Impact of oil ageing on gear scuffing in hybrid dual-clutch transmissions

As the clutch friction stability in dual-clutch transmissions has been continuously improved over the last decade, wear protection of high loaded mechanical contacts has become increasingly important and mechanical failures have been observed at the end of extended oil drain intervals. A test method was established based on standard equipment already used for transmission fluid development. A modified FZG four-square test rig allows ageing a reduced oil volume to not decelerate development times. Applicability was demonstrated by ageing four full formulated transmission fluids using the new approach and comparing the results to aged transmission fluids sampled from durability testing. The oils aged with the FZG rig exhibit the same characteristics of scuffing load capacity degradation as observed in full transmission systems. However, in the full transmission system, the degradation starts at lower applied specific work, most likely due to the different system complexity. It is pointed out that not only the thermal load but also the tribological stresses have to be considered to mimic oil ageing.