Back-up rings are used to prevent O-rings and lip seals from extruding into the sealing gap when subjected to high pressures. 1,500 PSI is a reasonable place to start your test. Darcoid Nor-Cal Seal has Back-up rings made from 90 durometer rubber, PTFE and plastic to keep to O-ring from extruding into the clearance gap causing seal failure.
Parker’s Parbak® back-up rings help prevent extrusion in high pressure service and compensate for loose fitting parts. The use of loose-fitting parts makes for cost reduction in the machining of unit components.
- Elastic memory permits Parbak rings to be stretched into place for assembly without preconditioning or cutting.
- Continuous construction prevents damage to the O-ring seal.
- Lubrication is enhanced by rubber which absorbs system fluid and does not plate out on rubbing surfaces.
In double acting seal assemblies, a Parbak is required on both sides of the O-ring. It is cheap insurance to use two Parbaks even in single acting installations. At assembly, it is too easy to place a single Parbak on the wrong side of the O-ring. By specifying one on each side of the O-ring, there will be one on the low-pressure side, where it is needed, and the extra Parbak does no harm. Parbaks are quick and easy to assemble, minimizing assembly costs, and they cannot fall out of the O-ring groove. Besides their advantages as anti-extrusion devices, Parbak rings help trap lubricant, preserving the O-ring and reducing friction. The standard sizes are listed in the Table
Parbak Elastomer Back-Up Rings
Hard rubber back-up rings combine most of the best features of both leather and PTFE anti-extrusion devices. Although no industrial or military standards have been issued for rubber back-up rings, they have been in use for a number of years. These are special devices designed to satisfy a specific problem. Parbaks in Parker Seal Group’s standard nitrile compound, N1444-90, are generally usable through a temperature range of -40°C to 121°C (-40° to 250°F). Hardening of this material due to high or low temperatures often improves performance as a back-up ring.