The lubrication of gear teeth is reviewed including some key aspects of recent theoretical research and current practice. A simple estimate of the thickness of the lubricating film in a typical pair of spur gears is presented on the basis of classical smooth-body isothermal, elastohydrodynamic lubrication theory. The deficiencies of this simple calculation are then discussed; these include roughness, friction, churning, starvation and contamination, all common features of practical gearing. Three simple methods are described for estimating the tooth temperature and its consequent effect on film thickness.
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