1. Modeling principle

A shaft line is made up of a set of parts connected either to each other – rotor-rotor connections – or to the frame – rotor-frame connections –. The actual distribution of torsional stiffness and inertia with respect to the axis of rotation is represented discretely, with the model consisting of a succession of undeformable inertia disks placed at the ends of massless torsion springs. The shaft line is divided into a set of sections, the number and size of which depend on the desired application and the complexity of the rotor geometry. For each section, we identify its equivalent torsional stiffness and its equivalent moment of inertia relative to the axis of rotation. The model of each rotor is obtained by assembling the preceding elements; a rotor-rotor connection may be represented by a torsion spring only, its moment of inertia being zero, as in the case of contact between two elastic...