O-ring
Encyclopedia
An O-ring, also known as a packing, or a toric joint, is a mechanical gasket

in the shape of a torus
Torus
In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...

; it is a loop of elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...

with a disc
Disk (mathematics)
In geometry, a disk is the region in a plane bounded by a circle.A disk is said to be closed or open according to whether or not it contains the circle that constitutes its boundary...

-shaped cross-section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...

, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal
Seal (mechanical)
A mechanical seal is a device which helps join systems or mechanisms together by preventing leakage , containing pressure, or excluding contamination...

at the interface.

The O-ring may be used in static applications or in dynamic applications where there is relative motion between the parts and the O-ring. Dynamic examples include rotating pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...

shafts and hydraulic cylinder
Hydraulic cylinder
A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...

pistons.

O-rings are one of the most common seals used in machine design because they are inexpensive, easy to make, reliable, and have simple mounting requirements. They can seal tens of megapascals
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

(thousands of psi) pressure.

## History

The US patent claim for the O-ring was filed in 1937 by a then 72-year-old Danish
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

-born machinist, Niels Christensen
Niels Christensen
Niels Christensen was a Danish-American inventor whose principal invention was the O-ring, the ubiquitous hydraulic seal.-Early years:...

. He came to the USA in 1891 and soon after that patented an air brake system for streetcars
Tram
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

(trams). Despite his legal efforts, his intellectual property
Intellectual property
Intellectual property is a term referring to a number of distinct types of creations of the mind for which a set of exclusive rights are recognized—and the corresponding fields of law...

rights were passed from company to company until they ended up at Westinghouse. During World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, the US government commandeered the O-ring patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....

as a critical war-related item and gave the right to manufacture to other organizations. Christensen received 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.

There is also a Swedish patent for the O-ring, that is dated May 12, 1896. It was the inventor J. O. Lundberg who received patent for the invention.

## Theory and design

O-rings are one of the simplest, yet most engineered, precise, and useful seal designs ever developed. They are one of the most common and important elements of machine design. O-rings are available in various metric and inch standard sizes. Sizes are specified by the inside diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

and the cross section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...

diameter (thickness). In the US the most common standard inch sizes are per SAE AS568C specification (i.e. AS568-214). ISO 3601-1:2008 contains the most commonly used standard sizes, both inch and metric, worldwide. The UK also has standards sizes known as BS sizes, typically ranging from BS001 to BS932. Several other size specifications also exist.

### 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
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...

at the O-ring contacting surfaces. As long as the pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

of the fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

being contained does not exceed the contact stress of the O-ring, leaking cannot occur. Fortunately, the pressure of the contained fluid transfers through the essentially incompressible o-ring material, and the contact stress rises with increasing pressure. For this reason, an o-ring can easily seal high pressure as long as it does not fail mechanically. The most common failure is extrusion through the mating parts.

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. But it is still important to maintain good surface finish of those mating parts, especially at low temperatures where the seal rubber reaches its glass transition temperature and becomes increasingly crystalline. Surface finish is also especially important in dynamic applications. A surface finish that is too rough will abrade the surface of the o-ring, and a surface that is too smooth will not allow the seal to be adequately lubricated by a fluid film.

### Vacuum applications

In vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

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 ring
Back-up ring
A back-up ring is a rigid ring that holds an elastomeric seal to its designed shape and in its correct place. Consider the problem of sealing the piston inside a pneumatic cylinder: in order to prevent leakage, a soft and flexible material is required, but the same properties may leave the seal...

s are used to save the ring from excessive deformation
Because the ring feels the ambient pressure
Ambient pressure
The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, which comes into contact with the object....

and the partial pressure of gases only at the seal, their gradients will be steep near the seal
Seal (mechanical)
A mechanical seal is a device which helps join systems or mechanisms together by preventing leakage , containing pressure, or excluding contamination...

and shallow in the bulk (opposite to the gradient of the contact stress
See: Vacuum flange#KF.2FQF. High-vacuum systems below 10−9 Torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...

use copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

or nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

O-rings. Also, vacuum systems that have to be immersed in liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

use indium
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...

O-rings, because rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

becomes hard and brittle
Brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...

at low temperatures.

