NPT thread strength - Stress Distribution / Tensile Area

[Bruno730]

Hi all

Having a debate with a few friends (some engineers and some non-engineers, but mechanically inclined). For an NPT threaded pipe connection under a tensile load, is the load applied to:

> ONLY the last engaged external thread;
> Distributed evenly across all engaged threads, or;
> Highest at the last engaged thread and then decrease with increasing distance from the end of the connection?

We look forward to the conversation!

 

[mfgenggear]

May be helpfull

Screw thread Calculations - Roy Mech

roymech.org

 

[TugboatEng]

The trouble with NPT is the variance in engagement. The cross section area varies based on engagement depth.

 

[3DDave]

mfgenggear, None of those are pipe threads.

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See

Strength of straight vs tapered threads - Welding, Bonding & Fastener engineering

In thread404-472367 @Compositepro wrote: Tapered threads are structurally stronger than straight threads. I'll admit to having never thought about that as a possibiity, however it is neither obvious nor intuitive to me that this is true. I'm not sufficiently motivated to do the math, but if...

www.eng-tips.com

https://www.sciencedirect.com/science/article/abs/pii/S0020740325002218

Tapered threads are more likely to better distribute the thread load, hence they are very popular in the down-hole drilling industry where a string of pipe sections that can be miles long are assembled with sections of pipe with tapered thread. They also engage much faster, which is also a source of popularity.

That said, the typical above ground tapered thread installation doesn't have nearly as much elastic conformance to provide load distribution, so they will be somewhere in the middle with the non-uniform distribution of straight threads on the other side. The taper is also shallow so they don't have a quick-engage mode.

Elasticity rules out the first conclusion; if it was only the first turn of thread then no other turn of thread could possibly be in contact. That would require both parts to be infinitely rigid or that there be only the first turn of thread.

Distributed evenly is also unlikely as there are small variations in thread form. They won't necessarily be as unevenly distributed as straight threads, but it won't be even.

Elasticity rules out the last one for the same reason it rules out the second one. It could happen but it doesn't have to happen.

 

[mfgenggear]

Yes I know , there are pipe thread calculation out there, maybe even machinist hand book.
But according to the first post I did, the author
Commented on the first couple of threads having the most stress. Is that not applicable?

 

[CWB1]

is the load applied to: Highest at the last engaged thread and then decrease with increasing distance from the end of the connection?

Yes, that's true of every bolted joint regardless of thread type. Your highest load, highest stress, and therefore largest thread concern is always in that first thread.

The difference between straight and tapered threads of identical major diameters in that circumstances isnt the loading, its the stress in each thread. As you move along the straight thread, stress decreases bc F/A; F decreases and A remains the same. As you move along the tapered thread both decrease.

Tapered threads are rarely used to support structural loads bc they're a compromise. You get a thinner sectional area vs straight threads bc you dont need a shoulder but OTOH the taper induces radial stress causing thread/material wear, and ultimately straight threaded joints are stronger and less likely to fail.