Strength of straight vs tapered threads 3

[MintJulep]

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 @Compositepro or someone else wants to, or has a reference for existing calculations I'd be interested in seeing them.

 

[BrianE22]

I did find this on the web:

https://www.baski.com/Portals/0/Documents/NPT Joint Strength Table.pdf

Hopefully someone else will chime in because I was one of the people that said "no problem" in the thread you mentioned. It seems to say that NPT threads don't have much pull force capability at all. I find it hard to believe that the 3/4 NPT thread can only handle 2390 Lbs. That can't be right???

 

[EdStainless]

There is a small catch to this, tapered threads can be stronger, but they are far more sensitive to engagement.
My experience is that in tapered threads the load tends to be carried across more threads than in straight cut.
But getting the depth of taper and engagement correct can be an issue (think of bad pipe threads where you either bottom out or can only engage a couple of threads).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[hydtools]

BrianE22, that chart states FS of 4.

Ted

 

[BrianE22]

Ted -

You are right - combine that with the material strength of 30,000 psi yield then the force they show does make sense to me.

 

[mfgenggear]

tapered threads are pain, don't use unless you really have to

 

[MintJulep]

This paper seems relevant. I havent't fully read it yet.

https://www.researchgate.net/public...n_of_Stress_Concentration_by_Tapering_Threads

 

[MintJulep]

Also relevant. Also TLDR yet.

 

[Ingenuity]

I did find this on the web:

https://www.baski.com/Portals/0/Documents/NPT Joint Strength Table.pdf

From BrianE22's linked document, if I look at the 3/4" pipe subset:

I assume (big assumption?) that all 3 tabulated schedules of 3/4" pipe would have the same NPT thread engagement, and that thread shear would govern the joint strength, and all 3 schedules would have the same strength. However, it appears that the joint strength is not governed by the shear strength of the threads, but based upon tensile strength of the joint. Is this correct?

I added a column (BLUE) with tensile yield capacity of the full pipe section, based upon 30 ksi material, factor of safety of 4, and applied a 12.5% reduction in wall thickness as per the notes on the data sheet to arrive at my tabulated value - just to compare joint strength with pipe strength.

I am new to this pipe-thread stuff - I usually work with structural nuts and bolts, where we don't give much thought to thread engagement,etc.

 

[Compositepro]

This is why I say tapered threads are stronger:

There is zero clearance in the threads when fully engaged. Engagement of straight threads is often designed to be only 75%.

In pipes and tubular structures with tapered threads, the wall thickness of each part of the joint tapers from about 100% at the start of the joint to nil at the end of the part (where the stress in that part is, by definition, zero). This allows the stretch of each part in the joint to more closely match the other. In a straight thread there is a large sudden reduction in wall thickness where the thread starts, causing a stress riser, and this is where failure will occur. The benefits of tapered threads in solid structures is not as great but still exist.