|II Boundary Lubrication
If the speed is very low there will be no pressure build up in the lubricant at all and hence the loading is for 100% carried by the asperities in the contact area, protected by adsorbed molecules of the lubricant and/or a thin oxide layer. Characteristic for boundary lubrication is the absence of hydrodynamic pressure. Dry contact is excluded from boundary lubrication.
|I/II Mixed Lubrication
A hydrodynamic pressure is build up in the lubricant when the speed increases. Characteristic for mixed lubrication is that the loading is carried by a combination of the hydrodynamic pressure and the contact pressure between the asperities of both surfaces. It is the intermediate region between boundary lubrication and hydrodynamic lubrication.
At high speed the hydrodynamic pressure increases such that the surface asperities are completely separated by a lubricant film. Characteristic for hydrodynamic lubrication is that the load and hydrodynamic pressure are in equilibrium.
Aquaplaning, where a thin water film separates the tyre and the road is a widely known example of elastohydrodynamic lubrication.
|left: Stribeck curve low
contact pressure (dispers contact
in journal bearing)
right: Stribeck curve high contact pressure (concentrated point or line contact)
The arrows direct the effect of running in. Optimal
running in conditions are located beyond the BL - ML
transition. Most asperities in the contact area are
in contact and well lubricated.
|To investigate lubricant additives,
surface treatments and coatings the transition diagram is favourable. In
this diagram, the region is visible where adhesion occurs (III)