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Thomas Abraham Chirathadam

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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 rig
15 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)

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

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

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