O-ring Seals

An o-ring, also known as a packing 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. O-rings are one of the most common yet important elements of machine design. They are available in various metric and standard sizes. The o-ring is one of the simplest, yet most engineered, precise, and useful seal designs ever developed. 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.

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