Grease Lubrication in Rolling Bearings E-Book

Piet M. Lugt

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Tribology in Practice Series (PEP)
• Sprache: Englisch

Seitenzahl: 724

Veröffentlichungsjahr: 2012

Beschreibung

The definitive book on the science of grease lubrication for roller and needle bearings in industrial and vehicle engineering.

Grease Lubrication in Rolling Bearings provides an overview of the existing knowledge on the various aspects of grease lubrication (including lubrication systems) and the state of the art models that exist today. The book reviews the physical and chemical aspects of grease lubrication, primarily directed towards lubrication of rolling bearings.

The first part of the book covers grease composition, properties and rheology, including thermal and dynamics properties. Later chapters cover the dynamics of greased bearings, including grease life, bearing life, reliability and testing. The final chapter covers lubrications systems – the systems that deliver grease to the components requiring lubrication.

Grease Lubrication in Rolling Bearings:

• Describes the underlying physical and chemical properties of grease.
• Discusses the effect of load, speed, temperature, bearing geometry, bearing materials and grease type on bearing wear.
• Covers both bearing and grease performance, including thermo-mechanical ageing and testing methodologies.

It is intended for researchers and engineers in the petro-chemical and bearing industry, industries related to this (e.g. wind turbine industry, automotive industry) and for application engineers. It will also be of interest for teaching in post-graduate courses.

