1

Gas Bearings for Microturbomachinery

Rotordynamic Performance & Stability

Turbomachinery Laboratory, Texas A&M University Mechanical Engineering Department

The 8th IFToMM International Conference on Rotordynamics

Luis San AndrésMast-Childs Professor

ASME, STLE Fellow

September 2010

2

Oil-Free Bearings for Turbomachinery

JustificationCurrent advancements in vehicle turbochargers and midsize gas turbines need of proven gas bearing technology to procure compact units with improved efficiency in an oil-free environment.

DOE, DARPA, NASA interests range from applications as portable fuel cells (< 60 kW) in microengines to midsize gas turbines (< 250 kW) for distributed power and hybrid vehicles.

Gas Bearings allow• weight reduction, energy and complexity savings• higher temperatures, without needs for cooling air • improved overall engine efficiency

3

Microturbomachinery as per IGTIMicroturbomachinery as per IGTI

ASME Paper No. GT2002-30404

Honeywell, Hydrogen and Fuel Cells Merit Review

Automotive turbochargers, turbo expanders, compressors,

Distributed power(Hybrid Gas turbine & Fuel Cell), Hybrid vehicles

Drivers:deregulation in distributed power, environmental needs, increased reliability & efficiency

International Gas Turbine Institute

Max. Power ~ 250 kWatt

4

Micro Gas TurbinesMicro Gas Turbines

100Turbec, ABB & Volvo

70, 250IngersollRand

175General Electric

35, 60, 80, 150Elliott EnergySystems

30, 60, 200Capstone

25, 80Bowman

OUTPUT POWER (kW)MANUFACTURER

Microturbine Power Conversion Technology Review, ORNL/TM-2003/74.

Cogeneration systems with high efficiency• Multiple fuels (best if free)• 99.99X% Reliability• Low emissions• Reduced maintenance• Lower lifecycle cost

60kW MGT

www.microturbine.com

Hybrid System : MGT with Fuel Cell can reach efficiency > 60%

Ideal to replace reciprocating engines. Low footprint desirable

5

MEMS MTM

Ultra MicroturbomachineryUltra Microturbomachinery

GT-2003-38866

Meso-scale MTM

PowerMEMS 2004

• Palm-size power source• Brayton cycle• Gas foil bearings

www.m-dot.com

Small unmanned vehicles and to replace batteries in portable electronic devices 100 Watt & less

• Silicon wafer• 1.2 Million rpm• Thrust 0.1 N• Spiral groove and hydrostatic gas bearings

6

MTM materials & fabricationMTM materials & fabrication

Materials & Reliability

• High temperature durability• Light weight

GT2004-53493

Fabrication

•Mold SDM•Precision 3D Milling•MEMS

GT2003-38866

DRIE process

Mold SDM process

GT2003-38933

GT2003- 38151

3D Milling

7

• Oil-Free • NO DN limit• Low friction and power loss• Thermal management

GAS BEARINGS

AIAA-2004-5720-984

Gas Foil Bearing

GT 2004-53621

Flexure Pivot Bearing

AIAA 2004-4189

Rolling element bearings

PowerMEMS 2003

• Low temperatures• Low DN limit (< 2 M)• Need lubrication system

NICH Center, Tohoku University

Herringbone grooved bearing

• Precision fabrication process• Low load capacity and stiffness and little damping

Available Bearing TechnologiesAvailable Bearing Technologies

8

Largest power to weight ratio, Compact & low # of parts

High energy density

Reliability and efficiency,Low maintenance

Extreme temperature and pressure

Environmentally safe (low emissions)

Lower lifecycle cost ($ kW)

High speed

Materials

Manufacturing

Processes & Cycles

Fuels

Rotordynamics & (Oil-free) Bearings & Sealing

Coatings: surface conditioning for low friction and wearCeramic rotors and components

Automated agile processesCost & number

Low-NOx combustors for liquid & gas fuelsTH scaling (low Reynolds #)

Best if free (bio-fuels)

MTM – Needs, Hurdles & Issues

9

Gas Bearings for Oil-Free MTM

Advantages of gas bearings over oil-lubricated bearings– Process gas is cleaner and eliminates

contamination by buffer lubricants– Gases are more stable at extreme temperatures

and speeds (no lubricant vaporization, cavitation, solidification, or decomposition)

– Gas bearing systems are lower in cost: less power usage and small friction, enabling savings in weight and piping

Gas Bearings Must Be Simple!Gas Bearings Must Be Simple!

10

Ideal gas bearings for MTM

Simple – low cost, small geometry, low part count, constructed from common materials, manufactured with elementary methods.

Load Tolerant – capable of handling both normal and extreme bearing loads without compromising the integrity of the rotor system.

High Rotor Speeds – no specific speed limit (such as DN) restricting shaft sizes. Small Power losses.

Good Dynamic Properties – predictable and repeatable stiffness and damping over a wide temperature range.

Reliable – capable of operation without significant wear or required maintenance, able to tolerate extended storage and handling without performance degradation.

+++ Modeling/Analysis (anchored to test data) available

11

Example: Turbochargers

Increased engine efficiency and performance relies on a robust rotor-bearing system

RBSWith Fully Floating

Bearing RBSWith Semi Floating

BearingRBS

With Ball BearingOil Less

Bearing System (2007)

DETC2007-34136

RBS: Rotor Bearing System

12

Bearing technologies• Preformed bushings• Ceramic rolling element bearings• Rigid geometry gas bearings • Flexure tilting pad gas bearings• Foil gas bearings • Solid lubricant films (coatings)• Active and passive magnetic bearings

Technical requirements for turbochargers

8.77 M6 MP V (psi ft/s)752800Tip speed (m/s) 5.75 M3.7 MDN (rpm-mm)61 to 157169 to 247Max speed (krpm)459 to 669068 to 182Rotor weight (gram)71 to 18341 to 60Compressor wheel diameter (mm)CV turboPV turbo

13

Preformed BushingsSelf-lubricating material, typically a thermoplastic or a graphite derivative. The bushings may have a rigid support backing (steel or bronze).

