The lambda ratio — a critical re-examination

Abstract

The reliable performance of heavily loaded contacts can only be sustained over long periods when a lubricant film fully separates the two bodies and asperities do not interact. Engineering surfaces do have a certain degree of roughness and this would then determine the required lubricant film thickness. Unfortunately thick lubricant films have disadvantages such as high power losses (oil churning) or may not be attainable because of prescribed lubricants or high operating temperatures. In order to optimize bearing selection against these conflicting parameters or in order to design specific surfaces for extreme operating conditions a thorough understanding of the mechanisms of micro EHL or asperity lubrication is required. This required level of understanding goes beyond the current one which employs Λ, the ratio between film thickness and combined surface roughness. When detailed analysis of the behaviour of surface asperities in heavily loaded elastohydrodynamic contacts includes non-Newtonian effects, two phenomena become evident. One phenomenon is the possibility to describe theoretically the collapse of an oil film and to determine when a lubricated rough surface in contact with another surface can come into solid contact through the lubricant film. The other phenomenon, which is closely related to the first one, is the explanation of the well-known fact that the oil film thickness needed to separate two elastohydrodynamically lubricated surfaces is strongly dependent on the structure of the surface roughness and not only on the values of the different surface roughness parameters. Both of these phenomena can only be explained if the pressure distribution in the lubricant film has such high frequency variation that local asperities in the lubricant film become elastically deformed by the pressure distribution, making the contact surfaces conform much more than in the unstressed state outside the high pressure contact zone. The analysis should include details of the surface topography, the amount of lubricant present on the track, the rheological behaviour of the lubricant, the thermal behaviour in the contact, the transient behaviour of non-smooth contacts and the degree to which the asperities are flattened under these conditions. These aspects are addressed in this paper.