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33 rd Turbomachinery Research Consortium Meeting. Identification of structural stiffness and material loss factor in a shimmed (generation one) bump type foil bearing. TRC-B&C-04-2013. Joshua Norsworthy Graduate Research Assistant. Luis San Andrés Mast-Childs Professor. May 2 013.

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33rd Turbomachinery Research Consortium Meeting

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


Joshua Norsworthy

Graduate Research Assistant

Luis San Andrés

Mast-Childs Professor

May 2013

TRC Project 32513/1519 FB

Metal Mesh Foil Bearings: Operation at high temperatures


Bump-type foil bearing (BFB)

BFB components: bearing cartridge, bump Foil strip (compliant), and top foil

Hydrodynamic air film pressure develops between the rotating shaft and the top foil

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

Top Foil Fixed End

Bump Foil

Top Foil



  • Adequate load capacity provided by the bump foil strip (compliant)
  • Frictional damping (due to relative motion of bump foils) dominates
  • Tolerant to misalignment



Bump Foil

issues with foil bearings
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 –

Example subsynchronous motions

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)


Original and shimmed GFBs

Shimmed GFB

Original GFB

Shimmed GFB

Original GFB

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


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


Test BFB specifications





L/D = 1.03


Shimmed bump-type foil bearings

The shims have an adhesive glue layer on the bottom

Shims are added at discrete locations circumferentially and stretch axially

Bump Foil

Top Foil

Shimmed bumps are pressed closer to the rotor than other bumps

Metal Shim

Bearing Cartridge


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)


Static load test rig

Lathe saddle

Load cell

Eddy current sensor

Test bearing

Shaft affixed in lathe chuck

Lathe chuck

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

Load cell

Eddy Current sensor

Stationary shaft

Lathe tool holder

Test bearing

Dr. Chirathadam, TRC 31st (2011)


Load-deflection results for original BFB

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

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

Region of bump compression

Regions with bump compression are fitted to a third order polynomial

Bearing reaction force:

Nonlinear F(X)

Small BFB hysteresis loop : little mechanical energy dissipation

Region of bump compression


Load vs. deflection – compare

Applied load vs BFB deflection

Original and Shimmed BFBs show nonlinear force behavior.

Shims effectively reduce bearing clearance.


Estimated bearing structural stiffness

Bearing structural stiffness:

Bearing with 100μm shims


Bearing with 50μm shims


Bearing stiffness increases with bearing deflection and shim thickness

Original bearing


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

Bearing with 100µm shims


Loss factor estimation

Loss factor

BFB with 100µm shims

Largest loss factor for the 50µm shim bearing due to increased sliding friction


Predictions (90º orientation)

Load vs BFB deflection

50 µm shims

100 µm shims


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




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 g ~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.

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


Proposal to TRC (1 year)

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

  • Install and test the original and shimmed BFB in a rotordynamic test rig.
  • Conduct dynamic load tests to determine force coefficients over a range of excitation frequencies
  • For increasing static loads, conduct rotor speed start-up and shut down tests and record drag torque and lift-shaft speed.

New Proposal

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

Joshua Norsworthy

Graduate Research Assistant

Luis San Andrés

Mast-Childs Professor

May 2013



TRC budget

Shimmed bump foil bearings

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