Experimental analysis on squeal modal instability

In this paper, an experimental analysis performed on a simplified brake apparatus is presented. Brake squeal is a major concern in braking design. During past years a common approach for squeal prediction was the complex eigenvalues analysis. Squeal phenomenon is treated like a dynamic instability. When two modes of the brake system couple at the same frequency, one of them becomes unstable leading to increasing vibration. The presented experimental analysis is focused on correlating squeal characteristics with the dynamic behavior of the system. The experimental modal identification of the set-up is performed and different squeal conditions and frequencies are reproduced and analyzed. Particular attention is addressed to the system dynamics in function of the driving parameters on squeal occurrence. Squeal events are correlated with the modal behavior of the system in function of the main parameters, like contact pressure, friction material properties and system geometry. The robustness of the obtained squeal events permits a further analysis on the triggering of the squeal instability during braking, including the values of parameters that bring to instability. The obtained results agree with the modal coupling approach for squeal prediction, and confirm the characterization of squeal as dynamic instability. Introduction Disc brake noise continues to be object of investigation for automotive manufacturers and researchers. Because of the complexity of the problem and the need of estimating the squeal tendency during brake design, many analytical and numerical [1] approaches have been proposed. The modal coupling approach (mode lock-in) between two system modes, proposed by Akay & all [2], is one of the most accepted. The complex eigenvalues analysis is a popular numerical tool for squeal instability prediction [3-4]: squeal propensity is quantified by the dynamic instability of certain system modes. This paper shows an experimental analysis aimed to validate this approach for squeal prediction and to identify squeal phenomenon as a modal instability of the system. Brake squeal is a strongly non-linear phenomenon, characterized by friction material and contact non linearities. The modal approach allows to predict the rise of squeal when it is still in linear conditions. The paper uses a simplified set-up that is particularly appropriate to identify and eventually modify its dynamic behaviour. A description of a modal analysis of the set-up is first presented. The dynamics of the system is studied by considering separately three main sub-structures of the brake(calliper, disc and pad). Different ways to shift the mode frequencies of the system are used to find different instability conditions. All squeal conditions are related to an appropriate dynamic configurations of the system with particular values of the parameters. Some comments and remarks are finally reported. Experimental rig Since a real brake apparatus is characterized by geometry and dynamics that can be hardly controlled and understood, an experimental and theoretical study of a simplified experimental set-up is preferred. The set-up consists in a rotating disc (the disc brake rotor) and a small friction pad pushed against the disc by weights positioned on a rigid support (figure 1). in su -0 03 55 49 1, v er si on 1 22 J an 2 00 9 Author manuscript, published in "International Modal Analysis Conference IMAC-XXIV (Missouri), Saint-Louis : États-Unis d'Amérique (2006)"