When building a model in an analysis involving rotordynamics, it is important to identify and separate rotating and non-rotating parts to:
Apply the rotational velocity (or velocities) to the rotating parts
Make sure the rotating parts are axisymmetric
Whether you construct your model in MAPDL, or you import it from an external CAD program, you will want to use the grouping and selecting capabilities in MAPDL to define areas of your model in ways that will optimize your analysis.
In the case of a rotordynamic analysis, this means identifying the spool, the bearings, the support structure and other areas as components or assemblies. See Selecting and Components in the Basic Analysis Guide for more information on how this capability can be applied to your analysis.