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 round 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 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 shafts and hydraulic cylinder 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
of pressure.
Manufacturing O rings can be produced by extrusion,injection
molding, pressure molding or transfer molding. History
The first patent for the O-ring, is dated May 12, 1896 as a Swedish patent. J. O. Lundberg,
the inventor of the O-ring, received the patent. The US patent for the O-ring was filed in
1937 by a then 72-year-old Danish-born machinist, Niels Christensen. He came to the USA 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 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.
Theory and design
O-rings are available in various metric and inch standard sizes. Sizes are specified by
the inside diameter and the cross section diameter. In the US the most common standard
inch sizes are per SAE AS568C specification. 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 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. 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 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 Because the ring feels the ambient pressure and 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 gradient of the contact stress See: Vacuum_flange#KF.2FQF.
High-vacuum systems below 10−9 Torr use copper or nickel O-rings. Also, vacuum systems
that have to be immersed in liquid nitrogen use indium O-rings, because rubber becomes
hard and brittle 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 requirements, durometer, size and cost.
Synthetic rubbers - Thermosets: Butadiene rubber
Butyl rubber Chlorosulfonated polyethylene
Epichlorohydrin rubber(ECH, ECO) Ethylene propylene diene monomer: good resistance
to hot water and steam, detergents, caustic potash solutions, sodium hydroxide solutions,
silicone oils and greases, many polar solvents and many diluted acids and chemicals. Special
formulations are excellent for use with glycol-based brake fluids. Unsuitable for use with mineral
oil products: lubricants, oils, or fuels. Peroxide-cured compounds are suitable for
higher temperatures. Ethylene propylene rubber
Fluoroelastomer: noted for their very high resistance to heat and a wide variety of chemicals.
Other key benefits include excellent resistance to aging and ozone, very low gas permeability
and the fact that the materials are self-extinguishing. Standard FKM materials have excellent resistance
to mineral oils and greases, aliphatic, aromatic and chlorinated hydrocarbons, fuels, non-flammable
hydraulic fluids and many organic solvents and chemicals. Generally not resistant to
hot water, steam, polar solvents, glycol-based brake fluids and low molecular weight organic
acids. In addition to the standard FKM materials, a number of specialty materials with different
monomer compositions and fluorine content are available that offer improved chemical
or temperature resistance and/or better low temperature performance.
Nitrile rubber: a common material for o-rings because of its good mechanical properties,
its resistance to lubricants and greases, and its relatively low cost. The physical
and chemical resistance properties of NBR materials are determined by the acrylonitrile
content of the base polymer: low content ensures good flexibility at low temperatures, but
offers limited resistance to oils and fuels. As the ACN content increases, the low temperature
flexibility reduces and the resistance to oils and fuels improves. Physical and chemical
resistance properties of NBR materials are also affected by the cure system of the polymer.
Peroxide-cured materials have improved physical properties, chemical resistance and thermal
properties, as compared to sulfur-donor-cured materials. Standard grades of NBR are typically
resistant to mineral oil-based lubricants and greases, many grades of hydraulic fluids,
aliphatic hydrocarbons, silicone oils and greases and water to about 80°C. NBR is generally
not resistant to aromatic and chlorinated hydrocarbons, fuels with a high aromatic content,
polar solvents, glycol-based brake fluids and non-flammable hydraulic fluids. NBR also
has low resistance to ozone, weathering and aging. HNBR has considerable improvement of
the resistance to heat, ozone and aging, and gives it good mechanical properties.
Perfluoroelastomer Polyacrylate rubber(ACM)
Polychloroprene Polyisoprene
Polysulfide rubber Polytetrafluoroethylene
Sanifluor Silicone rubber: noted for their ability to
be used over a wide temperature range and for excellent resistance to ozone, weathering
and aging. Compared with most other sealing elastomers, the physical properties of silicones
are poor. Generally, silicone materials are physiologically harmless so they are commonly
used by the food and drug industries. Standard silicones are resistant to water, aliphatic
engine and transmission oils and animal and plant oils and fats. Silicones are generally
not resistant to fuels, aromatic mineral oils, steam, silicone oils and greases, acids or
alkalis. Fluorosilicone elastomers are far more resistant to oils and fuels. The temperature
range of applications is somewhat more restricted. Styrene butadiene rubber
Thermoplastics: Thermoplastic elastomer styrenics
Thermoplastic polyolefin LDPE, HDPE, LLDPE, ULDPE
Thermoplastic polyurethane polyether, polyester: Polyurethanes differ from classic elastomers
in that they have much better mechanical properties. In particular they have a high resistance
to abrasion, wear and extrusion, a high tensile strength and excellent tear resistance. Polyurethanes
are generally resistant to aging and ozone, mineral oils and greases, silicone oils and
greases, nonflammable hydraulic fluids HFA & HFB, water up to 50°C and aliphatic hydrocarbons.
Thermoplastic etheresterelastomers copolyesters Thermoplastic polyamide Polyamides
Melt Processible Rubber Thermoplastic Vulcanizate
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, 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, packings or gaskets. See also washers.
Automotive cylinder heads are typically sealed by flat gaskets faced with copper.
Knife edges pressed into copper gaskets are used for high vacuum.
Elastomers or soft metals that solidify in place are used as seals.
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 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 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.
Elastomers are sensitive to ionizing radiation. In typical applications, O-rings are well
protected from less penetrating radiation such as ultraviolet and soft X-rays, but more
penetrating radiation such as neutrons may cause rapid deterioration. In such environments,
soft metal seals are used. 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. When an O-ring is cooled below
its Tg, 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 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, Feynman
observed a small out-gassing event from the Solid Rocket Booster 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. No O-ring issues have occurred since Challenger, and they did not play a
role in the Space Shuttle Columbia disaster of 2003.
Future An O-ring is one of the simplest, 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 O-ring. There are cottage
industries of elastomer 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
applications. The use of nano-PTFE in fluoroelastomers and perfluoroelastomers 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. 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.
Standards ISO 3601 Fluid power systems — O-rings
ISO 3601-1:2012 Inside diameters, cross-sections, tolerances and designation codes
ISO 3601-2:2008 Housing dimensions for general applications
ISO 3601-4:2008 Anti-extrusion rings See also
References
External links O-ring basics from C. Otto Gehrckens
Designing with Fluoroelastomers "Dichtomatik O-ring handbook", written at
an introductory level "Standard, Metric, Swedish, French and JIS
O-Ring Sizes" "O-Ring Gland Design, Static, Axial, Dynamic
Applications" "O-Rings & sealing solutions"