Leseprobe

Contents

Cover

Tribology Series

Title Page

Copyright

Dedication

Preface

Series Preface

List of Abbreviations

Subscripts

Chapter 1: Introduction

1.1 Why Lubricate Rolling Bearings?

1.2 History of Grease Lubrication

1.3 Grease Versus Oil Lubrication

Chapter 2: Lubrication Mechanisms

2.1 Introduction

2.2 Definition of Grease

2.3 Operating Conditions

2.4 The Phases in Grease Lubrication

2.5 Film Thickness During the Bleeding Phase

2.6 Feed and Loss Mechanisms During the Bleeding Phase

2.7 Film Thickness and Starvation (Side Flow)

2.8 Track Replenishment

2.9 Grease Flow

2.10 Wall-Slip

2.11 Oxidation

2.12 EP Additives

2.13 Dynamic Behaviour

2.14 Grease Life

Chapter 3: Grease Composition and Properties

3.1 Base Oil

3.2 Base Oil Viscosity and Density

3.3 Thickener

3.4 Additives

3.5 Solid Fillers/Dry Lubricants

3.6 Compatibility

3.7 Polymer Grease

Chapter 4: Grease Life in Rolling Bearings

4.1 Introduction

4.2 Relubrication Intervals and Grease Life

4.3 The Traffic Light Concept

4.4 Grease Life as a Function of Temperature in the Green Zone

4.5 SKF Relubrication and Grease Life

4.6 Comparison Grease Life/Relubrication Models

4.7 Very Low and High Speeds

4.8 Large Rolling Bearings

4.9 Effect of Load

4.10 Effect of Outer-Ring Rotation

4.11 Cage Material

4.12 Bearing Type

4.13 Temperature and Bearing Material

4.14 Grease Fill

4.15 Vertical Shaft

4.16 Vibrations and Shock Loads

4.17 Grease Shelf Life/Storage Life

Chapter 5: Lubricating Grease Rheology

5.1 Visco-Elastic Behaviour

5.2 Viscometers

5.3 Oscillatory Shear

5.4 Shear Thinning and Yield

5.5 Yield Stress

5.6 Wall-Slip Effects

5.7 Translation Between Oscillatory Shear and Linear Shear Measurements

5.8 Normal Stresses

5.9 Time Dependent Viscosity and Thixotropy

5.10 Tackiness

Chapter 6: Grease and Base Oil Flow

6.1 Grease Flow in Pipes

6.2 Grease Flow in Rolling Bearings

Chapter 7: Grease Bleeding

7.1 Introduction

7.2 Ball Versus Roller Bearings

7.3 Grease Bleeding Measurement Techniques

7.4 Bleeding from the Covers and Under the Cage

7.5 A Grease Bleeding Model for Pressurized Grease by Centrifugal Forces

Chapter 8: Grease Aging

8.1 Mechanical Aging

8.2 Grease Oxidation

8.3 The Chemistry of Base Oil Film Oxidation

8.4 Oxidation of the Thickener

8.5 A Simple Model for Base Oil Degradation

8.6 Polymerization

8.7 Evaporation

8.8 Simple Models for the Life of Base Oil

Chapter 9: Film Thickness Theory for Single Contacts

9.1 Elasto-Hydrodynamic Lubrication

9.2 Contact Geometry and Deformation

9.3 EHL Film Thickness, Oil

9.4 EHD Film Thickness, Grease

9.5 Starvation

9.6 Spin

Chapter 10: Film Thickness in Grease Lubricated Rolling Bearings

10.1 Thin Layer Flow on Bearing Surfaces

10.2 Starved EHL for Rolling Bearings

10.3 Cage Clearance and Film Thickness

10.4 Full Bearing Film Thickness

Chapter 11: Grease Dynamics

11.1 Introduction

11.2 Grease Reservoir Formation

11.3 Temperature Behaviour

11.4 Temperature and Film Breakdown

11.5 Chaotic Behaviour

11.6 Quantitative Analysis of Grease Tests

11.7 Discussion

Chapter 12: Reliability

12.1 Failure Distribution

12.2 Mean Life and Time Between Failures

12.3 Percentile Life

12.4 Point and Interval Estimates

12.5 Sudden Death Testing

12.6 System Life Prediction

Chapter 13: Grease Lubrication and Bearing Life

13.1 Bearing Failure Modes

13.2 Rated Fatigue Life of Grease Lubricated Rolling Bearings

13.3 Background of the Fatigue Life Ratings of Grease Lubricated Bearings

13.4 Lubricant Chemistry and Bearing Life

13.5 Water in Grease

13.6 Surface Finish Aspects Related to Grease Lubrication

Chapter 14: Grease Lubrication Mechanisms in Bearing Seals

14.1 Introduction

14.2 Lubrication Mechanisms for Elastomer Contact Seals

14.3 Sealing Action of Grease

14.4 Softening and Leakage

14.5 Compatibility

14.6 A Film Thickness Model for Bearing Seals

14.7 Some Examples Showing the Importance of Sealing and Grease

Chapter 15: Condition Monitoring and Maintenance

15.1 Condition Monitoring

15.2 Vibrations and Acoustic Emission

15.3 Lubcheck

15.4 Consistency Measurement

15.5 Oil Bleeding Properties

15.6 Oil Content

15.7 Particle Contamination

15.8 Spectroscopy

15.9 Linear Voltammetry

15.10 Total Acid Number

15.11 DSC – Differential Scanning Calorimetry

15.12 Oxidation Bomb

15.13 Water

Chapter 16: Grease Qualification Testing

16.1 Introduction

16.2 Standard Test Methods

16.3 Some Qualification Criteria for Grease Selection

16.4 Pumpability

Chapter 17: Lubrication Systems

17.1 Single Point Lubrication Methods

17.2 Centralized Grease Lubrication Systems

17.3 Pumps

17.4 Valves

17.5 Distributors

17.6 Single-Line Centralized Lubrication Systems

17.7 Dual-Line Lubrication Systems

17.8 Progressive Lubrication Systems

17.9 Multi-Line Lubrication System

17.10 Cyclic Grease Flow

17.11 Requirements of the Grease

17.12 Grease Pumpability Tests

Appendix A: Characteristics of Paraffinic Hydrocarbons

References

Index

Tribology Series

BhushanIntroduction to Tribology, 2nd EditionMarch 2013BhushanPrinciples and Applications to Tribology, 2nd EditionMarch 2013LugtGrease Lubrication in Rolling BearingsJanuary 2013Honary and RichterBiobased Lubricants and Greases: Technology and ProductsApril 2011Martin and OhmaeNanolubricantsApril 2008Khonsari and BooserApplied Tribology: Bearing Design and Lubrication, 2nd EditionApril 2008Stachowiak (ed)Wear: Materials, Mechanisms and PracticeNovember 2005LansdownLubrication and Lubricant Selection: A Practical Guide, 3rd EditionNovember 2003CartierHandbook of Surface Treatment and CoatingsMay 2003Sherrington, Rowe and Wood (eds)Total Tribology: Towards an Integrated ApproachDecember 2002Kragelsky andTribology: Lubrication, Friction and WearApril 2001Stolarski and TobeRolling ContactsDecember 2000Neale and GeeGuide to Wear Problems and Testing for IndustryOctober 2000

This edition first published 2013 © 2013 John Wiley & Sons, Ltd

Registered officeJohn Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom

For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com.

