Wear of sliding surfaces in machinery is one of the major causes of mechanical failure. Thus, detecting wear on sliding surfaces is very important for preventing mechanical failure. Current diagnosis methods for detecting wear particles such as a spectrometry oil analysis program, particle count method and ferrography method are very useful for it. However, these methods need high cost, expertise and long time for the analysis. Moreover, the particle count method can only measure the size and

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... e number of particles and it cannot indicate whether the lubricating oil is degraded by oxidation or solid particles. Therefore, the diagnosis method which can evaluate degradation cause and level of lubricating oil is required for effective maintenance. The authors have developed a new diagnostic method for lubricating oils by the colorimetric analysis of membrane patches. In the previous study, we reported that there is a good correlation between membrane patch color and contamination in lubricating oil. From the results of our previous study, it is possible to diagnose wear on sliding surfaces by using membrane patch color. The purpose of this study is to develop new wear monitoring method of sliding surface in machinery by using membrane patch color. First, we conducted filtration tests to investigate degradation causes of lubricating oil in actual machinery. The results showed that they are classified into solid particles and/or oxidation products. Secondly, we prepared oil samples which were artificially degraded by solid particles and/or oil oxidation products based on the filtration test, and we conducted wear tests in the degraded oils by using block-on-ring tester. The results presented that it is important to detect not only the number of particles but types of particles for the monitoring of wear on sliding surfaces. Finally, we investigated the relation between ISO cleanliness code and membrane patch color and we proposed a new parameter Ic using the ISO code. As a result, it is shown that it is possible to detect wear on sliding surfaces easily using Ic and membrane patch color.