Developments in Numerical and Experimental Methods Applied to Tribology. Proceedings of the 10th Leeds–lyon Symposium on Tribology Held at the Institut National des Sciences Appliquées, Lyon, France, 6th–9th September 1983

297 Pages·1984·26.67 MB·English

by  D Dowson

DEVELOPMENTS IN NUMERICAL
AND EXPERIMENTAL METHODS
APPLIED TO TRIBOLOGY
edited by
D. Dowson, C. M. Taylor, M. Godet and D. Berthe
Proceedings of the
10th Leeds-Lyon Symposium on Tribology
held at the
Institut National des Sciences Appliquées,
Lyon, France
6th-9th September 1983
D
gg
Butterworths
{London - Boston - Durban - Singapore - Sydney - Toronto - Wellington)
for the Institute of Tribology, Leeds University
and
The Institut National des Sciences Appliquées de Lyon
Copyright© Butterworth & Co (Publishers) Ltd 1984
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in
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ISBN 408 221 64X
Printed in Great Britain by
Robert Hartnoll Ltd. Bodmin, Cornwall
Introduction The two key-note speakers, Professor Barwell and Dr. Hamrock, one
for each topic, showed total respect for the participants more concerned
with the other subject, and did indeed set the tone of the conference.
Professor Barwell did so under great duress as part of his talk was given
in a dark amphitheatre during a power cut.
The key-note talks were followed by the traditional Leeds-Lyon
When in September 1973, during the conference on Turbulent Lubri- banquet which was held at the Becfigue Restaurant just outside of Lyon.
cation held at North Western University, Professor Dowson suggested All participants were very sorry that Professor Dowson could not attend
that an annual symposium be held alternately in Leeds and in Lyon, few the meeting because of the pressure of work brought about by his
of us imagined that we would reach number 10. Well the 10th Leeds- election as Pro-Vice Chancellor of the University of Leeds. His warm
Lyon symposium is history, the 11th is planned, and discussions messages were ably conveyed by Chris Taylor who led an active and
concerning the 12th have started early as Eurotrib '85 will also be held in friendly contingent from Leeds. The meeting convened the next morning
Lyon in September 1985. While much of the credit for this continuity at 8.30 and a quick pace was kept thanks to our energetic chairmen. A
should go to the personnel of both laboratories that host the conference, visit to the nuclear plant of Saint Alban proved particularly interesting;
we are all aware that these events could not be held without the the plant is being built and, therefore, it was possible to see the very heart
continued support of many friends in the Tribology community of the reactor. All tribologists toured the massive bearings of the turbine
throughout the world. The 10th anniversary seems an appropriate time assemblies even though this required some climbing.
to thank these friends for their faithfulness. The Leeds-Lyon symposium lived up to its social reputation. Dinners
The 10th Leeds-Lyon symposium, held at INSA in Lyon, was ended late and were followed by lively gatherings around the bar. Choirs
attended by 137 delegates from 16 countries. In something of a break and dart games were improvised and lasted well into the early morning.
with tradition, two topics, rather than one, were chosen; the conference Communication problems which seem to slow conversation during the
discussed new developments in both numerical and experimental day vanished then thanks to the determination of many and particularly
methods. The two subjects were chosen to appeal to a greater number of of the Imperial College delegation whose contribution to the conference
tribologists and make for a livelier conference. We ran into a certain was a round the clock affair. The Anglo-French spirit of the conference
number of difficulties as we received many more numerical than was held throughout the meeting as on the last day of the work sessions a
experimental papers, and found that a few experimentalists had little traditional "Lyonnais" game of petanque was held instead of the more
enthusiasm at having to sit through "esoteric" numerical presentations; "British" darts.
