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Different thread strengths 1

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EngineerPhil

EngineerPhil

Petroleum
Dec 19, 2002
22
Hello,

I'm trying to design a threaded part to a pressure vessel. The MAWP is 14.5k psi and the material has a yield of greater than 110k psi. Typically API P110. This is a casing grade.

I have worked out the maximum stress on the mandrel (longitudinal stress due to pressue and the effects of a differential pressure on the test piece reacting against an end stop). While not hugely convincing calcs. I have used plenty of SF's.

I can utilise any form of thread. I know that ACME 2G Stub threads are often used but I find them expensive to machine. But, the Machinery's Handbook on p1794 (Ed 26)provides some means of calculating the stress area and shear area of these types of threads.

The question is, can I use the same equations for other types of threads, namely British pipe threads, Witworth etc. If not, what is a general rule of thumb for calculating the stress area and shear area of any thread? There must be some literature on this but where? I bought MH as I thought it would tell me all, but I 'm struggling to get the info I require.

Any thoughts?

EngineerPhil.
 
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These two threads have more information:

thread404-32938

thread404-44696
 
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Without looking at the other threads corypad referenced, here is the short answer.

The shear area of a thread is based on it's minor diameter.

The stress (tensile) area of a thread is based on the mean material condition of the thread, plus the minor diameter. For most threads, this is close enough to the pitch diameter that you can just use that and not have to calculate the mean material.
 
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Many thanks CoryPad and Swertel, I have much reading and calculations to do.

I think my initial calcs are also inappropriate. I have a tensile load on the thread of a solid shaft in excess of a million pounds. Too much I think.

As a quick query, what is the logitudinal stress on a solid shaft due to a uniform external pressure? Depending what calcs I use it is either equal to the pressure or zero. I am somehat puzzled and fear that I am missing the obvious. An explanation would be great.

Cheers

 
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EngineerPhil,

The longitudinal stress on the solid shaft due to uniform external pressure is equal to the external pressure. The zero is the equivalent stress (e.g. Tresca or von Mises) that causes yielding - yielding is invariant with respect to a hydrostatic stress.
 
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E.Phil
I would use 7/8 squared for the approximate value
rather than the pitch diameter squared for a better
approximation. Metric threads are easier as you can
simply take the diameter minus the thread pitch
to determine the minor diameter. Go metrics!
 
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