Emilio Graff

Frequency plots---resultant displacement is senseless

Discussion created by Emilio Graff on Oct 20, 2009
Latest reply on Oct 24, 2009 by Joe Galliera

This is an announcement---a complaint if you will---maybe it will help someone who gets confused.


The so called "frequency" analysis has the purpose of finding the natural frequencies and mode shapes of your meshed body. This analysis is akin to finding the frequencies of a vibrating string, or, slightly more sophisticated, that of beams (which is related to column buckling). The results are (by default) shown as "resultant displacement" with real units.


Let it be made clear that it is impossible to calculate actual displacements with such analysis. It only gives you the frequency at which the mode occurs, and the shape of the mode. The actual displacements depend on the input excitation, as has been said previously in other posts. In a theoretically frictionless/dampingless world the displacements would be infinite, because by definition, these are the resonant frequencies of your system and all excitation at that frequency gets amplified. By this very fact, under random vibration, the system tends toward these frequencies. Moreover, the only two material properties that matter in this calculation is the Young's modulus and density.


So, Solidworks, shame on you for:


1. Putting real units on the scale bar of these plots.

2. Making the magnitudes completely absurd---I'm getting 3 meters of displacement on a structure not 1 meter tall.


For those of us who know this is nonsense, it can almost be ignored. But it inevitably means extra phone calls and raised eyebrows come design review time. You cannot expect your customers to first give a short lesson on the subject to their clients.