Stability of whirl and whip in rotor/bearing systems

Abstract

The paper presents a continuation of the analysis of lateral vibrations of the lightly loaded rotor/bearing system discussed in reference [1]. In the present paper, it is shown that the rotor self-excited vibrations (known as oil whirl and oil whip) due to fluid dynamic forces generated in the oil-lubricated bearing, can exhibit multiple regimes. These regimes were observed experimentally and were reported from the machinery field data. The analysis of stability of the self-excited vibrations of the whirl- and whip-type is outlined in this paper. It is shown that after the threshold of stability, the pure rotational motion of the shaft becomes unstable and the whirl regime is stable. With increasing rotational speed the whirl smoothly transforms into whip. At a certain speed, determined by the system parameters, whip may disappear and either with a “jump” in frequency and amplitude becomes replaced by the second mode whirl, or the rotor motion stabilizes for a limited range of the rotational speeds, and then the second mode whirl starts again. These phenomena were observed experimentally. The analysis is based on the four-degree-of-freedom model of the rotor (the first bending mode and two rotor modal masses taken into account) and on the rotating fluid dynamic forces in the bearing.