Dynamics of Rotors Influenced by Rubbing Contacts

Sammanfattning: This thesis concerns theoretical studies of the dynamical behaviour of a rotor interacting with its non-rotating surroundings. Rotor-stator rubs make the rotor system strongly non-linear, which leads to phenomena that are not seen for ordinary rotors.Three different rotor models have been used: 1) the Jeffcott rotor, 2) a two-mass rotor and 3) a finite-element model of a large disc-stack centrifuge. Gyroscopic effects were not included in the first two models. Isotropic bearings were used in all models and the stator was represented by a massless, rigid, spring-supported ring. The friction force was calculated from the Coulomb friction model.Jump phenomena may occur for these systems and this has been investigated. This phenomenon has specially been studied for the Jeffcott rotor. The radial clearance was found to have a major influence on its existence. The stability has been investigated by a method based on the principle of linearised stability. Parametric studies have been performed and stability was found to be improved by high external damping and low values of friction and stator stiffness. Further, the influence of stator offset on stability was also examined. For small values of offset the equations could be linearised, and this system was parametrically excited. The Floquet theory was adopted for the examination of stability properties. The offset was not found to have any major influence on the stability.The two-mass rotor model used is an extended version of the Jeffcott rotor, with only one mass interacting with the stator. Jump phenomena were found at both resonances, and the mass without rub showed rather complex behaviour at the second resonance. The stability was investigated and two speed regions with unstable solutions were found at the second resonance. The results were verified by a direct time-integration of the governing equations.The stability of a large disc-stack centrifuge interacting with its casing has also been investigated. A finite-element description of the rotating part, with gyroscopic effects included, was used. The gyroscopic effect impaired the stability of the centrifuge. The friction force needed for onset of instability was found to have a critical threshold value for a given rotor configuration.

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