Tightening torque for a "relieved" buttress thread

[printq]

HI.

I am having trouble with some relieved buttress threads (i.e. buttress threads with a "relief" slope instead of a square profile). I am looking to calculate the torque needed to pre-tighten a box/pin assembly such that it will not overtorque when in operation. This is the same as the make-up torque calculated for API threads.

I have been able to find some formulas based off of API spec 7 (which I don't have access to), but no explanation as to how they were derived (my threads are parallel, not tapered). At present, I am deriving the make-up torque by applying a stress of 62500 psi to the box/pin (this is what spec 7 suggests, but I don't know why - a proportion of the yield stress?), converting that to a force (since I know the areas), and then raising this load in the form of power screw calculations. I don't know how appropriate this is (wouldn't this give the torque before overtorque occurs, and hence failure?).

I don't know coeff. of friction, etc. but the intention is to see what altering some of the thread angles, pitch, etc. will do, so it isn't all that important (is it?).

Would appreciate any comments.

Cheers.

 

[CoryPad]

This isn't meant as personal criticism, but you do not have appropriate information nor experience to perform these calculations accurately. If this work is important, you need to obtain the appropriate information, such as industry specifications. Lastly, friction coefficient is very important, since small changes in it produce large changes in preload.

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