requests for the opening hours of the swimming pool were received. The Saturday outing took us through the Maçonnais, and the
Things turned out better than expected as the authors of numerical delegates visited the Abbaye Saint Philibert of Tournus, the churches
papers emphasised more what could be done with the new techniques and museums of Cluny,and of course one of the better cellars of Macon-
than the highly specialised points from which only a few could truly Viré where its owner Mr. Bonhomme gave us a very fine dissertation on
profit. Further, the dual session that had been set up — one on numerics, wine making.
the other on experimental aspects — was criticised as going against the We are looking forward to meeting all our friends again during the
Leeds Lyon spirit in which the delegates are supposed to reflect as a 11th Leeds-Lyon symposium which will be held in Leeds and will
group on the subject of the session. Possibly, as organisers, we were over discuss "Mixed lubrication and lubricated wear".
cautious and underestimated the resilience of tribologists to excursions
outside of their immediate field of endeavour. To these we extend our M. Godet
apologies. D. Berthe
Delegates attending the symposium
vii
Paper I (i)
The role of particle analysis - a review of
ferrography
F. T. Barwell
SYNOPSIS
Modes of formation of particles can be postulated to correspond with the characteristic mechanism
of wear, notably adhesion, cutting and fatigue. Chemical and thermal effects must be taken into
account and lead to non-metallic particles which are translucent. When transported within fluids,
particles tend to migrate inwards due to the tubular pinch effect. The analytical ferrograph is
well developed and improved methods of use are described. Also modifications to the direct-reading
method are recommended. The on-line ferrograph offers promise for automation of condition monitor-
ing. Application of ferrography to non-ferrous particles, notably in the medical field, can be
achieved by using solutions containing rare earth derivatives such as erbium chloride.
1 INTRODUCTION suggests that the formation of loose particles
may result from chemical changes in the adherent
The study of Wear as a specific activity is of fragments. Thus steel fragments may oxidise
comparatively recent origin, the first signifi- into a brittle easily removed material which
cant publications appearing in the fifties of exposes fresh surface areas for the formation of
this century. Even then very little attention new junctions. The rate of wear is governed by
was paid to the particles created by the wear the chemical activity of the environment in the
process although it might be expected that their manner elucidated by Kerridge [2]. Confirmation
nature and distribution would help to explain of the important effect of ambient conditions
the process whereby material was removed from is provided by the experiments with iron sliding
the wearing surface. This situation has been on iron and nickel sliding on nickel, reported
transformed by the event of 'TQASiOQncipky' which by Soda and Sasada [3] and Sasada et al. [4] in
constitutes a very convenient means for causing air and vacuum, respectively. Although the
particles to be deposited from liquids in an friction-coefficient in air was much lower than
orderly manner and in a dispostion which enables in vacuum, the wear-rate was much greater.
them to be studied by a variety of techniques. Sasada, Norose and Mishina [5] propose a
Ferrography has now become recognised as model wherein a metal-to-metal junction formed
an important tool for use in the study of the between intimate surfaces is sheared by fric-
fundamental processes of wear and also has pro- tional motion. A small fragment of either sur-
mise of significant economic benefits when face will be sheared off and will adhere to the
applied to the study of the condition of opera- mating surface. This 'transfer-element1 consti-
ting machinery. Many developments are possible tutes a new asperity which will, in turn, inter-
in this, notably the 'on-line1 instrument which act with the mating surface. Through constant
can be incorporated into a completely automatic repetition the transfer-particle gradually
system of condition-monitoring. grows to a considerable size until it is removed
Most revolutionary of all is the develop- suddenly "by a mechanical impulse from the oppo-
ment of medical ferrography where the technique site surface". At each interaction the trans-
is used not only to study the wear of artificial fer element will support a considerable propor-
joints but to elucidate the functioning of the tion of the applied load so that it will tend
natural joint itself. to be extruded sideways; thus the resulting
The promise offered by particle analysis wear particle may be expected to be rather flat.
is so attractive that publications in the field Electron-microscope studies by Kayaba and
are appearing at an increasing rate. This revue Kato [6] on model asperities show the creation
cannot therefore refer to them all but an att- of 'slip-tongues1 which can develop flake-like
empt has been made to select a limited number characteristics. Tsuya [7] et al. also illu-
which are representative of the main lines of strate the formation of flake-like extrusions
development of the subject. resulting from shearing action.
