Historically, the tapered roller [bearings online] 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 ina 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 bearings on line 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.

Indentation marks in rolling bearing raceways are known to be a source of surface damage, resulting in shorter bearing service life. SKF bearing life calculation has ­taken account of this for many years using the ηc contamination factor. Recently, SKF has developed advanced models and conducted experiments to predict the ­interaction of scratches and indentations, and the lubricant present in the raceways. This article explores in detail these interactions and the effect of sliding, to give a unique insight into the way the damage process takes place.