Principles and Applications of Tribology

388 Pages·1975·16.779 MB·English

by  Desmond F. Moore and D. W. Hopkins (Auth.)

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PRINCIPLES A ND
A P P L I C A T I O NS OF
T R I B O L O GY
DESMOND F. MOORE
B.E., M.S., Ph.D., C.Eng.,
F.I.E.I., M.I.Mech.E., M.A.S.M.E.
Lecturer in Mechanical Engineering,
University College, Dublin, Ireland
Director,
International Mechanical Consultants Ltd.
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Copyright © 1975 Desmond F. Moore
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First edition 1975
Library of Congress Cataloging in Publication Data
Moore, Desmond F. Principles and applications of tribology.
(International series in materials science and technology, v. 14)
Includes bibliographical references.
1. Tribology. I. Title. TJ1075. M59 1975 621.8 9 74-4261
ISBN 0-08-017902-9
ISBN 0-08-019007-3 (Flexi)
Printed in Great Britain by A. Wheaton & Co., Exeter.
PREFACE
TRIBOLOGY is defined as the science and practice of friction, lubrication and wear applied
to engineering surfaces in relative motion. In recent years it has become widely recognized
as a truly diverse and interdisciplinary field of study. Physics, chemistry, metallurgy, mate-
rials science, rheology, lubrication, elasticity, viscoelasticity, elastohydrodynamics, thermo-
dynamics and heat transfer play complex and interactive roles in determining the general
condition of surface friction. It has been estimated that approximately one third to one-half
of the world's energy resources in present use appears ultimately as friction in one form or
another. The importance of friction and wear in our modern world cannot therefore be
over-emphasized.
This book attempts to deal with the whole field of tribology in a single volume. While
some may consider this an impossible task, there are those, including the author, who feel
that today's engineering student deserves a relatively simple and unified treatment of the
subject area. In pursuing this objective, the contents of the book have been divided into two
sections dealing broadly with principles and applications respectively. The text is written
from the point of view of the mechanical engineer, and the section on principles emphasizes
a fundamental understanding of the subject matter before proceeding to a diversity of
practical applications.
Although the complete text is presented in systematic order, the chapters are to a large
extent independent and self-sufficient and may therefore be selected in a diffιrent order
suitable to a particular course of instruction. Chapter 1 deals with the immense scope of
tribology and range of applications in the modern world of technology. Chapter 2 is devoted
entirely to the evaluation and measurement of surface texture. Chapters 3, 4 and 5 present
the fundamental concepts underlying the friction of metals, elastomers and other materials
respectively. The principles of hydrodynamic lubrication are dealt with briefly in Chapter 6,
and the mechanisms of boundary and elastohydrodynamic lubrication receive a more
comprehensive treatment in Chapters 7 and 8 respectively. Chapter 9 is a generalized
treatise on wear and abrasion phenomena in metals and elastomers. Whereas surface inter-
actions are dealt with exclusively throughout Chapters 2 to 8, internal friction in solids,
liquids and gases is identified and explained in Chapter 10. Chapter 11 is an abbreviated
yet thorough treatment of experimental methods used in tribological studies. The remaining
five chapters in the book are devoted to specific applications, including manufacturing
processes, automotive applications, transportation, locomotion, bearing design and miscel-
laneous.
Because of the immense scope of the subject matter, omissions must inevitably occur in a
xi
χίί Preface
limited text on tribology, but it is hoped that the number of these has been kept to a mini-
mum. Above all, it is hoped to impress upon the student the relevance of tribology to
his entire mechanized society, and to stimulate his interest in this important new field
of engineering science. In attempting to achieve these objectives, a basic understanding
of tribological principles is combined with as many practical examples as possible in a text
of reasonable size.
The author expresses his sincere appreciation to Professor W. E. Meyer from the Penn-
sylvania State University, Professor H. C. A. van Eldik Thieme from the Technische Hoge-
school in Delft and Professor Dr.-Ing. A. W. Hussmann from the Technische Universitδt
in Munich who stimulated and promoted in different ways the author's continuing interest
in this subject. Particular acknowledgement is due my colleague and close friend Dr.-Ing.
