Types of lubrication and viscosity

Lubrication reduces friction and wear and carries away heat by putting a film of lubricant between sliding surfaces.

Types of lubrication

1. Hydrodynamic (thick film): the surfaces are completely separated by an oil film whose pressure is generated by the motion itself (wedge action). Friction is very low (coefficient 0.001 to 0.01) and there is no wear. Example: journal bearing running at normal speed.
2. Hydrostatic: oil is supplied under pressure from an external pump, so the film exists even at zero speed. Used for heavy slow machines such as telescopes and large turbines.
3. Elastohydrodynamic (EHL): in rolling contacts such as gears and rolling bearings, the high contact pressure deforms the surfaces elastically and raises the oil viscosity so a very thin film is formed.
4. Boundary: the film is only a molecular layer, and the surfaces touch at asperities. Friction coefficient 0.05 to 0.2. Occurs at low speed, high load or starting and stopping. Additives (anti-wear, extreme pressure) protect the surface.
5. Mixed (partial): a combination of boundary and thick film lubrication.

Friction against $ZN/p$

   f
   |\
   | \                 /
   |  \      mixed    /  thick film
   |   \_           _/
   |boundary \_____/
   +------------------------> ZN/p
              ^
          minimum friction
          (critical value)

Here $Z$ is viscosity, $N$ speed and $p$ bearing pressure. To the right of the minimum, the friction rises slowly with $ZN/p$ and the bearing is in stable lubrication: if the temperature rises, viscosity falls, $ZN/p$ falls, friction decreases and the bearing recovers. To the left, a fall in viscosity increases friction and heat further, leading to seizure (unstable). Journal bearings are designed so that $ZN/p$ is about three times the critical value.

Viscosity

• Dynamic (absolute) viscosity $\mu$ (or $Z$): the ratio of shear stress to velocity gradient, $\tau = \mu\,\dfrac{du}{dy}$. SI unit Pa.s (N.s/m$^2$); 1 poise = 0.1 Pa.s; 1 cP = 1 mPa.s.
• Kinematic viscosity $\nu = \mu/\rho$. SI unit m$^2$/s; 1 stoke = $10^{-4}$ m$^2$/s.

Viscosity falls with temperature; the viscosity index measures how little it changes.