The dynamical behaviour of the rotor is mainly due to the flexural vibrations, which can be excited by several different causes. One of the most common causes is the eccentricity of the centre of gravity, which is called the unbalance of the rotor. Here we have certain amount of unbalance and certain unbalance distribution along the rotor. Generally there are also some torsional vibrations, which may interest the dynamical behaviour of such system, but in this course we are mainly interested in flexural vibrations i.e. lateral vibrations.