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This article was downloaded by: [UNAM Ciudad Universitaria]On: 21 December 2014, At: 01:56Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
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Experimental Study of a New Compliant Foil Air Bearingwith Elastic SupportY. Hou a , L. Y. Xiong a & C. Z. Chen aa Institute of Cryogenic Engineering Xi'an Jiaotong University , Xi'an, China , 710049Published online: 12 Aug 2010.
To cite this article: Y. Hou , L. Y. Xiong & C. Z. Chen (2004) Experimental Study of a New Compliant Foil Air Bearing withElastic Support, Tribology Transactions, 47:2, 308-311, DOI: 10.1080/05698190490440911
To link to this article: http://dx.doi.org/10.1080/05698190490440911
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Tribology Transactions, 47: 308-311, 2004Copyright C© Society of Tribologists and Lubrication EngineersISSN: 0569-8197 print / 1547-397X onlineDOI: 10.1080/05698190490440911
Experimental Study of a New Compliant Foil AirBearing with Elastic Support
Y. HOU, L. Y. XIONG and C. Z. CHENInstitute of Cryogenic Engineering
Xi’an Jiaotong UniversityXi’an 710049, China
A new air-lubricated compliant foil journal bearing with
elastic support, which has uniform surface stiffness and is
much simpler in structure than previous compliant foil bear-
ings (CFBs), is introduced in this article. Experiments have been
conducted on the application of this type of CFB to a high-speed
test rig, and this CFB can operate stably at 151,000 rpm. From
the tests it is clear that the radial clearance C has a direct impact
on the performance of this CFB, so the numerical relationship
of structural parameters is listed in this article. Experimental
results indicate that the CFB presented here offers preferable
system dynamic and stability performance and has adequate
damping to effectively reduce the possibility of self-excited and
fractional frequency whirl.
KEY WORDS
Gas Bearings; Foil Bearings; Journal Bearings
INTRODUCTION
Self-acting gas bearings have many advantages such as conve-nience and simplicity because they do without the pressure supplysystem necessary for pressurized aerostatic gas bearings. As one ofthe most predominant types of self-acting gas bearings, compliantfoil bearings (CFBs) have played an important role in high-speedturbomachinery. Blok and Van Rossum first began the researchon CFBs in 1953 (1), and the research on CFBs in China wasbegun later in the 1980s. Xi’an Jiaotong University (XJTU) hascarried on elaborate investigations since 1990, and many tests andanalyses on CFBs have been undertaken (Xiong, et al. (2)).
NEW COMPLIANT FOIL BEARINGWITH ELASTIC SUPPORT
A traditional foil gas bearing employs a plurality of corrugatedthin metal plates as the resilient support and a bending thin metalplate of rectangular shape as the top foil. The support foils canbe deflected under the hydrodynamic gas film forces between thesurface of the shaft and the top foil, which introduces many excel-lent features to CFBs. Many remarkable achievements have been
Manuscript received by STLE November 7, 2003Final manuscript accepted January 31, 2004
Review led by William Marscher
obtained such as the tension type, bump type, and multi-wedgedtype (Gray, et al.(3)); they have been successfully used in turbo-machinery for commercial applications. However, for some smallcryogenic turboexpanders that have higher rotation speed andrather smaller shaft diameter, the use of these traditional CFBs isdifficult. For example, the rotation speed of a turbine often exceeds100,000 rpm while the shaft diameter can be less than 25.4 mm.The major disadvantages of the traditional foil bearing can beattributed to the following: the support foils are usually too com-plicated to be manufactured and installed easily, especially fortechnical limitations in China. Moreover, it is hard to control suit-able damping to support the high-speed rotary shaft.
Professor K. Hayashi developed the plate-foil gas bearing in1994 (Hayashi and Hirasata (4)), which has been studied in detailby Xiong and Hou at XJTU (Hou, et al. (5)). It seemed to havebetter performance but also the drawback that the stiffness of thefoil element is not equal in all directions. Recently, Xi’an JiaotongUniversity developed another new type of foil journal bearing, acompliant foil bearing with elastic support, to solve this problem(Chen, et al. (6)). As shown in Fig. 1, this CFB consists of a foilelement and a housing. The foil element is a smooth metal platethat has a rectangular elastic material attached to its backside and afixed pin at its end. The foil element wraps in the reverse directionaround the rotation of the shaft for one circumference with itspin inserted into a pinhole of the housing. MoS2 is adopted as thelubricant to achieve better durability. It seems to possess betterperformance because of its uniform surface stiffness and favorableelastic foundation and to be more convenient for manufacture andinstallation.
TEST METHOD AND DATA ACQUISITION
The performance of a rotor-bearing system was investigatedon a high-speed test rig. The rotor of the test rig is driven by areaction wheel with the diameter of 22.0 mm, shown in Fig. 2. Apair of this new type of foil journal bearings is used to supportthe rotor and externally pressurized gas thrust bearings with theinherent compensated restrictor used to equalize the axial force.The material and the dimensions of the shaft are as follows: mate-rial: 3Cr13Ni, diameter: 16.99 mm, length: 174.00 mm, and mass:0.320 Kg.
The radial motion of the rotor is measured via two eddycurrent–type displacement probes mounted on the bearing
308
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Experimental Study of a New Compliant Foil Air Bearing with Elastic Support 309
Fig. 1—Schematic view of compliant journal foil bearing with elasticsupport.
housing in the X-Y direction. All probe outputs are recorded anddisplayed on a computer and an oscillograph using a Keithley dataacquisition system. The rotor speed can be obtained by analyzingthe vibration signal (the distribution of the peaks of the oscillationfrequency) using the fast Fourier transform (FFT) method. Theoutput signals are analyzed in real time, so the rotational speedand the vibration of the rotor can thus be determined.
