Tribology Series

Grease Lubrication in Rolling Bearings

Piet M. Lugt

GREASE LUBRICATION IN ROLLING BEARINGS Tribology Series

Bhushan Introduction to Tribology, 2nd Edition March 2013 Bhushan Principles and Applications to Tribology, 2nd March 2013 Edition Lugt Grease Lubrication in Rolling Bearings January 2013 Honary and Richter Biobased Lubricants and Greases: Technology April 2011 and Products Martin and Ohmae Nanolubricants April 2008 Khonsari and Applied Tribology: Bearing Design and April 2008 Booser Lubrication, 2nd Edition Stachowiak (ed) Wear: Materials, Mechanisms and Practice November 2005 Lansdown Lubrication and Lubricant Selection: A November 2003 Practical Guide, 3rd Edition Cartier Handbook of Surface Treatment and Coatings May 2003 Sherrington, Rowe Total Tribology: Towards an Integrated December 2002 and Wood (eds) Approach Kragelsky and Tribology: Lubrication, Friction and Wear April 2001 Stolarski and Tobe Rolling Contacts December 2000 Neale and Gee Guide to Wear Problems and Testing for October 2000 Industry GREASE LUBRICATION IN ROLLING BEARINGS

Piet M. Lugt SKF, The Netherlands

A John Wiley & Sons, Ltd., Publication This edition first published 2013 C 2013 John Wiley & Sons, Ltd Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For detailsof 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. Ð (Tribology in practice series) Includes bibliographical references and index. ISBN 978-1-118-35391-2 (hardback) Ð ISBN 978-1-118-48396-1 (obook) Ð ISBN 978-1-118-48397-8 (epub) 1. Roller bearingsÐLubrication. 2. Lubrication and lubricants. I. Title. TJ1071.L78 2013 621.89Ðdc23 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 Typeset in 10/12pt Times by Aptara Inc., New Delhi, India Dedicated to: Marjo, Michiel and Marijn

Contents

Preface xvii Series Preface xix List of Abbreviations xxi

1 Introduction 1 1.1 Why Lubricate Rolling Bearings? 1 1.2 History of Grease Lubrication 2 1.3 Grease Versus Oil Lubrication 3

2 Lubrication Mechanisms 5 2.1 Introduction 5 2.2 Definition of Grease 6 2.3 Operating Conditions 6 2.4 The Phases in Grease Lubrication 7 2.5 Film Thickness During the Bleeding Phase 8 2.5.1 Ball Bearings 8 2.5.2 Roller Bearings 10 2.6 Feed and Loss Mechanisms During the Bleeding Phase 10 2.7 Film Thickness and Starvation (Side Flow) 11 2.8 Track Replenishment 12 2.9 Grease Flow 13 2.9.1 Non-Newtonian Rheology 14 2.10 Wall-Slip 15 2.11 Oxidation 16 2.12 EP Additives 16 2.13 Dynamic Behaviour 17 2.14 Grease Life 17 2.14.1 Temperature 18 2.14.2 Speed 19 2.14.3 Load 19 2.14.4 Bearing Type 20 2.14.5 Grease Type 20 2.14.6 Environment 21 viii Contents

3 Grease Composition and Properties 23 3.1 Base Oil 24 3.1.1 Natural Triglyceride and Wax Ester Base Oils 26 3.1.2 Mineral Oils 26 3.1.3 Synthetic Oils 30 3.2 Base Oil Viscosity and Density 41 3.2.1 ViscosityÐTemperature 44 3.2.2 ViscosityÐPressureÐTemperature 45 3.2.3 Density, Compressibility 47 3.3 Thickener 49 3.3.1 Soap Greases, Simple Greases 50 3.3.2 Complex Greases 51 3.3.3 Non-soap Thickeners 52 3.3.4 Mixed Thickeners 52 3.3.5 Mechanical Structure 53 3.3.6 Oil Retention 56 3.3.7 Properties of Different Types of Grease Thickeners 56 3.4 Additives 61 3.4.1 Corrosion Inhibitors 62 3.4.2 Antioxidants 62 3.4.3 EP/AW Additives 63 3.5 Solid Fillers/Dry Lubricants 66 3.5.1 MoS2 and Graphite 66 3.5.2 Nanoparticles 66 3.5.3 ZnO 66 3.5.4 Teflon (polytetrafluoroethylene) 66 3.5.5 Polyethylene 66 3.6 Compatibility 67 3.7 Polymer Grease 67

