Abstract
Keywords: Clutch, Squeal Noise, Mode-coupling Instabilities, Nonlinear Vibrations, Destabilization Paradox
In cars with manual transmission, different unforced vibrations may be observed during the sliding phase of clutch engagement. Such friction-induced self-generated vibrations can be explained by four general and independent mechanisms, namely stick-slip, speed dependent friction force, sprag-slip and mode coupling. The former two rely on tribological properties whereas the latter two are due to geometrical conditions. In the automotive industry, low frequency phenomena such as judder (10-20Hz), felt as jolts of the vehicle when starting, can often be attributed to tribological properties. However, squeal noise is a relatively high frequency phenomenon (up to several kHz), and may arise even when the friction coefficient is almost constant versus sliding speed. It cannot be related to stick-slip behaviour because of the speed range of the vibrations measured. In this case the assumption of a purely tribological origin appears unrealistic. Consequently, sprag-slip and to a greater extent mode coupling instabilities due to the intrinsic structure of the system are more likely to be responsible for this phenomenon. The present study deals with the analysis of the conditions for a spontaneous destabilization of the stationary sliding state and the rise of vibrations in clutches due to such a mode coupling mechanism. A minimal phenomenological model is proposed in order to illustrate such a behaviour. Then, both linear and non-linear parametric investigations are performed in order to define risk criteria, predictive models and rules for robust design. Important results are demonstrated and highlighted on the role of major physical parameters leading to a better understanding of the non-linear dynamic of systems subject to flutter instabilities. The proposed phenomenological model takes into consideration not only positional nonconservative coupling forces due to friction, but also gyroscopic and dissipative damping properties of the system. The results and the methodology used in this work can be largely applied to many different applications including disc brake squeal noise.