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 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.

“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.

The American AS 568B standard dates back over 50-years. Originally, the high cost of tools to produce O-Rings meant that inch O-Rings were used in metric housings. Elastomer O-Rings have a relatively large tolerance field between the minimum and maximum effective cross-section; where at the lower limit there will be sufficient squeeze to fill the sealing groove and at the higher, it will not extrude beyond the sealing gap. This characteristic meant that AS 568B O-Rings with widened tolerances could effectively fit into metric housings.

These wider tolerances were reflected in ISO 3601, initially drafted back in the early 1980s. America, who account for a third of the world’s market for O-Rings continued to use tolerances as specified by the AS standard while the rest of the world work to Europe’s widened tolerances.

“Discussions to harmonize the two standards have been ongoing for a number of years, continues Murthum. The revised standard not only successfully embraces both the AS and ISO standard tolerances in a single standard but also extend 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.”

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 there is now a table of tolerances covering cross-sections from 0.8 to 8.4 mm.

Until the harmonization 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. “One thing the working group ensured was that the ISO designation remained the same,” adds Murthum. “With such a long established standard this was important and means that documentation need not be changed, unless they include the standard’s date.”

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