A back-up ring is a rigid ring that holds an electrometric 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 Eastover 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 Eastover 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 reciprocating shaft seals.
Back-up rings are used with a primary seal such as an o-ring in both dynamic and static applications to prevent extrusion of the o-ring when it is subject to high pressures, there are excessive extrusion gaps present in the application, or the o-ring is exposed to elevated temperatures. Elevated temperatures (160°F and higher) impose a severe burden on the physical and mechanical properties of rubber O Rings which further increases their tendency to extrude. Two back-up rings should be used with one on each side of the primary seal.