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Research Papers: Gas Turbines: Structures and Dynamics

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

[+] Author and Article Information
Matthew O. T. Cole

Department of Mechanical Engineering,
Faculty of Engineering,
Chiang Mai University,
Chiang Mai 50200, Thailand
e-mail: motcole@hotmail.com

Lawrence Hawkins

Calnetix Technologies,
16323 Shoemaker Ave,
Cerritos, CA 90703
e-mail: lhawkins@calnetix.com

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 27, 2015; final manuscript received December 10, 2015; published online February 17, 2016. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(7), 072507 (Feb 17, 2016) (8 pages) Paper No: GTP-15-1551; doi: 10.1115/1.4032343 History: Received November 27, 2015; Revised December 10, 2015

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 end-face under axial load. This paper proposes a new analytical approach to model and predict such friction-driven 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 rotor-housing structure. This model is coupled with an evaluation of Coulomb friction associated with slip between the rotor and the supporting end-face of a thrust bearing. The resulting equations can be used to compute a set of possible whirl motions via a root-finding procedure. A case study is undertaken for a 140 kW energy storage flywheel. Model-based predictions are compared with measured data from spin-down tests and show a good level of agreement. The study confirms the role of friction-related forces in driving forward-whirl 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.

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References

Figures

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Fig. 1

Situation for axial and radial contact interaction between rotor and clearance bearing, as seen in a reference frame rotating about the bearing center Ob at the whirl speed Ω

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Fig. 2

Dimensionless functions used to compute friction forces

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Fig. 3

Schematic showing physical variables treated as inputsand outputs in dynamic model construction. Force inputs fn,mx,y  are applied to both the rotor and bearings (in opposite directions).

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Fig. 4

Cross section of flywheel unit showing (a) location of backup bearings and housing isolators and (b) thrust/radial backup bearing arrangement

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Fig. 5

Measured whirl behaviors during coast down for three (out of nine) drop tests. Dominant (and secondary) harmonic component of vibration has been calculated from rotor displacement measurements.

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Fig. 7

Circular whirl solutions (frequencies) are obtained from the condition b(Ω)=T. Three cases are shown for differing housing isolator characteristics (see Table 1): (a) model 1, (b) model 2, and (c) model 3.

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Fig. 8

Predicted bearing forces and orbit radii for model 3

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Fig. 6

Circular frequency response functions for nominal rotor-housing model (model 1)

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