Effect of journal out-of-roundness on stability of a symmetric hydrodynamic journal bearing system. Part 1: Theoretical analysis

Abstract

The study examines the effect of journal out-of-roundness on the dynamic performance of rotating machines supported by oil film bearings. A general model of a symmetric hydrodynamic journal bearing system allowing for journal out-of-roundness is presented. The results show that the journal out-of-roundness influences the oil film thickness and further influences the dynamic characteristics of the system. The journal out-of-roundness may incur an additional vibration component when the system operates. The amplitude of this vibration component is dependent upon the magnitude of the journal out-of-roundness, while the frequency of which is an integral multiple of the synchronous vibration frequency, depending upon the profile of journal out-of-roundness. Although journal out-of-roundness may increase overall system stability, when dimensionless out-of-roundness is below 0.2, there is little effect; between 0.2 and 0.4, stability is improved but vibration levels are high. When the dimensionless journal out-of-roundness is larger than 0.5, the theoretical analysis shows much uncertainty on the system stability and the boundary of oil film instability is not very clear, this needs further examination from both theoretical and experimental evidences. The system designer should balance between the stability and the vibration level at the design stage.