Engineering Tribology 2011 Part 1 potx

Stachowiak Department of Mechanical and Materials Engineering, University of Western Australia, Australia Andrew W.. Batchelor Department of Mechanical and Materials Engineering, Univers

E N G I N E E R I N G T R I B O L O G Y

E N G I N E E R I N G T R I B O L O G Y

Gwidon W Stachowiak

Department of Mechanical and Materials Engineering,

University of Western Australia, Australia

Andrew W Batchelor

Department of Mechanical and Materials Engineering,

University of Western Australia, Australia

To the most important persons in our lives Grazyna Stachowiak

VIII ENGINEERING TRIBOLOGY

CONTENTS IX

X ENGINEERING TRIBOLOGY

XII ENGINEERING TRIBOLOGY

Bearing inlet temperature and thermal interaction between pads of a

CONTENTS XIII

Computer program for the analysis of an infinitely long pad bearing in

Example of the analysis of an infinitely long pad bearing in the case of

Computer program for the analysis of an elastically deforming

Computer program for the analysis of vibrational stability in a partial arc

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CONTENTS XV

Contact area, pressure, maximum deflection and position of the

Approximate solution of Reynolds equation with simultaneous elastic

Effects of the non-dimensional parameters on EHL contact pressures and

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8 BOUNDARY AND EXTREME PRESSURE LUBRICATION 357

Influence of oxygen and water on the lubrication mechanism by

CONTENTS XVII

XVIII ENGINEERING TRIBOLOGY

Model of contact between solids based on statistical parameters of rough

Model of contact between solids based on the fractal geometry of rough

CONTENTS XIX

CONTENTS XXI

Influence of counterface roughness, hardness and material type on

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Effect of lubricant, corrosive agents and microstructure on wear and

P R E F A C E

Several years ago, the idea arose to write a general book on tribology Students oftenrequested a suitable book for the study of tribology and there were problems inrecommending any one text-book Existing text-books were either too specialized or tooliteral Many books provided exhaustive reviews of friction and wear data while othersprovided detailed description of the lubrication and wear problems occurring in machinery

A book which explains the concepts of tribology in terms useful to engineering students andengineers was, however, lacking The situation was rather like attempting to describe thefunction of a machine to a non-specialist using manufacturers drawings In many cases thebasic models of friction and wear were not explained adequately As a result moresophisticated concepts could not be understood The interdisciplinary nature of tribologywith knowledge drawn from different disciplines such as mechanical engineering, materialsscience, chemistry and physics leads to a general tendency for the chemist to describe indetail, for example, lubricant additives, the mechanical engineer to discuss, for example, padbearings and so on, with no overall guide to the subject In this book, the interaction betweenthese different fields of knowledge to achieve the final result, the control of friction andwear, is emphasized The interdisciplinary view of tribology was largely developed byProfessor Alastair Cameron about three decades ago and has proved to be the most successfulway of analysing friction and wear problems

In many cases tribology is viewed as an inaccessible subject which does not produce usefulanswers In this book we try to redress this problem Rutherford's maxim, that ‘any goodscientific theory is explainable to the average barmaid’ is applied in this book with variousconcepts explained in the simplest possible terms with supporting illustrations

In this second edition of Engineering Tribology we aim to update the contents of the firstedition while maintaining its style In this edition a number of extra topics have beenincluded to make the book more comprehensive as suggested by the reviewers of the firstedition The listings of literature citations have been extended to include recent findingsfrom tribology research Extra diagrams have also been included where it was found that thereadability of the original text could be improved Computer programs used in the numericalanalysis have been upgraded from BASIC to MatLab in order to make them more accessible

to current users Despite all these changes, the purpose of writing ‘Engineering Tribology’remains the same, i.e to provide a reader-friendly and comprehensive introduction to thesubject of tribology and its implications for engineering This edition, like the first edition, isintended for final year under-graduate and post-graduate students and professionalengineers The subject matter of the book is also relevant to mechanical and materialsengineering, applied chemistry and physics courses

