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

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

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 Metal mesh ring Top Foil

TAMU past work (Metal Mesh Dampers) 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 METAL MESH DAMPERS provide large amounts of damping. Inexpensive. Oil-free

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

MMFB dimensions and specifications Dimensions and Specifications Bearing Cartridge outer diameter, D Bo (mm)58.15 Bearing Cartridge inner diameter, D Bi (mm)42.10 Bearing Axial length, L (mm)28.05 Metal mesh donut outer diameter, D MMo (mm)42.10 Metal mesh donut inner diameter, D MMi (mm)28.30 Metal mesh density, ρ MM (%)20 Top foil thickness, T tf (mm)0.076 Metal wire diameter, D W (mm)0.30 Young’s modulus of Copper, E (GPa), at 21 ºC 110 Poisson’s ratio of Copper, υ0.34 Bearing mass (Cartridge + Mesh + Foil), M (kg) PICTURE

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

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

Test procedure  Sacrificial layer of MoS 2 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.

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

Varying steady state speed & torque Rotor starts 61 krpm Rotor stops 50 krpm 37 krpm 24 krpm 2.5 N-mm 57 N-mm 45 N-mm 2.4 N-mm2.0 N-mm 1.7 N-mm Manual speed up to 65 krpm, steady state operation, and deceleration to rest Drag torque decreases with step wise reduction in rotating speed until the journal starts rubbing the bearing Shaft speed changes every 20 s : 65 – 50 – krpm Side load = 8.9 N W D = 3.6 N

Startup torque vs applied static load Worn MoS 2 layer Fresh coating of MoS 2 Top foil with worn MoS 2 layer shows higher starup torques Larger difference in startup torques at higher static loads Startup Torque : Peak torque measured during startup Dry sliding operation

DRY friction coeff. vs static load Worn MoS 2 layer Fresh MoS 2 layer With increasing operation cycles, the MoS 2 layer wears away, increasing the contact or dry-frictioncoefficient. Enduring coating on top foil required for efficient MMFB operation! Friction coefficient f = (Torque/Radius)/(Static load) Dry sliding operation

Bearing drag torque vs rotor speed 8.9 N (2 lb) 17.8 N (4 lb) 26.7 N (6 lb) 35.6 N (8 lb) Rotor not lifted off Dead weight (W D = 3.6 N) Increasing static load (W s ) to 35.6 N (8 lb) Rotor speed [krpm] Bearing torque [N-mm ] Steady state bearing drag torque increases with static load and rotor speed airborne operation W D = 3.6 N Side load increases Data derived from bearingtorque and rotor speed vstime data

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

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

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 f rapidly decreases initially, and then gradually raises with increasing rotor speed Dry sliding Airborne (hydrodynamic)

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

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

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 MoS 2 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 speedoil-free rotorcraft applications