DNV GL’s accumulated know-how has enabled the classification society to develop state-of-the-art design rules for shaft alignment, including a highly detailed approach to verify the aft stern tube bearing design for both static and running conditions. The calculation method for the minimum shaft-rotational speed at which hydrodynamic lubrication of the aft stern tube bearing is ensured was introduced in 2013. Based on a quasi-empirical solution of the Reynolds equation for journal bearings, it accounts for the challenging conditions that are typical for stern tube bearings, such as very uneven load distribution and extensive misalignment between shaft and bearing. The calculation method aims to ensure hydrodynamic lubrication in the areas exposed to the maximum bearing pressure, setting a limit for the minimum allowable continuous operational shaft speed at dead slow ahead and engine maximum continuous rating (MCR).
EALs available on the market, the study has so far identified two main aspects where the tested biodegradable lubricants behave differently when compared with a reference mineral oil: pressure- related and temperature-related viscosity properties. Additionally, one of the EALs are found to display shear thinning properties at high shear rates.
Viscosity vs pressure The tested EALs show lower pressure vs viscosity coefficients than a similar-grade mineral oil throughout the relevant temperature range. This is confirmed by other published studies such as Akihiko Yano (2015). Consequently, the true viscosity in the aft stern tube bearing oil film at high load conditions is lower for EALs compared to mineral oils of equal grade. Given that oil film thickness and oil film load capacity are proportional to viscosity, the safety margins towards oil film breakdown and consequent bearing failure are reduced in these operating conditions. Increasing the specified viscosity of the EAL is one solution to avoid operating with a reduced viscosity in high load conditions as compared to a mineral oil.
Figure 1: Stribeck curve showing the three different operating regimes for a journal bearing
Originally, the specified kinematic viscosity was the only input parameter needed to define the required stern tube lubricant to be used. This was based upon the fact that almost all installations used various engine crankcase oils in the stern tube. Regardless of make, these traditional mineral oil lubricants are very similar with regard to their viscosity properties and load-carrying capabilities. The first phase of the EAL study was set up to investigate these viscosity properties and to quantify any potential differences between the new biodegradable lubricants and the traditional mineral oils.
Major findings of the EAL study While investigating the viscosity properties of various
Figure 2: Kinematic viscosity comparison of tested EALs with a reference mineral oil
Viscosity vs temperature EALs intended for stern tube applications have a significantly higher viscosity index than their mineral oil alternatives. Typical values range between 135 and 210 for stern tube EALs and 95–105 for mineral stern tube oils. For temperatures lower than 40°C the viscosity of an EAL will be lower than for an equal-grade mineral oil. This might be safety-critical in certain types of operation, such as cold start-up and mooring trials where the stern tube system will typically be at seawater temperature. For temperatures above 40°C the effect will be the opposite, resulting in a positive viscosity effect for EALs compared to mineral oils.
Continued on page 16 LUBE MAGAZINE NO.154 DECEMBER 2019 15
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