A Shimmed Bump Foil Bearing: Measurements of Drag Torque, Lift Off Speed, and Identification of Stiffness

and Damping Coefficients

Luis San AndrésPrincipal Investigator

Joshua NorsworthyGraduate Research Assistant

May 2014

TRC Project 32513/1519F3

TRC-B&C-01-2014

EFFECT OF SHIMMING ON THE ROTORDYNAMIC FORCECOEFFICIENTS OF A BUMP‐TYPE FOIL BEARING

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Introduction Gas Foil BearingsBump-type foil bearings (BFB) : a gas film in series with a compliant

under-spring is a choise support for microturbomachinery (<400kW)Typically top foil, shaft or both oated to minimize wear & reduce friction

Issues:• Expensive highly engineered

elements

• Nonlinear substructure: contributes to sub synchronous rotor whirl

motions

• Thermal management advised

• LOW load capacity (compared to oil lubricated bearings)

Justification & Past Work

Kim and San Andrés (2009) Trib. Trans., Vol. 52

Sim et al. (2012) J.Tribol. Vol.134Oil free turbocharger on shimmed foil bearings

Shimmed (mechanically preloaded) BFBs are a low cost way to ensure stable performance.

Sim et al. (2014) Proc. ASME Turbo Expo 2014Three pad BFB

Issue: BFB supported rotors often show large sub synchronous whirl motions.

Prior art:shimmed BFB

increases the onset speed of rotor instability and

reduces the amplitude of sub

synchronous whirl motions

Test BFB and shims

Shims placed 120° apart, stretch axially through bearing

Bearing dimensionsL= 38.1mmD = 36.5 mmL/D~ 1.03

Shims have one surface with adhesive.

Shims press some bumps closer to the rotor

L=38.1 mm, arc=12o,Thickness =30 µm,50 µm

Other BFB dimensions. Add slide

FB radial clearance, cnom=(DI -Ds)/2= 0.120 mm

cnom: Nominal bearing clearance

tS : Shim thickness

NS : Number of shims

θ : Angular coordinateθp : Angular

distance between consecutive

shimsθ1 : Angular

coordinate of the first shim

Clearance profile: ( ) 111 cos

2 2s

nom s S pnom

tc c t Nc

Clearance of shimmed BFB

The clearance of a shimmed bearing is periodic resembling a tri-lobe or three pad bearing. The bearing clearance reduces at shim locations.

Rotordynamic test rig

Max. operating speed: 80 krpmTurbocharger driven rotorRegulated air supply: 7.58 bar (110 psig)Test journal diameter:36.5 mm

TC cross-sectional view

turbocharger, Model T25, donated by Honeywell Turbo Technologies

Journal press fitted on shaft stub

Bearing

Drag TorqueSpeed up to 60 krpm, steady state operation, and deceleration to rest.

Lift off speed occurs at the lowest torque denoting airborne operation

Top shaft speed = 60 krpm

Start up drag torque (dry friction)

Bearing with 30 µm shims

Bearing with 50 µm shims

Original bearing

Torque max variability : ±5 N-mm Max. variability : ±2.5 krpm

Bearing with 30 µm shimsOriginal bearing

Bearing with 50µm shims

TfRW

Drag torque and rotor lift off

speed increase with specific load

Friction factor of shimmed BFBsincreases with shim thickness

and decreases with specific load

Peak startup torque Bearing lift off speed

Breakaway friction factor

fstartup

fbreakaway=Tbreakaway/RW

Original bearing

Breakaway f agrees well with f from startup tests.

Torque meter

Differences at W/LD~20 kPa are due to wear. Tests conducted after 200 cycles of rotor start and stop.

Friction coefficientf = (Torque)/(Radius*Static load)

Torque to turn manually shaft inside bearing.

Airborne friction factor

f for the BFB with 30µm shims is equal to that of

the original bearing.

f for the BFB with 50 µm shims is 15% larger than f

for the original bearing.

f ~ 0.1

f ~ 0.04 Friction factor decreases with specific

load

Rotordynamic test rig

Shaft speed: 50 krpm (833 Hz)Displacement amplitude: 20 µmVertical specific load W/LD:14.3 kPa

Test frequency range: up to 450 HzDynamic load: up to 250 N

Eddy current sensor

5 cm

Accelerometer

BEARING

Oil inlet

Oil outlet

TC center housing

Air outlet

Shaft stub

Turbine housing

Stinger connection to shaker

Load sensor

Accelerometer

Static load

Static load(force gauge)

Journal

Squirrel cage (Soft

elastic support)

Y X

Thermocouple

Thermocouple

BEARING

Parameter IdentificationApply: sine sweep load excitations (200-400 Hz), amplitude controlled (20 µm).Measure: bearing absolute accelerations and displacements relative to journal

X XX

Y YY

S S2SX XXX XX XY XY

YX YX YY YY S SY Y2S

C KMF AxK j C K j C j

yK j C K j C C KF AM j

( ) ( )

( ) ( )

( )

( )System EOM

Frequency domain analysis yields stiffness and damping coefficients

BFB stiffnesses, KOriginal bearing Bearing with 30 µm shims

Bearing with 50 µm shims

BFB direct stiffnesses increases with excitation

frequency, not significantly affected by shims

X

Y

14.3 kPaspecific

Shaft speed50 krpm (833 Hz)W/LD=14.3 kPa

BFB Damping, COriginal bearing Bearing with 30 µm shims

Bearing with 50 µm shims

Damping CXX, along static load (X) decreases with excitation frequency.

Direct damping increases modestly for shimmed BFB.

X

Y

14.3 kPa specific load

Shaft speed50 krpm (833 Hz)W/LD=14.3 kPa

BFB loss factor,

BFB loss factor (γ~0.39-0.48) is not affected by shim thickness or rotor speed

X

Y

14.3 kPa specific load

C KProportional structural damping model

0 0

end endt t

Mt t

T TVE dt E dt

z C z z K z

Viscous energy dissipation (Ev) = structural material energy dissipated (Em) over entire duration of load excitation (t=0 - tend)

Bearing

Configuration

Loss

Factor

0 krpm

Original 0.39

30 m

shims0.48

50 m

shims0.39

50

krpm

Original 0.43

30 m

shims0.45

50 m

shims0.43

0

0

end

end

t

t

t

Tt

T dt

dt

z C z

z K z

TC vibration measurements

Rotor on shimmed BFB does not show sub synchronous vibrations

Original bearing

Bearing with 30 µm shims

Bearing with 50 µm shims

FB post test inspection

• Shimmed (50 μm) BFB shows LARGEST DRY friction coefficient (f ~0.80-0.40)

• Once airborne, friction factor is small [ f < 0.04-0.10]. BFB with 50 μm shims has 15% larger f than original BFB and BFB with 30 μm shims).

• Rotordynamic coefficients: shim thickness does not affect BFB stiffnesses; however, it increases the damping coefficients.

• Shim thickness does not change the BFB loss factor γ~0.38-0.45.

• TC rotor supported on shimmed BFB (50 m) demonstrates operation free of sub synchronous whirl.

Conclusions

??

How? Yet unknown. MS thesis will provide rationale

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Questions(?)