Advantages: Disadvantages: No lubrication required Simple design and construction Moderate friction and wear Inexpensive Off the shelf items Low conductivity – electrical and

thermal insulation.

Dynamic behavior unknown Restricted to low P-V numbers. Relatively Low temperature limit

(Graphalloy bushings to 1000 F). Simple cylindrical configurations

molded Not suitable for high speeds Unknown hydrodynamic performance

Vespel, Torlon, PeekGraphalloy

Limits:PV 300,000 psi ft/minLow Temperature

14

Ceramic Rolling Element Bearingssilicon nitride (Si3N4) balls and hardened steel outer and inner raceways. Cage retainer made of thermoplastics (PTFE with bronze impregnated, Vespel™, Thorlon™, Peek™, Nylon)

Advantages: Disadvantages: High speed and acceleration - up to 3

million DN Increased stiffness Less friction, less heat Low thermal expansion, higher

accuracy Non-conductive Extended operating life Ability to support thrust (axial) loads.

Requires some form of lubricant for extended usage.

Higher cost for parts and lubricant Operating temperature limited by

lubricant Dissimilar heat expansion of materials

can cause seizure. Too stiff, little damping.

Dry friction μ= 0.17(compare to 0.42

steel/steel)Need lubrication to

last, Expensive

15

Rigid Geometry Gas Bearings

Gas film bearings (hydrostatic/hybrid) give low friction and support load (< 1 bar)

journal rotation

Plain journalbearing

pressure

Major issues: Little damping, Wear at start & stop,Instability (whirl & hammer)

16

Gas Foil BearingsFoil bearings integrate a gas film in series with an elastic substructure. Bump foil and multi-leaf foil bearings are used. Foils must be coated to avoid wear and seizure, and to reduce drag friction during frequent start and stops.

Issues:Excessive cost / Protected technology

Extensive testing for coatingsRotordynamic instability or forced

nonlinearityThermal management Issues

Complicated analysis tools

Currently used in micro power (<100 kW) systems and secondary cryogenic turbopumps

Source: NASA Oil Free MTM program

17

Solid lubricant films ensure low friction and reduced surface wear rates while promoting early rotor lift off.Applied with plasma spray, ion beam deposition, sputtering, and chemical vapor deposition.PS304, Near Frictionless Carbon Coating (NFC™) , UltraC Diamond™, Casidiam™ , Fluoropolymer (EMRALON™), Molybdenum and Graphite coatings

Low friction coefficients in air are as low as 0.10

Still costly for mass production !

Coatings (Solid Lubricant Films)

Procedure for depositing PS304 coating on shaft

Undercut shaft First plasma spray Second plasma spray Polished

Source: NASA Oil Free MTM program

18

Active Magnetic BearingsAttraction type – controlled 5 axes

Permanent magnet – reluctance bearings

Passive Magnetic Bearings

Limitations- Cost,

- Low temperature,- Complexity

19

Rankings for gas bearings Technology Details Cost/unit RRD RRA A Preformed

bushings Thermoplastics & Graphite derivatives

$30.00 unit 1 4

B Ceramic rolling element bearings

Ceramic – steel Contained lubricant

$33.00 unit 1 2

C Rigid geometry gas bearings

Hydrodynamic/ hydrostatic

$2-$4 unit

2 3

D Tilting pad gas bearings

Hydrodynamic/ hydrostatic

$50 Hypad© 2 3

E Foil bearings Hydrodynamic $ 100 MITI 3 2 G Active magnetic

bearings $50-100

system (bulk) 3 4

H Passive magnetic bearings

NA 5 5

F Solid film

lubricants. Applicable to products C-E

Coatings on shaft.

Diamond like coating (DLC). PS 304

$0.75 part $50 part + grinding

2

2

2

3

Foil coatings Teflon (Emralon™) $ 0.50 part (bulk)

1 2

RRD: relative ranking of current state of development. (1): off the shelf item with proven results over a wide range of applications(2): readily available technology, engineering analysis or experimentation required (3): known technology, both engineering andtesting required (4): some applications known, largely empirical development (5): unknown product, research phase.

RRA: ranking for readiness of application to oil-free TCs(1): readily available engineered product proven for TC application (2): technology known, engineering analysis in progress, manufacturing process and performance issues for TC application. (3): technology available, proven engineering analysis, product development needed to extend limits of application.(4): some applications known, largely empirical development at this time(5): unknown application to oil-free TC.

20

Gas Bearings for MTM

What are the needs?

Make READY technology for industrial application by PUSHING development to

• make of the shelf item with proven results for a wide range of applications;

• engineered product with well known manufacturing process;

• known (verifiable) performance with solid laboratory and field experiences

21

Gas Bearings for Oil-Free MTM

Thrust:Investigate conventional bearings of low cost, easy to manufacture (common materials) and easy to install & align.

