Lock-up clutches prevent slip under normal driving conditions, as a converter can in principle only transfer torque with slip, which would result in higher consumption. The lock-up clutch is actuated and controlled by the gearbox. In many applications, lower continuous slip is set selectively under certain load conditions, in order to prevent gearbox noise. Modern gearbox control units are able to selectively set a constant slip of approx. 5 rpm, which has practically no influence on consumption. Also, in certain driving situations, the dynamics of the vehicle can be improved in the short-time by selectively increasing engine speed.
The friction occurring during these operations (a few watts through to several kilowatts) can overheat the automatic transmission fluid (ATF) at certain points, therefore significantly reducing the life of the gearbox. An effective system for cooling friction linings is therefore required. Oil has to be selectively routed through or over the friction linings. Standard materials are high-performance paper linings or carbon fibre linings. Diaphragm cooling units are also used, whereby the oil is passed through a hole in the piston near to the friction surfaces. In this case, however, the efficiencies of lining flow cooling are not achieved.
Luk therefore always goes for flow-cooled systems. At LuK, non-fluted carbon linings that allow cooling oil to pass through the carbon fibres and/or fluted paper linings with impregnated continuous flutes etc. are now the accepted components. Flow-cooled conical clutches offering special benefits were also tried and tested (LuK TorCon System). A host of LuK patents in this area highlights the complexity and importance of efficient high-performance cooling. For optimal application, specially developed simulation programs are used for flute design.