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Bolt and Nut Failures

Thursday, 1 February 2024
Bolt and Nut Failures
When a Bolt and Nut assembly is over tightened or when a bolted joint is statically over loaded in tension, failure may occur in a number of ways.
  • The female of nut thread may strip.  This generally occurs when the bolt is harder than the nut, and is more likely when a fine thread is involved, and/or a thin nut.   Another common cause is “short bolting”, where the male thread does not protrude 2 threads through the nut.
  • The most unusual failure is stripping of the male bolt thread.  The most common cause of this is using a nut of a higher grade than is specified for the bolt in question.
  • The bolt may break.  Generally the bolt will break at the thread run out.  Many therefore assume that the breaking load can be calculated on the basis of the minor diameter of the thread.  Tests have proven however that the breaking load is more accurately estimated on the cross sectional area of the Pitch diameter which roughly equates to the mean of the Major and Minor diameters of the thread.  This is called the “Stress Area”, and is generally accepted as the basis for calculating the strength in tension of all externally threaded parts.  See sketch:
                                                                        
Thread stripping is not a sudden failure, and in fact is progressive.  The first stressed thread will strip and as the load goes on subsequent threads, they will also strip.  The progressive stripping may tale hours to reach total failure, and until this happens there is no visual warning that the joint is coming apart.  
The keys to preventing thread stripping are to provide maximum thread engagement; tighten bolts carefully within their design parameters; and wherever possible use matched strength bolts and nuts.  Be aware that a nut, because of is geometry is inherently stronger than a bolt made of the same material.  Matched nuts are always “softer” than the bolt.  Fore example, Class 8 nuts are not as “Hard” as Class 8.8 bolts.  To ensure fully thread engagement, a bolt thread should, after tightening always protrude beyond the nut face by two full threads.   Of course it also helps to specify a course thread.
For the reasons given above it is imperative that any bolt and nut assembly fail by the bolt breaking.  Since the failure is immediate and visible, and occurs during tightening, the operator can correct the problem when it occurs.