Effect of Bearing Constraints on the Whirling Characteristics of a Multi-rotor System

Abstract

The thesis describes an investigation of the behaviour of a symmetrical two-rotor shaft system supported in journal bearings. The rotors are of appreciable diametral inertia and overhung from. Oil is gravity fed to the bearings from a header tank. The thesis is divided into two parts. Part one describes a phenomenon in which the system exhibits sub-shaft speed resonances while part two describes a phenomenon in which the system becomes unstable and whirls in contact with the bearings, at half the shaft rotational speed. To the authors knowledge these phenomena have not been reported previously. The source of the phenomenon of sub-shaft speed resonances is attributed to a rubber flexible drive coupling. Treating the coupling as a non-linear spring, a theory is propounded which explains the shaft behaviour. Agreement between theory and experiment is quite good. The phenomenon of the unstable half shaft rotational speed motion described in part two is also attributed, though indirectly to the drive coupling. The coupling causes the rotors to move in antiphase so that gyroscopic couples, arising out of this movement, are able to control the shaft motion. The appearance of instability is given because the amplitude of the motion is beyond that limited by the bearings. Neglecting external forces, a theory is propounded which, provided the lubricant viscosity is known accurately; predicts the shaft speed at the onset of “instability".