TRC Project 32513/1519F3 EFFECT OFSHIMMING ON THE ROTORDYNAMIC FORCE COEFFICIENTS OF A BUMP TYPE FOIL BEARING TRC-B&C-01-2014 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
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) 2
Justification & Past Work Issue: BFB supported rotors often show large sub synchronous whirl motions. Kim and San Andrés (2009) Trib. Trans., Vol. 52 Sim et al. (2012) J.Tribol. Vol.134 Oil free turbocharger on shimmed foil bearings Sim et al. (2014) Proc. ASME Turbo Expo 2014 Three pad BFB Prior art: shimmed BFB increases the onset speed of rotor instability and reduces the amplitude of sub synchronous whirl motions Shimmed (mechanically preloaded) BFBs are a low cost way to ensure stable performance.
Test BFB and shims Shims have one surface with adhesive. Shims press some bumps closer to the rotor L=38.1 mm, arc=12 o, Thickness =30 µm,50 µm Shims placed 120 apart, stretch axially through bearing FB radial clearance, c nom =(D I -D s )/2= 0.120 mm Other BFB dimensions. Add slide Bearing dimensions L= 38.1mm D = 36.5 mm L/D~ 1.03
Clearance of shimmed BFB t 1 2cnom 2 s Clearance profile: c( ) 1 cnom ts cos NS 1 p c nom : Nominal bearing clearance t S : Shim thickness N S : Number of shims θ : Angular coordinate θ p : Angular distance between consecutive shims θ 1 : Angular coordinate of the first shim 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 Bearing TC cross-sectional view Max. operating speed: 80 krpm Turbocharger driven rotor Regulated air supply: 7.58 bar (110 psig) Test journal diameter:36.5 mm Journal press fitted on shaft stub turbocharger, Model T25, donated by Honeywell Turbo Technologies
Drag Torque Top shaft speed = 60 krpm Speed up to 60 krpm, steady state operation, and deceleration to rest. Lift off speed occurs at the lowest torque denoting airborne operation
Start up drag torque (dry friction) Peak startup torque Torque max variability : ±5 N-mm Bearing with 30 µm shims Bearing with 50 µm shims Bearing lift off speed Max. variability : ±2.5 krpm Bearing with 50µm shims Original bearing Original bearing Bearing with 30 µm shims f T 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
Breakaway friction factor Friction coefficient f = (Torque)/(Radius*Static load) f startup Original bearing fbreakaway=tbreakaway/rw Torque meter Torque to turn manually shaft inside bearing. Differences at W/LD~20 kpa are due to wear. Tests conducted after 200 cycles of rotor start and stop. Breakaway f agrees well with f from startup tests.
Airborne friction factor f ~ 0.1 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 ~ 0.04 Friction factor decreases with specific load
Rotordynamic test rig Oil inlet TC center housing Eddy current sensor Static load Journal BEARING Air outlet Shaft stub Accelerometer Oil outlet Turbine Squirrel housing Thermocouple cage (Soft BEARING Static load elastic (force gauge) support) Stinger connection to shaker X Y Load sensor Accelerometer Shaft speed: 50 krpm (833 Hz) Displacement amplitude: 20 µm Vertical specific load W/LD:14.3 kpa Thermocouple 5 cm Test frequency range: up to 450 Hz Dynamic load: up to 250 N
Parameter Identification Apply: sine sweep load excitations (200-400 Hz), amplitude controlled (20 µm). Measure: bearing absolute accelerations and displacements relative to journal System EOM CS K X S X S 2 K X X XX jcxx KXY jc x F M A XY j ( ) ( ) X( ) K y YX jcyx KYY jc YY ( ) F C Y S K Y S A Y Y M ( ) S ( ) 2 Y j Frequency domain analysis yields stiffness and damping coefficients
BFB stiffnesses, K Original bearing Bearing with 30 µm shims 14.3 kpa specific X Y Bearing with 50 µm shims W/LD=14.3 kpa Shaft speed 50 krpm (833 Hz) BFB direct stiffnesses increases with excitation frequency, not significantly affected by shims
BFB Damping, C Original bearing Bearing with 30 µm shims 14.3 kpa specific load X Y Bearing with 50 µm shims W/LD=14.3 kpa Shaft speed 50 krpm (833 Hz) Damping C XX, along static load (X) decreases with excitation frequency. Direct damping increases modestly for shimmed BFB.
BFB loss factor, Proportional structural damping model C K Viscous energy dissipation (E v ) = structural material energy dissipated (E m ) over entire duration of load excitation (t=0 - t end ) tend tend T T EV z C z dt EM dt z K z t0 t0 X Y 14.3 kpa specific load t0 t t0 BFB loss factor (γ~0.39-0.48) is not affected by shim thickness or rotor speed t end end z T C z T zk z dt dt Loss Bearing Factor Configuration Original 0.39 30 m 0 krpm shims 0.48 50 m shims 0.39 Original 0.43 30 m 50 0.45 shims krpm 50 m 0.43 shims
TC vibration measurements Original bearing Bearing with 30 µm shims Rotor on shimmed BFB does not show sub synchronous vibrations Bearing with 50 µm shims
FB post test inspection
Conclusions 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. How? Yet unknown. MS thesis will provide rationale
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