A dimensionless number, written as the Greek letter mu, found by dividing the force needed to slide one surface over another by the load holding them together. Clean dry steel on steel runs around 0.5 to 0.8, a boundary-lubricated contact around 0.1, and a full hydrodynamic film can fall below 0.005. Static friction, the force to start motion, is usually higher than kinetic friction, and that gap is what drives stick-slip.
Lab ran coefficient of friction on the new chain lube, the old stuff sat at 0.12 and this one's down near 0.08.
Also heard as
- COF
- mu
- friction coefficient
Related terms
Stribeck curve
Plot of friction against viscosity times speed over load, showing the boundary, mixed and full-film regimes.
Stick-slip
Jerky motion where sliding parts alternately grab and jump because static friction beats sliding friction.
Boundary lubrication
Regime where surfaces touch through the oil and protection comes from chemical surface films, not fluid pressure.
Tribology
The science of friction, wear and lubrication at every contact where parts slide or roll.