a "separated" one, or
make a test like I did, you end up
transferring hydraulic fluid form one tank to the other.
Further consequences may vary form disastrous
to hilarious...
That case taught me to pay additional attention to
all double pumps and systems using those pumps, and also to always
check if the supplied schematics corresponds to the reality. So
whenever you come across a double (triple) gear pump with the
suspiciously "fat" intermediate plate, check if the system has more
than one oil tank - consider this as another tip to keep you from unexpected hydraulic
trouble.
