Is it realistic to expect a simple solid modeling program like Alibre to have the chutzpah to also model the mating of screw threads? Heck, it can’t even model mating involute gear teeth!
As we all know, a screw thread is just an inclined plane wrapped around a cylinder. To expect two of these to mate perfectly over their length of engagement is naïve at best. So, given that there will be an interference fit between the bolt and the nut under tensile load in the bolt, the compressive forces at the interface will vary greatly depending on pitch variation, “bumpiness” of the profile, and friction between the two surfaces. A decent FEA program should include a “tool box” that allows the user to specify this geometry explicitly. I would not expect Alibre, or any other solid modeling program, to specify exactly how two mating threaded parts actually mated. In my opinion, that should be the job of the FEA software package’s mesh generator.
Plus, in Alibre, there is no way provided to specify the lead stack-up tolerance on helical mating threads, which will surely affect the tension and torque on the bolt. I am purchasing some ceramic ¼-20 threaded rods for use as high temperature electrical insulators in a vacuum arc system. The rod manufacturer cautions me not to expect more than five to ten turns of thread engagement before the lead stack-up gets large enough to crack the rod. Should I expect my Alibre model to include this factoid?
That’s just my 2¢. :| I wish someone would offer an inexpensive version of the COMSOL Multiphysics FEA modeling software. Anyone at Sourceforge listening? There is a free Linux/Unix package, SALOME, available for downloading here:
http://salome-platform.org/ Unfortunately, I don’t do Linux yet, although the new Ubuntu release is tempting.
For more clues on free FEA modeling read the attached PDF file.