In this paper, we analyzed influences of environmentally acceptable lubricants (EAL) with natural seawater on tribological characteristics of stern tube bearings by using 3-pin on disk frictional test apparatus with specimens for stern tube bearing material and propeller shaft material. We focused on 4 types of lubricants, traditional mineral oil, saturated ester-based EAL, unsaturated ester-based EAL and non-ester-based EAL, and evaluated the frictional characteristics under various operating conditions by using stribeck curves. As a result, even if natural seawater enters mineral oil, frictional characteristics of this oil do not change significantly. However, frictional characteristics of saturated ester-based EAL and unsaturated ester-based EAL are affected by mixed natural seawater. When natural seawater is mixed in this saturated ester-based EAL and roughly eight weeks passed, friction coefficients between stern tube bearing material and propeller shaft material are larger compared with that without natural seawater. When non-ester-based EAL is used as the lubricating oil for stern tube bearing and the propeller shaft, its friction coefficient becomes very large.
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