Fundamentals of Tribology

Ramsey Gohar, Homer Rahnejat

• Publisher: WSPC (EUROPE)
• Year: 2018
• ISBN:
  9781786345196
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

Fundamentals of Tribology deals with the fundamentals of lubrication, friction and wear, as well as mechanics of contacting surfaces and their topography. It begins by introducing the reader to the importance of tribology in everyday life and offers a brief history of the subject. It then describes the nature of rough surfaces and the mechanics of contacting elastic solids and their deformation under load and friction in their relative motion. The book goes on to discuss the importance of lubricant rheology with respect to viscosity and density. Then, the principles of hydrodynamic lubrication are covered with derivations of the governing Reynolds and energy equations. Applications of hydrodynamic lubrication in various forms of bearings — journal bearings, thrust bearings and externally pressurised bearings — are outlined. The important and still evolving subject of elastohydrodynamic lubrication is treated in some detail, both at its fundamentals and its applications in thin shell or overlay bearings, cam-followers and internal combustion engine pistons.

The fundamentals of biotribology are also covered, particularly its applications to endo-articular mammalian joints such as hip and knee joints and their arthroplasty. In addition, there is a treatment of the rapidly emerging knowledge of tribological phenomena in lightly loaded vanishing conjunctions (nanotribology), in natural systems and very small devices, such as MEMS and high density data storage media. There is also a new chapter on the rapidly emerging subject of surface texturing to promote retention of microreservoirs of lubricant, acting as microbearings and improving lubrication of otherwise poorly lubricated conjunctions.

This book targets the undergraduate and postgraduate body as well as engineering professionals in industry, where often a quick solution or understanding of certain tribological fundamentals is sought. The book can also form an initial basis for those interested in research into certain aspects of tribology.

Contents:

• Introduction to Tribology
• The Nature of Rough Surfaces
• Elastic Solids in Normal Contact
• Dry Friction and Wear
• Lubricant Properties
• The Reynolds and Energy Equations
• Thrust Bearings
• Journal Bearings
• Externally Pressurised (EP) Bearings
• Elastohydrodynamic Lubrication (EHL)
• Fatigue Life of Rolling Element Bearings
• Transient Elastohydrodynamic Lubrication
• Nanotribology
• Biotribology
• Surface Texturing for Enhanced Tribological Performance

Readership: Students in an undergraduate course in Mechanical, Chemical or Automotive Engineering or Material Science in later years of their courses. Postgraduate students on taught course within the same disciplines. Early career researchers (working towards a PhD degree or as Research Associates) as well as researchers in an R&D environment in industry.
Tribology;Lubrication;Wear;Friction;Biotribology;Nanotribology;Hydrodynamics;Elastohydrodynamics;Bearings0 Key Features:

• The book provides a broad scope in the field of tribology across the physical scale from nano-scale to micro-scale and onto system level investigations. It covers a broad range of traditional applications, such as bearings to modern research-led issues including MEMS and storage devices. It also includes biotribology and the new growing field of laser surface texturing. The breadth of applications as well as the multi-scale approach is rather unique within the books of tribology currently on the market
• The approach is also multi-disciplinary in terms of underlying physics, including the interactions between system dynamics, tribology and thermal analysis as well as aspects of physical chemistry and biomechanics

Chapter 1

Introduction to Tribology

1.1Introduction

The generic word tribology, in the title of this book, is derived from ancient Greek. Its root is found in a verb of that period trivein which itself was based on the word pedo, meaning ‘the character formation of privileged children by their home tutors’ (pedotriveis). Trivein meant rubbing in the context of shaping the personalities of these children (cf rub it in to someone).

More recently, a 1964 government-appointed committee was set up to find ways of reducing the untoward effects of friction on the British industrial economy. The committee invented the word tribology to emphasize the scientific nature of studying the interactions of solid contacting surfaces in relative motion, these being covered by the three disciplines of Friction, Lubrication and Wear, thus making the prefix Tri appropriate. Friction can be considered as a part of physics or mechanical engineering, Lubrication is covered by mechanical engineering and chemistry, whilst Wear is a part of material science.

Tribology is, therefore, a multidisciplinary subject that draws upon a large section of the syllabus of a typical undergraduate science or engineering course.

In our everyday encounters with the solid environment, we often encounter the disciplines embraced by tribology. For example, friction gives us the ability to walk and to grasp objects, whilst lubrication is essential for the proper functioning of the joints in our skeletons. Even though the skeleton’s joints have the property of healing through regrowth, the untoward effects of wear can sometimes prevent them from functioning properly if their lubrication mechanism breaks down.

By means of effective lubrication, mechanical design seeks to minimize friction in devices such as engines, skis and computer hard disc drives. On the other hand, high friction is essential for the traction and braking of rubber car tyres against the road surface.

For the ancient Egyptians, control of friction was important when they were hauling slabs of stone up inclined slopes during the construction of the pyramids. It is equally important today when seeking ways to improve efficiency of automobile internal combustion engines. These involve mechanisms that have sliding contacts, such as the reciprocating motion of the engine pistons in their cylinders (see Chapter 12). Here, the solution resides in reduction of friction through lubrication, an example of which is given below.

One of the earliest examples of the use of lubricants was in the transportation of immense monuments in ancient Egypt. Circumstances often compelled the Egyptians to drag their loads, as depicted in Figure 1.1. The bas-relief shows workers dragging a monumental statue, resting on a wooden sled, along a wooden track. A man on the front of the sled is pouring liquid from a flask onto the track ahead of it. This could well have been a vegetable oil or even plain water, either of which would reduce the sliding friction and wear of the contacting surfaces.

