Automotive OEMs are actively seeking solutions that can reduce torque, improve efficiency, and increase fuel economy. In today’s marketplace, virtually all automotive suppliers are placing more resources and emphasis on meeting these goals. The establishment by the U.S. Department of Transportation of new fuel economy standards of 54.5 mpg for model year 2025 has prompted OEMs to seriously consider all new technologies that can improve fuel economy. Meeting the 54.5-mpg fuel economy standard will not be easy or inexpensive. In fact, the U.S. EPA (Environmental Protection Agency) estimates that the cost for new technologies that will enable a common passenger car to meet the new standard will be likely to exceed $2200 per vehicle by 2025.
Some of these new technologies have already been or will soon be incorporated into passenger vehicles. Technologies such as gasoline direct injection, turbochargers, as well as 8- and 9-speed transmissions along with the first generation of plug-in hybrid vehicles represent the way forward. While the new fuel economy standards appear quite restrictive, the “54.5 mpg” standard represents a higher certification level than that which will be displayed on the window sticker, because tests will have to be carried out under laboratory conditions. The “label” value, which reflects real-world driving, is likely to be an average of 40 mpg. However, the best solutions are often greater than the sum of their parts. As a first step, technologies that can reduce engine torque, coefficient of friction, or rolling resistance will be adapted quickly by most vehicle OEMs.
One such technology can be seen in the new tandem bearing developed by Schaeffler Group to maximize driveline efficiency. The end result is a new generation of high-efficiency bearings that have proven to reduce drag torque by 50% and increase fuel economy up to 1.5% while helping to reduce CO2 emissions. Another benefit of this new bearing is that the coefficient of friction is much lower when compared to a traditional tapered roller bearing. The end result is that the bearings generate less heat while in operation. This, in turn, helps to keep the axle oil cooler, which will extend overall service life. Another benefit is that the amount of oil required for cooling can be reduced within the axle, and this helps to minimize churning losses. Both the tandem and tapered roller bearings offer the benefits of axial positioning and thrust management of loads; however, the tandem bearing has a much lower coefficient of friction via the use of rotating ball bearings. This difference can lead to much greater efficiency benefits.
The Group’s new tandem bearing was developed as a high-performance bearing, specifically to meet the needs of the automotive industry. Speaking about the bearing development, Michael Claassen, Director of Engineering for Schaeffler Automotive Chassis Systems Group, said: “The tandem is a good example of Schaeffler’s path forward for high-efficiency bearings. The tandem delivers a significant improvement in overall efficiency when compared to traditional tapered roller bearings.”
Historically, the tapered roller bearing has been the primary bearing option for axle and differential applications thanks to its robustness, the lack of alternatives, and low efficiency targets of the past. However, the use of tapered roller bearings comes at the expense of much higher sliding friction. The need to maximize fuel economy has opened the door to modern technologies that can deliver lower friction and higher efficiency.
The tandem bearing is a two-row angular contact ball bearing, with each row having a contact angle in the same direction. This arrangement of ball rows enables high stiffness and rigidity. Because the tandem utilizes ball bearings, rolling resistance comes from rolling friction between the balls and the grooved raceways they roll against. In comparison, a tapered roller bearing’s rolling resistance comes from a combination of rolling friction and sliding friction, most of which comes from the sliding motion between the end of the roller and the raceway support lip that it slides against. This results in much greater sliding friction. The end result means tandem bearings have about half the friction of tapered roller bearings, which allows tandems to run with higher overall efficiency and lower operating temperatures when compared with a taper bearing.
Another challenge that Schaeffler had to address with the tandem bearing was to choose an appropriate plastic material for the bearing cages. The requirements upon these cages were somewhat challenging. The plastic needed adequate stiffness to retain the ball bearings yet it also had to be flexible to facilitate loading of the balls into the bearing pockets. In addition, the polymer needed to withstand hypoid gear oil at temperatures of up to 150°C (302°F). Finally, it needed to be dimensionally stable in support of the differential and pinion assembly process. Schaeffler selected Stanyl from DSM Engineering Plastics for the cages, thanks to the material’s proven performance in other Schaeffler bearing applications.
Stanyl is a high-performance polyamide with a wide versatility and a unique set of properties, including high mechanical property retention and excellent resistance to wear and friction. DSM offers Stanyl in a wide variety of grades including unfilled (non-reinforced) as well as grades containing glass fiber, mineral, lubricants, and/or flame retardants.
Bill Burnham, Business Development Manager, DSM Engineering Plastics, wrote this article for SAE Magazines.