back-ups

MB

8700

5100

PAB

PDB

Back-up rings are the most common anti-extrusion devices in dynamic sealing. They provide simple solutions to safely increase system pressure or solve an existing seal extrusion problem. Back-up rings function by positioning a more robust material adjacent to the extrusion gap, taking the seal’s place and providing a barrier against high pressures. Back-ups can be used to offset the reduced pressure rating effects of wear rings or to improve seal life at increased pressures. They can also be used to protect seals against pressure spikes, or to ensure seal performance at higher temperatures.

Parker offers a wide range of back-up ring profiles and materials to complement each seal type and to suit every application. Modular back-up rings disperse pressure from the seal throughout the gland to fill the extrusion gap and protect the seal (see Figure 1). The use of Profile MB can increase a PolyPak’s pressure rating to 10,000 psi, while 8700 back-ups provide added extrusion resistance to u-cups with only a minimal increase in gland width.

Positively-actuated back-ups are actuated both axially and radially into the extrusion gap, guarding the seal against extrusion (see Figure 2).

For many profiles, positively-actuated back-ups can provide the ultimate extrusion resistance while retaining the seal’s original gland dimensions.

For extreme pressures, Parker can design custom back-up ring systems utilizing metal or engineered plastics technology and highly advanced geometries. In Figure 3 (angled back-ups), a single modular back-up is replaced with dual wedge-shaped back-ups, composed of UltraCOMP or bronze. As pressure increases, the angled back-ups are forced to bridge the clearance gap, eliminating extrusion.

This method has been used successfully at pressures as high as 100,000 psi. Please contact Parker or your authorized distributor for engineering assistance in designing custom back-up configurations.

When to Use Back-up Rings
• System operating pressure exceeds the limitations of the seal’s extrusion resistance.
• Pressure spikes in the system exceed normal operating conditions, risking damage to the seal.
• The use of wear rings has increased the extrusion gap, reducing the seal's pressure rating to
an unacceptable level.
• The system temperature is high enough to lower the seal’s extrusion resistance to an
unacceptable level.