Some Final Comments on Lubrication

Before closing the book on lubrication, there are a few additional points of interest that should be mentioned briefly. Two potentially important ones from a practical standpoint are the so-called Rehbinder effect and weeping lubrication. The Rehbinder effect relates to the effects of the adsorption on the mechanical strength of materials. While there exists some uncertainty on the matter, there is significant evidence that the adsorption of surfactants or other materials onto surfaces, especially in cracks and surface flaws, can reduce the mechanical strength of the material. 

The second potentially important point to be mentioned is that of weeping lubrication, in which a lubricant is trapped inside a porous material, in surface cracks and flaws, or absorbed by a polymeric material, to be slowly extruded as needed when subjected to pressures and temperatures as a result of friction (Fig. 18.14). Such a weeping action can provide a mechanism for the continuous renewal of a lubricating layer under conditions in which confinement of a normal lubricant would be impractical or impossible. 

Weeping lubrication has been suggested as one of the possible mechanisms by which body joints are lubricated by the synovial fluid. In that case, the 

FIGURE 18.14. For some porous or absorbent materials (i.e., I ubber rollers), it is possible to incorporate (impregnate) lubricant into the material so that it can be appUed continuously to the interacting surfaces by pressure release, temperature changes, or simply by diffusion. Such a process may be described as weeping lubrication. 

Lubrication is clearly an important practical topic that is continually changing and improving to meet the new demands of technology. The above discussion is obviously superficial (pardon the pun) but does introduce many of the more fundamental aspects of related problems and perhaps some solutions. While there exists a great deal of uncertainty and some disagreement on details, there are some general conclusions that can be drawn and that seem to have broad applicability. 

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