Number of threads pass the nut

[bowey]

Does anyone know a code or a mechanical requirement/justification to have two or more threads pass the nut when torquing hex nuts. I have been told that at lease two threads must show pass the nut and I am not sure why. Please respond. Thanks

 

[CoryPad]

DIN 78, Bolt End Protrusions, specifies threads extending beyond the nut end.

 

[TBP]

You must have at least one FULL thread showing past the nut, or you will not be sure that there is full thread engagement. If you can count two threads, then even if one is only half a thread (because you can't see the other side) then you can still be certain.

 

[RiBeneke]

Most codes of practice have this requirement for threads projecting past the nut.
This ensures that the full number of threads act in shear when the bolt is in tension, and also ensures that the pattern of stress in the threads at the tip of the bolt is the conventional pattern on which codes and design parameters are based.

 

[Kenneth]

This requirement is also typical in aerospace, where it also helps to assure that the locking element of "self-locking" nuts (the "norm" in aerospace; typically deformed threads located at the "top" of the nut, etc.) is fully engaged with fully formed bolt threads. This is also directly related to the fact that many aerospace bolt specifications permit a thread chamfer/point length of two thread pitches.

 

[Taro]

What type of bolts are you using? The Specification for Structural Joints Using ASTM A325 or A490 Bolts, Section 2.3.2 says "The bolt length used shall be such that the end of the bolt extends beyond or is at least flush with the outer face of the nut when properly installed". The two thread rule of thumb has been used, but is not required for these bolts.

 

[PAN]

In case of using bolt tensioner, two threads are not enough.

 

[diamondjim]

Two threads was probably asked by the design
guys to ensure because of stack up of tolerances
that the flush or extended case would exist.
You cannot design flush conditions unless every
thing is perfect and we are not there yet.

 

[dugal]

One of the good things about this practice is that as a supervisor it is one of those quick visual inspections you can do on a bolted joint to ensure it is OK. If you see negative thread protusion and/or excessive protrusion you investigate because you either have a tradesperson who does not know their trade or who does not care.

 

[evelrod]

I have stayed out of this thread until now because the original question has been fairly answered, but---As a good supervisor one should know the capabilities of his people. If they don't know their trade either teach them or replace them. If they don't care, turn them into 'cash'!
As to the one thread, two thread question--- in the heavy construction industry we have come from two thread (the accepted 'norm' of 30 years ago) projection to 'flush nut', the common practice in the L.A. area for the last few years. With the now common practice of using TS bolts, the problem of overtorque has largely been solved (along with the noise problem). In my practice with auto and aircraft restoration/racing, where safety wireing is not required, Nylocks work quite well at 'flush nut'.

Rod