The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

Library of Congress Cataloging-in-Publication Data

Lugt, Piet M., author. Grease lubrication in rolling bearings / Piet M. Lugt. pages cm. – (Tribology in practice series) Includes bibliographical references and index. ISBN 978-1-118-35391-2 (hardback) – ISBN 978-1-118-48396-1 (obook) – ISBN 978-1-118-48397-8 (epub) 1. Roller bearings–Lubrication. 2. Lubrication and lubricants. I. Title. TJ1071.L78 2013 621.8′9–dc23 2012031584

Cover photograph courtesy of SKF Maintenance Products

A catalogue record for this book is available from the Library of Congress.

ISBN: 978-1-118-35391-2

Dedicated to: Marjo, Michiel and Marijn

Preface

Technology development and bearing development have gone hand-in-hand. There are more than 50 billion bearings operating in the world at any time. They are the most widespread machine element after nuts and bolts [412]. The continuous increase in performance is placing very high demands on bearings in many applications. The load carrying capacity of bearings has increased enormously over the years and energy losses have been reduced. In practice this means that for the same type and size of bearing, the service life has become much longer and the frictional torque has been reduced. Long service life and low friction in bearings can only be obtained by proper lubrication, that is, by having a lubricating film separating the rolling elements from the rings such that roughness interaction is prevented. In the case of oil lubrication, the films can easily be calculated using classic Elasto-Hydrodynamic Lubrication (EHL) models. In the case of grease lubrication, this is much more difficult. Several aspects play a role here, such as oil bleeding, oil flow and starvation. But mechanical and thermal aging aspects of the grease or its components also have an influence on the ability to form a lubricating film.

The challenge in grease research is primarily three-fold. The first challenge is to develop greases that will provide longer life and/or are able to operate under more severe conditions (extreme low and high temperature and speed). The second challenge is the development of predictive tools, such as numerical models or expert systems. The third challenge is to design bearing-systems that will increase grease life by, for example, optimizing the grease flow. All these aspects require a fundamental understanding of the lubrication mechanisms of lubricating greases.

The bearing industry has a particular interest in understanding grease lubrication. More than 90% of all rolling element bearings are greased and sealed for life, effectively making grease a bearing component, similar to rolling elements and seals. In addition, the internal design of the bearing has an impact on the performance of the grease. This book gives an overview of the existing knowledge on the various aspects of grease lubrication and the state of the art models that exist in the public literature today.

In other words, this book reviews the physical and chemical aspects of grease lubrication, primarily directed towards lubrication of rolling bearings. It is intended for researchers and engineers in the petrochemical and bearing industries. It may also be of interest for teaching in postgraduate courses.

I have used material and information from various experts in the field of grease lubrication, rolling bearings, seals and lubrication systems. The following persons contributed to much of the material in the various chapters: Dave M. Pallister, Chapter 3, Grease composition and Chapter 8, Grease Aging; Pieter Baart, Chapter 7, Grease bleeding and Chapter 14, Sealing; Marco T. van Zoelen and Cornelis (Kees) H. Venner, Chapters 9 and 10, Film thickness; John H. Tripp and Slavco Velickov, Chapter 11, Grease dynamics; Antonio Gabelli, Chapter 13, Bearing Life; Raimund Stockhammer and Paul Conley, Chapter 17, Lubrication systems.

John H. Tripp is the main author of Chapter 12, Reliability. Much of the text from Chapter 16 originates from documents from Ben Huiskamp.

I utilized various experts to review parts of this book: Bas v.d. Vorst (rheology), Sebastien Blachere (reliability), Rihard Pasaribu (grease aging), John Tripp (grease flow), Pieter Baart (rheology), Brian Murray and Alan Thomson (Condition Monitoring and Maintenance) and Dick Meijer (grease composition). Marylou Rood created many of the figures and Walter Verhaert edited the full document.

Many thanks to the people of the SKF reference group: Alejandro Sanz, Håkan Lindgren, Domenico Bosco, Frank Berens, Frank Fiddelaers, Victoria van Camp, Gerwin Preisinger, Ferdinant Schweitzer, Filip Rosengren, Göran Lindsten, Cornelia Haag, Jürgen Kreutzkaemper, Risto Kuukkanen, Rihard Pasaribu and Steve Lane for their critical review of the document and constructive comments.

I would like to express my sincere thanks to Alejadro Sanz for originating this project and for his continuous support throughout the writing process.

I hereby acknowledge Alexander de Vries, Alan Begg, Edward Holweg and Eva Karlsson for their permission to commence this work and Alexander de Vries for his approval of the final document.

