Back-up rings are plastic or rubber rings used to prevent the O-ring from entering the clearance gap. When high pressures are exerted on the O-ring, its soft rubber material can be forced into the clearance gap, causing the O-ring to extrude (see our O-ring Design Guide for examples of O-ring failure).

The hard back-up ring material will keep the O-ring material from flowing into the gap, which will prevent premature failure. MIL spec numbers 27595, 28774, and 28782 are for reference purposes only. See back-up rings literature for more information.