Required Bolt Torque to Induce Given Stress

BertSmurf

Hi, I have a problem calculating the required torque that needs to be applied to a bolt.
I need to torque the bolt to a maximum of 75% of the yield strength of the bolt (which I have worked out)
However I am having trouble figuring out what torque will actually apply this stress to the bolt...
What is the relationship between applied torque and induced tensile stress on the bolt?

Nugatory

What's your application? Tell us more and you'll get a better answer.

But in general you can't do what you're trying to do (a known fraction of yield strength) just by controlling the torque - nearly all the applied torque goes to overcoming friction, so even a small uncertainty in the friction between male and female threads and under the head of the bolt means a large uncertainty in the tension of the bolt. Instead, for really critical applications you have to use load indicating washers, measure the actual elongation of the bolt, go with angle tightening techniques, and other methods that more directly measure the strain in the bolt.

If your application is bit less critical than, for example, connecting rod bolts in a Formula One racing motor there are a number of online resources that will give a close-enough answer for various combinations of materials, thread pitch, bolt diameter, thread lubrication.

Studiot

That's a pretty good answer, Nugatory.