I have developed a mathematical model to study the dynamic behavior of rotor (with 6 dof). It is assumed that rotor is rigidly mounted at the mid point of shaft, and both end of the shaft is attached to ball bearing. I have developed the FE model for it too, and the frequencies in X, Y, Theta, Phi (in plane translation & rotation) is same in analytical & FE model. However for Psi (out of plane rotation),I assumed that the restoring moment will be provided by the torsional stiffness of shaft (G.J/L) and got a frequency; but in FE model I got a rigid body mode for this dof. My doubt is, for a shaft whose one end is attached to a bearing, will there be any torsion / or in other words will it provide any restoring moment ?