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Slip-controlled lock-up clutch systems are very efficient and greatly improve fuel economy. On the other hand, these systems can cause unstable vibrations including those known as “shudder vibrations”. In this study, the authors made a theoretical analysis of these unstable vibrations to clarify the fundamental frictional properties of automatic transmission fluids (ATFs) required for slip-controlled lock-up clutch systems. Based on this analysis, we established lubricant technology having a sufficient anti-shudder property and high torque capacity. Further, we developed a new test apparatus to evaluate the anti-shudder durability for lubricant development.

The effect of volume viscosity or second vicosity on the traction coefficient has been investigated. The volume viscosity has been considered to cause the compression delay of the EHL film in the traction drive and as a result to decrease the shear force or the traction coefficient, because the oil film pressure changes from atmospheric pressure to several GPa in a shorter time interval than one millisecond. The volume viscosity and traction coefficient have been measured for five synthetic oils. The volume viscosity of oil has been estimated from the absorption coefficient of an ultrasonic wave which has been determined from the relation between a detected amplitude of ultrasonic pulse and the propagation distance. It has been found that the traction coefficient has increased as the volume viscosity has decreased.