### 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
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...

requirements, durometer, size and cost.

Synthetic rubber
Synthetic rubber
Synthetic rubber is is any type of artificial elastomer, invariably a polymer. An elastomer is a material with the mechanical property that it can undergo much more elastic deformation under stress than most materials and still return to its previous size without permanent deformation...

s - Thermosets:
• Butyl rubber
Butyl rubber
Butyl rubber is a synthetic rubber, a copolymer of isobutylene with isoprene. The abbreviation IIR stands for Isobutylene Isoprene Rubber. Polyisobutylene, also known as "PIB" or polyisobutene, n, is the homopolymer of isobutylene, or 2-methyl-1-propene, on which butyl rubber is based...

(IIR)
• Chlorosulfonated polyethylene (CSM)
• Epichiorohydrin rubber(ECH, ECO)
• Ethylene propylene diene monomer (EPDM)
• Ethylene propylene rubber
Ethylene propylene rubber
Ethylene propylene rubber is an insulation used for high voltage cables. It has improved thermal characteristics over more traditional cables, such as cross-linked polyethylene, enabling a smaller cross sectional area for the same load carrying capacity...

(EPR)
• Fluoroelastomer
Fluoroelastomer
A fluoroelastomer is a special purpose fluorocarbon-based synthetic rubber. It has wide chemical resistance and superior performance, especially in high temperature application in different media....

(FKM)
• Nitrile rubber
Nitrile rubber
Nitrile rubber, also known as Buna-N, Perbunan, or NBR, is a synthetic rubber copolymer of acrylonitrile and butadiene. Trade names include Nipol, Krynac and Europrene....

(NBR, HNBR, HSN)
• Perfluoroelastomer (FFKM)
• Polyacrylate rubber(ACM)
• Polychloroprene (neoprene
Neoprene
Neoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...

) (CR)
• Polyisoprene (IR)
• Polysulfide rubber (PSR)
• Sanifluor
• Silicone rubber
Silicone rubber
Silicone rubber is an elastomer composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations...

(SiR)

Thermoplastics:
• Thermoplastic elastomer
Thermoplastic elastomer
Thermoplastic elastomers , sometimes referred to as thermoplastic rubbers, are a class of copolymers or a physical mix of polymers which consist of materials with both thermoplastic and elastomeric properties...

(TPE) styrenics
• Thermoplastic polyolefin (TPO) LDPE, HDPE, LLDPE, ULDPE
• Thermoplastic polyurethane
Polyurethane
A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...

(TPU) polyether, polyester
Polyester
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...

• Thermoplastic etheresterelastomers (TEEEs) copolyesters
• Thermoplastic polyamide (PEBA) Polyamides
• Melt Processible Rubber (MPR)
• Thermoplastic Vulcanizate (TPV)

## Other seals

There are variations in cross-section design other than circular. These include the O-ring with an x-shaped profile, commonly called the X-ring, Q-ring, or by the trademarked name Quad Ring. When squeezed upon installation, they seal with 4 contact surfaces—2 small contact surfaces on the top and bottom. This contrasts with the standard 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 rings with a square profile, commonly called square-cuts, lathe cuts, or Square rings. When O-rings were selling at a premium because of the novelty, lack of efficient manufacturing processes and high labor content, Square rings were introduced as an economical substitution for O-rings. The Square ring is typically 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 molding
Compression molding
Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are...

, transfer molding
Transfer molding
Transfer molding, like compression molding, is a process where the amount of molding material is measured and inserted before the molding takes place. The molding material is preheated and loaded into a chamber known as the pot...

, injection molding
Injection molding
Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity...

), especially in low volumes. The physical sealing performance of Square rings in static applications is superior to that of O-rings, however in dynamic applications it is inferior to that of O-rings. Square rings are usually only used in dynamic applications as energizers in cap seal assemblies. Square rings can also be more difficult to install than O-rings.

Similar devices with a non-round cross-sections are called seals or packing
Packing
Packing may refer to:In Mechanical engineering:* Packing, also known as an O-ring or other type of Seal , a term for a sealing materialIn Chemical engineering:* Structured packing, i.e...

