Niels Christensen invented a basic component of modern industry—the O-ring—entirely alone and without fully understanding how it worked
Often the most useful inventions are the simplest. Uncomplicated, functional solutions to common needs are the essence of good design. The safety pin and the paper clip, for instance, are as ubiquitous as they are elegant. A less visible but equally important example of classic simplicity is the ordinary O-ring.
There are dozens of O-rings in every home and car, and they have been applied to everything from fountain pens and soap dispensers to hydraulic presses and bomb-bay doors. Total usage is in the billions. Their misapplication in the space shuttle Challenger led to a tragedy and focused national attention on this essential element in modern industry.
The O-ring’s somewhat redundant name does not just refer to the doughnut of hard rubber itself, which has been around in various forms for more than a century. It includes the ring’s housing as well, because what makes the O-ring so useful and effective is the placement of that doughnut in a straight-sided groove having a width roughly one and a half times the ring’s diameter. This simple combination is unsurpassed as a seal for pistons sliding in cylinders under the pressure of a fluid.
The width of the groove is crucial. If it is too big or too small, the seal will not be as effective, the piston will not slide as smoothly, and the ring will wear out much faster. The proper proportions were discovered in 1933 by an independent inventor in his late sixties named Niels Christensen. He had spent much of his life battling big corporations over the rights to his earlier inventions. Reaping the profits from the O-ring would prove nearly as difficult. After years of trying to get his new hydraulic seal accepted, Christensen saw it play an important role in America’s World War II effort. Yet it wasn’t until years after he died in 1952 that the government finally settled his claims. His story shows that for many independent inventors, getting credit for an invention can be more difficult than developing it.
Niels Anton Christensen was born in 1865 on a farm in Toerring, Denmark. On that farm he found a workshop with blacksmith’s and woodworking tools, which he used to build a spinning wheel, a windmill, and a gyroscope. At fourteen he was apprenticed to a machinist in the town of Viele. Four years later he completed his apprenticeship, entered the Technical Institute of Copenhagen, and began work as a journeyman machinist and draftsman. He helped make Denmark’s first Maxim gun and drew some of the plans for its biggest lighthouse. Eventually he became an engineer in the Danish navy. While touring the machine shops of Britain he jumped ship in 1888 to become a draftsman for the navel architect H. W. Culliford at Newcastle, on the Tyne River, then the world’s center of advanced power engineering.
Late in 1891 a Danish friend paid his passage to America, where he became “leading draftsman” at Fraser and Chalmers in Chicago, a manufacturer of machinery for industry, mining, and transportation. Within a year a reorganization cost him his job. He worked briefly on electrical systems for Chicago’s Columbian Exposition and then was hired by the E. P. Allis Company of Milwaukee. At the same time, he began looking around for other opportunities to apply his skills and training. A major 1893 streetcar crash in Oak Park, Illinois, turned his attention to improving the brake systems then in use.
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. With the bigger cars coming into use in the 1890s, this system was proving increasingly inadequate, especially since a loss of electric power was often the reason sudden braking was needed in the first place. A method of storing energy was necessary, and Christensen decided that compressed air, already used to brake railroad trains, was the most promising.
His design used a compressor and electric motor totally enclosed in a metal case, the 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 he secured patents on the key elements of his system: the sealed motor-compressor combination and a special triple valve to control the flow of compressed air. With financial backers including a Milwaukee bank president, he organized the Christensen Engineering Company of Milwaukee to produce and market the systems. 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.
Success brought disagreement with the company’s owners. They wanted to expand into manufacturing electrical equipment, while Christensen wanted to stick to compressors. Christensen’s uncompromising manner didn’t help smooth things over. “He had a very precise idea of what was true and what was not,” says his grandson John Young. “He was blunt and direct. There were people who were afraid of him.” In the end he left the company, taking with him some preferred stock and a 5 percent royalty on his patents.
This arrangement provided him with a nice income for a few years, but then in 1905 National Brake, the successor to Christensen Engineering, went bust following its president’s involvement in a failed attempt to corner the wheat market. Eventually Westinghouse interests got control of the bankrupt company and reorganized it as National Electric and Brake. It went back to manufacturing Christensen’s braking systems but did not recognize the validity of his patents. Thus began, in 1905, a twenty-five-year court battle.
Christensen licensed his patents to a competitor, Allis-Chalmers (which had been formed by the merger of two of his old employers), on the condition that it sue National Electric and Brake for patent infringement. He refused all offers to settle for anything less than complete vindication. “I won’t say that Grandpa Chris didn’t know what compromise was,” says John Young. “But if he thought he was right, that was the end of it.”
Christensen and his family learned to curse the name of Westinghouse, as the country’s largest manufacturer of electrical equipment used all the money and resources at its disposal to contest the suit. Westinghouse’s main argument was as follows: When Christensen first filed for a patent on his motor-compressor combination, the Patent Office mistakenly included a drawing from another patent. To rectify the error, a new patent was issued under a new number. Westinghouse charged that this “double patenting” invalidated Christensen’s claims.
This flimsy pretext, carefully kept alive with Westinghouse lawyers and money, sustained the giant corporation’s fight for two and a half decades. Finally, in 1930, came the vindication for which Christensen had held out so long: he got a $325,000 payment and complete control of his patents. But the victory was somewhat Pyrrhic, because by that time the patents expired, streetcars were disappearing, and the Great Depression had begun.
While the fight with Westinghouse was going on, Christensen continued to come up with inventions, all of them based in some way on pneumatic or hydraulic power. He developed a line of gasoline engines for farms, starters for automobiles and airplanes, and new braking systems. His earnings were enough to support his family comfortably, and by 1930 the Westinghouse settlement seemed likely to be the capper of a long and moderately distinguished career. But his most useful invention by far was still to come.
