An O-ring, also known as a toric joint, is a mechanical gasket in the shape of a torus. It is a ring of elastomer with a disc shaped cross section which is designed to be compressed in a groove between two or more mating parts. An O-ring can be used in two basic applications: static and dynamic, an example of a dynamic application is in a hydraulic pump or cylinder. They are extensively used in machine design because they are inexpensive and easy to assemble; they are capable of sealing and maintaining thousands of pounds of air pressure. As long as the pressure of the fluid being contained is not greater than the contact stress of the O-ring, leaking cannot take place. Fortunately the pressure of the fluid transfers through the incompressible O-ring material, and the contact stress rises with increasing pressure. It is for this reason that the O-ring can withstand high pressure as long as the part does not fail mechanically. The most common failure is caused by extrusion of the O-ring material through the mating parts.
