English Buttress Thread Shear Area 1

[subtechy]

All,
Looking for advice on a thread which took an absolute age to track down. As the title describes, it is an English Buttress thread. Before you start panicking, this is based on the traditional American Buttress thread and has 7deg and 45deg angles on the thread. The thread being designed to withstand high stresses along the thread axis in one direction only.
What I am trying to decipher, is how to calculate the shear area of this thread. The machinist handbook gives calculations for the general purpose acme threads shear area but none for the buttress threads. Can anyone tell me if this can still be worked using the same calculation, but instead of using the acme 14.5deg angle use the 7deg for the buttress?
I personally do not feel this is right, due to the taper of the thread not being symmetrical (ie. 7deg one side and 45deg the other).
If anyone else has any brighter ideas, let me know.
Or, if any further info is requested, let me know also.

Cheers in advance

Martyn

 

[Cockroach]

Yes, you're bang on!

In the Machinist Handbook, the 0.25862 entry is physically half the tangent of half the flank angle, tan(29/2) = tan(14.5). This appears again in the stress analysis for Unified National Threads (ANSI B1.1-1982), 0.57735 is half the tangent of the flank angles, tan(60/2) = tan(30).

Your problem is going to be with the equivalent material expression for the English Buttress thread, since the fore and aft angles are not equivalent. Afterall, 45 and 7 degree slope leading edge(s), then straight back, should be mechanically equivalent to a thread of equal material at another specified, solid angle. Then you could "estimate" thread shear based on the tangent of half the equivalent angle which represents same material volume.

This is only the first half of the solution. You must also account for the taper of thread, the American Butress is 3/4" per foot. If you follow the discussion presented for National Pipe Threads, you can use a similar approach noting the average pitch diameter of thread, then rectifying for a straight thread. Afterall, the Acme, Stub Acme and UN thread stress equations apply for straight and not tapered threads, a guy has to account for this effect in some manner.

I've only used this approach for NPT forms, my tensile tests in the lab were within 5% of theoretical (Hueston - Halliburton 1996). The method should also work for Buttress forms!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada