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.

Parents
• Another possible mode of failure that comes to mind, presuming the threads are triangular or similar, is that a pull force will tend to be translated into a crushing force on the inner threaded tube - if it's relatively thin walled or particularly malleable , I could see it collapsing allowing the internal thread to contract and pull out.

- Andy.

• Another possible mode of failure that comes to mind, presuming the threads are triangular or similar, is that a pull force will tend to be translated into a crushing force on the inner threaded tube - if it's relatively thin walled or particularly malleable , I could see it collapsing allowing the internal thread to contract and pull out.

- Andy.

Children
• True - the normal analysis assumes the 'bolt' is solid, or at least largely so. There is an identical  "bursting"  force in a nut,

However, to collapse a cylinder, even one weakened by a spiral groove, is hard - but depends on  the diameter as well as the  sidewall thickness, for a given external pressure a tube with a smaller diameter can have a thinner side-wall. The analysis is similar to that of collapsing a  vertical beam in compression, where the danger point is when if there is small assymetry, the energy gained if futher collapse occurs is greater than that needed to cause  it.

The text book formulae (Rourke and Young) above are for perfect defect-free thin walled tubes collapsing under external pressure. This probably is not the situation and the results should be considered with caution, or at least a factor of 5 safety.....