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Analysis of Concept and Result of Mean Rolling Speeds U in Lubrication Theory

U, which indicates the quantity of lubricants being dragged into contact area between surfaces, is of importance in both elastohydrodynamic lubrication and hydrodynamic lubrication. However, in nearly all gear lubrication references U is incorrectly defined as the mean entrainment speed of a pair of points which moving in an absolute space, and, consequently, results induced from such a U keep valid only in some specific occasions. A new definition is given that U is the mean relative speed of a pair of points to the contact area, the validity of the definition is checked and the factors influencing U are discussed.