Historically, the invention of the wheel was one of the greatest achievements of man because of its impact on transport. The low rolling friction, associated with the wheel, enabled loads to be transported both rapidly and cheaply compared with earlier times, when humans or animals had to carry or drag them. However, for its proper functioning, the wheel always requires a suitable surface upon which it can roll. This was not always the case in ancient times. For example, because their country’s mountainside tracks were poor, the early Peruvian civilization did not appear to utilize the wheel for transport.

Figure 1.1. Transporting an Egyptian statue. A wall drawing in the grotto of El Bersheh.

Figure 1.2. Bobbin-type separators designed by Leonardo (circa 1495).

An example of a device using rolling friction was conceived by Leonardo da Vinci [1] around 1495. His ingenious design was of a low-friction thrust ball bearing using bobbin-type separators shown in Figure 1.2. Leonardo’s full description of the device is given in Question 4.2 where we ask reader, with the aid of his own sketches, to elaborate further on the probable design of the bearing and its separators.

As we mentioned earlier, although sometimes high friction is needed in order to fulfil certain functions in our everyday lives, the need for reducing it is extremely important, not the least, because of the energy it consumes. Consequently, much scientific effort and ingenuity continue to be devoted to achieving this latter objective. Therefore, ways of reducing friction and wear between solid surfaces will be the main aim of our book. Apart from naturally occurring skeletal surfaces (when we discuss biomechanics), throughout the book, the solid surfaces will generally be of prepared steel, it being the material of choice in engineering and science.

Throughout the history of tribology, the approach for finding ways of reducing friction has mainly been by trial and error or by experiments. These have produced some fundamental scientific laws, which we still use today as the bases. However, with the complexity of modern engineering and the rivalry that pervades the marketplace, companies must continually develop existing products while conceiving new ones in a limited time period.

Often scientific research in tribology is based on complex computer programs that are not always freely available to industry. A solution is, therefore, to devise adequate and simple design and diagnostic and remedial methods to produce results as quickly as possible. To do this, the book sometimes utilizes existing research results from the literature to obtain a solution.

1.2Regimes of lubrication

As lubrication is an important component of tribology, here is a convenient point to introduce the reader to its structure in a very simple and approximate way. We will assume two rubbing (or rolling) solid surfaces, such as steel on steel. They are also rough, because a perfectly smooth surface cannot exist. The act of lubrication is to have between the surfaces a layer of a different material that reduces friction between them, either by being softer than the surfaces or by being a coherent liquid lubricant or gas entrained between the two surfaces by their relative movement. Now most engineering structures are exposed to the atmosphere, so the primary friction-reducing layer is a low-shear-strength oxide film formed naturally by the interaction of the steel surfaces with the atmosphere. (In a clean environment or a vacuum, the surfaces have no oxide film, resulting in high friction).

Stribeck [2] originally devised a convenient way of relating rough-ness and film thickness by a parameter

, the roughness height being some representative value of the undistorted roughness features and the film thickness being measured from it (Chapter 2). He devised the Stribeck curve, shown in Figure 1.3, where the coefficient of friction (

) is plotted against λ_s.

The frictional contact of clean surfaces is represented by point A at the beginning of the boundary lubrication regime in Figure 1.3. The subsequent formation of an oxide film; the controlled deposition of a softer material; and the insertion of special boundary lubricants such as tallow, castor oil or additives to a mineral oil — all these will reduce friction. Regime AB represents this behaviour.

Figure 1.3. Stribeck curve (not to scale).

At point B, where the surface roughness equals the film thickness, we enter the mixed lubrication or partial lubrication regime BC. In this case, the load is partly supported by a coherent flowing oil film and partly by a regime-AB-type contact or alternatively, by thin microfilms formed between the distorted surface features themselves. Regime BC is one of the most common regimes in practice. In some ways, it represents a design failure because a coherent lubricant film is not present everywhere. As in regime AB, in regime BC, the surfaces wear, perhaps producing debris. Analysis of the forces at such contacts is discussed in Chapter 4.

The progressive reduction in the value of μ carries further into the regime CD, where a coherent elastohydrodynamic (EHL) lubricant film is produced. Here, the values of λ are large enough so that the surface features do not influence much the lubricant film thickness. The high pressures encountered in this zone cause the surfaces to distort elastically and the lubricant viscosity to increase. Rolling element bearings and gears fall into this category. The word elastohydrodynamic clearly defines the lubrication mechanism. It is characterized by a film thickness of 1 μm or less, requiring very smooth surfaces. Modern manufacturing methods and design improvements have increased this zone at the expense of zone BC. Remarkably, Dowson [3] points out that it has also been Nature’s choice for the EHL regime of lubrication to occur in the synovial joints of creatures ranging from elephants to shrews!

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Table of contents

• Cover
• HalfTitle
• Notable Titles of Interest
• Title
• Copyright
• Preface
• Acknowledgements
• Notation
• Contents
• Chapter 1. Introduction to Tribology
• Chapter 2. The Nature of Rough Surfaces
• Chapter 3. Elastic Solids in Normal Contact
• Chapter 4. Dry Friction and Wear
• Chapter 5. Lubricant Properties
• Chapter 6. The Reynolds and Energy Equations
• Chapter 7. Thrust Bearings
• Chapter 8. Journal Bearings
• Chapter 9. Externally Pressurised (EP) Bearings
• Chapter 10. Elastohydrodynamic Lubrication (EHL)
• Chapter 11. Fatigue Life of Rolling Element Bearings
• Chapter 12. Transient Elastohydrodynamic Lubrication
• Chapter 13. Nanotribology
• Chapter 14. Biotribology
• Chapter 15. Surface Texturing for Enhanced Tribological Performance
• Questions
• Book of Solutions
• Index