Combine hybrid (hydrostatic/hydrodynamic) bearings with low cost coating to allow for rub-free operation at start up and shut down

Major issues:Little damping, Wear at start & stop,Instability (whirl & hammer)

22

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

23

Overview of experience with gas bearings

• Three Lobe Bearings

• Flexure Pivot Tilting Pad Bearings

• Bump-type Foil Bearings

• Metal Mesh Foil Bearings

24

Positioning Bolt

X

Y

Load

LOP

Rotor/motor

Bearing

Sensors

Load cell

Air supply

Thrust pin

Rotor: 826 gramsBearings: L= 30 mm, D=29 mm, C < 25 um

190 mm, 29 mm diam

Hybrid Gas Bearing Test Rig (1)

25

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0 10000 20000 30000 40000 50000 60000

Operating Speed (rpm)

Dis

plac

emen

t Am

plitu

de (m

mp-

p)

Pratio = 5.08

Pratio = 3.72

Pratio = 3.02

As supply Pressure

increases, critical

speed rises & damping ratio drops

displacement

20 psig

40 psig

60 psig

Imbalance response

C=66 um, r:0.32 , do: 1mm

Three lobe gas bearings

ASME GT2003-38859

26

Flexure Pivot Bearings

27

Flexure-Pivot Tilting Pad BearingsO-ring

Grove

1.5”

AlignmentPins

1.5”

O-ring Grove

1.5”

AlignmentPins

1.5”

X

Y

Load

Wire-EDM, low cost, common material, engineered design, no stack-up tolerances,

PROVEN record in HP compressors (oil lubricated)

External pressurization

28

Ps=3.8 bar (40 psig)

Three-lobe Bearing (WFR=0.37)

unstable

Stable to 99 krpm

Clean 1X rotor response(no instability –

hydrodynamic type)

60 KRPM

GT 2004-53621

Waterfalls of rotor motion

GT 2003-38859

29

Rotordynamic response

Predictions vs. test data

L R

V: verticalH: horizontal

Good correlation: tests and rotor-gas bearing model predictions

IJTC 2006-12026ASME GT2008-50393

0

5

10

15

20

0 5000 10000 15000 20000 25000 30000 35000 40000Speed [rpm]

Am

plitu

de [μ

m, p

k-pk

]

5.08 bar

3.72 bar

2.36bar TEST

PREDICT

μm

As supply Pressure

increases, critical speed

raises &damping

ratio drops

30

0

5

10

15

20

0 10000 20000 30000 40000 50000Speed [rpm]

Am

plitu

de [μ

m, 0

-pk]

5.08 bar3.72 bar2.36 barNo feed pressure

5.08 bar

No feed pressure

2.36 bar

3.72 bar

A way to control rotordynamics?

Question:If shaft speed

regulates feed

pressure, could large

rotor motions be

suppressed ?

ASME 2007-27127

31

Towards suppression of critical speeds

As pressure supply increases, critical speed raises anddamping ratio decreases

L R

V: verticalH: horizontal

0

5

10

15

20

0 5000 10000 15000 20000 25000 30000 35000 40000Speed [rpm]

Am

plitu

de [μ

m, p

k-pk

]

5.08 bar

3.72 bar

2.36 bar

5.08 bar

3.72 bar

2.36bar

20 psig

40 psig

60 psig

Speed range for Ps control

ASME GT2008-50393

32

Rotor speed during coast down

0

10

20

30

40

50

0 50 100 150 200Coast down time [sec]

Rot

or s

peed

[krp

m]

15 krpm ~ 14 krpm

58.0 sec ~ 62.7 sec

~ 2 minute

Speed decays exponentially with time. Little viscous drag.

2.36 bar

210 rpm/s

Speed region for control of feed

pressure

Towards suppression of critical speeds

ASME GT2008-50393

33

Control of feed pressure

Simple on/off valve ASME GT2008-50393

34

0

5

10

15

20

0 5000 10000 15000 20000 25000 30000 35000 40000

Speed [rpm]

Am

plitu

de [μ

m, p

k-pk

]

Controller activated system

Displacements at RB(H)

L R

V: verticalH: horizontal

5.08 bar

2.36 barBlue line: Coast downRed line: Set speed

2.36 bar5.08 bar

Rotor peak amplitude eliminated by sudden increase in supply pressure

Suppression of critical speeds ASME GT2008-50393

35

0

5

10

15

20

0 5000 10000 15000 20000 25000 30000 35000 40000Speed [rpm]

Am

plitu

de [μ

m, p

k-pk

]

Test-O.C. #5, active Ps control, coast down

Prediction

5.08 bar 2.36 bar Supply pressure

2 bar

5 bar

speed

Ps

TestPrediction

TEST

PREDICT

Tests vs predictions

Good agreement! (predictable RBS performance)

IJTC 2006-12026

ASME GT2008-50393

36

Closure: Stable to 99 krpm!• External pressurization stiffens bearings and reduces

their modal damping, thus raising critical speeds.• Speed coast down tests with controlled supply pressure

show displacement of critical speed and elimination of peak displacements.

• Predictive codes reproduce well test rotor response; even for large rotor motions and controlled supply pressure!

• Hybrid bearings demonstrated reliable rotordynamic performance free of sub sync. whirl.

• OTHER measurements show reliability of gas bearings to external shocks and base periodic motions.

Flexure Pivot Gas Bearings

37

Gas Foil Bearings

38

Gas Foil Bearings

Advertised advantages: high load capacity (>20 psig), rotordynamically stable, tolerance of misalignment and shocks

: Bump spot weld X

Rotor spinning

Housing

Bump strip layer

Top foil

Gas film

Y

Top foil spot weld

Ω

Ө

39

Gas Foil Bearings – Bump type• Series of corrugated foil structures (bumps)

assembled within a bearing sleeve. • Integrate a hydrodynamic gas film in series

with one or more structural layers.

