Boundary Lubrication

  • fluid film thickness basically molecular 10-7cm
  • depends on the molecular properties of the lubricant & solid members, often chemical
  • ( E.P. additives actuated by high temperature generated by rubbing)
  • independent of the shape & velocity in so far as these influence the temperature generated from rubbing.
  • obeys classical laws of the friction
  • Frictional coefficient = 0.05 to 0.1

Hydrodynamic Lubrication

  • self acting and having fluid thickness 1 mm to 0.01 mm
  • Depends on the viscosity of the lube oil & the shape & relative motion of the solid surfaces
  • Independent of the properties of the solid members; so long as the elasticity does not deform the shape. & thermal properties do not effect the temperature of the lubricant
  • does not obey the classical friction laws.
  • Friction coefficient = 0.001
  • film may form by self action or by the hydrostatic pressure of the lube oil.
  • externally pressurised as above except the separation of surfaces is caused by the fluid being injected under the pressure

Hydrostatic Lubrication:

Where the oil film cannot be formed due to the motion of the moving parts, oil pressure has to be supplied externally. Such type of lubrication is called as Hydrostatic lubrication.

  • For the slow moving heavy parts, their relative motion is not enough to provide self-generated pressure for the lubrication & hence pressure is provided externally with the help of a pump.
  • For example, many of the crosshead bearings design need an additional crosshead lubrication pump to boost the pressure for the crosshead bearing lubrication because the pressure cannot be self-generated.

Elastohydrodynamic Lubrication

This is the type of the lubrication used with the rolling element bearings. To resolve , the material of the running surface deforms under high pressure as the rolling element passes over it. The oil wedge forms in this deformation.

  • Deformation & increased viscosity with pressure are involved
  • Frictional coefficient is 0.05
  • film thickness less than the hydrodynamic