Shear-induced migration and clustering
mechanisms of kaolinite clay particles under the
oscillating face seal
by
Jinchul Hong
Submitted to the Department of Mechanical engieneering
in partial fulfillment of the requirements for the degree of
Master of Science in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Sep 2000
© Jinchul Hong, MM. All rights reserved.
The author hereby grants to MIT permission to reproduce and
distribute publicly paper and electronic copies of this thesis document
in whole or in part.
MASSACHU)SETTS INSTITUTE
OF TECHNOLOGY
SEP 2 0 2000
LIBRARIES
Author .........
Department of Mechanical engieneering
Aug 4, 2000
Certified by.....
L.Mahadevan
Associate Professor
Thesis Supervisor
A ccepted by ..........
.........
.............
Ain A. Sonin
Chairman, Department Committee on Graduate Students
BARKER
Shear-induced migration and clustering mechanisms of
kaolinite clay particles under the oscillating face seal
by
Jinchul Hong
Submitted to the Department of Mechanical engieneering
on Aug 4, 2000, in partial fulfillment of the
requirements for the degree of
Master of Science in Mechanical Engineering
Abstract
The migration and clustering mechanisms of clay particles under the contact band of
an oscillating face seal are investigated. There are two seperate stages in the wear,
which are called the break-in period and the aggressive wear period. First small particles in the slurry penetrate through a short distance under the seal lip, where a thin
oil film is present and there is no sign of significant wear. This period is called breakin period. However, after a number of cycles, the particles aggregate and start to
form clusters. These clusters grow and the seal wears rapidly. Thus cluster formation
plays a critical role in the whole wear process.
As a first step, we have compared the possible forces which drive particles inwards.
These include the effects of Van der Waals interactions, hydrodynamic shear, Brownian diffusion, etc. In the initial stages of the process, when the particle concentration
is not high, prior experiments are consistent with shear-induced migration as the primary mechanism. Particles migrate inwards due to gradients in concentration, shear
rate, viscosity and the effects of curved-streamline. Van der Waals force are likely
to be dominant when the concentration is high enough so that the distance between
the particles is small. In order to confirm these mechanisms, a new experiment using
steel seals was carried out and corroborates earlier studies.
Finally, some clustering and dispersion mechanisms such as the effects of dissolved
ions (leading to aggregation) and thermal expansion of the seal lip (leading to dispersion) were also studied.
Thesis Supervisor: L.Mahadevan
Title: Associate Professor
2
Acknowledgments
I would like to show my appreciation to Professor L. Mahadevan for his advice and
efforts on this work. I could learn the significance of physical meaning and importance
of logical way of thinking from him.
I wish to thank Professor D. P. Hart, who provides me nice environments for research
as well as his advice.
I was fortunate to work with Dr. Mart Tamre. I respect his attitude of balancing the
experiment and theory.
Essential part of this work was collaboration with other people, especially in our
project group and CAT people. Especially I wish to thank George Costa, Kristy
Johnson, Mark J Kiesel and Keith Beckman for their sincere help. Without them,
this work would be impossible.
I also was blessed to have many people who made my life enjoyable at MIT (Seonghwan hyung, Hyunjong nuna, Junmo, Seongmoo and other Korean students). I am
grateful with my office mates (Steve, Brian, Prakash, Hang, Heather, Tom, Ting,
Chengsu, Jeremy, Mats and Jin) for sharing times.
I thank many friends in First Korean Church in Cambridge for their prayer and love.
I want to thank Taekyun and Joosung hyung for being my spiritual role model. I
thanks for all fellowships, especially with 76's.
Thanks to my friends in Korea. The memories with them always cheer me up. Many
thanks to Jinduk hyung, Jungmin, other IVF hoobaes and friends in KAIST for their
concerns.
Most of all, I wish to express my deepest appreciation to my father, mother and
brother. I hope to thank my fiancee, Heeyoung for always being there for me. Their
love and support have been the origin of my strength through my efforts.
3
Contents
1
2
3
Introduction
12
1.1
O bjectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
1.2
Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
Analysis of prior experiments
16
2.1
Description of track seal assembly . . . . . . . . . . . . . . . . . . . .
16
2.2
Experimental Procedure . . . . . . . . . . . . . . . . . . . . . . . . .
18
2.3
Results from prior experiments
20
2.4
Aggregation and dispersion mechanisms of clay particles
. . . . . . .
21
2.5
Com paring forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
2.5.1
Electrostatic forces . . . . . . . . . . . . . . . . . . . . . . . .
24
2.5.2
Comparsion of shear force with buoyancy force . . . . . . . . .
26
2.5.3
Comparison of shear force with Van der Waals force . . . . . .
29
2.5.4
Comparison of shear force with Brownian motion . . . . . . .
30
. . . . . . . . . . . . . . . . . . . . .
Evidence for shear-induced migration
31
3.1
31
Mechanisms of shear-induced migration . . . . . . . . . . . . . . . . .
3.1.1
Shear induced migration by concentration gradient and shear
rate gradient
. . . . . . . . . . . . . . . . . . . . . . . . . . .
31
3.1.2
Shear induced migration by viscosity gradient
. . . . . . . . .
33
3.1.3
Shear-induced migration by curved-streamline effects . . . . .
34
3.1.4
Comparison of various contributions to the particle flux . . . .
35
3.1.5
Role of particle shape and structure . . . . . . . . . . . . . . .
36
4
3.2
4
4.2
6
38
3.2.1
Shear induced migration in different geometries
. . . . . . .
38
3.2.2
Particle front movement during break-in period
. . . . . . .
39
3.2.3
Particle migration after penetration . . . . . . . . . . . . . .
41
3.2.4
Outward migration of particles after the breakup of clusters
43
Steel seal experiment
4.1
5
Experimental evidence of shear induced migration . . . . . . . . . .
45
Experimental procedures . . . . . . . . . . . . . . . . . . . . . . . . .
45
4.1.1
Method for alignment and control of the gap thickness
. . . .
46
4.1.2
Considerations in oil leakage . . . . . . . . . . . . . . . . . . .
51
4.1.3
Waviness measurement . . . . . . . . . . . .
. . . . . . . . . .
51
. . . . . . . . . . . . . . . . . . . . . . . . . . .
53
Experimental results
Particle aggregation and dispersion mechanisms
56
5.1
Water-Clay system . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
5.2
Additional aggregation and dispersion mechanisms.
. . . . . . . . . .
58
5.2.1
Aggregation mechanisms . . . . . . . . . . .
. . . . . . . . . .
58
5.2.2
Dispersion mechanisms . . . . . . . . . . . .
. . . . . . . . . .
62
Conclusion
66
6.1
Summary of particle interactions through whole wear process . . . . .
66
6.2
Future w ork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
A Clay mineralogy
70
B Notation and Physico-chemical values
73
5
List of Figures
1-1
Face seal and shaft seal. [3]
1-2
The configuration of contact between the seal and a glass plate. The
. . . . . . . . . . . . . . . . . . . . . . .
13
area over which the seal and the glass bushing make contact is called
the contact band. Outer diameter(OD) and inner diameter(ID) was
shown.........
1-3
13
....................................
The wear process with the number of oscillating cycles( 1 cycles/second).
Up to 2,700 cycles, the particles are ingested but do not form the clusters. Clusters can be seen from 3,000 cycles. When the size of clusters
are large enough to fill the contact gap, cluster shearing starts (3,600
cycles). The image size is 0.57 mmx0.51mm ??. . . . . . . . . . . . .
2-1
15
The configuration of the proceeding links around the drive sprocket of
the undercarriage. A tractor in the field (Right) has tracks that are
shown in a magnified view (Left). . . . . . . . . . . . . . . . . . . . .
2-2
The elastomeric seal assembly which is made of a polyurethane lip, a
polycarbonate stiffener ring, and a nitrile rubber load ring.
2-3
2-4
16
[3]
. . . .
17
Real pin-joint arragement with face seal. Both end caps are bolted to
freely moving pin lubricated by oil inside the seal.[3] . . . . . . . . . .
18
Experimental setup with mudbox. . . . . . . . . . . . . . . . . . . . .
19
6
2-5
The penetration of particles into the contact band as a function of
oscillation cycles. Seal wear is measured as the percentage of the seal
lip that the abrasive slurry has penetrated.
Clusters start to form
around the transition from break-oin period to aggressive wear period,
about 7,000 cycles. [12]
2-6
. . . . . . . . . . . . . . . . . . . . . . . . .
Clustering mechanism due to bridging of particles by a second immiscible liquid com ponent. . . . . . . . . . . . . . . . . . . . . . . . . . .
2-7
20
22
Entropic repulsive force between two particles due to adsorbed organic
m olecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
2-8
Distribution of ions adjacent to a clay surface in double layer. [28] . .
25
2-9
Double layer due to the imperfections of the crystal, constant charge
double layer(Left). Double layer due to the adsorption of specific ions,
constant potential double layer(Right). [28] . . . . . . . . . . . . . . .
2-10 The configuration of particles
3-1
. . . . . . . . . . . . . . . . . . . . . .
26
27
The oil film shape of unfilled smooth seal using LIF. The top and lower
solid lines represent the maximum and minimum lubrication measure.
The center dashed line represents the average [3]. OD is indicated in
F igure 1-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-2
Rotation of doublet without viscosity gradient(a) and with viscosity
gradient(b). [25] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-3
32
33
The collision between two hard spheres approach along curved streamline. The repulsive force acts along a line connecting the centers of
particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
3-4
Various curved geometries. The inverted cone and plate geometry has
the severest shear rate gradient. . . . . . . . . . . . . . . . . . . . . .
3-5
38
Shape of oil film thickness for 2+2 filled seal(Top), simplified geometry
of the oil film (Middle) and particle front movement under the seal
lip(Bottom). The contact band regions are divided into three regions
as show n. [3]
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
40
3-6
Particle migration after penetration occurs (Left) and side view of cross
section (Right)
3-7
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
Configurations of real track seal joint. The space between steel bushing
and seal lip surface shows inverted cone and plate geometry
. . . . .
42
. . . . . . . . . . .
43
3-8
Cluster shearing (a) and cluster disappearance(b)
3-9
Glass bushing profiles of 2+2 filled seal when small clusters appeared(a)
and outward migration occured(b).
. . . . . . . . . . . . . . . . . . .
44
4-1
The geometries of two different steel seals. All dimensions are inches.
46
4-2
Configuration of steel plate. All dimensions are inches.
