Types of lubrication

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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

Elastohydrodynamic

This 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

How boundary lubrication got its name. This is remembering a while back so bear with me. A chap was hired to investigate the cause of failure or train carriage wheels. He run test on journal bearings using pressure gauges mounted around the circumference of the bearing. After repeatedly blowing these gauges due to the very high pressure created in the oil wedge, he eventually came up with the idea of dynamic lubrication and the oil wedge. I believe the cause of failure of the carriage bearings was insufficient clearance preventing the journal from lifting and leading to rub.
Anyway, this chap gave a lecture to his peers. Someone in the audience said that he could understand his theory of dynamic lubrication, but asked to him explain how the bearing was lubricated during starting. To which our chap replied something along the lines of- " ....there are boundaries of lubrication beyond our knowledge...".