lubrication - mechanical engineering · me 383s bryant january 24, 2005 5 stribeck curve defines...
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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