W Geyer, formerly from the Technische Universitδt in Munich, whose enthusiasm and
support have proved invaluable during the past three years. Mr. D. W. Hopkins from the
University College of Swansea as series editor for the book deserves special thanks for
reviewing the manuscript and offering helpful suggestions. Finally, a word of appreciation
to Miss Gιraldine Warren for her excellent typing skills, and to my wife Miriam for her
patience and understanding during the preparation of the manuscript.
Dublin D. F. MOORE
C H A P T ER 1
INTRODUCTION
1.1 Definition and Scope of Tribology
Tribology is defined as the science and technology of interacting surfaces in relative
motion, having its origin in the Greek word tribos meaning rubbing. It is a study of the
friction, lubrication, and wear of engineering surfaces with a view to understanding surface
interactions in detail and then prescribing improvements in given applications. The scope of
tribology is, in fact, much broader than this definition implies. According to Dr. Salomon,
former editor of the international journal Wear, "Tribology means a state of mind and an
art: the intellectual approach to a flexible cooperation between people of widely differing
background. It is the art of applying operational analysis to problems of great economic
significance, namely : reliability, maintenance and wear of technical equipment, ranging
from spacecraft to household appliances." The work of the tribologist is truly interdisciplin-
ary, embodying physics, chemistry, mechanics, thermodynamics, and materials science, and
encompassing a large, complex, and intertwined area of machine design, reliability, and
performance where relative motion between surfaces is involved.
It is estimated that approximately one-third of the world's energy resources in present use
appear as friction in one form or another. This represents a staggering loss of potential
power for today's mechanized society. The purpose of research in tribology is understand-
ably the minimization and elimination of unnecessary waste at all levels of technology where
the rubbing of surfaces is involved. Indeed, sliding and rolling surfaces represent the key to
much of the effectiveness of our technological society.
We may ask : What do rubbing surfaces cost the economy of our Western world? No one
has yet offered a precise figure, but an estimate may be obtained from British figures.
According to a report by the Committee on Tribology in Great Britain in 1965, approxi-
mately £500 million is lost annually in worn parts within the United Kingdom alone because of
the failure of industry to understand what happens to surfaces which must move across one
another. The problem is of such gigantic proportions that tribological programmes have
been established by industry and government in the United Kingdom, the Soviet Union,
and several western European countries, and by individual corporations in the United
States.
Since World War II, the rapid rate of technological advancement has required great
expansion in research on what to do about surfaces that rub. The obvious approach has
3
4 Principles and Applications of Tribology
focused on learning how to oil, grease, or otherwise lubricate them. Thus conventional
lubrication has been concerned with :
(a) evaluating lubricants in the light of standard specifications;
(b) compounding lubricants to meet new conditions, or
(c) determining how lubricants respond to cold, heat, nuclear radiation, and other en-
vironments and to the materials to be lubricated.
Much of the technology associated with rubbing surfaces still involves conventional
lubrication following the above procedure. Indeed, before the 1950s, most mechanical
devices were lubricated primarily by mineral oils and soap-thickened mineral-oil greases, and
the selection of lubricants for given uses was largely empirical. Now, however, synthetic
materials have found increasing usage in lubrication, and extensive programmes of research
and development make it possible to design lubricants to meet unusual or increasingly severe
requirements. Complex systems such as nuclear submarines, supersonic aircraft or Apollo
spacecraft demand the solution of critical lubrication problems. Thus advancing technology
has brought us to the point where every aspect of the phenomena associated with rubbing
surfaces must be investigated, and the new discipline—tribology—plays an increasing role
in our mechanized environment.
One of the important objectives in tribology is the regulation of the magnitude of frictional
forces according to whether we require a minimum (as in machinery) or a maximum (as in
the case of anti-skid surfaces). It must be emphasized, however, that this objective can be
realized only after a fundamental understanding of the frictional process is obtained for all
conditions of temperature, sliding velocity, lubrication, surface finish, and material pro-
perties. This book is written to explain the fundamentals of tribology in the simplest terms
and to illustrate the basic concepts with a variety of everyday applications.
