Eigenvalues of two similar structures

[stressoblivion]

Dear experts, I have two commercial aircraft interior galley structures. One of them has been in service for a while. The other is a new unit that needs to meet same requirements of vibration. So the objective is to compare these two structures to show compliance of the new unit with Part 25.

They both weigh between 400lbs and 500lbs. Looking at the first 10 modes of both structures using nastran, when I looked at the eigenvalues, the new str has a first mode value of 9.4, the old str has a value of 0.042. The way I interpret them is that the heavier unit needs a higher input frequency to excite the first mode, while the old unit needs a much lower input frequency (it is only slighly lighter). How can I use these values better? Why such a big difference?

Much Appreciated.

 

[electricpete]

0.042 hz would be an extremely low natural frequency for just about any simple system I can think of. I guess it can happen for a complex system, but I would think you would have to work very hard to tune it to do that... or else get very lucky. I would think it might be worth looking at the modeshape of that 0.024 hz mode to see if it lokos realistic.

Just a thought from someone without much experience analysing complex systems.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[GregLocock]

You almost certainly have a bad modelling error in the 0.042 Hz one.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[bigzeke]

The .042Hz is likely a "rigid body mode", check your model for accurately modeled restraints.