NPT thread strength - Stress Distribution / Tensile Area 1

[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]

ok I used AI to find me a lead.
can some one vet the following calculations
link

https://www.padtinc.com/wp-content/uploads/2022/03/PADT-ANSYS-Fastener_Simulation_Part1.pdf

 

[mfgenggear]

True - I was referring to the generalized use of tapered threads and their limited axial location capacity.

NPT on its own is optimized for pipes, not structures, and is not suitable for applications where broad axial adjustment is expected or wanted.

according to this web page 4 threads of engagement.

https://www.padtinc.com/wp-content/uploads/2022/03/PADT-ANSYS-Fastener_Simulation_Part1.pdf

 

[Bruno730]

Still no indication of what kind of "structural" joint was being discussed in the first place.

Because there was no specific application or existing system being evaluated. We were simply discussing a piece of pipe, an NPT coupling at the end of the pipe, and a tensile load applied to the coupling (or free end of the pipe).

 

[Bruno730]

according to this web page 4 threads of engagement.

https://www.padtinc.com/wp-content/uploads/2022/03/PADT-ANSYS-Fastener_Simulation_Part1.pdf

I had come across this too. On page 7 of the PDF, it says "tensile area of the aluminum part is As = 0.1414 in^2" However, they don't explain or show how they came up with that value. It does not appear to come from the typical tensile area formula of At = (π / 4) * (dn - 0.9743 / n)^2 used for bolts. I think they just calculated At as (π / 4) * ((Thread diameter at the beginning of external thread, or hand-tight engagement)^2) - (nominal outside diameter)^2). When I do so, I get similar numbers.

Also, isn't the number of threads engaged only relevant for shear? For tension, isn't the wall thickness at one location the limiting factor to calculate your cross-sectional area?

 

[3DDave]

What an interesting group that would discuss a use they don't have any plans for and which isn't used in practice.

FEA would ordinarily be used, but that would require an initial problem to solve so that a representative model could be made.

What calculations are your other engineers coming up with and what sources have they found? That would move this conversation along much better.

 

[Bruno730]

I did some back-of-the-napkin calculations using the tensile area at the last engaged thread diameter and backed into what the maximum applied tensile load could be based on the yield strength of steel, but I couldn't find any sources that my methodology was correct. No one else has provided any calcs, just conversation.

I think this thread has been very helpful with everyone's input and experiences. It seems my methodology was correct.

 

[3DDave]

Did they then make a puller with straight threads of similar size and the same materials and it worked?

I'm supposing there isn't Grade 8 material involved in random NPT threaded pipe or some random mating material.

What size thread? What load? All threads will fail if made of poor material or sufficient load is applied.

 

[mfgenggear]

The only other information i found is to actually run tension test. On tensile machine.
And pull til failure. For shear test of the threads or tension load on the pipe.

 

[mfgenggear]

Thread shear loads will depend on the parent material. Generally used low pressure system, like propane, or natural gas, water, air pressure system. But not for high pressure. Then flange design. Correct?

 

[Compositepro]

Enerpac uses NPT threads to connect their hydraulic rams to a multitude of fixtures and extensions. It is not unusual for NPT threads to be used for structural applications. For example, railings and floor flanges.

 

[Bruno730]

Enerpac uses NPT threads to connect their hydraulic rams to a multitude of fixtures and extensions. It is not unusual for NPT threads to be used for structural applications. For example, railings and floor flanges.

I am not familiar with the application. Is the Enerpac application using NPT on hollow pipes/tubes or something with a solid cross section?

 

[TugboatEng]

I happen to have an Enerpac jack right here. 10,000 psi jacks use 3/8 NPT into the cylindrical body. This is a low cycle load and would not be suitable for most other hydraulic applications operating at even a fraction of that pressure.

 

[mfgenggear]

I am not familiar with the application. Is the Enerpac application using NPT on hollow pipes/tubes or something with a solid cross section?

Here ya go, ya go post by compositepro

How to Select the Right Hydraulic Fittings and Couplers - Enerpac Blog

Hydraulic fittings and couplers including the types available and what you need to know to make the right decision.

blog.enerpac.com

 

[Compositepro]

I am not referring to hydraulic fittings, but mechanical.

enerpac attachment fittings images - Google Search

 

[mfgenggear]

I am not referring to hydraulic fittings, but mechanical.

enerpac attachment fittings images - Google Search

The hydralic fittings upto 10kpsi
My mistake

 

[TugboatEng]

The mechanical connections on Enerpac jacks are not NPT.

 

[Stress_Eng]

Thinking about the posted question, the thread load distribution through the length of the engaged thread will be influenced by the taper in the two parts of the joint. I would think however, the distribution would be similar to a typical thread, but with the first and last few threads increasing and decreasing slightly.

 

[ghostdigital]

These guys got a Pipe Thread Installation Torque and Hoop Stress calculator

Pipe Thread Installation Torque Table Chart

Brass and steel Pipe thread (NPT) installation procedure and torque specification table and chart.

www.engineersedge.com