A back-up ring is a rigid ring that holds an elastomeric seal to its designed shape and in its correct place. Back up rings are commonly used with O-rings, lip seals, and as reciprocating shaft seals.
When sealing the piston inside a pneumatic cylinder, a soft and flexible material is required to prevent leakage, but those same properties may leave the seal material vulnerable to being pulled out of its seat and then pinched or torn in the narrow space between piston and cylinder wall. If the joint cannot be redesigned, or a more resistant elastomer used, then the solution may be direct reinforcement with a stiffer material - in the form of a hard inner ring in this case. A second function can be to hold the elastomer in place while a machine is being assembled, as geometry may prevent the seal from being directly checked after assembly.
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