Modal balancing of flexible rotors 1

[heitor]

Hello.

I'm studying the modal balancing of flexible rotors and I found a small problem with this technique.
I found strange modelling the rotor as a non-rotating structure, i.e., neglecting the gyroscopic matrix, to find the modal shapes.
So my question is: the mode shape of the rotor at the first critical speed is the same of the first mode shape of the rotor at rest?

Thank you.

 

[GregLocock]

It is on simple linear systems. As you introduce more complexity into the model it will diverge.

None the less, it is very easy to predict the whirl speed of shafts by measuring the frequency of their non-rotating bending mode (I only ever need to worry about first mode, luckily).



Cheers

Greg Locock

 

[heitor]

But Greg, you are neglecting the gyroscopic effect by doing this, aren't you? So the prevision made by the non-rotating first bending mode will not be correct.

Thank you!

 

[GregLocock]

Yes I am neglecting gyroscopic effects. Perhaps because I deal with "simple linear systems", or perhaps for another reason. In a linear model the deflections are assumed to be negligible, therefore the angular deflections will be negligible, therefore the gyroscopic torques will be negligible.

Most classical balancing techniques rely on linearity, if only because the system will get more linear as the balance improves.




Cheers

Greg Locock

 

[JasonChen]

According to my experience, the gyroscopic effects would have significant affection when the rotor runs in high speed, such as 10k rpm or above. And the effect is also relative to the mass distribution of rotor. But generally speaking, the 1st mode of flexible rotor would not have obviously coupled critical speed in practice.

Jason Chen