Insane Hydraulics

Site theme image

Solenoid Valves, Hydraulic Jacks, Low Voltage and Broken Doors

In this post, I would like to describe a short troubleshooting episode that is interesting not only due to the peculiar malfunction but also due to the sheer coolness of the hydraulic equipment I had the pleasure of working on.

A couple of years ago, we were contacted by the local police department, asking for our help with designing a piece of tactical equipment, namely - a hydraulic door breaker for forced entry interventions. 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 that immediately caught everyone's attention with its "tactical" appearance. The neat-looking rucksack turned out to be a French-made hydraulic door breaker, which consisted of a small high-pressure HPU 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 criminal elements hiding from the justice behind the said door.

I've had my share of playing around with portable rescue spreaders, cutters, and rams, but nothing of what I'd seen before came close in coolness to this tool, or better - piece of assault gear, which, by the way, was extremely well built. So, the idea was to create a similar device using off-the-shelf components, making it ten times cheaper than the original, which proved to be effective but which also blew a large hole in the station's 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, the "men in black" would be in charge of testing the finished product "in the field."

The final solution consited of two standard high-pressure single-action spring-retracted cylinders connected to a small 12-volt power-pack through 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 how soon would the jacks be wearing out...

About a month later, however, the station workshop personnel reached out to us with a complaint 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 big concern because a tactical door opener stuck in the middle of a forced entry is like an erection lost halfway through 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 "apparatus" over for an inspection.

What a sight to behold it was! The custom-built solution lacked the perfect finish and factory-built look of the original breaker, but it was undoubtedly far more intimidating! The rugged steel frame with an exposed power pack and a motorcycle battery under it, along with the directional valves, the hoses, and the wiring, made a person wearing it look like a futuristic warrior wearing exoskeleton armor, with the massive hand-held double jack combination contributing even more to the illusion. I, personally, thought that most criminals would instantly surrender to the backpack-bearer, in the certainty that the Iron Man himself came to bring them down!

And, indeed, the problem was there - the jacks would open up a little and then stop. The electric motor was whining, but the cylinders would not move. The directional valves were disassembled and inspected but no damage was found. The valves did perfectly well on the test stand, but when they were re-mounted on the backpack - all we could "squeeze out" was a couple of inches of cylinder travel at best! It was clear that 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 DCV failure was happening due to the tension drop caused by the electric motor. The motor drew a decent amount of current, and since the motorcycle battery that powered the system was relatively small and, as it turned out, "relatively used" as well, the tension would quickly drop below the level sufficient to maintain the holding current of the DCV solenoids. And since the cylinders were spring-retracted, the motor wasn't running during the retraction cycle, which is why the problem only appeared during their extension.

There were two ways to solve the problem - a bigger battery or a smaller motor, but since the men didn't want to compromise the door-breaking speed, they opted for the bigger battery, and the tool has been breaking doors problem-free ever since.

This case is a nice illustration of why special care needs to be taken while choosing a power source for battery-powered hydraulic equipment that uses solenoid-operated valves, given that the tension drop caused by the motor's high current consumption can be significant. The battery (and the wiring) should be 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 components.