High Pressure Rheology for Quantitative Elastohydrodynamics

Scott S. Bair

• Publisher: Elsevier
• Year: 2019
• eBook ISBN: 9780128163108
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

High-Pressure Rheology for Quantitative Elastohydrodynamics, Second Edition, contains updated sections on scaling laws and thermal effects, including new sections on the importance of the pressure dependence of viscosity, the role of the localization limit of stress, and new material on the shear dependence of viscosity and temperature dependence viscosity. Since publication of the original edition, the experimental methods, the resulting property data and new correlations have resulted in a revolution in understanding of the mechanisms of film formation and the mechanical dissipation.- Describes lubricant rheology and dependence of lubricant viscosity and density on pressure and temperature- Provides a detailed description of the relationship of lubricant properties on pressure, temperature and shear stress- Includes data for many more liquids, including the recently characterized reference liquids

Chapter One

An Introduction to Elastohydrodynamic Lubrication

Abstract

Elastohydrodynamics has been characterized as a mature science, having begun about 70 years ago with the Ertel–Grubin solution for the film thickness, which had previously been thought to be insignificant compared with the surface roughness of the lubricated surfaces. Classical elastohydrodynamic lubrication, however, developed in manner that is odd for a quantitative science. The pressure dependence of viscosity, a property essential to the existence of the field, was not accurately described in the many books on this subject until recently. In spite of this deficiency, it will be helpful to introduce the subject with a cursory exposition of what has come before.

Keywords

Conformal contacts; concentrated contacts; piezoviscous liquids; line contacts; point contacts; isoviscous rigid; piezoviscous rigid; Isoviscous elastic; piezoviscous elastic; inlet zone; Hertz zone; starved inlet; assumptions of classical EHL; primary properties

1.1 Lubrication

It would be difficult to imagine any type of machinery without lubrication. There have been numerous analyses showcasing the energy savings by the application of the best lubrication practices. However, if we had to live with the high friction coefficients and wear rates that go with bare metal contacts, heat engines, gearboxes, and other common machines would be scarcely usable. A recent review [1] of the impact of friction and wear on global energy consumption found that 23% can be attributed to tribological contacts and that these energy losses could be reduced by 40%. Realizing that friction and wear are dependent upon the rheology of the lubricant brings recognition of the importance of rheology to this subject.

Elastohydrodynamics is a part of the broader field of lubrication. Lubricants are employed to generate a film of easily sheared material interposed between the surfaces of machine components to reduce friction and prevent wear. Early lubricants were often animal or vegetable fats and oils. Today the majority of liquid lubricants are based on highly refined mineral oils and synthetic oils such as polyalphaolefins (PAOs), polyolesters, polyglycols, and others are becoming more important with time. The synthetics tend to have a reduced temperature dependence of viscosity as compared with mineral oils. Mineral oils (and synthetics as well) are often blended with high molecular weight polymer to reduce the temperature dependence. Greases are liquid lubricants that have been given a solid-like yield strength by the addition of a solid thickener, which allows the grease to remain without flow in a desired location until needed, whereupon the liquid is delivered to the contact to function in the usual way.

Regimes of lubrication are identified by the mechanism of film formation. When a molecularly thin film formed from the oil itself or an additive is physically or chemically attached to the interacting surfaces, the regime is known as boundary lubrication. When the surfaces are separated by a film of pressurized liquid with sufficient thickness to completely prevent the contact between roughness features, full film lubrication is said to exist. If the liquid pressure is generated externally and applied through passages penetrating the surfaces into the space between the surfaces, this full film regime is called hydrostatic. If the generation of pressure is self-acting, that is, if the motion of the surfaces in combination with the viscous properties of the liquid and the shape of the conjunction acts to pressurize the film, this full film is termed hydrodynamic. Many machine components operate with the hydrodynamic mechanism active, but with an insufficiently thick film to prevent some interaction of the surface roughness. In this case the regime is said to be mixed, that is, mixed hydrodynamic and boundary lubri...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• Preface to the First Edition
• Chapter One. An Introduction to Elastohydrodynamic Lubrication
• Chapter Two. An Introduction to the Rheology of Polymeric Liquids
• Chapter Three. General High-Pressure Experimental Technique
• Chapter Four. Compressibility and the Equation of State
• Chapter Five. The Pressure and Temperature Dependence of the Low-Shear Viscosity
• Chapter Six. Correlations for the Temperature and Pressure and Composition Dependence of Low-Shear Viscosity
• Chapter Seven. Measurement Techniques for the Shear Dependence of Viscosity at Elevated Pressure
• Chapter Eight. The Shear Dependence of Viscosity at Elevated Pressure
• Chapter Nine. The Glass Transition and Related Transitions in Liquids Under Pressure
• Chapter Ten. Shear Localization, Slip, and the Limiting Stress
• Chapter Eleven. The Reynolds Equation
• Chapter Twelve. Application to Elastohydrodynamic Film Thickness
• Chapter Thirteen. Application to Elastohydrodynamic Friction
• Chapter Fourteen. Conclusion
• Appendix. Nomenclature
• Index