Journal Bearing Response to Excitation and Behavior in the Unstable Region

Abstract

Three types of journal bearing of circular bore, elliptical, and offset-halves designs were subjected to excitation tests over a wide range of frequency. It is shown that, in the stable region of operation, the amplitude of the response correlates reasonably well with the solutions of linear theory. The response was found to peak (strongly in some cases) at excitation frequencies in the range 0.43 to 0.75 of the shaft running speed, depending on design and distance of the operating point from the instability threshold. These observed natural frequencies were found to correlate well with linear theory solutions for lower range frequency values up to about 0.65. At higher predicted natural frequencies, the observed resonance was less than the theoretical solutions. Little is known of bearing behavior beyond the threshold of stability. It is shown experimentally that limit cycle motion occurs in the unstable region. The orbit size can be quite small, depending on preload and other design factors. The orbit size tends to increase with distance from the instability threshold.