A back-up ring is a ring of hard, extrusion resistant material such as high durometer nitrile, nylon, Hytrel, or PTFE (a generic compound of Teflon™, a trademark of the Chemours Company). 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, PTFE, even Viton. Nitrile and PTFE are the most commonly used. Typically, nitrile comes in a solid ring, with one side concaved to fit the O-Ring. Generally, PTFE 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
O-Rings Product Colors
Special order O-rings and seals in different colors are increasingly popular with OEMs for parts identification and are priceless if assembly or repair mistakes are avoided. Please call Rocket for specific information (lead times, minimums) about both custom colors and stock colors for your production or private labeled repair kits.