The objective of the work reported in this thesis was to increase understanding of thin film lubrication in elastohydrodynamic (EHL) contacts. Increasingly stringent cost demands upon the manufacturing of lubricated machine elements coupled with new environmental requirements have created a need for this knowledge. Even though the performance demands are continously being increased, requiring higher power density from components, it cannot be at the expense of reliability or cost. It is therefore important to reach an optimum balance where a minimum of effort should be put in making parts; instead, initial in-service operation should form final functional contact surfaces thus reducing the cost whilst improving performance. New lubricants that are environmentally friendly, yet more suitable in function, are needed to enhance this total product performance. When the thickness of a separating fluid is in the same order of magnitude as surface roughness, the EHL contact must be viewed as a system. The performance of such a tribological system is dependent on not only macro geometry and base fluid properties but also on surface topography and chemistry, the molecular configuration of the fluid and its formulation of additives. As a result, the behaviour of the contact is highly complex and difficult to predict theoretically in all lubricating regimes. The importance of each factor can have a great impact on a system’s in-service performance and must be considered under these thin film, high pressure conditions. Presented in this thesis is the development of optical and electrical techniques and their application in the study of EHL contacts under full film conditions and mixed lubrication where the mating surfaces come into direct contact. When possible, specific areas within thin film lubrication have been isolated. Also, complete EHL systems have been explored. Optical techniques have been evaluated and used to study the behaviour of very thin lubricant films. Using the same technique, the dimple phenomenon was investigated; which, due to severe shearing of the lubricant, form a local increase in film thickness and pressure. Shearing of the lubricant also affects overall film performance in the contact. As a rule, the thickness of the film decreases with increased shear. The decrease is, however, dependent on the lubricant type. Ester tended to be less sensitive to shear than a mineral oil partly due to advantageous thermal properties. In an extended investigation, several different types of Esters were tested using a mineral oil as a reference. Results showed that some of the Esters tested, which have mineral-like bulk properties, possess a corresponding film building ability at both low and high shear conditions. Hence, in a film building sense, an Ester may be chosen based on application; not only because of its biodegradability. The developed electrical measurement method simultaneously measures the EHL contact’s resistance and capacitance. These values can be used in determining the amount of direct contact between the mating surfaces and their separation. Even though this method is less resolving than optical methods, it can be directly applied to truly realistic contacts. The method was employed in a study on how surface topography influences run-in and film building properties when using a fully formulated oil. Under the tested conditions, surfaces with sharp asperities ran-in at a higher pace than smoother surfaces. Consequently, the lowest speed at which no direct metal- metal contact occurred was lower for surfaces normally considered as less advantageous. The suggested reason for this behaviour is that rougher surfaces produce higher localised pressures and temperatures that promote the formation of tribofilms that come from additives. Reference tests with pure base oil supported this theory and also found significant differences in run-in behaviour between the base and formulated oils.
Luleå: Luleå tekniska universitet, 2004. , 63 p.