Elastohydrodynamics - '96 Fundamentals and Applications in Lubrication and Traction, Proceedings of the 23rd Leeds-Lyon Symposium on Tribology held in the Institute of Tribology, Department of Mechanical Engineering

699 Pages·1997·27.541 MB·iii-viii, 3-757\699

by  D. Dowson, C.M. Taylor, T.H.C. Childs, G. Dalmaz, Y. Berthier, L. Flamand, J.-M. Georges and A.A. Lubrecht (Eds.)

TRIBOLOGY SERIES, 32
EDITOR: D. DOWSON
ELASTOHYDRODYNAMICS '96
-
FUNDAMENTALS AND
APPLICATIONS IN
LUBRICATION AND TRACTION
edited by
D. Dowson", C.M. Taylor, T.H.C. Childs and
G. Dalmaz, Y. Berthier, L. Flamand, J.-M. Georges, A.A. Lubrecht
* Principal Editor
Proceedings of the 23rd Leeds-Lyon Symposium on Tribology
held in the Institute of Tribology, Department of Mechanical Engineering,
The University of Leeds, UK
10th - 13th September, 1996.
ELSEVIER
Amsterdam Lausanne New York Oxford Shannon Tokyo 1997
For the Institute of Tribology, The University of Leeds
and
The lnstitut National des Sciences Appliquees de Lyon
ELSEVIER SCIENCE B.V.
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P.O. Box 21 1,1000 AE Amsterdam, The Netherlands
ISBN 0 444 82809 5 Vol. 32)
ISBN 0 444 41677 3 [Series)
(0 1997 ELSEVIER SCIENCE B.V. All rights reserved.
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V
INTRODUCTION
The 23rd Leeds-Lyon Symposium addressed the topic of “Elastohydrodynamics”. It was held
at the regular UK venue, Bodington Hall, The University of Leeds from Tuesday September
10th to Friday September 13th 1996. The meeting was attended by 140 delegates fiom twenty
countries and papers were presented in sixteen sessions with once again many parallel sessions
in order to accommodate the large numbers of papers. The organisers from Leeds were
magnificently supported by authors fiom the tribological community world-wide and we give
our thanks to them, to other delegates and particularly to our colleagues from the Laboratoire
de Mecanique des Contacts of the Institut National des Sciences Appliquees de Lyon who
once again turned out in force.
There have been three previous international meetings addressing the general area of
elastohydrodynamic lubrication.
-
“Elastohydrodynamic Lubrication” A Symposium arranged by the Lubrication
and Wear Group of the Institution of Mechanical Engineers from the 21st/23rd
September 1965.
“Elastohydrodynamic Lubrication” a meeting arranged by the Tribology Group of
-
the Institution of Mechanical Engineers from the 1 1 13th April 1972.
“Elastohydrodynamics and Related Topics” - The 5th Leeds-Lyon Symposium on
Tribology held from the 19th to the 22nd September 1978.
All these meetings were at the University of Leeds and the Institute of Tribology within the
Department of Mechanical Engineering and Leeds was particularly proud therefore to organise
this fourth world-wide gathering of tribologists. Four authors at the first meeting in 1965 also
-
presented papers at the 23rd Leeds-Lyon Symposium on Tribology Alastair Cameron,
Duncan Dowson, Chris Hooke and Jerry Kannel. It was a particular delight to have these
colleagues with us thirty one years to the month after the first EHL Meeting.
Professor Stathis Ioannides presented the Keynote Address before the Symposium Dinner on
the opening evening of the meeting. His paper “Tribology in Rolling Element Bearings,
Recent Advances and the Wider Implications“ was a tour de force reflecting his wide
experience of the influence of elastohydrodynamic lubrication studies on the development of
rolling element bearing technology with particular reference to his experience at SKF
Engineering and Research in the Netherlands. In addition to this Keynote Address there were
fifteen other sessions covering the general areas, AnalyticaVNumerical, Experimental, Soft
EHL, Lubricant Properties, Transmissions-Gears, Biotribology, Rolling Element Bearings,
vi
Boundary Lubrication, Friction, Traction and Wear, Metal Forming, Transmissions - CVT’s
and Plain Bearings. Nine other invited technical papers supported these sessions and we are
gratefbl to the Invited Authors for their carefblly prepared papers and valuable contributions.
