TRC-B&C-01-2014 A Shimmed Bump Foil Bearing: Measurements of - - PowerPoint PPT Presentation

A Shimmed Bump Foil Bearing: Measurements of Drag Torque, Lift Off Speed, and Identification of Stiffness and Damping Coefficients Luis San Andrés

Principal Investigator

Joshua Norsworthy

Graduate Research Assistant

May 2014

TRC Project 32513/1519F3

TRC-B&C-01-2014

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

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Introduction Gas Foil Bearings

Bump-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.134 Oil 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 2014 Three 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 dimensions L= 38.1mm D = 36.5 mm L/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

coordinate

θp : Angular

distance between consecutive shims

θ1 : Angular

coordinate of the first shim

Clearance profile:  

 

( ) 1

1 1 cos 2 2

s nom s S p nom

t c c t N c



                 

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 krpm

Turbocharger driven rotor Regulated 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 Torque

Speed 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 shims Original bearing Bearing with 50µm shims

T f RW 

Drag torque and rotor lift off speed increase with specific load Friction factor of shimmed BFBs increases 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 coefficient f = (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 µm Vertical specific load W/LD:14.3 kPa Test frequency range: up to 450 Hz Dynamic 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 Identification

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

X X X Y Y Y

S S 2 S X X XX XX XY XY YX YX YY YY S S Y Y 2 S

C K M F A x K j C K j C j y K j C K j C C K F A M j

   

 

                                                       

( ) ( ) ( ) ( )

( ) ( )

System EOM Frequency domain analysis yields stiffness and damping coefficients

BFB stiffnesses, K

Original 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 kPa specific

Shaft speed 50 krpm (833 Hz) W/LD=14.3 kPa

BFB Damping, C

Original 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 speed 50 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 K

Proportional structural damping model

end end

t t M t t T T V

E 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 shims 0.48 50 m shims 0.39 50 krpm Original 0.43 30 m shims 0.45 50 m shims 0.43

end end

t t t T t 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(?)