Applications: APUs, ACMs, micro gas turbines, turbo expanders

• Reliable • Tolerant to misalignment and

debris, also high temperature• Need coatings to reduce friction at

start-up & shutdown• Damping from dry-friction and

operation with limit cycles

40

Foil Bearings (+/-)• Increased reliability: load capacity (< 20 psi)• No lubricant supply system, i.e. reduce weight• High and low temperature capability (> 1,000 C) • No scheduled maintenance • Tolerate high vibration and shock load. Quiet

operation

• Endurance: performance at start up & shut down(lift off speed)

• Little test data for rotordynamic force coefficients & operation with limit cycles (sub harmonic motions)

• Thermal management for high temperature applications (gas turbines, turbochargers)

• Predictive models lack validation for GFB operation at HIGH TEMPERATURE

41

No. of finiteelementsbetween bumps

Axially distributed linear spring

Fixed end

Thin Shell

Shell Length Shell width

Finite element

Shell thickness

ASME JGT 2008, v130, 2010, v132; ASME GT 2007-27249ASME J. Tribol., 2006, v128

0

1

2

3

4

5

6

0 90 180 270 360Angular location [deg]

Dim

ensi

onle

ss p

ress

ure,

p/pa[-]

Predicted gas film pressure field

P/Pa

Ω

X

Y

Θ

W

Foil Bearing Models

Fast PC codes integrate foil

structure with gas film

hydrodynamics –GUI driven

42

Accuracy of Foil Bearing Model Predictions

100

1000

10000

100000

0 10 20 30

Time [sec]

Rot

or s

peed

[rpm

]

1

10

100

1000

0 10 20 30

Time [sec]

Torq

ue [N

-mm

] 10,000 cycle test data (Kim et al., 2003)

Prediction

1

10

100

1000

0 10 20 30

Time [sec]

Torq

ue [N

-mm

] 10,000 cycle test data (Kim et al., 2003)

Prediction

Prediction

Shutdown

Test data

StartupStartup

0

100

200300

400

500

600700

0 5000 10000 15000 20000

Rotor speed [rpm]

Load

cap

acity

[N]

Test data

Prediction

KIST test data (2003)

0

4

8

12

0 2000 4000 6000 8000 10000

Rotor speed [rpm]

Torq

ue [N

-mm

]

Static load: 52 N

Rotor speed decreases

Driving motor Shutdowns

Prediction10,000 cyclesTest

data:5,000 cycles

Model validation: Static load performance AIAA-2007-5094

43

Foil Bearing Test Rig

Driving motor (1HP, 50 krpm)

Flexible coupling

Optical Tachometer

Start motor(2HP, 25 krpm)

Foil bearing housing

Electromagnet loader

Test rotor

Shaft Diameter = 1.500”Rotor mass = 2.2 lb

ASME GT2006-91238

44Amplitudes of subsynchronous motions INCREASE as imbalance increases (forced nonlinearity)

Effect of imbalance on RBS response

Speed (-)

Imbalance +

0 100 200 300 400 500 600 700 800

20

40

60

80

100

Frequency [Hz]

Am

plitu

de [m

icro

ns]

Dis

plac

emen

t Am

plitu

de (μ

m)

Frequency (Hz)

u = 7.4 μm

1X0.5X

2X

u = 7.4 μm

0 100 200 300 400 500 600 700 8000

20

40

60

80

100

Frequency [Hz]

y ( )

1X0.5X

2X

25.7 krpm

2.6 krpm

12.5 krpm

20.5 krpm

u = 10.5 μm

Frequency (Hz)

u = 10.5 μm

26 krpm

Limit cycle: large subsync motions

aggravated by imbalance

ASME GT2006-91238

45

Dis

plac

emen

t Am

plitu

de (μ

m)

0 5 10 15 20 250

20

40

60

SUB SYNCSYNCHRONOUS

Rotor speed (krpm)D

ispl

acem

ent A

mpl

itude

(μm

) 0 5 10 15 20 25

0

20

40

60

SUB SYNCSYNCHRONOUS

Rotor speed (krpm)

u = 7.4 μm u = 10.5 μm

Synchronous and subsynchronous motions for two imbalances

Severe subsync motions (limit cycle) aggravated by imbalance. Non linear forced response

AIAA-2007-5094

SynchronousSubsynchronous

Effect of imbalance on RBS whirl motions

46

50 krpm0 200 400 600 800 1000

0

20

40

60

80

100

Frequency [Hz]

Am

plitu

de [m

icro

ns]

2 krpm

1X 26 krpmA

mpl

itude

(μm

, 0-p

k)

Subsynchronous motions

Large amplitudes locked at natural frequency (50 to 27 krpm) …… but stable limit cycle!

Coast down from 50 krpm (833 Hz)

Rotor speed

decreases

Rotordynamic instability?Rotordynamic instability?ASME JGT, 209, v31

47

Gas Foil Bearings

All GFB models predict (linearized) rotordynamic force coefficients.

No model readily available to predict nonlinear rotordynamic forced response

What causes the subsynchronous motions?

What causes the excitation of natural frequency?

48

FB: stiffness & energy dissipation

FB structure is non linear (stiffness hardening), a typical source of sub harmonic motions for large (dynamic) loads. Hysteresis loop gives energy dissipation

-300

-200

-100

0

100

200

300

-50 -40 -30 -20 -10 0 10 20 30 40 50

Static load [N]

Bea

ring

disp

lace

men

t [μm

]

Test 1Test 2Test 3Test 4Test 5Test 6Test 7Test 8

Eight cyclic load -unload structural

tests

Loading

Loading

Unloading

Unloading

F ≠ K X2 3

1 2 3sFBF K r K r K r= + +

AIAA-2007-5094

49

0 100 200 300 400 500 600 700 800

50

0

50

100

150Waterfall

Excitation frequency [Hz]

Am

plitu

de [u

m]

X1.2 krpm

36 krpm

Am

plitu

de [μ

m]

