bolt shankfails in tension before threads fail in shear

[electricpete]

I read that typical bolt sizes are designed so that for a bolt loaded in tension, the shank will fail before the threads shear.

I can understand why that's a desirable feature. But I can't understand why it's so.

Tensile Stress on shank is F / Ashank

Sheat Stress on threads is F / Athreads

I would think Athreads is < Astem since only a few threads are active (right?). That suggests the shank tensile stress is higher than the thread shear stress.

Further, the material limit for shear stress is likely lower for shear stress than for tensile stress.

I know I am obviously missing some big parts of the picture. Can someone please explain.

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[desertfox]

Hi electricpete

Not sure whether this is what your missing but the internal thread of any size always as a bigger shear area than its male counterpart.
Have a look at this site:-

http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 28.pdf

 

[hokie66]

It depends on how many threads are engaged. If, as you say, only a few threads are engaged, the threads may fail. But a normal size nut has enough threads to cause the bolt to fail in tension if the nut is correctly installed.

 

[CoryPad]

This is a simple force balance. The force to strip threads must be greater than the force to fracture the shank. The shank tension force is equal to the tensile strength times the shank sectional area. The thread stipping force is equal to the shear stress (which is only 60 % of the tensile strength) times the shear area. The shear area is proportional to thread diameter and engagement length. You must provide sufficient engagement length to insure threads won't strip before the shank fractures.

Here is a calculation:

thread725-35222

Regards,

Cory

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[sdz]

I believe what is meant is that the shank will fail in yield before the threaded part ruptures. That ensures ductile behaviour.

 

[rb1957]

i think cory and sdz are pointing you in the right direction. the shear strength is determined from the number of engaged threads, and this should be greater than the yield strength of the minimum thread area (rather than the shank ... there's a thought, if the shank yielded the extra elongation would jam the threads, no?

 

[unclesyd]

Here is a paper by Dose that may help clear up the tension/shear relationship.

http://www.hexagon.de/dose/dose-1e.pdf