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The new global O-ring standard: an update

01 February 2009

ISO 3601, the standard covering O-rings has been revised to meet American and European, as well as Asian needs. It consolidates the long established AS 568B inch standard with the existing metric ISO 3601 and, as it retains its original designation, documentation may not need to be modified

Until the harmonisation between the AS and ISO standards for O-rings, it was not possible to specify housing dimensions and retaining rings. Now two parts have been added to the standard that define these. 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.

“Discussions to harmonise the two standards have been ongoing for a number of year. Changes to the existing standard were needed to reflect what was happening in the market place,” says Bernd Murthum, O-ring product manager at Trelleborg Sealing Solutions, and convenor of the ISO working group responsible for this latest revision. The revised standard not only successfully embraces both the AS and ISO standard tolerance in a single standard, but also extends the cross-sections covered, as Mr Murthum explains.

“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 recognised in the revised standard.” The five parts of the now valid O-ring standard are as follows:

ISO 3601-1: 2008: inside diameters, cross sections, tolerances and designation codes all revised, with dimensions expressed in both metric and inch measurements. Class A is identical with AS568B, while 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. The range for O-ring cross section diameter tolerances is from 0.8 to 8.4mm.

ISO 3601-2: 2008: new here are housing dimensions for general applications. This part covers technology for measuring surface roughness of O-ring housings, and offers design recommendations for O-ring squeeze, elongation and compression as well as maximum groove fill. There is an equation for calculation of O-ring cross section reduction caused by the elongation of the O-ring inner diameter.

ISO 3601-3: 2005 is partially revised and concerns the quality acceptance criteria for O-ring surfaces.

ISO 3601-4: 2008: this is new and covers retaining ring specifications, including spiral back-up rings, solid type (cut/uncut), concave type (cut/uncut). Tolerances are given for outer and inner diameters, depending on rod or piston applications, and while there are design recommendations, there is no size list for preferred housing dimensions.

ISO 3601-5:2002 is currently in revision and concerns the suitability of elastomeric materials for industrial applications. This will replace general information on material groups, with detailed information of more practical use to design engineers. It will also comprise detailed specifications for important material groups.

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