Piet M. Lugt SKF Engineering & Research Centre, The Netherlands

Series Preface

There are more than 20 billion grease lubricated rolling bearings working in various mechanical devices across the world. Experience shows that about 80% of premature bearing failures are due to lubrication problems. This is a long-awaited book addressing the important topic of rolling contact bearing lubrication by greases.

The book opens with a discussion on grease lubrication mechanisms and then follows by describing grease composition and properties, grease life in rolling bearings, rheological properties, flow characteristics and grease ageing. The text then proceeds to calculations of grease film thickness in elastohydrodynamic contacts, beginning with the theory and ending with the temperature effects on grease dynamics. The next section explaining the theory of reliability is followed by a description of the effects of grease lubrication on bearing life. Grease lubricated seals are also discussed in a separate chapter. The book finishes with chapters on condition monitoring, grease testing standards and grease lubrication systems. The interested reader will be able to find all information relevant to greases and grease lubricated rolling bearings in this book.

The strength of this book is its comprehensiveness. The fundamentals of grease properties and the lubrication of rolling bearings are illustrated through practical applications, with an emphasis on bearing life and reliability. The topic has been thoroughly researched by the authors and all the relevant areas are meticulously covered. The material is presented in an easily accessible manner.

Based on the contents and the level of detail, this book can be recommended for advanced undergraduate and postgraduate courses in the subject areas of tribology, machine design, reliability and maintenance. Practicing engineers and designers will also find the book very useful as a reference. The book is a valuable addition to Wiley's Tribology Book Series.

Gwidon Stachowiak University of Western Australia

1

Introduction

1.1 Why Lubricate Rolling Bearings?

Rolling motion can be used to carry and transmit load while facilitating movement with very low friction and low wear rates, even in the absence of lubrication. The best known example where this is used is the wheel, invented by Mesopotamians in ca. 3500 BC. Lubrication of wheel–road (or later wheel–rail) contact is difficult, but even in the absence of lubrication wear rates are much lower than those of, for example, sledges or sliding shoes. The rolling bearing is based on this principle, although the configuration is more complex since, for carrying a single load, several rolling elements are used, which have a double contact (with the inner-ring and the outer-ring). Unfortunately, even in the apparently rolling contacts, slip occurs. This is partly due to the elastic deformation of the bodies in contact, which flattens the contacts to some extent, and partly due to the kinematics in the bearings. The first effect is usually very small (and can be decreased by using materials with a high elastic modulus). The second effect is more severe. The first effect is dominant in the contacts on tapered and cylindrical roller bearing raceways, which can run at very low friction levels (note that this does not apply to the contacts on the flanges in these bearings). For other bearing types, sliding profiles in the contacts between rolling element and rings typically show just one or two points of pure rolling. Positive slip occurs between these points and negative slip outside these points. This is shown in Figure 1.1 for a thrust spherical roller bearing.

Figure 1.1 Slip in a spherical roller thrust bearing. Reproduced from Olofsson, 1997 © Elsevier.

In the absence of lubrication, the surfaces will be in intimate contact, resulting in high friction and wear at the areas where slip occurs. This will produce high stresses close to the surface, leading both to reduction of the fatigue life of the bearing and also to wear.

The occurrence of wear in the slip zones and the absence of wear in the points of pure rolling will produce a nonuniform wear profile across the tracks, leading again to high stresses at the zero-sliding points where less wear has taken place, with a corresponding reduction in the life of the bearing. This does not mean that rolling bearings cannot run in the absence of lubrication, but with no lubrication the service life will be impaired.

Full separation of the surfaces in contact, or ‘full film lubrication’, is preferable. In this case there is virtually no wear and the life of the bearing will be determined by fatigue. If full film conditions are not possible the materials should preferably be ‘incompatible’, meaning that adhesion and ‘welding’ can be avoided. This can be done by using ceramic rolling elements, for example, or by applying a suitable coating (or surface treatment) on one or both surfaces in contact. Due to the local sliding conditions a coating will wear and the service life of the bearing is determined by the wear rate and thickness of the coating. Nevertheless, the relatively short unlubricated life can be increased substantially by this solution. The advantage of using fluid lubrication is its ability to repair itself after shear in the contacts due to its ability to replenish the contacts (a self-healing mechanism). If a sufficient quantity of lubricant is available, this will happen through churning, splashing or will be flow-induced by the geometry of the bearing (by the pumping effect and centrifugal forces). In the case of grease lubrication, it occurs primarily through oil-bleeding, spin, cage distribution and to some extent through a centrifugal force inducing flow in thin lubricant layers.

