Description

Back-up rings are washer-like devices that are used to prevent a seal from extruding through any gaps while under pressure. These seals are installed in the downstream side of the gland.  Back-up rings are not seals themselves, but are usually used in conjunction with O-rings in high-pressure applications (over 1500 psi) for support or as spacers. 

Applications

Typical applications include

•  Hydraulic cylinders
•  High-pressure hydraulic systems,
•  High-pressure valves.

 In many applications it is best to use two back-up rings; one above the o-ring and one below, to provide maximum support.
   

Configurations

   
Back-up rings are available in two distinct face design configurations – a flat, washer style, and contoured.  The simplest form has two flat, parallel surfaces, much like a washer. 

In contoured back-up rings, one face is straight and the opposite face is curved or contoured.  The contoured face is the one that comes in contact with the o-ring, providing better grip and support. 

Back-up rings may be of solid or spiral construction. 

Spiral Backup Rings

Spiral back-up rings are of overlapping, coiled construction.  As the o-ring exerts pressure upon them, the overlapping rings provide a number of contact levels against the shaft providing extra support. 
   

Specifications

   
When selecting between the numerous types of back-up rings available, the three most important specifications to be aware of are

• Inner Dimension of the Ring, or the outer dimension of the shaft on which the back-up rings will be applied;
• Radial Cross Section of the seal;
• Ring Thickness, or the axial width of the cross section of the ring, calculated as (ring O.D. - ring I.D.)/2.

Materials

  
Back-up rings are available with a number of material compositions, either rubber or polymer, metallic, or natural materials, each of which have specific advantages and disadvantages depending upon the application in which the back-up rings will be used. 

In some cases, seal manufacturers use their own proprietary materials in the manufacture of back-up rings.  When in doubt, it is best to check with the back up rings’ manufacturer as to which material would be best for the application at hand.

Standards

BS 5106 - Specification for dimensions of spiral anti-extrusion back-up rings and their housings

DIN ISO 3601-4 - Fluid power systems - o-rings - part 4

JIS B 2401-4 - o-rings - part 4: anti-extrusion rings (back up rings)

SAE AS5860 - retainers, (back-up rings), hydraulic and pneumatic, polytetrafluoroethylene resin, single turn, static gland

References

Image Credits:

Freudenberg-NOK Sealing Technologies Simrit | Hi-Tech Seals, Inc.