A back-up ring is a ring of hard, extrusion resistant material such as high durometer nitrile, nylon, Hytrel, or PTFE (Teflon). They are not seals themselves but are usually used in conjunction with O-Rings in high-pressure applications for support or as spacers.
A back-up ring is designed to fit between the O-Ring and the extrusion gap and prevent the extrusion of the O-Ring. Back-up rings are installed with O-Rings and quad rings. A Buna (also known as nitrile) back-up ring is typically 90-durometer (hardness). Back-up rings prevent O-Rings from gap extrusion in static radial sealing. The hard back-up ring will keep the O-Ring material from flowing into the gap which will prevent premature failure.
Back-up rings are made in many materials, including nitrile, nylon, Hytrel, Teflon, even Viton. Nitrile and Teflon are the most commonly used. Typically, nitrile comes in a solid ring, with one side concaved to fit the O-Ring. Generally, Teflon comes in single turn scarf cut (with an angle cut), solid or a spiral of two turns.
Inch sizes are produced to Aerospace Standard 568, just like O-Rings and quad rings. When selecting a back-up ring, the three most important specifications to be aware of are:
- Inner dimension of the ring, or the outer dimension of the shaft on which the back-up rings will be applied
- Radial cross-section of the seal
- Ring thickness, or the axial width of the cross section of the ring, calculated as (ring OD minus ring ID)/2