Lubrication is the process or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each other, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. The interposed lubricant film can be a solid, (e.g. graphite, MoS2) a solid/liquid dispersion, a liquid, a liquid-liquid dispersion (a grease) or, exceptionally, a gas.

Lubrication can also describe the phenomenon such reduction of wear occurs without human intervention.

Hydrodynamic (HD) lubrication, also known as fluid film lubrication has essential elements:

• A lubricant, which must be a viscous fluid.
• Hydrodynamicflow behavior of fluid between bearing and journal.
• The surfaces between which the fluid films move must be convergent.

Hydrodynamic (Full Film) Lubrication is obtained when two mating surfaces are completely separated by a cohesive film of lubricant.

The thickness of the film thus exceeds the combined roughness of the surfaces. The coefficient of friction is lower than with boundary-layer lubrication. Hydrodynamic lubrication prevents wear in moving parts, and metal to metal contact is prevented.

Hydrodynamic lubrication requires thin, converging fluid films. These fluids can be liquid or gas, so long as they exhibit viscosity. In computer components, like a hard disk, heads are supported by hydrodynamic lubrication in which the fluid film is the atmosphere.

The scale of these films is on the order of micrometers. Their convergence creates pressures normal to the surfaces they contact, forcing them apart.

3 Types of bearings include:

• Self-acting: Film exists due to relative motion. e.g. spiral groove bearings.
• Squeeze film: Film exists due to relative normal motion.
• Externally pressurized: Film exists due to external pressurization.

Conceptually the bearings can be thought of as two major geometric classes: bearing-journal (anti-friction), and plane-slider (friction).

The Reynolds equations can be used to derive the governing principles for the fluids. Note that when gases are used, their derivation is much more involved.

The thin films can be thought to have pressure and viscous forces acting on them. Because there is a difference in velocity there will be a difference in the surface traction vectors. Because of mass conservation we can also assume an increase in pressure, making the body forces different.

Hydrodynamic lubrication – Characteristics:

1. Fluid film at the point of minimum thickness decreases in thickness as the load increases
2. Pressure within the fluid mass increases as the film thickness decreases due to load
3. Pressure within the fluid mass is greatest at some point approaching minimum clearance and lowest at the point of maximum clearance (due to divergence)
4. Viscosity increases as pressure increases (more resistance to shear)
5. Film thickness at the point of minimum clearance increases with the use of more viscous fluids
6. With same load, the pressure increases as the viscosity of fluid increases
7. With a given load and fluid, the thickness of the film will increase as speed is increased

8. Fluid friction increases as the viscosity of the lubricant becomes greater