Should higher grade nuts be specified for use with galvanized or sherardized set screws and bolts on CE approved assemblies? - Andrews Fasteners CE Approved Bolts, Nuts & Washers
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Should higher grade nuts be specified for use with galvanized or sherardized set screws and bolts on CE approved assemblies?

To accommodate the thickness of zinc it is normal practice to over-tap the nut thread. This operation reduces the strength of the nut and the overall strength of the nut and bolt assembly.

To improve the strength of the assembly manufacturers use either a thicker nut (increasing the number of threads between the nut and the bolt in the assembly) or a higher grade nut.

BS EN 15048-1: To achieve full tensile resistance, hot dip galvanized assemblies can require special measures, see EN ISO 10684, Annex F.

EN ISO 10684, Annex F: Strength of hot dip galvanized bolt or screw and nut assemblies.

b) Nuts manufactured with thread tolerance class 6AZ in accordance with ISO 965-5.

To archive full assembly strength with nuts tapped oversize to thread tolerance class 6AZ two options may be considered:

1) Fit the bolts and screws with nuts of one property class higher, i.e. bolt or screw of property class 8.8 with nut of property class 10, etc.

2) Using Style 2 nuts – thicker nuts instead of Style 1

Thicker nuts are not generally available and therefore it is normal practice for manufacturers to fit a higher grade nut.

The following are recommendations regarding higher grade nuts:

BS EN 15048 – Non-preloaded bolting assemblies

  1. Property class 8.8 bolts, assembled with property class 10 nuts
  2. Property class 10.9 bolts, assembled with property class 12 nuts*

*) EN ISO 10684, Annex F is only recommendation for consideration. There is nothing against of using same grade of nut as grade of bolt (i.e. Gr 10.9 bolt with Gr 10 nut), considering, that bolt and nut passing assembly test at specified value.

BS EN 14399 – Preloaded bolting assemblies

  1. Property class 8.8 bolts, assembled with property class 8 nuts. In this option the nuts are supplied in accordance with BS EN 14399-3 for hot dip galvanizing and have higher hardness limits and a different tolerance class to self-colour property class 8 nuts to BS EN 14399-3
  2. Property class 8.8 bolts, assembled with property class 10 nuts
  3. Property class 10.9 bolts, assembled with property class 10 nuts. The grade 10 nuts according to BS EN 14399-3 have been developed to gives sufficient strength after over-tapping

You can read more about this matter in BCSA memorandum No 191-12.

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