# Why the routine you proposed in the example for rotordynamic prestressed analysis fails?

Hi, I am a student developing computations on rotordynamics for my thesis. I am using ansys 2020 R2. I find a problem in the solution of the prestressed (where the prestress is given by the centrifugal force) case for a NON symmetric geometry. In particular, the solution in the rotating reference system seems to produce negative and zero frequencies as I was dealing with a symmetric part. This holds both for the example at https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v202/en/ans_str/strllinpertrotmach.html and, also for my case study which is a rotating bladed disc with a shaft attached and a cantilever mount (in other words a stodola-green rotor with blades). The error that comes out is that of a large negative pivot found. In the attachment I send you the routine used for the solution and some images describing the problem.

I would really appreciate your help since this problem is taking me away from the conclusion of my master thesis.

Ps: jsut for completeness, the part is meshed with solid185 elements and they are no distorted elements.

## Answers

Spin softening is a linear representation of the additional centrifugal force produced by radial displacements of the model. For the finned model shown, a very large rotational velocity might produce such a large increase in the centrifugal force that the spin softening is greater than the stiffness of the fin. A test would be to do a static analysis of the angular velocity gradually applied with large deflection turned on. The force deflection curve should indicate whether spin softening is sufficient to overcome the stiffness of the fin.

Unfortunately we tested the script on another Ansys version and it seems to work there.