Insane Hydraulics
Bold and audacious blog about fluid power
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 sheer coolness of the hydraulic equipment involved.
A couple of years ago we were contacted by a local police department, asking for our help with designing a piece of tactical hydraulic equipment, namely - a hydraulic door breaker, which was to be used, naturally, in 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 "military" aspect. The neat-looking rucksack turned out to be a French-made hydraulic door breaker, which consisted of a small high-pressure power-pack placed in the backpack, and two hand-held hydraulic jacks, fixed at 90 degrees to one another. One of the jacks was supposed to spread the door frame wide, while the other would push the door out - right to the feet of unsuspecting criminal elements hiding from the justice behind the said door.
Being an assault gear it left "trivial" backpack based hydraulic equipment I had known before in the shade. 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 a similar device using common in-stock components, making it ten times cheaper than the original, which proved to be effective but which also blew a large hole in the police station budget. We were supposed to supply the parts and provide hydraulic guidance, while the police workshop would take care of the construction and, of course, testing the finished product "in the field.
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 tactical door opener stuck in the middle of a forced entry resembles pretty much an erection lost halfway through sexual intercourse - it is annoyingly embarrassing and it also makes all the "preliminaries" a waste of time, so we kindly asked the mechanics to bring the finished apparatus 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", it was undoubtedly far more intimidating! The rugged steel back frame with a power-pack and a motorcycle battery inside, along with the directional valves, the hoses, and all the wiring made a person wearing the backpack look like a futuristic exoskeleton, 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 the certainty that the Iron Man himself came 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 well on the test stand. However, when they were 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 OK, 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 cylinders were spring-retracted, the motor wasn't running during the retraction cycle, making it 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 ever since.
The technical lesson behind the described case is important - special care should be taken while choosing a power source for battery-powered hydraulic equipment that uses solenoid-operated valves or any other additional electric/electronic gear, given that the tension drop caused by the motor's high current consumption can be significant. The battery (and the wiring) should be adequately sized to ensure that the tension under no circumstances drops below the level necessary for the correct operation of solenoid valves and/or any other "onboard" electric equipment.