Dedicated high-tech axle lubricants can help the automotive industry to reduce the total cost of vehicle ownership by improving fuel economy and reducing operating overheads
Courtesy Transmission Technology International

Extensive testing conducted on Lubrizol’s lubricant additive and viscosity modifier technologies has revealed that changing to dedicated axle lubricants can generate fuel economy improvements of up to 1.5%. This can equate to significant savings for commercial vehicle fleets. Taking an average commercial vehicle haul of 140,000km per year can result in 21,000km of savings over a 10-year period, equivalent to traveling more than half the distance around the world with the total fuel saved.

Dedicated axle lubricants in next-generation drivetrains challenge the ‘one lubricant for all systems’ approach, thereby prompting OEMs and their suppliers to seek new and improved methods to optimize performance, maintenance schedules and gain overall drivetrain efficiency.

End-user requirements dictate the need for a balanced axle lubricant that offers improved efficiency for added fuel economy, without compromising durability. Robust additive and viscosity modifier technology, coupled with a move from Group I to Group III and synthetic base stocks, enables the extended drain intervals that end users have now come to expect. Additionally, axle-specific lubricants provide wear-protection for increasing load weight, which helps with overall durability. Improved durability also helps to decrease vehicle operating costs by providing maximum component life without additional service-interval maintenance.

With axles operating in extremely harsh environments, as well as the added requirements for efficiency across all hardware areas, the focus on axle lubricants continues to increase. Today, advancements in axle lubricants, viscosity modifiers and additive technology are continuing to enable longer drain intervals, which have stretched to 600,000km in Europe and approaching 1,000,000km for long-haul trucks operating in North America. “Significant resources have been, and continue to be, invested in the design and testing of axle-specific fluids because they are able to provide efficiencies without overhauling drivetrain mechanical design,” says Daimler’s Dr Eric Menthe. “By formulating vehicle-specific axle fluids, vehicle and fleet owners can expect higher gains in fuel economy and durability beyond that of standard API GL-5 gear oils. This can be achieved only by using next-generation axle and lubricant technology.”


Laying Foundations
API Category GL-5 (ASTM D7450-08) is a fundamental building block for the minimum acceptable lubricant performance for hypoid axles. ASTM D7450-08 contains laboratory bench and mechanical tests that designate the type of service characteristic of gears, particularly hypoids, in automotive axles under high-speed and/or low-speed, high-torque conditions. However, ASTM D7450-08 has certain limitations since it does not address thermal stability and cleanliness, which are important factors for keeping elastomers free from debris that can damage radial shaft seals, resulting in leaks.

Axle manufacturers recognize the benefits of SAE J2360, an axle lubricant specification that goes beyond ASTM D7450-08, and are responding by stipulating this as the baseline performance for their service fill fluids. This global specification contains additional proof of performance for the additive technology and other components in the lubricant, which include a controlled field trial in both light- and heavy-duty vehicles. The extensive data generated in this testing is independently reviewed and approved by the Performance Review Institute (PRI). An allocated license number is then issued to the company and that specific formulation.

Recognition and availability of SAE J2360 approved axle lubricants is continually increasing in both the factory fill and service fill market. For example, axle manufacturer, Meritor, now stipulates SAE J2360 axle lubricants as the minimum quality level for service fill application.

Stringent Requirements
OEM and Tier 1 axle manufacturers are employing more stringent requirements for dedicated axle lubricants. The trend is toward lower viscosities (e.g. SAE 75W-85), particularly in first fill applications, to enable fuel economy improvements. New tests are also being developed to screen in areas beyond ASTM D7450-08 and SAE J2360 to ensure there are no compromises in axle durability with lower viscosity fluids.

Commercial vehicle manufacturers are taking steps to reduce aerodynamic drag, which is a significant factor for improving fuel economy. Diversion of underbody airflow can lead to less cooling of the axles, thereby necessitating axle additive technology systems that reduce friction and minimize wear while providing lower operating temperatures. For commercial vehicles, the impact of carrying heavy loads over long distances means that in standard operations axle temperatures are typically in the 140-150°C range, and in the most severe of applications these can rise to levels approaching 185°C. As a result, laboratory and field testing is conducted in extreme temperature and hauling conditions to prove the effective thermal characteristics of an axle lubricant.


Optimized Axle Lubricants
As vehicle owners look to reduce cost of ownership and increase vehicle life, optimized axle lubricants help to provide better fuel economy, improved wear protection and extended lubricant drain intervals.

Axle lubricants consist of base oil, additives and viscosity modifiers. All three have an interconnected impact on efficiency. Lubrication-related efficiency losses include fluid traction, friction between mating component surfaces, and viscous drag. The latter relates to churning and pumping losses, which are reduced with a lighter viscosity grade lubricant. Higher viscosity lubricants, however, can be more effective at higher power as they minimize friction between mating surfaces.

A lubricant with a high viscosity index (VI) offers a great compromise by providing lower viscosity at low temperature, while maintaining higher viscosity as temperature increases. A high-shear stable viscosity modifier system is necessary to maintain film thickness over the life of the lubricant.

In commercial vehicle testing, Lubrizol has demonstrated fuel economy improvements of up to 3% when combining dedicated fluid technologies for both axle and manual transmission, which could yield annual savings of US$3,400 (€2,600) of fuel for an average truck driven 140,000km.

The lubricant is an integral part of the axle design process. Dedicated axle lubricants protect equipment through enhanced load-carrying capabilities, improved fuel economy and thermal stability, as well as enabling extended lubricant drain intervals. Lubrizol’s proven axle additive and viscosity modifier technologies can be customized to provide axle performance for maximum end-user benefits, and are tailored to meet OEM’s specifications, designs and materials. All requirements can be met when the axle lubricant is included as an integral part of the design process, with all parties working in a close and cooperative environment to meet the common goal.