Brake noise or squeal is a major problem experienced by brake system manufacturers, which leads to customer dissatisfaction and the questioning of quality. Research on the NVH performance of brake systems is performed to study and eliminate this problem using advanced testing tools, design modification, dynamometer testing etc. But, brake squeal still persists. These research advancements led to one of the most important reasons for squeal which is the complexity of the brake pad. Therefore, it is very imperative to understand the behaviour of the brake pad, in terms of its dynamic characteristics (eigenfrequencies, damping and mode shapes). Experimental Modal Analysis (EMA) is commonly used to analyse brake pads for studying these characteristics. EMA is generally performed under free-free boundary conditions. However, as squeal happens only during a braking event, analysing brake pads under pressure condition means a step towards reality. Furthermore, the dynamic characteristics of the brake pads are dependent on the pressure and the boundary conditions (location of piston and caliper). Therefore, an approach to study the behaviour of brake pads under pressure is a decisive process for advancing in solving the squeal issue.
The Brake Impedance Test stand (BIT) is developed to analyse brake pads under varying compression loads. The test rig is equipped to provide a specified piston compression, equivalent to that on a squealing brake event, on brake pads. EMA is performed by employing a Scalable Automatic Modal hammer (SAM) to excite the brake pad and provide repetitively precise force input. Laser Doppler Vibrometry on its single-point and 3D Scanning variants is used to collect the velocity output of the pads vibration. This research is engineered towards analysing different brake pads, differing in geometry and manufacturing process and to find any significant variances amongst each other. The BIT provides in-depth knowledge in deviations, in terms of mode shape analysis, eigenfrequency shifts and complexity analysis, among brake pads under pressure. These deviations among brake pads cannot be observed by testing under free-free boundary conditions.
The data derived from the BIT characterizes dynamic parameters of a brake pad under pressure. Using this data and further R&D it would be possible to determine dynamic parameters closer to the reality of a squealing brake event, and correlate those with the results of road/dyno testing. The conclusions will lead to effective brake pad parameters for a robust brake against squeal research.
Blaschke Peter, Mallareddy Tarun Teja, Schneider Sarah, Edel Carl-Heinz, Alarcón Daniel J., