The failure of perspex in lubrication contacts.

The catastrophic failure of lubricated Perspex disks and cylinders rotating in heavily loaded point and line contacts in conditions of both rolling and sliding has been investigated. Observation of specimens in various stages of failure suggested that both thermal effects and mechanical stresses were involved in the failure mechanism. It has been shown that temperatures generated by conventional "flash temperature" mechanisms are not likely to be a significant factor in the mode of failure. Moreover, since this form of failure occurs under conditions of pure rolling the generation of heat by hysteresis loss appears to be a significant factor. The quasi-elastic theory of rolling friction developed by Greenwood, Minshall and Tabor has been used to derive the intensity of heat generation, and hence the temperature distribution, in the sub-surface regions; this theory has also been developed to cover a wider range of materials and conditions. The theoretical sub-surface temperatures have been compared with those measured, over a range of loads and speeds, in experiments using embedded thermocouples. The heat transfer coefficient at the surface of a rotating disk (which is a factor influencing sub-surface temperatures) has been estimated, using the Chilton-Colburn relationship, from experimental measurements of the mass transfer coefficient. Experiments to measure the coefficient of rolling friction are described, and the relationship between hysteresis loss factor, frequency and temperature is derived from the results. The effect of the variation of hysteresis loss factor with temperature upon the subsurface temperature has been discussed. The relevance of these theories and experiments to the original observations of failure of Perspex, and to the successful operation of rolling systems using polymers, has been discussed.