We can take into consideration the Jeffcott rotor again, composed by a rigid disk mounted on a symmetrical flexible rotating shaft supported by two rigid bearings. This rotor system has only two degrees of freedom (the displacements of the disk in horizontal and vertical directions) or, if we use a rotating co-ordinate system, still one d.o.f.: the radial displacement of the disk. The unavoidable excentricity of the centre of mass of the disk creates a centrifugal force due to the rotational speed, which deflects the flexible shaft as shown. Due to the deflection the centrifugal force increases: for each rotating speed the centrifugal force will be balanced by the elastic reaction force of the shaft

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