. . . . . . . .
46
4-3
Glass plate with very thin silver coating 1000A.
. . . . . . . . . . . .
47
4-4
Setup of steel seal experiment for alignment and control of the gap
between the glass plate and the steel seal . . . . . . . . . . .
4-5
. . . .
48
Cross section of the steel seal and the glass plate. The distance between
the steel seal and the silver coating and the thickness of the silver
coating is exaggerated. . . . . . . . . . . . . . . . . . . . . . . . . . .
4-6
48
(a) The non-coincidence of the volatage V, V2 and V3 at the three
locations on the glass shows that the gap is not constant, i.e the gap
between the seal surface and glass plate is not aligned. (b) Voltage is
checked after alignment. The coincidence of V1, V2 and V3 shows that
the steel seal and glass plate are aligned. . . . . . . . . . . . . . . . .
4-7
Typical steel seal experiment. Slurry is provided from top or side with
a rate of 5ml/min. The observed part is indicated . . . . . . . . . . .
4-8
50
51
Waviness measurement for flat steel seal. A curve 1 is for the outmost
circle. The larger number of curve corresponds to the inner measurem ent circle.
4-9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
Waviness measurement for concave steel seal. A curve 1 is for outmost
circle. The larger number of curve corresponds to the inner measurem ent circle.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
53
4-10 Part of each seal after 3,500 cycles. The particles can migrate only
small part of the contact band for flat seal(Left) while they migrate
whole contact band except bottom part of seal for concave seal(Right).
54
4-11 Close view of particle front near the interface. The particle front penetrates immediately after slurry supply and stays at certain distance
from the outer edge.
. . . . . . . . . . . . . . . . . . . . . . . . . . .
55
4-12 Particle front movement for concave seal. The particle front moves
gradually with time, then stops near the inside diameter of the concave
seal. The data was fitted to third order polynomial. . . . . . . . . . .
5-1
55
FTIR Spectrum results and metallic ions by I. C. P. Spectro. (a) The
strong and medium intensity bands of the spectrum are consistent with
an aliphatic hydrocarbon, the base oil. Based on the presence of boron
in the oil, the oil is likely to contain potassium borate additives. (b)
The band at ~ 1165cm
1
is likely due to the existence of sulfonate,
used usually as a rust inhibitor. Based on the presence of sulfonate,
the oil is likely to contain calcium sulfonate
5-2
59
Interface area captured by CCD camera. Clusters starts to form from
the interface.
5-3
. . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
The above images show a seal operating with a dry mixture of clay
and sand added to its periphery and surfactant added to the oil with
number of cycles. No clusters were observed even after 40,000 cycles.
However, surfactant does not prevent clustering formation for a slurry
mixture based on water [3].
5-4
. . . . . . . . . . . . . . . . . . . . . . .
61
The location of thermocouples at three different parts, top, middle and
bottom. The right image shows the enlarged cross section where the
thermocouple is fitted to the hole. After inserting the thermocouple,
polyurethane sealant is spread on the seal lip surface for smoothing
surface and preventing particle concentration around the thermocouple. 62
9
5-5
Temperature at three different loacation is measured in 'C (a-c). Clusters disappear when temperature exceeds approximately 60 'C. Two
measurements at every two minutes were done to check the spatial
variation of temperature. Observe that outward migration occurs at
the same time when normal force(d) reaches a local peak. . . . . . . .
5-6
64
Average normal force is shown with temperature in three different
parts. Note that the normal force shows a sudden increment when
the temperature increases rapidly. Outward migration occurred when
the normal force reached local peaks. . . . . . . . . . . . . . . . . . .
6-1
65
Summary of abrasive wear process due to particle ingestion and cluster
formation. Related forces and effects are written for each stage.
. . .
67
A-i Silicon tetrahedron and silica tetrahedra arranged in a hexageonal
netw ork.[19] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
A-2 Octahderal unit and sheet structure of octahedral units.[19] . . . . . .
71
10
List of Tables
2.1
Summary of clustering and dispersion mechanisms for clay suspension
23
5.1
Anions in Water investigated by ion chromatography. ASTM D4327 .
57
A.1
Physical properties and dimensions of common clay minerals . . . . .
72
11
Chapter 1
Introduction
Mechanical parts which have relative motion during contact are subject to wear and
failure. The traditional solution to delaying the wear process of mechanical parts is
the use of seals containing liquid lubricants.
Seals are classified according to their design configuration as shaft seals and face
seals [3, 30].
The main difference between the shaft seal and the face seal is the
manner in which they contact their running surface. A shaft seal contacts its running
surface on the perimeter of a shaft, while a face seal contacts its running surface
on the end face as shown in Figure 1-1. Typical applications of shaft seals involve
situations associated with high sliding speeds and low contact pressure area. On the
other hand, the face seal can sustain a much wider range of contact pressures.
1.1
Objectives
It is essential to prolong seal life to reduce the cost and time for maintenance of joints.
Improvement in seal design is aimed at delaying the wear process and eventually
leading to less frequent replacements of the seal. The price of a seal itself represents
only a small fraction of the maintenance cost, but frequent replacement means a loss
in productivity due to down time and the cost of labor. To achieve long working life
for seals, we need to understand the onset and evolution of the wear process.
12
Air side
Air sidS
Side
SI
Shaft Seal
Face Seal
Figure 1-1: Face seal and shaft seal. [3]
Stiff ner Ring
Oil
OD
Contact Band
ID
Glass Bushing
Figure 1-2: The configuration of contact between the seal and a glass plate. The area
over which the seal and the glass bushing make contact is called the contact band.
Outer diameter(OD) and inner diameter(ID) was shown.
13
Earlier work [3, 30, 22] shows that the wear process consists of three stages as
shown in Figure 1-3. The first stage is the particle ingestion process when small
particles in the mud mixture move into the contact band where a seal surface contacts
with a glass bushing plate. The second stage includes the formation of clusters as
the particles aggregate. The size of the clusters grows as more particles penetrate
into the contact band. During the third stage clusters start rolling between the seal
lip and the running glass plate. Cluster shearing seems to push the debris further
into the contact band. It is evident that cluster formation plays a critical role in the
wear process. To prevent or delay cluster formation, we need to understand particle
interactions in the oil film in the contact zone.
This thesis deals with the behavior of clay particles in a sheared suspension. Two
main subjects of our research are particle migration and cluster formation mechanisms
in the oil film under contact band. The primary goals of this work are as follows.
First, we analyze prior experimental data on the onset and evolution of face seal wear
and show that they are consistent with our hypothesis that shear-induced migration
is the dominant mechanism by which particles are ingested into the seal. Secondly, we
will present new experimental results in a simplified geometry to consolidate our hypothesis of shear induced migration. Finally, we will present some particle clustering
mechanisms that are validated by experiment.
1.2
Thesis outline
The organization of the thesis is as follows. In Chapter 2, we will analyze prior experimental work by comparing various forces. By characterizing and non-dimensionalizing
forces, we show that the dominant migration mechanism in the contact zone is associated with shear induced migration. Chapter 3 deals with shear induced migration
and evidence of supporting experimental results. To confirm shear induced migration
theory, we have devised a new steel seal experiment with simplified surface profiles
and discribe these experiments in Chapter 4. Chapter 5 will deal with some mechanisms for particle clustering in the face seal and present experimental evidence for
14
the same. The last chapter includes a brief summary, conclusions with suggestions
for future work.
1,200
2,100
1,500
2,400
1,800
2,700
Particle
Front
Cluster
Shearing
Cluster Starts
3,000
3,300
3,600
Figure 1-3: The wear process with the number of oscillating cycles( 1 cycles/second).
Up to 2,700 cycles, the particles are ingested but do not form the clusters. Clusters can
be seen from 3,000 cycles. When the size of clusters are large enough to fill the contact
gap, cluster shearing starts (3,600 cycles). The image size is 0.57 mmx0.51mm ??.
15
Chapter 2
Analysis of prior experiments
2.1
Description of track seal assembly
The oscillating face seal is used in the pin-jointed tracks of various type of machines
at Caterpillar, Inc. The pin joint can rotate freely within the bushing when the link
proceeds around the drive sprocket of the undercarriage (Figure 2-1).
Link
Joint
Figure 2-1: The configuration of the proceeding links around the drive sprocket of the
undercarriage. A tractor in the field (Right) has tracks that are shown in a magnified
view (Left).
Each pin joint fits into a steel bushing which contacts the seal lip surface (Figure 23). After the seals are pressed into the end caps, the seal assembly (seal and end
caps) is bolted to the both ends of the pin. Finally, the seal surface contacts the
steel bushing with a high normal pressure, which maintains the lubricant between
16
the pin and bushing and is supposed to prevent contamination of the lubricant from
the environment of mud, soil etc.
84 mm
Stif
ner ring
load ring
Figure 2-2: The elastomeric seal assembly which is made of a polyurethane lip, a
polycarbonate stiffener ring, and a nitrile rubber load ring. [3]
The seal itself consists of three separate components (Figure 2-2). They are the
load ring, the stiffener ring and seal lip. The load ring is made of nitrile rubber and
pressed onto the end cap to maintain high contact pressure between the seal and
bushing surface. The stiffener ring is made of a glass fiber filled polycarbonate and
is much harder than seal lip material (E, = 0.45MPa), polyurethane. The stiffener
ring fits into the load ring to transfer the force from the load ring to the seal lip. The
seal lip (E = 3.45MPa) is softer compared to the stiffener ring and directly contacts
the steel bushing surface and contains the lubricant.
The motion of the seal is oscillatory as the links move around the wheel (Figure
2-1). The frequency of oscillations is as low as the speed of link movement. In our
experiment, the typical frequency is 1 cycle/sec. The tractors are subject to various
operating condition, usually submerged in clay or sand particles. Such particles are
responsible for the abrasive wear of the seal.
17
end cap-
bushing
10 cm
Figure 2-3: Real pin-joint arragement with face seal. Both end caps are bolted to
freely moving pin lubricated by oil inside the seal.[3]
2.2
Experimental Procedure
Our experimental setup is shown in Figure 2-4. The main difference between the
experimental setup and the real track joint is the bushing surface. In our experimental
setup, a glass plate was used instead of a steel bushing to enable the recording of the
whole wear process via CCD camera. Although the surface characteristics of two
surface should be different, it is believed that the abrasion patterns and whole wear
process are the same by Ayala [4]. The camera is connected to a frame grabber and
send a video signal to a 8-bit gray scale picture 512 by 460 pixels in size. When we
change the microscope, we calibrate the image size by using a micro-ruler.
