Extrusion of O-rings may be prevented by the use of anti-extrusion (back-up) devices. These are thin rings of much harder material fitted into the gland between the seal and the clearance gaps, which essentially provide zero clearance. The groove must be widened to provide space for either one or two Back-Ups. If only using one Back-Up, be certain to position opposite the pressure source next to the O-ring the groove.
Available in hard elastomer materials, PTFE, PEEK, the Back-Up Rings can be an effective means for eliminating extrusion in an application. Material selection will be much like an O-ring in that the Back-Up must also be compatible with the fluid and lubricants in the application.
Parker Back-Up sizes correspond to each AS568 O-ring size. The O-rings have a prefix of 2-xxx and the corresponding Back-Up will have an 8-xxx prefix for the same code number. For example, a 2-214 O-ring (Inner diameter 0.298 ± 0.010 and thickness 0.139 ± 0.004 inches) will use a 8-214 Back-Up. The most common Back-Up materials are (links to product series) E0652-90(EP), N0300-90 (NBR), KB163-90(HNBR), V0709-90(FKM); and V1238-95(FKM).