1

Identification of structural stiffness and material loss factor in a shimmed (generation one) bump type foil bearing

Luis San AndrésMast-Childs Professor

May 2013

Joshua NorsworthyGraduate Research Assistant

TRC-B&C-04-2013

33rd Turbomachinery Research Consortium Meeting

TRC Project 32513/1519 FBMetal Mesh Foil Bearings: Operation at high temperatures

2

Bump-type foil bearing (BFB)

BFB components: bearing cartridge, bump Foil strip (compliant), and top foilHydrodynamic air film pressure develops between the rotating shaft and the top foil

Bearing Cartridge

Top FoilBump Foil

Top Foil Fixed End

Bump Foil

Applications: ACMs, micro gas turbines, turbo expanders, turbo compressors, turbo blowers, soon in automotive turbochargers

Adequate load capacity provided by the bump foil strip (compliant)

Frictional damping (due to relative motion of bump foils) dominates

Tolerant to misalignment

Bearing Cartridge

Issues with foil bearings Endurance: performance at start up & shut down Little test data for rotordynamic force coefficients Thermal management for high temperature

applications (gas turbines, turbochargers)Prone to subsynchronous whirl and limit

cycle operation –

RudDloff, L., Arghir, M., et al., 2011, “Experimental Analysis of a First generation foil Bearing. Start-Up

Torque and Dynamic Coefficients,”ASME GT2010-22966

Unloaded FB: “Self-Excited” whirl motions at speed 30 krpm (500 Hz) with whirl frequency=165 Hz (WFR=0.33)

Example subsynchronous motions

5

(a) Gas foil bearing (b) Gas foil bearing with three shims

JournalY

Θl

Ω Ω

JournalY

X

Θp

ts

Θl

Θs Housing

Structural bump

Thin foil

Shim g

XOriginal GFB Shimmed GFBShimmed GFBOriginal GFB

Inserting metal shims underneath bump strips introduces a preload (centering stiffness) at low cost – typical industrial practice. Preload produces centering stiffness at small loads

Original and shimmed GFBs

Kim, T.H., and San Andrés, L., 2009, Trib. Trans.

Test BFB specifications

L

Do

Di

2cnom

Parameters MagnitudeBearing cartridge outer diameter, DO 50.74 mmBearing cartridge inner diameter, DI 37.98 mmBearing axial length, L 38.10 mmTop foil thickness (Inconel X750), tT 0.1 mm

foil length , 2πDI 110 mmNumber of bumps, NB 26 Bump foil (Inconel X750)

Thickness, tB 0.112 mmPitch, s0 4.3 mmLength, lB 2.1 mmHeight, hB 0.50 mm

Shim (AISI 4140) length Length 38.1 mmThickness, ts 50 µmWidth 7.87 mmAngular extent 11.8°

Shaft diameter, Ds 36.5 mmMeasured inner diameter of the FB (assembled)

36.74 mm

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

Weight of test bearing and outer cartridge 1.1 kg (10 N)

L/D = 1.03

7

Top Foil

Bump Foil

Metal ShimBearing Cartridge

Shimmed bump-type foil bearings

Shimmed bumps are pressed closer to the rotor than other bumps

Shims are added at discrete locations circumferentially and stretch axially

The shims have an adhesive glue layer on the bottom

8

Clearance in a shimmed BFB

Clearance profile:

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

(all angular coordinates taken from the middle

of the shim)

Eddy current sensor

Lathe saddle

Test bearingShaft affixed in lathe chuck

Load cell

Lathe chuck

Lathe tool holder

•

•

Eddy Current sensor Load cell

Test bearing

Stationary shaft

Lathe chuck holds shaft & bearing during loading/unloading cycles.

Static load test rig

Dr. Chirathadam, TRC 31st (2011)

10

Load-deflection results for original BFB

Region of bump compression

Region of bump compression

Bearing reaction force:

Regions of low load (<50 N) evidence the diametral clearance region

Regions with bump compression are

fitted to a third order polynomial

4 cycles of push and pull loads directed 90o to top foil fixed

end

Nonlinear F(X)

Small BFB hysteresis loop : little mechanical

energy dissipation

F K 0 K1x K 3x3

x

11

Load vs. deflection – compare

Original and Shimmed BFBs show nonlinear force behavior.Shims effectively reduce bearing clearance.

Applied load vs BFB deflection

12

Bearing with 100μm shims Push/Pull

Original bearingPush/Pull

Bearing with 50μm shimsPush/Pull

Bearing stiffness increases with bearing deflection and shim thickness

Bearing structural stiffness:

Estimated bearing structural stiffness

K ≠0 at x=0: assembly interference fit (no

clearance)

Bearing with 100µm shims

2( ) 1 33x

FK K K xx

13

BFB with 100µm shims

Loss factor estimation

Loss factor Edis

Ke2 1Ke

2F dx

Largest loss factor for the

50µm shim bearing due to

increased sliding friction

14

Predictions (90º orientation)

Stiffness

Load vs BFB deflection

Original

50 µm shims 100 µm shims

Predictions from a simple model agree well with experimental results for all test configurations

Shimmed BFB (without an assembly interference fit) shows larger energy dissipation than original BFB.

Bearing structural stiffness increases with increasing shim thickness and hardens with bearing displacement.

• The loss factor for the test (generation one) BFB is small.

• Highest loss factor ~0.20 for bearing with 50 µm shims due to increase in sliding of foil strips wit bearing cartridge.

• Predictions from model agree well with experimental results.

Conclusions

a) Install and test the original and shimmed BFB in a rotordynamic test rig.

b) Conduct dynamic load tests to determine force coefficients over a range of excitation frequencies

c) For increasing static loads, conduct rotor speed start-up and shut down tests and record drag torque and lift-shaft speed.

Proposal to TRC (1 year)

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

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

Objective: Quantify rotor lift off and shut down events for shimmed BFBs, as well as determine force coefficient

17

Effect of Shimming on the Rotordynamic Force Coefficients of a Bump-type Foil Bearing

Luis San AndrésMast-Childs Professor

May 2013

Joshua NorsworthyGraduate Research Assistant

New Proposal

TRC budget

Year ISupport for graduate student (20 h/week) x $ 2,050 x 12 months $ 24,600Fringe benefits (0.6%) and medical insurance ($185/month) $ 2,378Travel to (US) technical conference $ 1,200Tuition three semesters ($362 credit hour x 24 h/year) $ 8,688Supplies for test rig (machine parts $1,000 + portable data storage $140)

$ 1,140

$ 37,996

Research will characterize, qualitatively and quantitatively, shimming as a cheap and effective way of improving rotordynamic performance of rotors supported on BFBs.Thework is important for manufactures of turbochargers, turboexpanders and micro gas turbines

Shimmed bump foil bearings

Korean Institute of Science and Technology (KIST)

Turbomachinery Research Consortium

Questions (?)

More information athttp://rotorlab.tamu.edu

Acknowledgments/ Thanks to