Before the experiment, the glass plate is cleaned with alcohol to remove any dirt
on the surface. The glass plate is glued to the steel plate using epoxy and cured for
two hours. The steel plate is held in place by tightening three bolts at the ends of
three long rods. Once we observe first contact of seal lip surface with the glass plate,
we place three displacement sensors on the steel plate as shown in Figure 2-4. Each
bolt is adjusted manually to ensure the alignment of the contact surface between
the glass plate and the seal assembly. The seal assembly is pushed with a typical
18
Turning
Arm
Steel rod
Bolt
Mud
box
V?
CCD Camera
Displacement
sensor
Seal
Sens r
Adhesive
sealant
Glass plate
Figure 2-4: Experimental setup with mudbox.
normal load 1700N and monitored by a sensor which also can measure the torque and
normal force in real-time. The sensor was calibrated with prior experiment data for
0
the displacement and normal load, and showed good agreement.
After imposing the load, a mud box is inserted to contain the mud mixture outside
edge of the seal. Oil of viscosity 0.32 Pa s is filled into the inside of the seal assembly
through a cavity in the rig. After the sealant is cured, we pour slurry into the mud
box. By weight, the slurry is made of approximately 30 % of kaolinite clay, 40 %
bank sand and 30 % water with very small amount of Cabosyl and salt. The mud
box environment is believed to provide realistic field conditions. The seal assembly
and seal mount oscillate with an amplitude of 30
0
at a frequency of 1Hz (1 cycle/sec).
With typical loading conditions, the mean velocity of the seal is 22 mm/sec and the
19
maximum shear velocity is 70mm/sec. The amplitude can be controlled by adjusting
the length of the turning arm. The frequency of oscillation can be controlled by
changing the speed of electric motor which is connected to the arm.
2.3
Results from prior experiments
The wear process can be divided into two periods according to the wear rate as
shown in Figure 2-5. During the particle ingestion stage, there is no significant wear.
On cluster formation, penetration of particles into the contact band proceeds rapidly.
The clusters of particles press against the seal lip locally reducing the contact pressure
between the seal lip and bushing plate allowing larger particles to penetrate under
the contact band [12]. These two distinct wear periods are called the break-in period
and the aggressive wear period [10].
1
30%
:
25%
Break-i
20%
-
per _d
we r peri
peri
v wrtaggr
pggres.ive
C
10% -----------
Cluster formation
(D
I
C
%
1% Break-in
0
2,000
------
period-
4,000
_
6,000
8,000
10,000
12,000
Number of oscillation
Figure 2-5: The penetration of particles into the contact band as a function of oscillation cycles. Seal wear is measured as the percentage of the seal lip that the abrasive
slurry has penetrated. Clusters start to form around the transition from break-oin
period to aggressive wear period, about 7,000 cycles. [12]
Ayala also investigated the chemical composition of clusters by using an electron
microscope and a x-ray analyzer [3].
He extracted clusters carefully after the ex-
20
periment with normal loading conditions and removed oil in clusters by rinsing with
acetone. Comparing the spectrum for each sample, he found that clusters were made
of mainly clay particles with small amount of sand. From the experimental results,
we can focus on the clay particles since we are interested in ingestion mechansisms
and cluster formation.
2.4
Aggregation and dispersion mechanisms of clay
particles
The clay particles experience various types of interactions between the particles and
the suspending medium. The interactions can be categorized into the favorable and
unfavorable mechanisms for particle aggregation. The various factors can be summarized as the followings.
Van der Waals attraction force exists between dispersed particles. The magnitude of this force depends on various factors, such as the size and shape of particles
and the nature of the suspending medium. For instance, the potential energy due
to Van der Waals attraction between induced dipoles is U
-
while U ~
1
between parallel plates [23].
Kaolinite particle can have positive charges at the edges depending on the dispersion medium. On the other hand, the flat face of kaolinite particle is negatively
charged due to the isomorphous substitution (See Appendix A for details). When the
particles are dispersed in a liquid medium which has ions, they carry double layers
of opposite charge and attract one another electrostatically. Such type of clustering
mechanisms due to screened electrostatics can occur in both hydrous medium and
organic media if additives which form ions are present [28].
Another important mechanism for particle aggregation in our system is the bridging of particles by a second immiscible liquid component [28]. If the dispersed
particles are wetted by polar liquid such as water, the liquid is believed to envelop
the particles with very thin film. When two particles come together in the immisci-
21
Oil
Oil
Particle
Particle
Particle
Particle
Water Film
Junction
Figure 2-6: Clustering mechanism due to bridging of particles by a second immiscible
liquid component.
ble liquid medium such as oil, their water films tend to combine together to reduce
interfacial area. As our clay particle first contacts water then penetrates the oil film
under the contact band, this mechanism can be important.
Electric double layer repulsion is regarded as the most important repulsive
force in common clay systems. When two particles carrying same kind of electric
double layer approach each other in the suspension, the interference leads to a repulsive force between the particles. Electric repulsive forces are not restricted to hydrous
system, but can also exist in organic dispersions with enough ions as well.
When long chain of molecules or polymers are adsorbed on the clay surface especially in organic compounds, entropic repulsion can lead to dispersion of particles
as shown in Figure 2-7. If the particles with adsorbed molecules approach each other
to within a distance (d) which is shorter than twice the length of adsorbed molecules
(L), the molecules begin to interfere. The adsorbed molecules can move freely before
interference, which means the system has less entropy after interference. Since the
system changes in the direction of increased entropy, the particles will tend to separate again [19, 28]. Such entropy effects are manifested as the repulsive force between
the particles. This repulsive force can occurs for small particles at moderately large
distance to overcome strong van der Waals force. Entropic repulsion force is expected
22
Adsorbed organic compounds
Clay particle
L
VVVV~
d
Figure 2-7: Entropic repulsive force between two particles due to adsorbed organic
molecules.
to contribute to repulsive forces in oil film under the contact band due to adsorption
of organic compounds.
Brownian motion due to the thermal motion of the liquid molecules surrounding
the particles can be contributed to disperse the particles when the particle size is small
and shear rate is low [24].
Clustering mechanism
Dispersion mechanism
Van der Waals force
Screened electrostatics
Inter-surfacial tension
Electric double layer repulsion
Entropic repulsive force
Brownian motion
Table 2.1: Summary of clustering and dispersion mechanisms for clay suspension
23
2.5
Comparing forces
Next we compare the order of magnitude of the various forces associated with aggregation and dispersion.
2.5.1
Electrostatic forces
The origin of the electric attraction and repulsive force comes from diffusive double
layer. The double layer consists of the particle charge and an equivalent amount of
ionic charge which is accumulated in the liquid near the particle surface [28]. There
are two kinds of electric double layers, namely constant-charge layers and constantpotential double layers.
The constant-charge double layer is formed as follows. Clay has a net negative
charge as a result of isomorphous substitution. Such a net negative charge is compensated by cat-ions which are located on the layer surfaces. If the particles dissolve
in liquid with the cat-ions, these compensating cat-ions tend to concentrate near the
particle surface due to electrostatic attraction. At the same time, due to the concentration gradient, the cat-ions surrounding the particle surface tend to diffuse away
from the particles. These two opposing trends result in the creation of an atmospheric
distribution of the compensating cat-ions in a diffuse double layer on the exterior layer
surface of a clay particle. The distribution of ions adjacent to a clay surface which
represents the concept of the diffuse double layer is shown in Figure 2-8. The electric
double layer on the layer surface has a constant charge which is determined by the
type and degree of isomorphous substitution. As a result, the layer-surface charge
does not depend on the presence of electrolytes in the suspension.
The constant-potential double layer is created by the preferential adsorption of
certain specific ions on the particle surfaces. Adsorbed ions are called potential determining ions. These ions consist of the inner coating of the electric double layer and
result in equivalent amount of counter ions accumulation near the charged surface.
The charging process requires the presence of a sufficient amount of an electrolyte
containing the kind of ions which are adsorbed on the particle surface. Therefore,
24
1
0E
0
0
(D0-
G0
0G_
s
(D_
_
Distance
Figure 2-8: Distribution of ions adjacent to a clay surface in double layer. [28]
this electric double layer depends on the presence of electrolytes in the suspension.
These two different kinds of double layer are shown in Figure 2-9.
For our clay, kaolinite, the flat layer surfaces has net negative charge by isomorphous substitution. However, at the edges of the plates, the tetrahedral silica sheets
and the octahedral alumina sheets are disrupted and have broken bonds. At such a
broken edge, it is known that an electric double layer can be created by the adsorption of potential-determining ions under certain conditions [20]. Such conditions are
determined by pH of the solvent and amount of specific ions available in the suspension. It can be concluded that the clay particle has the complicated structure with
two different signs of particle surfaces carrying different types of electric double layer
under certain conditions.
When two particles approach each other in suspension due to shear motion or
Brownian motion, their electric double layer begins to overlap. Such interference
results in the change of the ion distribution in the double layer of both particles,
which involves an increase in the free energy of the system. It means work must be
done to bring the particles together and a repulsive force exists during the process.
Such repulsive forces will decrease with increasing electrolyte concentration due to
the compression of the double layer thickness [19]. The Debye screening length 1/r's,
the distance from the surface to the center of gravity of the double layer, is taken as
25
its thickness [19].
PARTICLE /
SOLUTION
PARTICLE /
SOLUTION
Figure 2-9: Double layer due to the imperfections of the crystal, constant charge
double layer(Left). Double layer due to the adsorption of specific ions, constant
potential double layer(Right). [28]
In addition to the repulsive force, we can expect there exists electrostatic attraction forces due to opposite sign of electric double layer adjacent to particle surface.
These particles will attract each other electro-statically. Such electrostatic attractive
forces would occur due to the different signs of double layer at the edge and on the
flat surface of clay particles.
2.5.2
Comparsion of shear force with buoyancy force
We compare the hydrodynamic shear force with the buoyancy force. In order to
simplify the model, we assume kaolinite particle is a circular plate. In that case, the
buoyancy force acting on the particle is calculated [2],
Fb ~ $(pp - pf)g2tR 2
(2.1)
where pp is a density of a particle, pj is a density of suspending fluid, q is the
concentration of particles, R is a radius of particle and t is a thickness of particle.
The next step is to assume the configuration of particles.
