how much can you torque a bolt

I am just doing some quick back of the envelope calculations on how
much you can torque a bolt. Let's use 60kpsi as the tensile strength,
this is just approximate. if I have a 1/4" x 20tpi bolt how much can
I torque it? I've tried calculating and I think I'm making an error
area = pi*.125^2 ~= .05 in^2.
.05 in^2 * 60k lb/in^2 = 3000 lb.
sounds reasonable enough. here's where I get into trouble
pitch = 1/20 tpi = .05
lets treat the radius as 1 foot to normalize it to ft-lbs of torque.
the advantage of a screw is circumference/pitch, which gives:
pi * 24 in. / .05 in. ~- 1500
So, does that mean I can only torque it to 2 ft-lbs before I snap it?
I doubt it. Clearly I'm making a mistake somewhere.
Of course I am ignoring things like friction, shear stress, etc. just
looking at tensile strength.
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The effective diameter in the threads is less than the 0.25 in. It's called the tensile diameter and is the average of the pitch and minor diameters. For UN threads, Dt = D - 0.9743/n (n = ? threads/inch =1/pitch).
Torque = K*Dia*Pre (neglecting uncertainties), K = 0.2 for unlubed
T = 0.2*(0.25 in)*(3000 lb) = 150 in-lb
I suggest you look up bolted joint calculations in a book like, Mechanical Engineering Design by Shigley. Or look for manuals, reports, etc by NASA and others.
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Jeff Finlayson
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Mr. Finlayson,
Thanks this is what I was looking for. I found the formula K*Dia*Pre on the net and couldn't figure out why the pitch didn't enter into it, now I see where it does from the tensile dia. formula. I'm still a little confused by the concept that the pitch does nothing more than affect the tensile diameter, after all the pitch is what decides how much torque gets applied as tensile stress, right?
Viktor Mikhailovich Polesov
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I have one question. What is standard for allowed stress qualification in the US, for bolts?
Reply to
So it's clear the bolt diameter, D is the major diameter. The pitch diameter is at the midpoint between the minor (root) diameter and the major (outer) diameter for a bolt (or external threads).
The pitch is included in the nut/friction factor, K. The friction terms generally dominate though.
Installation torque is generally based on the fastener material yield strength. Like 30%-80% of yield, depending on what the joint is for.
Preloaded bolted joints is a large and complex subject. There's no way cover much of that here. Go through the Nasa files if want to get into the details.
Good luck..
Reply to
Jeff Finlayson
The pitch decides how tightening torque converts to stress, in part. But the efficiency of the conversion has to be less than 50% or the nut simply unscrews again. Friction is in that sense helpful.
If you meter torque increase with rotation of the wrench, at some point the straight line flattens somewhat. At that point the screw is yielding plastically, and cannot provide more stiffness to resist. Some bolts are specified at that point, a little above or a reasonable fraction below.
If the friction is high, the bolt shears in torsion before its tensile yield point.
Brian Whatcott Altus OK
Reply to
Brian Whatcott
When I was a young engineer I was taught that you can't rely on friction to help you.
When it comes to a bolted joint design, high friction will increase the torque require to achieve a desired preload. Low friction will increase the tensile stress in the bolt for a given torque. Both extremes must be considered when designing a bolted joint.
For applications where the clamping force is critical you can eliminate the uncertainty in clamping force associated with friction by requiring a specific angle of turn, rather than torque.
As Brian states friction will help retain a bolt. However, but if the bolted joint is critical, e.g. aerospace application or location where a liberated bolt or nut would cause extensive damage, don't rely on friction alone to retain the bolt. Options for retaining the bolt can range from thread compound, lock nuts, to welded, staked or safety wired nuts.
Dave Parker
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Try this site
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Jim Y

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