SAE J306 Explained
The need to ensure adequate lifetime protection of transmission and driveline equipment is an increasing challenge for manufacturers. With ever-tightening fuel economy regulations requiring engineers to explore every avenue to reduce weight and mechanical losses in their products, lubricant volume and viscosity require special attention.
At the same time, the market imperative to offer products with reduced total cost of ownership requires that the fluids are specified to offer protection at extended service intervals.It is crucial, therefore, that manufacturers can have confidence that the fluids they specify will afford the required performance and reliability. Standards defining the rheology of lubricants are fundamental to effective Driveline equipment design, so that it operates with the performance and durability that customers demand.
The name SAE — the widely recognized acronym of the Society of Automotive Engineers — has long been synonymous within the lubricants industry as the internationally recognized and popularly accepted classification and categorization system for lubricants. While the SAE J300 and SAE J306 Standards may at first glance appear broadly similar, SAE J300 is for motor oils and SAE J306 is for automotive gear oils; the latter comprises both axle and manual transmission lubricants.
These applications represent product categories that are clearly very different and which fulfill completely different sets of requirements. It is therefore essential that the grade scales defined under each standard are not confused or compared, as the use of an inappropriate fluid can lead to catastrophic and highly expensive equipment failure.
Under the SAE J306 standard, lubricants are defined in terms of a grade denoting their minimum kinematic viscosity at 100˚C, as measured according to ASTM D445, while also demonstrating shear stability over 20 hours using CEC L-45-A-99 (Method C). Some lubricants are further designated with the letter “W” (Winter), signifying a low-temperature viscosity grade. In addition to their high-temperature definition, these “W” grades are further defined as providing a maximum temperature — ranging from -12 to -55˚C — at which they retain a threshold level of viscosity.
Balancing blends for performance
Achieving the optimal lubricant for a given driveline application requires a thorough understanding of both the equipment application and the properties of the base fluid and additive package. Even for a comparatively simple SAE J306-compliant monograde lubricant, performance additives will be used. The additive mix will seek to reduce friction and remove heat, and will include extreme pressure anti-wear additives to prevent wear, pitting, spalling, scoring, scuffing and other types of distress that can result in equipment failure and downtime. Protection against oxidation, thermal degradation, rust, copper corrosion and foaming also must be provided.
The viscosity of lubricants tends to decrease with increasing operating temperature. At elevated temperatures, the liquid becomes increasingly thin, providing a lower level of protection. Conversely, at lower temperatures the fluid thickens; the increased viscosity reduces the efficiency of the equipment it is protecting. It follows, then, that for a driveline required to operate only at moderate temperatures, a monograde product may provide adequate protection at an optimal price point. However, for operation across wider temperature extremes, a multigrade fluid engineered for a more balanced viscosity profile is required.
To achieve the required performance, multigrade fluids need additional additive components. For example, depending on the extent of cold-temperature operation, multigrade lubricants will require the addition of a pour point depressant, and in the most extreme cases, additional viscosity modifier. The chart below provides an example of a range of typical formulations and properties of SAE 90 from monograde to a wide-span multigrade SAE 75W-90.
Pour point depressants and viscosity modifiers
In cold temperatures, the wax in base oil tends to separate out and form crystals that interlock and lead to fluid thickening. As the fluid drops below the pour point, this thickening increases significantly, leading to increased mechanical losses in the equipment as well as reduced lubricant effectiveness. Pour point depressants modify the shape of the wax crystals that form at low temperature, preventing them from interlocking and thus reducing the pour point by as much as 40˚C.
The selection of the correct pour point depressant will be influenced by the choice of base oil, the potential interaction with the performance additive package and any viscosity modifier used, and the performance requirements and operating environment of the equipment.
The viscosity modifier must also be selected to ensure compatibility with the other lubricant components. For many specific applications, a range of polymer structures have been developed to provide the desired viscosity modifier performance. Selection of the right product for the right application is crucial in order to avoid compromising equipment performance and durability.
Shear stability is a key aspect of SAE J306 compliance. Lubricants must “stay in grade” after testing for 20 hours in order to confirm adequate shear stability. With the increasing popularity of wide-span multigrade lubricants that require the use of viscosity modifiers, some equipment manufacturers are specifying extended-hour testing as a part of their approval process. This is because some viscosity modifier technologies can continue to shear beyond the 20 hours specified under the SAE J306 standard.
Reflecting changing demands
SAE J306 was originally defined in 1991 but was extensively revised in 2005 to provide new grades and tighter classifications. These changes reflected the increasing requirements for fuel economy and the trends for increased numbers of gears in manual transmissions and for longer service drain intervals.
Lubrizol is ideally placed to assist its customers in navigating the complexities of engineering the optimal fluid for their SAE J306-compliant applications, however challenging these may be in terms of efficiency, performance, durability and temperature extremes. With its world-class knowledge of additive technology and its extensive range of viscosity modifiers and pour point depressants, Lubrizol can provide solutions tailored to the requirements of any product or application.