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Research Papers: Gas Turbines: Structures and Dynamics

Enhanced Groove Geometry for Herringbone Grooved Journal Bearings

[+] Author and Article Information
J. Schiffmann

Laboratory for Applied Mechanical Design,
Ecole Polytechnique Fédérale de Lausanne,
EPFL IMT LAMD,
Jaquet Droz 1,
Neuchâtel CH-2002, Switzerland
e-mail: jurg.schiffmann@epfl.ch

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received June 30, 2013; final manuscript received July 6, 2013; published online August 27, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 135(10), 102501 (Aug 27, 2013) (8 pages) Paper No: GTP-13-1211; doi: 10.1115/1.4025035 History: Received June 30, 2013; Revised July 06, 2013

Although gas-lubricated herringbone grooved journal bearings (HGJB) are known for high rotordynamic stability thresholds, small clearance to diameter ratios are required for stable rotor operation. Tight clearances not only increase bearing losses but also yield challenging manufacturing and assembly tolerances, which ultimately translate into cost. Traditionally, the grooves of HGJB are of helical nature with constant cross section and pitch. The current paper aims at increasing the clearance to diameter ratio and the stability threshold of grooved bearings by introducing enhanced groove geometries. The axial evolution of groove width, depth, and local pitch are described by individual third order polynomials with four interpolation points. The expression for the smooth pressure distribution resulting from the narrow groove theory is modified to enable the calculation of bearing properties with modified groove patterns. The reduced order bearing model is coupled to a linear rigid body rotordynamic model for predicting the whirl speed map and the corresponding stability. By introducing a critical mass parameter as a measure for stability, a criterion for the instability onset is proposed. The optimum groove geometry is found by coupling the gas bearing supported rotor model with a multiobjective optimizer. By maximizing both the clearance to diameter ratio and the rotordynamic stability it is shown that with optimal groove geometry, which deviates from helicoids with constant pitch and cross section, the critical mass parameter can be improved by more than one order of magnitude compared to traditional HGJB geometries. The clearance to diameter ratio can be increased by up to 80% while keeping the same stability margin, thus reducing both losses and manufacturing constraints. The optimum groove pattern distributions (width ratio, angle, and depth) are summarized for a variety of L/D ratios and for different compressibility numbers in a first attempt to set up general design guidelines for enhanced gas-lubricated HGJB.

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Figures

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Fig. 2

HGJB nomenclature including the position of the interpolation points for the enhanced groove geometry

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Fig. 1

Scaling effect on impeller, motor, and bearing diameters, on rotor speed and on heat fluxes as a function of shaft power

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Fig. 6

Traditional and enhanced groove geometry for a clearance to diameter ratio of 0.5‰

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Fig. 7

Unperturbed pressure of the bearing at concentric position as a function of axial position and selected clearance to diameter ratios for Pareto-optimum enhanced and traditional grooves

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Fig. 3

Rotordynamic model for rigid-body analysis

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Fig. 4

Pareto curves for traditional and enhanced pumping grooves

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Fig. 5

Pareto-optimum variable distribution for traditional and enhanced pumping grooves as a function of clearance to diameter ratio

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Fig. 8

Critical mass as a function of the highest operation compressibility number for different L/D ratio

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