Thread Stripping

[PW]

How is the shear stress calculated in a threaded plug subject to internal pressure (fitted into a pressure vessel). What force would actually strip the thread if only say 5 threads were engaged.

Cheers

 

[Adrian2]

Dear PW;

There are many answers to these questions. I am not sure which country you live in but I am sure you have a governing body for the constrution of steel structures and pressure vessels which lays down the rules for the use of fasteners.

In Canada, the Canadian Institute of Steel Construction publishes their steel manual which gives guidelines and rules for metric fastener applications. In the USA, the American Institute of Steel Construction publishes the Manual of Steel Construction which provides similiar information for imperial fasteners.

You must adhere to the rules laid down for your respective area. It is possible that different manuals will give you differing values for allowable stresses. My training has been to calculate pullout force based on the bolt root diameter. For the thread engagement I believe we need to know the bolt diameter. But how things are done in your location may be different to my experience.

Regards Adrian

 

[PW]

Thanks Adrian.

We do have regs etc in the UK (BS5500) but this problem is more of a fit for purpose requirement and does not come into any code. I know ASME VIII specifies minimum number of threads to be engaged for pipe threads but to satisfy my self that the plug will not shear I wanted to calculate the shear stress in the thread.
This is not as easy as it appears and I do not believe it is as simple as dividing the pressure force by the bolt pitch area.
I need to know if the angle of the thread pitch etc has any bearing on the calc.

Thanks
pw

 

[JamesBarlow]

The problem you are looking to solve is more complicated than you have guesed. When you get down to an actual analysis of the threads it will involve all the dimensions. The thread profile, along with the pitch, all factor in. You can, for simplicity, assume that if your angles are small the stress induced by angle is small compared to the stress caused by the transition from thread face to root (stress concentration).

You should also know that when dealing with large diameter threads, such as those in a pressure vessel, the load is not carried evenly along all the threads. The threads on the inside of the vessel carry more load than those towards the outside.

As a general rule, and I can only say that this applies to the modified buttress thread shown in the ASME code, that taking the pitch area divided by the force does work. We compare the shear in the threads to a modified yield strength ( 1/4 or less of Sy).This will not give you an acurate prediction of failure but if you design to this you will be safe. I admit that this is overdesigned, but until I can work through a formal analysis I would rather add more material than deal with a failed application.

 

[Kenneth]

You might want to take a look at Thread404-11447 for a (somewhat) related question.

The formulas referenced in that thread (found in Appendix B of FED-STD-H28/2B -- available for free at

http://astimage.daps.dla.mil/docimages/0001/34/77/H28-2B.PD9

-- start reading at about page 56) can be used to calculate the (theoretical) load required to shear or "strip" a thread at a given pitch diameter, length of engagement, material shear strength, etc. This calculation is based on the geometric shear area of the internal thread at minimum material condition (equal to the area of that thread which is intersected by a cylinder with a diameter equal to the minimum major diameter of the mating external thread over the length of engagement) and shear strength of the parent thread material.

If you don’t know the shear strength of your materials, two possible sources of free online information are MatWeb (usually “typical” values) at

http://www.matweb.com/

or in MIL-HDBK-5 (usually statistically adjusted values to a given confidence level) at (warning huge download!)

http://astimage.daps.dla.mil/docimages/0001/73/51/MILHBK5H.PD5

or (smaller download; pick your chapter/material)

http://www.grantadesign.com/cgi-bin/stat-tree.cgi?src=MIL5-H.xml&br=0.

; Alternately, you wish to use an applicable material specification “minimum guaranteed” value.

 

[GregPerry]

This is not that difficult. Check out your old varsity design book:

Mine was "Fundamentals of Machine Component Design" and gives the following equation for unified and ISO threads:

F=(pi)d(0.75t)(0.58Sy)

F=Tensile load
d=majour diameter of the thread
t= nut thickness (or depth into a vesel)
Sy=yield strengh

If your thread is different just correct the area portion of the equation ( d(0.75t) )

Greg