An active control of bearing is proposed as a new method to improve the stability characteristics of the externally pressurized air journal bearing and the results of numerical investigations of the stability characteristics of the actively controlled air journal bearing are presented. The synchronous control, where the bearing center whirls with the same frequency as the journal center with respect to the position in the steady state, is proposed as a control algorithm of the active air bearing. The step jump method is used to calculate the locus of the journal center which whirls or vibrates relative to the whirling bearing. The stability characteristics of the high speed rotor supported by actively controlled bearing system for zero steady-state eccentricity are investigated for various phase difference between the bearing and journal. It is shown that the stability of the air bearing can be greatly increased by the controlled synchronous motion of the bearing, and there is an optimum phase difference, which gives the maximum stability limit of the system.

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