Wet clutch friction reliability : influence of water contamination and system design
Wet clutches are machine components using friction to transfer torque and providing interruptible connection between rotating shafts in different automobile applications including automatic transmissions. Like any friction generating machine components, wet clutches are susceptible to continuous wear and degradation during sliding. This regular deterioration process as well as the choice of operating conditions, ultimately change the overall system performance during operation due to resultant change in the system parameters.
The first part of this thesis summarizes some of the notable studies on the wet clutch tribological performance and clarifies goals of the investigation.
Previously, plenty of experimental studies on wet clutches have been reported but still some effects regarding the water contamination problem and the influence of mechanical design factors are not covered thoroughly. The thesis aims to experimentally analyse these two different aspects for improving wet clutch performance regarding frictional characteristics and reliability. These two investigations are focused on wet clutches in automatic transmission applications.
For evaluating friction behaviour during a long clutch engagement period, suitable test equipment is designed where standard paper based friction plates and steel separator plates are tested with commercially available ATF. To investigate a clutch operated in a controlled environment is one of main the research objectives. The vital concern while designing the test rig is to monitor the clutch parameters for achieving the desired operating conditions for individual tests. Instead of using a multiple clutch plate configuration, as in real applications, a single friction and reaction plate arrangement is considered to simplify the analyses.
An experimental study on wet clutch frictional behaviour under water contaminated lubrication condition reveals the change in the friction level for a water contaminated lubricant. It was shown that the friction level increased for the addition of water in the system. This is not a desirable clutch frictional behaviour for maintaining frictional stability. The increase of friction for added water was influenced by the water amount but not by the water exposure time. The test results also showed a higher change in the separator plate's roughness parameter (Ra) for water contaminated systems compared to an uncontaminated wet clutch.
In the second part of this thesis, the influence of the clutch’s output shaft’s stiffness and inertia on the clutch system‘s friction reliability is experimentally evaluated. Test results show that the choice of these design factors can provide different outcome concerning clutch frictional performance and shudder sensitivity. Shudder tendency is seen to be increased for decreased torsion shaft stiffness. High frictional losses and clutch degradation are observed for systems with less inertia.