The phenomenon of vehicle park brake rollaway

摘要

When a vehicle is parked on a slope with hot brakes, what appears to be a sufficient parking brake action can sometimes become insufficient. When the brakes cool down, the braking force reduces due to relaxation of the entire parking brake system, and the vehicle may start to move, leading to obvious catastrophic consequences. This phenomenon is known as vehicle rollaway. This thesis describes the problem in detail and postulates a mechanism that accounts for the occurrence of the rollaway event on vehicles using integrated rear callipers. Different testing methods are presented that are used to investigate the propensity of a vehicle's parking brake system to rollaway. These include on-vehicle evaluations and laboratory based measurements. A description is given of a novel dynamometer facility that was developed for this research that is capable of testing vehicle parking brake systems for rollaway. Two mathematical modelling techniques are presented that demonstrate how the parking brake system parameters influence the likelihood of rollaway occurring. A finite element model was used to simulate the change in contact pressure at the frictional interface during a rollaway event. A numerical model was also used to predict the change in torque developed by a parking brake system cooling from an initial elevated temperature. The change in clamp load at the frictional interface was modelled using an essentially I-D quasi-static system that showed how the stiffness and the thermal properties of the system qualitatively affect the phenomenon. The research found that rollaway does not always start with a uniform motion, but with a stick/slip motion. The likelihood of rollaway occurring was found to be directly linked to the temperature of the brake when the vehicle is parked. Rollaway can be reduced by lowering the initial temperature of the brake prior to parking. Rollaway can also be reduced by increasing the input load to the system when applying the parking brake. The research identifies the key design parameters of the brake system components whose values require close control within the real system if rollaway is to be avoided.