Direct lubrication tilting pad journal bearings (TPJBs) require less oil flow, reduce power consumption, and offer cooler pad temperatures for operation at high surface speeds. Although apparently free of hydrodynamic instability, the literature shows that direct lubrication TPJBs exhibit unexpected shaft vibrations with a broadband low frequency range, albeit small in amplitude. Published industrial practice demonstrates the inlet lubrication type, a reduced supply flow rate causing film starvation, and the bearing discharge conditions (evacuated or end sealed) affect the onset, gravity, and persistency of the subsynchronous vibration (SSV) hash motions. The paper presents a physical model to predict the performance of TPJBs with flow conditions ranging from over flooded to extreme starvation. Lubricant starvation occurs first on an unloaded pad, thus producing a (beneficial) reduction in drag power. As the supplied flowrate decreases further, fluid starvation moves toward the loaded pads and affects the film temperature and power loss, increases the journal eccentricity, and modifies the dynamic force coefficients of each tilting pad and thus the whole bearing. For a point mass rotor supported on a TPJB, the analysis produces eigenvalues and frequency response functions (FRFs) from three physical models for lateral rotor displacements: one with frequency reduced (4 × 4) bearing stiffness (K) and damping (C) coefficients and another with constant K–C–M (inertia) coefficients over a frequency range. The third model keeps the degrees of freedom (DOF) (tilting) of each pad and incorporates the full matrices of force coefficients including fluid inertia. Predictions of rotordynamic performance follow for two TPJBs: one bearing with load between pads (LBP) configuration, and the other with a load on a pad (LOP) configuration. For both examples, under increasingly poor lubricant flow conditions, the damping ratio for the rotor-bearing low frequency (SSV) modes decreases, thus producing an increase in the amplitude of the FRFs. For the LOP bearing, a large static load produces a significant asymmetry in the force coefficients; the rotor bearing has a small stiffness and damping for shaft displacements in the direction orthogonal to the load. A reduction in lubricant flow only exacerbates the phenomenon; starvation reaches the loaded pad to eventually cause the onset of low frequency (SSV) instability. The bearing analyzed showed similar behavior in a test bench. The predictions thus show a direct correlation between lubricant flow starvation and the onset of SSV.
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A Flow Starvation Model for Tilting Pad Journal Bearings and Evaluation of Frequency Response Functions: A Contribution Toward Understanding the Onset of Low Frequency Shaft Motions
Luis San Andrés,
Luis San Andrés
Professor
Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77843
e-mail: Lsanandres@tamu.edu
Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77843
e-mail: Lsanandres@tamu.edu
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Bonjin Koo,
Bonjin Koo
Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77843
e-mail: Bjkoo@tamu.edu
Texas A&M University,
College Station, TX 77843
e-mail: Bjkoo@tamu.edu
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Makoto Hemmi
Makoto Hemmi
Center of Tech. Innov.-M.E.,
R&D Group. Hitachi, Ltd.,
Hitachinaka 312-0034, Japan
e-mail: Makoto.hemmi.js@hitachi.com
R&D Group. Hitachi, Ltd.,
Hitachinaka 312-0034, Japan
e-mail: Makoto.hemmi.js@hitachi.com
Search for other works by this author on:
Luis San Andrés
Professor
Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77843
e-mail: Lsanandres@tamu.edu
Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77843
e-mail: Lsanandres@tamu.edu
Bonjin Koo
Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77843
e-mail: Bjkoo@tamu.edu
Texas A&M University,
College Station, TX 77843
e-mail: Bjkoo@tamu.edu
Makoto Hemmi
Center of Tech. Innov.-M.E.,
R&D Group. Hitachi, Ltd.,
Hitachinaka 312-0034, Japan
e-mail: Makoto.hemmi.js@hitachi.com
R&D Group. Hitachi, Ltd.,
Hitachinaka 312-0034, Japan
e-mail: Makoto.hemmi.js@hitachi.com
Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 21, 2017; final manuscript received August 1, 2017; published online January 3, 2018. Editor: David Wisler.
J. Eng. Gas Turbines Power. May 2018, 140(5): 052506 (14 pages)
Published Online: January 3, 2018
Article history
Received:
July 21, 2017
Revised:
August 1, 2017
Citation
San Andrés, L., Koo, B., and Hemmi, M. (January 3, 2018). "A Flow Starvation Model for Tilting Pad Journal Bearings and Evaluation of Frequency Response Functions: A Contribution Toward Understanding the Onset of Low Frequency Shaft Motions." ASME. J. Eng. Gas Turbines Power. May 2018; 140(5): 052506. https://doi.org/10.1115/1.4038043
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