Development of Long Life Pulley-Supporting Bearing for Belt-CVT

The belt-type continuous variable transmission (b-CVT) consists of a simple structure that transmits power by using a steel push belt in combination with a pulley. One important factor that leads to the deterioration of rolling bearing life is influence of additives in the special traction oil (CVT fluid). CVT fluid is mixed various additives to increase friction coefficient with the aim of maximizing sliding performance between the metal belt and pulley surface areas. In order to restrain heat generation due to friction driving between the metal belt and the pulleys, and to minimize churning resistance of the gears in the unit, viscous resistance of CVT fluid is designated at a lower class than that of gear lubrication oil used in manual transmissions. As a result, formation of an oil film is impeded throughout the bearing interior, creating harsher operating conditions that those found in conventional transmissions. During the developmental stage of b-CVTs, automakers confirmed that premature flaking occasionally occurred before the calculated life of the bearings. This report describes the process of premature flaking and the development of long life bearings as a measure against flaking life and dimensional stability under high operating temperatures as one of the more important functions of transmission bearings.