Thread Sizing 1

[DBCox]

Hello Everyone,

I have a question about thread shear/failure. I have always based my calculations on assuming 2 to 3 threads handle the full load placed on whatever threaded fastener I am using. A bolt shank should fail due to tension before the threads fail by design (typically).

Now, fine threads are generally thought to be stronger. I always assumed this was due to better thread engagement, higher torques, and a slighly larger throat. But, if you use the same assumption that the first 2-3 threads handle the load (somewhat contradictory by my first assumption above), the opposite would happen because the shear area is reduced due to the thread shear area being smaller than a coarse thread bolt...

So I ran some numbers...

For a 1" grade 8 bolt, the tensile strength should be 118,000 lbs.

# of course threads necessary to handle that load assuming 8 threads/in, minor dia of 1" (i know thats large), and a shear of .707*ultimate. You get 3 threads - seems OK for the 2-3 thread rule.

# of fine threads (14 pitch)- you need 5 threads - seems like threads would fail before shank fails if only 2-3 can are holding the load.

So, whats the issue here? Is the 2-3 threads rule wrong? What if I need to determine the threads necessary for a gr8 bolt to be screwed into a material with a lower yield? Obviously the threads required will be greater than 3-5. It seems a thread engagement ratio would be a little better, but I am not sure where to find this data. Any help?

Thanks

 

[CoryPad]

The "2-3 threads rule" is wrong.

If you want to calculate the required nut height to insure bolt fracture (usually the preferred failure mode, then look here for the equations you need:

thread725-35222

Regards,

Cory

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

 

[DBCox]

Cory,

Thank you for the information. I looked over the post you suggested, and it helped, but I am still confused. What is the second part of the shear stress equation for ( [P/2 + (d - D2)/?3])? I tried to derrive this with no luck. At some point in the calculation, a thread engagement must be considered because I cannot safely assume 100% of the threads are engaged. This second part may be taking that into account somehow, but I am not seeing it.

Personally, I think about a 30% thread engagement is a safe assumption, but I would like some sort of documentation to prove that. Machining tolerances play a big role in that too of course. I may be way off, but it is something that has to be considered whether it is 10% or 99%

 

[CoryPad]

The second part accounts for the variable geometry presented by threads.

I don't understand why you say "a thread engagement must be considered because I cannot safely assume 100% of the threads are engaged". Why? Are you intending to have an overly long thread engagement?

There have been several studies of force distribution vs. number of threads, and usually ~ 5-6 threads are taking almost 100% of the applied force. If you use the equations in the other thread, they tell you the required engagement length. You can add a safety factor to that (plus chamfers, countersinks, etc.) for overall nut/tapped hole length.

Regards,

Cory

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

 

[blfarrar]

The basic rule for length of thread engagement is at least on to one & a half time the diameter of the thread.
Very critical on alloys that this is not reduced.

The actual tensile strength of a fastener is well shown in most screw makers data sheets, but it must be realsied that the fastener has to be tensioned (stretched) to effect a joint between the materials.

This accepts that the number & size (dia) of fasteners is correct or exceeded & the fastener is torqued to corect value by legitimate method of tensioning either torque spanner / driver. The alternative is to use torque angle method that some engine makers etc use.

Be mindfull of lubrication of the fastener if it has to be torqued, equally "thread loc" can radically affect torque values, especially small dia fasteners (less that 10mm dia)

Hope this helps

Bruce L Farrar.
Works Engineering Manager
Marshalls Mono PLC.Brookfoot Works.
Halifax W.Yorks UK

 

[Cockroach]

BlakRapter, the issue here is that your entire model for analysis of threads is wrong.

Shear is based upon engagement of threads at the pitch diameter. Any load placed on the threaded joint naturally makes the external screw fail BEFORE the nut gives way. The reasoning here is that the Nut has much more shear area than the screw.

The finer the thread, the shallower the height. In general a nice coarse thread takes a higher load, that is why full Acme forms at 4 TPI are preferred in the oil patch over alternate forms.

You need to consult the AISI thread manuals for the proper analysis, the Machinist Handbook is alright but rather simplistic in the analysis. There are several good reference textbooks out on the subject, but a few of the responses here have put you on the proper path of enlightenment.

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

 

[Screwman]

Cory's formulas are correct. We have used the same ones for years and they yield results very close to reality.
When calculating length of engagement required to obtain fastener shank failure (usualy the prefered failure mode) you have to start out comparing the relative strength of the fastener and nut materials. The formulas take into account the fact that there isn't a perfect 100% root to crest engagement of the threads.
When looking at lengths of engagement you should think of diameters of engagement instead of threads. As you found, it takes more fine threads to break the bolt than it does coarse threads, but they both work out to be the same Length of Engagement, which when you think about it makes sense. When you have a low strength nut material like a tapped aluminum hole, it will take significantly more LE to break the bolt than into steel.

 

[rorschach]

To add to screwman's and cory's posts, fine threads allow higher torques because the root diameter is larger than for coarser threads. and the reason why around 5 threads will take the load is simple elastic (or in severe instances, plastic) deformation allowing the threads to distort until either a large enough contact surface area is developed, or the UTS is exceeded, whichever comes first.