1X Subsynchronous

rotor motions

Rotor speed: 30 →1.2 krpm (600 →20 Hz)Imbalance displacement, u = 12 μm (Vertical motion)

Subsynchronous (sub harmonic) whirl motions of large amplitude

AIAA-2007-5094

Waterfall of predictions for rotorWaterfall of predictions for rotor--GFB systemGFB system

50

0 100 200 300 400 500 6000

20

40

60

80

PredictionExperiment

Frequency [Hz]

Am

plitu

de [u

m]

Am

plitu

de [μ

m]

Comparison to test data

Whirl frequencies within a narrow band enclosing natural frequency (132 Hz) of RBS

0 5 10 15 20 25 300

40

80

120

160

200

PredictionExperiment

Rotor speed [krpm]Fr

eque

ncy

[Hz]

Amplitude vs. frequency Frequency vs. rotor speed

Test dataPredictions

Test data

Predictions

Rotor speed (krpm)Frequency (Hz)

AIAA-2007-5094

Amplitude and frequency of whirl motionsAmplitude and frequency of whirl motions

51

6 8 10 12 14 16 18 20 22 24 26 28 300

0.25

0.5

0.75

1

Rotor speed [krpm]

Whi

rl fr

eque

ncy

ratio

(WFR

)

WFR

P& T show bifurcation of motion into 1/2 & 1/3 WFRs

Test data(San Andres et al, 2006)

Predictions Test data(Kim and San Andres et al, 2007)

Rotor speed (krpm)

AIAA-2007-5094

Whirl frequency ratio of rotor bearing systemWhirl frequency ratio of rotor bearing system

52

• FB structure is highly non linear, i.e. stiffness hardening: a common source of sub harmonic motions for large (dynamic) loads.

• Subsynchronous frequencies track shaft speed at ~ ½ to 1/3 whirl ratios, locking at system natural frequency.

• Model predictions agree well with rotor response measurements (Duffing oscillator with multiple frequency response).

Gas Foil Bearings

Closure 1 Instability or forced nonlinearity?

AIAA-2007-5094

53

AIR SUPPLY

Effect of cooling flow on rotordynamics

Ps

Rotating journal

Pa

Bump spring

Top foilBearing housing

Circumferential velocity

ΩRJAxial

velocity ΩRJ

Outer gap

Inner gas film

XY

zx

Typically foil bearings DO not require pressurization.

Cooling flow is for thermal management: to remove

heat from drag or to reduce thermal gradients in hot/cold

engine sections

Side effect: Axial flow retards evolution of circumferential flow velocity

ASME JGT, 209, v31

54

Effect of side flow on rotordynamics

0

20

40

60

0

20

40

60

Am

plitu

de [μ

m, 0

-pk]

0

20

40

60

0 100 200 300 400 500 600

Frequency [Hz]

(a) 0.35 bar

(b) 1.4 bar

(c) 2.8 bar

Whirl frequency locks at RBS

natural frequency ( not

affected by level of feed pressure

For Ps ≥ 2.8 bar rotor subsync. whirl motions

disappear;(stable rotor

response)ωsub= 132 Hz

ωsub= 147 Hz

ωsub= 127 HzSubsynchronous ωsyn= 508 Hz

Synchronous

FFT of shaft motions at 30 krpm

ASME JGT, 209, v31

55Onset of subsynchronous whirl motions

0

20

40

60

0

20

40

60

Am

plitu

de [μ

m, 0

-pk]

0

20

40

60

10 15 20 25 30

Rotor speed [krpm]

(a) 0.35 bar

(b) 1.4 bar

(c) 2.8 bar

SynchronousSubsynchronous

NOS: 25 krpm

NOS: 30.5 krpm

NOS: 27 krpm

Delay of large

amplitude subsynchro

nous rotor motions

with increase in

axial cooling flow

Effect of side flow on rotordynamics

ASME JGT, 209, v31

56

(a) Gas foil bearing (b) Gas foil bearing with three shims

JournalY

Θl

Ω Ω

JournalY

X

Θp

ts

Θl

Θs Housing

Structural bump

Thin foil

Shim g

XOriginal GFB Shimmed GFBShimmed GFB

Effect of preload on rotordynamics

Original GFB

Inserting metal shims underneath bump strips introduces a preload (centering stiffness) at low cost – typical industrial practice

Trib Trans 2009, v52

57

400 200 0 200 400 600 800 10000

20

40

60

80

100Free end, vertical direction

Frequency [Hz]

Am

plitu

de [u

m]

50 krpm

2 krpm

25 krpm

1X

Am

plitu

de [μ

m]

(a) Waterfall

0 10 20 30 40 500

20

40

60

SUB SYNCSYNCHRONOUS

Rotor speed [krpm]

Ampl

itude

[um

, 0-p

k]

27 krpm

Am

plitu

de [μ

m, 0

-pk]

Original GFBs0.35 bar (5 psig)

Am

plitu

de (μ

m)

Rotor speed (krpm)

Am

plitu

de (μ

m, 0

-pk) Frequency (Hz)

400 200 0 200 400 600 800 10000

20

40

60

80

100

Frequency [Hz]A

mpl

itude

[mic

rons

]A

mpl

itude

[μm

]

50 krpm

2 krpm

25 krpm

1X

0 10 20 30 40 500

20

40

60

SUB SYNCSYNCHRONOUS

Rotor speed [krpm]

Am

plitu

de [

um, 0

-pk]

40 krpm

Am

plitu

de [μ

m, 0

-pk]

Shimmed GFBs0.35 bar (5 psig)

Rotor speed (krpm)

Am

plitu

de (μ

m, 0

-pk)

Am

plitu

de (μ

m)

Frequency (Hz)

Gas Foil Bearing with Metal Shims Trib Trans 2009, v52

58

• Predictive foil bearing FE model (structure + gas film) benchmarked by test data.• (Cooling) side pressure reduces amplitude of sub sync whirl motions• Preloads (shims) increase bearing stiffness and raise onset speed of subsync. whirl.•Predicted rotor 1X response and GFB force coefficients agree well with measurements.30

Gas Foil Bearings

Closure 2

Foil Bearings survive severe subsynchronous motions and abusive operation!

