A study evaluating the use of environmentally acceptable lubricants (EALs) in stern tube bearing installations has concluded oil viscosity is key to selecting an EAL, reports Motorship.
Interest in studying EAL performance was crucial after an increase in the number of new build stern tube bearing failures in 2014.
Vickers Oil technical director Chris Wholley says that their team was able to develop a deeper understanding of lubricant behavior in a journal bearing, which has helped them us to better advice the ship owners and operators on how to reduce the adverse effects of shaft misalignment.
More stable than mineral oil
ABS’ Joint Development Project (JDP) with Vickers Oil and Imperial College London identified key characteristics of EALs compared to mineral oil lubricants. It was seen that the viscosity of EALs is more stable than that of mineral oils, with respect to changes in temperature.
Also, EALs demonstrated relatively lower pressure-viscosity coefficients than mineral oils, indicating that EALs may form thinner oil films, potentially insufficient to protect surfaces from wear under edge loading conditions.
Derek Novak, ABS senior vice president of global engineering and technology, says that oil viscosity is the main determinant in choosing the optimal EAL.
Ideal for sensitive shaft alignment
The project also found that using EALs with higher pressure-viscosity coefficients, compared to other EALs of the same viscosity, may provide some safety margin in shaft-alignment sensitive installations.
EALs offers less margin for shaft misalignment conditions, therefore using EALs with higher viscosity when compared to a mineral oil of lower viscosity may provide an additional tolerance for edge loading wear.
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Source: The Motorship
