Tilting-Pad Journal Bearings—Frequency-Dependent Dynamic Coefficients and Pivot Flexibility Effects

Laurence F Wagner,Paul E Allaire

doi:10.3390/lubricants10020020

Abstract

Tribologists have generally accepted that the dynamic modeling of tilting-pad journal bearings (TPJB) must consider the frequency dependency of the dynamic properties. Industrial compressors, turbines, and other rotating machines are subjected to instability drivers, such as blades, impellers, and seals, leading to dominant vibratory frequencies that are generally quite different from rotational frequency. Though the literature has provided related methods and numerical results, full understanding of the physics of TPJB frequency dependency is not generally available to the design community, and theorists and experimentalists are often not in agreement. This investigation hinges on a single-pad, two degree-of-freedom model that creates a basis for understanding the various geometries and operating conditions related to frequency dependency for a full bearing. The analytical results indicate that both stiffness and damping coefficients show frequency dependency, and that the dependency is primarily associated with the pad rotational damping and the flexibility of the pivot contact region that provides support for the pad. Understanding the role of pivot flexibility in combination with the fluid film provides a key to improving agreement between theory and experiment. This article is a revised and expanded version of the paper presented at the ASME 2019 Turbo Expo in Phoenix, Arizona from 17 to 21 June. The paper number was GT2019-90195 and it was titled “On the Frequency Dependency of Tilting-Pad Journal Bearings”.

Highlights

High-speed rotating machines with flexible rotors, such as compressors, turbines, and other machines supported on fluid-film bearings, are often subject to instability drivers, such as forces developed by annular-clearance seals, blade rows, or impellers
This study indicates that for physical understanding, for rotordynamic modeling purposes, and with concurrence of many researchers, tilting-pad bearings must be considered as inherently frequency-dependent components
The principal physical reason for frequency dependency related to film mechanics is centered around the coupling between journal translation and pad rotation, and the associated damping, pad rotational damping

Summary

Introduction

High-speed rotating machines with flexible rotors, such as compressors, turbines, and other machines supported on fluid-film bearings, are often subject to instability drivers, such as forces developed by annular-clearance seals, blade rows, or impellers. The properties of the destabilizing elements usually cannot be determined with a confidence level on par with the properties of the bearings, so it is often desirable to include some stability margin in the chosen bearing design. Such a margin has traditionally been found with the tilting-pad journal bearing (TPJB). Even understanding, of tilting-pad journal bearing dynamic behavior has been a source of debate for several decades. A vexing topic has been frequency dependency, an issue allied to rotordynamic stability. All theorists have concluded that frequency dependency is definite. But many say they are unable to find this dependency, especially the dependence of damping on frequency

Objectives

Findings

Discussion

Conclusion