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Research Papers

On the Design, Manufacture, and Premature Failure of a Metal Mesh Foil Thrust Bearing—How Concepts That Work on Paper, Actually Do Not

[+] Author and Article Information
Travis A. Cable

Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77845
e-mail: cable.travis@gmail.com

Luis San Andrés

Fellow ASME
Mast-Childs Chair Professor
Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77845
e-mail: Lsanandres@tamu.edu

Manuscript received July 3, 2018; final manuscript received July 18, 2018; published online October 22, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 140(12), 121007 (Oct 22, 2018) (13 pages) Paper No: GTP-18-1437; doi: 10.1115/1.4041137 History: Received July 03, 2018; Revised July 18, 2018

Oil-free microturbomachinery (OFT) implements compliant foil bearings because of their minute drag and ability to operate in extreme (high or low) temperature. Prominent to date, bump-foil thrust bearings integrate an underspring thin metal structure that provides resilience and material damping, and while the rotor is airborne, it acts in series with the stiffness and damping of the gas film. The design and manufacturing of foil bearings remain costly as it demands extensive engineering and actual experience. Alternative foil bearing configurations, less costly and easier to manufacture, are highly desirable to enable widespread usage of OFT. This paper details the design and manufacturing of a novel Rayleigh-step metal mesh foil thrust bearing (MMFTB) as well as its testing on a dedicated rig. Metal mesh structures offer significant material structural damping and can be easily procured at a fraction of the cost of a typical bump-foil strip layer. The MMFTB consists of a solid carrier, a number of stacked annular copper mesh sheets (wire diameter = 0.25, 0.3, and 0.41 mm), and a steel top foil (0.127 mm thick) that makes six pads (ID = 50.8 mm, OD =2 ID), each 45 deg in extent. A 3 μm polymer coats each pad, and a photochemical process etches a step 20 μm in height. Static and dynamic load measurements (without rotor speed) demonstrate that the MMFTB has structural stiffness and material damping similar to that of a publicized bump-type foil thrust bearing. A maiden test of the MMFTB with rotor speed of Ω = 15 krpm (∼80 m/s at bearing outer diameter (OD)) proved briefly the bearing operation when applying a tiny thrust load. Further tests with ambient air, a rotor speed of 40 krpm (∼212 m/s at bearing OD), and a very light load/area <7 kPa failed several of the prototype bearings, all exhibiting significant wear on one or more pads. The source of the failure is the inherent unevenness of the metal mesh stacked substructures, thus causing the pads to bulge toward the rotor collar surface before a load applies. A deficient anchoring method exacerbates the unevenness. Incidentally, a high rotor speed induced large windage that lifted the top foils pushing them against the spinning collar. As the bearing moved toward the rotating collar to begin applying thrust, the local high spots rubbed against the collar before a hydrodynamic wedge could form to separate the surfaces. Without a robust sacrificial coating, metal-to-metal contact quickly disfigured the contacting top foil pads, erasing the etched step, and leading to failure. In concept, and on paper, the mesh sheets and the top foil lay flat against the bearing carrier, giving a false sense of uniformity in the design process. In actuality, a designer must consider the manufactured states of the individual components and how they assemble. A redesign of the bearing intends to overcome the existing flaws (highlighted herein) by incorporating a thicker top foil that is well anchored (to better withstand the effects of windage), a robust sacrificial coating, and a hydrodynamic wedge accomplished via a circumferential taper on each pad.

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References

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Figures

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Fig. 1

Schematic (XZ plane) view of a Rayleigh-step metal mesh thrust foil bearing

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Fig. 2

(a) Assembled front view, (b) an exploded view, and (c) assembled back view of a Rayleigh-step metal mesh thrust foil bearing

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Fig. 3

Photographs of several mesh substructures with various wire diameters and densities (OPI: opening per inch)

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Fig. 4

Schematics and photographs of three six-pad Rayleigh-step top foils: (a) ΘLP = 1/3, (b) ΘLP = ½, and (c) 0 ≤ ΘL ≤ 45 deg

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Fig. 5

Isometric view of a test rig for the evaluation of hydrodynamic foil thrust bearings

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Fig. 6

A schematic cross section view of a test bearing installed on a load shaft and assembled into an aerostatic plenum

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Fig. 7

Two photographs of a prototype MMFTB assembled with three sheets of 40 OPI mesh and a Rayleigh-step top foil with steps located 15 deg from the pads leading edges: (a) top-down (bird-eye) view and (b) front view

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Fig. 8

Photograph of a metal mesh thrust bearing installed on test rig

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Fig. 9

Specific load versus bearing displacement for three MMFTBs (# sheets and OPI vary) and a BFTB from Ref. [14]

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Fig. 10

Estimated structural stiffness (Ks) versus bearing deflection for several MMFTBs and a BFTB from Ref. [14]

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Fig. 11

A photograph of the thrust foil bearing test rig setup for dynamic analysis

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Fig. 12

Axial dynamics model for test foil thrust bearing and load shaft

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Fig. 13

MMFTB dynamic stiffness (K) versus excitation frequency. Applied specific load W/A = 7.7, 19.7, and 32.9 kPa and three distinct mesh types Z¯12=5μm: (a) bearing with three sheets of 20 OPI metal mesh, (b) bearing with three sheets of 30 OPI metal mesh, and (c) bearing with three sheets of 40 OPI metal mesh.

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Fig. 14

MMFTB material loss factor (γ) versus excitation frequency for three applied specific loads of W/A = 7.7, 19.7, and 32.9 kPa and three different mesh types.Z¯12=5μm: (a) bearing with three sheets of 20 OPI metal mesh, (b) bearing with three sheets of 30 OPI metal mesh, and (c) bearing with 3 sheets of 40 OPI metal mesh

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Fig. 15

Average foil bearing stiffness and material loss factor versus applied specific load (W/A) for four thrust foil bearings. Results valid for 40 Hz ≤ f ≤ 300 Hz. (a) Stiffness versus applied load and (b) loss factor (γ) versus applied load.

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Fig. 16

Magnified image of a single pad and corresponding profile produced with an industrial profilometer. (a) magnified image of a single pad and (b) corresponding profile along scan.

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Fig. 17

Post-test condition of a Rayleigh-step top foil with steps located 15 deg from the pads leading edges. Wear marks visible at leading edges and pads centers. (a) Front side of top foil and (b) back side of top foil.

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Fig. 18

Photographs of (a) test bearing installed on load shaft and (b) a torque measurement system for the test bearing

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Fig. 19

Bearing clearance (from loaded position), drag torque, and top foil temperature versus time. Rayleigh-step bearing with ΘL = 15 deg, Ω = 40 krpm (212 m/s at bearing OD). (a) bearing drag torque and center clearance and (b) top foil temperatures (see Fig. 18(a) for T notation).

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Fig. 20

Photographs depicting post-test conditions of three Rayleigh-step top foils implemented on a prototype MMFTB

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Fig. 21

Photograph of a single layer of 20 OPI mesh depicting the inherent waviness in the thin structure

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