Back-up Rings are washer-like devices that are used to prevent a seal from extruding through any gaps while under pressure. Back-up rings are not seals themselves, but are usually used in conjunction with o-rings in high-pressure applications for support or as spacers. They prevent extrusion of the o-ring when it is subjected to high pressures, or when the extrusion gaps are excessive. Wherever two back-up rings should be possibly used, one should be put on each side of the o-ring.
