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The Hurdy-gurdy

Summer 1994 | Volume 10 |  Issue 1

“This is a replacement human hip part,” says Michael Coladonato, showing me a curved metal bar about eight inches long shaped so gracefully it looks like sculpture. “We made it of a cobalt-chrome-nickel alloy called MP35N. And these things over here hold the space shuttle together. You spend a lot of money for these. We’re the world leader in precision fasteners.”

Michael Coladonato, the supervisor of contract research for SPS Technologies, in Jenkintown, Pennsylvania, runs an in-house lab that does independent testing both for SPS’s products and for other companies as well. “Let me show you this machine we used to test our space-shuttle fasteners,” he says.

We walk through a wide, airy, immaculate room full of large, modern equipment—fatigue testers, spectrographs, even a scanning electron microscope—to an iron, steel, and brass machine from another era. “We call this the hurdy-gurdy,” he says. He’s not sure of the reason it’s called this: possibly because it works by turning a crank, makes a lot of noise, and is very old. “It’s our most powerful torsion tester,” he remarks. He rests a hand on its rounded iron shoulder. “Notice that plate on it there.”

The brass plate lists four dates for patents covering the machine, the earliest 1879, the most recent 1891. “We think it was actually built in 1906. But last week it broke this bolt for us, which holds the rocket-motor sections together on the space shuttle.” The four-hundred-dollar advanced-alloy bolt—about five inches long and surprisingly heavy—had fractured just above where it screws into a nut.

“How the hurdy-gurdy works is really very simple,” Coladonato explains. “You see these two iron arms reaching across from either end of it? You attach something between them, and this arm twists it while the other holds it still. You could twist this pen here with it, for instance.” He points to his shirt pocket. “In fact, we did some work for Mark Cross pens a few years ago. That rule across the top shows how much torque you’re applying.” The brass rule is engraved with numbers indicating inch-pounds, from zero up to 60,000. An iron truck riding on a screw along the top of the rule points down to indicate the reading.


“That 60,000 inch-pounds is quite a bit of torsion. The most powerful V-8 automobile engine you can buy, in a Cadillac, produces about 480 footpounds. That’s less than a tenth of this. But all this machine is, really, is an electric motor geared way down and attached to an extremely rugged base and accurate measuring device. Watch.”

He presses a button, and the motor housed in one end turns over with a rumbling, grating sound. The iron arm from that end very slowly rotates, more slowly than the second hand on a clock, at 0.8 rpm. “When a piece is attached,” Coladonato says, “the stationary arm here will visibly rock slightly. That force is transferred via a series of moment arms resting on knife edges down to another arm across the bottom, and to the top, where the measurement reads out.”

“The beauty is that this machine is dead accurate up to 30,000 inchpounds and only a little less accurate above. We’re at the practical limit, within 2 percent. That’s why we used it to test for NASA—that and the fact that a machine like this would cost us about two hundred dollars an hour to rent time on or maybe a hundred thousand dollars to buy new. We bought this one used in 1957 for $2,333. We got it from the Tinius Olsen Company, which made it.” He pushes a button to flip off the power, and the ancient motor rattles to a stop.

“The NASA work came out of the Challenger disaster. After that, new designs and new tests had to be imposed on many parts to ensure that no such thing could happen again. An outfit called Aerojet got the job building the two rocket motors, and here at SPS we make the fasteners that hold those motors together.” He picks up the broken nut and bolt again. “This sample gave out well beyond its minimum required torsional strength of 36,000 inch-pounds, so it’s very strong. The very best bolt you could buy retail might handle 20,000 inch-pounds.”

How do you feel testing crucial aerospace components on a machine built three years after Orville Wright first got his wood-and-cloth Flyer to lift off a North Carolina beach? “You feel confident,” Coladonato says. “When we started making these NASA fasteners, the hurdy-gurdy became an essential part of torsion testing. We were using it weekly for a few years there. We’ve beaten the devil out of it too, as you might imagine. But the only repair it has ever needed is a replacement clutch lining in the motor drive. You know, I ask myself, for a purpose like this, why did anyone ever stray away from this simple, brilliant design?

“And you know, I don’t have an answer.”

We hope you enjoyed this essay.

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