Three-Dimensional FEM Rotordynamics and the So-Called Centrifugal Softening of Rotors

Centrifugal softening effect is an alleged and elusive reduction of the natural frequencies of a rotating system with increasing speed sometimes found in finite element rotordynamics. This reduction may, in some instances, be large enough to cause some of the natural frequencies to vanish, leading to a sort of elastic instability. Some doubts can be however cast on the phenomenon in itself and on the mathematical models causing it to appear. The aim of the present work is to shed some light on centrifugal softening and to discuss the assumptions that are at the basis of 3-dimensional FEM modeling in rotordynamics. Two degrees of freedom models, like the ones introduced by Rankine and Jeffcott, are first studied and then the classical rotating beam, ring, disc and membrane are dealt with. Some numerical models, built using the FEM, are then solved using both dedicated and general purpose codes. In all cases no strong centrifugal softening is found.