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.

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.....

To a good first approximation the in ward pressure may be assumed to be related to the thread tension resolved along the active ramp of the thread angle (under load one side of the thread is unloaded). For normal 60 degree triangle threads, this is ~ twice the load force on the threads, i,e,  the tension divided by the length of thread bearing load - usually only the top turn or two of a thread are doing any work, more in elastic materials. less in harder ones although the usual advice is that thread engagement of about one bolt diameter (6-8 turns) is best, in reality half that is doing about 75% of the heavy lifting....

Mike.