4 Grease Life in Rolling Bearings 71 4.1 Introduction 71 4.2 Relubrication Intervals and Grease Life 71 4.3 The Traffic Light Concept 72 4.3.1 Low Temperatures 74 4.3.2 Extreme Low Temperature 75 4.3.3 Extreme High Temperature 75 4.4 Grease Life as a Function of Temperature in the Green Zone 75 4.5 SKF Relubrication and Grease Life 76 4.6 Comparison Grease Life/Relubrication Models 78 4.7 Very Low and High Speeds 82 4.7.1 Speed Ratings and Speed Factors 82 4.7.2 High Speed 82 4.7.3 Very Low Speeds 85 4.8 Large Rolling Bearings 85 4.9 Effect of Load 86 Contents ix

4.9.1 Varying Load 86 4.9.2 Direction of Load 89 4.9.3 Very Heavy Loads 89 4.10 Effect of Outer-Ring Rotation 90 4.11 Cage Material 90 4.12 Bearing Type 91 4.12.1 Roller Bearings 91 4.12.2 Hybrid Bearings 91 4.13 Temperature and Bearing Material 92 4.14 Grease Fill 94 4.15 Vertical Shaft 95 4.16 Vibrations and Shock Loads 96 4.17 Grease Shelf Life/Storage Life 97

5 Lubricating Grease Rheology 99 5.1 Visco-Elastic Behaviour 99 5.2 Viscometers 102 5.2.1 Parallel Plate and Cone-Plate Viscometers 103 5.2.2 Errors in Rheometry Measurements 103 5.2.3 Errors in Thin Film Parallel Plate Rheometry Measurements 105 5.3 Oscillatory Shear 108 5.3.1 Theory 108 5.3.2 Application to Grease 110 5.3.3 Effect of Thickener Concentration 112 5.4 Shear Thinning and Yield 112 5.4.1 Grease 112 5.4.2 Lubricating Oil 116 5.5 Yield Stress 118 5.5.1 The Concept 118 5.5.2 Influence of Temperature 119 5.5.3 Consistency 120 5.6 Wall-Slip Effects 122 5.7 Translation Between Oscillatory Shear and Linear Shear Measurements 125 5.7.1 Viscosity 125 5.7.2 Yield Stress 126 5.8 Normal stresses 126 5.9 Time Dependent Viscosity and Thixotropy 128 5.10 Tackiness 133 5.10.1 Introduction 133 5.10.2 Tackifiers 134 5.10.3 Pull-Off Test 135 5.10.4 Other Tests 136

6 Grease and Base Oil Flow 137 6.1 Grease Flow in Pipes 137 6.1.1 Approximation Using the Newtonian Pipe Flow Equations 137 x Contents

6.1.2 Non-Newtonian Fluid 138 6.1.3 Bingham Rheology 139 6.1.4 Sisko Rheology 140 6.1.5 Power Law Rheology 140 6.1.6 HerschelÐBulkley Rheology 140 6.1.7 The Darcy Friction Factor 142 6.1.8 Transient Effects 144 6.1.9 Air in Grease 144 6.1.10 Entrance Length 145 6.1.11 Solid Particles in Grease Flow 145 6.1.12 Wall-Slip/Slip Layer 145 6.1.13 Impact of Roughness 147 6.1.14 Grease Aging in Pipes 149 6.2 Grease Flow in Rolling Bearings 149 6.2.1 Churning 149 6.2.2 Flow Through Bearing Seals 152 6.2.3 Relubrication 152 6.2.4 Grease Flow Around Discontinuities 153 6.2.5 Creep Flow 153 6.2.6 Flow Induced by Vibrations 155

