BACK UP RINGS
The anti-extrusion rings (also called back-up-rings or more briefly BK) are used together with the O-ring inside of static and dynamic applications. For static seals, the maximum permissible pressures are up to 400 bar, for dynamic seals up to 350 bar.
Their main function is to prevent the O-ring extrusion in the backlash between the metal elements. The use of anti-extrusion rings is necessary in case of elevated pressures (usually higher than 100 bars), significant backlashes, high temperature of the fluid to be contained, high number of cycles, a high forward speed, high temperature or pressure variations and in case of non-clean fluids.
If the pressure acts only on one side of the O-ring, it is sufficient to mount only one back up ring on the opposite side with respect to the direction of the pressure. If, instead, the pressure acts on both sides of the O-ring or in the presence of a non-clean fluid, it is necessary to mount back up rings, one for each side of the O-ring.
There are plenty of types of back-up rings according to the criticality of the application.
The most used material for back-up rings is PTFE (virgin or loaded with carbon graphite or bronze). For special applications, other materials might be more suitable such as polyamides (PA 6, PA 12, Devlon), PEEK, POM, the PCTFE and PE.