The formation of wear particles from inter-
acting solids involves the creation of addi-
2 FORMATION OF PARTICLES
tional surface energy which must come from the
frictional work developed at those interactions
The shearing of the junctions postulated in the
[8]. It is assumed that the additional surface
adhesion theory of wear is unlikely to give rise
energy is equal to the elastic energy which is
directly to a wear particle because material
released when the particle is finally torn from
sheared from one surface may be expected to ad-
its substrate. This energy would be proportional
here to the other surface. Rabinowicz [1]
3
to the volume of material affected and therefore sufficient stress must be available at the crack
a given quota of energy per unit volume, so pre- tip to support crack growth and, furthermore,
dicting a certain ratio of area to volume and sufficient new energy must flow to the growing
thus a particular particle size. There would crack to supply the work involved in the creation
also be a minimum load below which particles of new surfaces. The chemical environment may
would not be formed. That this is an oversimpli- exert a significant influence on both these fac-
fied picture is apparent from the following rea- tors [15].
sons: Spherically-shaped particles are not nor- Although most industrial metals possess a
mally found in rubbing-wear situations; particle polyphase crystalline structure this does not
size distributions are usually exponential in appear to directly influence the form taken by
character; and finally, wear appears to occur wear particles, which generally take a form
at all values of load. which would be expected to arise from uniform
Asperity contacts may not necessarily in- isotropic material. An exception was the obser-
volve adhesion particularly if one of the inter- vation by electron-microscope of rectilinear
acting surfaces presents material which is harder particles [16] . These can be assumed to be com-
than the other. Shearing action can produce posed of cementite arising from decomposition of
chisel-like particles [9] or indeed the contin- pearlitic structures. Preferential etching-away
uous coils of deformed material which constitute of the ferrite by acid [17] could leave cemen-
'cutting-wear' particles. The same laws as those tite unsupported and prone to removal by sliding
governing machining action will apply [10] notably action.
the existence of a critical angle below which The electrical action taking place between
material removal does not take place. Work- the interacting surfaces has been inadequately
hardening may also be important. explored because it might be expected that the
It must not be forgotten that, once formed, transfer of electrical energy by conduction may
a wear particle may suffer deformation and break- result in wearing action. This generally occurs
age by repeatedly being presented to the inter- by disruption of conducting bridges giving finely
acting system. The fact that the majority of dispersed particles. However, recent work [9]
particles are thin and long may be attributed to has revealed characteristic string-like particles
this. which presumably arise from conducting bridges.
In considering interacting surfaces in mach- Summarising, there appear to be three prin-
inery it is important to realise that stresses cipal modes whereby mechanical action may lead
may be applied and released literally millions of to the creation of wear particles. These are
times and that in the confined space between the adhesion, cutting, and fatigue. These may be
interacting asperities stress may be hydrostatic affected by chemical and thermal action. In
(or three-dimensional) causing extension of nor- particular, frictional heating may lead to oxi-
mally brittle material of many times the value dation so that the resulting particles consist
which would result in rupture under uni-axial of oxides rather than material in the metallic
stress. The very large number of very thin par- state.
ticles (under 10 y) usually observed with well
run-in machines are attributable to this, which
3 MANIPULATION OF PARTICLES WITHIN FLUIDS
is consistent with the delamination theory of
wear [11].
