The revised global O-Ring standard - background & explanations

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 Trelleborg Sealing Solutions has a high level of expertise in specification of O-Rings and other sealing elements within a wide range of industrial, aerospace and automotive applications. Design engineers can contact one of over 40 marketing companies worldwide for further information on this O-Ring standard or selecting the optimum sealing element for their specific application.