Elsevier

Tribology International

Volume 120, April 2018, Pages 115-126
Tribology International

Component level study of an actively lubricated LEG Tilting Pad Bearing: Theory and experiment

https://doi.org/10.1016/j.triboint.2017.12.024Get rights and content

Highlights

  • A novel active bearing design is proposed, based on the LEG TPJB geometry and oil supply flow regulation via servovalves.

  • A simplified rotor - bearing setup provides experimental data on the proposed design.

  • A “proof of concept” test campaign demonstrates the viability of the proposed active bearing design.

  • The simulation tool for the active bearing design is validated against the experimental data, providing good performance.

Abstract

This article constitutes the second step in a research effort aiming at evaluating the feasibility of introducing active characteristics into standard leading edge groove (LEG) tilting pad journal bearings. The strategy proposed is to control the LEG inlet flow using a servovalve. This work portrays the first experimental study for the“proof of concept” of this configuration, as well as a comparison with theoretical results. A simplified setup, featuring a rigid rotor supported by a single pad arrangement is the subject of study. The obtained results prove the viability of the proposed active bearing design, validate the available simulation tool and exemplify on a conceptual level the operational benefits from introducing this technology into standard LEG Tilting Pad Bearings.

Introduction

The requirements arising from the so called 4th industrial revolution call for an expansion of the usual manner in which rotating machinery is designed and operated. Following these new trends, it is desirable that machines feature a higher degree of adaptability to new operational conditions, by introducing “smart” characteristics into their design. As such, the conception of new mechatronic machine elements is an alternative to comply with these requirements.

Bearings are key components to determine the steady state and dynamic characteristics of the rotating machinery [1,2]. Consequently, by introducing “smart” characteristics into their design, it is feasible to transfer those properties to the whole machine. For high speed and load applications, tilting pad journal bearings are the support elements of choice, due to their improved stability behavior, compared to other oil film bearing designs [3]. Hence, the improvement of their characteristics by means of the introduction of active characteristics would entail an expansion of the operational limits for turbomachinery as a whole.

Within this scope, a number of investigations have been carried out over the years, considering several strategies for achieving an active tilting pad journal bearing design. The introduction of piezoelectric actuators affecting the tilting pad radial position and resulting bearing clearance was introduced in Ref. [4]. Linear and rotational actuators acting directly over the tilting pads were studied in Refs. [[5], [6], [7]]. The usage of electromagnetic actuators installed within the tilting pads was explored by Ref. [8].

The usage of hydraulic actuation for achieving active properties in tilting pad journal bearings was introduced by Santos in Ref. [9]. Two methods were proposed, being the so called active lubrication concept [10] the one that has received more attention and development in terms of follow-up publications. It proposed the usage of servovalve controlled pressurized oil injection into the bearing clearance, by means of nozzles located on the tilting pad surface, in order to generate controllable forces over the supported rotor. Latest developments in this research effort consider theoretical and experimental investigations. A multiphysics theoretical model for this bearing design has been refined over the years [[11], [12], [13]], including a ETHD tilting pad bearing model coupled with the effect of the radial oil injection and dynamics of the hydraulic supply system. The theoretical model has been applied to synthetize model-based controllers to alter the dynamics of a flexible rotor setup in Ref. [14]. The feasibility of using this active bearing as an excitation source for carrying out operational modal analysis testing in rotating machinery was experimentally studied in Ref. [15].

Recently, the application of the active lubrication concept in tilting pad journal bearings featuring the leading edge groove lubrication system has been theoretically studied in Ref. [16]. Under such configuration, the injection nozzles located in the tilting pad surface are replaced by the leading edge groove, as the method to provide controllable pressurized oil supply to the bearing clearance. The leading edge groove has already been studied in terms of its benefits considering reduced oil consumption and reduced bearing losses, when compared to other lubrication methods [[17], [18], [19], [20]].

The results obtained from the mathematical model for the proposed active bearing design validated the initial concept. However, the presented results were only of theoretical nature, being their experimental validation the following step to continue the development of this idea. This is the main motivation for the present publication.