7 Grease Bleeding 157 7.1 Introduction 157 7.2 Ball Versus Roller Bearings 158 7.3 Grease Bleeding Measurement Techniques 158 7.4 Bleeding from the Covers and Under the Cage 159 7.5 A Grease Bleeding Model for Pressurized Grease by Centrifugal Forces 161 7.5.1 Oil Bleeding Model 162 7.5.2 Quality of the Model 166

8 Grease Aging 171 8.1 Mechanical Aging 172 8.1.1 Softening of Grease in Rolling Bearings 172 8.1.2 Hardening of Grease in Rolling Bearings 179 8.2 Grease Oxidation 179 8.3 The Chemistry of Base Oil Film Oxidation 181 8.3.1 Chemical Reactions 181 8.4 Oxidation of the Thickener 183 8.5 A Simple Model for Base Oil Degradation 185 8.6 Polymerization 186 8.7 Evaporation 186 8.8 Simple Models for the Life of Base Oil 187 8.8.1 Booser’s Oil Life Model 187 8.8.2 Two Phase Model 188 Contents xi

9 Film Thickness Theory for Single Contacts 191 9.1 Elasto-Hydrodynamic Lubrication 192 9.1.1 History 192 9.1.2 The NavierÐStokes Equations 193 9.1.3 The Reynolds and Thin Film Equation 194 9.1.4 Cavitation 198 9.2 Contact Geometry and Deformation 198 9.2.1 Rigid Bodies 199 9.2.2 Elastic Deformation 200 9.3 EHL Film Thickness, Oil 202 9.3.1 Example: 6204 Bearing 205 9.4 EHD Film Thickness, Grease 205 9.4.1 Measurements 205 9.4.2 Film Thickness Models for Grease Rheology 207 9.5 Starvation 212 9.5.1 Starved Oil Lubricated Contacts 212 9.5.2 Starved Lubrication EHL Models 213 9.5.3 Base Oil Replenishment 219 9.5.4 Starved Grease Lubricated Contacts 222 9.6 Spin 225

10 Film Thickness in Grease Lubricated Rolling Bearings 227 10.1 Thin Layer Flow on Bearing Surfaces 228 10.1.1 Contact Replenishment in Bearings 228 10.1.2 Thin Layer Flow Induced by Centrifugal Forces 231 10.1.3 Combining the Thin Layer Flow on the All Bearing Components 233 10.2 Starved EHL for Rolling Bearings 234 10.2.1 Central Film Thickness 234 10.2.2 Combining Lightly Starved and Severely Starved 237 10.3 Cage Clearance and Film Thickness 239 10.4 Full Bearing Film Thickness 241

11 Grease Dynamics 245 11.1 Introduction 245 11.2 Grease Reservoir Formation 245 11.3 Temperature Behaviour 246 11.4 Temperature and Film Breakdown 249 11.5 Chaotic Behaviour 249 11.5.1 Reconstruction of the Temperature Dynamics Using Time Delayed Embedding 249 11.5.2 Estimation of the Time Delay τ 251 11.5.3 Calculation of the Dimensions d and m 251 11.5.4 Calculation of the Lyapunov Exponents 252 11.6 Quantitative Analysis of Grease Tests 253 11.7 Discussion 254 xii Contents

