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Continued from page 15


Figure 3: Temperature vs Kinematic viscosity relationship for tested EALs compared to a reference mineral


Shear rate vs viscosity properties (65°C) One of the six EALs tested in the study was found to display shear-thinning properties due to the breakdown of long-chain polymer additives used in the lubricant formulation under high shear loads. However, this effect is less significant than the pressure and temperature effects within the expected shear-rate range applicable for a stern tube application. Any potential detrimental effects of lubricant shear thinning will be further scrutinised in the next testing phase of the EAL study.


EAL study conclusions “The overall conclusion we can draw from the joint


development project Phase 1 results is that (new and non-degraded) EALs provide safety margins equivalent to those of mineral oils in most operating conditions,” Alnes summarises. “However, there are transient conditions involving high oil film pressures and/or low oil temperatures where EALs will have a reduced load-carrying capacity.” The vast majority of stern tube bearing failures in recent years occurred in the same type of transient conditions, e.g. during hard manoeuvring at high ship speeds (high pressure), during mooring trials (low temperature and high pressure), and when operating with a partly submerged propeller (high pressure). Thus, the observed damage picture fits well with the findings of the study.


EAL study results prompt update of DNV GL shaft alignment rules To ensure that the aft stern tube bearing operates safely within the hydrodynamic lubrication regime, several parameters must be kept in mind - in particular the viscosity of the lubricant and the local peak pressure. “The tests performed during the EAL study revealed that under certain operational conditions the viscosity properties of biodegradable lubricants


Viscosity vs. temperature


Figure 4: Graph depicting kinematic viscosity vs temperature Continued on page 18 16 LUBE MAGAZINE NO.154 DECEMBER 2019


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