by Mike Wright » Wed Nov 29, 2006 11:06 pm
I can't answer your question directly but a general formula that is used for metric bolts is:
T = 0.2Fd
where T = Torque in Nm, F = bolt preload required in Newtons, and d = bolt diameter in m (NOT mm).
Assuming your problem is all in imperial units you should be able to convert to metric, apply the above formula and convert back to imperial.
Please note that this formula is only an approximation as there are a number of variables, for example how well lubricated the thread is, and its pitch. In practice the differences in thread forms, pitch etc. for a particular diameter make little difference.
It is normal to preload bolts to 80% of their yield strength, and you should check that for the grade of bolt you are using, and size, you do not exceed this figure by much. Bolt manufacturer's etc. have such data on there websites. They also feature in engineering data books.
Please also note that despite common usage as such, neither lbs (or kg) are actually a force, they are actually units of mass. To convert to Newtons, kg must be multiplied by acceleration due to gravity, g, which is 9.81m/sÂ².