12 Reliability 257 12.1 Failure Distribution 258 12.2 Mean Life and Time Between Failures 261 12.3 Percentile Life 264 12.4 Point and Interval Estimates 265 12.4.1 Graphical Methods for Point Estimates 265 12.4.2 Suspended Tests, Censored Data 267 12.4.3 Weibull Parameters η and β: Maximum Likelihood Method 269 12.4.4 Bias of Point Estimates 272 12.4.5 Confidence Intervals for β 273 12.4.6 Confidence Intervals and Unbiased Point Estimates for Life Percentiles 273 12.4.7 Estimate Precision 274 12.5 Sudden Death Testing 275 12.5.1 Maximum Likelihood Method for a 3-Parameter Weibull Distribution 280 12.6 System Life Prediction 281

13 Grease Lubrication and Bearing Life 283 13.1 Bearing Failure Modes 283 13.2 Rated Fatigue Life of Grease Lubricated Rolling Bearings 285 13.2.1 Introduction 285 13.2.2 The Lubrication Factor 287 13.2.3 The Contamination Factor ηc 288 13.2.4 The Stress-Life Modification Factor aslf 289 13.3 Background of the Fatigue Life Ratings of Grease Lubricated Bearings 289 13.3.1 Fatigue Life and Endurance Testing in the Period 1940Ð1960 289 13.3.2 Fatigue Life and Endurance Testing After 1960 291 13.3.3 The Reliability of Grease Lubricated Bearings 292 13.4 Lubricant Chemistry and Bearing Life 296 13.4.1 Anti-Wear Additives 297 13.4.2 EP Additives 297 13.4.3 The Influence of Lubricant Additives on Bearing Life 297 13.5 Water in Grease 304 13.5.1 Introduction 304 13.5.2 Film Thickness 304 13.5.3 Water in Oil and Bearing Life 304 13.5.4 Concentration of Water 305 13.5.5 Water in Grease 306 13.6 Surface Finish Aspects Related to Grease Lubrication 306

14 Grease Lubrication Mechanisms in Bearing Seals 309 14.1 Introduction 309 14.2 Lubrication Mechanisms for Elastomer Contact Seals 309 14.3 Sealing Action of Grease 312 14.3.1 Migration of Contaminant Particles in the Pocket 313 Contents xiii

14.3.2 Migration of Contaminant Particles in the Vicinity of the Sealing Contact 316 14.4 Softening and Leakage 319 14.5 Compatibility 320 14.6 A Film Thickness Model for Bearing Seals 320 14.6.1 Oil Feed 321 14.6.2 Oil Loss 321 14.7 Some Examples Showing the Importance of Sealing and Grease 324

15 Condition Monitoring and Maintenance 327 15.1 Condition Monitoring 327 15.2 Vibrations and Acoustic Emission 328 15.3 Lubcheck 331 15.4 Consistency Measurement 331 15.5 Oil Bleeding Properties 332 15.6 Oil Content 332 15.7 Particle Contamination 332 15.8 Spectroscopy 333 15.8.1 Infrared (IR) Spectroscopy 333 15.9 Linear Voltammetry 334 15.10 Total Acid Number 335 15.11 DSC Ð Differential Scanning Calorimetry 335 15.12 Oxidation Bomb 336 15.13 Water 336

16 Grease Qualification Testing 339 16.1 Introduction 339 16.2 Standard Test Methods 339 16.2.1 Penetration/Grease Consistency 339 16.2.2 Worked Penetration 341 16.2.3 Shell Roll Stability 341 16.2.4 Dropping Point 343 16.2.5 Emcor 344 16.2.6 Oil Separation 346 16.2.7 Water Resistance 347 16.2.8 Low Temperature Torque 348 16.2.9 Flow Pressure 349 16.2.10 4-Ball Weld Load 349 16.2.11 4-Ball Wear Scar 350 16.2.12 High Speed Grease Life Testing, RHF1 351 16.2.13 R0F 353 16.2.14 R0F+ 354 16.2.15 R2F, Using the Special Spherical Roller Bearing 356 16.2.16 R2F, Using Standard Bearings 357 16.2.17 V2F 358 16.2.18 FE8 359