Those who chaired sessions are recorded in this Volume of Proceedings and to them likewise
we would like to express our thanks. Not only did those who chaired the sessions keep the
busy programme to its time schedule but they also ensured valuable discussions which are
recorded along with the Written Discussion in this Volume of Proceedings.
The Symposium Dinner was held at Nidd Hall Country House Hotel immediately after the
Keynote Address of Professor Ioannides on the first evening. The dinner was graced by the
presence of the Pro-Chancellor of the University of Leeds, Colonel A C Roberts, who has been
a great supporter of the Department of Mechanical Engineering and the aRer dinner speech
was made by Professor David A Parkes, Director of Shell Research Limited who entertained
and informed delegates in a sparkling presentation. On the afternoon of Thursday 12th
September 1996 the delegates repaired to the new Royal Armouries Museum in the City of
Leeds. The visit had been prepared by a presentation the previous evening by Mr Chris
O’Boyle, Chief Executive of the Royal Armouries, and the visit to this new E42 million
development was much enjoyed by delegates and rounded off in the evening by an excellent
meal. After the meeting many delegates remained in Leeds to enjoy a walk in the Yorkshire
Dales on Saturday 14th September. Two years previously those who had undertaken a similar
walk had scaled one of the three Yorkshire Peaks, Whernside. On this occasion walkers scaled
the second of the three peaks, Ingleborough, enjoying a repast and drink or two in the same
Public House, The Old Hill Inn, as previously. It was observed that at the next Leeds-Lyon
Symposium to be held in Leeds there would be some sense in addressing the third of the three
-
Yorkshire Peaks Pen-y-ghent.
Yet again we are sincerely gratehl to our colleagues at the University of Leeds who have
joined us in the detailed planning and execution of the Symposium. These have included, Mrs
Cath Goulborn. Mrs Sheila Moore, Dr Peter Dearnley, Mr Ron Harding, Mr David
Hawkridge, Mr Brian Jobbins, Mr David Jones, and other academic colleagues, clerical staff,
technical staff,r esearch fellows and research students who have played their part. We would
like in particular to note that David Jones retired from the Department of Mechanical
Engineering on the 30th September 1996. His association with, and contribution to, the series
of Leeds-Lyon Symposia since their inception has been exceptional including taking
responsibility for all aspects surrounding the audio-visual presentations at Leeds. We should
also like to record that one of our tribological colleagues Dr Chris Radcliffe has also left us but
it is with pleasure that we wish to record that two new academic tribological colleagues have
now joined us in Leeds, Dr Pascal Ehret on the 1st July 1996 and Dr Martin Priest on the 1st
September 1996. Once again Elsevier Science Publishers BV, Amsterdam have supported our
endeavours tremendously and we gratehlly acknowledge them and the following organisations
vii
who made financial contributions to the meeting. These financial contributions enabled us to
achieve our goals and in particular to reduce the registration fee for many younger research
workers.
BP International Ltd
-
Charities Aid Foundation Unilever's Research and Engineering Division
DePuy International Ltd
Elsevier Science Publishers BV
FAG OEM und Handel AG
NSK Limited
Shell Research Ltd
SKF Engineering & Research Centre BV
The range of topics covered by the Leeds-Lyon series of meetings is now extensive as will be
seen from the following list.
1. 'Cavitation and Related Phenomena in Lubrication'
2. 'Superlaminar Flow in Bearings'
3. 'The Wear of Non-Metallic Materials'
4. 'Surface Roughness Effects in Lubrication'
5. 'Elastohydrodynamic Lubrication and Related Topics'
6. 'Thermal Effects in Tribology'
7. 'Friction and Traction'
8. 'The Running-In Process in Tribology'
9. 'The Tribology of Reciprocating Engines'
10. 'Numerical and Experimental Methods Applied to Tribology'
1 1. 'Mixed Lubrication and Lubricated Wear'
12. 'Global Studies of Mechanisms and Local Analyses of Surface Distress
Phenomena'
13. 'Fluid Film Lubrication - Osborne Reynolds Centenary'
14. 'Interface Dynamics'
15. 'Tribological Design of Machine Elements'
16. 'Mechanics of Coatings'
...
