Quick way to go from course thread torque to fine thread?

[BlackEngineer13]

I find myself in situations in which our crews in the field realize that the studs available are fine thread rather than the standard course thread we generally use. Clearly there are issues with this happening, but that's a separate concern for me to deal with.

I know that fine thread studs have a different torque requirement than course threads to achieve the same bolt load, but I was wondering if there is a known ratio that our established torque can be multiplied by to make the change from course thread to fine thread. It would be nice to let the guys know quickly what the new spec would be rather than using b1.1 and pcc-1 to calculate it every time for custom required forces.

This is a new issue for us, but it's come up three times this year, and I just want to know if I am missing something easy. Thanks for any insight you might be able to give!

 

[bcd]

My machine design textbook states that there is evidence that for UNF threads, the torque coefficient, C, is 6 to 10% less than for coarse threads. So for general rule of thumb, to generate same clamping force, torque for fasteners with fine threads is about 8% lower than coarse.

That is about as good as a general rule as you can get. Only way to get something better is to test with your specific conditions.