When choosing a gear oil for marine thrusters or a stern tube lubricant for propeller shaft bearings, there’s more to consider than the price-performance ratio. The 2013 Vessel General Permit requires the use of environmentally acceptable lubricants (EALs) that meet EPA standards in applications with an oil-to-sea interface.
While EALs are formulated to meet environmental requirements, they’re still designed to protect marine equipment in the long term. To prevent bearing and gear failure, choose EALs that resist shearing under load. Using a lubricant with good shear stability protects components and extends oil change intervals.
To evaluate the performance of an EAL, determine the following:
- Estimate projected life of the oil under operating conditions based on thermal, oxidative and hydrolytic stability.
- Measure its ability to protect components from wear, scuffing, pitting and corrosion.
- Evaluate the kinematic viscosity of the oil and its ability to withstand shearing under pressure.
The high loads, speed of rotation and heat generated by stern tube systems and thrusters place significant stress on lubricants. A lubricant relies on viscosity to protect components and improve operational efficiency. Any reduction in viscosity can increase micropitting and gear wear.
Viscosity is key to EAL performance
The performance and life of your marine equipment depends on EAL viscosity. Temperature is one factor that affects viscosity. Lubricants tend to thicken in lower temperatures and thin at higher temperatures. This change in viscosity due to temperature is called Viscosity Index (VI). EALs with a higher VI experience less viscosity change due to temperatures.
One way to increase the VI of a lubricant is to use viscosity improvers when formulating it. However, lubricants with a high VI due to viscosity improvers can experience a temporary or, in some cases, permanent loss of viscosity in high shear, high pressure and extreme temperature conditions.
Another way to increase the VI of a lubricant is to use high viscosity base stocks in the formulation. This method of increasing the VI of a lubricant tends to result in better shear stability.
There are two ways to measure EAL viscosity:
- A good sampling program – Measuring the viscosity of an EAL over time will identify any loss of viscosity due to shearing.
- Manufacturer lab testing – A standard test method like the KRL Tapered Roller Bearing test subjects lubricants to severe shear stress similar to what they’d experience in actual field applications.
EALs should withstand high shearing forces
When we tested various EALs using the KRL Tapered Roller Bearing method, we found a wide range of results among the sample lubricants.
- Two of the samples with viscosity modifiers showed a minimum of 50% loss in viscosity.
- Synthetic lubricants that used a high viscosity ester base stock with no viscosity modifiers maintained a consistent viscosity.
Even a small drop in viscosity can drop VI permanently. Using a lubricant with a low VI can compromise shear stability and lead to premature wear, scuffing and pitting and, ultimately, component failure.
While EALs are formulated to protect the environment, lubricant performance shouldn’t be sacrificed. Choosing an EAL formulated with a high VI base stock is necessary to prevent bearing and gear failure due to shearing stress. Doing so will ensure a high level of shear protection that meets environmental and equipment requirements.
Learn about our EALs for marine applications.