Can someone explain to me gyroscopic stiffening and softening

[tricha122]

hi,

I’m trying to understand why in a rotor bearing system that the critical speeds of the rotor are higher for forward whirl, and lower for backward whirl.

My general understanding is that forward and backward whirl frequencies diverge due to gyroscopic effects, and for forward whirl the gyroscopic effect is “stiffening” - and so the critical speeds are higher, and the opposite is true for backward whirl.

I’ve never been able to rationalize this with a FBD or by looking at the equations of motion. Does anyone have a good way to visualize / explain this phenomenon?

Any help would be greatly appreciated, thanks!

 

[anorlunda]

I’m trying to understand why in a rotor bearing system that the critical speeds of the rotor are higher for forward whirl, and lower for backward whirl.

Why do you think that is true? Provide like to what you have been reading please.

 

[tricha122]

https://goo.gl/images/ZBfE4J

In a whirl frequency plot, it is always the case that forward whirl modes trend upward with rotor speed, and backward whirl modes trend downward. This is due to the gyroscopic effect. At zero speed these modes coincide / are the same. But at high speed, if there’s significant gyro effects, these modes can even look different and have different strain energy percentages between the shaft and the bearings.