Theoretical analysis of composite-film hydrodynamic bearings

Abstract

Major aspects that influence quality and efficiency of the hydrodynamic lubrication of tribological contacts are the surface properties of the contact, the properties of the lubricant, and the structure of the lubricant film. In search for efficiency, past approaches focused on surface modification techniques (surface texturing) and modification of lubricant properties (lubricant additives). The idea in this article is to modify the structure of the lubricant film itself. In contrast to a traditional bearing, the composite-film bearing advocated here relies on a double-layer, composite, lubricant film for separating the load-bearing surfaces. (One of the Reviewers pointed out that ‘the functions of friction modifiers and antiwear additives are to form a boundary lubrication layer, which has a higher apparent viscosity than the bulk lubrication oil, and resulting in a sandwich structure of lubrication film. In this sense, the composite film lubrication has been used in many practical tribosystems already’.)  Our first version of the composite-film bearing was based on an observation that to minimize frictional losses, two immiscible liquids of distinct viscosities will arrange into (stable) configurations that separates the component liquids while concentrating shear deformation in the low viscosity component. However, particularly in turbulent flow of the lubricant, reliance on the above principle might not be adequate, and, in this article, we offer a new composite-film bearing construct in which actual physical boundary separates the two component films. For journal bearings, this naturally leads to the concept of the ‘floating-ring’ bearing, with two distinctly different lubricants occupying the inside and the outside clearance spaces. We show here that the new version of the composite-film bearing, i.e. a floating-ring bearing lubricated with two distinctly different liquids, performs not unlike its first kind; specifically, its application leads to substantial reduction of viscous losses, over 90% in some cases, while maintaining safe film dimensions.