Stress/Deformation on 3/4" NPT Fitting 2

[wallace24]

Hello,
I'm looking to determine, for a given torque on a plug, tightened into a 3/4" NPT coupling - to calculate the circumferential stress. Trying to determine whether the fitting is experiencing permanent deformation.
Thanks

 

[MintJulep]

Strain gauges.

 

[Cockroach]

You need to apply the pressure vessel theory, thick walled, and solve for interference fitting. This falls under Lame's Theory and there are several excellent articles on the web that discuss your problem.

Circumferential stress is not the problem. You need to look at thread load verses make-up torque, you need to examine stripping the cone of threads from the base metal, you need to settle the account on transverse loading or shear, bending stress. Bending is the number one killer on NPT threads.

Look at the mechanical engineering articles from the University of Tennessee. This will help you on your quest for the truth.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[wallace24]

Thank you both for your replies.
This gives the ammo to approach this problem.
Very enlightening on the actions of NPT threads!

 

[cnuk]

I have a very similar and related question. I need to calculate the required diameter for a threaded connection used to transmit torque. More specifically, I have two shafts joined on the same axis by a threaded connection. One shaft is obviously threaded male and the other female, and they shoulder together when tightened. The concern is that the OD of the female thread needs to be as small as possible, yet strong enough to transmit the torque without belling or mushrooming the end of the female connection. For API standard tapered threads used in the oil industry, API has a spec that shows the calculation but for straight thread Acme or UN threads I'm struggling with the calculation. A lot of it is determined by the friction factor on the shoulder, but the thread angle has to play a role in the hoop stress generated in the female part.

Any helpful hints to get me started? I'm reviewing my engineering texts but know this is a question I've looked into before and never found a sufficient answer. I have access to advanced nonlinear FEA but that seems overkill to me. I'd like the spreadsheet solution.

Thank You