1. Thomas Abraham Chirathadam
29th Turbomachinery Research Consortium Meeting
May 2009
Measurements of Drag Torque, Lift-off Speed and Identification of Structural Stiffness and Damping in a Metal Mesh Foil Bearing
TRC-B&C-3-09
Luis San Andrés
Thomas Abraham Chirathadam
Tae-Ho Kim
Project title : Metal Mesh Foil Bearings for Oil-Free Turbomachinery: Test Rig for prototype demonstrations
TRC Funded Project, TEES #32513/1519 V2

2. TRC Project: Tasks 08/09 Construction of
Metal Mesh Foil Bearing (MMFB) Test Rig
MMFB performance characteristics
Bearing drag torque
Lift- Off Speed
Top Foil Temperature
Identification of force coefficients (Impact load tests) with and w/o shaft rotation
Structural stiffness and
equivalent viscous damping
Current research builds upon earlier work on metal mesh dampers conducted
by Prof. John Vance and students

3. Metal Mesh Foil Bearing (MMFB)
MMFB COMPONENTS: Bearing Cartridge, Metal mesh ring and Top Foil
Hydrodynamic air film develops between rotating shaft and top foil.
Potential applications: ACMs, micro gas turbines, turbo expanders, turbo compressors, turbo blowers, automotive turbochargers, APU
Metal mesh ring
Top
Foil
Large damping (material hysteresis) offered by metal mesh
Tolerant to misalignment, and applicable to a wide temperature range
Suitable tribological coatings needed to reduce friction at start-up & shutdown
Cartridge

4. Metal Mesh Foil Bearings (+/-)
No lubrication (oil-free). NO High or Low temperature limits.
Resilient structure with lots of material damping.
Simple construction ( in comparison to bump-type foil bearings)
Cost effective, uses common materials
Metal mesh creeps or sag with operation & time
Near absence of predictive models (bearing mainly)
Damping is NOT viscous. Modeling difficulties 4

5. Simple construction and assembly procedure
MMFB assembly
Simple construction and assembly procedure
BEARING CARTRIDGE
METAL
MESH RING
TOP FOIL 5

6. MMFB dimensions and specifications
Bearing Cartridge outer diameter, DBo(mm)
58.15
Bearing Cartridge inner diameter, DBi(mm)
42.10
Bearing Axial length, L (mm)
28.05
Metal mesh donut outer diameter, DMMo (mm)
Metal mesh donut inner diameter, DMMi(mm)
28.30
Metal mesh density, ρMM (%) 20
Top foil thickness, Ttf (mm)
0.076
Metal wire diameter, DW (mm)
0.30
Young’s modulus of Copper, E (GPa),
at 21 ºC
110
Poisson’s ratio of Copper, υ
0.34
Bearing mass (Cartridge + Mesh + Foil), M (kg)
0.316
PICTURE

7. MMFB rotordynamic test rig15 10 5 cm
Journal press fitted on Shaft Stub
(a) Static shaft
TC cross-sectional view
Ref. Honeywell drawing #
Max. operating speed: 75 krpm
Turbocharger driven rotor
Regulated air supply: 9.30bar (120 psig)
Twin ball bearing turbocharger, Model T25, donated by Honeywell Turbo Technologies
Test Journal: length 55 mm, 28 mm diameter , Weight=0.22 kg 7

8. Preloading using a rubber band
Test Rig: Torque & Lift-Off measurements
Thermocouple
Force gauge
String to pull bearing
Shaft
(Φ 28 mm)
Static load
MMFB
Top foil fixed end
Positioning (movable) table
Torque arm
Preloading using a rubber band
Eddy current sensor
5 cm
Calibrated spring 8

9. Test procedure Sacrificial layer of MoS2 applied on top foil surface
Mount MMFB on shaft of TC rig. Apply static horizontal load
High Pressure cold air drives the ball bearing supported Turbo Charger. Oil cooled TC casing
Air inlet gradually opened to raise the turbine shaft speed. Valve closing to decelerate rotor to rest
Torque and shaft speed measured during the entire experiment. All experiments repeated thrice.