For simplicity, we
assume a uniform distribution of particles and regular lattice structure(See Figure
26
Useal
Contact Gap
Glass plate
R
U(ho) + dU
Z-
t
H
U(ho)
zi -
U(ho) - dU
R
H
t
w
Figure 2-10: The configuration of particles
2-10).
The important assumption for this model is the ratio of gap between the
particles in horizontal direction(w) to in vertical direction(H) is same as the ratio of
horizontal dimension of particle(radius, R) to vertical dimension of particle(thickness
t). This assumption corresponds to the uniform concentration of particles. Then the
horizontal gap between the particles is
RH
t
(2.2)
With our model, concentration of particles in suspension can be defined in equation (2.3)
27
2wrR 2 t
2$
(2R + HR/t)2 (2t + H)
=t
(2.3)
From the equation for characteristic shear force, we define the dimensionless number as the ratio of characteristic shear force to buoyancy force as
dUR
2
(2.4)
H
phgt
N
so-Fb
where
1
(2.5)
$(p - Pf )gt
/H is the shear rate assuming linear velocity
= Usea/Contact Gap = dU
profile.
On the other hand, there is another repulsive hydrodynamic force due to the
squeezing of oil between the particles. From mass conservation, the order of the
squeezing velocity and hydrodynamic repulsive force can be characterized as,
(2.6)
UseaH
Usq
R
(27
R 33
fpUseai
2
Fs
(2.8)
s= =H
FsPuseaIR
N
2
$(pp - pf)gtH
Fb
where Nq is the ratio of characteristic squeezing force to buoyancy force. To get
the order of dimensionless number Nsb and Nsq, we only need the value of H as the
range of size, thickness of particle and other properties are already known. For H, we
need to solve equation (2.3). Typical values for each parameter are listed in Appendix
B.
For kaolinite clay particle,
Nsh
106
-
10' and Nsq
-
106
-
10.
It shows that the
hydrodynamic forces, shear force and squeezing force dominate the buoyancy force
over the entire range of concentration.
28
2.5.3
Comparison of shear force with Van der Waals force
The attractive energy per unit area between two infinite parallel plates yields the
following equation [5].
A
Va
1
A
48w d2
+
1
(d + 6)2
2
2
(d + 6/2)2
(2.9)
where d is a half distance between plates, 6 is the thickness of the plate and A is
a Hamaker's constant ~~0(10-20).
In our case, kaolinite, the theoretical values of Hamaker constant is 3.1 x 10-
20 j
[21]. Equation (2.9) is not exact due to the frequency dependence of Van der Waals
force, but is a good approximation at sufficiently high concentrations. Using (2.9),
the attractive force per unit area is
F
= Dan
Va ~~
d3
Od
(2.10)
The dimensionless number which characterizes the ratio of the shear force to Van
der Waals force is
F
Area
NV
p
h
Fvan
(2.11)
pyd3
A
(2.12)
Using the same model for particle configuration in Figure 2-10, H=2d(Figure 210). The results show that hydrodynamic shear force dominates the Van der Waals
force for the lower concentrations and large particles, since Nv ~ O(103)
-
O(105).
However, as the concentration increases and the size decreases, Van der Waals
force can become dominant. Ayala's experiment [3] shows that particles immediately
penetrate under the seal lip and the concentration of particles started to increase as
time goes by. After a certain number of cycles, it starts to form clusters when the
concentration of particles increases.
In addition, the size of the ingested particles
29
before cluster formation seems to be small enough to be in the region where Van der
Waals force dominates. It is consistent with the prediction.
2.5.4
Comparison of shear force with Brownian motion
Clay particles in suspensions experience random motion as a result of the thermal
motion of the liquid molecules surrounding them. The stability of clay suspensions is
controlled by Brownian motion at low shear rates.
In order to compare the effects of shear force with Brownian motion, we use the
Peclet number defined as the ratio of the Brownian time scale to the time scale
characterized by the reciprocal of the mean shear rates [24].
k3
Pe-
(2.13)
where p is the viscosity of the suspending medium, R the characteristic particle size,
- the shear rate, k the Boltzmann constant(= 1.380 x 10- 2 3 J/K -molecule), and T
is the absolute temperature by Kelvin.
As the change of viscosity with temperature is not known in our oil, we decided
to compare only the lower limit of Peclet number. The viscosity of our oil at 100 0 C is
approximately 0.032 Pa - s. The minimum of Peclet number can be obtained at high
temperature and small particles. The order of Peclet number is O(107). Therefore
the effects of Brownian motion are negligible compared to the shear force.
30
Chapter 3
Evidence for shear-induced
migration
In this chapter, we will review some of the basic mechanisms of shear-induced migration with the goal of understanding how they relate to the problem of particle
ingestion and cluster formation.
In our geometry (Figure 3-1) , the shape of oil film thickness can be approximated
to parallel plate geometry. There are three mechanisms of shear-induced migration
for parallel plate geometry. We will review about shear induced micro-structures to
get insight into their role in shear induced migration. Then we will present several
pieces of evidence of shear induced migration in our system.
3.1
3.1.1
Mechanisms of shear-induced migration
Shear induced migration by concentration gradient and
shear rate gradient
To explain shear-induced migration of particles in concentrated suspensions in a Couette device, Leighton and Acrivos [17] suggested a migration mechanism due to the
effects of a spatially varying inter-particle interaction frequency. The interaction frequency is related with concentration of particles(o) and shear rate()
31
as the former
0.8
086
E
0,7
03
0,2
0
0
0.2
0.4
0.6
distance from OD [mrn
0.8
1
Figure 3-1: The oil film shape of unfilled smooth seal using LIF. The top and lower
solid lines represent the maximum and minimum lubrication measure. The center
dashed line represents the average [3]. OD is indicated in Figure 1-2.
represents the number of particles and the latter characterizes the frequency of collisions.
With the assumption [25] that the particles in a sheared concentrated suspension
move affinely, on average, the number of collisions per unit time per unit volume
is
0. Then the variation in the collision frequency over the characteristic length
4
which is the particle radius, a, is aV(#). If we assume again the particle migration
velocity is linearly proportional to this variation of the interaction frequency and the
displacement per collision is of the order of a, we obtain an expression for the particle
flux.
Jc = -Kca
2
(0 2Vy + #$,7#)
(3.1)
The first term in (3.1) represents the migration due to shear-rate gradients while
the second term expresses the effects due to concentration gradients.
32
3.1.2
Shear induced migration by viscosity gradient
The viscosity of a suspension shows a strong dependence on the concentration. This
results in the migration of particles due to viscosity gradients as shown in Figure 3-2.
When two particles come into contact in a shear flow, they rotate until they are
separated. The center of rotation of the doublet is the midpoint of the line connecting
the sphere centers in the absence of a viscosity gradient. A viscosity gradients leads to
a differential resistance to rotation, so that the center of rotation moves the direction
of higher viscosity. This results in a net displacement from the higher viscosity region
to the lower viscosity region, as shown in Figure 3-2(b).
o
O
Low
Viscosity
~High
Viscosity
(a)no viscosity gradient
(b)viscosity gradient exists
Figure 3-2: Rotation of doublet without viscosity gradient(a) and with viscosity gradient(b). [25]
With the assumption [25] that the migration velocity is proportional to the change
in viscosity over the characteristic length scale(a), and the assumption that the displacement per interaction scales as the particle size, the resulting migration velocity
is ('#)(a2 /77)V71.
Multiplying the migration velocity with the concentration
#
which
represents the number of particles per volume, the particle flux due to the viscosity
gradient is
J=K
-2a 2}
(3.2)
1
If we use a phenomenological equation for the viscosity [16] as a function of concen-
33
tration,
(3.3)
77 qO(1 - o/m)-1.82
where rhO is the viscosity of the suspending medium and
#m is the
maximum packing
fraction, which is about 0.68 for spheres. Using (3.3), (3.2) we can write Vo
J7
3.1.3
V
2ad2
,q do
= -K
(3.4)
Shear-induced migration by curved-streamline effects
For a parallel plate geometry of uniform gap with uniform initial concentration, the
previous two shear induced migration mechanisms will induce radially inward particle
migration. However, the experimental observations of Champman [6] and Chow [1]
with parallel plate geometry showed uniform concentration profiles for monodisperse
suspension suggesting that there is no net migration from high shear regions to lower
shear regions after a short initial transition.
It was suggested by Krishnan et al
[9]
that particles interacting in a shear flow
with curved streamlines of non-uniform curvature, would migrate towards regions
of lower streamline curvature, i. e. radially outward in the parallel plate geometry.
Figure 3-3 shows the interaction between two particles along a curved streamline.
When two equal-sized particles come to close, the repulsive force due to the presence
of surface roughness on the particles [18] and hydrodynamic squeezing forces keeps
two particles apart. The repulsive force acts along a line connecting the centers of two
particles and can be divided into two components. The direction of one component
is tangent to the streamline and the other is normal to it. These repulsive forces
result in a net radially outward displacement in a curved-streamline shear flow. The
migration velocity, Us, scales as a2 /R and follows from the geometry of the situation.
Therefore, the flux due to curved streamline effects is
Jcse =Kcse
ya2 02
R
(3.5)
in a radially outward direction. When the total flux, Jc + J,, + Jcse = 0, we get a
34
Repulsive
Force
Un
a
Ut
Streamline
R: Curvature of
Streamline
Figure 3-3: The collision between two hard spheres approach along curved streamline.
The repulsive force acts along a line connecting the centers of particles
steady radial distribution of particles in a parallel plate geometry.
3.1.4
Comparison of various contributions to the particle flux
We compared the effects of shear induced migration mechanism due to concentration
gradient with viscosity gradient. The appropriate dimensionless number, which is the
ratio of flux due to concentration gradients to the flux due to viscosity gradients is
-
J,
$ do
(3.6)
0 dr
In order to estimate the order of Nc,,, we use the effective viscosity equation (3.3)
which leads to
Nc m -- c _ O
NiJ,
#
-
1
1) 1
1.82
(3.7)
If a different effective viscosity equation [27] is used, this leads to
S= rO
1 - 1.3510
4
)2.493
1 - 0.349#
35
(3.8)
NcL
N
-
Jc
itJ,
_
__-248(3.9)
(1 - 1.351#)(1 - 0.349#) 1
2.498
#
Now N, is solely a function of concentration. Both of the fluxes have comparable
effects over a range of concentrations. As the direction of flux due to concentration
gradient and viscosity gradient is same, they enforce the migration of particles from
high concentration region to low concentration region. We will treat them as one flux
due to concentration gradient afterwards.
