Grease Lubrication In Rolling Bearings

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 NC 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.




CONTENTS

Preface xvii

List of Abbreviations xix

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

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 Viscosity:Temperature 44
3.2.2 Viscosity:PressureNCTemperature 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 Inorganic 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 Anti-Oxidants 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
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
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 HerschelNCBulkley 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 184
8.6 Polymerization 186
8.7 Evaporation 186
8.8 Simple Models for the Life of Base Oil 186
8.8.1 Booser's Oil Life Model 186
8.8.2 Two Phase Model 187

9 Film Thickness Theory for Single Contacts 191

9.1 Elasto-Hydrodynamic Lubrication 192
9.1.1 History 192
9.1.2 The Navier:Stokes 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 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

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 ƒÅ and ƒÀ: Maximum Likelihood Method 269
12.4.4 Bias of Point Estimates 272
12.4.5 Confidence Intervals for ƒÀ 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 āc 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 1940-1960 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 Radial Lip Seals 309
14.3 Sealing Action of Grease 312
14.3.1 Migration of Contaminant Particles in the Pocket 313
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 Importance of Sealing Grease Inside the Bearing 324

15 Condition Monitoring and Maintenance 327

15.1 Condition Monitoring 327
15.2 Acoustic Emission 328
15.3 Lubcheck 330
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 DCS NC 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 345
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
16.2.19 FE9 360
16.2.20 A-Frame Cycle Test 360
16.2.21 Cold Chamber Test 361
16.2.22 BeQuiet+ 362
16.2.23 Fafnir Friction Oxidation Test 364
16.2.24 Copper Corrosion Test 365
16.2.25 EP Reaction Test 366
16.2.26 Compatibility with Preservatives/Process Fluids 367
16.2.27 Compatibility Tests for Polymeric Materials 367
16.2.28 Remaining Oil Percentage, or Thickener/Oil Ratio 368
16.2.29 ROF/ROF+ 369
16.2.30 R2F and FE8 Comparison 370
16.2.31 ASTM D 3527 Life Performance of Wheel Bearing Grease 372
16.2.32 ASTM D 5483 Oxidation Induction Time of Lubricating Greases by Pressure Differential Scanning Calometry 372
16.2.33 Linear Sweep Voltammmetry 374
16.3 Some Qualification Criteria for Grease Selection 374
16.3.1 Low Temperature Limit 374
16.3.2 Low Temperature Performance Limit 374
16.3.3 High Temperature Performance Limit 374
16.3.4 High Temperature Limit 375
16.3.5 Minimum Speed 375
16.3.6 Maximum Speed 375
16.4 Pumpability 375

17 Lubrication Systems 377

17.1 Single Point Lubrication Methods 379
17.2 Centralized Grease Lubrication Systems 380
17.3 Pumps 382
17.3.1 Shovel Pump for Pumping High Viscous Grease 382
17.3.2 Method to Create a Positive Head Pressure by Using a Follower Plate 384
17.4 Valves 384
17.5 Distributors 386
17.6 Single-Line Centralized Lubrication Systems 386
17.6.1 Single-Line System and Venting 387
17.6.2 Prelubrication Distributors 387
17.6.3 Relubrication Distributors 390
17.6.4 Strengths and Weaknesses of Single-Line Systems 392
17.7 Dual-Line Lubrication Systems 393
17.7.1 Description 393
17.7.2 Strengths and Weaknesses of the Dual-Line System 394
17.8 Progressive Lubrication Systems 394
17.8.1 Description 394
17.8.2 Strengths and Weaknesses of Progressive Systems 397
17.9 Multi-Line Lubrication System 397
17.10 Cyclic Grease Flow 397
17.11 Requirements of the Grease 398
17.11.1 Grease Pumpability 398
17.11.2 Venting Pressure for Single-Line Systems 399
17.11.3 Oil Separation/Bleeding 400
17.11.4 Cleanliness 400
17.11.5 Compressibility 401
17.11.6 Homogeneity 401
17.11.7 Additives 401
17.11.8 Compatibility 402
17.11.9 Delivery Resistance or Pressure Losses 402
17.12 Grease Pumpability Tests 402
17.12.1 Flow Ability 403
17.12.2 Delivery Test 408

A Characteristics of Paraffinic Hydrocarbons 413
References 415
Index