Wet Clutch Degradation Monitored by Lubricant Analysis 2010-01-2232

In the competitive market of the car industry today, companies need to continuously strive to optimize the performance, price and environmental properties of their products in order to survive. Wet clutches, as parts of transmission components of passenger cars are no exception. An understanding of how the wet clutch system functions and fails is necessary to optimize price and service life. The friction characteristics of the wet clutch system are determined by lubricant-surface interactions in the contact between the friction discs. Wet clutch failure can often be associated with the deterioration of friction characteristics which eventually leads to stick-slip or shudder. Consequently, knowledge of why and of how friction characteristics change over time is of the outermost significance to enable the understanding and prediction of wet clutch performance. As the lubricant is an essential component of the wet clutch system, lubricant ageing is a factor of importance. Oxidation, thermal degradation, shearing, additive degradation and water contamination could all be considered to influence lubricant ageing. The aim of this work was therefore to find suitable ways of measuring the remaining useful life of wet clutch lubricants and to correlate changes in friction characteristics with changes in lubricant properties. Both field trials and measurements in a wet clutch test rig were performed. Viscosity, acid number, additive degradation, water contamination, particle content and metal content were measured for the lubricant as it degraded. Particle content results showed a rapid increase early in the ageing process. However, as ageing progressed particle levels actually decreased and this was probably a result of particles slowly grinded between contacting surfaces. On the other hand, metal content increased as ageing progressed, which could indicate slowly progressing wear. Water levels were found to be higher in field trials than in lubricants used in wet clutch test rigs. It is concluded that this was due to the severe and accelerated operating conditions of the wet clutch test rig.