Lubrication - Mechanical Engineering · ME 383S Bryant January 24, 2005 5 Stribeck Curve Defines fluid lubrication regimes o Boundary lubrication: asperity contacts support load,

ME 383S Bryant January 24, 20051

Lubrication

• Lubricant: material placed between surfacesto reduce friction and/or wear. Can be liquid,solid, or gas.

• Pressures/forces separate surfaces• Reduced surface contact reduces friction &

wear• Lubricant sacrificed• Mechanically compliant• Moderate friction: µ ≈ 10-2 to 10-1

ME 383S Bryant January 24, 20052

SOLID LUBRICANTS• Contact between surfaces reduced

⇒ Reduced surface wear ( & maybe friction )

• Lubricant sacrificed

• Examples

Powders

Graphite, MoS2 lubrication: easy shear

Rocks, sand

• Stiff contact

ME 383S Bryant January 24, 20053

Lubrication in Ancient Eqypt

Figure pouring sand (solid lubricant) in front of rail

ME 383S Bryant January 24, 20054

FLUID FILM BEARINGS

• Relative motionof surfaces

• Fluid sheared

• Pressures generated in film

• Pressures separate surfaces,reducewear & friction

• Problemsload support limitednot "stiff", compliant

journal

bearing

fluidpressures

rotation

Example: Journal bearing

• Effectiveness of liquid lubricant depends onfilm thickness

• Film thickness gauged by Stribeck curve

ME 383S Bryant January 24, 20055

Stribeck Curve Defines fluid lubrication regimes

o Boundary lubrication: asperity contactssupport load, despite thin film

o Mixed film: asperity contacts +hydrodynamic support of load

o Hydrodynamic lubrication: full film formed,surfaces do not contact

Friction vs non-dimensional stribeck numberηN/P

0.001

0.01

1

10

0.1

frictioncoefficient

stribeck number η N/P10 205

hydrodynamiclubrication

mixed film lubrication &elastohydrodynamic

boundarylubrication

η: dynamic viscosity, N: speed, P: pressure