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Here is my contribution to Wiki. sysop of www.oringsUSA.com
http://en.wikipedia.org/wiki/O-ring
The History of the O-Ring
An O-ring is a loop of elastomer with a round (O-shaped) cross section used as a mechanical seal. They are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface. The joint may be static, or (in a few circumstances) have relative motion between parts and O-ring (rotating pump shafts and hydraulic cylinders, for example). Joints with motion usually require lubrication of the O-ring to reduce wear. This is often accomplished with the fluid being sealed. O-rings are one of the most popular seals used in machine design because they are inexpensive and easy to make, reliable, and have simple mounting requirements. They can seal tens of megapascals (thousands of psi) pressure. In some cases, O-rings are used with back-up rings.
The O-ring was invented in 1936 by then 72-year-old Danish-born Niels Christensen. During World War II, the US government "bought" critical war-related patents after finding out big businesses were in violation of Christensen's patent right. Christensen got a lump sum payment of $75,000 for it.
Theory and Design: Successful O-ring joint design requires a rigid mechanical mounting that applies a predictable deformation to the O-ring. This introduces a calculated mechanical stress at the O-ring contacting surfaces. As long as the pressure of the fluid being contained does not exceed the contact stress of the O-ring, leaking cannot occur. The seal is designed to have a point contact between the O-ring and sealing faces. This allows a high local stress, able to contain high pressure, without exceeding the yield stress of the O-ring body. The flexible nature of O-ring materials accommodates imperfections in the mounting parts. O-rings are available in a large number of standard sizes and materials. Manufacturers or reference books supply application and machining data for the mounting. O-rings are one of the most common and important elements of machine design.
Material: O-ring selection is based on chemical compatibility, application temperature, sealing pressure, lubrication requirements, quality, quantity and cost. Typical O-ring materials: Nitrile (NBR or Buna-N), Silicone (VMQ), ®Fluorocarbon (Viton, FKM), Perfluoroelastomer (Kalrez® (FFKM), Fluorosilicone (FVMQ), Ethylene Propylene (EPM, EPDM, EP, EPR), Neoprene (CR, Chloroprene), Polyurethane (AU, EU).
Other Seals: Similar devices with a non-round cross section are called seals or gaskets. See also washer (mechanical).
Challenger Disaster: The failure of an O-ring seal was determined to be the cause of the space shuttle Challenger disaster on January 28, 1986. A contributing factor was cold weather prior to the launch. This was famously demonstrated on television by a Cal Tech professor (physics) Richard Feynman, when he placed a small O-ring into his ice water, and subsequently showed its loss of pliability before an investigative committee.
The material of the failed O-ring was Viton (registered trade name with DuPont) and the manufacturer of that particular O-ring was Morton-Thiokol in Utah, USA. Viton is not a good material in cold temperature applications. When an O-ring is frozen there is a TG (transition-to-glass) point where it will not bounce back. Even when O-ring does not reach TG point, the O-ring once compressed, like most any other material in cold temperature, will take longer time to bounce back to its original shape, Viton doubly so. The O-rings (and all other seals) do their work by creating positive push against the surface in turn blocking the leak. The night before the launch showed freezing temperature. Concerned with this, the NASA technician has measured the temperature prior to launch. The ambient temperature was within the launch range, and the shuttle got the green light to launch. However, NASA did not account for the fact the temperature "at the O-ring" was still below the launch range. The air was warm enough, but the O-ring hadn't thawed yet. What Dr. Feynman observed during his video forensic investigation was he saw a puff of black smoke come out of the side of the solid fuel rocket. He deduced that that must have been failed zinc-oxide putty and O-ring bits at the joint expelled out by hot gas. The tiny leak when the flame reached it acted as a torch against the external tank and booster. Freed booster struck the main tank and pierced the tank's side. Liquid hydrogen and liquid oxygen fuels ignited. The Challenger was completely destroyed 73 seconds after the launch.
The rubber industry has gone through its share of transformation after the accident. All O-rings come with batch number and serial date, just like the medicine industry, to control and track precise distribution. O-rings can be recalled off the shelf if needed.
Courtesy of Wikipedia.
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either be thermoplastic-meaning they can be heated and cooled without changing propertiesor - thermoset, where an increase in temperature changes the chemical structure and properties. As a class, plastics have low elongation and high elongation set.
