O-Ring - General Information

Acushnet O-Rings Standard sizes and Tolerances

An O-Ring functions as a seal through the mechanical deformation of the elastomeric compound by mating surfaces. The resilient rubber conforms to the shape of the metal sections and blocks the passage of gases or liquids. If the pressure increases, the O-Ring is further deformed and provides tighter sealing.

The application of O-Rings is based on a volume relationship between the O-Ring and the gland. Experience has shown that O-Ring volume is not adversely affected by wider dimensional tolerances. Furthermore, Type II tolerances long in use for the fluoroelastomers and similar high shrinkage materials have proven to be as effective as seals molded to tighter tolerances. This is attributable to constant seal volume.

As a result of this, the latest revision of AS 568, formerly ARP 568 and now AS 568A, is a combination of Class I and II tolerances. This change is being incorporated in all military specifications and drawings as they come up for revision. Therefore, in the future, most AN, MS and NAS tolerance standards will conform to AS 568A. It should be recognized that standards apply both to dimensions, as shown, to drawing numbers, and to compounds, and that the military specifications will not be withdrawn, but updated to conform to a single dimensional specification. Various company standards probably also will be revised to meet this standard.

Through O-Ring industry sponsored technical committees, the United States is currently working with the International Standards Organization (ISO) to adapt AS 568A standard sizes to international metric requirements.

Tentative agreement has been reached on a size standard incorporating the existing five AS 568A cross sections and a series of ID dimensions determined by a logarithmic progression of the numbers. The result has been to increase the number of small sizes, i.e., to make the steps between sizes smaller, and to decrease the number of large sizes by widening the steps between sizes. This action is consistent with the assembly requirements of an O-Ring seal, and the elastic properties of O-Ring seal compounds.

Although some U.S. sizes will be deleted or modified in the new document, Acushnet will continue to manufacture existing sizes as long as there is demand.

The new ISO sizes will become available when approved by member nations. In the meantime, AS 568A contains metric equivalents in International System (SI) units. Calculation of these metric equivalents was done in accord with the rules appearing in SAE J390, Method A, rounding off tolerances as recommended.

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