Gwidon W StachowiakAndrew W Batchelor

A C K N O W L E D G E M E N T S

Any book depends on the efforts of many different people and this book is no exception.Firstly we would like to thank Professor Duncan Dowson for his personal input, enthusiasm,encouragement and meticulous checking of the manuscript and very many constructivecomments and remarks We would also like to thank Mrs Grazyna Stachowiak for verydetailed research, review of technical material, proof-reading, many constructive discussions,SEM micrographs and preparation of index; Dr Pawel Podsiadlo for his help in convertingthe computer programs into Matlab, useful discussions on wavelets and scanning of theimages; Gosia Wlodarczak-Sarnecka for the design of the book cover; Longin Sarnecki for thecover photo; Alex Simpson for thorough checking of some of the chapters; and Dr NathanScott for the preparation of the illustrations Without Nathan's illustrations the book wouldnot be the same We also would like to thank the Library of the University of WesternAustralia for their help in finding all those references and the Department of Mechanical andMaterials Engineering, University of Western Australia, for its help during the preparation

Figure 16.6: The American Society of Mechanical Engineers From Transactions of the ASME,Journal of Lubrication Technology, Vol 101, 1979, pp 212-219

Figures 11.41 and 16.22 were previously published in Wear, Vol 113, 1986, pp 305-322 andVol 17, 1971, pp 301-312 respectively

by assuming that motion was always to the top of the spheres The relatively low priority oftribology at that time meant that nobody really bothered to question what would happenwhen motion between the spheres was in a downwards direction Unlike thermodynamics,where fallacious concepts like ‘phlogiston’ were rapidly disproved by energetic researcherssuch as Lavoisier in the late 18th century, relatively little understanding of tribology wasgained until 1886 with the publication of Osborne Reynolds' classical paper on hydrodynamiclubrication Reynolds proved that hydrodynamic pressure of liquid entrained betweensliding surfaces was sufficient to prevent contact between surfaces even at very low slidingspeeds His research had immediate practical application and lead to the removal of an oilhole from the load line of railway axle bearings The oil, instead of being drained away by thehole, was now able to generate a hydrodynamic film and much lower friction resulted Thework of Reynolds initiated countless other research efforts aimed at improving theinteraction between two contacting surfaces, and which continue to this day As a resultjournal bearings are now designed to high levels of sophistication Wear and thefundamentals of friction are far more complex problems, the experimental investigation ofwhich is dependent on advanced instrumentation such as scanning electron microscopy andatomic force microscopy Therefore, it has only recently been possible to study these processes

on a microscopic scale where a true understanding of their nature can be found

Tribology is therefore a very new field of science, most of the knowledge being gained afterthe Second World War In comparison many basic engineering subjects, e.g.thermodynamics, mechanics and plasticity, are relatively old and well established Tribology

is still in an imperfect state and subject to some controversy which has impeded the diffusion

2 ENGINEERING TRIBOLOGY

of information to technologists in general The need for information is nevertheless critical;even simple facts such as the type of lubricant that can be used in a particular application, orpreventing the contamination of oil by water must be fully understood by an engineer.Therefore this book is devoted to these fundamental engineering tribology principles

1.2 MEANING OF TRIBOLOGY

Tribology, which focuses on friction, wear and lubrication of interacting surfaces in relativemotion, is a new field of science defined in 1967 by a committee of the Organization for

Economic Cooperation and Development ‘Tribology’ is derived from the Greek word ‘tribos’

meaning rubbing or sliding After an initial period of scepticism, as is inevitable for anynewly introduced word or concept, the word ‘tribology’ has gained gradual acceptance As theword tribology is relatively new, its meaning is still unclear to the wider community andhumorous comparisons with tribes or tribolites tend to persist as soon as the word ‘tribology’

is mentioned

Wear is the major cause of material wastage and loss of mechanical performance and anyreduction in wear can result in considerable savings Friction is a principal cause of wear andenergy dissipation Considerable savings can be made by improved friction control It isestimated that one third of the world's energy resources in present use is needed toovercome friction in one form or another Lubrication is an effective means of controllingwear and reducing friction Tribology is a field of science which applies an operationalanalysis to problems of great economic significance such as reliability, maintenance and wear

of technical equipment ranging from household appliances to spacecraft

The question is why ‘the interacting surfaces in relative motion’, (which essentially meansrolling, sliding, normal approach or separation of surfaces), are so important to our economyand why they affect our standard of living The answer is that surface interaction dictates orcontrols the functioning of practically every device developed by man Everything that manmakes wears out, almost always as a result of relative motion between surfaces An analysis

