axial force generated by 3/8-16 bolt with a given torque 1

[aroundhere]

I have seen the simplified formulas of bolt clamping which is similar to what I am trying to figure, but wanted to know if this is correct. If want to find out how much force is generated on the end of the .375-16 bolt being screwed into a tapped hole when metal hits metal (the end of the .375 screw, not the head), or if I can compress a spring etc... I understand that friction may be a large variable. Either the steel bolt bottoming out on a sleet flat, or if compressing a spring... if I am applying a known torque of 120in-lb.

Do I not need to find the mechanical advantage first of the thread first? (thd dia*pi/pitch) .375*pi/.062= 19 MA
Then divide 120in-lb by thread radius.. 120in-lb/.1875in=640lb then multiply by the mech advantage 640lb*19=12,160lb ? this sounds high. Is this wrong? I understand that friction has not yet been accounted for. Before friction and assuming the head of bolt of screw does not bottom out is what I am curious of. Or, how far off is the above calculation. Please explain if I have missed the boat. If I am close, please comment on how the friction coefficient would then be applied. I have done this before years ago and had our ME check and I believe I was correct. I have forgot much since then. thanks


Dia=.375"
Torque=120 in-lbs
pitch=.062"

 

[tbuelna]

First, I would point out that the axial force at the contact between the screw tip and the plate at the point of initial contact is essentially zero.

As for the resulting max net axial force at the bolt tip that would be produced by torquing the 3/8-16 bolt to 120 in-lbs, you would need to reduce the mechanical advantage of the threads by the friction losses in the thread contacts and the tip contact. At higher torques applied to the bolt head, you would also need to allow for some torsional deflection in the bolt body. And lastly, there is usually a large difference between static (breakaway) and sliding friction coefficients. This difference can be up to 100%. Personally, I would also use a bare-metal-on-bare-metal friction coefficient of 0.30.

 

[MarkTEng]

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