Improved stability with pressure and shims

59

Foil Bearings Thermal Management

What is effect of rotor temperature on dynamic performance of rotor-GFB system?

• Measure bearing & rotor temperatures & rotordynamic performance for increasing shaft temperatures

• Realize effectiveness of side cooling flow on thermal management

Gases have little thermal capacity. Cooling flow needed for thermal management: to remove heat from shear drag or to reduce thermal gradients from hot to cold engine sections

60

Temperatures in test rig

Foil bearings

Cartridge heaterHeater stand

T14

T10

T12

T13

Coupling cooling air

Drive motor

T15T16Th

Hollow shaftT5

T11

TambΩ

T1

T4

T3

T2

45º

Free end (FE) GFB

g

45º

Ω

T6

T7

T8

T9

Drive end (DE) GFB

g

Insulated safety cover

Cooling air

Thermocouples: 1 x heater, 2 x 4 FB outboard, 2 x Bearing housing outer surface, 1x Drive motor, 1 x ambient + infrared thermometers 2 x rotor, rotor surface temperature (Total = 17)

61

Rotordynamic response for hot rotor

As Tc increases, critical speed increases by ~ 2 krpm and the peak amplitude decreases.

0

20

40

60

80

100

0 5000 10000 15000 20000 25000 30000

Rotor speed [rpm]

Am

plitu

de [μ

m, 0

-pk]

Tc= 22 °C

Tc= 93 °C

Tc= 132 °C

Cartridge temperature (Tc) increases

Coastdown

ASME J. Tribol., 2010, v132

DEV

62

FH

FV

DH

DVg

Baseline. No forced cooling

Similar responses – free of sub synchronous whirl motions

Waterfalls of rotor motionDEH

(a) Heater off

(a) Heater on, Ths=360 C

0 200 400 600 800 10000

0.02

0.04

0.06

0.08

trace 1

X_leftWF

0.707

X_leftXaxis0 200 400 600 800 1000

Frequency [Hz]

80

60

40

20

0

Am

plitu

de [µ

m, 0

-pk]

1X2X

30 krpm

2 krpm

14 krpm

0 200 400 600 800 10000

0.02

0.04

0.06

0.08

trace 1

WF

X_leftXaxis

0 200 400 600 800 10000

0.02

0.04

0.06

0.08

trace 1

WF

X_leftXaxis

1X

0 200 400 600 800 1000Frequency [Hz]

80

60

40

20

0

2X

ASME GT 2010-22981

63

0

10

20

30

40

50

0 5 10 15 20 25 30

Speed [krpm]

Am

plitu

de [μ

m, 0

-pk]

DVDHFVFH

0

10

20

30

40

50

0 5 10 15 20 25 30

Speed [krpm]

Am

plitu

de [μ

m, 0

-pk]

DVDHFVFH

No forced cooling

Rotor 1X response for cold & hot conds

No heating Ths=360C

Critical speed (Rigid body) ~ 13 krpmElastic mode at 29 krpm

No major differences in responses between cold and hot

Free End Drive EndNo heating

T11=37º C T12=26º C

Free End Drive End

T11=157º C T12=107º C

Ths=360º C

ASME GT 2010-22981

64

Free End Drive EndNo heating

T11=37º C T12=26º C

Free End Drive End

T11=157º C T12=107º C

Ths=360º C

Free End Drive End

T11=92º C T12=70º C

Ths=200º C

FH

FV

DH

DVg

0

10

20

30

40

50

0 5 10 15 20 25 30

Speed [krpm]

Am

plitu

de [μ

m, 0

-pk]

No heatingThs=100ºCThs=200ºCThs=300ºCThs=360ºC

Heater up to 360C. No forced cooling

Cartridge temperature (Ths) increases

As Ths increases to 360ºC, peak amplitudes between 7~15 krpm decrease

natural frequency

Rotor 1X response for cold & hot condsD

HASME GT 2010-22981

65

1

10

100

0 10 20 30 40 50 60 70 80Coast down time [sec]

Rot

or s

peed

[krp

m]

No heatingThs=100ºCThs=200ºCThs=300ºCThs=360ºC

Time for rotor coast down

Heater up to 360C.No forced cooling

Coastdown time reduces as rotor heats (lesser clearance)

Cartridge temperature (Ths) increases

Exponential decay

Long time to coastdown :

very low viscous drag

Effect of temperature

Coastdown time (s)

Speed (krpm)

ASME GT 2010-22981

66

Foil Bearings survived high temperature operation– Still working !

Thermal management with cooling streams works best at high temperatures and flow rates ensuring turbulent flow.

As rotor and bearing temperatures increase, air becomes more viscous and bearing clearances decrease; hence coastdown time decreases.

For operation with hot shaft, rotor motion reduces while crossing critical speed.

