Effects of disc thickness variation and out of roundness on electromechanical brakes

Imperfections in the brake geometry, such as the disc thickness variation (DTV) or the out of roundness (OOR) of drum brakes are well examined in the context of hydraulic brake systems in passenger cars. Geometric imperfections can cause discomfort due to brake torque oscillations, chassis vibrations and brake pedal pulsations. These effects also influence the interactions between the basic brake and the actuation mechanism and vice versa.
Electromechanically actuated brakes (EMB) have some unique properties in that respect. For example, the actuator motor inertia is directly coupled to the brake pad by a relatively stiff gearing. In contrast, the coupling of a hydraulic brake system to the foundation brake is different, leading to different stiffness and damping properties. This paper examines these differences and their consequences, especially regarding the vibration load on the EMB gears and bearings.
A theoretical analysis shown in the paper predicts some challenges in the design and endurance testing of future EMBs. First experimental results obtained with eccentric drums and electromechanical brake actuation partly confirm the theoretical analysis. The measurements also raise some questions about the relative motion of the brake shoes in a rotating eccentric drum. Even if the brake shoes are mounted in a floating manner, the friction at the lower and upper abutment prevents the shoes to follow freely the eccentric movement. Small eccentricities may be completely adsorbed by deformations, without any sliding motion of the brake shoes. Additional sensors on the brake shoes are used to gather information about the motion relative to the backplate.
The results indicate that a deeper analysis of DTV, OOR and other geometry imperfections must be conducted for EMB-s in passenger cars. Their effect may have to be considered in future endurance test specifications.