THE LAMBDA RATIO - A CRITICAL REEXAMINATION

The reliable performance of heavily loaded contacts can only be sustai ned over long periods when a lubricant film fully separates the two bo dies and asperities do not interact. Engineering surfaces do have a ce rtain degree of roughness and this would then determine the required l ubricant film thickness. Unfortunately thick lubricant films have disa dvantages such as high power losses (oil churning) or may not be attai nable because of prescribed lubricants or high operating temperatures. In order to optimize bearing selection against these conflicting para meters or in order to design specific surfaces for extreme operating c onditions a thorough understanding of the mechanisms of micro EBL or a sperity lubrication is required. This required level of understanding goes beyond the current one which employs LAMBDA, the ratio between fi lm thickness and combined surface roughness. When detailed analysis of the behaviour of surface asperities in heavily loaded elastohydrodyna mic contacts includes non-Newtonian effects, two phenomena become evid ent. One phenomenon is the possibility to describe theoretically the c ollapse of an oil film and to determine when a lubricated rough surfac e 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 lubricate d surfaces is strongly dependent on the structure of the surface rough ness and not only on the values of the different surface roughness par ameters. 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 deforme d by the pressure distribution, making the contact surfaces conform mu ch 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 behaviou r of the lubricant, the thermal behaviour in the contact, the transien t behaviour of non-smooth contacts and the degree to which the asperit ies are flattened under these conditions. These aspects are addressed in this paper.