Duncan Dowson: Pioneer of elastohydrodynamic lubrication of gears

The paper briefly reviews Duncan Dowson's ground-breaking contribution to the theory of elastohydrodynamic lubrication in relation to the understanding of lubrication of gear tooth contacts. His early work with Higginson on numerical modelling of elastohydrodynamic lubrication finally explained how gears can operate successfully, and avoid wear, due to the generation of a stiff, protective oil film. The resulting minimum film thickness equation stands as a reliable reference formula for calculations in gear design standards. The paper includes examples of how elastohydrodynamic lubrication theory has been developed by the present authors and their co-workers, and applied to aid the design of engineering components such as worm gears, thrust rims and profile-modified helical gears. Also included is its extension to include the important effects of surface roughness at the asperity level (micro-elastohydrodynamic lubrication) and its relevance to the current, troublesome problem of micropitting

Running-in and micropitting behaviour of steel surfaces under mixed lubrication conditions

The paper investigates the running-in of hardened steel surfaces under mixed lubrication conditions. Pairs of surfaces of both equal and differing hardness were loaded together under rolling/sliding conditions in a twin-disk rig, and the evolution of surface topography was investigated using in-situ profilometry. Evaluation of roughness parameters, height distributions and profile relocation showed that the running-in of these surfaces is a rapid process where the most prominent asperity tips undergo plastic deformation during the initial loading cycles. Finally, the pair of equal hardness disks, following further running in a separate series of experiments, was found to suffer from micro-pitting. This micropitting predominantly occurred along the tips of prominent asperities, and the potential link between running-in and surface failure is discussed

An investigation into mixed lubrication conditions using electrical contact resistance techniques

Electrical contact resistance (ECR) techniques are widely used to study mixed-elastohydrodynamic lubrication conditions, where direct asperity contact takes place in conditions of very thin lubricant films. This paper presents the use of the ECR technique to study realistic mixed-EHL contacts, identifying the high frequency variation of instantaneous contact resistance on a repeatable basis between two superfinished surfaces. The variation of mean ECR measurements with operating conditions for ground surfaces in contact is investigated, and it is shown that they are strongly related to the lubricant film thickness and lambda ratio. Thermal effects are considered and shown to be highly influential on both the mean and instantaneous contact resistance. The influence of load on contact resistance is also investigated

Transient elastohydrodynamic point contact analysis using a new coupled differential deflection method, Part 1: theory and validation

The paper presents a transient analysis technique for point contact elastohydrodynamic (EHL) lubrication problems using coupled elastic and hydrodynamic equations. Full coupling is made possible by use of a novel differential deflection formulation. The way in which the differential deflection is incorporated into the overall solution method for a point contact is discussed. A range of spatial and temporal discretization methods are incorporated and compared. The method is validated under transient conditions by a detailed comparison with published work produced using a different, independent method incorporating a moving roughness feature. A comparison of the results with different discretization methods leads to the conclusion that spatial central differencing with a Crank-Nicolson temporal discretization is the most effective finite difference scheme, and this is generally equivalent to the finite element discretization given in detail in the paper. A comparison of the results produced for moving rough surfaces suggests that the finite element formulation is preferred

Theoretical and experimental investigations of lubrications at point contacts.

A theoretical and experimental study of hydrodynamic lubrication between surfaces which touch, nominally, at a single point has been carried out. The first theoretical study involves the most general case of two surfaces of any curvature and having any directions of sliding and rolling motions. The theory has been applied to derive the dependence of oil film thickness between hypoid gears upon the design parameters. The second theoretical study is concerned with the hydrodynamic lubrication of a sphere spinning about its point of contact with a cylindrical groove. This motion is a simulation of one element of the kinematic behaviour of a ball in an angular contact bearing and calculations have been made to show the relative effect of this motion in the generation of a hydrodynamic film under these conditions. Experimental investigations have been made, using a crossed cylinders machine, into the nature of the oil film under conditions of elastohydrodynamic lubrication at point contacts. The optical interference technique has been used to derive the dependence of film thickness between cylinders of steel and glass upon speed and load using a medium viscosity mineral oil as a lubricant. The factors which affect the overall shape of the film under these conditions have also been investigated. Finally, results are presented of a direct comparison between the optical and capacitive methods of measuring the film thickness at point contacts