Model Based Analysis of Friction Induced Subsynchronous Whirl for a Rotor Contacting With Clearance Bearings Under Axial Load

Abstract

For rotors supported by active magnetic bearings (AMBs), clearance bearings are commonly used to provide backup support under loss of AMB functionality. Test data from real machines shows that vibration during touchdown on backup bearings may involve steady forward whirling of the rotor with a subsynchronous frequency. This excitation is believed to be due to friction forces transmitted between the rotor and a bearing endface under axial load. This paper proposes a new analytical approach to model and predict such frictiondriven forward whirl behaviors. A set of constraint equations are derived that relate a circular whirl motion of arbitrary orbital speed to the frequency response functions for the rotorhousing structure. This model is coupled with an evaluation of Coulomb friction associated with slip between the rotor and the supporting endface of a thrust bearing. The resulting equations can be used to compute a set of possible whirl motions via a rootfinding procedure. A case study is undertaken for a 140 kW energy storage flywheel. Modelbased predictions are compared with measured data from spindown tests and show a good level of agreement. The study confirms the role of frictionrelated forces in driving forwardwhirl response behaviors. It also highlights the key role of housing and machine support characteristics in response behavior. This influence is shown to be complex and not open to simple physical interpretation. Therefore, the proposed analytical method is seen as a useful tool to investigate this influence while avoiding the need for time consuming numerical simulations.