This work inherits the knowledge developed in previous investigations related to tilting pad journal bearings under active lubrication regime. In particular, the test rig and experimental methodologies synthetized previously in Refs. [[13], [21], [22], [23]] constitute the foundation on which this work is developed. As such, the main original contributions of this article are twofold. Firstly, it provides the first set of experimental results concerning the application of the active lubrication concept to a LEG Tilting Pad Bearing arrangement. The experimental data thoroughly describes the steady state and dynamic properties of this novel active bearing design. Secondly, a comparison with theoretical results is provided, obtained by means of the theoretical model presented in Ref. [16]. Consequently, this article results enable to validate this simulation tool, and to determine its limitations and range of applicability.

The experimental data reported here was obtained in a simplified setup (single pad arrangement, supporting a rigid rotor with vertical degree of freedom), when compared to the real geometry and application of a tilting pad bearing arrangement. Furthermore, the range of applied loads and journal rotational speeds are within the “low” range, when considering industrial applications. In this respect, in Ref. [16] the mathematical model in passive configuration (no active LEG included) was already validated against experimental results reported in Ref. [24], obtained under operational conditions resembling an industrial application of a passive tilting pad bearing. Therefore, the scope of this study is to prove experimentally the feasibility of transforming the LEG Tilting Pad Bearing into a mechatronic machine element, and to verify the simulation tool available for such system. Considering these objectives, the simplified test rig setup is sufficient and convenient for a “proof of concept” test campaign, as it reduces the amount of variables that can introduce errors in the experimental results. Based on the results presented here, one can determine the viability of testing the proposed active bearing design in a more complex test facility, resembling in a closer manner an industrial application.

Section snippets

Test facilities

The experimental results reported in this article were obtained at the Laboratory of Mechatronic Machine Elements at the Technical University of Denmark (DTU). The experiments were conducted using the test facilities depicted in Fig. 1. A list of the parameters that describe the setup are provided in Table 1. It consists of a rigid rotor supported in the vertical direction by a tilting pad bearing arrangement. The rotor is attached to a tilting arm by means of roller bearings. The arm

Mathematical model: active LEG bearing

The mathematical model for the Actively Lubricated LEG Tilting Pad Bearing was thoroughly formulated and presented in Ref. [16]. A detailed description of it is outside the scope of the current article, hence the reader is advised to refer to the provided reference for a complete discussion.

See Fig. 5 for a schematics of the relevant nomenclature used in the current article. The domains under study are: hydraulic supply system (servovalve, oil feedline), leading edge groove cavity, oil film,

Experimental and theoretical characterization of the active LEG bearing

This section aims at characterizing the behavior of the Actively Lubricated LEG Tilting Pad Journal Bearing (named Active LEG Bearing in the following discussion). Results include steady state properties, active force measurement in quasistatic and dynamic operation, and equivalent dynamic coefficients.

The experimental test results are compared with the ones delivered by the mathematical model presented in Ref. [16], in order to portray its capabilities concerning the accurate prediction of the

Examples of application scope of the active LEG bearing

The previous section has portrayed a experimental and theoretical characterization of the Active LEG Bearing. The results enabled to analyze the effect of the servovalve controlled LEG inlet flow over the bearing performance, namely steady state properties, active force and equivalent dynamic coefficients. In general, good agreement between theory and experiment was achieved.

In the following discussion, the focus will be set in presenting the operational benefits arising from the introduction

Conclusion and future aspects

This article constitutes the second step in a research effort aiming at evaluating the feasibility of introducing active characteristics into standard leading edge groove (LEG) tilting pad journal bearings. By means of theoretical and experimental results, it has been proven that the servovalve controlled LEG inlet flow is a viable alternative to transform a tilting pad bearing into a calibrated actuator, able to generate controllable forces in a wide frequency range. Consequently, by combining

Acknowledgments

This work has been financed by CONICYT Chile, project FONDECYT Iniciacion No. 11150112.

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