As I had a slack day today, I checked a sample of bolts for thread form. For
coarse threads most had a fully formed thread within 90 to 180 degrees. The
worst was one turn, with the exception of a few horrors that I had ground
the ends of to help drive home. This was for rolled threads. For cut threads
full form was achieved sooner.
Without a doubt, at least 90% of the thread failures that I have seen are
stripping of the bolt thread, not the nut thread and this can be
substantiated by theory, assuming equal material properties of nut and bolt
(more meat in a nut thread than a bolt thread). I have never seen a bolt
fail by thread stripping in service. All the failures I have seen have been
failure of the bolt, normally at a thread root. However I have seen far more
bolts broken during tightening and mainly untightening, in a form of
torsional failure. You all will have seen them. I reckon a number of the
thread root failures are a result of bolts being overtorqued , weakened and
then snapping when subjected to the intended tensile load. You know the
drill; tighten it till it breaks and then back off 1/4 turn.
O.K. suppose we have one thread less engagement that the nut would otherwise permit. What torque setting do I use ?
If all nuts are one pitch longer than they need be, why not make them the right length at manufacture? It would save a lot of metal. I know there is normally a 120 degree chamfer on both sides of most nuts
While I generally trust standards (we all have to trust something), this
"less one thread" business has got me beat and I would like to know more
about the reasoning behind it.
Does anyone know how you go about lodging a question with the ANSI/ASME
guys?
I think the "two threads showing" bit is overly conservative, but the "minus 1 thread" is worrying and I am far more comfortable with the former. I would prefer the specification to say that a fully formed thread profile must be present at the outer face of the nut.
Cheers
Steve McK
> B31.3 (asme not astm) says
> "... if the lack of full engagement is not more than one thread." Note the
> word "full" and then get a sample of a stud bolt and nut. Take a closer
look
> at the first few pitches of the thread/nut and what do you notice. Right.
> Its only partially formed because of the chamfer, etc.
>
> It has to "stick-out" (in the words of Christopher) a min. of one thread
> just to meet the lack of one full thread engagement. But why look at the
> last sentence when its just easy to meet the first sentence - bolt should
> extend completely through their nuts. By so doing, you at least avoid
thread
> stripping(says Bill Eccles of boltscience.com).
>
> -----Original Message-----
> From: m.apte@ticb.com [mailto:m.apte@ticb.com]
> Sent: 19 March, 2002 5:32 PM
> To: PipingDesign@yahoogroups.com
> Subject: RE: [PipingDesign] Bolt Thread Engagement
>
>
> The ASTM B 31.3 states that the bolting is acceptable even if the amount
of
> dis engagement is upto 1 thread.
> i.e. the bolt can be inside the nut by not more than one thread.
> it means even if it does not come out of nut it is acceptable ( though not
> prefered.)
>
> Have a nice day.
> -Mandar.
>
>
> > Does anyone know of a ASTM on ANSI standard that
> > discusses the number of<BR>
> > threads that must be engaged in a bolt
>
>
>
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>
>
Received on Thu Mar 21 02:31:00 2002
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