In 1926 Midland Steel Products, of Cleveland, Ohio, had hired Christensen to develop automobile and truck brakes. He was working in its lab in 1933 when he decided to attack a problem that had bothered him throughout his career in hydraulics: 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 too quickly. Christensen decided to experiment with different dimensions for the ring and the groove it sat in.
In keeping with the practical nature of his training, Christensen’s 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. When he got the size of the groove right, the results were remarkable. “This packing ring has been tested for 2,790,000 ½” 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 sizes 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.
Essentially, the groove wider than the ring size allows the ring to roll by about twenty degrees when it is pushed from one side. That rolling motion lays down a very thin layer, perhaps one ten-thousandth of an inch, of hydraulic fluid between the rubber and the cylinder wall. The thin layer of 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. This lubrication protects the ring from wear and thus lengthens its life. The width of the groove makes all the difference. 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.
Whatever the mechanism behind it, Midland Steel wasn’t impressed with Christensen’s advance. The manager of its lab had different ideas about hydraulic seals. Again, Christensen’s lack of diplomacy didn’t help. His attitude toward those he felt had made technical errors was contemptuous. “He’d cut them down to nothing and then jump on the bloody remains,” John Young recalls. In 1934 Midland’s president wrote Christensen to say that his services were no longer needed.
Through the remainder of the 1930s Christensen tried to interest manufacturers in his new type of 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 build-up. Tens of thousands of airplanes were to be built, and each would have 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 were a pair of Army Air Corps engineers in charge of hydraulic seals. They immediately agreed to give O-rings a try. They installed some on the worn, rusty landing gear on a Northrop A-17A, and the seal held up through eighty-eight 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.
So a man in his seventies, written off by his employer and largely ignored by the technical establishment, had come up with an invention that met a pressing need of the entire aviation industry. The O-ring would save millions of dollars, and perhaps lives as well, by making hydraulic seals simpler, cheaper, and more reliable. How much of that savings the device’s inventor would get was not settled until long after the war ended.
Christensen’s efforts to profit from the patent got off to an auspicious start. In April 1941 he licensed his patent to United Aircraft for a payment of fifteen cents to two dollars per O-ring used, depending on the size. Other aircraft companies tried to evade the patent with such earlier types of seal as the V-ring and the U-ring, each employing a different geometry of groove and seal. But nothing else worked as well.
Christensen looked set to collect fees from the entire industry. After Pearl Harbor, though, the government bought rights to many key military-related patents and made them available royalty-free to all manufacturers. Christensen got $75,000 for use of his 0-ring patent in military applications for the duration of the emergency.
That emergency turned out to last until 1952, when President Truman declared the official end of World War II. By that time only four years remained on the patent. Two major makers of the rings signed new licensing agreements and agreed to collect fees for the patent from their customers. But dozens of others did not. Most military contractors just went on using the O-ring as if they still had unrestricted wartime rights. Other manufacturers had already expanded the O-ring’s uses beyond military applications, and it showed up in fountain pens, soap dispensers, plumbing systems, hydraulic presses, automobile brakes, washing machines, and hundreds of other places.
Niels Christensen had invented a basic component of modern industry, but his royalties amounted to only tens of thousands of dollars per year. All the stubbornness that had sustained him through the long fight with Westinghouse was still there, and he was determined to get his rightful share of the profits from the O-ring. Christensen traced his troubles back to the government’s wartime appropriation of his invention, and Franklin D. Roosevelt replaced George Westinghouse as the family devil. “I remember, late in his life, Grandpa being given an FDR dime in change at a gas station,” recalls Niels Owen Young. “Take this back. I won’t have it,’ he roared.”
After Christensen died in 1952, his attorney, Joseph C. Calhoun, pressed the claims on behalf of his estate. In 1955 he filed suit against the government to recover lost licensing revenues for the period 1952–56. The government claimed that Christensen’s original patent was invalid, citing previous patents that it said had included O-ring-type devices. One was an American patent for a rotary shaft seal and the other was a British patent covering the float valve for a flush toilet. Each one included a picture of a rubber ring resting in a square-sided slot slightly wider than the ring.
The O-ring is particularly ill suited for rotary seals, so the American patent was no problem. The British one was less easily disposed of, but CaIhoun got around it by appealing to the precise wording of Christensen’s patent. It specified a ring “compressed into somewhat ellipsoidal cross section” by the piston and cylinder walls. The British patent did not mention that compression, which was essential for an O-ring in any application to work.
This small distinction was enough to win the case for Christensen’s heirs. The government’s experts concluded that the old patents gave “no teaching that such a packing would provide an effective seal for a reciprocating piston and cylinder.” Thus Christensen’s patent was a genuine advance, not merely a revival. In 1964 the court awarded the plaintiffs “reasonable and entire compensation.”
Establishing how much that compensation would amount to took another seven years. Perhaps the court used that interval to take an exact count, because in the end it declared that precisely 24,221,745 infringing O-rings had been used in the years 1952–56. Christensen’s estate collected one-quarter of a cent per outlaw ring, plus interest. It added up to about a hundred thousand dollars.
The story of Niels Christensen shows that an independent, self-taught inventor could continue to make important contributions well into the twentieth century. Hands-on shop training, mechanical intuition, and a tenacity honed through many technical and legal battles were more than a match for formal education and corporate resources. Christensen was able to sustain his inventive drive through struggles against first big business and then big government. He kept up a steady stream of inventions throughout both battles, and in the period in between, he reached the peak of his creativity with the small but vital 0-ring.