Effect of lubricant darkening on tribology characteristics of the tin-based white metal bearing in the 2-stroke marine diesel engine has been investigated. Through a reciprocating wear test, friction and wear characteristics were evaluated as a function of lubricant darkening. The darkness of the lubricant increased with wear test time even under the condition of less possibility of soot contamination. The principal cause responsible for darkening was found to be the very fine tin particles dispersed in the lubricant. The formation of these particles is attributed to the wear of the bearing. However the wear loss of the bearing metal obtained from the wear test with the darkened lubricant was similar to that with the initial clean lubricant. This means that the lubricant darkening is not a main factor responsible for the degradation of white-metal bearing durability.

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