2009 ASME/STLE International Joint Tribology Conference Texas A&M University Mechanical Engineering Department Paper IJTC Material is based upon work supported by NASA NNH06ZEA001N-SSRW2, Fundamental Aeronautics: Subsonic Rotary Wing Project and the Texas A&M Turbomachinery Research Consortium Luis San Andrés Mast-Childs Professor Fellow ASME, Fellow STLE Keun Ryu Research Assistant October 2009 Experimental Structural Stiffness and Damping of a 2nd Generation Foil Bearing for Increasing Shaft Temperatures

Series of corrugated foil structures (bumps) assembled within a bearing sleeve. Integrate a hydrodynamic gas film in series with one or more structural support layers. Use coatings to reduce friction at start-up & shutdown Applications: APUs, ACMs, micro gas turbines, turbo expanders Tolerant to misalignment & contamination High temperature capability Damping from dry-friction and operation with stable limit cycles Gas Foil Bearings – Bump Type

Gas Foil Bearings (+/-) Proven reliability with load capacity Reduce system weight & volume. High temperature (jet engine hot) No scheduled maintenance Tolerate high vibration and absorb shock loads Less load capacity than rolling & oil lubricated bearings Wear during start up & shut down (coating survival) Still little test data for rotordynamic force coefficients at TAMU, predictive models benchmarked to test data, including thermal management schemes

Motivation GFB load capacity, stiffness and damping depends mainly on its underspring structure. Tests & analysis verified. High temperature affects GFB force response. Changes in clearance & material properties, coating endurance. Operation temperature range (low to high to low) modifies structural properties.

Objectives - Identify FB structural stiffness and structural loss factor from dynamic load tests Measure dynamic force performance Of 2 nd generation foil bearing – no shaft spinning & at increasing temperatures - Quantify the effect of bearing temperature and dynamic load (amplitude & frequency) on FB force coefficients. - Support with reliable test data concurrent development of GFB predictive computational tool.

Overview – GFB dynamic load Salehi et al. (2003): Identify FB dynamic stiffness and equivalent viscous damping. Damping increases with static load but decreases with amplitude of motion and frequency. ROOM TEMPERATURE Heshmat and Ku (1994): Dynamic load tests: FB dynamic forced performance depends on frequency. Rubio and San Andrés (2005): From dynamic load tests, obtain FB energy dissipation parameters: equivalent viscous damping OR structural loss factor OR dry-friction coefficient: FB stiffness decreases with dynamic load amplitude. Viscous damping decreases with frequency. LOSS FACTOR represents best FB mechanical energy dissipation Structure only – no shaft spinning

Lee et al. (2006): develop FE model accounting for thermal effects. Operating temperature to 500°C reduces FB stiffness and damping. Predictions agree with test data. Howard et al. (2001): Determine experimentally GFB forced performance for increasing shaft speed, static load, and temperature. From impact load tests: viscous damping is dominant for lightly loaded GFB at high temperature. Dry- friction type damping is + significant for heavily loaded GFB. HIGH TEMPERATURE Kim, Breedlove and San Andrés (2009): Identify FB structural stiffness and loss factor for operation at elevated shaft temperature (200 C). Tests with dynamic loads varying in amplitude and frequency. Overview – GFB dynamic load

Top foil Cartridge sheet Bumps FB nominal dimensions Parameter [Dimension]SymbolValue Cartridge inner diameter [mm] D37.92 Cartridge outer diameter [mm] DODO Axial bearing length [mm] L25.40 Number of bumps NBNB 24× 3 Bump pitch [mm] s4.64 Bump length [mm] 2l o 3.95 Bump foil thickness [mm] t0.102 Bump height [mm] h0.51 Top foil thickness [mm]tTtT Bump arc radius [mm] rBrB 4.08 Bearing Top foil inner diameter [mm] DTDT Generation II FB Three (axial) bump strip layers, each with 24 bumps. Patented solid lubricant coating (up to 800°F) on top foil surface. Test foil bearing Other data proprietary Shaft OD mm: Highly preloaded FB

Test setup for static load Apply load and measure FB displacement to determine FB static structural stiffness Room temperature tests 90° bearing orientation FB loose fit into 3.08 mm thick bearing shell Uncoated rigid shaft supports floating FB. Load cell Direction of static load g Displacement sensor

FB deflection vs static load F ≠ K X Nonlinear F(X) : Stiffness hardening Large hysteresis loop = mechanical energy dissipation due to dry-friction between top foil contacting bumps and bump strip layers contacting bearing cartridge sheet Room temperature tests Shaft OD mm: Highly preloaded FB

FB static structural stiffness Cubic polynomial curve fit over span of applied loads F=F 0 +K 1 X+K 2 X 2 +K 3 X 3 K=K 1 +2K 2 X+3K 3 X 2 Distinctive hardening effect as FB deflection increases Room temperature tests

Test setup for dynamic loads Bearing housing Eddy current sensor Load cell Test shaft Cartridge heater Thermocouples Shaker Single frequency dynamic load in horizontal direction Test bearing Bearing housing Index fixture 90° bearing orientation Uncoated rigid, non- rotating, hollow shaft supports floating FB. FB Displacement controlled [µm]7.4, 11.1, 14.8, and 18.5 Frequency Range, Hz (increment: 25 Hz) Shaft Temperature, °C23, 103, 183, and 263 Bearing Mass M, kg0.785 (load cell + attachment hardware) FB press fitted onto 15.5 mm thick bearing housing!