1.2 Macroscopic and Microscopic Viewpoints
The friction force generated between two bodies having relative tangential motion can be
considered as a macroscopic or microscopic mechanism, depending on the interest and
orientation of the reader. The microscopic or molecular mechanism can also be described
as causative, since the molecular interaction of surface molecules of the sliding pair is
treated in some detail and for a variety of experimental conditions to establish the true cause
of the friction mechanism. This approach has obviously more appeal for the physicist or
physical chemist. On the other hand, the macroscopic mechanism is often referred to as
resultant, being based on a relatively crude simulation of frictional events and usually follow-
ing a simple yet adequate model representation. Here, the simplistic treatment and emphasis
on applications rather than underlying causes are of more interest to the engineer.
It is not the purpose of this text to extol the virtues of either approach at the expense of the
other. Indeed, both mechanisms should be treated in unison, each complementing the other
where appropriate, so that a full comprehension of tribological principles emerges. This text
is written with such an objective in mind. It is interesting to note that the most commonly
Introduction 5
accepted welding-shearing-ploughing theory of metallic friction (1) can be classified as
macroscopic since there is no mention of surface molecular activity. On the other hand,
most of the current theories of elastomeric friction <2) describe adhesion as a thermally
activated molecular-kinetic exchange mechanism, so that the microscopic viewpoint is
adopted.
The essential différence between the macroscopic and microscopic approaches, apart from
the scale of events, is the distinction between a continuum and reality. In the vicinity of a
frictional interface, each surface molecule pervades a volume greater than atomic dimen-
sions and is continually vibrating and twisting with thermal energy. The interchange of these
molecules as one surface captures and in turn loses some of its surface molecules to the
mating surface is, of course, the reality which gives rise to adhesional friction. For a finite
surface area, however, the number of degrees of freedom associated with such molecular
activity is prohibitively large even for the modern computer to handle, and we therefore use
either of two approaches :
(a) we may apply statistical techniques to evaluate mean or averaged values of molecular
motion and forces, or
(b) we may neglect molecular activity entirely and treat both sliding materials as a con-
tinuum having the same general properties as those observed from experiment.
Thus the microscopic approach, while relatively precise in its representation of surface
interactions, is severely limited in terms of applications, whereas the macroscopic approach
has the opposite characteristics. The use of mechanical models (embodying springs and
dashpots in different combinations) is widespread in the simulation of continuum behaviour,
and it must be realized that they are merely a tool for predicting dynamic performance in
viscoelastic bodies. We might generally conclude that a microscopic understanding of the
frictional process is of inestimable value in establishing the validity of a macroscopic model
for practical applications.
1.3 Internal and External Friction
It is desirable at this stage to distinguish between internal and external friction in bodies.
Surface interactions in general can be classified under the general term external friction for
obvious reasons, whereas molecular-kinetic events and bulk energy dissipation occurring
within the body of a material are the cause and result respectively of internal friction. The
physicist will define a surface as a boundary having zero thickness, and of course, such do
not exist in practice. We must therefore modify this definition and permit a surface to
occupy an infinitesimal thickness layer perhaps measurable in Angstrom units. The origin
of "surface" frictional forces is molecular-kinetic interchange as described previously, and
this occurs within such layers which can be measured in both sliding members. This com-
posite activity layer approaches zero dimensions in practical terms.
Internal friction in solids is a direct consequence of the forced motion of bulk molecules
which under equilibrium are closely spaced and exhibit a strong mutual attraction or
2 M: PAT: 2
6 Principles and Applications of Tribology
repulsion for each other (see Chapter 10). Such motion causes internal shearing in the
material of the body and results in internal heat generation. It is common to express
internal friction as damping capacity, loss tangent, or tangent modulus. In the case of liquids
and gases, internal friction is expressed as absolute or kinematic viscosity. Perhaps a
pneumatic tyre offers the clearest example of both internal and external friction. During
normal rolling, the body or carcass of the tyre increases in temperature as a result of con-
tinual flexing and recovery of tread elements entering and leaving the contact patch. This is
the internal friction generation mechanism. At the same time, the localized "squirming"
of tread rubber over road asperities during rolling, braking, driving, and cornering gives
rise to the external friction component which largely controls and steers the vehicle. The
relative magnitude of the internal and external frictional mechanisms in this example
depends to a large extent on the mode of operation of the vehicle and experimental con-
ditions.
