Low adhesion coefficient of wheel/rail gives rise to some problems with driving force to achieve a high speed, as well braking force to stop safely within the distance regulated by railway standard. Moreover, most of surface damages on wheel-treads such as flats, skidding marks and shelling due to low adhesion coefficient, will occur to cause noise and vibration of vehicle and deteriorate the riding quality. According to some questionnaire, the machining wheel-tread due to flats damages accounted for 75% of all wheel machining work in Japan [1]. This means the low adhesion may be one of important factors to increase the maintenance cost of railway. In order to solve such a low adhesion problem, many interesting and important findings about the mechanism of adhesion and the method of improving the adhesion between wheel and rail have been so far obtained [2-5]. However, considering the trend of speed up more than 300km/h in recent years and the trouble of wheel idle still generates in mountain line in autumn, it is still a very important issue to make clear of the behavior of adhesion coefficient and its control particularly under various contamination conditions. Study on the adhesion of wheel/rail in railway system includes many research fields such as tribology, rolling contact mechanics, material science, structural dynamics, heat transfer and others. The author focused on several contaminations, which play very important roles in the adhesion coefficient of wheel/rail interface from the tribological point of view. Those contaminations are rainwater, lubricant, rust, leaves and so on. This paper describes the influence of those contaminations on the adhesion coefficient by means of both theoretical and experimental approaches. Numerical analysis was based on mix-lubrication theory in case of liquid (rainwater, oil) exists between wheel and rail, and laboratory experiments were conducted with a twin-disc rolling contact machine.
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