Also referred to as anti-extrusion rings, these are not actual seals themselves but are used in conjunction with ‘O’ Rings and are installed to prevent the extrusion of an elastomeric ‘O’ Ring in its housing groove under certain operating conditions. They can be used in all applications but primarily are used when higher pressures or pressure spikes, higher temperatures and / or larger extrusion gaps form part of the operating conditions.
They are normally recommended for applications where;
- Fluid media pressure exceeds 10 MPa
- ‘O’ Rings of low strength elastomers are used
- Adverse mechanical conditions prevail
- Virgin PTFE
- Filled PTFE – Fillers are sometimes added to the Virgin PTFE to improve some of its basic properties and characteristics, such as, enhanced compressive strength and thermal conductivity, reduced thermal expansion and reduced wear
- PEEK™ (from Victrex plc)
This range of materials, the chemical resistance of which is almost universal, with the exception of molten alkali metals, fluorine gas and elemental fluorine, making them the ideal material choice for ‘O’ Ring Back-Up Rings.
There are three main manufactured styles of Back-Up Rings
Spiral Wound – A spiral of two turns is the most commonly used type of construction. This method of production allows the back-up ring to be easily opened to enable easy installation over a shaft and it also ensures the ‘O’ Ring is supported around its entire diameter.
Solid – Generally used on very small diameters but can be difficult to install
Endless Split – Single turn can also be supplied as an endless configuration to ensure good support. These can also be scarf split if required although we do not generally recommend this as extrusion can occur at the split
Alternative Material Options
Nitrile or Viton Back-Up rings are also available and have the following characteristics
- Molded in one-piece contoured design which enables the ‘O’ Ring to keep its shape under high pressure
- Can be stretched easily into place
- Do not cause the localised wear on ‘O’ Rings