You may not know exactly how many frequencies will be needed, but results from your first pass study can be used to check by accessing the percentage of Mass participation. First, just run a linear frequency study with, say, 5 or 10 modes. After completion, right click the Results folder and choose "List Mass Participation Factor". Check the MPF at the bottom of each column (x. y, & z directions). The coarse rule of thumb is that if the MPF is under 80%, then more modes may be called for. Note that for perfectly symmetrical motion (e.g., both tines of a tuning fork moving toward and away form each other symmetrically) the MPF in that axis is listed as "0," so a low mode number with 0 MPF may mean that you'll never get to 80% MPF in that direction.
If the total MPF can be raised significantly by adding a few more modes (say going from 50% to 65% by adding 5 modes), then keep the higher number of modes. If, on the other hand, you add modes and the total MPF does not increase, then you already have enough modes.
Here's an interesting article by Jaime Martinez with a partial explanation of using the MPF in SW Simulation Frequency studies:
And a nice example by the same engineer:
Much appreciated, Jaime.
Jaime uses a general rule of thumb for a given mode's participation of 1% MPF as a minimum if it is to be considered to be a significant mode. This SolidWorks blog post says to consider a mode significant if the MPF is > 2%:
These are guidelines and not absolute answers. Engineering judgement comes into play here.
thanks a lot for your helps
i run the study with 10 mode of frequencies
and i got theses results of 80% mass participation
but my question now is should i now calculate frequencies closet to the (330.44 Hz ) where i got the higher value of mass participation on the (z) direction ( the dynamic study is 300g in the Z direction) ?? or what to do now ??