A Flow Starvation Model for Tilting Pad Journal Bearings and Evaluation of Frequency Response Functions: A Contribution towards Understanding the Onset of Low Frequency Shaft Motions

[+] Author and Article Information
Luis San Andrés

Mast-Childs Chair Professor Mechanical Engineering Dept. Texas A&M University College Station, TX 77843, USA

Bonjin Koo

Research Assistant Mechanical Engineering Dept. Texas A&M University College Station, TX 77843, USA

Makoto Hemmi

Senior Researcher Center of Tech. Innov.-M.E., R&D Group. Hitachi, Ltd., Hitachinaka, 312-0034, Japan

1Corresponding author.

ASME doi:10.1115/1.4038043 History: Received July 21, 2017; Revised August 01, 2017


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 sub synchronous (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 towards 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). Predictions of rotordynamic performance follow for two TPJBs. 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.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 on a test bench. The predictions thus show a direct correlation between lubricant flow starvation and the onset of SSV.

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