Some physical aspects of lubrication and wear

The application of physical principles in some studies of lubrication and wear is described. The states of boundary and hydrodynamic lubrication are contrasted and the importance of identifying the state of lubrication of a system is emphasized. Experiments which demonstrate that the lubrication of gears and roller bearings is essentially hydrodynamic are described and the consequences of solid-solid contacts through the hydrodynamic film are discussed. The function of boundary additives such as are present in E.P. oils in mitigating the damage consequent upon solid-solid encounters is described and so is the influence upon damage of the surfaces themselves. Lastly the importance is indicated of wear experiments conducted without lubrication. Such experiments have a relevance to solid-solid encounters through hydrodynamic films and are of direct application to systems in which fluid lubrication cannot be tolerated. So often a little oil suffices to still a squeal or to restore a neglected machine to use that sometimes, without conscious effort, the scientific interest of lubrication and wear and their economic importance pass unmarked.