Gas Foil Bearings

Closure 3 Thermal management ASME GT2010-22981

67

Novel developments

Metal Mesh Foil Bearings

68

Metal Mesh Foil Bearing (MMFB)

Bearing Cartridge, metal mesh ring and top foilHydrodynamic air film between rotating shaft and top foil

Applications: ACMs, micro gas turbines, turbo expanders, turbo compressors, turbo blowers, automotive turbochargers, APUs

• Large damping (material hysteresis) offered by metal mesh

• Tolerant to misalignment,• Wide temperature range• Coatings to reduce friction at start-up &

shutdownCartridgeMetal mesh ring

TopFoil

69

MMFB Assembly

BEARING CARTRIDGE

METAL MESH RING TOP FOIL

Simple construction and assembly procedure

ASME GT2009-59315

70

Metal Mesh Foil Bearings (+/-)• NO temperature limits• Resilient structure with lots

of material damping.• Simple construction ( in

comparison to other foil bearings)

• Cheap!

Metal mesh tends to sag or creep over timeDamping NOT viscous. Modeling difficultiesUnknown rotordynamic force coefficients

71

MMFB rotordynamic test rig

Max. operating speed: 75 krpmTurbocharger driven rotorRegulated air supply: 9.30bar

Journal: length 55 mm, 28 mm diameter , weight=0.22 kg

Journal press fitted on Shaft Stub

TC cross-sectional view

Twin ball bearing turbocharger Model T25

MMFB

ASME GT2010-22440

72

020406080

0 5 10 15 20 25 30Time [sec]

Spee

d [k

rpm

]

0

50

100

0 5 10 15 20 25 30Time [sec]B

earin

g to

rque

[N-m

Lift off speed and torqueRotor starts

Constant speed ~ 65 krpm

Valve open

Valve close

3 N-mm

Rotor stops

START UP to 65 krpm and SHUT DOWN

Once airborne, drag torque is ~ 3 % of

Startup ‘breakaway’torque

Iift off speed

Lift off speed at lowest torque : airborne operation

Load: 18 N

WD= 3.6 N

2009 AHS 65th Annual forum

73

0

5

10

15

20

25

30

35

40

45

50

0 10000 20000 30000 40000 50000 60000Journal speed [rpm]

Pow

er lo

ss [W

]Load 8.9 NLoad 17.8 NLoad 26.7 NLoad 35.6 N

MMFB power loss vs rotor speed

8.9 N (2 lb)

17.8 N (4 lb)

26.7 N (6 lb)35.6 N (8 lb)

Rotor accelerates

Power loss decreases to min during mixed lubrication, then increases with increasing rotor speed

Dead weight

(WD= 3.6 N)

Increasing static load (Ws) to 35.6 N (8 lb)

Mixed lubrication

Hydrodynamiclubrication

ASME GT2010-22440

74

0

0.01

0.02

0.03

0.04

0.05

0 10 20 30 40 50 60Rotor speed [krpm]

Fric

tion

coef

ficie

nt [-

]

Rotor accelerates

8.9 N (2 lb)

17.8 N (4 lb)26.7 N (6 lb)35.6 N (8 lb)

Friction coefficient vs rotor speed

f ~ 0.01

Friction coefficient ( f ) decreases with increasing static load

Dry sliding Airborne

f = (Torque/Radius)/(Static load)

f ~ 0.3-0.4

2009 AHS 65th Annual forum

75

Waterfall of rotor-MMFB motions

H load = 18 N

Rot

or s

peed

[krp

m]

Frequency [Hz]

Rot

or d

ecel

erat

ing

to re

st

Reverse whirl Forward whirl

Large sub harmonic

motions locked at natural

frequency

½ frequeny whirl absent with

larger applied loads

Metal mesh foil bearings have similar “forced”nonlinearity issues as bump-type foil bearings

ASME GT2010-22440

76

Metal Mesh Foil Bearings

ClosureWhile airborne, MMFB power loss increases with rotor

speed (little friction). Min power loss found.

MMFB structural stiffness and damping coefficients are frequency and amplitude dependent. Predictive model benchmarked against test data.

Measurements of MMFB rotordynamic force coefficients underway.

MMFBs are inexpensive gas bearings for oil-free MTM. Use cheap commercially available materials.

What we’ve learned so far

77

Turbocharger Systems

Increased engine efficiency and performance relies on robust rotor-bearing system

conventionalOil-Bearing

Oil LessBearing System (2007)

DETC2007-34136

Foil Bearingsor Metal Mesh Bearingsor Flex Pivot Bearings

Bearing Cartridge Metal mesh

donut

Formed top foil

A challenge!

78

Gas Bearings for MTM

The road ahead

79

Closure: Gas Bearings for MTM

Dominant challenges for gas bearing technology:

– Low gas viscosity requires minute clearances to generate load capacity.

– Damping & rotor stability are crucial– Inexpensive coatings to reduce drag and wear – Bearing design & manufacturing process well

known– Adequate thermal management to extend

operating envelope into high temperatures

80

Closure: Gas Bearings for MTM

Other pressing challenges for gas bearing technology:intermittent contact and damaging wear at

startup & shut down, andtemporary rubs during normal operating

conditions

Current research focuses oncoatings (materials), rotordynamics (stability) & high temperature (thermal management)

Need Low Cost & Long Life Solution!

81

Largest power to weight ratio, Compact & low # of parts

Reliability and efficiency,Low maintenance

Extreme temperature and pressure

Environmentally safe (low emissions)

Lower lifecycle cost ($ kW)

High speed

Materials

Manufacturing

Processes & Cycles

Fuels

Rotordynamics & (Oil-free) Bearings & Sealing

Coatings: surface conditioning for low friction and wearCeramic rotors and components

Automated agile processesCost & number

Low-NOx combustors for liquid & gas fuelsTH scaling (low Reynolds #)

Best if free (bio-fuels)

MTM – Needs & issues reassessed

82

AcknowledgmentsThanks toNSF (Grant # 0322925)NASA GRC (Program NNH06ZEA001N-SSRW2),Capstone Turbines, Inc.,Honeywell Turbocharging Systems,Korea Institute of Science and TechnologyFoster-Miller, MiTI, TAMU Turbomachinery Research Consortium (TRC)

Learn more:http://rotorlab.tamu.edu

83

References

Gas Bearings for MTM

84

References

Osborne, D.A., and San Andrés, L., 2006, “Comparison of Rotordynamic Analysis Predictions with the Test Response of Simple Gas Hybrid Bearings for Oil Free Turbomachinery,” ASME J. Eng. Gas Turbines and Power, v128.