Vlll
17. ‘Vehicle Tribology’
18. ‘Wear Particles : From the Cradle to the Grave’
19. ‘Thin Films in Tribology’
20. ‘Dissipative Processes in Tribology’
2 1. ‘Lubricants and Lubrication’
22. ‘The Third Body Concept: Interpretation of Tribological Phenomena‘
23 ‘Elastohydrodynamics- ‘96’
Over the years the organisers of the meeting and the editors of the Volume of Proceedings
have taken great care to ensure the quality of the papers presented orally and included in the
final Volume of Proceedings. A view on the quality of all material is taken at the presentations
and all manuscripts are reviewed by international tribological authorities in Europe and
occasionally from elsewhere. The editors would like to acknowledge the support of referees
and to record their commitment to the continuation and improvements of the high quality of
papers which appear in the Proceedings.
Exceptionally in 1997 there is to be a World Tribology Congress held from the 8th-12th
September in London, the organisation being led by the Tribology Group of the Institution of
Mechanical Engineers. All major tribological research societies have committed to the support
of this World Tribology Congress in different ways. This includes the Leeds-Lyon Symposium
which on this unusual occasion will be held at Imperial College of Science, Technology and
Medicine from September 4th-6th immediately preceding the World Tribology Congress. We
are grateful to our colleagues in the Department of Mechanical Engineering and the
-
Department of Chemical Engineering at Imperial College Professor Brian Briscoe, Dr
Philippa Cann and Professor Hugh Spikes for their positive and energetic support of Leeds and
Lyon on this occasion and we are already looking forward to this 24th Leeds-Lyon meeting
which will be entitled “Tribology for Energy Conservation”.
Duncan Dowson, Chris Taylor, Tom Childs
Elastohydrodynamics '96 / D. Dowson et al. (Editors)
0 1997 Elsevier Science B.V. All rights reserved. 3
EHL in rolling element bearings, recent advances and the wider implications.
E Ioannides
SKF Engineering & Research Centre, and
Imperial College of Science, Technology and Medicine
Rolling element bearings were introduced industrially in the latter part of the nineteenth century and
rapidly became the main machine element used in the support of rotating shafts and machinery, mainly
because of their low frictional losses. Todays' demands for increased reliability and downsizing, combined
with the requirement for high speed development of equipment, have encouraged the development of
predictive tools rather than following the usually more expensive and slower testing approach. This in turn
has spurred the refinement of EHL predictions that model more realistic operational conditions in rolling
bearing and other machine elements. In this paper, lubrication of real surfaces with roughness or damage
from solid contaminants is discussed, together with frictional losses. The implications of treating a rolling
element bearing as a full dynamic system are considered and the need for further research is identified.
1. INTRODUCTION
The rolling element bearing is the most
widespread machine element (after nuts and bolts)
with some fifty billion bearings in operation
throughout the world. The origin of rolling
bearings can be traced in antiquity where bearings
supporting platforms in Roman ships had rolling
elements made of bronze and wood. Even earlier a
Celtic cart was found with hubs. having spacing for
supporting wooden elements, rolling or possibly
sliding [l]. Despite these early origins rolling
bearings were introduced industrially only during
the latter part of the nineteenth century after a
flurry of patents during the previous century. The
chief reason for this is that low surface roughness
of the rolling elements and the contacting raceways
is needed to ensure satisfactory separation of these
components by the very thin lubricant film formed
Fig.1: Photoelastic model of a radially loaded
during rotation of the bearing. This, together with
bearing
the use of special steels of high cleanliness, is
necessary to ensure long and trouble-free operation
As a consequence the modem rolling element
under the high contact pressures that develop in the
bearing is a precision machine element, usually
minute contact areas between the relatively small
mass produced at high speed with very demanding
rolling elements and the raceways, asmay be seen
dimensional and working contact surface
in the photoelastic model of Fig. 1. Thus,
requirements. The high demand on dimensional
commercialization of rolling element bearings had
accuracy of the different bearing components
to await the capability of manufacturing smooth
originates from the requirements for running
surfaces made of strong steel.
