Full definition
A rigid, non-elastic ring installed adjacent to an O-ring in its groove on the low-pressure side to prevent the O-ring from extruding (being forced) into the clearance gap between mating metal surfaces under high pressure. Without backup rings, O-rings begin to extrude above approximately 7 MPa (1,000 psi) in static applications and 3.5 MPa (500 psi) in dynamic applications, depending on clearance gap and rubber hardness. Materials: PTFE (most common — low friction, chemical inertness, -200 to +260°C), nylon/polyamide (economical), PEEK (high temperature and pressure), and hard rubber (90 Shore A). Types: continuous solid ring (requires stretch over bore — spiral-cut backup ring avoids this), spiral-cut (two-turn PTFE, most popular for field installation), and split ring. Per AS568 and ISO 3601 groove dimensions must accommodate both O-ring and backup ring. For pressures above 14 MPa, use backup rings on both sides of the O-ring. Backup rings extend O-ring pressure rating from 7 to 35+ MPa. Brands: Parker, Trelleborg, Freudenberg.