Foil Gas Bearing With Compression Springs: Analyses and Experiments

[+] Author and Article Information
Ju-ho Song

Mechanical Engineering Department, Texas A&M University, College Station, TX 77843-3123

Daejong Kim1

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

Each spring bump is designated as an “elastic foundation” hereafter as a general term for orbit simulations.

Average radius of orbit in Fig. 1 is 0.0502 (1.256μm), which is slightly larger than but very close to bump deflection magnitude, indicating very high gas film stiffness at high speeds. The orbits at higher speeds, if instability does not occur, would be close to circles with imbalance radius of 0.95μm (for imbalance of 570mgmm).


Corresponding author.

J. Tribol 129(3), 628-639 (Mar 02, 2007) (12 pages) doi:10.1115/1.2736455 History: Received May 04, 2006; Revised March 02, 2007

A new foil gas bearing with spring bumps was constructed, analyzed, and tested. The new foil gas bearing uses a series of compression springs as compliant underlying structures instead of corrugated bump foils. Experiments on the stiffness of the spring bumps show an excellent agreement with an analytical model developed for the spring bumps. Load capacity, structural stiffness, and equivalent viscous damping (and structural loss factor) were measured to demonstrate the feasibility of the new foil bearing. Orbit and coast-down simulations using the calculated stiffness and measured structural loss factor indicate that the damping of underlying structure can suppress the maximum peak at the critical speed very effectively but not the onset of hydrodynamic rotor-bearing instability. However, the damping plays an important role in suppressing the subsynchronous vibrations under limit cycles. The observation is believed to be true with any air foil bearings with different types of elastic foundations.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Photo of foil gas bearing with compression springs: (a) Spring bumps (nine bumps along axial direction) and (b) assembled foil bearing

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Figure 2

Free-body diagram of spring bump

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Figure 3

Coordinate system for analyses of spring bumps

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Figure 4

Free-body diagram of partial bump

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Figure 5

Definition of normal vector for decomposition of total moment into bending moment and torsion

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Figure 6

Free-body diagram of bump foils without friction

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Figure 7

Schematic view and photo of a test rig to measure stiffness of springs

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Figure 8

Measured and predicted load-deflection curve of spring bumps; numbers shown are stiffnesses per unit area

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Figure 9

Static structural stiffness and load versus deflection: (a) Mean deflection versus static load and (b) structural stiffness versus mean deflection at loading

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Figure 10

Bearing structural impedance (Loads: 4N, 8N, 12N): (a) Real part of bearing structural impedance and (b) imaginary part of bearing structural impedance

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Figure 11

Equivalent damping coefficient-frequency curve (Loads: 4N, 8N, 12N)

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Figure 12

Structural loss factor of the spring bumps structure calculated from measured stiffness and equivalent viscous damping coefficients (Loads: 4N, 8N, 12N)

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Figure 13

Photo of load capacity test rig

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Figure 14

Bearing cooling method: Split bearing housing with cooling jacket, surrounding the bearing sleeve: (a) Holes for cooling air on the bearing sleeve and (b) photo of cooling jacket

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Figure 15

Bearing temperature without cooling, numbers are loads in newtons

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Figure 16

Bearing temperature with cooling (1350cm3∕s, 3scfm), numbers are loads in newtons

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Figure 17

Coordinate system for orbit simulations

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Figure 18

Top foil deflection model at bearing edge

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Figure 19

Exemplary pressure distribution, bump deflection, film thickness using the edge deflection model (20,000rpm, 26N static load): (a) nondimensional pressure and (b) Nondimensional film thickness

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Figure 20

Simulated peak-to-peak coast-down imbalance response and phase angle

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Figure 21

Simulated orbits at various speeds (dotted curves represent clearance circles): (a)6000rpm, (b)7600rpm, (c)12,000rpm, (d)14,000rpm, (e)14,400rpm, and (f)16,000rpm

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Figure 22

Contours of bump deflection magnitudes at different speeds: (a)6000rpm, (b)9123rpm, (c)14,000rpm, and (d)15,000rpm

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Figure 23

Bearing centerline pressures at 14,000rpm at different rotor positions; solid line is static pressure under static load corresponding to rotor weight

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Figure 24

Total dissipated damping energy from elastic foundation for one cycle

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Figure 25

Simulated orbits for bearing with external load of 10N (dotted curves represent clearance circles): (a)23,000rpm and (b)24,000rpm(WFR=0.34)




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