Back-Up Rings and Seals
In a sealing application, backup rings provide added extrusion resistance, and prevent damage to O-rings and seals when subjected to high pressures, larger extrusion gaps, and/or high temperatures.
Extrusion failure is one of the most common types of O-ring failure. When the internal pressure of an application becomes too great, the O-ring will actually extrude in to the clearance gap. This extrudate will quickly be nibbled away causing a loss of material, and once sufficient material is lost, seal failure will quickly follow.
There are three options to prevent this, the first of which is to reduce the clearances to lower the extrusion gap. This is obviously an expensive option, so a cheaper solution is to raise the durometer of the O-ring. Even though a higher durometer O-ring offers superior extrusion resistance, this is often not a feasible solution due to material availability, and to the fact that harder durometer materials have limited low pressure sealing ability. The last and best option is the addition of a backup ring. A backup ring is a ring of hard, extrusion resistant material such as high durometer nitrile, Viton (FFKM), or PTFE.
A backup ring is designed to fit between the O-ring and the extrusion gap and prevent the extrusion of the O-ring. Depending upon the direction of pressure in the sealing application, you can either use 1 backup ring or 2 backup rings. If unsure, it is always best to use two backup rings per l O-ring.