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# Calculate the whirling/critical speed of a shaft

## Calculate the whirling/critical speed of a shaft

(OP)
I was asked to calculate the critical speed of a complex shaft with blades (for an helicopter) and since I've never done it before I'm doing a smaller model with a beam and a lump mass at the end. I have the equation to calculate its critical speed so I can double check with ABAQUS.
The problem is that I don't know how to calculate this value through ABAQUS. Any idea? It's a direct-Solution steady-state dynamic analysis and I read already the documentation but I'm missing something I guess.

Thanks!

### RE: Calculate the whirling/critical speed of a shaft

In order to find critical speed you should create Campbell diagram (natural frequency vs rotation speed for different modes). Use complex eigenfrequency extraction preceded by general static step with rotordynamic load (ROTDYNF). Also before complex eigenfrequency exctraction you will need real natural frequency extraction step.

### RE: Calculate the whirling/critical speed of a shaft

(OP)
According with the documentation I should have Complex Frequency as choice in the Linear Perturbation step but I don't have it.

### RE: Calculate the whirling/critical speed of a shaft

You have to define regular „frequency” step first. Then complex eigenfrequency extraction will become available from linear perturbation procedures group.

### RE: Calculate the whirling/critical speed of a shaft

(OP)
Can you confirm I have to apply the load via input file? apparently it's not supported in CAE.

Also, is there a way to get Campbell diagram via ABAQUS or I have to create them by changing the input value of rpm?
Thanks!

### RE: Calculate the whirling/critical speed of a shaft

Yes, ROTDYNF load is not supported by CAE and you have to add it manually to the input file.

Unfortunately Abaqus won’t generate Campbell diagram automatically and you have to create it by running several analyses with different rotation speed.

### RE: Calculate the whirling/critical speed of a shaft

(OP)
Thank you. I got to that point, I'm trying different rpm (or rad/sec). Unfortunately I don't see much change in the amplitude, i.e. I can't figure out which speed is the critical one. In other words the peaks are way too small to assume that's the critical speed.

I created a static with Rotordynamics load, a modal analysis and complex analysis. With animation one I can see the whirling.

### RE: Calculate the whirling/critical speed of a shaft

(OP)
I wonder if I should plot a different amplitude, since I'm in the complex domain

### RE: Calculate the whirling/critical speed of a shaft

Does your Campbell diagram feature lines for different modes ? Critical speed is found on the intersection of two lines.

If you search for "Rotordynamics Analysis
using Abaqus/Standard" you will find a nice document with example of such analysis.

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