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Research Papers: Gas Turbines: Structures and Dynamics

Structural and Rotordynamic Force Coefficients of a Shimmed Bump Foil Bearing: An Assessment of a Simple Engineering Practice

[+] Author and Article Information
Luis San Andrés

Mast-Childs Chair Professor
ASME Fellow
Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77843
e-mail: Lsanandres@tamu.edu

Joshua Norsworthy

Borg-Warner Turbo Systems,
Arden, NC 28704
e-mail: Jnorsworthy@borgwarner.com

1Work conducted as a graduate student at Texas A&M University, College Station, TX.

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received June 26, 2015; final manuscript received July 30, 2015; published online August 25, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(1), 012505 (Aug 25, 2015) (8 pages) Paper No: GTP-15-1223; doi: 10.1115/1.4031238 History: Received June 26, 2015; Revised July 30, 2015

High speed rotors supported on bump-type foil bearings (BFBs) often suffer from large subsynchronous whirl motions. Mechanically preloading BFBs through shimming is a common, low cost practice that shows improvements in rotordynamic stability. However, there is an absence of empirical information related to the force coefficients (structural and rotordynamic) of shimmed BFBs. This paper details a concerted study toward assessing the effect of shimming on a first generation BFB (L = 38.1 mm and D = 36.5 mm). Three metal shims, 120 deg apart, are glued to the inner surface of the bearing cartridge and facing the underside of the bump foil strip. The shim sets are of identical thickness, either 30 μm or 50 μm. In static load tests, a bearing with shims shows a (nonlinear) structural stiffness larger than for the bearing without shims. Torque measurements during shaft acceleration also demonstrate a shimmed BFB has a larger friction coefficient. For a static load of 14.3 kPa, dynamic loads with a frequency sweep from 250 Hz to 450 Hz are exerted on the BFB, without and with shims, to estimate its rotordynamic force coefficients while operating at ∼50 krpm (833 Hz). Similar measurements are conducted without shaft rotation. Results are presented for the original BFB (without shims) and the two shimmed BFB configurations. The direct stiffnesses of the BFB, shimmed or not, increase with excitation frequency, thus evidencing a mild hardening effect. The BFB stiffness and damping coefficients decrease slightly for operation with rotor speed as opposed to the coefficients when the shaft is stationary. For frequencies above 300 Hz, the direct damping coefficients of the BFB with 50 μm thick shims are ∼30% larger than the coefficients of the original bearing. The bearing structural loss factor, a measure of its ability to dissipate mechanical energy, is derived from the direct stiffness and damping coefficients. The BFB with 50 μm thick shims has a 25% larger loss factor—average from test data collected at 300 Hz to 400 Hz—than the original BFB. Further measurements of rotor motions while the shaft accelerates to ∼50 krpm demonstrate the shimmed BFB (thickest shim set) effectively removes subsynchronous whirl motions amplitudes that were conspicuous when operating with the original bearing.

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Figures

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Fig. 1

Schematic representation of a typical BFB and a shimmed BFB

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Fig. 2

(a) Schematic view of a bump foil and geometric parameters, and (b) a photograph of a BFB with a metal shim layered axially through the bearing

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Fig. 3

Bearing deflection (top) and derived structural stiffness (bottom) versus static load for original BFB and bearing with two shim sets (50 μm and 100 μm). Four cycles of push and pull static loads, 90 deg bearing orientation.

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Fig. 4

Schematic view of test rig used to measure drag torque [9,10]

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Fig. 5

Drag friction factor (f) versus shaft speed for original BFB and bearing with shim sets of thickness 30 μm and 50 μm. Operation with specific load W/LD ∼ 20 kPa. (Reproduced with permission from San Andrés, L., and Norsworthy [13]. Copyright 2014 by Springer International Publishing.)

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Fig. 6

Breakaway friction factor versus specific load (W/LD) for original BFB and BFB with shim sets of thickness 30 μm and 50 μm. (Reproduced with permission from San Andrés, L., and Norsworthy [13]. Copyright 2014 by Springer International Publishing.)

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Fig. 7

Schematic view of rotordynamic test rig with softly supported bearing and connections to shakers for applying dynamic loads to a test bearing

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Fig. 8

BFB (identified) stiffnesses versus excitation frequency for (a) original bearing, and bearing with (b) 30 μm shims and (c) 50 μm shims. Specific load W/(LD) ∼ 14.3 kPa. Top: no journal rotation. Bottom: shaft speed = 50 krpm (833 Hz). Bottom graphs from Ref. [13]. (Reproduced with permission from San Andrés, L., and Norsworthy [13]. Copyright 2014 by Springer International Publishing.)

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Fig. 9

BFB (identified) damping coefficients versus excitation frequency for (a) original bearing, and bearing with (b) 30 μm shims and (c) 50 μm shims. Specific load W/(LD) ∼ 14.3 kPa. Top: no journal rotation. Bottom: shaft speed = 50 krpm (833 Hz). Bottom graphs from Ref. [13]. (Reproduced with permission from San Andrés, L., and Norsworthy [13]. Copyright 2014 by Springer International Publishing.)

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Fig. 10

Loss factor (γ) versus excitation frequency for original bearing, and bearing with 30 μm shims and 50 μm shims. Specific load W/(LD) ∼ 14.3 kPa. Top: no journal rotation. Bottom: shaft speed = 50 krpm (833 Hz). (Reproduced with permission from San Andrés, L., and Norsworthy [13]. Copyright 2014 by Springer International Publishing.)

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Fig. 11

Waterfalls of shaft motion during a rapid speed run up and coast down test. Specific load W/(LD) ∼ 0 kPa: (top) with original BFB, (middle) BFB with 30 μm shims, and (bottom) BFB with 50 μm shims.

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