Explain How Thread is Worn Down

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Thread Starter

Mark86

Explain what happens to the integrity of the NPT thread as it is put together and taken apart multiple times. The items are under about 1,000 psi on average.

Any information I could pass along and put into my presentation for the meeting would be GREAT!!! Thanks in advance!
 
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Bob Peterson

The integrity of the thread is not typically affected much by being put together and taken apart multiple times. I suppose if by multiple times you mean hundreds or thousands you might get an issue. But if you had such a situation, it would probably be better not to use an NPT connection if it could be avoided.

You can often repair damaged threads with a file if the threads become damaged, such as by being dropped, or mangled with a tool.

Corrosion can also damage the threads.

My guess is mechanical and corrosion damage are going to be bigger issues than just the act of being put together and taken apart.
 
Bob Peterson,

I would agree, except that the original poster stated these threads are under about 1000 psi. I have seen large springs on various valves which kept decreasing pressure on the threads of the bolts use to compress them as the bolts were being loosened (and which applied increasing pressure as the bolts were being tightened). If the proper amount of anti-seize lubricant hadn't been applied during original assembly/compression, and/or if any rust was present and wasn't removed from the threads and some lubrication applied before loosening they would become damaged very quickly, or over time.

If this or something like this is the case, I would say the threads would experience a higher wear rate during assembly/disassembly and if the assembly/disassembly occurred fairly frequently AND metal of the wrong type was used and/or failure to use anti-seize/thread lubricant during assembly/disassembly would contribute to increased wear.

But, there's a lot we don't know about the application.
 
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Bob Peterson

CSA:

I am inclined to agree that if something was improperly assembled or disassembled that damage could easily result, but the poster did not ask about that, just about repeated assembly and disassembly, which by itself should not damage it, although anything will wear out eventually.

As is often the case with questions asked here, the question often leaves a lot to be desired as far as what is really being asked.
 
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Curt Wuollet

That's pretty complicated depending on material, fit, and how much they are (over) tightened. The fit tends to get better for a few cycles as the threads wear in to each other. But over torque can distort the taper and cause fewer threads to actually engage at sealing pressures. And the threads, unless very carefully cut or formed are significant stress raisers and can cause fatigue at the root. Because it is (deliberately) a jam fit, there can be considerable plastic deformation and sheared burrs can prevent sealing. If you look at a much used thread you can see all of these and even pitch variations. The deformation that allows the thread to seal simply keeps happening until the thread shape is non-ideal. This means they are kind of like torque to yield bolts and not designed for multiple reuse. But, used as intended they do work well. Even the rough and ragged threads you see on "hardware store" fittings somehow manage to seal more often than not. The short answer is the threads change shape until reasonable torque will no longer seal them.

Regards
cww
 
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