The dimensionless number, which is the ratio of flux due to concentration gradient
to curved streamline effects is
Ns =
--
RV
Jcse
(3.10)
0
Near the outer edge, there is a high concentration gradient initially until particle
penetrate resulting in Nc, >> 1. The dimensionless number, which is the ratio of
flux due to shear rate gradient to curved stremaline effects is
Ns =
-s-
Jcse
RV
'
(3.11)
For the flat plate, the shear rate at a radius r is wr/h where w is the angular
velocity and h is the contact gap height. Then,
Ns =
RV
0(1)
(3.12)
We will use above results to explain particle front movement in Section 3.3.2.
3.1.5
Role of particle shape and structure
Disk-like anisotropic particles have not been understood well in spite of their significance to industrial applications.
Recent experimental results for concentrated
kaolinite suspension using the x-ray scattering technique [14] and suspension conductivity [13] provided evidence of particle alignment increments with increasing shear
rate. The physical reason for particle alignment is the restriction of particle motion
36
due to particle interactions such as geometrical restriction, electrostatic repulsion, etc
[15].
For anisotropic particles with a large length/thickness ratio, the excluded volume
of particles is much larger than the actual volume. Thus, particle motion would be
geometrically restricted at a much lower volume fraction than the maximum packing
fraction. With 0* defined as the concentration at which the particles begin to interfere,
1/Kas the Debye screening length, d as the particle diameter and h as its thickness,
the particle has an effective diameter of d + 2/K and an effective thickness h + 2/K.
Particle interactions occur when the excluded volume fraction 0e is equal to the
maximum packing fraction of a sphere, 0.64:
e
=
4jr N d + 2K
)
(
3 V
2
(3.13)
0.64
=2K-
The actual volume fraction is given as,
lrNhd)
V2=
( )2
(3.14)
Using equations (3.13) and (3.14), 0* can be approximated by
0* = 0.96
When # >
(3.15)
(1
d (I + 2/dK) 3
#*, the particle might not rotate freely, and its motion is restricted
by other
particles. In addition to geometric restriction and electrostatic repulsion, there are
other factors for ordering such as long range interactions and hydrodynamic forces.
If the clay particles are aligned parallel to each other, their collision of will be more
similar to that of spheres. This implies that the theory of shear-induced migration
applies qualitatively to the anisotropic clay suspension.
When the orientation of
individual particles is random, their collision of particles would be very complicated
and difficult to analyze.
The aligned structure of particles corresponds to our simplified model for comparison of forces in Figure 2-10. Particle alignment also implies that particle interactions
37
would be dominated by electrostatic repulsive forces and van der Waals forces between
the basal planes which have negative charges [13].
3.2
Experimental evidence of shear induced migration
The previous section on the mechanisms of shear-induced migration leads us to consider the evidence for such phenomena in seals. The mechanisms are valid for noncolloidal monodisperse hard spheres in micro-hydrodynamics, i.e. Re < 1, and exclude the effects of inertia and anisotropic structures. However, our clay suspension
consists of kaolinite minerals which are small and anisotropic in addition to being polydisperse. Furthermore, the clay particles are unevenly charged. We present several
supporting experimental results of shear induced migration in our clay suspensions
in this section.
3.2.1
Shear induced migration in different geometries
Shear induced migration in curved streamline geometries with parallel plate, cone
and plate and inverted cone and plate devices have been examined previously[6].
Measurement of the torque for each geometry supports the theory of shear induced
migration, although the results were not conclusive. Figure 3-4 shows the configurations of various curved geometries.
_I
Flat
_
I
Cone and plate
Inverted cone and plate
Figure 3-4: Various curved geometries. The inverted cone and plate geometry has
the severest shear rate gradient.
38
For the inverted cone and plate geometry, the velocity is increases linearly with
radius, while the gap is decreases linearly. Thus, this is the most favorable geometry
for radially inward particle migration due to severe shear rate gradients. On the other
hand, the cone and plate geometry has no shear rate gradient. Outward migration
might occur due to curved streamline effects with an initially uniform distribution
of particles. However, for seals immersed in mud, there is a concentration gradient
directed radially inward, leading to inward migration.
3.2.2
Particle front movement during break-in period
Prior experiments[3] show particle front movement during the break-in period. The
results in Figure 3-5(Top) show that the particle front stops at a certain distance
from the edge. Shear induced migration theory can explain why the particle front
stops.
To understand this, consider the contact band region which is divided into three
regions according to oil film shape. In the region I, the profile of the oil film is
a cone and plate geometry leading to radially outward migration if the suspension
has a uniform concentration.
However, the seal environment has a high particle
concentration outside the contact band, leading to some inward migration. We showed
that the flux due to concentration can dominate near the outer edge. In region II, the
profile of the oil film is an inverted cone and plate. The effect of particle concentration
gradients is smaller in this region than region I as the concentration is very small
inside the contact band initially. However, shear rate gradients cause particles to
move further with the effect of concentration gradients. Curved streamline effects
will induce a radially outward flux, but it is smaller than the combining flux due
to the concentration gradient and the shear rate gradient. In region III, the profile
becomes that of a flat plate geometry, and the effect of the shear rate gradient is
still favorable to that of radially inward migration. However, the effect of the shear
rate gradient is smaller compared to region II(inverted cone and plate geometry). We
showed that the fluxes due to shear rate gradient and curved streamline effects are
comparable. In addition, the effect of the concentration gradient is negligible as the
39
concentration is lower than in regions I and II. We can conclude that the decreased
effects of the concentration gradient and the shear rate gradient are canceled by the
curved streamline effect near the boundary between regions' II and III.
0R
g
0. Region 1I1 Region 11
Region III
-----
Cone
and
Plate
I
Ilnverted
ned
Cone
-
Flat Plate
Oil Film
-I
- -P
a rticl e
Front
Stop
Region II
Region I
LLa
II
0 I
Figure 3-5: Shape of oil film thickness for 2+2 filled seal(Top), simplified geometry
of the oil film (Middle) and particle front movement under the seal lip(Bottom). The
contact band regions are divided into three regions as shown. [3]
40
3.2.3
Particle migration after penetration
The cross section after particle penetration through the contact band is shown in
Figure 3-6. Particles can penetrate over the whole seal surface, but stop at some
radial location. From that location, particles migrate in the tangential direction as
shown in Figure 3-6.
Seal
Load Ring
Figure 3-6: Particle migration after penetration occurs (Left) and side view of cross
section (Right)
From the figure, we can recognize the shape of oil film changes from inverted cone
and plate geometry to flat plate geometry at the end of seal surface. The shape of seal
surface (inverted cone and plate) is favorable for radially inward migration. On the
other hand, radially inward migration can be canceled in the flat plate geometry with
the same argument as in the previous section. As time goes by, the concentration
of particles inside the seal increases and particles migrate further inward. However,
at a certain point, the particles migrate in the tangential direction. This point is
the equilibrium point where the flux due to the concentration gradient, shear rate
gradient and curved streamline effect cancel one another.
Figure 3-7 shows the configuration of the real track joint seal and bushing. The
41
Steel Bushing
Inverted Cone and Plate
Steel Bushing
/
geometry
goer
Pin Joint
Seal
Load Ring
Figure 3-7: Configurations of real track seal joint. The space between steel bushing
and seal lip surface shows inverted cone and plate geometry
seal surface against flat steel bushing has inverted cone and plate geometry. After the
particles penetrate the contact band, this geometry drives particles inward and causes
severe damage. Our experimental results show that the oil does not leak significantly
even after particles penetrate through the contact band. If we can control the particle
migration after penetration through the contact band, we may reduce the damage on
the joint part by delaying the migration of particles. Therefore, the shape of the
seal surface and bushing profile should be modified to reduce the inward migration
of particles.
42
3.2.4
Outward migration of particles after the breakup of
clusters
Earlier experiments by Ayala [3] were performed under normal load of approximate
900N. However, the typical normal load in actual situations is in the range of 16001800 N. Under these loads, an additional mechanism leads to cluster breakup followed
by outward migration. The two figures show the initial cluster patterns after (a) 1,800
cycles which eventually disappear after (b) 7,000 cycles. After outward migration has
occurred, the wear of the seal is solely due to friction between the seal and glass
bushing.
(a)After 1,800 cycles
(b)After 7,000 cycles
Figure 3-8: Cluster shearing (a) and cluster disappearance(b)
We investigated the profile of the seal and glass bushing for a filled seal when
small clusters exist and outward migration occurs. The results are shown in Figure
3-9.
When small clusters appear, some particles stick to the surface in Figure 3-9(a).
This is probably due to electrostatic attraction between the particles and the broken
glass surface. The clay particles have a net negative charge, and the broken glass
surface typically has positive charges due to broken silicate bonds. Under high normal
pressure, the electrostatic attraction force could result in strong bonds between the
particles and the glass plate.
After a while, wear process leads to the profile of glass bushing which is similar
to a cone and plate geometry. The seal lip will be flattened under high normal forces
43
so that the glass profile shape decides the direction of migration. When outward
migration occured after disappearance of the clusters, the glass bushing plate has
cone-plate geometry (Figure 3-9(b)).
As discussed in Section 3.3.1, the cone-plate
geometry favors outward migration, since shear rate gradients and curved-streamline
effects will drive particles radially outwards.
Ayala [3] did not observe cluster breakup and outward migration of particles.
The reason for different results with prior experiments is the higher normal loads
that we operate with. This leads to wider contact bands get and the frictional torque
increases. Particles take longer to penetrate into the seal giving to wear the glass
bushing.
lMoao
Imjde
Moss:Outside
Outside
4000A
........
.........
2000A
...
..
..
1? h
......
/V
........
AMA
....
....
IO
......
..
....
...
A
.........
...... ......
10s00
720
910
1001
12?3
1405
163?
1$10
M0
SomLoo~0 ftM)
(a)Small clusters appeared
M1
124
122
154
Is"4
Loanlo4n)
M
2404
M27
310
(b)Outward migration occured
Figure 3-9: Glass bushing profiles of 2+2 filled seal when small clusters appeared(a)
and outward migration occured(b).
44
Chapter 4
Steel seal experiment
In the previous chapter, we showed that ealier experiments are consistent with our
hypothesis. To study this further, we devised new experiments using a polished steel
seal with different geometries; flat plate and inverted-cone-plate geometry. We will
call them as flat seal and concave seal for each. This chapter discusses the results
using steel seals. As we will see, our studies gave further support for shear induced
migration in our system.
