Nut Thread Bending Factor 1

[guidingengr]

This is a late follow-up question to thread725-35222. I was wondering where values are obtained for dimensionless nut thread bending factor C3. The original post was for a steel to steel bolted connection, and recommended using .897. In my case, I have a steel bolt (grade 8), and 6061-T6 threaded plate.

I tried googleing and had no luck.

Just joined this forum, and think it's great.

 

[CoryPad]

VDI 2230 Systematic calculation of high duty bolted joints has the following:

C₃ = 0,728 + 1,769 R_s – 2,896 + 1,296

for 0,4 < R_s < 1

or

C₃ = 0,897 where for R_s ? 1

and

R_s = ([&tau;]_BM · A_SGM)/([&tau;]_BS · A_SGS)

where

C₃ is the correction factor for thread engagement length
R_s is the strength ratio
[&tau;]_BM is the nut shear strength
A_SGM is nut thread shear area during axial loading
[&tau;]_BS is the bolt shear strength
A_SGS is the bolt thread shear area during axial loading

Regards,

Cory

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

 

[guidingengr]

Cory,

Thanks for the formulas.

I have an application where some tapped 3/8" UNF holes in an aluminum plate got stripped out by grade 8 screws. My first thought was to increase tapped hole size in aluminum to 1/2" UNF. I performed the calculations, and for fun I performed again using 1/2" UNC. I determined that the 1/2" coarse threads can actually handle a higher load.

My calculations show that shear areas for nut and screw both increased in the coarse screw which is a good thing for my application.

Does this make sense?

Thanks!

 

[guidingengr]

Corypad,

Please disregard my last post. I found a mistake in my calculation and am re-evaluating. My nut shear area did not increase with the coarse thread.

Sorry,
ron

 

[guidingengr]

As mentioned, I am using tapped aluminum hole with steel bolt, thus I need longer length of engagement to ensure bolt will fail before nut is stripped out.

My question is however, when determining the maximum load that the nut can handle, should I base nut and bolt shear areas on the Le from Q=J*Le, where Q is the length of engagement required because of the difference in material strength.

I had always believed the first 5 to 6 threads are taking most of the load, so I would think using the smaller Le (not Q value) should be used.

I am also trying to determine whether to use coarse or fine threads for my application. From my nut (tapped hole)calculations so far, fine threads have more shear area as compared to coarse threads, but when I perform final calculation with the nut bending factor, the coarse threads end up being the stronger option (C3 is larger value for coarse thread).

Any comments?

 

[CoryPad]

While the first 5 or 6 threads take most of the force due to preloading, the thread engagement calculation is done assuming accidental overstressing. This causes yielding of the threads, which can provide transfer to more threads.

I don't understand your Q equation. VDI 2230 uses the length of full thread engagement as its basis for maximum force to strip internal threads.

Regards,

Cory

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