The Thermo-Mechanical Behavior in Automotive Brake and Clutch Systems

Automotive brakes and clutches involve bodies that are in contact and move relative to each other. Typically in the clutch, the contacting bodies take the shape of an axisymmetric disk. As similar as the contacting parts may be from system to system, their functions often vary. While they are used to decelerate or stop the motion of a rotating disk in the automotive brake, in the clutch system they are a mean of transmitting motion between two rotating parts. In the brake system the contact usually takes place between a rotating disk and a stationary friction pad. In the clutch system, however, the contacting disks are rotating at relative speeds and the contact results in a sliding motion over a short period of time till the two bodies are at the same speed. The length of the sliding motion depends on the amount of contact pressure applied, as well as the friction coefficient. The contact engagement in these systems takes place between the friction material of the friction disk and a steel surface. The main problem associated with these types of systems is the variation of contact pressure distribution during engagement, which leads to areas of high-pressure concentration. As a result of sliding motion and friction, areas of high heat generation or hot spots may result which can in turn damage the contact surfaces. The damage can take different forms such as variation in the contacting disk thickness and surface cracks. The variation in disk thickness is expressed as disturbance to the applied load resulting in a low frequency vibration. Overheating of materials at the contact points, on the other hand, can lead to material degradation, which effectively reduces the lifetime of the effected system. Fig. 1 shows an example of hot spots patterns found in a clutch disk. The problem of hot spots has imposed design constraints to the brake and clutch systems in the past. Recently, it has become crucial to investigate the problem of hot spots promoted by the use of new materials and design improvements. The main objective of these investigations are to, if at all possible, completely eradicate the hot spots. This requires identification and examination of the parameters responsible for hot spots and simulation of the engagement process. The sliding motion from the contact lasts for a short period of time that might not exceed half of a second, as is the case in the clutch system. Considering the short time of engagement the transient solution is indeed the key to understanding the process of hot spot formation. This will help to recognize how the problem fields, such as the temperature and the contact pressure evolve with time, in addition, to determine the possibility of material yielding through the computation of thermal stresses. Furthermore, design sensitivity analysis can be carried out with the availability of a transient solution.