Standard for how many threads to be out from a nut 4

[Cristodul]

Good morning everybody
I run in to a problem that some of the customers request to have a minim of 3 threads to be out of every fastener set.
I know that there is a standard for this situation but I can't find it. I would like to know what the reason is and actually what is the formula to calculate how many threads to have out.
Can somebody guide me in this matter?

Thank you!

C

 

[desertfox]

Hi Cristodul

Never seem a standard for it but normaly I would have 3 to 4 threads coming out of the nut, the reason is to ensure you have full thread engagement of the nut.
Again no standard formula just add up your joint dimensions + nut thickness and add on appropriate length to
fastener.

regards

desertfox

 

[Cristodul]

Thank you for the post!

I was thinking the same way, but I was told to look for a standard...

 

[jetmaker]

Cristodul,

Be careful of having too many threads out the bottom. The standard I use is usually 2 threads when doing a visual check or calculating grip length. The reasons for doing so are to ensure full nut engagement while minimizing weight and ensuring the bolt shank does not run into the nut.

The last one is critical for machine-type bolts that have a smooth shank. If the shank should run into the nut, then poor clamp-up and possible bolt and/or nut damage may occur. This condition presents significant fatigue issues for the fastener system.

jetmaker

 

[swertel]

I'll concur with jetmaker because my rule-of-thumb, passed down to me from generations of gurus, has always been 2-1/2 threads. Why the extra 1/2, because no one counts the threads correctly. Do you count that lead-in thread, the one that is only partially formed, or the one up from it? 2-1/2 threads gets your there!

--Scott

http://wertel.eng.pro

 

[ewh]

2-1/2 threads is also the rule of thumb taught to me when I first started designing, based on the reasoning given by swertel.

 

[MintJulep]

You should have "at least" for the reasons noted above.

You should have "no more than" to prevent excessivly long protruding fastener ends that could pose a safety hazard (scrapes, head bangs and such), could prevent standard sockets from being used at assembly, and that generally make maintenance a pain.

 

[civilperson]

AISC says flush with the end of the nut is sufficient in steel connections.

 

[KENAT]

Well first off I'd think you should have done a tolerance study to make sure you don't get the situation jetmaker describes with the smooth shank.

You should also probably support whatever figure you want with a tol analysis.

I'd probably determine my minimum bolt length so I had at least 1 full thread at worst case tolerances and then go to the nearest standard length up which means most of the time in reality you'll have significantly more than 1 or probably 2 threads.

I assume AISC requirement of 'flush' is based on worst case tol stack up, not nominals?

Within reason though the customer gets what the customer wants, if they ask for 3 I'd give them 3 unless I have a really good reason not to.

Out of interest the 2 1/2 figure a lot of you use, is this on worst case or on nominal calcs, or is it for measuring actual joints?

 

[ewh]

It was worst case in my situation.

 

[CoryPad]

This question has been answered before:

thread725-57019

thread725-37409



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swertel]

So who's going to write up the FAQ?

--Scott

http://wertel.eng.pro

 

[MintJulep]

I actually found a standard, or at least a citable source.

Naval Ships' Technical Manual Chapter 075
Fasteners

075-7.5 LENGTH OF THREAD PROTRUSION

075-7.5.1 MINIMUM THREAD PROTRUSION.
A minimum thread protrusion length is given to ensure that
all the threads are engaged. This is required to ensure a full-strength fastener. Equipment component drawings
should specify the type, length, and size of bolt, stud, bolt-stud, etc. to be used to obtain the proper thread protrusion.
If not specified on the drawings, threaded fasteners of commercially stocked lengths should be used.
Threaded fasteners, when installed and tightened, should protrude a distance of at least one thread beyond the top
of the nut or plastic insert. Excessive protrusion should be avoided, particularly when necessary clearances,
accessibility, and safety are important. Thread protrusion is considered excessive if it could cause damage to
machinery or harm to personnel. Where practicable, the number of threads protruding should not exceed five. In
no case should thread protrusion exceed ten threads. In the case of a stud, excessive thread protrusion may indicate
that the stud has not been properly driven in the blind hole. In self-locking nuts where the distance from the
top of the nut to the locking element (plastic insert) is equal to or greater than the chamfer, the bolt or stud end
may be flush with the top of the nut. For existing or reused fasteners where the thread protrusion exceeds 10
threads, verification should be made that the proper length fastener was installed. For existing installations utilizing
standard nuts, acceptable minimum thread protrusion would be where the male thread, below any
unthreaded chamfer or crown, is flush with the top of the nut. Washers should not be added to reduce protrusion
except as specifically required by equipment component drawings or technical manuals.

075-7.5.2 MAXIMUM THREAD PROTRUSION.
There is no maximum thread protrusion from the standpoint
of the function of the fastener. Obviously, too much is a waste of material and weight. There is also a safety
problem from long, sharp, threaded rods sticking out from equipment, and protruding threads can get in the way
of other parts of the equipment. A good rule to follow is to always use the shortest standard length fastener that
gives a minimum one thread protrusion. Small and/or short bolts come in 1/8- or 1/4-inch increments. As the fastener
gets larger and longer the increments change to 1/2 inch. Up to a five-thread protrusion from a fastener is
certainly reasonable, you shouldn’t take a hacksaw to it for that; but if you have ten or more threads, you could
probably have done a better job of selecting the length. There is no requirement to reduce the length of protrusion
of existing fasteners before reinstalling them. There is also no reason you cannot cut off excess protrusion
if it is obviously too long or in the way. However, you cannot use washers solely to reduce thread protrusion.

 

[Cristodul]

Thank you all for your help!
I got the answer I was looking for.