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O-ring

[oh-ring]

An o-ring, also known as a packing, or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a disc-shaped cross-section, 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 some designs) have relative motion between the parts and the 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 typically accomplished with the fluid being sealed.

O-rings are one of the most common 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.

History

The o-ring U. S. patent claim was filed in 1937 by a then 72 year old Danish-born machinist, Niels Christensen He came to America in 1891 and soon after that patented an air brake system for streetcars. Despite his legal efforts, his intellectual property rights were passed from company to company until they ended up at Westinghouse During World War II, the US government commandeered the o-ring patent as a critical war-related item and gave the right to manufacture to other organizations. Christensen got a lump sum payment of US$75,000 for his efforts. Litigation resulted in a $100,000 payment to his heirs in 1971, 19 years after his death.

Theory and design

O-rings are one of the most common yet important elements of machine design. They are available in various metric and standard sizes. The UK standards sizes are known as BS Sizes and typically range from BS001 to BS932. The most common standard sizes in the US are controlled by SAE AS568A. In general o-rings are specified by the inside diameter and the cross section diameter (thickness). The o-ring is one of the simplest, yet most engineered, precise, and useful seal designs ever developed.

Typical applications

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. Maintaining good surface finish of those mating parts is still important, however, especially at low temperatures where the seal rubber reaches its glass transition temperature and becomes increasingly crystalline.

Vacuum applications

In vacuum applications the permeability of the material makes point contacts quite useless. Instead, higher mounting forces are used and the ring fills the whole groove. Also round back-up rings are used to save the ring from excessive deformation As the ring feels the ambient pressure only at the seals and the ring feels the partial pressure of gases only at the seal, their gradients will be steep near the seal and shallow in the bulk (opposite to the gradients of the point contact ). See: Vacuum_flange#KF.2FQF. For high vacuum systems below 10-9 Torr, copper or nickel o-rings have to be utilized. As rubber becomes hard and brittle at low temperatures, in vacuum systems that have to be immersed in liquid nitrogen, indium o-rings are used.

High temperature applications

In some high temperature applications, o-rings may need to be mounted in a tangentially compressed state to compensate for the Gow-Joule effect.

Material

O-ring selection is based on chemical compatibility , application temperature , sealing pressure , lubrication requirements, quality, quantity and cost.

Synthetic rubbers - Thermosets:

Thermoplastics:

Other seals

There are variations in cross-section design other than circular. These include o-rings with x shaped profiles, commonly called x-rings or quad rings. When squeezed upon installation, they seal with 4 contact surfaces – 2 small contact surfaces on the top and bottom. This contrasts with the o-ring's comparatively larger single contact surfaces top and bottom. X-rings are most commonly used in reciprocating applications, where they provide reduced running and breakout friction and reduced risk of spiraling when compared to o-rings.

There are also o-rings with a square profile, commonly called square-cut. When o-rings were selling at a premium because of the novelty, lack of efficient manufacturing processes and high labor content, square-cuts were introduced as an economical substitution for o-rings. The square-cut is manufactured by molding an elastomer sleeve which is then lathe-cut. This style of seal is sometimes less expensive to manufacture with certain materials and molding technologies (compression, transfer, injection), especially in low volumes. The physical sealing performance of square-cut rings is inferior to the o-rings. Today the price of o-rings has decreased to the point that the square-cut design is nearly obsolete.

Similar devices with a non-round cross-sections are called seals or packings. See also washer (mechanical).

Failure modes of O-rings

O-ring materials may be subjected to high or low temperatures, chemical attack, vibration, abrasion, and movement. Materials are selected according to the situation.

O-ring materials exist which can tolerate temperatures as low as -200 C or as high as 250+ C. At the low end nearly all engineering materials will turn rigid and fail to seal, at the high end the materials will often burn or decompose. Chemical attacks can degrade the material, start brittle cracks or cause it to swell. For example, NBR seals can crack when exposed to ozone gas at very low concentrations unless protected. Other failures can be caused by using the wrong size of ring for a specific recess, when extrusion of the rubber will occur.

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 Caltech physics professor Richard Feynman, when he placed a small O-ring into ice-cold water, and subsequently showed its loss of pliability before an investigative committee.

The material of the failed O-ring was FKM which was specified by the shuttle motor contractor, Morton-Thiokol. FKM is not a good material for cold temperature applications. When an O-ring is cooled below its Tg (glass transition temperature), it loses its elasticity and becomes brittle. More importantly, when an O-ring is cooled near, but not beyond, its Tg, the cold O-ring, once compressed, will take longer than normal to return to its original shape. O-rings (and all other seals) work by creating positive pressure against a surface thereby preventing leaks. On the night before the launch, exceedingly low air temperatures were recorded. On account of this, NASA technicians performed an inspection. The ambient temperature was within launch parameters, and the launch sequence was allowed to proceed. However, the temperature of the rubber O-rings remained significantly lower than that of the surrounding air. During his investigation of the launch footage, Dr. Feynman observed a small out-gassing event from the Solid Rocket Booster (SRB) at the joint between two segments in the moments immediately preceding the explosion. This was blamed on a failed O-ring seal. The escaping high temperature gas impinged upon the external tank, and the entire vehicle was destroyed as a result.

The rubber industry has gone through its share of transformation after the accident. Many O-rings now come with batch and date coding, as in the medicine industry, to precisely track and control distribution. O-rings can, if needed, be recalled off the shelf. Furthermore, O-rings and other seals are routinely batch-tested for quality control by the manufacturers, and often undergo Q/A several more times by the distributor and ultimate end users.

As for the SRBs themselves, NASA and Morton-Thiokol redesigned them with a new joint design, which now incorporated three O-rings instead of two, with the joints themselves having onboard heaters which can be turned on when temperatures drop below 50 °F (10 °C). No O-ring issues have occurred since Challenger, and they did not play a role in the Space Shuttle Columbia disaster of 2003.

Future of the O-Ring

An o-ring is one of the most simple, yet highly critical, precision mechanical components ever developed. However, there are new advances that may take some of the burden of critical sealing away from the exclusive domain of o-rings. There are cottage industries of elastomer consultants assisting in designing o-ring-less pressure vessels. Nano-rubber is one such new frontier. Presently these advancements are increasing the importance of o-rings. Since o-rings encompass the areas of chemistry and material science, any advancement in nano-rubber will affect the o-ring industry.

Already there are elastomers filled with nano-carbon and nano-PTFE and molded into o-rings used in high performance applications. For example carbon nanotubes are used in electrostatic dissipative applications and nano-PTFE is used in ultra pure semiconductor applications. The use of nano-PTFE in fluoroelastomers and perfluoroelastomers such as Viton improves abrasion resistance, lowers friction, lowers permeation, and can act as clean filler.

Using conductive carbon black or other fillers can exhibit the useful properties of conductive rubber, namely preventing electrical arcing, static sparks, and the overall build-up of charge within rubber that may cause it to behave like a capacitor (electrostatic dissipative). By dissipating these charges, these materials, which include doped carbon-black and rubber with metal filling additives, reduce the risk of ignition, which can be useful for fuel lines.

See also

References

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