Estudo de geração de trincas térmicas em discos de freios de veículos comerciais

Abstract

The disc brake systems, when compared with the drum brake systems currently used in heavy-duty vehicles, resent significant advantages, such as: better performance in relation to temperature and speed and better braking comfort. On the other hand, the use of those systems submits the brake discs to much higher mechanical and thermal stress than that in brake disc systems used in light vehicles. Such stress frequently causes thermal cracks in the brake discs, which can limit the safety of the vehicle, the durability of the brake pads and also of the brake disc itself. The search for solutions to develop brake pads less likely to generate thermal cracks has been carried out constantly by manufacturers of friction materials. However, the methods of analyzing materials for their potential to generate hermal cracks usually take a long time and involve the use of equipment with a high operational cost. Through the optimization of test parameters and the development of a procedure for small scale evaluation of friction materials in regard to their potential for generating thermal cracks, this study aims to reduce the time and cost of development of friction materials. Various discs and brake systems found in the market, as well as the existing test procedures, were characterized. After this, test parameters were chosen to be used in a small-scale test procedure, which used a brake disc with physical characteristics and a chemical composition similar to the disc used in the full-scale test. Three friction materials whose capability of generating cracks is well-known were tested in order to validate the small-scale test procedure. The parameters for the validation of the small-scale test procedure took into account: the frequency of thermal spots on the brake disc friction surface measured with an infrared camera; the rate of growth of the cracks; the test time and the life of the brake disc. In addition, relations between the occurrence of thermal cracks and the physical characteristics of the friction materials, such as compressibility, hardness and mechanical resistance, were established. As a result of this research, we have demonstrated that it is possible to use, for the development of friction materials, a small-scale test procedure with physical parameters similar to those of the full-scale test, but with reduced time when compared with the standard testing method. Besides, we have shown that the generation of thermal cracks is associated directly with the characteristic of structural rigidity of the friction materials, which, along with the deformations and non-uniform heat flows encountered in the brake disc, cause thermal cracks on the friction surface of the brake discs of commercial vehicles.