Wet clutch transmission fluid for AWD differentials: influence of lubricant additives on friction characteristics

In recent years, several electronically controllable automotive transmission systems using wet clutches as intelligent differentials have emerged on the market. These applications place great demands on the anti‐shudder properties of the transmission fluids used. The aims of this study were (i) to investigate the influence of different additives on the friction characteristics of a transmission fluid for all‐wheel drive systems featuring wet multi‐plate clutch with a sintered brass‐based friction material and, based on this knowledge, (ii) to formulate a new transmission fluid with the desired frictional properties. In addition to excellent anti‐shudder properties, the new fluid was required to lubricate hypoid gears under high load. To meet this requirement, it is necessary to add significant amounts of extreme pressure additives to the base oil, which are known to have an unfavourable influence on anti‐shudder properties, necessitating the adoption of novel additive technologies. The additives studied include anti‐wear additives, friction modifiers, corrosion inhibitors, detergents, antioxidants and extreme pressure additives. This paper shows how different additives affect friction in different ways, and that the interactions between the different additives are important to consider. It was concluded that it is feasible to combine good anti‐shudder properties for wet clutches with good lubrication of hypoid gears. Copyright © 2006 John Wiley and Sons, Ltd.