1.2 History of Grease Lubrication

The word ‘grease’ is derived from the Latin word ‘crassus’ meaning fat. As far back as 1400 BC, both mutton fat and beef fat were used as axle greases in chariots (journal bearings). Early forms of grease lubricants before the 19th century were largely based on natural triglycerides, animal fats and oils, commonly known as ‘grease’ (Polishuk [475]).1 Partial rendering of fats with lime or lye would produce simple greases that were effective as lubricants for wooden axles and simple machinery. Triglycerides are good boundary lubricants that show low coefficients of friction but they show poor oxidative stability at elevated operating temperatures.

After the discovery of oil in the USA (Drake) in 1859, most lubricants were based on mineral oil [450]. The first ‘modern’ greases were lime soaps or calcium soaps, which today are not much used in rolling bearings. They may however be used providing that the temperatures stay low. Later, aluminium and sodium greases were developed, which could accommodate higher temperatures. Until the Second World War only these calcium, sodium and aluminium greases were used.

In the 1930s–1940s new thickeners were discovered for multipurpose greases, based on calcium, lithium and barium [450]. In 1940 the first calcium complex grease and lithium grease patents [182] were issued. Today, over 50% of the market still consists of lithium grease. Aluminium complex greases were developed in the 1950s and lithium complex greases in the 1960s. Polyurea use, started in the 1980s, especially in Japan.

In 1992 a new type of grease was invented by Meijer [414], where the thickener comprises a mixture of a high molecular and low molecular weight polymer of propylene. A grease structure could be obtained through rapid quenching. This type of grease has been successfully tested and is used today in, for example, paper mill bearings [73]. Another example is nanotube grease[271,272].

Grease lubricants are used in a large variety of environments. Operating temperatures for grease lubricated applications range from subzero, −70 °C to temperatures exceeding 300 °C for high temperatures applications. They are also used in vacuum atmospheres encountered by space applications. More often, the operating environment involves wet and humid atmospheres, exposure to salt water and many other types of corrosive agents that affect the performance of rolling bearings and machine elements. The chemical composition of grease lubricants varies considerably to accommodate the large variety of applications and extremes in operating environments. Grease is commonly used for rolling bearing lubrication as a cost-effective and convenient source of lubrication.

1.3 Grease Versus Oil Lubrication

As mentioned above, the longest service life can be obtained if the lubricant film fully separates the contacting surfaces. In a rolling bearing this is achieved through hydrodynamic action where the lubricant is sheared inbetween the roller–ring contacts. Once inside these contacts the viscosity becomes so high, due to the high pressures, that leakage (pressure-driven flow) out of the contact will remain very small. It will be shown later, in Chapter 9, that this film thickness depends on oil viscosity and bearing speed. Obviously, a film can only be maintained if sufficient oil is available. In oil bath lubrication this is not a problem, but in the case of grease lubrication this is more difficult. The lubricating grease will generate a thick film at the beginning of bearing operation, formed by the combination of thickener and base oil. Side flow occurs due to the pressure difference inside the bearing contacts and next to the tracks. There may be very little reflow back into the track and the bearing may suffer from starvation, with thinner films then expected based on EHL (Elasto-Hydrodynamic Lubrication) theory.

Inside the bearing contacts (micro) slip occurs and heat will be generated. In the case of oil lubrication, the oil will act as a coolant for the bearing, reducing the temperature rise and therefore maintaining a sufficiently high viscosity and film thickness. Unfortunately, this is not possible in grease lubrication. There is generally no flow here and therefore no cooling effect by the lubricant.

High temperatures, mechanical work and the build-up of contaminants cause aging of the lubricant. In the case of oil lubrication this will be small due to the cooling and replenishment action. Unfortunately the effect of aging cannot be neglected in grease lubrication. Aging will primarily occur through oxidation of the base oil and thickener and through the breakdown of the structure. A long service life therefore often requires periodic replenishment through active relubrication (systems). Sometimes, the specific rheological behaviour of grease creates difficulties in centralized lubrication systems (pumpability).

Despite the above mentioned drawbacks, there are also clear advantages in using grease as a lubricant. Generally, friction levels are lower than in the case of oil lubrication, primarily due to the absence of churning, apart from the start-up phase. The next advantage is the ease of operation. Sealed and greased-for-life bearings do not require oil baths, which may leak. A well designed bearing with good quality grease requires no maintenance. In addition, the grease will fulfil a sealing function and form a barrier against entry of contaminants onto the raceway, extending the service life of the bearing.

