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 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 during assembly.

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-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.