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“Tribology in Mechanical Engineering”
MAE 493N/593T
Dr. Konstantinos
A.
Sierros
West Virginia University
Mechanical & Aerospace Engineering
ESB Annex
263
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Biotribology
Biotribology: Tribological phenomena occurring in either the human body or in
animals and possibly plants
Biotribology
Tribological processes naturally
occurring in or on the tissues and
organs
of
animals
Tribological processes that may
occur after implantation of
artificial
device
in
human
body
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Biotribology‐Examples
http://www.ocw.cn/NR/rdonlyres/Biological‐Engineering/20‐441Fall‐
2003/6E544DF6‐4361‐46B0‐B9A9‐A74F2B171A03/0/chp_artificial_hip.jpg
EXAMPLES
•Wear of skin and its replacement by new skin cells
• Lubricated sliding
of
eyelids
over
the
eye
• Wear of orthopaedic implants
‐Artificial hips
‐Artificial
knees
‐Synovial joint lubrication
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Other examples
• Wear of dentures
• Friction of skin and garments, affecting the comfort of clothes, socks and shoes
• Tribology of contact lenses
• Wear of
replacement
heart
valves
• Lubrication of pump in total artificial hearts
• Wear of screws and plates in bone fracture repair
• Lubrication in pericardium and pleural surfaces
The pericardium is the thin
sac enclosing
the
heart
http://www.finejewelrydesigns.com/images/pleura‐
pericardium‐chart.jpg
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Tribology of artificial joints
•Long term issue: Loosening of the joint caused by osteolysis and
adverse tissue
reactions to wear debris in artificial joints
• Osteolysis refers to an active resorption of bone matrix
• Biological reactions
depend
on
‐Size of wear particles
‐Worn volume
• Example: In
hip
implants
where
metal
‐on
‐metal
bearings
are
used
the
size
of
wear debris is generally much smaller than polyethylene‐on‐metal contact
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Tribology of artificial joints
• Smoothest
bearing
surface
for
artificial
joints
is
usually
found
on
ceramics
(0.005 μm)
• Metallic bearing surfaces: 0.01 μm
Rα
‐
Arithmetical mean
deviation
‐
Centre line average
Average roughness
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Tribology of artificial joints
• A
comparison
of
the
Rα
is
shown
in
the
table
below
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Tribology of artificial joints
• Typical
roughness
values
for
bearing
surfaces
used
in
artificial
hip
joints
are
presented below;
( ) ( )2_
2
_ cuphead R R R α α α +=Composite surface roughness
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Tribology of artificial joints
• Friction: Resistance
to
motion
•
Remember laws of friction:
1.
Friction force is directly proportional to applied load
2.
Friction force is independent of apparent area of contact
3.
Friction
force
is
independent
of
sliding
speed
Amontons (1699)
Coulomb (1785)
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Tribology of artificial joints
• Friction coefficient
depends
strongly
on
nature
of
bearing
surfaces
in
the
presence of biological lubricants and is different from the values listed
previously
•
Early hip replacements by Sir John Charnley utilized PTFE (Teflon) but failed
very quickly
because
of
wear
•
Early hip implants based on metal‐on‐metal also failed very quickly due to
equatorial contact which resulted in high friction and high frictional torque
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Tribology of artificial joints
•
Frictional force at the interface between the outside of the acetabular cup
and the
cement
(or
bone)
2
1
R
WRF
μ =
•
In order to reduce the probability of interface failure is important to minimize
the stress transmitted at the interface by;
‐
Reducing the friction coefficient μ
‐
Reducing the femoral head radius R
1
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Tribology of artificial joints
• Frictional testing
is
conducted
using
a simulator
• Single‐station
servo
‐hydraulic
machine
•
Controlled by a PC
•
3D loading and motion patterns
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Tribology of artificial joints
•
Measured
frictional
torque
T
is
used
to
calculate
the
friction factor f
•
Friction factor is used to compare the effect of different variables such as;
‐
Material combination
‐
Implant size and design
‐
Lubricant
‐
Load and motion profiles
W R
T f
1
=
dimensionlessparameter
•
Some of the above parameters are combined to form the Sommerfeld number z
W
nuR z 1=
n is lubricant viscosity
u is entraining velocity of bearing surfaces
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Tribology of artificial joints
•
Friction factors associated with different lubrication regimes are shown below
•
Variation of friction factor f vs Sommerfeld number z can be plotted
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•
Variation
of
friction factor f vs
Sommerfeld number z
•
Stribeck curve (Stribeck 1920s)
Tribology of artificial joints
W R
T f
1
=
W
nuR z 1=
