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AD 426: Bolt head protrusion through nuts and threads in grip lengths

January 2, 2019 by NSC in Advisory Desk, Technical
To ensure that bolt threads are fully engaged in the nut, BS EN 1090-2 clause 8.2.2 specifies that the protrusion must be at least one thread pitch. This is because the very end of the bolt may be slightly convex, leading to a reduced resistance if threads are not fully effective.

The same clause specifies the necessary numbers of threads within the grip length (between bolt head and the nut). For non-preloaded bolts, one full thread is required – to ensure the nut can be properly tightened. For preloaded bolts according to BS EN 14399-3 (HR system, generally used in the UK in preference to the HV system) or according to BS EN 14399-10 (HRC system, commonly known as a ‘tension control bolt’), a minimum of four threads within the tensioned length is specified. The reason for the threads in the tensioned length is to encourage ductile behaviour – AD 268 (which related to the BS 5950 requirements) reproduces a figure from Owens and Cheal (Butterworths), showing significantly more elongation when there are more threads in the tensioned length. Incidentally, BS 5950-2 required three and five threads in the tensioned length, for class 8.8 and 10.9 bolts respectively.

Contact: Richard Henderson
Tel: 01344 636525

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