Influences of bearing damping on critical speeds of flexible rotors are investigated. First, the damped critical speed and the damping ratio of a uniform shaft supported at both ends are examined. It is shown that there exists an optimum bearing damping that gives the maximum damping ratio for each order of critical speed, and that with increasing support stiffness the optimum damping increases but the maximum damping ratio decreases. The meaning of critical speed map is discussed on the basis of the attainable maximum damping ratio. Next, the critical speed characteristics of a uniform shaft supported by three or four bearings are examined. Especially, for the case of 3-bearing-support, the relationship between the maximum damping ratio or the optimum bearing damping and the support stiffness is discussed. Finally, experiments to verify the existence of the optimum bearing damping are carried out for two cases of support stiffness using a test rotor supported by ball bearings and oil film damper bearings. The test results reveal that for each support stiffness there exists an optimum bearing damping, under which the vibration amplitude at the critical speed becomes minimum.

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