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A piston back up ring is a rigid ring that holds an elastomeric seal to its designed shape and in its correct place. Consider the problem of sealing the piston inside a pneumatic cylinder: in order to prevent leakage, a soft and flexible material is required, but the same properties may leave the seal material vulnerable to being pulled out of its seat and then pinched or torn in the (too 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. Back up rings are commonly used with O-rings, lip seals, and (as reciprocating shaft seals.