minimum exposed thread 2

[tomrivet]

I've been told that at least 3 turns of thread should be visible at the end of a bolt to ensure that the nut will maintain full tension. Can anyone explain the mechanism behind this? I'd like to know as I need to cut some bolts short in order to weld a small nut (o.d. similar to the diameter of the original bolt) to them. A transducer will then be mounted in this smaller nut.

Will the small nut provide "support" to help keep the large nut at tension?

Thanks

 

[GregLocock]

Hmm, I don't think you'll find any proof of this, I think it is an inspection/QC/craftsmanship tradition. I've certainly come across it.

Basically you need to ensure that the first thread on the nut is engaging with a fully formed thread on the bolt. Given typical lead-ins on bolts 3 turns seems a bit much.

Theoretically there's also the stress flow, which will be somewhat affected by the amount of bolt protruding.



Cheers

Greg Locock

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[Drej]

I have seen some building codes stipulate that "at least one thread should be exposed above the nut", but asides from that I think you just have to apply common sense. Obviously you won't want to waste material, so we're not talking about ten threads exposed, but also you must bear in mind that the bolts/nuts are standardised with failure of the bolt in mind and not stripping of the thread. Hence, you just need to ensure that the full nut thread is engaged. Bear in mind also that the first thread or so of a bolt/nut is usually taken up by a chamfer, and so you wouldn't - strictly speaking - expect full nut engagement with this thread. Hence the one thread rule is probably sufficient (three is definitely sufficient).

-- drej --


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[TBP]

The ASME B31.1 Power Piping Code calls for the stud or bolt to extend completely through the nut on flanged joints. If you're working in a plant where lines are required to be blanked-off for maintenance, you'll need what appear to be "extra" threads on the studs to permit the flange to be made-up after the blank is inserted.

In addition, every pipefitters handbook I've ever seen has tables showing the number, diameter, and length of studs for various classes & sizes of flanges, when using 2H (heavy) nuts. Many people think the tables are wrong - usually showing lengths that are FAR too long, but that's because they're using Gr 5 or 8 SAE bolts, and matching nuts, which are typically NOT approved for the service. If studs & matching heavy nuts are used, the lengths on the tables work out just fine.

 

[CoryPad]

This subject is not new. Using the search function at this site is recommended before creating a new thread. Here are three threads that answer your question:

thread725-57019

thread404-41951

thread725-37409

Regards,

Cory

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[UcfSE]

On the structural side, sheet metal screws connecting cold-formed steel framing need three threads of exposed screw beyond connected parts. That is based on manufacturer recommendations. I suppose it helps with the tilting of the screw under shear loading. For high-strength structural bolts the threads are required to be at least flush with the end of a standard nut. I believe a standard nut is the same width as the diameter of the bolt it is connecting. "Stick-through" is not required. Other types of bolts or screws may or may not have different requirements.

 

[Cockroach]

I think this fallacy may come from the allowance of three (3)full threads between the full shoulder drill depth and tap distance.

Of course drill/tap has nothing to do with the overhang allowance, but three (3) seems to be a convenient number to remember. I have never, never seen anything in the literature on it, but recall as a Junior been instructed to allow three (3) threads overhang on the nut.

It makes for a prettier AutoCad picture, but I guess five (5) full threads works just as well, aye?

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[BillPSU]

When making threaded components imperfect threads are allowed for the first 2 1/2 turns. this applies both to an external thread and an internal thread. On a rolled thread bolt you will see the end of the bolt is extruded by the thread rolling process thus making the first couple of turns imperfect threads. The same conditions occur when a roll thread tap goes through the ID of the nut. I don't have the spec which details this but I'm sure your fastener supplier can get the detailed specification.