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

E FFECT OF S HIMMING ON THE R OTORDYNAMIC F ORCE TRC Project 32513/1519F3 C OEFFICIENTS 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 Joshua Norsworthy Graduate Research Assistant Principal Investigator 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. Prior art: Kim and San Andrés (2009) Trib. Trans., Vol. 52 shimmed BFB increases the onset Sim et al. (2012) J.Tribol. Vol.134 speed of rotor instability and Oil free turbocharger on shimmed foil bearings reduces the amplitude of sub Sim et al. (2014) Proc. ASME Turbo Expo 2014 synchronous whirl motions Three pad BFB 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 Bearing dimensions L= 38.1mm Other BFB dimensions. Add slide D = 36.5 mm L/D~ 1.03

Clearance of shimmed BFB c nom : Nominal     t 1     Clearance profile:         c 1 s c t cos N      ( ) nom s S 1 p 2 c 2   bearing clearance nom 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 Journal press fitted on shaft Turbocharger driven rotor stub Regulated air supply: 7.58 bar (110 psig) turbocharger, Model T25, Test journal diameter:36.5 mm donated by Honeywell Turbo Technologies

Drag Torque Speed up to 60 krpm, steady state operation, and Top shaft speed = 60 krpm deceleration to rest. Lift off speed occurs at the lowest torque denoting airborne operation

Start up drag torque (dry friction) Peak startup torque Bearing lift off speed Torque max variability : ± 5 N-mm Max. variability : ± 2.5 krpm Bearing with 30 µm shims Bearing with 50µm shims Bearing with 50 µm shims Original bearing Original bearing Bearing with 30 µm shims Drag torque and rotor lift off T  f speed increase with specific load RW Friction factor of shimmed BFBs increases with shim thickness and decreases with specific load

Breakaway friction factor Friction coefficient f = (Torque)/(Radius*Static load) Original bearing f startup f breakaway =T breakaway /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. Friction factor f ~ 0.04 decreases with specific load

Rotordynamic test rig Eddy current sensor Oil inlet Static load TC center housing Journal BEARING Shaft stub Accelerometer Oil outlet Turbine Squirrel Air outlet housing cage (Soft Thermocouple Static load elastic BEARING (force gauge) support) Stinger connection to shaker X Load sensor Y Accelerometer Thermocouple 5 cm Shaft speed: 50 krpm (833 Hz) Test frequency range: up to 450 Hz Displacement amplitude: 20 µm Dynamic load: up to 250 N Vertical specific load W/LD :14.3 kPa

Parameter Identification Apply: sine sweep load excitations (200-400 Hz), amplitude controlled (20 µm). Measure: bearing absolute accelerations and displacements relative to journal   C K   M S S       F X X A      x    K j C K j C  j  2 S X X X  ( )     XX XX XY XY   System EOM    ( ) ( )             y K j C K j C C K F A           ( )   YX YX YY YY S S Y  M  Y  ( ) Y Y (  )  j   2  S Y 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 Shaft speed W/LD =14.3 kPa 50 krpm (833 Hz) BFB direct stiffnesses increases with excitation frequency, not significantly affected by shims

BFB Damping, C Bearing with 30 µm shims Original bearing 14.3 kPa specific load X Y Bearing with 50 µm shims Shaft speed W/LD =14.3 kPa 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,     C K Proportional structural damping model Loss Viscous energy dissipation ( E v ) = structural material Bearing Factor energy dissipated ( E m ) over entire duration of load Configuration excitation ( t =0 - t end ) Original 0.39 t t  end end   30  m     T   T  E z C z dt E z K z dt 0.48 V M  0 krpm shims   t 0 t 0 50  m 14.3 kPa 0.39 t specific load shims end     T  z C z dt Original 0.43   t 0 X 30  m t end 50 0.45  Y T   z K z dt shims krpm  t 0 50  m 0.43 BFB loss factor ( γ ~ 0.39-0.48 ) is not shims affected by shim thickness or rotor speed

TC vibration measurements Original bearing Bearing with 30 µm shims Rotor on shimmed BFB Bearing with 50 µm shims does not show sub synchronous vibrations

FB post test inspection

Conclusions • Shimmed (50 μ m) 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 μ m shims has 15% larger f than original BFB and BFB with 30 μ m 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

Questions(?) 19