Changes on the Way to ISO Nut property Standard

ISO/FDIS 898-2 Mechanical properties of fasteners made of carbon steel and alloy steel – Part 2 Nuts with specified property classes – Coarse thread and fine pitch thread

The final draft of what will become the new ISO 898-2 has been circulated by ISO and has been approved by the BSI Fastener Standards Committee FME/9. This is the basic mechanical property standard for nuts with metric threads, and there are some significant changes from the previous issue.

The most obvious change is that it replaces two existing standards. BS EN 20898-2:1994, ISO898-2:1992 which covers nuts with coarse metric threads is now combined with BS EN ISO 898-6:1996 for metric nuts with fine pitch thread, and both are updated into the same format. This change will be welcomed, not least because only one standard will need to be purchased! So this document will cover standard ISO metric thread nuts with coarse threads of diameters M5 to M39, and fine threads from M8 to M39.

This is a comprehensive revision of the existing standards and there are several technical changes. Style 0, for thin nuts of heights between 0.45D and 0.8D, has been introduced (D being the nominal diameter) alongside the existing style 1 for regular nuts and style 2 for high nuts. There have been changes to the ranges of nominal diameters that apply to nut styles and property classes, and the guidance for matching the available nut designs and property classes with those of ISO metric bolts has been enlarged and clarified.

The material compositions table for the various grades and styles has been enlarged, with changes to the minimum manganese, and maximum phosphorus and sulphur in some grades. Mechanical property tables have been rearranged and are easier to understand, and the hardness tables now include Brinell and Rockwell values rather than referring to a conversion standard to use for the existing Vickers hardness values. The section on mechanical property test methods has been amended and expanded, and there are now specifications for dimensions and hardness of some of the test tooling, which did not appear previously.

The modifications to the existing two standards appear to be sensible and logical, but manufacturers will need to ensure that they comply with the adjustments to steel chemistry and modifications to mechanical test requirements. There is also the possibility that some parts in stock may not totally comply with the changes. But the basic property requirements of the nuts have not been modified. So provided the previous mechanical tests were conducted with adequate test tooling, there should be some confidence that parts in stock that complied with the mechanicals in the previous standards should also comply with this one. The main concern these days is that any customers or nations that insist that parts be made to the latest issue of the ISO standard will take the option to reject previously manufactured parts once this new standard is issued. This arises because of ISO’s policy of not dating any reference standards in their product drawings, so this standard will apply as soon as it issues.