Abstract
Seal stability is often evaluated by Whirl frequency ratio (WFR) and Effective damping coefficient (Ceff) calculated on the assumption of synchronous whirl. However, the natural frequency of the rotor system must be used for calculation of WFR and Ceff when determining self-excited vibration. This paper discusses the evaluation of seal stability using WFR and Ceff. First, the stability analysis is performed by FEM for a simply supported Jeffcott rotor model, having a seal near the disk. Next, WFR and Ceff are calculated for the following two frequencies; (1) the natural frequency of the rotor system, (2) rotational frequency. As the reference of stability, the logarithmic decrement is used for comparing with WFR and Ceff. Effective damping coefficient (Ceff) calculated using the natural frequency of the rotor correctly estimates the seal stability. Whirl frequency ratio (WFR) for the natural frequency of the rotor can judge the rotor stability, but cannot evaluate how good the seal is. Both Ceff and WFR calculated by using the rotational frequency cannot give correct criterion for the rotor stability.
