How to constrain an FEA model for modal analysis
How to constrain an FEA model for modal analysis
(OP)
Hey,
Any FEA guys out there? I have modeled a gear in PATRAN in order to determine the natural frequencies and mode shapes. My question is: How should I constrain the model? I noticed that the results are very sensitive to how the model is constrained. I simply constrained one node (completely) to fix the model in space, but is this right?
Anyone?
Anyone know of a good reference book(s) on doing modal/vibration analysis using FEA?
Any FEA guys out there? I have modeled a gear in PATRAN in order to determine the natural frequencies and mode shapes. My question is: How should I constrain the model? I noticed that the results are very sensitive to how the model is constrained. I simply constrained one node (completely) to fix the model in space, but is this right?
Anyone?
Anyone know of a good reference book(s) on doing modal/vibration analysis using FEA?





RE: How to constrain an FEA model for modal analysis
Is your gear circularly symmetric? The first two modes of a ring-like structure which is circularly symmetric occur at exactly the same frequency and modeshape, but one of the mode shapes is rotated 45 degrees around the circle with respect to the other. In the perfect case the "angle" of the modeshapes will be arbitrary (although the 45 degree separation remains). Introducing a slight asymmetry (your constraint at 1 node) will tend to force the node of one of these first two modes to occur at the constraining point and the other mode may not be exactly 45 degrees different.
However I would expect the frequencies and modeshapes to not change too much. You say your results are sensitive to a single constraint. But how sensitve? How much do the frequencies change? do the modeshapes look very different?
Michael
RE: How to constrain an FEA model for modal analysis
For your case, there is no need to apply any constraints to your model.Normally; After model preparation, boundary conditions are defined (constraints etc.), forces are applied and you pass to the solution.But in modal analysis you do not need to do all these.
After modelling you should select 'free body dynamics' option.(it should be in the boundary conditions part) Once you select it, the program will automatically disable the force buttons.Now you are ready to analyse your model.In the solution set do not forget to change the number of natural modes that you want to visualize.The first 4-5 modes will probably be static rigid body modes, with very small frequencies on the order of 10^3 or so.
As reference material:
For the basics: www.sem.org (experimental mechanics society)
Text Book:D. J. Ewins,Modal Testing
For Patran: go over the examples, workshops provided in the manuals, they would really help.
Öncü
RE: How to constrain an FEA model for modal analysis
like all other have stated, actually you need no boundary counstrains for doing modal (eigenvalue) analysis.
Of course, may be you would get very little eigenvalue (near to zero) for the 1-3 first eigenvalues, these are referred to free body modes. If you want to avoid this, then simply constrained it, as you said, or take over the constrains defined for static analysis.
Hope this would help you.
cheers
RE: How to constrain an FEA model for modal analysis
Also you will always get 6 rigid body modes if the item is truly free-free, not 1-3!
Cheers
Greg Locock
RE: How to constrain an FEA model for modal analysis
Thanks,
Goodvibes