Thread Strength Calculation

Hello all,

I am in need of a means to calculate thread strength.

In the assembly of hydraulic actuators, a thread will be machined into the bore of the tube, and onto the mating component. Although we have not suffered any failures, our thread sizes and lengths are always worked, arbitrarily, from the fact that they have worked before. It would be far better to have a definite figure of what the breaking strength of the thread will be.

In looking it up, it appears hard to find a definitive answer. Likewise, to me, it is unclear whether the failure mode of the thread would be under tensile/compressive load, or shear. Either way, I am unsure of quite how the area would be determined.

I would be most grateful if anyone could help me find an answer.

  • Can't help you with a formula I'm afraid. But am thinking the weakest point would be that part of the tube which has been threaded, but does not have the component thread inside it. So you wouldn't want to have more thread in the tube than the length of the component thread. I know tap threads are tapered, so you will have some scoring of the inside of the tube. Might be worth thinking about a sleeve on the outside of the tube covering the length of pipe which is cut into on the inside. 

    Just my tuppence worth

  • I hadn't thought of it in that way, but I see exactly where you are coming from. The mating part screws up to a shoulder, so do have the internal thread extending beyond. I might have to look at that... Thanks!

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