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“Tribology in Mechanical Engineering”

MAE 493N/593T

Dr. Konstantinos

A.

Sierros

West Virginia University

Mechanical & Aerospace Engineering

ESB Annex

263

[email protected]

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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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• 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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• A

comparison

of

the

Rα

is

shown

in

the

table

below

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• 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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• 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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• 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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•

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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• 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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•

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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•

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)


W R

T f

1

=

W

nuR z 1=

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• Typical friction

factors

in

various

hip

joints

are

shown

below;

W R

T f

1

=

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•

Lubrication regimes

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•

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

•


‐

Based on the λ

ratio

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•


‐

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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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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Example

λ

ratio for UHMWPE‐on‐metal is 0.025‐0.62

(less than 1)

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•

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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•

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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•

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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Gecko effect

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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

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Summary

• Introduction

• Examples

• Artificial joints

‐

Surfaces‐Friction

‐Lubrication

‐Wear

• Gecko effect

• Diatoms

Documents

MAE 493N 593T Lec17