1.4 Dry and Lubricated Surfaces
The most important criterion from a design viewpoint in a given application is whether
dry or lubricated conditions are to prevail at the sliding interface. In many applications such
as machinery, it is known that only one condition shall prevail (usually lubrication), al-
though several régimes of lubrication may exist. There are a few cases, however, where it
cannot be known in advance whether the interface is dry or wet, and it is obviously more
difficult to proceed with any design. The commonest example of this phenomenon is again
the pneumatic tyre. Under dry conditions it is desirable to maximize the adhesion 1" compo-
nent of friction by ensuring a maximum contact area between tyre and road—and this is
achieved by having a smooth tread and a smooth road surface. Such a combination, how-
ever, would produce a disastrously low coefficient of friction under wet conditions. In the
latter case, an adequate tread pattern and a suitably textured road surface offer the best
conditions, although this combination gives a lower coefficient of friction in dry weather.
The several lubrication régimes which exist may be classified as hydrodynamic, boundary,
and elastohydrodynamic. The different types of bearing used today (journal, slider, thrust,
and foil bearings) are the best examples of fully hydrodynamic behaviour, where the sliding
surfaces are completely separated by an interfacial lubricant film. Boundary or mixed
lubrication is a combination of hydrodynamic and solid contact between moving surfaces,
and this régime is normally assumed to prevail when hydrodynamic lubrication fails in a
given product design. For example, ajournai bearing is designed to operate at a given load
and speed in the fully hydrodynamic region, but a fall in speed or an increase in load may
cause part solid and part hydrodynamic lubrication conditions to occur between the journal
and bearing surfaces. This boundary lubrication condition is unstable, and normally
recovers to the fully hydrodynamic behaviour or degenerates into complete seizure of the
surfaces. The pressures developed in thin lubricant films may reach proportions capable of
t Friction comprises two principal components, adhesion and deformation, as shown in Chapter 3.
Introduction 7
elastically deforming the boundary surfaces of the lubricant, and conditions at the sliding
interface are then classified as elastohydrodynamic. It is now generally accepted that elasto-
hydrodynamic contact conditions exist in a variety of applications hitherto considered
loosely as belonging to the hydrodynamic or boundary lubrication régimes; for example,
the contact of mating gear teeth, or that of ball-bearings in races, or lip seals on machined
rotating shafts, etc. Solid lubricants exhibit a compromise between dry and lubricated
conditions in the sense that although the contact interface is normally dry, the solid lubricant
material behaves as though initially wetted. This is a consequence of a physico-chemical
interaction occurring at the surface of a solid lubricant lining under particular loading and
sliding conditions, and these produce the equivalent of a lubricating effect.
1.5 The Range of Applications
One useful method of classifying tribology applications is to distinguish between rigid-
rigid, rigid-flexible, and flexible-flexible surface pairings, as indicated in Table 1.1. The
rigid-rigid pairing appears to be the most common type, the usual application being the
friction of metal-on-metal. Table 1.1 illustrates the application of tribological principles to
manufacturing technology, the central column indicating the particular manufacturing
TABLE 1.1. TRIBOLOGICAL PROCESSES IN MANUFACTURING
Tribological
Manufacturing processes Related industry
classification
Metal-on-metal Forging Reaming Wire manufacture
(rigid-rigid) Stamping Guillotining Iron and steel manufacturing
surface pairing" Grinding Drawing Metal processing
Milling Extrusion Tool design
Lapping Forming Mechanical components
Spinning Swaging Machine design
Plastic6-on-metal Injection and blow moulding Tyre manufacture
(flexible-rigid) Cold working Extrusion Plastics industry
surface pairing" Thermo forming Drawing Building and construction
Vacuum forming Coating Electrical insulation
Laminating Shoe manufacture
Flooring materials
Solid lubricants
Fibre-on-fibre Spinning Weaving, etc. Textile industry
(flexible-flexible) Carding Plastics
pairing Hosiery and knitwear
Cable manufacture
β Includes vertical or tangential motion.
b Includes elastomers, solid lubricants, and rubbers.
Note: All operations may or may not have a lubricant.
2*