A slip-controlled lock-up clutch system has been widely adapted in automatic transmissions for improving the fuel efficiency of the vehicle. The slip-control might induce a vibration, a so-called "shudder", caused by a stick-slip between the clutch surfaces. To prevent shudder from occurring for a long time, ATFs are required having a good anti-shudder property and improving anti-shudder durability.

In this study, the anti-shudder effect of ATF additives and their mechanisms have been investigated.

The addition of over-based Ca-sulfonate and friction modifiers (FMs) remarkably improved the anti-shudder durability of ATF.
From the results of surface analyses, it was found that the shudder occurrence was correlated with the contact area roughness and the boundary frictional property of the steel plate surface. Large contact area roughness and low boundary friction coefficient were preferred to prevent shudder. It was suggested that the increase in the contact area roughness was caused by the over-based Ca-sulfonate and that lowering the boundary friction coefficient was caused by the over-based Ca-sulfonate and the FMs.
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