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August 1, 2000

A public radio commentary

There exists a tiny device without which we wouldn't easily have air planes, automobiles, tractors, or air conditioners - just about anything with motion. In fact you use one every time you step on the brake in your car. Its costs fractions of a penny and has made headlines only once. It is the tiny, yet mighty o-ring.

You've likely heard about o-rings during the Space shuttle disaster of 1986, and I'm sure you've seen one, the name is self-descriptive - they are usually just a thin tube of rubber shaped into a ring. It seems so simple and obvious that it couldn't have been really invented - yet it isn't that obvious, there is a trick to making an o-ring work. Usually we think of such a world changing invention as coming from some inspirited young turk, but the o-ring came from a senior citizen, Niels Christensen.

Christensen, a danish immigrant, was an expert on brakes. He'd came to America at age 26 to be the leading draftsman for a Chicago engineering firm, but after a year or two the company reorganized and he lost his job. While unemployed he read of a major streetcar crash where the breaks failed. How, he wondered, could he improve the breaking system. At that time the break shoes were pressed against the wheels by the strength of the conductor, amplified by the electricity that ran the train. The problem: A sudden loss of electrical power and the breaks were out. Chirstensen realized there needed to be a way to store the energy and release it later - to do this the ingenious inventor used air.

Before the car started out Chirstensen used an electric motor to force air into a cylinder, which when released drove the break drums. Because the air was stored and released mechanically it didn't matter if the electricity shut off. How does this relate to the tiny o-ring?

Christensen needed to seal the compressed air cylinder, the seal he used was a cumbersome and tricky triple value; he didn't use an o-ring, but it put the problem of sealing foremost in his mind. Some forty years later in 1933 Christensen, now sixty-eight, was still working on sealing the fluid in breaks - this time though for cars.

He tried this time a simple rubber ring. He cut a groove into his piston, slipped the o-ring over it and pressurized the container; he found, as others before him did, that it failed. If he had been a younger man he wouldn't have had the insight and intuition to continue. Patiently Christensen changed the size of the groove, cutting new ones with slightly different dimensions. In time he found the magic to an o-ring: Make the groove one and half times the o-ring radius. The result was remarkable: "This packing ring", he wrote in his notebook, "has been tested" nearly three millions times and "has never leaked and is still tight."

It was so simple that no one believed it would work until finally two World War Two Army Air Crops engineers used it to fix some leaking breaks on the landing gear of Northrop bomber. It worked like magic and was used in all military aircraft - and soon this simple, but ingenious o-ring seal appeared everywhere: fountain pens, soap dispensers, plumbing systems, hydraulic presses, automobile breaks, washing machines, and hundreds of other places.

Copyright 2000 William S. Hammack Enterprises