Thermomechanics of block brakes

Doctoral thesis, 2006

Friction in sliding contacts plays an important role in tribology. Applications can be found in many engineering fields  such as braking of vehicles  in which friction brakes are by far the most common. Thus, an understanding of the mechanisms involved in friction brakes has become more important as the demands on brakes increase. Today, the braking systems used are acknowledged to have problems related to thermal loading. From a theoretical as well as experimental perspective, little is actually known about the location, magnitude and distribution of pressure and temperature between sliding bodies. This is especially true when considering high energy sliding contacts, which can be found in brakes and clutches. This thesis concerns the temperature and pressure distribution between two sliding bodies. Investigations of how various parameters influence contact pressure and temperature have been carried out. An instability phenomenon known as thermoelastic instability, which is frequently observed in experiments, was found to be the main driving force in terms of excessive pressure and temperature. Thermoelastic instability on the friction material appears as moving contact points, caused by the interaction between wear and thermal expansion. A two-dimensional finite element model and a pin model were used to calculate interface pressure and temperature. These models can handle temperature dependent variables, such as the friction and wear coefficients. The models and methods employed in the present thesis were chosen in order to illuminate essential aspects of a braking system and, coupled with numerical results, to quantify the effects of different materials properties. A less complex pin model of a brake was employed to reduce the computational effort, which is practical when simulating several minutes of high speed braking or if the brake model forms a part of a larger simulation model. Temperature measurements in a full-scale block brake test rig were also performed, in order to investigate block temperature and to verify the mathematical models. Both measurements and simulations show an unstable temperature distribution under the studied conditions. Comparison with experiments and the finite element model revealed that the pin model may be an alternative to more complex models for studying aspects of sliding contact, in particular contact pressure, temperature and thermoelastic instability.