Billions of O-Rings are used as basic seals for pressures
below 1500 psi. Where large clearance gaps or higher pressures
are necessary, O-Rings are assisted by leather, PTFE, hard
rubber, backup or other close clearance anti-extrusion devices
to prevent extrusion and nibbling.
PTFE Back-up Washers
PTFE back-up washers have been used since 1952 to fill a
need for better chemical and heat resistance. They reduce
friction because small particles of antifrictional material
coat the rubbing surface.
The spiral cut, which was the earliest, costs less and is
the most popular. It is Government approved for -65° to 225°F
for 3000 psi. The single turn scarf-cut is approved at -65°F
to 275°F for 3000 psi and offers somewhat better extrusion
protection. Both should be installed without overlays at the
gap. They may be clipped in the field to fit non-standard
sizes. They must be installed carefully to prevent shearing
The solid no-cut type generally requires two-piece glands
although, in larger sizes, they may be fold-fitted with care
into a rod groove and then sized to correct for distortion.
This type may be used to 400°F and 5000 psi.
Hard Rubber Back-up Washers
Hard rubber back-ups, having a concave side, are available
in many sizes corresponding to
standard AS568 O-Ring sizes. The curvature of the backup
helps maintain the O-Ring shape under the pressure. Rubber
backups are easy to install since they can be stretched into a
groove. O-Rings cannot be damaged by a split or sharp surface
since these back-ups are of a continuous molded surface.
Back-ups are available in standard nitrile, ethylene
propylene, and fluorocarbon.