Boundary(i) fluid film thickness basically molecular 10-7cm
(ii) depends on molecular properties of lubricant and solid members, often chemical( E.P. additives actuated by high temperature generated by rubbing)
(iii) independent of shape and velocity in so far as these influence the temperature generated from rubbing.
(iv) obeys classical laws of friction
(v) Frictional coefficient = 0.05 to 0.1
Hydrodynamic-self acting (i) fluid thickness 1 mm to 0.01 mm
(ii) Depends on the viscosity of the lube oil and the shape and relative motion of the solid surfaces
(iii) Independent of properties of the solid members; so long as the elasticity does not deform the shape. and thermal properties do not effect the temperature of the lubricant
(iv) does not obey classical friction laws.
(v) Friction coefficient = 0.001
(vi) film may form by self action or by Hydrostatic pressure of the lube oil.
Hydrodynamic-externally pressurised as above except the separation of surfaces is caused by fluid being injected under pressure
ElastohydrodynamicThis is the type of lubrication used with rolling element bearings. To clarify, the material of the running surface deforms under high pressure as the rolling element passes over it. The oil wedge forms in this deformation.
(i) Deformation and increased viscosity with pressure are involved
(ii) Frictional coefficient = 0.05
(iii) film thickness less than Hydrodynamic