Coming back to the different exciting causes, which act on system of rotor, their list is composed by unbalance, bow (permanent bow or thermal bow), misalignments of the bearings and also of the coupling, axial unsimmetry, crack (which is a very dangerous defect, since it can develop to high depth without giving very strong indication about its presence). Other causes are rotor to stator rub, instabilities and also some other causes. The model of the exciting causes can be generally represented by a system of equivalent forces and couples.

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