Fluid film lubrication

Hamrock, B. J. (1994). Fundamentals of Fluid Film Lubrication. New York McGraw-Hill. 

In gears and ball bearings, extremely high local pressures occur, at least for short periods of time for which the lubricant has no time to flow away from such a high-pressure region. Under these conditions, the above theory of hydrodynamic lubrication would predict a lubrication layer thickness that is smaller than the surface corrugation. However, experiments show that fluid film lubrication still holds under such conditions. To understand this phenomenon, we have to take two effects into account, which lead to elasto-hydrodynamic lubrication ... 

Lu JC. Fluid film lubrication in chemical mechanical planarization. Master s Thesis Tufts University 2001. 

Szeri AZ. Fluid Film Lubrication Theory and Design. Cambridge University Press 1998. 

Herzig" also studied the effect of dispersed molybdenum disulphide in a hydrodynamic bearing, and showed that, where full fluid film lubrication Is not present, dispersed molybdenum disulphide can decrease friction. The coefficient of friction in an oil-lubricated journal bearing is related to the Sommerfeld Number ZN/P where Z Is the viscosity, N the rate of rotation, and P the bearing pressure or specific load. Figure 13.2 shows some empirical relationships between coefficient of friction and the Sommerfeld Number for an oil with different concentrations of dispersed molybdenum disulphide in a foil bearing . 

Vol. 11 Fluid Film Lubrication - Osborne Reynolds Centenary (Dowson et al.. Editors)... 

Bernard J. Hamrock, Fundamentals of Fluid Film Lubrication", P141, McGraw-Hill, 1994... 

The problem with thin fluid film lubrication occurs when the relative motion of the solid surfaces either stops completely, stops at reversal in reciprocating motion or the dynamic loading of a cam on its follower, one gear tooth on another or on a journal within a bearing such that this lubrication mechanism fails and the surfaces make contact. Under boundary lubrication conditions the role of adsorbed molecular films of protective additives is crucial in protecting against wear. 

The Breakdown of Fluid Film Lubrication in Elastic Isoviscous Point Contacts, Wear, 1980, 63, 1, 25. ... 

It is obvious from scrutiny of the nature and mechanism of fluid film lubrication that in order for the to lubricate the rubbing of one solid surface against another it must separate them. Hydrodynamic calculations will show whether or not a fluid film of the requisite thickness theoretically can exist under the given pressure distribution over the opposing surfaces. We may therefore take as an idealized criterion of fluid film failure a calculated film thickness of zero. The physical consequence implied by this criterion is that the solid surfaces can then come into direct contact. 

The problem of wear when the fluid film lubricant is no longer intact is associated with the asperity contact of structured surfaces. The contact behavior of such surfaces was discussed in Chapter 12 wear models governed by asperity contact were described in Chapter 13. Theoretically the laws controlling fluid film thickness can be coupled with asperity contact models to yield quantitative descriptions of the course of wear. In this section we shall deal with those cases in which the function of the lubricant is only to provide a fluid film separating the two rubbing bodies, and the events at the contact, once it is established, are determined by the interaction of mechanical parameters such as load and rubbing speed with the properties of the contacting interface. 

As PTFE so readily forms transferred layers, this type of gross "lubrication" failure is an acute prbblem. In many applications it has to be accepted as a low dry friction is required when full fluid film lubrication is lost. This is sometimes the case at the ends of the stroke in a reciprocating contact. 

In hydrodynamic lubrication, the load is supported by the pressure developed due to relative motion and the geometry of the system. In the regime of hydrodynamic or fluid film lubrication, there is no contact between the solids. The film thickness is governed by the bulk physical properties of the lubricants, the most important being viscosity friction arises purely from shearing of viscous lubricant. 

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