Optimised Tooth Flank Geometry for High Performance Transmissions 2007-01-0118

The recent development of dual clutch transmissions (DCT) has focused upon the development of the sub-systems with the greatest novelty and hence risk, namely the clutch module and its control. A conflict arises between the oil properties required for clutch control such as low viscosity, and appropriate variation of the dynamic friction co-efficient with slip speed, overall transmission efficiency and correct lubrication of the other transmission components. It can be argued that this emphasis has biased the lube properties to the detriment of its other functions such as the lubrication and cooling of the synchronisers and gears.

Accepting the oil properties required for the clutch, the paper investigates ameliorative solutions to the issue of gear life in conditions of low viscosity lubrication using alternative gear geometry, with specific interest in gear pitting. A potential solution involves adopting tooth profiles which permit an increased relative radius of curvature and thus decreased contact stresses. The involute is known to have disadvantageous curvature radii near the base circle. A new optimised hybrid tooth profile has been used in the researches carried out at the Technical University of Munich (FZG). Details relating to the new tooth profile are presented. Further, the paper gives detailed description of the manufacturing and testing method and the results which have been obtained.

The profile improved significantly the pitting resistance of the gears which have been tested on a standard FZG test rig. Some improvements were observed for micro-pitting but further research would be necessary to obtain certitude. The possible implications of the inaccuracy in centre distance, the main objection point for using non-involute profiles, are also analysed in the paper.