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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.

The conventional rolling bearing life equation (1), which is based on the theory of Lundberg and Palmgren, has a problem in that it does not match the actual bearing life in all operating conditions. For instance, while the actual life of a bearing under clean lubrication is 20 times longer than the calculated life, actual life under contaminated lubrication is as low as one-tenth of the calculated life. To solve this problem, the following life equation (Eq. 1: Advanced Bearing Life Equation) was developed with the aNSK life modification factor: The new aNSK factor is based on data from bearing life aNSK tests involving over 450 roller bearings and over 550 ball bearings under a variety of operating conditions. The new life equation with the aNSK factor showed a satisfactory fit between calculated life and actual life.

Bearings used in electrical engine-driven accessories, such as alternators and air conditioner (A/C) compressor pulleys, generate flaking related to non-metallic inclusions of unusual microstructure (white structure flaking), which occurs at one-tenth to one-twentieth of the calculated bearing life. In this paper, we will describe our investigation of the mechanism of white structure flaking, and report on how we achieved extended service life of engine-driven accessory bearings by implementing an evaluation of white structure flaking based on the discovered mechanism. We verified that the tribochemical reaction of grease, and statistic electricity generated by friction between the drive belt and pulley were the main culprits of white structure flaking. White structure flaking was further augmented by hydrogen invading the bearing steel, which further impaired bearing life.