Very small particles tend to be retained in sus-
Under more intensive loading the effect of
pension within fluids because of viscous forces.
repeated stressing is felt more deeply within
Thus, a particle of spherical shape 1/20 y m in
the substrate. Tsuya [12] reports that contin-
diameter propelled at 1 m/s within a fluid of
uous plastic working of the interacting bodies
1 Pa. S. viscosity (this gives rise to a Reynolds
leads to the formation of micronised crystals.
number which is well within the range of Stoke's
Cracks originating at the boundary of the micro-
law) would exert a drag of 3 7rn.dU. The mass
nised region develop in the direction of flow
would be π p d^/6 where p is the density of
until a particle is released.
the particle. Then
This leads to consideration of the possible
role of fatigue as a basic mechanism of wear, as
drag/mass = 18 η U/p d2 (1)
put forward by Endo and Fukida [13]. The effect
of ambient atmosphere on wear may be equally
which comes to well over a million-million. Alt-
well explained by the action of oxygen in accel-
ernatively, the limiting velocity under the
erating the flow of fatigue cracks as by the
action of gravity is
anti-welding action of oxide films referred to
earlier 14 .
g P d2/18n (2)
When contact between interacting bodies is
concentrated to give rise to Hertzian conditions, -21
which in the case assumed amounts to 2.8x10
fatigue is to be expected to occur in the region
m/s. It is difficult to envisage the applica-
of maximum shear strain and will take the form
tion of sufficient force to such a particle to
of »pitting'. This gives rise to 'chunky' parti-
remove it from the fluid in reasonable time by
cles and usually to very many spherical particles
either centrifugal, electromagnetic or electro-
which can provide a distinct early warning of the
static means. It must therefore be assumed that
imminence of pitting failure. This is very fre-
the smallest particles will be retained within
quently experienced with rolling contact bearings
samples of oil.
but in some disc machine tests, designed to simu-
Particles migrate in a radial direction
late the action of gears, spherical particles
when entrained in a fluid flowing in a cylindri-
have not been observed.
cal tube under laminar conditions. Segre and
When a particle is formed, some form of
Silberberg [18] concluded that if the distribu-
fracture between it and the parent material must
tion of particles was initially uniform across
occur and further understanding of the mechanism
the section of the flowing mass, as flow proceeds
of wear must be sought from application of the
the particles become concentrated more and more
laws of fracture mechanics. These imply that
within a concentric annular region having a
4
radius equal to about 0.6 of the tube radius. on the aforementioned principle have been com-
This phenomenon is known as the 'tubular pinch pared with those obtained by Ferrography by
effect1. The motion of particles has been shown McCullagh and Campbell [21]. In a system indi-
by Aoki et al [19] to be governed by two Reynolds cated by ferrography to have a low level of met-
numbers, that relating to the motion of the fluid allic particles it was considered that high HIAC
within the tube and that describing the motion of counts were attributable to the presence of non-
the particle within the fluid. These authors metallic contaminant in the fluid.
observed two patterns of particle movement, one
in which particles move laterally either toward
4 THE ANALYTICAL FERROGRAPH
the tube axis or the tube wall and the other
where they tend to concentrate within an annular
For the results of any analytical system applied
region, as represented by the tubular pinch
to fluids containing wear debris to be of value
effect. The particle Reynolds number is defined
in assessment or prognostication of wear they
as a \^ where a is particle diameter, v is
must be reproducible and capable of distinguish-
its velocity relative to the fluid, and v is
ing between different categories of particles,
the kinematic viscosity of the fluid. A negative
if only by size[22] [23]. This places two basic
Reynolds number indicates that the particle lags
functional requirements on the requisite appara-
behind the flowing liquid and a positive value
tus, namely regulation and classification.
indicates that it is moving faster than the
The regulation function is provided for in
fluid. When the value is greater than one or
the standard apparatus by a peristaltic pump
less than minus one, movement is unilateral, but
which provides a constant flow of the sample
when it is in the region intermediate between
fluid. Jones, Kwon and Vaughan [24.] have demon-
these values, bilateral motion occurs causing
stated that in the course of preparing a ferro-
the tubular pinch effect to occur. The particles
gram using the existing system, the original
were observed to rotate with an angular velocity
wear particles were degraded mechanically during
corresponding to one-half of the vorticity of the
their passage through the oil delivery system
fluid at the position of the centroid and that
due to the compressive squeezing effect of the
they continued to do so after attaining the equi-
peristaltic pump action. They recommend a re-
librium radial position.
