Abstract
KEYWORDS – brake disk, heat localization, frictional heating, hot spot, FE analyses
ABSTRACT–Frictional heat is generated on the rubbing surface in disk brake equipment, and various types of thermal localization occur during sliding contact between the disk and pad surfaces during braking. Thermoelastic expansion due to the frictional heat generated affects the contact pressure distribution. The disk surface temperature is frequently localized, and such thermal behaviour causes surface damage. An FE analysis method for evaluating disk surface temperatures should be developed with the aim of reducing disk damage. The objectives of this paper are to understand the origin of hot spot generation and reproduce it by FE analyses.
To evaluate this phenomenon, braking tests were conducted, and the localized temperature on the disk surface was measured using an infrared camera. Contact force was also measured by a 3D piezoelectric force sensor device, and the friction coefficient was calculated from the normal and tangential forces. By analyzing the contact force frequency, the correlation between the temperature distribution and contact force variation was evaluated. To simulate localized temperature on the disk surface, FE analyses for the braking tests were conducted. The FE analysis method combined with contact analysis and heat transfer/thermal stress analysis was applied. The disk surface temperature was analyzed using the heat flux distributions obtained from the results of the contact analysis. The heat flux distributions were calculated in consideration of frictional heating, including the introduction of both actual contact force variation and friction coefficient variation between the disk and pad. As a result of the braking tests, hot spots appeared on the disk surface, and number of hot spots was varied during the braking tests. Frequency analysis results of the contact force indicated that localized temperature was correlated with the frequency response for contact force variation. The disk surface temperature distribution obtained from the FE analyses closely coincided with the temperature measured during the braking tests, and the FE analyses reproduced same number of the hot spot behaviours in the tests. In addition, disk and pad temperature was also in good agreement between the test and analysis. Therefore, the proposed method is believed to be useful for evaluating the rubbing surface temperature. This paper found the correlation between hot spot generation and contact force, and applies a new numerical analysis model for localized temperature evaluation. In order to reproduce localized temperature behaviour, contact analysis should be conducted, but such analysis requires too much time. Combining contact analysis and heat coupled heat transfer/thermal stress analysis can reduce the calculation time. This new method is more practical than the method using contact analysis only. To clarify the mechanism of hot spot generation, braking tests and FE analyses were conducted. The test results showed good correlation between hot spot generation, contact force and friction coefficient variation. The disk temperature obtained from the tests and FE analyses also showed good agreement. In addition, the time duration and location of the hot spots on the disk surface are not regular. These results suggest that hot spots are generated not only by a specific vibration but also by the superimposing of some other vibration frequency components.