Air/oil static test system used lead weights and air pressure to produce required forces.
The Old Timer of Royal Oak, Mich., was a regular contributor to H&P years before we ever even heard of the internet. But most of his advice is just as useful — and interesting — today.
So rather than leave his wisdom printed on pages archived in our storage room, I pulled out issues from the late 1980s and early 1990s and have been reproducing relevant entries in this blog. Here is my 17th entry, which was originally published in the May 1989 issue:
We’ve all probably looked at a machine or piece of equipment at one time or another and wondered just why the designer chose that particular configuration or that combination of components. Maybe he didn’t have much of a choice. Here’s a case in point:
We all got a rush call to attend a strategy meeting for a big project at one of our testing labs. They had contracted to put together and upgrade 30 old rotary side-loading test machines from a defunct government plant. These were to be used to test new products for another government agency.
On a railroad siding outside sat six freight cars loaded with disassembled original stands. There were racks of air-over-oil tanks, dozens of special cylinders, lengths of hose and tubing, and a multitude of gauges, regulators, valves, fittings, and miscellaneous items. It was a fluid power bonanza — if you could figure out how to use it.
Everything was there except money. This project was very light on money. But if it was a success, there was a flock of similar projects on the horizon and, eventually, money galore. So we were given the green light to go ahead full bore. Our orders: scrounge, convert, rework, and update — but don’t spend any money. Looks are totally unimportant.And, of course, have these machines ready to run in 30 days.
Make it functional, simple, and versatile. You know — a few weights, springs, levers, or what have you. And make each station independent. We don’t want to depend on a central system. Don’t worry if it takes a couple of hours to set up, because we’ll be running the same test for months after it’s ready to go.
Test loads would range from ½ to 500 tons. The force cylinders on the stands had 16-in bores, so we needed a pressure range from 5 to 500 psi. We decided that the pressure had to be hydraulic, for fear that if a test part ruptured the sudden release of the force cylinder could intensify trapped air pressure and possibly destroy the cylinder or our machine. The system used static loading, so we didn’t need 30 power units — which we couldn’t afford anyway. We did have 105-psi supply air and our pile of junk.
We brainstormed through a few dozen schemes — some wild, some goofy, some with merit — and finally came up with the diagrammed arrangement. With valve A open, air through regular B pushes oil into the work cylinder. When the work cylinder has extended up against the test load, continuing oil flow floats the intensifying cylinder up about 4 in, lifting the weight spindle with it. If valve A now is closed to trap the oil in the two cylinders, pressure can be intensified by manually placing weights on the spindle. We provided ten 50-lb weights, and each one increased the force at the work cylinder by ½ ton. Regulator C comes into play ror forces about 5 tons. It’s fairly easy to figure out the combinations of air pressure through C, and weights on the spindle that will produce the desired forces.
You’ve probably noticed that we could have run this circuit with air alone and no weights, but the lab chief didn’t trust the regulators at the low settings for smaller loads. Besides, the weights gave us an easy visual floating reference.
The intensifier cylinder trapped about a quart of oil. Failure of a part produced a fraction of an inch of slow travel. No slams, no bangs, no damage! A nice, quiet, little static system running on low pressure and driven by shop air.
To reset the work cylinder, regulator B was turned down to zero and valve A opened. The system bled down slowly, which was of no concern.
The lab people loved it and, best of all, the government loved it. They eventually sprang for a force strain gauge with digital readout and a printer for permanent records. Our cost was mostly labor plus the lead weights. We eventually put the three valves in a locked cabinet to keep them safe from curious fingers.
Last I head, these units ran for more than five years with little or no maintenance — and no problems.