EXPERIMENTAL STUDY
The main purpose of the test is to study the dynamic and sta-bility performance of this new type of compliant foil air journalbearing with elastic support. As is known, the bearing structureparameters, especially the radial clearance C, play an importantrole in the stability of the foil bearings. So the radial clearance C
Fig. 2—The rotor-bearing system in the test rig.
TABLE 1—DESCRIPTION OF STRUCTURAL PARAMETERS FOR FOIL BEARING
Parameter Value
d: Diameter of rotor (mm) 16.99L: Axial length of foil bearing (mm) 20.0D: Inside diameter of bearing housing (mm) 18.0∼18.5C: Radial clearance of foil bearing (um) 0∼80t : Thickness of foil (mm) 0.04, 0.05, 0.06h: Thickness of support material (mm) 0.4∼0.6Numerical relationship of structural parameters D = 2[C+
(t + h)] + d
is expressed as follows, and it can be controlled by using bearinghousings of different diameter:
C = (D − d)2
− (t + h)
There exists a relationship between the radial clearance C andthickness of the plate foil t , the thickness of the support materialh, the diameter of rotor d, and the inside diameter of the bearinghousing D. The scope of the bearing structure parameters is shownin Table 1.
The test results are as follows. In this compliant foil bearingwith elastic support, we take smooth beryllium bronze slice plateas the metal foil and thin fluoride rubber cushion as the elasticfoundation. The thickness of the foil plate t has no observable in-fluence on the stiffness of the foil element and the performanceof the bearing because the plate just acts as a smooth surfacein the scope of the test use. Even so, it plays a role in the foilpre-deformation. That means if the foil plate is a bit thick, it istoo hard to deform. The thickness of the elastic support materialh will be an important parameter because it determines the de-formation of the foil element under the pressure gradient. Theradial clearance C is the dominant factor in the dynamic and sta-bility performance of this bearing. If the clearance is too small,a great deal of rub heat and drag heat will be created duringthe start-up and running period. The heat will be difficult to dis-perse and this would probably lead to performance decline andeven damage of the foil element. Conversely, damping will be in-adequate with excessive bearing radial clearance because of theloss of pressure gradient. It can be obtained from the test thatthere exists an optimized radial clearance value of 50 µm for thetest rotor. Because of length limitations, only the properties ofthis CFB with optimal parameters (C = 50 µm, t = 0.05 mm,h = 0.5 mm) are reported here.
The start-up waterfall diagram is shown in Fig. 3. The recordof rotor vibration at a steady maximum speed and the time-baserecord of the rotor vibration at this speed are shown in Fig. 4.Figure 5 shows vibration peak amplitudes as a function of theshaft speed. The test results indicate that the main portion of therotor response is synchronous (the fundamental frequency), andthe amplitudes of rotor vibration are small in most of the runningperiod. The subsynchronous resonance is very small in the wholeoperating region and exists only at the start-up moment. Due tothe limitation of rotor dynamic balance, the synchronous vibra-tion increases significantly after the speed surpasses 140,000 rpm.Even at this moment, the orbit of the rotor center is clear, and thesubsynchronous resonance is restrained.
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310 Y. HOU, L. Y. XIONG AND C. Z. CHEN
Fig. 3—Start-up waterfall diagram.
Fig. 4—Time-base record of rotor vibration at steady running.
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Experimental Study of a New Compliant Foil Air Bearing with Elastic Support 311
Fig. 5—Rotor vibration with rotor speed.
CONCLUSION
The new compliant foil air journal bearing with elastic supporthas uniform surface stiffness and is easy to fabricate because ofits unique configuration. The test results presented here indicatethat this type of CFB has the favorable attributes of CFBs, namelyexcellent dynamic and stability performance. It is useful for appli-cation to small high-speed turbomachinery.
ACKNOWLEDGMENTS
This project is supported by the National Nature Science Foun-dation of China (50206015) and the Research Fund of DoctoralProgram of Higher Education of China. The authors also wouldlike to thank Mr. Liu Guo-jun, et al. of Shuzhou Oxygen PlantManufactory for supplying the experimental parts.
REFERENCES(1) Blok, H. and Van Rossum, J. J. (1953), “The Foil Bearing—A New
Departure in Hydrodynamic Lubrication,” Lubr. Eng., 9, 6, pp 316–320.
(2) Xiong, L. Y., et al. (1997), “Development of Aerodynamic Foil Journal Bear-ings for High Speed Cryogenic Turbo-Expander,” Cryogenics, 37, pp 221–230.
(3) Gray, S., Heshmat, H. and Bhushan, B. (1981), “Technology Progress onCompliant Foil Air Bearing Systems for Commercial Applications,” BHRAFluid Eng., pp 69–97.
(4) Hayashi, K. and Hirasata, K. (1995), “Development of Aerodynamic FoilBearings for Small High-Speed Rotor,” In Lubricants and Lubrication,Elsevier Science, New York, pp 291–200.
(5) Hou, Y., et al. (2000), “The Experimental Study of Aerodynamic Plate-FoilJournal Bearing for High Speed Cryogenic Turboexpander,” Tribol. Trans.,43, 4, pp 681–684.
(6) Chen, C. Z., et al. (1998), “Self-Acting Foil Journal Bearing with ElasticSupport,” Chinese Patent No. 98233296.3.
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