Unacceptable Frequency

[Mariah]

Hi,
I did analyze a MOL pump padestal and its motor experimentally using Modal Analysis technique. We got 5 natural frequencies and mode shapes. Then, I modeled the structure in MSc/Nastran and Abaqus. I asked for 20 modes when running the analysis.Yes, I got 5 of them were quite similar to the experimental values. HOWEVER, the low modes values were too low and unacceptable - about 0.6 and 0.5 Hz. Why I got such values?What are the reasons for that?

 

Hi mariah.
check your model,s boundary conditions once..There may not be sufficinet restraints...or those low frequency modes may be local modes,which U can negelect and see only see global modes.Take alook at th e animated mode shapes.

cheers!

 

[zuardy]

Yes, I agree with GoodWishes, you should take a look to the boundary conditions. Because a very low eigenvalue is normally due to the insufficiency of boundary definition (constrains) and referred as rigid body modes. It is used some times to verify a model, wether it is constrained sufficiently.

cheers

 

[Mariah]

Thanks a lot guys!