O-Ring standard ISO 3601

Paul Boughton

ISO 3601, the standard covering O-Rings has been revised to meet American and European, as well as Asian needs, says Trelleborg Sealing Solutions.

ISO 3601 consolidates the long established AS 568B inch standard with the existing metric ISO 3601 and importantly, as it retains its original designation, means modification of documentation may not be required.
Until the harmonisation between the AS and ISO standards, it was not possible to specify housing dimensions and retaining rings. Now two parts have been added to the standard that define these, while still in revision is a section on elastomer materials.

Class A corresponds to the American standard AS 568B in its current format. Class B allows production of O-Rings in technically acceptable and economical metric sizes and inch sizes, which can then fit into metric grooves.

Also, in Class B, the five standard cross sections are replaced with a table of tolerances using a formula.
“Changes to the existing standard were needed to reflect what was happening in the market place,” says Bernd Murthum, O-Ring Product Manager for Trelleborg Sealing Solutions, who is the convener on the ISO working group responsible for this latest revision of the ISO 3601 O-Ring standard. “Discussions to harmonize the two standards have been ongoing for a number of years.”

The revised standard not only successfully embraces both the AS and ISO standard tolerance in a single standard, but also extends cross sections covered.

“ISO 3601 and AS 568B only describe five standard cross sections. In reality, in the market place there are many more. Common practice was to use the principles of the ISO over a broader range of cross sections. This is now recognized in the revised standard.”

The five parts of the now valid O-Ring standard ISO 3601 are as follows:

ISO 3601-1: 2008. Revised - Inside diameters, cross sections, tolerances and designation codes:
* Dimensions are expressed in both metric and inch measurements
* Class A is identical with AS568B
* Class B is similar to the tolerances of the previous ISO 3601-1: 2002 and includes an equation to calculate the tolerance for the O-Ring inside diameter
* Range for O-Ring cross section diameter tolerances from 0.8 mm to 8.4 mm

ISO 3601-2: 2008: New - Housing dimensions for general applications:
* Covers technology for measuring surface roughness of O-Ring housings
* Design recommendations for O-Ring squeeze, elongation and compression as well as maximum groove fill
* Equation for calculation of O-Ring cross section reduction caused by the elongation of the O-Ring inner diameter

ISO 3601-3: 2005.  Partially revised - Quality acceptance criteria for O-Ring surfaces

ISO 3601-4: 2008.  New – Retaining ring specifications
* Covers spiral Back-up Rings, solid type (cut/ uncut), concave type (cut/ uncut)
* Gives tolerances for outer and inner diameters depending on rod or piston applications
* Design recommendations but no size list for preferred housing dimensions

ISO 3601-5:2002, Currently in revision – Suitability of elastomeric materials for industrial applications
* Will replace general information on material groups with detailed information of more practical use to design engineers
* Will comprise detailed material specifications for important material groups

ISO 3601

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