Extrusion is one of the most common types of O-Ring failure. When the internal systemic pressure of an application becomes too great, the O-Ring will frequently extrude into the clearance gap between mating parts. In many instances, this extrudate will be nibbled away causing a loss of material, with seal failure quickly following. 

There are three options to prevent extrusion failure, the first of which is to tighten the clearances to reduce the extrusion gap. This can be a costly option; a less expensive solution is to increase the hardness of the O-Ring.  Although a higher durometer O-Ring offers superior extrusion resistance, it is often not a feasible solution due to material availability. What’s more, harder durometer materials have limited low pressure sealability.  The third and best option is the addition of a back-up ring, made from a hard, extrusion resistant material such as high durometer nitrile, nylon, Hytrel®, or PTFE. Back-up rings are designed to fit between the O-Ring and the extrusion gap, thus minimizing the probability of extrusion failure. 

While back-up rings are used in conjunction with O-Rings in both static and dynamic sealing applications, the gland they fit in must be specially designed to accomodate both the O-Ring and the back-up. For more information on gland design please visit All Seals's static or dynamic gland design guides.

STANDARD BACK UP RING

Used primarily as an anti-extrusion device, standard back-up rings are used in both static and dynamic sealing. Made of materials that are resistant to pressure induced flow, the back-up ring keeps a seal from extruding into the clearance gap between mating parts.

All Seals's standard Hytrel® back-up rings are equivalent to AN6224, AN6246, MS28774, MS28782, and MS28783 specifications.  The PTFE back-up rings are specifically cut to MS28774 specifications.  Both types of back-up rings share a common rectangular cross-section.

CONTOURED BACK-UP RING

Contoured back-up rings are anti-extrusion devices used in both static and dynamic sealing environments. They’re also useful when mating part clearances are not tightly controlled. In either case, they can extend the useful life of O-Rings. Contoured back-up rings have a slightly concave face on one side, providing uniform contact with the entire diameter of the O-Rings they are used with. Sizes correspond with AS568 O-Rings. Whenever possible, the use of two back-up rings is recommended, even in single-acting applications.

Stock material is 90 durometer nitrile. The temperature range is -40°F to +250°F