Abstract
There is growing urgency to reduce fuel consumption and CO2 emissions for automobiles. This drives the need for high efficiency in axle differentials. This paper focuses on drive pinion bearings in a rear axle differential. Because approximately 50% of power loss in the axle differential can be attributed to the pinion bearings, decreasing the friction loss of the pinion bearings can contribute to improved fuel efficiency, which reduces burdens on the global environment.
Tapered roller bearings are generally used to support the drive pinion. However, some automotive manufacturers have recently been considering the use of double row angular contact ball bearings to reduce bearing friction. Since life, stiffness and static safety factor of ball bearings are inferior as compared to comparable-sized tapered roller bearings, much larger ball bearings are required to provide the equivalent performance of tapered roller bearings.
We adhered to reducing friction torque of tapered roller bearing to the torque level of ball bearing, and then we have developed the super-low friction torque tapered roller bearings, which have a friction torque reduction up to 75% compared with conventional low friction torque tapered roller bearings. The developed bearings consist of four basic technologies, that is optimized internal geometry, special raceway crowning profiles, lubricating oil flow control and compactness with high-capacity technology.
This developed technology has been applied to drive pinion bearings in a rear axle differential for passenger cars, and the super-low friction performance of developed bearings has been evaluated by the bearing test under combined load simulating operating conditions and the axle differential test under preload only. As a result, it has been confirmed that the developed bearings has an excellent performance such as low friction torque and low temperature rise as well as the ball bearings under low-loaded conditions. The developed bearings has 40 to 50% lower torque and about 20℃ lower temperature rise than the conventional bearings. Furthermore, the developed bearings maintains to have the low friction performance even under high-loaded conditions, although the torque of the ball bearings is increased remarkably to be close to that of the conventional bearings with the load increasing.
As just described, regardless of load and speed, the developed bearings have the super-low friction torque which is as low as, or lower than, the ball bearings. Therefore, the developed bearings are expected to improve fuel efficiency by as much as 1.5% and reduce CO2 emissions of 3.5g/km, same as the ball bearings.
Keywords: tapered roller bearing, friction torque, rear axle differential, fuel efficiency, power loss