An O-ring is a loop of elastomer with a round (o-shaped) cross-section used as a mechanical seal. 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 a few circumstances) have relative motion between parts and 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 often accomplished with the fluid being sealed.

O-rings are one of the most popular seals used in machine design because they are inexpensive and easy to make, reliable, and have simple mounting requirements. They can seal many thousands of psi (tens of megapascals) pressure.

In some cases, O-rings are used with back-up rings.

Theory and design

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 doesn't 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.

O-rings are available in a large number of standard sizes and materials. Manufacturers or reference books supply application and machining data for the mounting.

Material

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

Typical o-ring materials;

• Nitrile (NBR or Buna N)
• Silicone (VMQ)
• Fluorocarbon (Viton®, FKM)
• Kalrez® (FFKM)
• Fluorosilicone (FVMQ)
• Ethylene Propylene (EPM, EPDM, EP, EPR)
• Neoprene (CR, Chloroprene)
• Polyurethane (AU, EU)

Similar devices with a non-round cross-section are called seals.

O-rings are one of the most common and important elements of machine design.

The failure of an O-ring seal was determined to be the cause of the Space Shuttle Challenger disaster. A contributing factor was cold weather. This was famously demonstrated on television by physicist Richard Feynman, when he placed a small O-ring into his ice water, and subsequently showed its loss of pliability before an investigative committee.