arrangement of the apparatus wherein the sample
One method of examining a fluid containing does not pass through the peristaltic pump but
particles is to arrange for it to flow at a slow, is expelled by air pressure from a closed sample
steady rate within the field of view of a suit- bottle. Air alone passes through the peristaltic
able microscope. In their "flow ultra-microscope" pump wherein it is pressurised before being fed
Mikhin et al [20] arranged for a sample of lubri- to the sample bottle.
cant (diluted if necessary to adjust viscosity)
In what he describes as a "short Communica-
to flow through capillary channel C as shewn in
tion" and within the space of two and a half
Fig.l. This is contained in a chamber K which
pages, Jones [25] has made a major contribution
is illuminated by convergent light from source S.
to the development of ferrography. Regarding
This beam is directed normally to the axis of
the harmful effects of the peristaltic pump on
liquid flow and is brought to a focus within
the particles within a sample, he proposes re-
channel C. A microscope M is arranged so that placing the pump by the syphoning arrangement
its optical axis coincides with the axis of flow. shown in Fig.2. (Other aspects of this communi-
Fluid with dispersed particles is caused to cation are referred to later in their appropriate
flow through the space where the light and the context.)
flow axes intersect and scintillations of diff-
The classification function is emphasised
used light are observed in the microscope. The
by the inclination of the glass slide relative
total number of large particles passing within a
to the permanent magnet, so that the intensity
given time may be detected by this means. Par-
of the magnetic field increases as the fluid
ticles below about 3x10"^ m cause light to be
sample passes down the slide. This provides a
scattered, producing scintillations, whereas convenient distribution of ferrous particles by
particles with dimensions greater than one-half size as well as ensuring that weakly magnetic
of the wave-length of light produce bright refl- or non-magnetic particles are spacially separa-
ections which are easily distinguished in the ted from ferrous particles of the same size. It
microscope. sometimes happens that the concentration of
In a series of experiments in which a copper- large particles in the immediate vicinity of the
zinc alloy was rubbed against steel in the pres- entry is such that identification of individual
ence of either vaseline or glycerine, particles particles for counting etc. is not possible. In
were counted in the flow ultra microscope as these circumstances a more convenient distribu-
well as being separated by centrifuging for tion may be obtained by raising the glass slide
spectrographic study. When glycerine was used by about one millimetre further from the magnet
as a lubricant many particles were formed initi- at the entry end.
ally but the rate fell off as wearing-in took
A novel device has been patented to over-
place. When an inactive lubricant (vaseline oil)
come this problem wherein a sample is caused to
was applied the wearing-in process did not occur
flow on a small square plastic slide which is
and the total number of particles continued to
constantly rotated so as to introduce a centri-
increase.
fugal effect. An annular magnet is located
Commercial instruments are available which under the slide which withdraws ferrous parti-
operate by the blockage of light. The sample cles from the sample fluid as it flows radially
fluid passes through a cell interposed between a outwards. This method provides very convenient
source of light and a photo-sensitive element. deposits for microscopic examination but does
Each particle passing through the cell blocks an not provide one of the most important features
amount of light proportional to its projected of ferrography, that is, the classification of
area. The output is processed to evaluate the ferrous particles by size.
diameter of an equivalent sphere which is stored Jones [25] has suggested that if the barr-
for counting. Results from examination of hydr- ier to oil spreading, which is normally disposed
aulic fluids by the HIAC system which operates
5
centrally, is offset as in Fig.3, a 'histogram' after heating to 500 C. Microprobe analysis has
type of deposit of wear debris will occur which then to be used to differentiate between these
should be easier to interpret. elements.