10. Lift off speed at lowest torque : airborne operation
Shaft speed and torque vs time
Constant speed
~ 65 krpm
Valve open
Valve close
3 N-mm
Applied Load: 17.8 N
Rotor starts
Rotor stops
WD= 3.6 N
Manual speed up to 65 krpm, steady state operation, and deceleration to rest
Iift off speed
Startup torque ~ 110 Nmm
Shutdown torque ~ 80 Nmm
Once airborne, drag torque is ~ 3 % of Startup ‘breakaway’ torque
Lift off speed at lowest torque : airborne operation
Top shaft speed = 65 krpm 10

11. Varying steady state speed & torque
Rotor starts
Rotor stops
Manual speed up to 65 krpm, steady state operation, and deceleration to rest
61 krpm
50 krpm
37 krpm
24 krpm
Drag torque decreases with step wise reduction in rotating speed until the journal starts rubbing the bearing
57 N-mm
45 N-mm
2.5 N-mm
2.4 N-mm
2.0 N-mm
1.7 N-mm
Side load = 8.9 N
WD= 3.6 N
Shaft speed changes every 20 s : 65 – 50 – krpm 11

12. Startup Torque : Peak torque measured during startup
Startup torque vs applied static load
Top foil with worn MoS2 layer shows higher startup torques
Worn MoS2 layer
Fresh coating of MoS2
Larger difference in startup torques at higher static loads
Startup Torque : Peak torque measured during startup
Dry sliding operation 12

13. Friction coefficient [-]
DRY friction coefficient vs static load
Friction coefficient
f = (Torque/Radius)/(Static load)
0.1
0.2
0.3
0.4
0.5 10 20 30 40
Static load [N]
Friction coefficient [-]
With increasing operation cycles, the MoS2 layer wears away, increasing the contact or dry-friction coefficient.
Worn MoS2 layer
Enduring coating on top foil required for efficient MMFB operation!
Fresh MoS2 layer
Dry sliding operation 13

14. Increasing static load (Ws) to 35.6 N (8 lb)
Data derived from bearing torque and rotor speed vs time data
Bearing drag torque vs rotor speed
Side load increases
WD= 3.6 N
Steady state bearing drag torque increases with static load and rotor speed
4.5 4
35.6 N (8 lb)
3.5
Rotor not lifted off
26.7 N (6 lb) 3
17.8 N (4 lb)
Bearing torque [N-mm]
2.5
8.9 N (2 lb) 2
1.5
Increasing static load (Ws) to 35.6 N (8 lb) 1
0.5
Dead weight (WD= 3.6 N) 20 30 40 50 60 70 80
Rotor speed [krpm]
airborne operation 14

15. Increasing static load (Ws) to 35.6 N (8 lb)
Friction coefficient vs rotor speed
Friction coefficient
f = (Torque/Radius)/(Static load)
Friction coefficient
f increases with rotor speed almost linearly
Increasing static load (Ws) to 35.6 N (8 lb)
Dead weight (WD= 3.6 N)
8.9 N (2 lb)
17.8 N (4 lb)
26.7 N (6 lb)
35.6 N (8 lb)
f decreases with increasing static load
airborne operation 15

16. Bearing drag torque vs rotor speed
Max. Uncertainty ± 0.35 N-mm
Bearing drag torque increases with increasing rotor speed and increasing applied static loads. Lift-Off speed increases almost linearly with static load
Lift-off speed
8.9 N (2 lb)
17.8 N (4 lb)
26.7 N (6 lb)
35.6 N (8 lb)
Rotor accelerates 16

17. Lift-Off speed vs applied static load
Side load increases
WD= 3.6 N
Lift-Off Speed: Rotor speed beyond which drag torque is significantly small, compared to Startup Torque
Lift-Off Speed increases ~ linearly with static load 17

18. Airborne (hydrodynamic)
Friction coefficient vs rotor speed
Friction coefficient ( f ) decreases with increasing static load
f ~ 0.01
8.9 N (2 lb)
17.8 N (4 lb)
26.7 N (6 lb)
35.6 N (8 lb)
f rapidly decreases initially, and then gradually raises with increasing rotor speed
Rotor accelerates
Dry sliding
Airborne (hydrodynamic)

19. INCREASING STATIC LOAD
Top foil temperature (bearing outboard)
Room Temperature : 21°C
Top foil temperature measured at MMFB outboard end
INCREASING STATIC LOAD
8.9 N (2 lb)
17.8 N (4 lb)
26.7 N (6 lb)
35.6 N (8 lb)
Side load increases
Top Foil Temperature increases with Static Load and Rotor Speed
Only small increase in temperature for the range of applied loads and rotor speeds 19

20. Accelerometers (Not visible in this view)
Test Setup : Impact Load Test
IMPACT HAMMER TC
Top foil fixed end
MMFB
Force gauge
Journal (28 mm)
Flexible string
Eddy current sensor
Accelerometers (Not visible in this view)
(FRONT VIEW)
Positioning table
(SIDE VIEW) TC
MMFB
Accelerometer
Eddy current sensor
Journal (28 mm)
5 cm
(TOP VIEW) 20

21. SHAFT STATIONARY: NOT ROTATING
Identification model
1-DOF mechanical system
Assembly mass, M = 0.38 kg
Impact along Y direction only
In frequency
domain
Identification model, in frequency domain
SHAFT STATIONARY: NOT ROTATING 21

22. Impact force Shaft not rotating
Time domain
Frequency domain averages of 10 impacts along vertical (Y) direction Y
Frequency domain 22

