Measurements of Drag Torque, Lift-off Speed, and Temperature in a Metal Mesh Foil Bearing

1. American Helicopter Society, 65th Annual Forum May 28, 2009 Measurements of Drag Torque, Lift-off Speed and Temperature in a Metal Mesh Foil Bearing AHS Paper No. 080173 Luis San Andrés Tae-Ho Kim Thomas Abraham Chirathadam Keun Ryu This material is based upon work funded by the TAMU Turbomachinery Research Consortium and donations from Honeywell Turbocharging Technologies

2. Gas foil bearings for rotorcraft applications • Elimination of complex oil lubrication system • Can operate at elevated temperatures • Elimination of the requirement for sealing • Reduced system overall weight ( High power density) • Enhanced reliability at high rotating speeds • Large inherent damping prevents potentially harmful rotor excursion • Extended maintenance intervals • Low power loss • Simple assembly procedure using cheap, commercially available materials

3. Top Foil Metal mesh ring 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 • 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. 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-2001-0247 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

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

6. MMFB dimensions and specifications PICTURE

7. MMFB Rotordynamic test rig Journal press fitted on Shaft Stub cm 15 10 5 0 (a) Static shaft TC cross-sectional view Ref. Honeywell drawing # 448655 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

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

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. Journal 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

11. Varying steady state speed & torque Rotor starts Rotor stops • Manual speed up to 65 krpm, steady state operation, and deceleration to rest 50 krpm 61 krpm 24 krpm 37 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 – 37 - 24 krpm

12. Startup torque vs applied static load Top foil with worn MoS2 layer shows higher starup 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

13. DRY friction coeff. vs static load 0.5 0.4 0.3 Friction coefficient [-] 0.2 0.1 0 0 10 20 30 40 Static load [N] Friction coefficient f = (Torque/Radius)/(Static load) 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

14. 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 35.6 N (8 lb) 4 Rotor not lifted off 26.7 N (6 lb) 3.5 3 17.8 N (4 lb) 2.5 Bearing torque [N-mm] 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) 0 20 30 40 50 60 70 80 Rotor speed [krpm] airborne operation

15. Friction coefficient vs rotor speed 8.9 N (2 lb) 17.8 N (4 lb) 26.7 N (6 lb) 35.6 N (8 lb) 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) f decreases with increasing static load airborne operation

16. Bearing drag torque vs rotor speed Lift-off speed 35.6 N (8 lb) 26.7 N (6 lb) 17.8 N (4 lb) 8.9 N (2 lb) Rotor accelerates 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

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

18. Lift-Off speedvs 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

19. Top foil temperature (bearing outboard) 35.6 N (8 lb) 26.7 N (6 lb) 17.8 N (4 lb) 8.9 N (2 lb) INCREASING STATIC LOAD Room Temperature : 21°C • Top foil temperature measured at MMFB outboard end 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

20. Conclusions • Metal mesh foil bearing assembled using cheap, commercially available materials. • Bearing break away torque, during start up, 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 Metal mesh foil bearing : Promising candidate for use in high speed oil-free rotorcraft applications