Tribodynamics of clutch system cold and hot judder

The thesis focuses on dry clutch system performance. Clutch is used to transfer and control power from the engine and is considered as a main source contributing to a number of noise, vibration and harshness phenomena, such as clutch judder, clutch in-cycle vibration (whoop) driveline shuffle and clonk. The main focus of the thesis is on the study of root causes of clutch judder phenomenon. This is a friction-induced vibration, originated from clutch friction lining sliding contacts with flywheel and pressure plate in the frequency range of 5-20 Hz. These torsional vibrations introduce undesired dynamic loads, increased slip condition, generated heat and wear of lining material. The judder phenomenon is also a main caused of driver and vehicle occupant discomfort. Judder is induced predominantly by the variations in the frictional characteristics of the clutch facing material and its variation with clamp load, interfacial slip speed and temperature, or by mechanical tolerances and misalignment.

The judder phenomenon has hitherto been tackled mainly through two approaches; (i)- application of control means to actively dampen the oscillations in the driveline., or (ii)- to change vehicle driveline properties by means of mechanical adjustments or improving the characteristics of the clutch friction material. A fundamental approach requires understanding of the root causes of judder with in-depth understanding of frictional behaviour of clutch lining material under different load, speed and temperature conditions, and at different stages of clutch lifetime. No comprehensive research has hitherto been reported in combined tribological assessment of friction lining, dynamics analysis through engagement as well as thermal behaviour of the lining with repeated clutch actuation. The thesis highlights such an in-depth integrated study and its effects upon the propensity of clutch judder.

The thesis reports on tribological assessment of clutch lining material through extensive pinon-disc tribometry and in line with established standards. The findings of the undertaken measurements are employed is establishment of lining tribological characteristics, which are integrated with a clutch system dynamics analysis as well as a thermal model. The integrated thermo-tribodynamic analysis constitutes the main contribution of this thesis to knowledge.

The major finding of the study shows that bulk temperature of the frictional pair affects the gradient of the µ-v characteristics, resulting in a reduction in its gradient at bulk temperatures rising from ambient to 90°C.This shows the inherent nature of the judder phenomenon with a greater propensity to stick-slip variations at lower bulk clutch face temperatures. In addition, the significant role of the change in thermal properties of clutch lining material during its lifetime and its effect on judder phenomenon is also established.