Cracked rotors: Industrial machine case histories and nonlinear effects shown by simple Jeffcott rotor

Horizontal rotors are always imposed to periodic stresses and, therefore, a crack due to a fatigue is unavoidable. The proper diagnosis of machinery is necessary to prevent tragic accidents and the vibration monitoring is the most important tool for such a diagnosis system. In order to develop a monitoring system that can detect a crack in an early stage of propagation, it is important to know the vibration characteristics of a cracked rotor. A crack opens or closes due to the direction of the lateral deflection. Therefore, a cracked rotor has nonlinear spring characteristics of a piecewise linear type. In order to diagnose the vibration characteristics properly, it is essential to understand the behavior caused by the nonlinearity. These piecewise linear characteristics make a directional difference in stiffness and this difference rotates with the rotor. As the result, the coefficients of linear and nonlinear terms in restoring forces become time dependent. According to the physical characteristics, cracked rotors can be classified into a class of nonlinear parametrically excited system. At first, this article introduces case histories of cracks found in industrial machines. Secondly, it explains the vibration characteristics of various kinds of resonances due to cracks using simple Jeffcott rotor with nonlinear spring characteristics. The utilization of the nonstationary vibrations for monitoring system is also explained.