The O-Ring (by Wikipedia)- 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 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. In some cases, O-rings are used with back-up rings.