He also draws attention to problems assoc- In order to avoid the need for coating
iated with the quantitative analysis of wear ferrograms by gold for good scanning-electron-
debris when a high energy electron beams beyond microscopy, carbon-filled plastic slides can be
the sample into the glass substrate. Elements used as the basis for ferrograms. However,
detected will include a background of silicon, because these are opaque, the ferrogram cannot
zinc, titanium, calcium and numerous other trace be studied by transmitted light. This objection
elements making up the glass. Variations in does not apply, however, when carbon-coated
analysis may also arise from variations in the translucent material (methyl-methacrylate) is
electron beam energy levels. Jones reports that used.
these problems are eliminated when the slide is The most complete and advanced method of
made from Perspex. This may be particularly studying ferrographs is the QUANTIMET as used
important when the reaction products of additives by Roylance et al [29]. After preliminary micro-
are being investigated because they are often scopic inspection to identify regions of inter-
similar to the background elements, calcium sul- est, reflection and transmission light sources
phate being an example. are used to establish optimum conditions of
Naturally, the intensity of concentration contrast for exhibiting particles relative to
of particles within a sample will affect the the background. By varying the conditions of
congestion or otherwise of the deposit on the illumination, particles of different composition
microscope slide. Depending on the purpose of can be distinguished. The size of the field of
the investigation, some control of the density observation can be varied from 0.16 x 0.13 mm to
of deposition is desirable, and this can most 1.6 x 1.3mm. The minimum detectable size which
readily be effected by control of dilution of can be identified, known as the 'picture point'
the sample. Jones [25] proposes a preliminary varies from 0.2 to 2.0ym.
assessment of the particle concentration in a Pocock [30] drew attention to the applica-
sample using the Direct-Reading ferrograph (see tion to wear particle analysis of the cumulative
Section 5). The quantity of sample required to distribution function proposed by Rosin and
give a DR reading of between 30 and 50 units is Rammler [31] to describe the size distribution
determined and this is used as a basis for a of particles resulting from grinding coal. The
ferrogram sample made up to contain three times similar but more familiar form associated with
that quantity. The initial sample should be the name of Weibull has been shewn to be appli-
subjected to at least 30 seconds in an ultra- cable. The value of the exponent 'n' is usually
sonic bath to ensure the dislodging of particles about unity, indicating a large preponderance of
which frequently settle-out, becoming attached the smaller particles. In the case of a heli-
to the bottom of the container. copter gearbox [32], a bi-modal distribution
Great care should be taken to ensure that was indicated.
spurious results do not arise from the introduc- The movement of grease within rolling-
tion of stray particles from nominally clean contact bearings is normally confined to a small
solvents, the absolute cleanliness of which quantity in close contact with the rolling ele-
often leaves a lot to be desired. Jones [25] has ments and races, whereas the remainder is sub-
observed a number of times that the absence of stantially unaffected by the movement of the
stabiliser in a solvent has resulted in wear bearing. In these circumstances extreme care
debris being attacked by acid arising from sol- is necessary to ensure that samples are repre-
vent degradation. sentative. However, Bowen et al [33] have
The completed ferrogram may be studied using demonstrated that, once adequate precautions
a wide variety of techniques. Simple numerical have been taken, useful information may be
data can be obtained using an optical densito- obtained concerning the condition of grease-
meter to assess the increase in optical density lubricated appliances.
arising from the deposition of particles on the A number of laboratories have now developed
slide. Visual examination at low magnification procedures for the study of functional machines,
(through a 100X lens for example) can enable an notably diesel-engines and gear-boxes, and a
evaluation to be made of the general distribution consensus of practice is now beginning to emerge
of particles, whereas higher magnifications (using [34]. Table I summarises some early results
a 1000 X lens) enables individual particles to be published by Senholzi [35].
studied. It is instructive to view samples by A convenient source of reference regarding
transmitted light as well as by reflected light the nature and origin of wear particles has
because many particles are translucent, being been produced by Anderson. The reference is as
composed of oxides or friction-polymers [23]. follows:- Anderson,D.P. (1982) 'Wear Particle
The microscope supplied with the ferrograph is Analysis (Revised)', Naval Air Engineering Center
provided with a system of bi-chromatic illumina- Report No 92-163, Washington, D.C. (Copies are
tion permitting the simultaneous use of trans- also obtainable from Telus Corporation, 4 Dear-
mitted and reflected light together with polari- born Road, Peabody, MA 0160, U.S.A.)
