Seals
An O-ring is a loop of elastomer with a round (o-shaped) cross-section used as a
mechanical seal or gasket. 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 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.
O-ring mounting for an ultra-high vacuum application . Pressure distribution within
the cross-section of the O-ring. The red lines are hard surfaces, which apply high
pressure. The fluid in the seams has lower pressure. The soft O-ring bridges the
pressure over the seams.
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 AS568. 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.
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 orings
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.
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.