accuracy of machinery whilst, as indicated above,
4
the need for low roughness originates from the need dominant in field studies of causes of failure, Fig 2,
to build a separating lubricating film. see for example [lo]. It should however be noted
in this connection that the vast majority of
Optimising the selection of rolling bearings in bearings, 99.5%, are either scrapped with the
machinery is an important issue, not only because equipment or replaced during maintenance without
of the large numbers of bearings involved, but also having failed.
because of the critical role these components play
in many applications, including safety, for CAUSES OF FAILURE FOR BEARING LIFE c L1
example in transport systems. Today's high %
performance and reliability requirements are also a
result of keen competition for all manufactured
equipment combined with the need for high speed
development from concept to finished product. This
in turn implies that numerical modelling is usually
preferable to testing because of both speed and
cost. Considerable effort has therefore been put into
modelling bearing performance in increasingly
realistic terms by including, for example. effects of
roughness and lubricant contamination. This is
particularly important in modem machinery where
the trends of higher speed and increased power
density (downsizing) have been eroding the
Fig. 2: Main Causes of Bearing Failure
traditional safety factors built into earlier designs.
The issue of rheology is also further discussed
The paper discusses three of the main
in the next section in relation to the friction torque
performance criteria of bearings, life, friction and
of bearings. This aspect of bearing performance is
dynamic behaviour, concentrating on EHL-related
gaining importance in the world of today with the
issues affecting these. The current issues of life
keen interest in cnergy conservation.
calculation are addressed first, including the
introduction of a fatigue limit for bearing steels
Finally, in addressing the overall
and the representation of realistic rough or
performance of bearings, the dynamic effects of
contaminated lubrication conditions. Some recent
the whole bearing operating as a system are
contributions to these topics may be found in Refs.
discussed. Again, as forces are transmitted through
12-61.
EHL contacts, these contacts dctcrmine the
dynamic behaviour of the rolling elements and thc
In addition, advances in EHL calculations and
cage. A 'systems approach' to modelling the
rheology modelling, which extend the
dynamics of the complete bearing therefore relies
understanding of asperity interactions and more
heavily on EHL simulation. Results from the
generally the effects of topography on film building
dynamic modelling of complete bearings at various
and contact stresses, are also discussed. Some of
levels of complexity are reported in Refs. [ 11 -12 1.
the difficulties and the progress in understanding
grease lubrication are also addressed, as sealed or
2. LIFE AND LUBRICATION
greased-for-life bearing applications have been
steadily increasing in past years. Recent work in
As indicated in the Introduction, the
this area is described in Refs. [6-91.
commercialization of rolling bearings had to wait
These broad issues of life, lubrication and
until manufacturing practices could deliver surfaces
contamination of rolling bearings, are also
smooth enough to be sufficiently separated by EHL
5
films. This is necessary to guarantee long life for 2.1. EHL of rough surfaces
rolling bearings which are expected to survive Lubricated rough contacts are characterised by
typically lo9 contacts at each point of the pressure oscillations which describe the pressure
contacting surface of the raceways, each at a high increase at the top of the asperities and the pressure
contact pressure between 1 to 4 GPa. With decrease in the intervening valleys compared to a
microslip present (Heathcote - i.e. curvature smooth contact. The EHL solution for a rolling
-
originated slip, slip from manufacturing engineering rough surface must in most cases
tolerances etc.) such separation is necessary since involve a simplified description of the surface or of
when a degree of metallic contact occurs, reduction the roughness, [15-181, as the solution must be time
of service life of the bearings has been observed. dependent to *account for the varying amounts of
Empirically this has been introduced into bearing entrained lubricant and its squeezing by the asperity
life calculations, [13], and into bearing approach velocities. In contrast, pure sliding of a
manufacturers' catalogues. rough surface against a smooth one produces a
steady state, where there is no approach velocity.