sers for the study of biréfringent non-metallic
particles. 5 THE DIRECT READING FERROGRAPH
Laboratory studies [27] have shewn that the
nature of particles on ferrograms can be deter- In contrast to the analytical ferrograph which
mined from temper colours which result from depends upon scientific interpretation of a
heating the slide on a hot plate for say 90 wealth of data, the 'Direct-Reading' ferrograph
seconds. It has been shewn [28] in diesel is intended for use by staff at technician level
engine studies that by heating the ferrogram to and is arranged to provide results in numerical
320°C for 90 seconds, low carbon steel goes blue, form. This is achieved by passing a sample of
cast-iron becomes straw-coloured, whereas alumin- the fluid containing wear debris through a glass
ium, chromium and molybdenum remain white even tube situated within a magnetic field which
6
causes ferrous wear particles to be deposited. control devices. The concentration readings
The magnetic field is so proportioned that nearly are available in two ranges, a coarse range
all of the larger particles (greater than 5 mi- from 0 to 1000 parts per million, and a fine
cron) are deposited at the entry region and range from 0 to 100 parts per million for use
smaller particles are distributed along the tube. when low rates of wear are involved.
Two optical paths are provided, the first being Experimental evaluations of the on-line
arranged to measure the large particles, and the instrument have been published[36] [37]. Whilst
second, situated 5mm further down the tube, pro- wear concentration may be employed to assess
vides a second measure which can be used to pro- the amount of wear debris present in a system,
vide some indication of changes in particle the quantity L-S is recommended as a measure of
distribution. A convention has arisen of refer- the severity of wear where L and S are the
ring to the density readings as DL and D5 res- recordings of the concentrations of 'large' and
pectively; DL being the reading taken from a 'small' particles respectively.
position nearest to the entry of the fluid. The
sum (DL + D$) measures, of course, the total
7 FUTURE DEVELOPMENTS - MEDICAL FERR0GRAPHY
deposit and a large value for the quantity
C^L - D§) might represent a severe wear situa-
The fact that ferrography is limited to the
tion wherein large particles might predominate.
study of magnetic particles is generally advan-
The product of these two quantities possesses
tageous insofar as most samples of lubricating
some plausibility as a "Wear Severity Index" but
oil contain many particles of all descriptions
the fact that this is equal to (DL) - (D$) may
which complicates analysis. The ferrograph
have merely algebraical significance. Neverthe-
acts as a sorting device, selecting those parti-
less the parameter in this form has been gener-
cles which are usually of greatest interest
ally accepted.
from the wear point of view. There are occa-
The reproducibility of early models of the
sions, however, where interest centres on non-
direct-reading ferrograph has been enhanced by
ferrous and even non-metallic particles. Thus
increasing the average area of light density
in the case of a hydraulic system, the main
from a circular shape to a rectangle with a
hazard to be safeguarded against is the deterior-
larger major dimension than the original dia-
ation of polymeric seals. Particles arising
meter. Reproducibility of better than 5% is now
from such deterioration may be made to act
attainable [25].
magnetically by being reacted upon by certain
The procedure known as the 'opaque oil
salts of the rare-earths, Erbium for example.
method' is now recommended for all determinations.
