Thread strength calculation

I just found a good paper on length of engagement when materials of different strengths are used.
I should be able to make use of that I think.

It's located here :

Hope the attached shows up... From "An Introduction to the Design and Behavior of Bolted Joints" third edition, John H. Bickford.

In short, shear stress on the threads depends on which material is stronger, the bolt or nut. Equations are given for each case. Suggestion: make a spreadsheet or use your fav. math software program.

Thank you iainuts. I just read trough what you posted and have a few questions.

Let's say I calculate the cross-sectional trough which the shear occurs, and I use an engagement length equal the bolt diameter.

Can I divide the force F, applied to the bolt with this calculated area and compare the result with the bolt material?

It's funny how one can calculate engagement without concidering forces acting on the fasteners..

Thanks again

Can I divide the force F, applied to the bolt with this calculated area and compare the result with the bolt material?

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Yes and no. You are taking the force, F, and dividing by the thread shear area as given by A(sub TS) as calculated in the attachment I've provided. Note that this A(sub TS) is dependant on the material strength as indicated in that attachment. This gives you shear stress. However, the shear strength of a material is only roughly 60% of the tensile strength. So yes, you can take the force needing to be resisted and divide by area, but that shear stress has to be compared to the shear strength of the material to get the safety factor, not the tensile strength.

Safety factor for bolting is typically 3 to 4. Note that by torquing a bolt, the shear stress created simply by torquing the nut/bolt may be SIGNIFICANTLY higher than the shear stress calculated by taking the force being resisted and dividing by area since the force created by torquing a fastener is generally higher than the force needing to be resisted - and that's ok and is in fact, preferable.

Very nice.
In my case there is little tourquing, the cylinder with the threads will just be aligned with a plane and only has to withstand pressure.

If I use the following equation for tensile stress area ;

A(sub t) = pi/4*[d-(0.9743/n)]^2

could I divide the force by this and then compare directly with the tensile strength?

Thank you

If I use the following equation for tensile stress area ;

A(sub t) = pi/4*[d-(0.9743/n)]^2

could I divide the force by this and then compare directly with the tensile strength?

Click to expand...

Yes, but then you're calculating a tensile (or compressive) normal stress on the fastener, not the shear stress in the threads.

Would you say it is sufficient to show that the shear stress in the threads does not exceed 60% of the materials tensile strength? I tried to find material data on Titanium grade 2 that gives the shear strength as well..

Would you say it is sufficient to show that the shear stress in the threads does not exceed 60% of the materials tensile strength?

Click to expand...

Yes, you should do that if you want to shear off the threads on roughly 50% of all your parts and turn the plugs into projectiles.

Happy hunting.

Hehe, good one. Thanks