For the selection of oil, the main parameters are: viscosity, boundary lubrication properties (lubricity) and type of additives. In the selection of grease the properties of the thickener dominate, but again the oil base stock properties are important. The main parameters are: consistency, operating temperature range, oil bleeding properties, viscosity of the base oil, corrosion inhibiting properties (additives) and load carrying capacity. This makes grease selection much more complex than oil selection. In this book the various aspects of grease lubrication in rolling bearings will be described, that is the lubrication of the bearing, the lubrication of the seal, lubrication systems, condition monitoring techniques and test methods. In the next chapter (Chapter 2) the lubrication mechanisms will be described. This chapter will touch upon many items that will be described in the following chapters, such as ‘film thickness’ (Chapters 9 and 10), ‘rheology’ (Chapter 5), ‘flow’ (Chapter 6), ‘oil bleeding’ (Chapter 7), ‘aging’ (Chapter 8) and ‘dynamic behaviour’ (Chapter 11). A large chapter in this book is dedicated to grease composition and properties for the various grease types (Chapter 3). A very important topic is bearing service life, which is given by the life of the grease (Chapter 4) and the life of the bearing (Chapter 13) supported by a separate chapter on reliability (Chapter 12).

Finally, separate chapters are dedicated to seal lubrication (Chapter 14), condition moni-toring (Chapter 15), test methods (Chapter 16) and lubrication systems (Chapter 17).

1Lard was used for the lubrication of traditional windmills in the Netherlands.

2

Lubrication Mechanisms

2.1 Introduction

Compared to oil lubrication, the physics and chemistry of lubricating grease in a rolling bearing is today not well understood. Howevere, it is certain that grease provides the bearing with a lubricating film that is initially thick enough to (at least partly) separate the rolling elements from the raceways. Unfortunately, generally the thickness and/or the ‘lubricity’ of this film changes over time, leading to a limited period in which the grease is able to lubricate the bearing, generally denoted as ‘grease life’. This time is preferably much longer than the fatigue life of the bearing. It is still not fully understood how this film is generated or how it deteriorates over time and leads to bearing damage and ultimately failure.

Although an exact prediction of the film thickness and ‘lubricity’ cannot be made, it is certain that a number of aspects are very important in the prediction of the performance of the grease and/or in selecting the optimum grease for the specific bearing application. Examples are the rheology (flow properties of the grease), the bleeding characteristics, EHL oil film formation, boundary film formation, starvation, track replenishment, thermal aging (such as oxidation) and mechanical aging [374].

Another important aspect in grease lubrication in rolling bearings is that the ‘grease life’ is not deterministic, that is, there is no absolute value for this and it is given by a statistical distribution. Even if bearings are running under very well controlled conditions, such as in a laboratory situation, there is the usual significant spread of failures. The ‘grease’ life is therefore usually defined as L10, that is, the time at which 10% of a population of bearings is expected to have failed [280], similar to bearing life. If a higher reliability is required, a correction is needed. To prevent grease failures, a bearing may be relubricated. If possible, this should be done well before failure is to be expected. Generally, the relubrication interval is defined as L01, that is the time at which 1% of a population of bearings is expected to have failed [280].

All this, and more, will be treated in this book in separate chapters. To give the reader a summary and an introduction to these chapters, the possible mechanisms in combination with the physical aspects of grease lubrication will first be given in this chapter.

2.2 Definition of Grease

Grease is defined as ‘a solid to semi-fluid product or dispersion of a thickening agent in a liquid lubricant. Other ingredients imparting special properties may also be included’ [450]. The base oil is kept inside the thickener structure by a combination of Van der Waals and capillary forces [70]. Interactions between thickener molecules are dipole-dipole including hydrogen bonding [282] or ionic and Van der Waals forces [197]. The effectiveness of these forces depends on how these fibres contact each other. The thickener fibres vary in length from about 1–100 microns and have a length diameter ratio of 10–100, where this ratio has been correlated with the consistency of the grease for a given concentration of thickener [518]. Sometimes grease is called a thickened oil (rather than a thick oil) [226, 230]. Generally, a lubricating grease shows visco-elastic semi-plastic flow behaviour giving it a consistency such that it does not easily leak out of the bearing.

2.3 Operating Conditions

The lubrication process is different for different speeds and temperatures and even for different bearing types. At high temperatures, oxidation and loss of consistency play a major role. At very low temperatures, the high values for consistency and/or viscosity may lead to too high start-up friction torque. The temperature window at which a grease can operate is given by the grease manufacturer or by the bearing manufacturer and is determined by life- and functional tests.