free free dynamics chech
free free dynamics chech
(OP)
I am carrying out a free free dynamic check to an assembly model to verify that it acts as a rigid model, honestly I don't have clear how to interpret my output results.
I mean the first 6 modes have got a frequenty of 1E-4 magnitude or lower, but the shape of the 6 animated models don't recall me the 6 modes of a beam (example).
The others modes (the 7th the 8th etc) have got values higher that 1 Hz and I thought that a rigid body without contraints shouldn't have futher modes.
Someone could clarify the subject or advice me some papers.
Thanks
Regards
Engroma
I mean the first 6 modes have got a frequenty of 1E-4 magnitude or lower, but the shape of the 6 animated models don't recall me the 6 modes of a beam (example).
The others modes (the 7th the 8th etc) have got values higher that 1 Hz and I thought that a rigid body without contraints shouldn't have futher modes.
Someone could clarify the subject or advice me some papers.
Thanks
Regards
Engroma





RE: free free dynamics chech
The 7th, 8th, etc. modes are probably meaningless for frequency, but may start to look like what you would expect from the bending mode shapes (a jumprope about each of the three major axes followed by two max/mins, etc.)
As for a text...do a google search and you will probably get a google of hits.
Garland E. Borowski, PE
Engineering Manager
Star Aviation
RE: free free dynamics chech
corus
RE: free free dynamics chech
Modes 7 and above should correspond to the free vibration modes of the unrestrained ("floating") object. Think of tossing a long flexible stick violently in the air - as well as flying as a projectile, it will probably "flex" in a shape similar to the first mode of a simply-supported beam, but in a way such that the centre of mass does not move laterally. That is, as the mid-span flexes to the left, the tips flex to the right. You can see this behaviour in slow motion footage of a javelin throw, for example.
Whether any of these higher modes are relevant to your analysis depends on what you are trying to achieve.
Hope this helps!
RE: free free dynamics chech
In a free-free modal analysis, the first six modes are all rigid body modes that will have infinite translations with an essentially meaningless frequency that is in effect 0Hz. These modes are the six translations and six rotations. You are not interested in these and for the sake of analysis, you should set your first shift point for the eigenvalue solver to 1Hz or higher. The higher modes are real free-free natural frequencies of the structure.
All continuous structures have an infinite number of natural frequencies and there is no such thing as a structure that will behave as a rigid body over the entire frequency spectrum. Say your FEA model gives the first non-zero free-free frequency as 50Hz then that is a real resonant mode of the structure. The structure will NOT behave as a rigid body when excited at or near 50Hz.
Now if a structure has a fundamental frequency at least 3X the forcing frequency, the structure will behave approximately as a rigid body, that is the vibration maginfication will be ~1.
I think you may be confusing the concept of "rigid body" with "static" response. When the ratio of the driving frequency to the natural frequency of the structure is below say .3 then response=input. The structure behaves in a static (or quasi-static to be more correct) manner. It may also behave as a rigid body, but not necessarrily.
RE: free free dynamics chech
The first 6 modes should be of very low frequency. The mode shapes may line up with the axes of the model or not. It doesn't much matter.
Your other modes will be flexural, and will be similar to the ones you'd expect from a more heavily constrained system. They should look 'sensible', and will be somewhat higher in frequency than a restrained model, usually.
If your model contained a hinge or an additional free body then you would have more than 6 of the very low frequency modes, so it sounds to me like you have proved that the model is not a mechanism and has only one free body.
Cheers
Greg Locock
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