Specific film thickness

Non-conforming contacts (in which the shapes of the bodies are dissimilar enough that under zero load, they only touch at a point or along a line and the stresses are highly concentrated in this contact area) such as; gear teeth, cam-follower joints and rolling-element bearings (balls, rollers) can operate in three modes of lubrication:

• boundary,
• mixed,
• EHD.

The specific film thickness is a principal factor that determines which of these situations will occur. It is denoted as: Λ.

Λ is defined as the ratio of minimum film thickness at the patch center to the composite rms (root mean square) surface roughness of the two surfaces.

, where

h_c – film thickness at center of contact patch
R_1q, R_2q  – rms average roughness of two contacting surfaces.

Λ<1 – the surfaces are in continuous metal-to-metal contact (i.e. boundary lubrication)

Λ>3÷4 – there is essentially no asperity contact

Figure below shows the experimentally measured frequency of asperity contact. Within an EHD gap as a function of specific film thickness.

Figure 10-13.
Effect of Specific Film Thickness Λ on the Asperity Contacts and Fatigue Life

It also shows the effect of specific film thickness on fatigue life of a rolling bearing.

Reference: Machine Design An Integrated Approach, 4th edition - Robert L. Norton

Elastohydrodynamic lubrication

As we all know, bearings can roll, slide or do both simultaneously, therefore lubrication is essential in any bearing to reduce friction (thus wear) and remove heat.

maxxtorque.com

Elastohydrodynamic, is one of the types of the Full-Film Lubrication. In this type, the contacting surfaces are nonconforming (the gear teeth, cam and follower), then it is more difficult to form a full-film of lubricant, since the nonconforming surfaces tend to expel rather than entrap the fluid. 

• At low speed these joints will be in boundary lubrication, and high wear rates can result with possible scuffing and scoring. 
• The load creates a contact patch from the elastic deflections of the surfaces.
• This small contact patch can provide enough of a flat surface to allow a full hydrodynamic film to form if the relative sliding velocity is high enough.

This is all I wanted to tell about such a kind of lubrication. The name itself is really scary-sounding, but not that strange if you know what it means. 

Reference: Machine Design An Integrated Approach, 4th edition - Robert L. Norton

Peter Deak