Heat Generation and Temperature Rise

Abstract

Heat generation in the oil film and the accompanying temperature rise are the most important factors in bearings. For example, temperature rise is the factor indicating the operating conditions of a bearing most directly, and if the temperature rise is small, the bearing is probably in a good operating condition. Generally speaking, the problems of heat generation and temperature rise are hard to handle, and so they were not considered in, for example, the early theory of Reynolds. It is thanks to the later development of computers that this kind of problem can now be handled theoretically.Let us first consider the meaning of heat generation and temperature rise in bearings. To begin with, the heat generation essentially corresponds to the loss of mechanical energy due to shear in the lubricant film (solid friction is sometimes also present) of a bearing. Therefore, the less heat generated the better.The effects of temperature rise constitute a bigger problem than the heat generation. The temperature rise decreases the viscosity of the lubricating oil, and thus the minimum film thickness and allows seizure to occur more easily. Further, the temperature rise changes the bearing clearance through the thermal deformation of the bearing metal and casing, thus changing bearing performance.Furthermore, an even bigger problem is that the boundary lubrication performance of the lubricant film will suddenly and almost completely be lost if the oil temperature exceeds a certain critical temperature. A lubricant film has in effect a kind of transition temperature. If the oil temperature is lower than this, the molecules of lubricant combine with a metal surface strongly, and also with the adjacent molecules of lubricant, and form a strong lubricant film on the metal surface. However, if the temperature exceeds the transition temperature, these combinations are lost and the strength of lubricant film will fall markedly. Thus the performance of boundary lubrication of the oil film will be lost and seizure can take place very easily. Therefore, the oil temperature must be kept under the transition temperature, which is unfortunately relatively low (for example 100 • C for low-cost oils and 160 • -170 • C for high-quality oils).In addition, if the oil temperature exceeds 150 • C, the rate of oxidization (or degradation) of the lubricating oil is markedly increased. Also at 100 • C, the tensile