4.1
Experimental procedures
Two different kinds of steel seals which have flat and concave surface geometries were
used. The configurations of the seals are shown in Figure 4-1. The concave seal has
an inverted cone-plate geometry in contact with the glass bushing plate. As shown,
the concave seal has an angle of 2 'with the horizontal. Each seal is attached to the
steel plate using epoxy, which is then bolted to a torque sensor. The steel plate has
a circular extruded part which fits to the inner diameter of the steel seal as shown in
Figure 4-2.
There are four major issues that are critical in designing the experiments. First,
we need to align the steel seal and the glass plate. Secondly, we need to control the
gap thickness to equalize the condition for the ingestion of particles with different
geometries. Thirdly, we will consider the effects of oil leakage for observation. In
45
addition, we need to show the effect of shear induced migration in circumferential
direction is negligible compared to that in radial direction. We measured the waviness
of the seal surface to prove it.
StI
0
6
Figure 4-1: The geometries of two different steel seals. All dimensions are inches.
0
0
3/
1.49
0.96
0.42
0.29
Rear View
Front View
Side View
Figure 4-2: Configuration of steel plate. All dimensions are inches.
4.1.1
Method for alignment and control of the gap thickness
Our solution for the alignment and control of the gap thickness as follows. Three parts
on the glass plate are coated with very thin silver film(1000A) using a co-evaporator,
46
as shown in Figure 4-3.
Silver
Coating
1 000
A
Covered Area
with Steel Plate
V1
Contact Area
with Steel Seal
it
V3
V
Glass
Plate
Figure 4-3: Glass plate with very thin silver coating
1000A.
The steel seal is connected to an AC signal generator and each coated part on the
glass plate is wired to switches. Each switch is connected with a capacitor whose value
is known. The capacitor is also connected to an AC signal generator. The complete
scheme of our circuit is shown in Figure 4-4. We measure the voltage between the
ends of the capacitor (Vap) to check the contact between the steel seal and the coated
parts on the glass.
For alignment and control of the gap, we implement an additional steel plate
combined with three micrometers. The micrometers contact the plate to which the
glass plate is attached. Two plates are connected with strong springs to hold them
tightly. The plate with micrometers is fixed so that the gap between the glass plate
and steel seal can be controlled by adjusting the micrometers.
V
47
Spring
Glass plate
Steel seal
Cseal
AC
Vseal
Micrometer
AC
Switch
Ccap
Vcap
Vcap
(b)Simplified electric circuit
(a)Steel seal experiment setup
Figure 4-4: Setup of steel seal experiment for alignment and control of the gap between
the glass plate and the steel seal
To measure the first contact accurately, we need to estimate the capacitance between the steel seal and the coated part on the glass. The capacitance of the seal
(Cseai)
should be comparable with that of a capacitor
(Ccap)
in Figure 4-4 for a high
sensitivity. The cross section of space between the steel seal and the glass plate is
shown in Figure 4-5.
Steel plate
I
Steel seal
-
I
-L/ Silver coating
d2
di
Glass plate
Figure 4-5: Cross section of the steel seal and the glass plate. The distance between the steel seal and the silver coating and the thickness of the silver coating is
exaggerated.
We have two parallel capacitances. One is due to the space between the steel seal
and the coated parts on the glass plate, and the other is due to the space between
48
the steel plate and the coated parts. Neglecting fringe effects around the edges,
each capacitance can be calculated where Cplate = edi,
10mm, Cseal = ed2
Aseai
1
Apiate
3 x 12.74mm x
3 x 12.74mm x 5.5mm. d, is the distance between the
steel plate and the glass plate, E is the permittivity of oil (=2.3 x 8.85 x 10- 1 2 F/m, [8])
and d2 is the distance between the steel seal and the coated parts. Since d, = 1.94cm
is much larger than
d2
order of Cseai is Cseai
e 10pm as shown in Figure 4-5,
Cpiate
can be neglected. The
0(10-10). Thus we used 100 pF as the capacitance
-
where the voltage is measured in Figure 4-4.
Our process for alignment is to find the first contact of the steel seal with three
different regions of the glass plate. Once they are located, we can regard the gap
between the seal surface and glass plate as zero neglecting the thin thickness of the
silver coating. When there is a finite gap between the coated parts and the steel seal,
the capacitance between the steel seal and glass plate can be calculated by using
v
Cseai
Vcap = Celx
Ctotai
Cseai
Vtotal =
Cseai + Ccap
x~a
1 Votal,
(4.1)
when we neglect the resistance of the contact and of the electric wires. When the
first contact occurs,
Cseai
becomes oc, so that Vcap=Vtot leading to a sudden change
in Vcap-
We adjust each micrometer to detect the first contact at each three coated parts.
The results in Figure Figure 4-6(a) show that the first contact occurs at different
distances. After finding the first contact at three regions, we adjust all of the micrometers with the same distance, maintaining the alignment. The results in Figure
Figure 4-6(b) show that the contact surface is aligned.
We want to make inlet condition for particle ingestion equal for two different seals.
The gap at the outer edge between the steel seal and glass bushing plate is fixed to
30 pm by adjusting micrometers after alignment, which is the minimum gap for the
concave seal.
49
5.5
*'
0'
0
4.5 F
*:v1
+:V2
o:V3
First contact
3.5
2.5 *
*
+
*
1.5
0
20
40
60
80
100
+
+
140
160
0
120
180
distance(pm)
(a)Before alignment
0
4.5
*:Vi
+:V2
o:V3
cc
3.5
8
0-
2.5
0
1.51
0
20
a
40
60
80
100
120
140
160
180
distance(gm)
(b)After alignment
Figure 4-6: (a) The non-coincidence of the volatage V, V2 and 3 at the three locations
on the glass shows that the gap is not constant, i.e the gap between the seal surface
and glass plate is not aligned. (b) Voltage is checked after alignment. The coincidence
of V1 , V2 and V3 shows that the steel seal and glass plate are aligned.
50
4.1.2
Considerations in oil leakage
The development of oil flow due to gravity is observed during the experiment(See
Figure 4-7). The speed of oil film flow from the boundary between oil and air tends
to increase from the upper part to the lower part. Such flow can effect on the migration
process of particles where the speed is significant. We observed the contact region
near the boundary of oil and air to minimize the oil flow effect.
Air
Slurry
Observed
Area
oil
Oil Leakage
Figure 4-7: Typical steel seal experiment. Slurry is provided from top or side with a
rate of 5ml/min. The observed part is indicated
4.1.3
Waviness measurement
Alignment of the two surfaces is only a crude measure of the oil film thckness. It
is necessary to check the profiles of the steel seals to find out that detailed shape of
the oil film between the glass plate and seal surface. When there is a considerable
difference of thickness along the circumference, the shear induced migration occurs in
that direction. To compare the effects of shear induced migration in radial direction
only, we need to prove the effects of waviness is negligible.
51
We used point probe method to measure waviness profiles. The measurements are
done along four concentric circles with 1 mm radial spacing at Caterpillar, Inc. The
outermost circle was within 0.1 mm of the outer diameter of the steel seal surface.
The measurement points are 4 degrees apart and 90 measurements are done for each
circle. Finally, total numbers of measurements are 360 points for each kind of steel
seal.
Waviness measurements of the flat seal and are shown in Figure 4-8. The maximum gap difference(amax) in the circumferential direction does not exceed 10pum.
Note that the gap difference in radial direction is very small except near the outer
edge. Thus the flat seal surface can be assumed flat compared to concave seal.
Flat Steel Seal
0.03
0.025
S0.02-
amax
S0.015
0.01-
g
2
0. 005.
50
100
150
200
Degreees COW
250
300
350
400
Figure 4-8: Waviness measurement for flat steel seal. A curve 1 is for the outmost
circle. The larger number of curve corresponds to the inner measurement circle.
Waviness measurement of concave seal is also shown in Figure 4-9.
amax
does not
exceed 20[tm. The height difference in radial direction between measurement 1 and
4(arad) is
shown. To compare the gradient of height, we define following dimensionless
number,
Nrc =
arad/D
amax/A
0.1 x 10-3/0.4 x 10-3
20 x 10 6 /(7r/2)42 x 10-3-
0(102)
(4.2)
where D is the radial distance between measurement 1 and 4. Eqn. (4.7) shows
52
that the height difference in circumferential direction is negligible.
Concave Steel Seal
0.12
'
E
0
0.08 --
arad-
E=0.06 -
10.04-
:3
3
4*4
0.02
--
0
50
100
150
200
Degreees
250
300
350
400
CCW
Figure 4-9: Waviness measurement for concave steel seal. A curve 1 is for outmost
circle. The larger number of curve corresponds to the inner measurement circle.
4.2
Experimental results
In Figure 4-10, we show that the penetration of particles with two seal geometries
after the same number of cycles. For the flat seal, we can observe that particles have
penetrated partially from outside. For the concave seal, the particles have penetrated
the whole contact band except near the bottom. This is because of oil leakage which
prevents particle migration(See Section 4.1.3).
We also observed small area near the interface of oil and air. In case of the flat
seal, the particles move inward immediately, and stay as shown in Figure 4-11. The
particle front does not penetrate in the radial direction further. The usual penetration
length is approximately 0.5 mm.
For the concave seal, the particle front moves radially inward until it passes whole
contact band. One of the results is shown in Figure 4-12. The penetration length
was fitted to third order polynomial by least square, then to the square function of
time. When we consider the penetration as diffusion, the diffusion coefficient(D) is
0.362 from 9 =atDa.r 2
53
(a)Flat Seal
(b)Concave seal
Figure 4-10: Part of each seal after 3,500 cycles. The particles can migrate only small
part of the contact band for flat seal(Left) while they migrate whole contact band
except bottom part of seal for concave seal(Right).
It is very likely that our experimental results support shear induced migration
theory in our geometry although it is still complicated to analyze. We simplified
surface profile using steel seal, but there are still other factors.
We need to consider the effect of gravity on the particles in addition to shear induced migration. The effect of oil flow on the migration of particles in circumferential
direction can be significant as the speed increases.
In spite of complications, the dramatic difference between flat seal and concave
seal clearly supports our hypothesis. With the experimental evidence in chapter 3, we
can insist that shear induced migration of clay particles is responsible for migration
of particles under the contact band.