Bearings are selected (type and size) such that The problem is time independent and many
the percentage probability (n) to fail in an solutions exist, [ 19-2 11.
application within a prescribed number of Millions
of revolutions L,, i.e. the expected life of the Estimating the life of such real rough surfaces
bearings, is known. A number of conditions is of primary interest in the effort to predict the life
influence the bearing life L, such as steel quality, of rolling bearings. The approach to this, given the
contacting surface quality, lubricating conditions, present day calculation limitations described above,
load, operating temperature, internal stresses from is to use expressions or design curves derived from
fittings, bearing kinematics, etc. and an increasing test [ 131 or to utilise stress fields obtained from dry
number of these are taken into account in the contact solutions to estimate the expected life.
prediction methodologies for L, [ 141. The latter approximation is valid for thin films
where the pressure distribution approaches
The lubricant in the bearing is the low shear asymptotically the dry contact one as the film
strength "third body" which is used to separate the thickness diminishes. In the absence of pressure
two otherwise contacting bodies. Thus shearing fluctuation attenuation by the lubricant film the
takes place in the lubricant and the shear stresses dry contact pressure distributions provide a
exerted on the metallic surfaces are reduced as the conservative limit for the purposes of life
separation grows. Under a variety of operating calculation. Such dry contact calculations have
conditions, the real engineering (rough) surfaces in been reported by a number of authors, [22-231, and
rolling bearings experience different degrees of more recently the extension of the methods to
metallic contact. Traditionally this has been layered contacts has been reported [24-251. The
described by the ratio, A, of the film thickness to dry contact subsurface stress fields were used in
[26] to assess the relative effect of roughness
the composite roughness or by the viscosity ratio K
parameters on life. In Fig. 3, from this reference,
preferred by bearing manufacturers. Loss of bearing
the computed surfaces show that averaged L,,
life has been empirically related to these
values increase dramatically when the rms slope of
parameters. In fact, this is the major cause of
the surface is reduced or when the skewness is
bearing failure as can be seen in Fig 2, showing the
made more negative. The benefit deriving from a
percentage share of the major causes of bearing
&
reduction in the roughness amplitude itself is, by
failure. Efforts to model real surface lubrication
contrast, more modest.
have thus intensified in recent years. Three areas of
Moreover, in attempting to model the rough
this effort are discussed below.
rolling and sliding lubricated contact present in
rolling bearings the non-Newtonian rheology of the
6
lubricants must be addressed. At the typical In general, the complexity of EHL
pressures and temperatures of the contacts in these calculations for real rough surfaces is prohibitive.
bearings such lubricants can solidify and react to However, in recent times progress in this direction
the small amounts of sliding present in bearings by has been greatly assisted by Multigrid techniques
leaking sideways from the top of the asperities, which have resulted in dramatically shorter
allowing oil film collapse, [27-281. This behaviour computational times and smaller memory
can also be modelled by treating the lubricant film requirements. It is therefore expected that these
as a perfectly plastic solid, [29]. Fig. 7 shows the computational techniques will further expand the
calculated collapse of the oil film thickness with ability to understand film formation and stress
passage time through the contact for a smooth on levels at the asperity contact level (micro EHL)
rough surface combination with four different within a rough lubricated contact, e.g. [30].
asperity wavelengths (h) in the direction However, the true performance of rolling bearings
transverse to the rolling/sliding [29]. under mixed or boundary lubrication conditions
depends also on the chemical composition of the oil
and any additives, which can have a profound
effect on life. Thus, for the foreseeable future,
additional experimental work will be needed to
support life predictions.
2.2. EHL of damage
In the past, a considerable amount of
experimental work has been reported in which
contaminated lubricants were used in rolling
element bearings and rolling contacts. An overview
of this work is given in [3]. In geneml, such
experimental work points to a wide range of life
reductions, usually severe, indicating the need for a
predictive model which can account for the effects
of such realistic conditions. For this reason
theoretical studies have recently been emerging in
which an effort is made to systematize the many
,parameters that govern the effects of particulate
contamination. These studies have concentrated on
two main aspects: (a) the mechanisms and the
prediction of damage by particles to the raceways
and rolling elements [4-71. and (b) a theoretical
confirmation of how such localised damage can
reduce the fatigue life of a bearing [8-131. For the
latter, fatigue models such as the one reported in
[14] have been used to predict the life reductions
associated with damaged raceways and rolling
elements. An alternative approach has been the use
of Fracture Mechanics and Micromechanics
modeling, e.g. [6].
Fig. 3: Relative life L,, variation versus roughness, Again in many cases the starting point for life
slope and skewness predictions is the dry contact stress calculations.