Control of hysteretic instability in rotating machinery by elastic suspension systems subject to dry and viscous friction

Most of the undesired whirling motions of rotating machines can be efficiently reduced by supporting journal boxes elastically and controlling their movement by viscousDampers or by dry friction surfaces normal to the shaft axis, which rub against theframe. In the case of dry dampers, resonance ranges of the floating supportconfiguration can be easily cut off by planning a motionless adhesive state of thefriction surfaces. On the contrary, the dry friction contact must change automaticallyinto sliding conditions when the fixed support resonances are to be feared. Moreover,the whirl amplitude can be restrained throughout the speed range by a proper choice ofthe suspension-to-shaft stiffness ratio and of the support-to-rotor mass ratio.This theoretical research deals firstly with the natural precession speeds and looksfor Campbell plots in dependence on the shaft angular speed, for several rotor-suspension systems. Then, the steady response to unbalance is investigated, in terms ofrotor and support orbits and of conical path of the rotor axis. In this search, the ranges ofadhesive or sliding contact are identified in particular for system with dry frictiondamping. At last, the destabilizing influence of the shaft hysteresis in the supercriticalregime is focalized and the counterbalancing effect of the other dissipative sources isverified. In the nonlinear case of dry friction dampers, the control of linear stability isfulfilled by a perturbation procedure, checking the magnitude of Floquet characteristicmultipliers on the complex plane. Moreover, the nonlinear stability far from steadymotion is tested by the direct numerical solution of thefull motion equations. Thecomparison configuration of suspension systems with viscous dampers and no dryfriction is examined through an analytical first approximation approach and closed-form results for stability thresholds are derived in particular for the symmetric case.