54
Figure 4-11: Close view of particle front near the interface. The particle front penetrates immediately after slurry supply and stays at certain distance from the outer
edge.
5
4
3
0
(D
2
-
50
100
-
150
-
0.602 sqrt(t) 4.3
200
250
300
Time(sec)
Figure 4-12: Particle front movement for concave seal. The particle front moves
gradually with time, then stops near the inside diameter of the concave seal. The
data was fitted to third order polynomial.
55
Chapter 5
Particle aggregation and dispersion
mechanisms
This chapter deals with the physical and chemical mechanisms for particle aggregation
and the dispersion. As discussed in Chapter 1, after the break-in period, particles
start to form clusters leading to aggressive wear period. The cluster composition
was investigated by Ayala [3] by using electron microscopy and x-ray diffraction.
These results, he showed that clusters were mainly composed of clay particles. In
our system, clay particles first contact water, then oil when they penetrate under the
contact band. We will first discuss the water-clay system, then move toward wateroil interactions to explain particle aggregation. We will also consider mechanisms
for disappearance of the clusters before outward migration begins under high normal
loads, as discussed in Chapter 3.
5.1
Water-Clay system
In a dry clay, adsorbed cat-ions due to isomorphous substitution are tightly held by
the negatively charged clay particles [19]. However, when clay particles are dissolved
in water, the adsorbed cat-ions tend to diffuse away and form an electric double layer
as explained in Section 2.1. It is well known that the broken edge can have a positive
charge under certain conditions which depend on pH [7, 11, 20, 26].
56
Anions in water
Fluoride
Chloride
Nitrate
Phosphate
Bromide
Nitrate
Sulfate
pH
Amount
< 20ppm
< 20ppm
< 20ppm
< 20ppm
< 20ppm
< 20ppm
< 20ppm
7.0 - 7.1
Table 5.1: Anions in Water investigated by ion chromatography. ASTM D4327
At pH values lower than the point of zero charge of the edges, i. e 7, the edges
have positive charge and there is no potential barrier for face-edge interaction to
aggregation [7, 11]. At pH values around 7, both face-edge and edge-edge flocculation
due to Van der Waals force can be expected. Where the pH > 7, the edges also have
negative charges and there is a considerable potential energy barrier to all modes of
flocculation.
We investigated the pH values and the presence of ions using pH paper and ion
chromatography. The results are summarized in Table 5.1. From the value of the
pH, our clay particles can have edge-edge bonding due to Van der Waals force and
some edge-face bonding due to opposite charge. However, in our system, there is
a considerable amount of anions which include Phosphate, Bromide, etc. It is well
known that some types of anions can be attracted to and neutralize the positive edge.
Especially anions which have similar size and geometry as the silica tetrahedron, such
as phosphate and borate, are very effective in neutralizing the edges of clay particles.
Such mechanisms decrease the possibility of edge-face bonding due to opposite charge.
Our mixture appears to be homogeneous, displaying some rigidity and elasticity.
This implies that particles are connected to one another and make network structures.
We can conclude that clay particles in our slurry mixture are connected by mainly
edge-edge bonding and, possibly, some face-edge bonding.
57
5.2
Additional aggregation and dispersion mechanisms
5.2.1
Aggregation mechanisms
In Chapter 2, some particle aggregation mechanisms were discussed and compared
using estimation of order of forces. Van der Waals forces and electrostatic attractive
forces due to opposite charges on the surface and edge were responsible for particle
aggregation. From the comparison of forces, Van der Waals forces were shown to be
dominant at moderately high particle concentrations. In addition the effects of dissolved ions at the water-oil interface and bridging of particles by a second immiscible
liquid can also lead to clustering.
The effects of dissolved ions at the interface was found in the experiments
when we used two different kinds of oils. They are Pennzoil Gearplus 80W-90 oil and
Mobil CAT Gear Oil SAE 80W-90. Ayala [31 used Pennzoil oil for his experiment, but
we found that different wear mechanisms could be observed when we used different
kinds of oils as lubricants.
As explained in Chapter 3, we observed that clusters
disappeared and particle outward migration occurred when we used higher normal
load (1700N) and Mobil CAT oil. However, when we used Pennzoil oil with same
normal load, we could not observe breakup of clusters.
We investigated the two oils using FTIR (Fourier Transformed Infrared Spectra)
and ion chromatography method. The analysis and interpretation for chemical components is given by Herguth Laboratories, Inc. The results for spectrum analysis and
metallic chemical compounds are shown in Figure 5-1 (a), (b).
The analysis shows that Mobil CAT oil has calcium sulfonate and Pennzoil oil has
potassium borate as additives in the oil. Potassium borate is soluble in water, but
calcium sulfonate is rather insoluble in water.
When potassium borate dissolves in water near the interface, it will increase the
concentration of electrolytes, which leads to the reduction of electrostatic repulsion
due to the compressed electric double layer thickness. This helps cluster formation
58
PRIMARY SAMPLE ID 713845
[
e
b
als
.'
.inum
IMARY
A
n
P
e0(XL.M 1
SAMPL
A..
C ilvr
Red Scan i sampie
,115.111101--1
7
.
. .
n;
p
-
.....
n
..
ppi
..-....
.......
Calc
-
m
ly d n
Mo
um
a
-(
.
-2
....-.
0
p
a
'
ppmn
(a) Pennzoil Gearplus SAE 80W-90 oil.
PRIMARY
m
s--
AM-;mttnum1
)by
SAMPLE ID713844
I
.
C . P,
c'8
ndsa
HTl 1
ap
115-3- 1
ppm
PpmYI
ppm
(A!.)-
*11*ml
............
PPM
Ni
0?l'N
;prm
10'm
"V1
(b) Mobil CAT oil.
Figure 5-1: FTIR Spectrum results and metallic ions by I. C. P. Spectro. (a) The
strong and medium intensity bands of the spectrum are consistent with an aliphatic
hydrocarbon, the base oil. Based on the presence of boron in the oil, the oil is likely
to contain potassium borate additives. (b) The band at _ 1165cm- 1 is likely due to
the existence of sulfonate, used usually as a rust inhibitor. Based on the presence of
sulfonate, the oil is likely to contain calcium sulfonate
59
due to reduced repulsive force explains why Penzzoil oil, which contains potassium
borate, leads to particle aggregation. On the other hand, when we use Mobil CAT oil
which contains calcium sulfonate, long range electrostatic repulsion prevents cluster
formation at the interface.
The experimental evidence for the effects of solubility near the interface was found
in Figure 5-2.
The figure shows the region around the oil-water interface under
oscillating steel plate. Clusters are formed around the interface between the water
and the oil. On the other hand, no clusters are found at the region far from the
interface.
Clusters
Steel Plate
Inte
Figure 5-2: Interface area captured by CCD camera. Clusters starts to form from the
interface.
60
However, more information about the compound in oil is required to fully understand the different results.
If the dispersed particles are preferentially wetted by polar liquids such as water
and the added liquid is immiscible with the organic dispersion medium such as oil, the
added water envelops the particles with a thin film. When two particles come together,
their water films will flow together at the contact point since the arrangement results
in a decrease of the total free interfacial energy of the system due to a reduction of
the interfacial area. When the clay particles first contact water, they adsorb a few
molecular to develop a thin film around the particle. When they penetrate into the
oil film, the water layer is immiscible with the surrounding oil medium, so that the
bridging of particles due to thin adsorbed water layer can be observed. These
ideas explain the disparate results obtained when Pennzoil oil with 5% of surfactant
which was used for dispersing dry clay in oil-based paint(See Figure 5-3) [3]. When
he used dry clay, he did not observe any clusters. However, when he used clay-water
mixture, he could observe clusters and wear.
398
40,854
Figure 5-3: The above images show a seal operating with a dry mixture of clay and
sand added to its periphery and surfactant added to the oil with number of cycles. No
clusters were observed even after 40,000 cycles. However, surfactant does not prevent
clustering formation for a slurry mixture based on water [3].
Such different results can be explained with two mechanisms, effects of dissolved
ions at the interface and bridging of particles by an adsorbed water film. The adsorbed
water film also helps clustering due to the reduction of interfacial energy. We can insist
61
that the different results are partially due to two additional clustering mechanisms.
5.2.2
Dispersion mechanisms
Experiments with flat seals reveal that the temperature on the glass surface changes
significantly and correlated with an accelerated wear of the seal. The large change in
temperature is due to increased friction with the wide contact band.
In order to quantify the effects of temperature variations, we embed thermocouples
in the seal lip surface as shown in Figure 5-4. We make three holes at the top,
middle and bottom part of the seal surface. Then we fix thermocouples in the holes
and fill the holes with polyurethane sealant to make the seal surface as smooth as
possible. Thermocouples are connected to a data acquisition system to record the
temperatures. At the same time, a CCD camera is used to record particle behavior.
We use insulated fine wire thermocouples (K type, OMEGA). The diameter at the
junction of the thermocouple is 0.020 inch. The space between two wires near the
junction was filled with silicon sealant to prevent a contact of wires at other parts
besides the junction.
Polyurethane
sealant
Adhesive sealant
Thermocouple junction
Insulated wire
Figure 5-4: The location of thermocouples at three different parts, top, middle and
bottom. The right image shows the enlarged cross section where the thermocouple is
fitted to the hole. After inserting the thermocouple, polyurethane sealant is spread
on the seal lip surface for smoothing surface and preventing particle concentration
around the thermocouple.
62
The temperature at each location is measured every two minutes during the experiment. 5 measurements at every minute were averaged to get an average normal force.
The measurement results are shown in Figure 5-5. Once cluster formation begins, the
wavelength between clusters changes until permanent cluster breakup and outward
migration occurs. The trends of change in all temperatures and average normal force
are plotted in Figure 5-6.
The temperatures show a rapid increase in the beginning of oscillation while the
normal force shows a rapid decrease in magnitude due to lubrication by the oil film.
After cluster formation starts, the temperature shows a gradual increase. After a
while, the temperatures at the middle and the bottom part increases rapidly and
clusters disappear. Our experimental results are suggestive the existence of a threshold temperature for cluster breakup.
Concomittantly, the normal force seems to increase significantly with the increase
in temperature. Simultaneous increments in the normal force with temperature above
a threshold, thermal expansion of seal lip material is likely to contribute to be the
dominant cause for cluster breakup. The thermal expansion coefficient of seal lip
material(a) is 1.62 x 10- 4 /C.
The change of temperature is approximately 16 'C
when a rapid increase of temperature and normal force occurs. Then the strain is
c = aAT = 2.59 x 10-.
