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Standard O-Ring Sizes andTolerances
O-Ring Dimensional Data
An O-Ring functions as a seal through the mechanical
deformation of the elastomeric compound by mating
metal 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the military specifications will not be withdrawn, but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, manufacturers 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 metr ic equivalents in International
System (SI) units. Calculation of these metr ic
equivalents was done in accord with the rules
appear ing in SAE J390, Method A, rounding off
tolerances as recommended.