First investigations on controlling unbalanced flexible rotors using shaft seals

To analyze the bending vibration behavior of an unbalanced flexible rotor considering both internal damping and a constant driving torque, the Laval-rotor-model is applied. The resulting set of nonlinear ordinary differential equations is transformed into a dimensionless formulation to enable numerical integration using an explicit Runge-Kutta-schema. Applying this approach it is possible to study two different phenomena, the well known "stalling" effect of the shaft that occurs just below the bending critical speed when the drive torque is too weak to pass the resonance as well as the effect of self-excitation that occur at very high speeds because of negative damping. It is shown that both problems can be solved, if the radial displacements of the shaft are controlled by a shaft seal. The latter is modeled as a lumped system - considering mass, stiffness and viscosity. The seal influences both the horizontal and vertical displacement of the shaft, and the speed of rotation. Because shaft seals are already used as sensors to determine the speed of rotation, it should be possible to develop a control approach to adjust the pressure between seal and shaft in such a way that stalling and self-excitation can be avoided or at least reduced.