Convener explains the logic of O-ring standard
T he recent introduction of the international O-ring standards ISO 3601-1 and 3601-2 was included in the August Sealing Technology. As previously discussed this standard has had a long and erratic path to fruition. Bernd Murthum, O-Ring Product Manager for Trelleborg Sealing Solutions, the convener of the ISO working group responsible for this standard, has been explaining the reasoning behind the current document.
ISO 3601, the standard covering O-rings, the most commonly used seal, has been revised to meet American, European, as well as Asian needs. It consolidates the long established AS 568B inch standard with the existing metric ISO 3601 and importantly, as it retains the original O-ring designations, means modification of documentation may not be required.
Changes to the existing standard were needed to reflect what was happening in the market place and reflect current common practice. 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 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. The revised standard not only successfully embraces both the AS and ISO standard tolerances in a single standard but also extend the cross-sections covered. ISO 3601 and AS 568B include only Bernd Murthum has succeeded in guiding the O-ring standard parts ISO 3601-1&2 to publication after over 30 years of discussion and numerous discarded draft documents.