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Backup (Parback)

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.

  1. Elastic memory permits Parbak rings to be stretched into place for assembly without preconditioning or cutting.
  2. Continuous construction prevents damage to the O-ring seal.
  3. 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.

Image of round thin plastic or rubber rings

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