134 mm Ø 25 mm Indexing fixture Ø 25.4 mm Shaft heating using electric heater Ø mm T1 T3 Th Significant temperature gradient along shaft axis. Cartridge heater warms unevenly shaft and bearing T4 Steady state temperature (heater 1 hr operation) Test Bearing Cartridge heater Bearing housing T2 76 mm

Control shaker load to keep FB motion amplitude at 7.3 µm Waterfalls of dynamic load and FB displacement 1X T h = 23°C 25 Hz 400 Hz 25 Hz 400 Hz Amplitude of load decreases with frequency. Single frequency FB motion ( a measure of linearity ) Frequency (Hz) Motion amplitude (  m) Dynamic load (N) Room temperature tests

Dynamic load vs excitation frequency FB motion amplitude increases T h = 23°C FB motion amplitude increases T h = 263°C At high frequency, less force needed to maintain same motion amplitude Amplitude of dynamic load decreases with frequency and increases with FB motion amplitudes

Parameter Identification (no shaft rotation) Equivalent Test System: 1DOF K stiffness, C eq viscous damping OR  loss factor Harmonic force & displacements Impedance Function Energy dissipated by either viscous damping or material structural losses

Real part of (F/X) decreases with FB motion amplitude and increases with shaft temperature Real part of (F/X) vs frequency Motion amplitude increases T h = 23°C Motion amplitude increases T h = 263°C System natural frequency decreases as FB motion amplitude increases (typical of nonlinear system with softening stiffness)

T h = 23°C Highly preloaded FB : K decreases as FB motion amplitude increases due to decrease in # of active bumps Dynamic structural K compared to static structural K Motion amplitude increases Dynamic load Static load At larger FB deflections, static K is larger than dynamic K FB stiffness: effect of freq. & amplitude Room temperature tests

T h = 23°C Equivalent viscous damping decreases with excitation frequency and FB motion amplitude. FB viscous damping: effect of freq. & amplitude Motion amplitude increases Room temperature tests

FB motion amplitude: 14.8 µm TEST FB cartridge OD is constrained within thick bearing housing. FB radial clearance decreases as shaft temperature raises! FB stiffness and viscous damping increase with shaft temperature and decrease with frequency. Heater temperature increases K & C eq : effect of shaft temperature

Loss factor vs frequency FB motion amplitude: 14.8 µm Heater temperature increases Structural (material) loss factor represents best energy dissipation in a FB FB loss factor increases with excitation frequency and decreases slightly with shaft temperature. Large damping expected in rotordynamic measurements Effect of temperature on loss factor 

Post-test condition of test FB Before operation Distinguishing “wear” marks on bump foils and cartridge ID After extensive dynamic load tests Marks evidence dry- friction of bumps against top foil and cartridge ID

Conclusions  FB structural stiffness and equivalent viscous damping decrease with frequency. As FB motion amplitude increases, less underlying bumps become active, thus reducing FB stiffness and damping  As shaft temperature increases (max 263 C), FB structural stiffness and equivalent viscous damping increase. As temperature increases, shaft OD grows while FB ID contracts; thus reducing the FB bearing radial clearance.  FB structural loss factor decreases slightly with temperature ; yet it increases with frequency, a desirable feature for high rotor speed operation. 2 nd gen FB with assembly preload  Test results WILL further anchor available GFB predictive tool (XL_GFB_THD©)

2009 hot rotor-GFB test rig Gas flow meter (Max. 500 LPM). Drive motor (max. 65 krpm) ) Instrumentation for high temperature. Insulation casing Insulated safety cover Infrared thermometer Flexible coupling Drive motor Cartridge heater Test GFBs Test hollow shaft (1.1 kg, 38.1mm OD, 210 mm length) Tachometer Eddy current sensors Hot heater inside rotor spinning 30 krpm Max. 360 °C

Video: Operation of hot rotor-GFB test rig

Acknowledgments/ Thanks to NASA GRC Dr. S. Howard & Dr. C. DellaCorte Dr. Tae Ho Kim at KIST (Korea) TAMU Turbomachinery Research Consortium NSF REUP MiTi© Learn more at:

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BACKUP SLIDE

System motion of equation Parameter identification procedure Work (W) by the shaker on the test FB Energy dissipated by equiv. viscous damping Energy dissipated by FB dry friction Equating external work input to energy dissipation ( W ~ E v or W~E F ) FB dynamic structural stiffness and equivalent viscous damping (frequency domain): <= Dry-friction coefficient