In this post I would like to describe a short troubleshooting episode,
which is interesting not only due to the malfunction itself, but also
due to the outmost coolness of the hydraulic equipment involved.
A couple of years ago we were contacted by a
local police department, asking our help with manufacturing a piece of
tactical hydraulic equipment, namely - a hydraulic door breaker, which
was to be used, naturally, for forced entry police situations. A group
of four husky square-jawed men with keen eyes, thin lips and,
strangely, alike black leather jackets, showed up on our doorstep
carrying a black backpack which immediately caught everyone's attention
with it's "weapon-like" aspect. The neat looking rucksack turned out to
be a French made hydraulic door breaker, which consisted of a small
electrical high pressure power pack in the backpack, and two hydraulic
jacks, mounted at 90 degrees to one another. One of the jacks was
supposed to spread wide the door frame, while the other would push the
door/hinges out right to the feet of unsuspecting criminal elements
hiding from the justice behind the aforementioned door.
Being an assault gear it definitely left other trivial hydraulic
rescue equipment in the shade. I confess that through years of dealing
with hydraulics I've had my share of toying around with all kinds of
fireman spreaders, cutters and rams, but nothing of what I'd seen
before came close in coolness to this tool, which, by the way, was
extremely well built. The idea was to create an alike device using
common in-stock hydraulics, making it ten times cheaper than the
"original prototype", which proved to be effective but which also blew
a large hole in the small police station budget. We were supposed to
supply the parts and hydraulic guidance, while the police workshop
would take care of the construction and, of course, field tests.
The quick and painless solution was found in pairing a
couple of standard high pressure single action spring-retracted jacks
with a small 12 volt power-pack, connected to the jacks through a
couple of solenoid operated directional valves. The parts were
supplied, all the necessary hydraulic advice was given, and with our
part of the deal done we were left wondering if the machine would
really work and how soon would the jacks wear out...
About a month later we were contacted by the station
workshop personnel, complaining about the directional valves that
wouldn't work in one direction. According to them, sometimes the jacks
would extend fully, but sometimes they would move only a couple of
inches and then stop. The retraction of the cylinders was working fine
and hadn't failed a single time. It was, naturally, a concern for them,
because a stuck tactical door opener in the middle of a forced entry
operation resembles pretty much an erection lost half way through sex -
it is annoyingly embarrassing and it also makes all the previous
preparations a waste of time, so we kindly asked the mechanics to bring
the finished gizmo over for an inspection.
The first glance at the custom-built device revealed
that while it was lacking the perfect finish and factory built look of
the "original prototype", it was undoubtfully far more intimidating -
the rugged steel back frame with a power-pack and a motorcycle battery
inside, along with directional valves, hoses, and all the wiring made a
person wearing the backpack look like a futuristic exoscheleton
equipped soldier, with the massive hand-held double jack combination
contributing even more to the illusion. I am more than sure that the
psychological impact alone would make most criminals instantly
surrender to the backpack-bearer, in certainty that the Iron Man
himself came along with the force to hunt them down!..
Indeed, the problem was there - the jacks
would move a little, and then would stop, the motor would remain
running but there was no jack movement at all! On suspicion of
directional valve malfunction the directional valves were disassembled
and inspected - no damage was found and the valves did perfectly normal
on the test stand. However, when re-mounted on the backpack the
situation repeated itself - all we could get was a couple of inches of
jack travel, and since we were sure the directional valves were
functional, we had to look for the malfunction somewhere else.
I don't want to bore you with fault finding
details and will go straight to the cause - the failure was being
caused by the tension drop, which in its turn was being caused by the
electric motor of the power-pack. The motor, despite being relatively
small, drew a decent amount of current, and since the motorcycle
battery used to power the tool was relatively small and relatively
"used", the tension in the circuit would quickly drop below the level
sufficient to maintain holding current of the directional valve
solenoid, yet it was still enough to keep the motor running. Since the
retraction of the cylinder was made by means of a spring, the motor
wasn't running during the retraction and the voltage wouldn't drop,
making the retraction cycle problem-free.
To solve the problem either a more powerful power
source (a bigger battery, or a similar battery connected in parallel
with the existing one) or a smaller motor (with less current draw) had
to be applied. Since the men didn't want to compromise the
door-breaking speed, they opted for a bigger battery, and the tool had
been breaking doors trouble-free ever since.
The technical lesson behind the described case
is simple - a special care should be taken while choosing a power
source for mobile battery powered hydraulic equipment that uses
solenoid operated valves or any other additional electric/electronic
gear, seeing that the tension drop caused by the motor's high current
consumption can be significant. The battery should be adequately sized
to ensure that the tension under no circumstances drops below the level
necessary for correct operation of solenoid valves and/or any other
"onboard" electric equipment.
