American Helicopter Society, 65th Annual Forum

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

Foil Bearing

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

Foil Bearing

Luis San AndrésTae-Ho Kim

Thomas Abraham Chirathadam Keun Ryu

AHS Paper No. 080173

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 speedsLarge 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 FoilHydrodynamic 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

TopFoil

TAMU past work (Metal Mesh Dampers)

Zarzour and Vance (2000) J. Eng. Gas Turb. & Power, Vol. 122

Advantages of Metal Mesh Dampers over SFDsCapable of operating at low and high temperaturesNo 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

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 SpecificationsBearing 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) 42.10

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

Bearing cartridge

Top foil

Donut shaped metal mesh

Rotating shaft

Gas film

Ω

MMFB Rotordynamic test rig

(a) Static shaft

Max. operating speed: 75 krpmTurbocharger driven rotorRegulated 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 viewRef. Honeywell drawing # 448655

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 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.

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

WD= 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-mm 2.0 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 – 37 - 24 krpm

Side load = 8.9 N

WD= 3.6 N

Startup torque vs applied static load

Worn MoS2 layer

Fresh coating of MoS2

Top foil with worn MoS 2

layer shows higher starup

torques

Larger difference in

startup torques at

higher static loadsStartup Torque : Peak torque measured during startup

Dry sliding operation

0

0.1

0.2

0.3

0.4

0.5

0 10 20 30 40Static load [N]

Fri

ctio

n c

oe

ffic

ien

t [-

]DRY friction coeff. vs static load

Worn MoS2 layer

Fresh MoS2 layer

With increasing operation

cycles, the MoS 2 layer

wears away, increasing the contact or dry-

friction coefficient.

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 (WD= 3.6 N)

Increasing static load (Ws) to 35.6 N

(8 lb)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

20 30 40 50 60 70 80

Rotor speed [krpm]

Be

ari

ng

to

rqu

e [N

-mm

]

Steady state bearing drag

torque increases

with static load and rotor

speed

airborne operation

WD= 3.6 N

Side load increases

Data derived from bearing torque and rotor speed vs time data

Friction coefficient vs rotor speed

Dead weight

(WD= 3.6 N)

Increasing static load (Ws) 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 speed and increasing applied static loads. Lift-Off speed increases almost linearly with static load

0

20

40

60

80

0 5 10 15 20 25 30Time [sec]

Sp

ee

d [

krp

m]

0

50

100

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

ea

rin

g t

orq

ue

[N

-mm

]

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

WD= 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 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