BIOE 4120- Lecture 11: Tribology

Tribology

-interdisciplinary study of the science and technology of interacting surfaces in relative motion

-study of friction, lubrication, and wear of engineering surfaces with a view to understanding surface interactions in detail and then prescribing improvements in given applications

Wear

Progressive loss of material from a surface by means of some mechanical action

Contact geometry

Conforming vs non-conforming

Kinematic and loading conditions

- Velocity

- load distribution

Lubrication

- Fluid viscosity

- friction

- molecular adsorption

Material Properties

- hardness

- elastic (viscoelastic) properties

- surface properties

Ideal Tribological Properties

- minimal wear

- minimal friction

- Ideal lubrication

Overall objective of tribology is

Control of friction and wear using lubrication

Coefficient of friction

A simple ration between the force necessary to make a body slide on a surface and the force pressing the body against the surface

A coefficient of friction = 0.1 means

A body will slide on a horizontal surface if a force equal to 1/10 its weight is applied to it

Measuring (Predicting) friction

- Lubrication regimes (solid, fluid film thickness)

- Lambda ratio

Lambda Ratio

Function of lubricant thickness and surface roughness

Origin of Friction

- Surface Roughness

- solid to solid contact

- adhesion

- deformation

Lubrication

Replace sold to sold contact (solid rubbing ) with a fluid layer (viscous shearing)

Lubrication uses

A film of layer to separate opposing surfaces during relative motion

Lubricants

Substances that separate rubbing surfaces and readily shear while adhering to surfaces (solid, fluid, grease)

The precise effect of lubricant on bearing performance depends on

- viscosity of the lubricant

- geometry of the bearing surfaces

- entraining velocity at which the bearing operates

- material properties of the surfaces

- surface roughness

To be useful, lubricants must

Support some load

Lubrication Regimes

- Boundary

- Mixed

- Hydrodynamic (fluid film)

Boundary Regime

Performance depends entirely on boundary film

Mixed Regime

both bulk lubricant and boundary film play a role

Hydrodynamic (fluid film) Regime

surfaces separated by a bulk lubricant film

Hydrodynamic lubrication

Occurs when rigid bearing surfaces which are not parallel, and are separated by a fluid film, slide tangentially in relation to each other

The viscosity within the wedge of fluid produces ....

A lifting pressure between the two surfaces

squeeze film lubrication

Occurs when bearing surfaces are moving perpendicularly

toward each other

Viscosity of fluid in the gap between the surfaces produces...

pressure which tends to force the lubricant out

Squeeze film lubrication is capable of carrying ....

High loads for short lengths of time

As the fluid is forced out, so the layer of fluid lubricant becomes...

thinner and the joint surfaces come into contact

Variation of fluid film lubrication occurs in articulations where the...

Bearing surfaces are not rigid (like articular cartilage or metal on polymer)

Fluid pressure deforms the ...

Soft articulate surfaces

Deformation causes..

Increase in surface area of bearing surfaces

Deformation reduces ...

Fluid escape between surfaces, so lubricating film

lasts longer, increasing load bearing capacity of surfaces

Deformation alters gaps between surfaces...

h_min (fluid film thickness) in a Hertzian contact system

In human synovial joints, lubricant fluid fluid film thicknesses

~ 0.1 microns to 0.5 microns

Viscosity (n)

- Most important property of lubricating fluid

- Measure of resistance of fluid to shearing flow

- defined as the shear stress on a plane within the fluid, per unit velocity gradient normal to that plane

Dynamic (absolute) viscosity

Tangential force per unit area required to move one horizontal plane with respect to the other at unit velocity when maintained a unit distance apart by the fluid

Stribeck Curve

Method for determining what type of lubrication regime is expected

Target elastohydrodynamic regime

Few material pairs use for artificial joint bearings can achieve hydrodynamic lubrication

Hydrodynamic (fluid film) lubrication is considered...

An “ideal” mode for minimizing wear between two articulating surfaces