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Back-up rings are also known as anti extrusion rings. A back-up ring is a rigid ring that holds an elastomeric seal to its designed shape and in its correct place. Back-up rings are often used with O-Rings to prevent extrusion at high pressures.
When sealing the piston inside a pneumatic cylinder, a soft and flexible material is required to prevent leakage. However, those same properties may leave the seal material vulnerable to being pulled out of its seat and then pinched or torn in the narrow space between piston and cylinder wall. If the joint cannot be redesigned, or a more resistant elastomer used, the solution may be direct reinforcement with a stiffer material. In this case, in the form of a hard inner ring.
Back-up rings should generally be used for dynamic applications over 100 bar, and static applications around 950 bar. Each application can vary, therefore it is essential to use this as a general guideline only.
There are three types of PTFE back up rings: – solid, spiral wound, and split. Solid PTFE back up rings can be difficult to install. Since PTFE is not resilient, the back up rings must be “reformed” after being installed in the groove to return them to their original dimensions. Spiral wound and split PTFE back-up rings overcome this problem. They can be opened slightly during installation without deforming them.
Traditionally PTFE has been used but in recent years, contoured elastomer rings have been developed and are now widely used. Eastern Seals supplies a wide range of back-up ring profiles and materials to complement each seal type and to suit every application. We hold large stocks of Contoured rings in both Nitrile and Viton®.