Corresponding change in normal force is AN = EcAsea,
,
13N where Aseai is the area of contact band. The measured increment in the normal
force is as shown in 5-5d. This is consistent with our hypothesis that the thermal
expansion of the seal lip material is responsible for breakup of the clusters.
Outward migration of particles occurs after cluster breakup. Note that outward
migration of particles occurs when the normal force reaches its maximum which results
in an outward migration of particles.
63
Cluster breakup
Outward migration
Outward migration
/
tb
60
CL40
30
(a) Top temperature
(b) Middle temperature
Cluster breakup
Outward migration
ILk
Outward migrathi
1
/
970960
950
940
c
930 2
2
920<
Ur>
0
(c) Bottom temperature
(d) Averaged normal force
Figure 5-5: Temperature at three different loacation is measured in 'C (a-c). Clusters
disappear when temperature exceeds approximately 60 0 C. Two measurements at
every two minutes were done to check the spatial variation of temperature. Observe
that outward migration occurs at the same time when normal force(d) reaches a local
peak.
64
980
-- Tepmerature at Top
-
-
Temperature at Middle
..w.. Temperature at Bottom
970
Z
2960
Average Normal force
Outward migra ion
L950
C
Cluster breakup(Middle)
940
-
breakup(Bot bm)
/-- ACluster
930
920
-~
I
0
50
TIme(mln)
100
Figure 5-6: Average normal force is shown with temperature in three different parts.
Note that the normal force shows a sudden increment when the temperature increases
rapidly. Outward migration occurred when the normal force reached local peaks.
65
Chapter 6
Conclusion
The goal of our thesis is to understand the fundamental behavior of particles for the
improvement of seal design. We investigate two categories of fundamental behavior
of particles: migration and clustering of particles. Shear induced migration explains
prior and current experiments on the migration of particles. Particle aggregation and
dispersion mechanisms were also investigated and validated by experiments. We will
review the interactions between particles and discuss future work.
6.1
Summary of particle interactions through whole
wear process
The particle interactions are summarized in Figure 6-1. Clay particles first contact
with water to make a slurry mixture. With the investigation of pH and containing
ions in water, we can conclude that edge-edge bonding and some edge-face bonding
combine particles together and make a network structure.
When particles are ingested under the contact band, the clay particles contact
with the oil film. The particle front moves and stops at certain distance due to shear
induced migration. Up to a certain number of cycles during break-in period, the
particles are dispersed by the dominant shear force.
When the concentration of particles are high enough, the van der Waals force is
66
Clay-water system
Edge to edge, edge to face bonding
Shear induced migration
Dispersed by shear force
Clay-oil system
I
Cluster formation
Breakup of clusters
{
Outward migration
Van der Waals force
Effects of dissolved ions at interface
Bridging of particles by adsorbed water
molecules and compouns in oil
Thermal expansion of seal surface
Electrostatic repulsive force
Entropic repulsion
Shear induced migration
Increased normal pressure
Figure 6-1: Summary of abrasive wear process due to particle ingestion and cluster
formation. Related forces and effects are written for each stage.
dominant and particles start to aggregate. In addition to the van der Waals force,
bridging of particles by adsorbed water film and organic compounds are likely to help
cluster formation. Clusters start to form around the interface of water and oil, which
suggests that dissolved ions at the interface help particles aggregate.
When clusters are large enough to fill the gap between the bushing plate and
seal surface, they start to roll due to friction. The wavelength between the clusters
changes with time
[3].
After a large number of cycles, the temperature of the seal surface shows a rapid
increase. The normal force also shows a sudden increment, which implies thermal
expansion of the seal lip surface. With other factors for dispersion of particles, thermal
expansion of seal surface is very likely to help breakup of clusters.
After the disappearance of clusters, the dispersed particles start to migrate outward.
The investigation of the glass bushing profile shows a favorable profile for
outward migration due to the shear induced migration mechanism. In addition, the
67
normal force shows a local maximum when outward migration occurs. The shape of
the surface profile and increased normal pressure drive particles radially outward.
6.2
Future work
Although shear induced migration is the dominant mechanism for particle migration,
in order to fully understand the migration mechanism, we need a simplified system.
The diversity of size, shape and the complex electrical and chemical interactions
between clay particles in a complex setup geometry make analysis difficult.
For
quantitative analysis of shear induced migration, we need to devise experiments with
particles of well-defined size and shape. The setup should be modified to exclude
the effects of gravity by using a horizontal setup and neutrally buoyant suspending
medium. The behavior of particles can be understood quantitatively with a change
of parameters, such as amplitude, frequency of oscillation and normal force. A deep
understanding of particle behavior in various geometries will give a way to control
the migration of particles and finally delay the wear process.
The role of the temperature change should be investigated in the future. The
change of temperature leads to dramatic change of wear properties. For our seal lip
material, polyurethane, 80 'C is usually considered as the top working limit under
continuous dynamic conditions [291. From our measurement results, it is possible
to exceeds limiting temperatures after a long running time. In order to guarantee
a durable temperature range, heat transfer should be considered to dissipate heat
from the surface to the outside. On the other hand, for more accurate research in
the role of temperature change, the non-contact measurement method such as laser
diagnostics is required. Any measurement by direct contact is not accurate enough
due to complicated heat transfer process and severe environment in our system. The
DELIF technique, which can measure the oil film thickness and the temperature
change simultaneously using two different dyes, is one of the solutions to our problem.
Besides the control of migration and clustering processes in a mechanical way such
as finding the optimal shape of oil film and efficient heat transfer, there is a chemical
68
way to control the clustering formation process. The lubricant oil should be developed
with considerations of interactions between particles and oil. Various techniques for
dispersing the particles using interactions between clay particles and metallic, organic
compounds in oil are available [28]. The information about the lubricant should be
known prior to improvement.
69
Appendix A
Clay mineralogy
The standard for classification of clay is somewhat ambiguous. In terms of size and
shape, the clay refers to constituents of a soil smaller than 0.002 mm(2 Pm) and has
platy shapes(in few cases with needle or tubular shape) [19]. However, it should be
kept in mind that not all clay particles are smaller than 2 Mm and not all non-clay
particles are coarser than 2 pm.
The principal building elements of the clay minerals are two-dimensional arrays of
silicon-oxygen tetrahedra and two-dimensional arrays of aluminum or magnesium
oxygen-hydroxyl octahedra. In most clay minerals, such sheets of tetrahedra and of
octahedra are superimposed in different ways [281. Clay minerals are classified by the
arrangement of these two element sheets.
In silica tetrahedra sheet, three of the four oxygens in each tetrahedron are shared
by three neighboring tetrahedra to form hexagonal network (See Figure A-1). The
unit structure can repeat indefinitely and has composition (Si 4 010) 4-.
Electrical
neutrality can be achieved by replacement of four oxygens by hydroxyls.
Octahedral sheet is composed of magnesium or aluminum coordinated octahedrally with oxygens or hydroxyls as shown in Figure A-2. In some conditions, other
cations such as Fe2 +, Fe3+, Mn2+, Ti 4+ can replace A1 3+ and Mg 2 +. When the cat-
ion is trivalent, such as A1 3+, then normally only two thirds of the possible cationic
spaces are filled, and referred gibbsite sheet. If cation is divalent like Mg2+, then
normally all possible cation sites are filled and called brucite sheet.
70
C4
0
0
Silicons
Silicon tetrahedron and silica tetrahedra arranged in a hexageonal
Figure A-1:
network.
*
Oxygen
[19]
Hydroxyls
*
Aluminums, magnesiums, etc.
Figure A-2: Octahderal unit and sheet structure of octahedral units.[19]
In the real clay minerals, however, some of tetrahedral and octahedral spaces are
filled with cations other than those in the ideal structures. Such replacement of a
cation other than normally found, without change in crystal structure is called isomorphous substitution[19]. Most common examples of isomorphous substitution are
replacement of Si 4 + with Al'+ in silicate tetrahedra sheet and Al'+ with Mg 2+ in
octahedral sheet. Such isomorphous substitution results in net 'egative charges of
clay minerals and electric double layer in clay suspension.
Our clay mineral, kaolinite, is composed of alternating silica and octahedral sheets.
The physical shape of well-crystallized particles of kaolinite is known as well-formed
71
six-sided plates. The lateral dimensions of plates are from about 0.1 to 4 pm, and the
vertical dimensions are from about 0.05 to 2 pm. The aspect ratio of the plate is usually ~ 12 although it may vary with particle size [15]. The edges of kaolinite particles
are positively charged when the surrounding medium is in a low pH condition, while
negatively charged in a high pH condition [7], [19],[20],[26]. Such reversal of charge
at the edges can contribute to the change of micro-structures in clay suspensions. In
addition, the anisotropy and different carrying charges at faces and edges of the clay
particles make the interactions complicated.
Mineral
Isomorphous substitusion
Kaolinite
Al for Si
Mg for Al
As Above
Typical particle
shape
Hexagonal sheets
Plate
Sheets
Variable
Illite
Al foi Si
Maybe Mg, Fe for Al
Mainly Al for Si
Also Mg, Fe for Al
As above
Typical particle
size
diameter= 0.3 - 10 p
thickness= 1 dia
outer diameter=.07 p
inner diamter=.04 p
length=.5 p
Very large
Flakes
Montmorillonite
Mainly Mg for Al
Sheets
Chlorite
Al for Si
Fe,Mg for Al
Plate
diarneter=.1-2 p
thickness <
dia
diameter=.1-1 p
thickness < 1 dia
Variable
Halloysite
Muscovite
Vermiculite
Hollow tubes
Table A.1: Physical properties and dimensions of common clay minerals
72
Appendix B
Notation and Physico-chemical
values
A
w
Useai
P
u-
The oscillation amplitude of the seal in radians
Typical value: A= 7r/6
Oscillation frequency in strokes per second
Typical value: 1 stroke/second
Seal shear velocity.
Typical mean velocity: U.=22 mm/s
Viscosity of 80W90 Penzzoil Gear oil
Value at STP: 0.32 Pa - s
Surface tension of oil
Typical value: 38 x 10-3N/m
P
E
Er
a
Average normal force
Typical value: P= 1700N
Stiffness of the seal lip material(3.45 MPa)
Stiffness of the load ring material(0.45 MPa)
Thermal expansion coefficient(from -30 'C to 110 'C)
Value = 8.995 x 10- / 'F
73
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