Free body analysis

[shushu007]

Is it good practice to model body that have constraint as a free body for resonance analysis?

 

[rb1957]

if resonance analysis means modal analysis, then I don't think it matters. I think if there are rigid body freedoms in a modal analysis then either the results make it clear (and you disregard), or the results understand these are rigid body motion and doesn't output them.

another day in paradise, or is paradise one day closer ?

 

[GregLocock]

Typically we used to restrain the model, as we would in the lab test, by using some small compliances to anchor it. But your FEA should be able to handle rigid body modes.

Cheers

Greg Locock


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[NRP99]

For finding out natural frequency of system to check resonance, it is required to know the natural frequency of system and not particular free body of that system. Body has different natural frequencies and mode shapes when behaving alone and when behaving in a system.

E.g. Cantilever beam(System) that is fix-free beam have different mode shapes and natural frequencies than only beam(Body) with no support that is free-free beam. Here the fixity of one end increases the stiffness of beam and give different mode shapes and natural frequencies than free-free beam.

 

[GregLocock]

True, but not what he was asking.

For double points, why does the first /flexural/ mode of a beam drop in frequency when pinned at each end, compared with the free-free result?

Cheers

Greg Locock


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[NRP99]

GregLocock

I am not getting your point here or OP's question. Could you explain your point with more information?

Pin-Pin beam is allowed to rotate only in the axis normal to depth of beam keeping all other movements fixed. How is the frequency of Pin-Pin beam will be lower than the free-free beam? Free-free beam will have near zero frequency with random mode shapes which is not at all comparable to pin-pin beam which have frequency of positive number and mode shape across/along the flexural direction.

 

[GregLocock]

Maybe you need to reread what i wrote.

Cheers

Greg Locock


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[shushu007]

here calculation of beam natural frequencies at different fixing and free beam

http://web.mit.edu/2.75/fundamental...ctures Design Spreadsheets/Beam_frequency.xls

in beam fixing one end of the beam reduce the natural frequency compare to free-free beam

 

[NRP99]

Still not understood the point here.

Maybe I need to understand what is double point that you mentioned.

Maybe I need to understand more about mathematical assumption of sinusoidal waveform of natural vibration which prove that free-free beam have high frequency than pin-pin beam except translation modes and zero frequency of free-free beam and Clamp-Clamp beam have same frequency as that of free-free beam.

But what about the actual scenario?