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Tribology Lecture I. Tribology. From:  = rubbing. Friction Wear Lubrication. Tribology deals with all aspects of . interacting surfaces in relative motion. - bearings. Friction. Loss of energy due to rubbing. Energy is converted to heat

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Presentation Transcript
from rubbing

Tribology

From:  = rubbing
  • Friction
  • Wear
  • Lubrication

Tribology deals with all aspects of

interacting surfaces in relative motion

- bearings

friction
Friction
  • Loss of energy due to rubbing. Energy is converted to heat
  • Extra energy and force required to overcome friction
  • Causes wear and failure
slide5

Origin of Friction

  • Surface Roughness
  • Solid to solid contact
  • Adhesion
  • Deformation
slide6

Lubrication

Replace Solid to solid contact

Fluid Layer

with a fluid layer - i.e. a lubricant

slide7

Lubrication

Solid rubbing replaced by

Fluid Layer

viscous shearing

slide10

To be useful must support some load

W

Fluid Layer

p

Need pressure in the fluid to support the load

slide11

Hydrodynamic Lubrication

W

Fluid Layer

p

Pressure is generated by motion and geometry of the the bearing in concert with the viscosity of the lubricant

slide13

z

h

h0

p

t

Infinitesimal element

U

p

x

0

B

slide14

Force balance

Viscosity equation

Combine:

slide15

Integrate wrt z

Apply BC’s:

No-slip: ux = U at z = 0, ux = 0 at z = h

yields

Volumetric flow rate (per unit width)

Incompressible flow, q = const. Evaluate at dp/dx = 0:

Solve for dp/dx

1 d reynolds equation16
1-D Reynolds Equation

wn

z

h(x)

ho

x

U

Reynold’s Equation

  • Integrate over x to get p(x)
  • Integrate over x again to get Wn
  • Result gives hoin terms of U, , Wn
example exponential h

U

Example Exponential h

B

wn

z

h(x)

ho

x

integrate wrt x; apply BC’s

p = 0 at x = 0 and at x = -B

solve for p(x), integrate to get

Wn/L, then solve for h0

2 d reynolds equation

U

P(x)

2-D Reynolds Equation

w

Sphere

R

z

Fluid Layer

hc

x

For sphere

Exact solution

slide19

U

Hydrodynamic LubricationPoint Contact

W

Sphere

R

Fluid Layer

hc

slide20

Hydrodynamic Lubrication(Refinement: Both surfaces moving)

W

Sphere

R

U1

Fluid Layer

hc

U2

“Entrainment”

or

“Rolling Velocity”

slide21

Hydrodynamic Lubrication(Refinement: two spheres)

W

R1

U1

hc

U2

Where R is now

“reduced” radius

R2

1

slide22

Hydrodynamic Lubrication

W

R1

U1

Nice theory but as a rule it

greatly under estimates hc

hc

U2

  • Pressure is very high near contact
    • P >>1000atm ( 108 Pa)
  • Pressure Dependence of 
  • Elastic Deformation of Sphere

R2

slide23

Hydrodynamic Lubrication

Elasto-Hydrodynamic Lubrication