Backup Rings 

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 into 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® (FKM), 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 one backup ring or two backup rings. If unsure, it is always best to use two backup rings per one o-ring.

Backup Ring Type

Basically, there are 4 types of back-up rings: flat/uncut, split/angle cut, spiral and contoured back-up rings. PTFE is considered to be the most commonly used material for back up rings. NBR rubber back-up rings are generally for contoured back-up ring to conform to an o-ring. 

Backup Rings  Material

Material : PTFE-Teflon

Material : Nitrile (NBR)

 

Material :VITON (FKM)

Material : POM

Material : POLYURETANE

What are Back-up Rings and how to use them?

What are Back-up rings / Anti-extrusion rings?

A Back-up Ring is an inflexible ring that holds an elastomeric seal to its outlined shape and in its right place. They are also called Anti Extrusion Rings and are typically used with O Rings to eliminate extrusion at high pressures.

For single acting applications – 1 Back-up Ring is sufficient, however for double acting applications – 2 Back-up Rings are necessary in order to prevent extrusion on both sides.

Back-up Rings are demanded in applications where:

• O Rings of low endurance are installed
  
• Unfavourable mechanical circumstances are inherent
  
• Dynamic applications over 100bar
  
• Static applications around 950bar
  
• Hydraulic cylinders
  
• High-pressure hydraulic systems
  
• High-pressure valves
  

There are 3 types of PTFE Back-up Rings: Solid, Single Turn and Spiral Cut.

Solid PTFE Back-up Ring may be hard or almost impossible to install, depending on the configuration due to PTFE not being elastic enough. The Back-up ring must be ‘mended’ after it’s installed in the groove to return to its normal dimensions. Single Turn and Spiral Cut Back-up Rings overpass this issue as they can both be opened a little in process of installation without deforming them. PTFE rings in a spiral cut are very popular as it simplifies proper fitting and relatively rejects the extrusion gap in the ring. Furthermore, all split rings are liable to damage during the installation process.

 

Specifications:
When choosing an adequate Back-up Ring for your application, it is important to be aware of most important specifications:
• Inner dimension of the Ring or the outer dimension of the shaft
• Radial cross-section of the elastomer
• Ring thickness

The use of Back-up Ring may be a practical solution to block the diametrical clearance gap and provide support for the O ring when the stress on it is increased to the point when it can no longer withstand being extruded into the diametrical gap.

A Back-up ring is created from a firmer and more rigid material than the O Ring. It is still however elastic enough to deform under stress to close the extrusion gap. O Ring grooves are dimensionally enlarged in size to contain the use of Back-up Ring(s).
A number of materials can be used for Back-up Rings such as PTFE, metal or hard rubber. PTFE has the benefit of a very low friction in dynamic applications.