Influence of Slight Gravitational Effect on the Characteristics of Onset Speeds of Instability and Stability in the Vertical Rotating Shaft Supported by Journal Bearing

In rotating machinery such as vertical pumps supported vertically by sliding bearings, it is difficult to predict the dynamic characteristics because of the strong influence of the nonlinearity of the sliding bearings. In addition, because the direct stiffness coefficient is small in a vertical shaft, the effect of unbalance causes large whirling amplitude and its effect on the dynamic behaviors becomes large. The authors have recently shown these characteristics in a vertical rotor system. The stability of the vibration response changes qualitatively between destabilizing and stabilizing conditions depending on the magnitude of the unbalance, and a characteristic curve that represents the change of stability was obtained. This paper investigates the effect of a small amount of gravity acting on the vertical rotor system. In particular, the effect of this small gravity effect on the characteristic curve representing the change in stability of the vibration response is investigated.