One of the main interesting problem in the dynamical study of the rotor systems is the determination of the critical speeds. We can show the concept of critical speed with aid of a very simple model (Laval or Jeffcott rotor). The disk has an eccentricity of its centre of gravity and the shaft deflects elastically. This deflection can be calculated as a function of the rotating speed and in this model you have in correspondence to the critical speed an amplitude which can reach infinite value. In real machine, in correspondence of a critical speed, very high vibration levels can be reached.

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