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Gas Turbines: Structures and Dynamics

A Metal Mesh Foil Bearing and a Bump-Type Foil Bearing: Comparison of Performance for Two Similar Size Gas Bearings

[+] Author and Article Information
Luis San Andrés

Mechanical Engineering Department,  Texas A&M University, College Station, TX 77843-3123Lsanandres@tamu.edu

Thomas Abraham Chirathadam

Mechanical Engineering Division,  Southwest Research Institute®, San Antonio, TX 78228-0510thomas.chirathadam@swri.org

Carbon footprint is a measure of the greenhouse gases emitted in mass units of carbon dioxide equivalent and is a combination of primary and secondary carbon footprints. While the primary carbon footprint measures the emission of CO2 after burning fossil fuels in automobiles, for instance, the secondary carbon footprint considers the indirect emissions, such as that during manufacturing and final waste decomposition [6].

Donated by Korea Institute of Science and Technology (KIST), South Korea.

Prior attempts to preform the top foil at lower temperatures (∼200 °C) did not yield satisfactory results.

Commercially available in the form of copper gauze/copper cloth.

Honeywell Turbocharging Technologies donated the Garrett T25 turbocharger.

J. Eng. Gas Turbines Power 134(10), 102501 (Aug 14, 2012) (13 pages) doi:10.1115/1.4007061 History: Received June 20, 2012; Revised June 21, 2012; Published August 14, 2012

Gas bearings in oil-free microturbomachinery for gas process applications and power generation (<400 kW) must be reliable and inexpensive, ensuring low drag power and thermal stability. Bump-type foil bearings (BFBs) and overleaf-type foil bearings are in use in specialized applications, though their development time (design and prototyping), exotic materials, and excessive manufacturing cost still prevent their widespread usage. Metal mesh foil bearings (MMFBs), on the other hand, are an inexpensive alternative that use common materials and no restrictions on intellectual property. Laboratory testing shows that prototype MMFBs perform similarly as typical BFBs, but offer significantly larger damping to dissipate mechanical energy due to rotor vibrations. This paper details a one-to-one comparison of the static and dynamic forced performance characteristics of a MMFB against a BFB of similar size and showcases the advantages and disadvantages of MMFBs. The bearings for comparison are a generation I BFB and a MMFB, both with a slenderness ratio L/D = 1.04. Measurements of rotor lift-off speed and drag friction at start-up and airborne conditions were conducted for rotor speeds to 70 krpm and under identical specific loads (W/LD = 0.06 to 0.26 bar). Static load versus bearing elastic deflection tests evidence a typical hardening nonlinearity with mechanical hysteresis, the MMFB showing two to three times more material damping than the BFB. The MMFB exhibits larger drag torques during rotor start-up, and shut-down tests though bearing lift-off happens at lower rotor speeds (∼15 krpm). As the rotor becomes airborne, both bearings offer very low drag friction coefficients, ∼0.03 for the MMFB and ∼0.04 for the BFB in the speed range 20–40 krpm. With the bearings floating on a journal spinning at 50 krpm, the MMFB dynamic direct force coefficients show little frequency dependency, while the BFB stiffness and damping increases with frequency (200–400 Hz). The BFB has a much larger stiffness and viscous damping coefficients than the MMFB. However, the MMFB material loss factor is at least twice as large as that in the BFB. The experiments show that the MMFB, when compared to the BFB, has a lower drag power and earlier lift-off speed and with dynamic force coefficients having a lesser dependency on whirl frequency excitation.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic representations of (a) MMFB and (b) BFB (not to scale)

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Figure 2

MMFB-applied static load and structural stiffness versus displacement for loads applied along (a), (c) 45 deg and (b), (d) 90 deg from the top foil fixed end

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Figure 3

MMFB-applied static load and structural stiffness versus displacement for loads applied along (a), (c) 45 deg and (b), (d) 90 deg from the top foil fixed end

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Figure 4

BFB-applied static load and structural stiffness versus displacement for loads applied along (a), (c) 45° and (b), (d) 90° from the top foil fixed end

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Figure 5

Mechanical hysteresis loop and structural linear stiffness (KL ) from load-displacement measurements in a MMFB. Load applied along 45 deg from the top foil fixed end.

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Figure 6

Schematic view of a test bearing, rotating journal, and instrumentation for static (pull) load and drag torque measurements. Inset shows a side view of the test rig.

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Figure 7

Rotor speed (Ω) and bearing drag torque (To ) versus elapsed time during a lift-off test cycle for operation with net static load W= 35.6 N. Metal mesh foil bearing (a), (b) and bump-type foil bearing (c), (d). Manual rotor speed-up to ∼ 70 krpm and deceleration to rest.

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Figure 8

Drag torque (To ) for (a) MMFB and (b) BFB versus rotor speed (Ω) and for increasing static loads (W). Measurements during rotor speed-up tests. Rotor speed when bearing lifts off noted.

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Figure 9

Drag friction coefficient (f) for (a) MMFB and (b) BFB versus rotor speed (Ω) and increasing static loads (W). Test data for rotor speed-up tests.

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Figure 10

Specific drag power (P′) for (a) MMFB and (b) BFB versus rotor speed (Ω) and for increasing static loads (W). Test data for rotor speed-up tests.

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Figure 11

Peak (maximum) start-up torque during dry sliding condition versus applied static load for both MMFB and BFB

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Figure 12

Schematic view of a gas foil bearing mounted on shaft of turbocharger drive system. Inset shows two stingers for application of dynamic loads along two orthogonal directions [8].

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Figure 13

Dynamic stiffness coefficients for (a) MMFB [8] and (b) BFB versus excitation frequency. Net applied static load W = 22 N (5 lbf ). Rotor speed = 50 krpm (833 Hz).

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Figure 14

Equivalent viscous damping coefficients for (a) MMFB [8] and (b) BFB versus excitation frequency. Net applied static load W= 22 N (5 lbf ). Rotor speed = 50 krpm.

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Figure 15

Estimated loss factor (γ) for a MMFB [8] and a BFB versus excitation frequency. Net static load W= 22 N (5 lbf ). Rotor at rest and spinning at 50 krpm.

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