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ASME TURBO EXPO 20 10 , 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

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Keun ryu research assistant

ASME TURBO EXPO 2010,Glasgow, Scotland, UK

DynamicResponse 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



ASME paper GT2010-22277

Supported by TAMU Turbomachinery Research Consortium

Microturbomachinery (< 250 kW)

Turbo Compressor

100 krpm, 10 kW


  • 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

Gas bearings for microturbomachinery


Metal Mesh Foil Bearing

  • Little friction and power losses

  • Simple configuration

  • High rotor speeds (DN value>4M)

  • Operate at extreme temperatures


  • Small damping

  • Low static load capacity

  • Prone to instability

GT 2009-59315

Gas Foil Bearing

Flexure Pivot Bearing


GT 2004-53621

Gas Bearing Research at TAMU

2001/2 - Three Lobe Bearings

2003/4 - Rayleigh Step Bearings

2002-09 - Flexure Pivot Tilting Pad Bearings

2004-10:Bump-type Foil Bearings

2008-10: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

Objective & tasks

Evaluate the reliability of rotor-air bearing systems to withstanding periodicbase or foundation excitations

  • Set up an electromagnetic shaker under the base of test rig to deliver periodic load excitations

  • Measure the rig acceleration and rotordynamicresponses due to shaker induced excitations

  • Model the rotor-air bearing system subject to base motions and compare predictions to test results


Load cell



Gas Bearing Test Rig

Thrust pin

Air supply

Positioning Bolt

190 mm, 29 mm diam


Rotor: 826 grams

Bearings: L= 30 mm, D=29 mm

Rotor and hybrid gas bearings


  • 0.826 kg, 190 mm in length

  • Location of sensors and bearings noted

Flexure Pivot Hybrid Bearings:Improved stability, nopivot wear

Clearance ~42 mm, preload ~40%.Web rotational stiffness = 62 Nm/rad.

Test rig tilted by 10°.

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!

Gas bearing test rig

Base excitation

Shaker & rod push base of test rig

Front and side views (not to scale)

Hybrid gas bearing test rig

Rod pushes base plate!

(no rigid connection)

Waterfalls in coast down

No base excitation

Ps = 2.36 bar

Subsynchronous whirl > 30 krpm,

fixed at system natural frequency = 193 Hz

Rotor speed coast down tests (35 krpm)

No base excitation

1X response

Feed pressure increases natural frequency and lowers damping ratio

Pressure 2.36bar 3.72bar 5.08bar

Natural Freq 192Hz 217Hz 250Hz

Natural frequency whole test rig (5 Hz)

Acceleration (g)

Soft mounts (coils) produce low natural frequency

Delivered excitations (6 Hz)

Rotor speed:

34 krpm (567 Hz)

Acceleration (g)

Acceleration (g)

Acceleration (g)

Due to electric motor


Shaker transfers impacts to rig base! Super harmonic frequencies excited

Waterfalls in coast down

Shaker frequency: 12Hz

Ps = 2.36 bar (ab)

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!

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!





Offset by

0.01 mm


Rotor motion amplitude at system natural

frequencydecreases as feed pressureincreases

Effect of rotor speed

26, 30 & 34 krpm

Shaker frequency: 12Hz

Feed pressure: 2.36 bar (ab)

12Hz, 24Hz, 36hz, etc






Rotor motion amplitude at system natural

frequencyincreases as rotor speed increases

Effect of base frequency

0, 5, 6, 9, 12 Hz

Rotor speed: 34 krpm (567Hz)

Feed pressure: 2.36 bar (ab)




NOT due to base motion!

Rotor motion amplitude at natural frequencyincreases as excitation frequency increases

Rigid rotor model

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

Rigid rotor model

Predicted natural frequencies

Measured from 1X response tests

Good agreement shows predicted bearing force coefficients are accurate

For predictions: input RECORDED BASE accelerations (vertical)

Predictions vs. measurements

Shaker input frequency: 12Hz

Feed pressure: 2.36 bar (ab)

Rotor speed: 34 krpm (567 Hz)

Nat freq.


Excitation freqs.

Above natural frequency,

RBS is isolated!

Predictions in good agreement! Test rotor-bearing system shows good isolation.


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.


  • Thanks support of

  • TAMU Turbomachinery Research Consortium

  • Bearings+ Co. (Houston)

Learn more

Questions ?