I'm running a free free analysis (modal analysis of an assembly without external constrains, to verify the “rigid body modes”) with FFEPlus solver.
For the connections between the parts of the assembly, I've deleted the global contact of the entire assembly and I've manually chosen the contact set between each component.
The problem is that i get the 4th, 5th and 6th mode of vibration with frequencies of 5Hz, 6Hz and 9Hz respectively, while I expected to obtain frequency very close to 0Hz (as for the first three modes of vibration).
I've verified the same problem with a small assembly (see attached images): if I set the global contact set, I get the expected 6 rigid body modes.
If I manually set the contact set between the parts, I get different values for the rotational modes.
Can someone help me?