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Tribology of artificial joints
• Typical friction
factors
in
various
hip
joints
are
shown
below;
W R
T f
1
=
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Tribology of artificial joints
•
Lubrication regimes
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Tribology of artificial joints
•
Fluid film lubrication is needed in order to minimize wear
•
However, wear cannot be fully eliminated even with fluid film lubrication due
to erosion and fatigue
•
Assessment of lubrication mechanism in artificial joints
‐
Experimental
measurements‐
Theoretical predictions
•
Experimental measurements
‐
Friction
measurements
related
to
Stribeck
plot‐
Detection of bearing surface separation by electrical resistance
measurements (metal‐on‐metal cases) – For insulating surfaces a conductive
coating is needed
•
Theoretical predictions
‐
Based on the λ
ratio
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Tribology of artificial joints
•
Theoretical predictions
‐
Based on the λ
ratio
( ) ( )[ ] 2 / 12
_
2
_
minmin
cupahead a R R
h
R
h
+== α λ
•
Rα
must be measured accurately
•
hmin
(fluid film
thickness)
must
be
predicted
as
accurately
as
possible
21.0
2'
65.0
'
min 8.2
−
⎟ ⎠
⎞⎜⎝
⎛ ⎟ ⎠
⎞⎜⎝
⎛ =
R E
W
R E
nu
R
h
R is equivalent radius
E’
is elastic modulus
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Tribology of artificial joints
21.0
2'
65.0
'
min 8.2
−
⎟ ⎠
⎞⎜⎝
⎛ ⎟ ⎠
⎞⎜⎝
⎛ =
R E
W
R E
nu
R
h
( )⎟⎟ ⎠
⎞⎜⎜⎝
⎛ +=
+=
d d
d
c
d d
c
cd d R 1
22
4d u ω =
⎥⎥⎦
⎤
⎢⎢⎣
⎡ −+
−=
cup
cup
head
head
E
v
E
v E
22
'11
/ 2
d is femoral head diameter
cd
is clearance between head and cup
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Tribology of artificial joints
Example
λ
ratio for UHMWPE‐on‐metal is 0.025‐0.62
(less than 1)
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Tribology of artificial joints
•
Wear
is
progressive
loss
of
material
from
the
operating
surface
as
a result
of
relative motion
•
Wear is important because It is related to;
‐
Decreased function of the component
‐ Component
replacement
cost
‐
Adverse effects of wear particles (Adverse tissue reactions, osteolysis,
loosening)
WEAR•
Abrasive
•
Adhesive
•
Fatigue
•
Erosive
Mechanical action
Chemical action
Wear types may occur simultaneously
or sequentially
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Tribology of artificial joints
•
Wear
assessment
of
total
replacement
hip
joints
‐
Pin‐on‐disc machines (steady‐state load, sliding speed, environment)
‐
Pin‐on‐plate machines (reciprocating movement of hip joint)
‐
Joint simulators (3D loading and motion pattern simulation)
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Tribology of artificial joints
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Tribology of artificial joints
•
UHMWPE‐on‐metal: Lubrication regime is predominantly boundary
lubrication
• Metal‐on
‐metal:
Mixed
lubrication
regime
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Gecko effect
•
Gecko is an animal that can rapidly climb up vertical smooth surfaces (such as
glass) and ceilings
•
The Gecko foot has about 14400 hairs (setae)/mm2
as revealed by
microscopy
•
These hairs are covered with even smaller projections (hundreds of nm in
diameter)
•
Adhesive force of single Gecko foot‐hair is 600‐fold greater than that of
frictional measurements of the material
•
Highly oriented setae reduce the detachment force of the foot by
simply
detaching above a critical angle with the opposing surface
•
Adhesion force in the Gecko is mediated via van der Waals interactions
• Gecko
foot
also
exhibits
self ‐
cleaning
properties•
Self ‐cleaning of Gecko setae is a result of geometry and not chemistry
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Diatom tribology
• Diatoms
are
unicellular
microalgea
with
a
cell
wall
consisting
of
a
siliceous
skeleton enveloped by a thin organic case
•
Cell walls of each diatom form a pill‐box like structure consisting of two parts
that fit within each other like a shoebox
• They vary
greatly
in
shape
ranging
from
box
shaped
to
cylindrical
•
They can be symmetrical or non‐symmetrical
•
They can serve as models for micro/nano tribological investigations
Di t t ib l
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Diatom tribology
•
Bacillaria
paxillifer
specie is capable
of movent
• No sign
of
wear
has
ever
been
found
on diatom cells
•
Entire colonies of 5‐30 cells expand
and contract in coordination
• Natural adhesives
Di t t ib l
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Diatom tribology
http://www.youtube.com/watch?v=FO5MPaIbS9U
References
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References
G. Stachowiak, A. Batchelor, Engineering Tribology, 3rd
Edition, Elsevier, 2005
Z. M. Jin et al, Biotribology, Current Orthopaedics (2006) 20, 32‐40
D. Dowson, V. Wright, Introduction to the biomechanics of joints and joint replacements
London: mechanical Engineering Publications Ltd; 1981
I. Gebeshuber, Biotribology inspires new technologies, Nanotoday
(2007) 2, 30‐37
I. Gebeshuber
et al, Tribology in biology, Tribology (2008) 2, 200‐212
Summary