Washer (mechanical)
A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a screw or nut. Other uses are as a spacer, spring , wear pad, preload indicating device, locking device, and to reduce vibration...

.

## Failure modes

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

There are O-ring materials which can tolerate temperatures as low as -200 C or as high as 250+ C. At the low end, nearly all engineering materials become rigid and fail to seal; at the high end, the materials often burn or decompose. Chemical attack can degrade the material, start brittle cracks or cause it to swell. For example, NBR seals can crack when exposed to ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

gas at very low concentrations, unless protected. Swelling by contact with a low viscosity fluid causes an increase in dimensions, and also lowers the tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

of the rubber. Other failures can be caused by using the wrong size of ring for a specific recess, which may cause extrusion of the rubber.

## Challenger disaster

The failure of an O-ring seal was determined to be the cause of the Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

Challenger
Space Shuttle Challenger
Space Shuttle Challenger was NASA's second Space Shuttle orbiter to be put into service, Columbia having been the first. The shuttle was built by Rockwell International's Space Transportation Systems Division in Downey, California...

disaster
STS-51-L
STS-51-L was the twenty-fifth flight of the American Space Shuttle program, which marked the first time an ordinary civilian, schoolteacher Christa McAuliffe, had flown aboard the Space Shuttle. The mission used Space Shuttle Challenger, which lifted off from the Launch Complex 39-B on 28 January...

on January 28, 1986. A contributing factor was cold weather prior to the launch. This was famously demonstrated on television by Caltech
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...

physics professor Richard Feynman
Richard Feynman
Richard Phillips Feynman was an American physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics...

, 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
FKM
FKM is the designation for about 80% of fluoroelastomers as defined in ASTM D1418. Other fluorinated elastomers are perfluoro-elastomers and tetrafluoro ethylene/propylene rubbers . All FKMs contain vinylidene fluoride as a monomer...

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
Space Shuttle Solid Rocket Booster
The Space Shuttle Solid Rocket Boosters were the pair of large solid rockets used by the United States' NASA Space Shuttle during the first two minutes of powered flight. Together they provided about 83% of liftoff thrust for the Space Shuttle. They were located on either side of the rusty or...

(SRB) at the joint between two segments in the moments immediately preceding the disaster. 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 cure date coding, as in the medicine industry, to precisely track and control distribution. For aerospace and military/defense applications, O-rings are usually individually packaged and labeled with the material, cure date, and batch information. 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
Space Shuttle Columbia disaster
The Space Shuttle Columbia disaster occurred on February 1, 2003, when shortly before it was scheduled to conclude its 28th mission, STS-107, the Space Shuttle Columbia disintegrated over Texas and Louisiana during re-entry into the Earth's atmosphere, resulting in the death of all seven crew members...

of 2003.

## Future

An O-ring is one of the most simple, yet highly critical, precision mechanical components ever developed. But, 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
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...

consultants assisting in designing O-ring-less pressure vessels. Nano-technology-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
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

applications. The use of nano-PTFE in fluoroelastomer
Fluoroelastomer
A fluoroelastomer is a special purpose fluorocarbon-based synthetic rubber. It has wide chemical resistance and superior performance, especially in high temperature application in different media....

s and perfluoroelastomers improves abrasion
Abrasion (mechanical)
Abrasion is the process of scuffing, scratching, wearing down, marring, or rubbing away. It can be intentionally imposed in a controlled process using an abrasive...

resistance, lowers friction, lowers permeation
Permeation
Permeation, in physics and engineering, is the penetration of a permeate through a solid, and is related to a material's intrinsic permeability...

, and can act as clean filler.

Using conductive carbon black
Carbon black
Carbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface-area-to-volume ratio, although its...

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.

### ISO 3601 Fluid power systems — O-rings

• Kremer's O-Ring Theory of Economic Development
• Labyrinth seal
Labyrinth seal
A labyrinth seal is a type of mechanical seal that provides a tortuous path to help prevent leakage. An example of such a seal is sometimes found within an axle's bearing to help prevent the leakage of the oil lubricating the bearing....

• Ozone cracking
Ozone cracking
Cracks can be formed in many different elastomers by ozone attack, and the characteristic form of attack of vulnerable rubbers is known as ozone cracking...