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Numerical Simulation of Transient Resonace Vibrations of a Rigid Rotor in Roller Bearings with Clearance

Irretier H.

Machine Dynamics Problems

2007

Vol. 31, No. 2

80-83

Extended abstract. For the compensation of a line fault of the general power supply system until the start of emergency power generators mechanical energy storage systems are used which consist of rotating flywheels with large mass moments of inertia fixed on thick shafts with attached generators (Fig. 1). The rotors are arranged in a vertical position and ran, supported in roller bearings, in a vacuum casing set up on weak vibration isolators. Inspections of the rotors during run-up and run-down show that transitions through unbalance resonances in the range of the nominal speed of the rotor exhibit vibration jump phenomena as they are known from systems with nonlinear spring characteristics under harmonic excitation. In case of clearances of the roller bearings due to long time of operation damages, similar effects are expected. By a numerical simulation it is proved that the observed unsteady transient resonance vibrations are produced by the clearances in the roller bearings and the correlation between the amount of the clearances and the observed vibrations of the rotor are investigated. First, the natural frequencies and the associated mode shapes of the rotor are calculated with help of a finite element model (FE) to give information if it is possible to apply a rigid body model (RB) for the simulation of the vibrations of the rotor in the operational speed range (Fig. 2). The results show that up to the nominal speed of 3400 rpm the bending vibrations of the rotor can be calculated as rigid body oscillations of the rotor shaft and the attached flywheel supported in the two roller bearings and the vibration isolated casing. Then, the equations of motion of the rigid body model are numerically integrated under consideration of the clearances of the roller bearings (Fig. 3). By several numerical studies the influence of the amount of the clearances on the transient resonance responses of the rotor during run-up and run-down through various critical speeds is investigated. The numerical results show that during the passages through the 1. and 2. forward critical speed of the rotor large transient unbalance vibrations occur. At high levels of clearances of the bearings they are superimposed by significant higher frequency vibrations (in particular the 2nd harmonic) which remain on a high magnitude even after the resonances and partly coupled with sudden changes of the related amplitudes. For sufficiently small values of the clearances of the bearings these effects recede into the background and they vanish completely for bearings without clearance.[...]