40 pROcESSINg tEchNOlOgy
Revised global O-ring standard
harmonises US, Europe and Asia
ISO 3601, the standard
covering O-Rings, the
most commonly used seal,
has been revised to meet
American, European, as
well as Asian needs.
ISO 3601, la norme
portant sur O-Rings, le
plus couramment utilisé
le sceau, a été révisé pour
répondre à l’Amérique,
d’Europe, d’Asie, ainsi que
les besoins.
ISO 3601, die Norm für
O-Ring, die am häufigsten
verwendeten Siegel,
wurde überarbeitet, um
in Amerika, Europa sowie
I
SO 3601 consolidates the long established Fig. 1. A selection of O-Rings. The revised
Asien Bedürfnisse.
AS 568B inch standard with the existing standard embraces AS and ISO tolernaces.
metric ISO 3601 and importantly, as it
retains its original designation, means “Discussions to harmonise the two standards
modification of documentation may not be have been ongoing for a number of years,”
required. continues Murthum. The revised standard not
“Changes to the existing standard were only successfully embraces both the AS and ISO
needed to reflect what was happening in the standard tolerances in a single standard but also
market place,” says Bernd Murthum, O-Ring extend cross-sections covered. “ISO 3601 and AS
Product Manager for Trelleborg Sealing Solutions, 568B only describe five standard cross-sections.
who is the convener on the ISO working group In reality, in the market place there are many
responsible for this latest revision of the ISO 3601 more. Common practice was to use the principles
O-Ring standard. of the ISO over a broader range of cross-sections.
The American AS 568B standard dates back This is now recognised in the revised standard.”
over 50-years. Originally, the high cost of tools Class A corresponds to the American standard
to produce O-Rings meant that inch O-Rings AS 568B in its current format. Class B allows
were used in metric housings. Elastomer O-Rings production of O-Rings in technically acceptable
have a relatively large tolerance field between the and economical metric sizes and inch sizes,
minimum and maximum effective cross-section; which can then fit into metric grooves. Also, in
where at the lower limit there will be sufficient Class B there is now a table of tolerances covering
squeeze to fill the sealing groove and at the cross-sections from 0.8 to 8.4 mm.
higher, it will not extrude beyond the sealing gap. Until the harmonisation between the AS and
This characteristic meant that AS 568B O-Rings ISO standards, it was not possible to specify
with widened tolerances could effectively fit into housing dimensions and retaining rings. Now
metric housings. two parts have been added to the standard that
These wider tolerances were reflected in ISO define these, while still in revision is a section on
3601, initially drafted back in the early 1980s. elastomer materials.
America, who account for a third of the world’s “One thing the working group ensured was
market for O-Rings continued to use tolerances as that the ISO designation remained the same,”
specified by the AS standard while the rest of the adds Murthum. “With such a long established
world work to Europe’s widened tolerances. standard this was important and means that
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