μ-EHL STUDIES ON LINE CONTACTS

Abstract

This thesis deals with the subject of micro-elastohydrodynamic lubrication (μ-EHL). This type of lubrication prevails when the roughness of lubricated surface is comparable with the thickness of fluid film generated by elastohydrodynamic (EHD) action. Recently, due to the down-sizing of machine elements and increased severity of operating conditions, the lubricating films have reduced to a level where the asperity interaction has started to play an important role. Under such conditions the collision of asperities results in significant rise in contact pressure with reduced value of film thickness, thereby resulting into the failure of lubricated contacts. The failures such as scuffing and micro-pitting arise due to the collision of asperities and can be averted by thorough understanding of μ-EHL phenomenon within the lubricated contacts. μ-EHL occurs in between the non-conformal contacts of anti-friction bearings, gears, cam-followers etc. In particular, this study is directed towards the numerical solution of the μ-elastohydrodynamically lubricated line contact problems. Experimental investigations pertaining to the performance variability of lubricants and the influence of surface textures on the performance of the contacts have also been investigated. The thesis begins with an introduction to the subject followed by a brief historical review going from the earliest achievements in the field of μ-EHL. As μ-EHL has its roots embedded into the elastohydrodynamic lubrication (EHL), various aspects of EHL too has been reviewed. From the review it is concluded that, in spite of the considerable progress that has been made over the years, the actual and realistic predictions for the μ-EHL problems considering realistic input data still has not been investigated fully. As a consequence of this accurate prediction pertaining to the lubricant film thickness, contact pressure and friction could not be made. The influence of surface roughness considering Newtonian and non-Newtonian Eyring model has been extensively studied over the years. However, the non-Newtonian Eyring fluid model is not recommended for shear thinning lubricants. Hence, for an accurate analysis, the practical conditions along with suitable non-Newtonian model should be taken into consideration. This also calls for a fast and viii stable numerical solver. The solver should take into consideration the characteristic properties of present generation lubricants and their constitutive relations regarding non-Newtonian behavior, geometric irregularities and the textures of the surfaces, operating conditions and the characteristics of present generation engineering materials. In μ-EHL contacts the amount of lubricant used continues to decrease and therefore so-called starved lubrication conditions may arise. Therefore, in the present work this is also accounted for. Further, the transient nature of the problems has been considered, as the movement of asperities within the contact is a time dependent phenomena. The development of such solver for the line contact problem is the subject of the first part of this thesis. Following the explanation of a physical mathematical model describing the lubrication of μ-EHL contact, the high order Discontinuous Galerkin (DG) method that basically enable fast solution with lesser number of grid points has been outlined. Subsequently, the spatial and temporal discretization schemes have been outlined for the solution of transient μ-EHL problems. This result in solver that enables the solution of μ-EHL line contact problem using lesser number of grid points on a small capacity computer. Further, the test facilities used and the procedures followed for the experimental evaluation of the lubricants and textured μ-EHL contacts has been discussed. The second part of this thesis basically deals with the findings obtained from the theoretical and experimental investigations. For the line contact situation, initially the “standard” EHL problems assuming perfectly smooth surfaces are solved for a wide range of load conditions. Thereafter, the “standard” EHL problem was upgraded to μ-EHL problems by incorporating surface roughness and the non-Newtonian behavior of lubricants. The surface roughness has been incorporated following both the stochastic and deterministic approaches. The non-Newtonian behavior of lubricants has been modeled using Power law fluid and Carreau fluid models. The newly developed algorithm is solved for more realistic inputs of geometric, material and operating parameters. Also transient effects of individual surface irregularities and the moving two-sided roughness are discussed. The influence of shape and size of the roughness feature on the contact pressure and the lubricant film thickness have been presented. ix Experimental investigations on performance variability of lubricants and the influence of surface texture on the performance of the contact have been discussed. The theoretical and experimental studies undertaken have led to some interesting new insights both in general as well as specific with respect to the lubricant and materials in μ-EHL line contacts. Finally, this thesis is concluded with some key recommendations for future research.