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Marine Why Shear Stability Is a Critical Factor in EAL Gear Oil Selection

Analyzing lubricants for marine applications isn’t as easy as it once was. Why?

Because selecting a gear oil for use in marine thrusters or a stern tube lubricant for use with propeller shaft bearings is typically based on analyzing price/performance characteristics of the lubricant and its availability.

What is different about selecting EALs?

Now with the implementation of the 2013 Vessel General Permit, this analysis must be expanded to include environmentally acceptable lubricants (EALs) that meet the new EPA standards. What does that mean for different EAL formulations?

  • To meet environmental requirements, EALs are blended from base oils other than mineral oil.
  • They must still meet the same lubricant performance objective: to provide long-term equipment protection.
  • They must also demonstrate meeting one critical analysis criterion: their ability to resist shearing under load.

Shear stability: A critical performance factor

One measure of a lubricant’s protective value is its ability to withstand shearing under pressure. Shear stability describes a lubricant’s ability to resist a decrease in viscosity due to exposure to mechanical loads.

Lubricant thinning under stress is the result of the breakdown of viscosity modifiers. A reduction in viscosity decreases the lubricant’s ability to create a fluid film to separate the shaft from the bearing. Furthermore, when the correct viscosity is not maintained, an increase in micro-pitting and gear wear may occur.

The best way to maintain viscosity to ensure shear stability

Internal testing of various EALs using the test method CEC L-45-A-99 has produced a wide range of results among the sample lubricants. Synthetic ester-based products that did not use viscosity modifiers produced the best results. Selecting an EAL using a base oil with an inherently high Viscosity Index (VI) without modifiers ─ assisted by a good oil sampling program ─ will ensure the high level of shear protection that meets today’s environmental and equipment requirements.

Want to learn more? Post a comment below and see test results in our article on how EALs with a high inherent Viscosity Index in the base oil provide superior shear protection.

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