Damped versus undamped critical speed calculations

[VibFrank]

Hy experts,

I often have the job to calculate lateral critical speeds for pumps. The one who did the job before me only did undamped calculations whereas I do both damped and undamped. Now the problem: most specifications still have the sentence: the distance to rotational speed must be greater than 20%. The (old) undamped calculation fullfills that, the damped calculation does often not. What are you doing in such cases ?

1.) Doing only undamped analysis.
2.) Discussing with the customer about allowable damping ratios

Thanks for your comments
Frank

 

[VibFrank]

You can find references to 1% modal damping in the references of the API Publication 684...but certainly: measurements are the best...