Friction and contact phenomena of disc brakes related to squeal

Abstract: Brake squeal is a sound with a main frequency between 1 and 20 kHz, generated by an oscillation of the components of the brake system. The energy driving the oscillation is supplied by the friction between the disc and the pads. Obviously, the friction characteristics have a major influence on the propensity for squeal generation in any given brake assembly.In the present thesis, friction phenomena and squeal generation of a brake system have been studied and correlated to the surface structure of the brake pads. Furthermore, the processes for the formation and disintegration of contact plateaus, occuring on the pad surface, have been investigated in detail.It is found that brake squeal can not be generated if the coefficient of friction isbelow a critical level, depending on design and friction characteristics of the brakesystem. Above the critical level, squeal generation is apparently stochastic, and theprobability for squeal dependent on the set of conditions. It is concluded that theseemingly stochastic behaviour of squeal generation only can be explained by thesubtle surface phenomena affecting the friction characteristics.It is further shown that contact plateaus form on the surface of organic brake pads. These are constituted by the wear resistant structural components of the pad, and wear debris compacting against the leading edge of the structural components. The expressions primary and secondary plateaus are introduced for the structural components and the compacted debris, respectively. The work has clarified some of the surface interaction phenomena responsible for the friction behaviour of organic brake pads.

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