Study on the load carrying capacity of sliding bearing lubricated by synthetic ester oils

To meet with the latest environmental regulation, as environmentally acceptable lubricant (EAL) in marine field, biodegradable oils based on synthetic ester oils, have been applied to stern tube bearings of many new building vessels since the end of 2013, when the US EPA∗ effectuated the 2013 VGP∗∗ regulation. These synthetic ester oils are of the same viscosity grade, ISO VG100, as commonly used mineral oil. However, up to now, there has been no report on verification of tribological characteristics to use them for stern tube bearings, including the 2013 VGP regulation. Therefore, physical properties of them were measured in tribological aspects, and load carrying capacities while mooring trial condition were clarified by the tests using 100 mm inner diameter plain bearing, comparing to mineral oil. As the result, the following are derived. 1) There are three major differences in viscosity characteristics of synthetic ester oils depending on temperature, shear stability and pressure, which gives them lower load carrying capacity. 2) Sliding velocities at friction rising points of synthetic ester oils in Stribeck curve are larger than the one of mineral oil by the effects above, which also indicates lower load carrying capacity of them. ∗)EPA: Environmental Protection Agency ∗∗)VGP: Vessel General Permit. Copyright © 2015 Japanese Society of Tribologists.