23. Bearing displacement
Shaft not rotating
Time domain
Frequency domain averages of 10 impacts along vertical (Y) direction Y
Time [s]
Rapidly decaying amplitude shows large damping from MMFB
Frequency domain 23

24. Bearing acceleration TC shaft stub is flexible AY ≠ -ω2 Y
Journal not rotating
Time domain Y
TC shaft stub is flexible
AY ≠ -ω2 Y
Acceleration derived from relative displacement of bearing cartridge and shaft
Measured acceleration
Frequency domain 24

25. Curve Fit – Identifying Static Stiffness and Mass
Journal not rotating
Estimated test system natural frequency
fn = (KY/M)0.5 = 89 Hz
Re (F/Y) = Kest + Mest* Re (A/Y)
Critical damping
Ccrit=2*(KYM)0.5 = 423 N.s/m
Kest = * 105 N/m
Mest = kg
Curve fit 25

26. Identified MMFB structural stiffness
Journal not rotating
Structural stiffness decreases (10%) initially ( Hz), but increases with further increase in frequency.
KY = Re {(F - MestAy)/Y}
Kest = * 105 N/m 26

27. Identified eq.viscous damping
Journal not rotating
Equivalent viscous damping decreases with increasing frequency
CYY = Im
MMFB shows lots of damping, making test system just below critically damped
Ccrit=2*(KYM)0.5 = 423 N.s/m 27

28. Aluminum foam bearings
DONATED by CIATEQ A.C.
Aluminum foam is stiff & brittle – Not recommended for use as structural support of foil bearing
Parameters
Bearing 1
Bearing 2
Bearing Cartridge Outer Diameter [mm]
58.03 ± 0.01
58.02 ± 0.01
Bearing Cartridge Inner Diameter [mm]
42.11 ± 0.01
41.60 ± 0.01
Aluminum Foam inner Diameter [mm]
[28.20, 29.30]
[28.04, 28.15]
Bearing Axial Length [mm]
28.07 ± 0.01
28.15 ± 0.01
Bearing Mass [kg] ± ±
Porosity % 86 93

29. Conclusions
TC driven MMFB rotordynamic test rig to measure bearing drag torque, bearing displacements and acceleration. Operates up to 70 krpm
Bearing startup torque, increases with applied static loads. A sacrificial coating of MoS2 reduces start up torque
Bearing drag torque, while bearing is airborne, increases with static load and rotor speed
Top foil steady state temperature – increases with static load and rotor speed
Impact tests: shows MMFB has large damping; its stiffness gradually increases with frequency, except while traversing the bearing natural frequency 29

30. Backup Slides

31. TAMU past work (Metal Mesh Dampers)
METAL MESH DAMPERS provide large amounts of damping. Inexpensive. Oil-free
Zarzour and Vance (2000) J. Eng. Gas Turb. & Power, Vol. 122
Advantages of Metal Mesh Dampers over SFDs
Capable of operating at low and high temperatures
No changes in performance if soaked in oil
Al-Khateeb and Vance (2001) GT
Test metal mesh donut and squirrel cage( in parallel)
MM damping not affected by modifying squirrel cage stiffness
Choudhry and Vance (2005) Proc. GT2005
Develop design equations, empirically based, to predict structural stiffness and viscous damping coefficient

32. 08 TRC: MMFB Research at TAMU
San Andres, L., Chirathadam, T.A., and Kim, T.H., (2009) GT
Static Load Test Setup
Load cell
Eddy Current sensor
Stationary shaft
Lathe tool holder
Test MMFB
Lathe tool holder moves forward and backward : push and pull forces on MMFB 32

33. 08 TRC: MMFB Research at TAMU
MMFB wire density ~ 20%
3 Cycles: loading & unloading
Large hysteresis loop : Mechanical energy dissipation
Push load
Start
Pull load
Hysteresis loop
Displacement: [-0.06,0.06] mm
Load: [-130, 90 ]N 33

34. 08 TRC: MMFB Research
MMFB wire density ~ 20%
During Load reversal : jump in structural stiffness
Push load
Pull load
Start
Max. Stiffness ~ 4 MN/m 34

35. 08 TRC: MMFB Research Dynamic Load Test Setup
Electrodyamic shaker
MMFB
Accelerometer
Force transducer
Test shaft Fixture
Test shaft
Eddy Current sensors
Motion amplitude controlled mode
12.7, 25.4 &38.1 μm
Frequency of excitation :
25 – 400 Hz (25 Hz interval) 35

36. 08 TRC: MMFB Research
Structural stiffness decrease with increasing motion amplitudes
Stiffness increases gradually with Frequency

37. 08 TRC: MMFB Research
Eq. Viscous damping decreases with increasing motion amplitudes
Damping decreases rapidly with frequency