Impact of water contamination and system design on wet clutch tribological performance
Wet clutches are lubricated machine components which utilize friction to transfer torque and thus establish connection between shafts rotating at different speeds. They are used in automatic transmission as shifting device or lockup clutch. As well as cooling the contacting surfaces, the lubrication provides controlled friction, smooth performance and long life. Because of this advantage, wet clutches are applicable for heavy duty vehicles (e.g. construction equipment, tractors) to handle high power. However, like any other tribological system, wet clutch performance is dependent on several factors concerning operating conditions, system design etc. As a friction generating machine component, the degradation of the lubricant and the contacting clutch surfaces are continuous process and the friction behavior also changes consequently with the progression of the system deterioration and that directly influences the clutch performance. So, to investigate the clutch influencing factors and fundamental understanding of the interaction mechanism is of great interest.
Though wet clutches were immensely studied earlier to investigate different issues, there was lack of clear understanding and knowledge concerning water contamination problem and the system design parameters. The main aim of this thesis is to experimentally evaluate and explain the impact of water contamination and to verify different system design parameters’ influence on wet clutch tribological performance. The study focused on the wet clutches in automatic transmissions.
Initially the thesis reviews the state of the art in wet clutch research, discusses the motivation for the current research aim and clarifies the goals of the investigation. Later the details of the experimental method and findings are discussed in this thesis.
Since evaluating the friction behaviour in desired environment is the key requirement, an automated wet clutch test rig was utilized to carry out the friction test where operating conditions (slip time, oil temperature, contact pressure/load, sliding speed) can be controlled and the output torque, contact temperature etc. can monitored. This suitable test equipment was designed for standard paper based friction plate and steel separator plate lubricated with automatic transmission fluid. 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.
In the first part of the thesis the effect of water contamination on wet clutch tribological performances is experimentally investigated. The study reveals that both the friction reliability and the friction-velocity relation are influenced by the change in the friction behaviour for a water contaminated lubricant. It is 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 is influenced by the water amount but not by the water exposure time. Then the study is continued for longer test to evaluate the ageing of the friction interfaces and the lubricant. Though water does not show any long term effect on water contaminated ATF as it is seen in case of friction interfaces, the short term effect in the surface active additives is observed in the current research. This finding also supported the investigation on the water contaminated friction interface degradation and the effect of water on additive adsorption of the contacting surfaces. It was found that the addition of water in ATF intensifies the mean friction coefficient, but in the long run the loss of friction much faster and high, which can in fact degrade the clutch life faster as well. Two simplified ATF was also analysed along with the commercial fully formulated ATF for investigating the water impact to get a simple and comprehensive overview of the consequence. 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 change in the clutch frictional behaviour with time and degradation can largely influence by the choice of these design factors. This 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.