1. Keun Ryu Research Assistant
ASME TURBO EXPO 2010, Glasgow, Scotland, UK
Dynamic Response of a Rotor-Hybrid Gas Bearing System due to Base Induced Periodic Motions
Luis San Andrés
Mast-Childs Professor
Fellow ASME
Yaying Niu
Research Assistant
Keun Ryu
Research Assistant
TURBOMACHINERY LABORATORY
TEXAS A&M UNIVERSITY
ASME paper GT
Supported by TAMU Turbomachinery Research Consortium

2. Microturbomachinery (< 250 kW)
Turbo Compressor
100 krpm, 10 kW
Advantages
Compact and fewer parts
Portable
High energy density
Lower emissions
Low operation/maintenance costs
Micro Turbo
500 krpm, 0.1~0.5 kW
Oil-free turbocharger
120 krpm, 110 kW

3. Gas bearings for microturbomachinery
Advantages
Metal Mesh Foil Bearing
Little friction and power losses
Simple configuration
High rotor speeds (DN value>4M)
Operate at extreme temperatures
Issues
Small damping
Low static load capacity
Prone to instability GT
Gas Foil Bearing
Flexure Pivot Bearing
AIAA GT

4. Gas Bearing Research at TAMU
2001/2 - Three Lobe Bearings
2003/4 - Rayleigh Step Bearings
Flexure Pivot Tilting Pad Bearings
: Bump-type Foil Bearings
: Metal Mesh Foil Bearings
Stability depends on feed pressure.
Stable to 80 krpm with 5 bar pressure
Worst performance to date with grooved bearings
Stable to 93 krpm w/o feed pressure. Operation to 100 krpm w/o problems. Easy to install and align.
Industry standard. Reliable but costly.
Models anchored to test data.
Cheap technology. Still infant. Users needed

5. Objective & tasks
Evaluate the reliability of rotor-air bearing systems to withstanding periodic base or foundation excitations
Set up an electromagnetic shaker under the base of test rig to deliver periodic load excitations
Measure the rig acceleration and rotordynamic responses due to shaker induced excitations
Model the rotor-air bearing system subject to base motions and compare predictions to test results

6. Gas Bearing Test Rig LOP Rotor/motor Load cell Sensors Bearing
Positioning Bolt
Thrust pin
Air supply
190 mm, 29 mm diam
LOP
Rotor: 826 grams
Bearings: L= 30 mm, D=29 mm

7. Rotor and hybrid gas bearings
0.826 kg, 190 mm in length
Location of sensors and bearings noted
Flexure Pivot Hybrid Bearings: Improved stability, no pivot wear
Clearance ~42 mm, preload ~40%. Web rotational stiffness = 62 Nm/rad.
Test rig tilted by 10°.

8. Previous work (GT 2009-59199) Intermittent base shock load excitations
Drop induced shocks ~30 g. Full recovery within ~ 0.1 sec.
Ps=2.36 bar (ab)
Rotor motion amplitudes increase with excitation of system natural frequency. NOT a rotordynamic instability!

9. Gas bearing test rig Base excitation
Shaker & rod push base of test rig
Front and side views (not to scale)

10. Hybrid gas bearing test rig
Rod pushes base plate!
(no rigid connection)

11. Waterfalls in coast down
No base excitation
Ps = 2.36 bar
Subsynchronous whirl > 30 krpm,
fixed at system natural frequency = 193 Hz

12. Rotor speed coast down tests (35 krpm)
No base excitation
1X response
Feed pressure increases natural frequency and lowers damping ratio
Pressure bar 3.72bar 5.08bar
Natural Freq 192Hz Hz Hz

13. Natural frequency whole test rig (5 Hz)
Acceleration (g)
Soft mounts (coils) produce low natural frequency

14. Delivered excitations (6 Hz)
Rotor speed:
34 krpm (567 Hz)
Acceleration (g)
Acceleration (g)
Acceleration (g)
Due to electric motor
zoom
Shaker transfers impacts to rig base! Super harmonic frequencies excited

15. Waterfalls in coast down
Shaker frequency: 12Hz
Ps = 2.36 bar (ab)

16. Rotor speed coast down Shaker frequency: 12Hz
Ps = 2.36 bar (ab)
Subsynchronous frequencies:
24 Hz (2 x 12 Hz)
Natural frequency 193 Hz
Synchronous motion dominates!
Excitation of system natural frequency does NOT mean instability!

17. Effect of feed pressure
Ps: 2.36, 3.72 & 5.08 bar
Shaker frequency: 12Hz
Rotor speed: 34 krpm (567 Hz)
12Hz, 24Hz, 36hz, etc
NOT due to base motion!
Pressure
increases
243Hz
215Hz
Offset by
0.01 mm
193Hz
Rotor motion amplitude at system natural
frequency decreases as feed pressure increases

18. Effect of rotor speed Rotor motion amplitude at system natural
26, 30 & 34 krpm
Shaker frequency: 12Hz
Feed pressure: 2.36 bar (ab)
12Hz, 24Hz, 36hz, etc
Speed
increases
193Hz
180Hz
180Hz
Rotor motion amplitude at system natural
frequency increases as rotor speed increases

19. Effect of base frequency
0, 5, 6, 9, 12 Hz
Rotor speed: 34 krpm (567Hz)
Feed pressure: 2.36 bar (ab)
193Hz
Frequency
increases
NOT due to base motion!
Rotor motion amplitude at natural frequency increases as excitation frequency increases

20. Rigid rotor model Equations of motion (linear system)
Rotor 1st elastic mode: 1,917 Hz (115 krpm)
Equations of motion (linear system)
U, Ub: rotor and base (abs) motions, Z=U-Ub
M,G: rotor inertia and gyroscopic matrices
W: rotor weight
Fimb: imbalance “force” vector
K, C: bearing stiffness and damping from gas bearing model (San Andres, 2006)
Rework equations in terms of measured variables:
System response = superposition of single frequency responses

21. For predictions: input RECORDED BASE accelerations (vertical)
Rigid rotor model
Predicted natural frequencies
 Rotor speed
26 krpm
30 krpm
34 krpm
Conical
191 Hz
200 Hz
208 Hz
Cylindrical
184 Hz
192 Hz
Measured from 1X response tests
Cylindrical
180 Hz
182 Hz
193 Hz
Good agreement shows predicted bearing force coefficients are accurate
For predictions: input RECORDED BASE accelerations (vertical)

22. Predictions vs. measurements
Shaker input frequency: 12Hz
Feed pressure: 2.36 bar (ab)
Rotor speed: 34 krpm (567 Hz)
Nat freq. 1X
Excitation freqs.
Above natural frequency,
RBS is isolated!
Predictions in good agreement! Test rotor-bearing system shows good isolation.

23. Conclusions
Base Excitations on Gas-Rotor Bearing Syst
Rotor response contains 1X, excitation frequency (5-12 Hz) and its super harmonics and system natural frequency.
Rotor motion amplitudes at natural frequency are smaller than synchronous amplitudes.
Excited rotor motion amplitude at system natural frequency increases as gas bearing feed pressure (5.08~2.36bar) decreases, as rotor speed (26~34krpm) increases, and as the shaker input frequency (5~12 Hz) increases.
Predicted rotor motion responses obtained from rigid rotor model show good correlation with test data.
Demonstrated isolation of rotor-air bearing system to withstand base excitations at low freqs.

24. Acknowledgments Questions ? Learn more http://rotorlab.tamu.edu
Thanks support of
TAMU Turbomachinery Research Consortium
Bearings+ Co. (Houston)
Learn more
Questions ?