8-311 BLACK BUNA 90D BACK-UP RING

APG stocks the HB90311, an AS568-311 black Buna-N (Nitrile) 90 Shore A Back-Up Ring built to an inside diameter of 0.537 in (13.64 mm), a cross section of 0.21 in (5.33 mm), and an outside diameter of 0.957 in (24.31 mm). A back-up ring installs on the low-pressure side of an o-ring to prevent extrusion into the clearance gap at elevated pressure.

The compound resists water, propane and butane, and many alcohols. That behavior comes from the compound itself: Buna-N (Nitrile) is valued for low compression set under sustained load, reliable performance in both static and dynamic service. The material carries a published service range of -40°F (-40°C) to 250°F (121.1°C). It is not the right choice where strong acids or polar solvents are present.

Typical settings include construction equipment, fluid power, and general industrial maintenance.

HB90311 ships individually. Unit weight is approximately 0.634 g and nominal inside diameter is listed as 9/16 inch. Check the compound against your process media, pressure and temperature before committing to a design. APG supports cross-referencing from drawings, samples and competitor numbers when a listed size is not an exact fit.