How to determine max torque of threaded fitting

[RobertsBrown]

I am trying to determine the failure mode of an aluminum aircraft fitting. It's a standard AN814-3D, if that matters.
Material is 6061-T6. Book tensile strength is 45,000 psi.
Thread is a 0.375-24 standard 60 degree UN thread.
Thread relief is 0.310 diameter.
I.D. of fitting is 0.125 diameter.

The assembly spec. is 25 ft-lb when assembled dry. Failure point is between 24 and 26 ft-lb in actual practice. I am trying to persuade my client that their process engineer didn't do his homework, but the only way to do that is to 'show my math'. I can easily determine the effective area at 0.064 in^, and the tensile strength is known, and if I wuss out and forget about friction and sticktion, then I just need to find a formula that gives me linear tension from rotational input into a screw system. This is where I am currently stuck.
This is probably as basic as it gets, but I can't seem to work it out.
Any help is appreciated.

 

[JBA]

You can use any standard threaded fastener torque vs tensile load calculation for this analysis with the exception that you will deduct the area of the thru port from the bolt tensile area of the thread Minimum I.D.
If you don't have references for the standard calculation, use a Google search for references.

 

[RobertsBrown]

That is helpful. I have done searches for torque values, but searching for the argument as stated yielded a few different sources. The basic torque value from pitch seems to be Tp= Force[ pitch/2 pi]
This means that a tensile strength of 45,000 psi leaves me at 2,835 for my 0.063 cross section, which results in a Tp of 18.8, which agrees nicely with the physical results. It also agrees with most torque charts I can find.

The two friction coefficients are still out there. The thread to thread friction and the bearing friction from under the head of the fitting. I have no real good idea how to calculate those values since the fitting is anodized aluminum and the mate is cad plated threads and painted aluminum casting. That's why I am just running with the 'pure' torque value.

So, a philosophical question: If I submit findings without taking into account friction, do I have to wear the pointy hat and sit in the corner?

 

[JBA]

The primary issue is that bolt head friction can result in an excessive amount of measured torque for a given tensile loading failure on the bolt shank and threads.

If you trying to establish or verify a maximum allowable torque for the fitting installation, I would suggest you perform a torque to break test with a well lubricated bolt head face to see how much your standard dry installation torque varies from the lubricated one. The point being that the well lubricated arrangement will give a better indication of the minimum torque required for fitting failure and how your standard dry case compares.

To be on the safe side, it might be better to use the lubricated head result as the recommended maximum torque; since that most closely represents the worst case zero friction fitting installation.

 

[Tom.G]

So, a philosophical question: If I submit findings without taking into account friction, do I have to wear the pointy hat and sit in the corner?

If you do, be sure to post a photo!