Diameter around Female Pipe thread

[allen1]

I am designing an aluminum adapter that will braze on one and and have 1/2" female NPT pipe threads on the other. My question is how large does the diameter need to be around the female threads to stop the adapter from cracking or splitting when the steel male pipe is threaded into it?

Anybody have experience with this or know a rule of thumb? I will not be doing any FEA analysis. Maybe a hand calculation or an internet calculator.

[PinkertonD]

Hi Allen,

A quick and dirty way to guess might be to measure a 1/2" PVC threaded coupling. If it is strong enough for PVC then it should well exceed that in aluminum. No??

Dave
Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.

[allen1]

Hi Allen,

A quick and dirty way to guess might be to measure a 1/2" PVC threaded coupling. If it is strong enough for PVC then it should well exceed that in aluminum. No??

Dave
Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.

I like that idea but they say don't use plastic female and male metal or the female will crack. This gave me another idea though. Look at the thickness of a steel female and make the aluminum 3-5 times thicker, since steel has 3-5 times more tensile strength. The steel may be over designed but that's another discussion.

Thanks for your help.

[PinkertonD]

Hi Allen,

Usually people over-tighten tapered pipe joints thinking they need to come to a stop as with nuts and bolts. Steel being steel it copes, but for PVC fitting they will split easily. I always tighten PVC only with bare or gloved hands. Never have a leak and never broken a fitting.

If you watch a competent qualified Plumber they too will only tighten steel very lightly even when using pipe wrenches.

Without checking, I suspect that PVC couplings are probably slightly larger in diameter than steel and that diameter would be fine for aluminum too, providing you heed the tightening warning above.

Dave
Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.