These fluids are now commercially available and
This involves passing solvent through the instru-
have been used successfully, but results in the
ment to set the initial readings before passing
engineering field have not yet been published.
the sample to be evaluated. After passage of
Results in the medical field are more read-
the sample, the tube is washed by solvent before
ily available and merit study, not only because
the final density readings.
of their own great inherent importance, but
because of the potential of the methods for
6 THE "ON-LINE" FERROGRAPH application to engineering studies.
The first medical ferrograms were made
The analytical ferrograph and the direct-reading from synovial fluids [38] [39] aspirated from
ferrograph both require to be provided with prosthetic hip joints. The wear particles pro-
samples drawn from the machine under observation, duced from the metallic components of prostheses
require laboratory conditions and the attention were readily recovered on ferrograms. In addi-
of trained operators. The on-line ferrograph tion, many non-metallic wear particles were
requires none of these, is installed on the deposited, probably due to the presence of min-
machine itself and provides its output in elec- ute ferromagnetic inclusions arising from the
trical form which can be arranged to automati- prosthetic material.
cally operate protective apparatus. The extension of the study to natural
Although the on-line ferrograph relies on joints was made possible by the development of
the extraction of debris from a sample by mag- solutions which can produce positive magnetic
netic means, the method of measurement is diff- susceptibilities in cartilage, bone and even
erent from that used in either the analytical soft tissues. Ferrograms made from synovial
or direct-reading instruments. Instead of taking fluid may be examined by the usual techniques
a fixed volume of sample and determining its including scanning-electron-microscopy. Mechan-
content of debris, the on-line instrument contin- ical erosion is a prominent feature of the
ues to pass fluid until a prescribed amount of destruction of most osteoarthritic joints. By
debris has been precipitated. This is measured harvesting wear particles, ferrography selecti-
by a capacitance sensor. When the accumulated vity gathers pathological material from the
debris reaches a predetermined level, the flow is complex intra-articular milieu. Analysis of
interrupted and the unit is automatically flushed these aberrant particles may yield clues about
before the cycle is repeated. The severity of their mode of generation and so contribute fur-
wear is therefore inversely proportional to the ther to the understanding of arthritus [40].
length of the cycle. A further development of ferrographic tech-
The magnetic field is graduated and the nique in the biological field is the concentra-
sensing device is duplicated so that both large tion and separation of bacteria and cells.
and small particles may be investigated. The Several solutions containing erbium chloride
duration of the measuring cycle can vary from have been developed and used to precipitate
30 seconds to 30 minutes, depending on the con- various species of bacteria by ferrography [41].
centration of wear debris within the oil.
Instrumental displays of concentrations of small
8 DISCUSSION
debris (less than 2 microns), large debris and
wear concentration are provided and appropriate
The analytical ferrograph has now become esta-
outputs can be provided to operate warning or
blished as a basic working tool for wear studies.
7
It is essentially a scientific instrument requir- enhance the use of ferrography)" published in
ing skill and experience on the part of the oper- Wear, volume 84 (1983), pages 111-113 4 repro-
ator although much of the labour of analysing duced by permission.
results may be avoided by using the Quantimat. References
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under well established conditions, there may be ZHANG, L.R. and VOOK, R.W. 'Wear debris
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For the output to be of value it must be related Massachusetts Institute of Technology, 1978,
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8 ACKNOWLEDGEMENT particle in laminar pipe flow', Bulletin
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Figures 2 and 3 from "Advances in ferrographic
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International Conference on Advances in (40) MILLS, G. and HUNTER, J. Ά preliminary
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can (1974), 230, 88-97. gation of iron content of lubricating oil
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3
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(29) ROYLANCE, B.J. and POCOCK, G. 'Application
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(30) POCOCK, G. 'Machinery health monitoring
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(32) ROYLANCE, B.J. and VAUGHAN, D.A. 'Multiple
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shown in inset. .
Ferrography, University College, Swansea,
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(33) BOWEN, E.R., BOWEN, J.P. and ANDERSON, D.P.
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cated systems', National Lubricating Grease
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Height of siphon
(34) HAMPSON, L.G. 'The interpretation of data
positioned to achieve
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Small bore capillary
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wear measurements by on-line ferrography', "Direct Read*
Proc. First International Conference on kit. -^
Short section cut from the
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Fig. 2. Syphoning arrangement for ferrogram preparation, after
Jones [25]
9