Luis San Andrés Mast-Childs Professor

1. 33rd Turbomachinery Research Consortium Meeting Identification of structural stiffness and material loss factor in a shimmed (generation one) bump type foil bearing TRC-B&C-04-2013 Joshua Norsworthy Graduate Research Assistant Luis San Andrés Mast-Childs Professor May 2013 TRC Project 32513/1519 FB Metal Mesh Foil Bearings: Operation at high temperatures

2. Bump-type foil bearing (BFB) BFB components: bearing cartridge, bump Foil strip (compliant), and top foil Hydrodynamic air film pressure develops between the rotating shaft and the top foil Applications:ACMs, micro gas turbines, turbo expanders, turbo compressors, turbo blowers, soon in automotive turbochargers Top Foil Fixed End Bump Foil Top Foil Bearing Cartridge • Adequate load capacity provided by the bump foil strip (compliant) • Frictional damping (due to relative motion of bump foils) dominates • Tolerant to misalignment Bearing Cartridge Bump Foil

3. Issues with foil bearings • Endurance: performance at start up & shut down • Little test data for rotordynamic force coefficients • Thermal management for high temperature applications (gas turbines, turbochargers) • Prone to subsynchronous whirl and limit cycle operation –

4. Example subsynchronous motions RudDloff, L., Arghir, M., et al., 2011, “Experimental Analysis of a First generation foil Bearing. Start-Up Torque and Dynamic Coefficients,” ASME GT2010-22966 Unloaded FB: “Self-Excited” whirl motions at speed 30 krpm (500 Hz) with whirl frequency=165 Hz (WFR=0.33)

5. Original and shimmed GFBs Shimmed GFB Original GFB Shimmed GFB Original GFB Inserting metal shims underneath bump strips introducesa preload (centering stiffness) at low cost –typical industrial practice. Preload produces centering stiffness at small loads 5 Kim, T.H., and San Andrés, L., 2009,Trib. Trans.

6. Test BFB specifications Di L 2cnom Do L/D = 1.03

7. Shimmed bump-type foil bearings The shims have an adhesive glue layer on the bottom Shims are added at discrete locations circumferentially and stretch axially Bump Foil Top Foil Shimmed bumps are pressed closer to the rotor than other bumps Metal Shim Bearing Cartridge

8. Clearance in a shimmed BFB Clearance profile: cnom:Nominal bearing clearance tS : Shim thickness NS : Number of shims θ: Angular coordinate θp : Angular distance between consecutive shims θ1 : Angular coordinate of the first shim (all angular coordinates taken from the middle of the shim)

9. Static load test rig Lathe saddle Load cell Eddy current sensor Test bearing Shaft affixed in lathe chuck Lathe chuck Lathe chuck holds shaft & bearing during loading/unloading cycles. Load cell Eddy Current sensor Stationary shaft Lathe tool holder Test bearing Dr. Chirathadam, TRC 31st (2011)

10. Load-deflection results for original BFB 4 cycles of push and pull loads directed 90o to top foil fixed end Regions of low load (<50 N) evidence the diametral clearance region Region of bump compression Regions with bump compression are fitted to a third order polynomial Bearing reaction force: Nonlinear F(X) Small BFB hysteresis loop : little mechanical energy dissipation Region of bump compression

11. Load vs. deflection – compare Applied load vs BFB deflection Original and Shimmed BFBs show nonlinear force behavior. Shims effectively reduce bearing clearance.

12. Estimated bearing structural stiffness Bearing structural stiffness: Bearing with 100μm shims Push/Pull Bearing with 50μm shims Push/Pull Bearing stiffness increases with bearing deflection and shim thickness Original bearing Push/Pull K ≠0 at x=0: assembly interference fit (no clearance) Bearing with 100µm shims

13. Loss factor estimation Loss factor BFB with 100µm shims Largest loss factor for the 50µm shim bearing due to increased sliding friction

14. Predictions (90º orientation) Load vs BFB deflection 50 µm shims 100 µm shims Original Predictions from a simple model agree well with experimental results for all test configurations Stiffness

15. Conclusions Bearing structural stiffness increases with increasing shim thickness and hardens with bearing displacement. • The loss factor for the test (generation one) BFB is small. • Highest loss factor g ~0.20 for bearing with 50 µm shims due to increase in sliding of foil strips wit bearing cartridge. • Predictions from model agree well with experimental results. Shimmed BFB (without an assembly interference fit) shows larger energy dissipation than original BFB.

16. Proposal to TRC (1 year) Objective:Quantify rotor lift off and shut down events for shimmed BFBs, as well as determine force coefficient • Install and test the original and shimmed BFB in a rotordynamic test rig. • Conduct dynamic load tests to determine force coefficients over a range of excitation frequencies • For increasing static loads, conduct rotor speed start-up and shut down tests and record drag torque and lift-shaft speed.

17. New Proposal Effect of Shimming on the Rotordynamic Force Coefficients of a Bump-type Foil Bearing Joshua Norsworthy Graduate Research Assistant Luis San Andrés Mast-Childs Professor May 2013 17

18. TRC budget Shimmed bump foil bearings Research will characterize, qualitatively and quantitatively, shimming as a cheap and effective way of improving rotordynamic performance of rotors supported on BFBs.The work is important for manufactures of turbochargers, turboexpanders and micro gas turbines