ASME GT2003-38859

Osborne, D.A., and San Andrés, L., 2006, “Experimental Response of Simple Gas Hybrid Bearings for Oil-Free Turbomachinery,” ASME J. Eng. Gas Turbines and Power, v128. 2003 BEST Rotordynamics Paper Award –ASME (IGTI)

ASME GT 2003-38833

San Andrés, L., 2006, “Hybrid Flexure Pivot-Tilting Pad Gas Bearings: Analysis and Experimental Validation,” ASME J. Tribology, 128.

IJTC 2006-12026

Zhu, S. and L., San Andrés, 2007, “Rotordynamic Performance of Flexure Pivot Hydrostatic Gas Bearings for Oil-Free Turbomachinery,” ASME J. Eng. Gas Turbines and Power, v129

ASME GT 2004-53621

San Andrés, L., and K. Ryu, 2008, “Flexure Pivot Tilting Pad Gas Bearings: Operation with Worn Clearances and Two Load-Pad Configurations,” ASME J. Eng. Gas Turbines and Power, v130

ASME GT2007-27127

San Andrés, L., and Ryu, K., 2008, “Hybrid Gas Bearings with Controlled Supply Pressure to Eliminate Rotor Vibrations while Crossing System Critical Speeds,” ASME J. Eng. Gas Turbines and Power, v130

ASME GT2008-50393

San Andrés, L., and Ryu, K., 2009, “Dynamic Forced Response of a Rotor-Hybrid Gas Bearing System Due to Intermittent Shocks.”

ASME GT2009-59199

San Andrés, L., Niu, Y., and Ryu, K, 2010, “Dynamic Response of a Rotor-Hybrid Gas Bearing System Due To Base Induced Periodic Motions.”

ASME GT2010-22277

Flexure pivot gas bearings

85

References Foil Bearings

San Andrés, L., D. Rubio, and T.H. Kim, 2007, “Rotordynamic Performance of a Rotor Supported on Bump Type Foil Gas Bearings: Experiments and Predictions,” ASME J. Eng. Gas Turbines and Power, v129

ASME GT2006-91238

Kim, T.H., and L. San Andrés, 2008, “Heavily Loaded Gas Foil Bearings: a Model Anchored to Test Data,” ASME J. Eng. Gas Turbines and Power, v130

ASME GT2005-68486

San Andrés, L., and T.H. Kim, 2007, “Issues on Instability and Force Nonlinearity in Gas Foil Bearing Supported Rotors,” 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cincinnati, OH, July 9-11

AIAA-2007-5094

San Andrés, L., and Kim, T.H., 2009, “Analysis of Gas Foil Bearings Integrating FE Top Foil Models,” Tribology International, v42

ASME GT2007-27249

Kim, T. H., and San Andrés, L., 2009, “Effect of Side End Pressurization on the Dynamic Performance of Gas Foil Bearings – A Model Anchored to Test Data,” ASME J. Eng. Gas Turbines and Power, v131. 2008 Best PAPER Rotordynamics IGTI

ASME GT2008-50571IJTC2007-44047

Kim, T.H., and San Andrés, L., 2009, "Effects of a Mechanical Preload on the Dynamic Force Response of Gas Foil Bearings - Measurements and Model Predictions," Tribology Transactions, v52

IJTC2008-71195

San Andrés, L., and Kim, T.H., 2009, “Thermohydrodynamic Analysis of Bump Type gas Foil Bearings: A Model Anchored to Test Data,” ASME J. Eng. Gas Turbines and Power, v132

ASME GT2009-59919

Kim, T. H., and San Andrés, L., 2010, “Thermohydrodynamic Model Predictions and Performance Measurements of Bump-Type Foil Bearing for Oil-Free Turboshaft Engines in Rotorcraft Propulsion Systems,” ASME J. of Tribology, v132

AHS 2009 paper

San Andrés, L., Ryu, K., and Kim, T-H, 2010, “Thermal Management and Rotordynamic Performance of a Hot Rotor-Gas Foil Bearings System. Part 1: Measurements.

ASME GT2010-22981

86

References

Gjika, K., C. Groves, L. San Andrés, and G. LaRue, 2007, “Nonlinear Dynamic Behavior of Turbocharger Rotor-Bearing Systems with Hydrodynamic Oil Film and Squeeze Film Damper in Series: Prediction and Experiment.”

ASME DETC2007-34136

Other

San Andrés, L., Kim, T.H., Chirathadam, T.A., and Ryu, K., 2009, “Measurements of Drag Torque, Lift-Off Journal Speed and Temperature in a Metal Mesh Foil Bearing,” American Helicopter Society 65th Annual Forum, Grapevine, Texas, May 27-29

AHS Paper

San Andrés, L., Chirathadam, T. A., and Kim, T.H., 2009, “Measurements of Structural Stiffness and Damping Coefficients in a Metal Mesh Foil Bearing.”

ASME GT2009-59315

San Andrés, L., and Chirathadam T.A., 2010, “Identification of Rotordynamic Force Coefficients of a Metal Mesh Foil Bearing Using Impact Load Excitations.”

ASME GT2010-22440

Metal mesh foil bearings

Learn more:http://rotorlab.tamu.edu

87

Gas Bearings for MTM

Copyright©Luis San Andres 2010