Author Topic: Internal threading Strength Requirements  (Read 420 times)


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Internal threading Strength Requirements
« on: April 24, 2018, 03:59:37 AM »
I have some 1.25" X .092" ti tube that I want to make into an HPA reservoir. I'm planing on 4300-4500 psi charge pressure and using 7075 alum as the threaded end plug. What is the thread pitch and length of thread needed for this project?
Thx, aerotulz

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Re: Internal threading Strength Requirements
« Reply #1 on: April 24, 2018, 07:08:54 AM »
The standard threading used on an MRod tube, which is 1.125" x 20 TPI should be fine, using a 1/2" thread engagement.... The threads are large enough at the root to have to bore the end of the tube slightly (about 0.004" per wall), and yet leave sufficient material above the top of the threads in the tube wall to resist the axial loads of the end plug.... You could also use 24 TPI, but you would have to bore the tube ID out more, so no advantage.... and the minimum diameter of the female threads in the tube would be less than the tube ID (without boring) if you used 16 TPI, so that is not as good....

Using 1.125" OD threads, at 4,500 psi, the minimum safety factor for the installation is for the remaining tube wall, and that is 5.5:1.... This requires that the O-ring that seals the plug be INBOARD of the threaded portion of tube, of course, so that the O-ring bears on the full thickness tube wall.... and the thinner portion of the thread root is not under pressure.... and hoop strength at that point is not an issue....

« Last Edit: April 24, 2018, 07:10:54 AM by rsterne »
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