In 1933 the 68-year-old inventor Niels Christensen finally tackled a problem that had bothered him throughout his long career in hydraulics. His elegant and simple solution—the O-ring—would become such a ubiquitous part of so many technologies that it is present by the dozens in every home and car, and applied to everything from fountain pens and soap dispensers to hydraulic presses and bomb-bay doors.
Forty years earlier, a horrible streetcar accident in Oak Park, Illinois, had turned the Danish-born inventor’s attention toward improving braking systems. The street railways of the time used the conductor’s muscle power, amplified by the electricity that ran the cars, to squeeze brake shoes against the wheels. When bigger cars appeared in the 1890s, this system proved inadequate, especially because the loss of electric power often caused the sudden braking in the first place. A means of storing energy was necessary; Christensen decided that compressed air, already used to brake railroad trains, was the most promising solution.
He created a compressor and electric motor entirely enclosed within a metal case, its moving parts surrounded by a bath of oil. (By contrast, railroad air brakes used a mechanically driven compressor rather than an electric motor.) In 1895 and 1899 Christensen secured patents for the sealed motor-compressor combination and a special triple valve to control the flow of compressed air. His first big sale came in 1897, when Frank Sprague chose Christensen braking systems for his South Side Elevated in Chicago. Soon they were being used nationwide and in Europe.
Despite his success developing hydraulic brakes, Christensen remained frustrated by the lack of a simple, reliable seal that would let a piston slide easily but block the flow of fluid. Rubber rings had been tried before, but they tended to wear out quickly. Christensen, now working for Midland Steel Products of Cleveland, decided to experiment with using different dimensions of rubber rings and explore different configurations of the grooves that they sat in.
His methods of research were not particularly refined. “He’d put a ring through a test, then look at it under a magnifying glass to see where it was scratched. No complicated analysis at all,” says his grandson Niels Owen Young.
What he finally settled on was a ring with a circular cross section, in a groove approximately one and a half times the ring’s radius. With the groove sized appropriately, the results were remarkable. “This packing ring has been tested for 2,790,000½” strokes at 600 psi and 2,790,000 return strokes at atmospheric pressure,” Christensen wrote in his notebook. “This packing ring never leaked and is still tight.”
It is unlikely that Christensen knew why that particular combination of groove and ring size worked. His 1937 patent application speaks of the ring’s being “continuously kneaded or worked to enhance its life,” as if the rubber ring could get stronger through exercise, like a human muscle. This is the opposite of what actually happens to the rubber. The real mechanism behind the O-ring’s action was discovered in 1941 with transparent cylinders and slow-motion photography by researchers at Vought-Sikorsky and Lockheed Aircraft.
With the groove wider than the ring size, the ring can roll about 20 degrees when pushed from one side. That rolling motion deposits a thin layer, perhaps one ten-thousandth of an inch, of hydraulic fluid between the rubber and the cylinder wall. The fluid lubricates the subsequent motion of the ring as it first squeezes against the end of the groove and then slides along with the piston, protecting the ring from wear and substantially lengthening its life. The width of the groove is critical: if it’s too narrow, the ring can’t roll and doesn’t get lubricated; if it’s too wide, the ring rolls too much and quickly wears out.
Remarkably, Midland Steel wasn’t impressed with Christensen’s advance. A year later they fired him. Through the remainder of the 1930s he tried to interest manufacturers in his new seal. A few tried it, as much out of curiosity as anything else. But for the most part the O-ring went nowhere.
Then came World War II and President Franklin D. Roosevelt’s defense buildups, which required the construction of tens of thousands of airplanes. Each aircraft needed hydraulic systems controlling doors, landing gear, and surface actuators—millions of hydraulic shafts would need to be sealed. Christensen wrote the Army Air Corps for an interview, loaded his car with O-rings, and drove across Ohio to Dayton’s Wright Field (now Wright-Patterson Air Force Base) in June 1940.
Nicholas Bashark and Elsworth M. Polk, the Army Air Corps engineers in charge of hydraulic seals, immediately agreed to try O-rings. They installed some on the worn, rusty landing gear on a Northrop A-17A. The seal held up through 88 bumpy landings. They ran controlled, quantitative laboratory tests, and the results were just as good. Bashark and Polk were convinced. Within two years the O-ring had been specified as the seal of choice in virtually every application where it could be used. The O-ring would save millions of dollars, and perhaps lives as well, by making hydraulic seals simpler, cheaper, and more reliable.