of machine break-downs shows that in the majority of cases failures and stoppages areassociated with interacting moving parts such as gears, bearings, couplings, sealings, cams,clutches, etc The majority of problems accounted for are tribological Our human body alsocontains interacting surfaces, e.g human joints, which are subjected to lubrication and wear.Despite our detailed knowledge covering many disciplines, the lubrication of human joints

is still far from fully understood

Tribology affects our lives to a much greater degree than is commonly realized For example,long before the deliberate control of friction and wear was first promoted, human beings andanimals were instinctively modifying friction and wear as it affected their own bodies It iscommon knowledge that the human skin becomes sweaty as a response to stress or fear Ithas only recently been discovered that sweating on the palms of hands or soles of feet ofhumans and dogs, but not rabbits, has the ability to raise friction between the palms or feetand a solid surface [2] In other words, when an animal or human senses danger, sweatingoccurs to promote either rapid flight from the scene of danger, or else the ability to firmlyhold a weapon or climb the nearest tree

A general result or observation derived from innumerable experiments and theories is thattribology comprises the study of:

manifested by severe friction and wear

Film formation between any pair of sliding objects is a natural phenomenon which can occurwithout human intervention Film formation might be the fundamental mechanism

INTRODUCTION 3

preventing the extremely high shear rates at the interface between two rigid sliding objects.Non-mechanical sliding systems provide many examples of this film formation Forexample, studies of the movement between adjacent geological plates on the surface of theearth reveal that a thin layer of fragmented rock and water forms between opposing rockmasses Chemical reactions between rock and water initiated by prevailing high temperatures(about 600°C) and pressures (about 100 [MPa]) are believed to improve the lubricatingfunction of the material in this layer [3] Laboratory tests of model faults reveal that slidinginitiates the formation of a self-sliding layer of fragmented rock at the interface with solidrock A pair of self-sealing layers attached to both rock masses prevent the leakage of waternecessary for the lubricating action of the inner layer of fragmented rock and water [3].Although the thickness of the intervening layer of fragmented rock is believed to be between

1 - 100 [m] [3], this thickness is insignificant when compared to the extent of geological platesand these layers can be classified as ‘films’ Sliding on a geological scale is therefore controlled

by the properties of these ‘lubricating films’, and this suggests a fundamental similaritybetween all forms of sliding whether on the massive geological scale or on the microscopicscale of sliding between erythrocytes and capillaries The question is, why do such films formand persist? A possible reason is that a thin film is mechanically stable, i.e it is very difficult

to completely expel such a film by squeezing between two objects It is not difficult to squeezeout some of the film but its complete removal is virtually impossible Although sliding isdestructive to these films, i.e wear occurs, it also facilitates their replenishment byentrainment of a ‘lubricant’ or else by the formation of fresh film material from wearparticles

Film formation between solid objects is intrinsic to sliding and other forms of relative

motion, and the study and application of these films for human benefits is the raison d'etre

Lubrication

Thin low shear strength layers of gas, liquid and solid are interposed between two surfaces inorder to improve the smoothness of movement of one surface over another and to preventdamage These layers of material separate contacting solid bodies and are usually very thin

and often difficult to observe In general, the thicknesses of these films range from 1 - 100

[µm], although thinner and thicker films can also be found Knowledge that is related toenhancing or diagnosing the effectiveness of these films in preventing damage in solid

contacts is commonly known as ‘lubrication’ Although there are no restrictions on the type

of material required to form a lubricating film, as gas, liquid and certain solids are alleffective, the material type does influence the limits of film effectiveness For example agaseous film is suitable for low contact stress while solid films are usually applied to slowsliding speed contacts Detailed analysis of gaseous or liquid films is usually termed

‘hydrodynamic lubrication’ while lubrication by solids is termed ‘solid lubrication’ A

specialized form of hydrodynamic lubrication involving physical interaction between the

contacting bodies and the liquid lubricant is termed ‘elastohydrodynamic lubrication’ and is

of considerable practical significance Another form of lubrication involves the chemical

interactions between contacting bodies and the liquid lubricant and is termed ‘boundary and

extreme pressure lubrication’ In the absence of any films, the only reliable means of