Paper 5: Some Factors Affecting the Whirl Instability of a Journal Bearing (Part 2)

Abstract

Experiments have been carried out on a simple unloaded shaft-bearing system to provide values for the onset conditions of oil whirl which can be readily compared with theoretical estimates making the minimum assumptions. Further evidence has been found to support the previous conclusion (Part 1) that, contrary to current theories, the ratio of elastic to hydrodynamic forces has a significant effect on the stability in the laminar flow region, and has a similar effect in the Taylor vortex region. It is suggested that this is because the axis of a journal rotating above its critical speed is not stationary, as assumed in current theories, but is rotating at shaft speed about its mean position. The extension of this work to include an investigation of the influence of clearance ratio, length to diameter ratio, journal mass and shaft diameter has confirmed that there is a substantial increase in stability with reduction of B. With Taylor vortex flow, the results are consistent with the onset of whirl occurring at twice an effective natural frequency, which is determined by the shaft stiffness and an oil film stiffness acting together. On this basis estimates are made of the effective oil film stiffness in the Taylor vortex region. In the laminar flow region, there is a similar variation of stability with B. The transition from laminar to Taylor vortex flow has a pronounced effect on the stability.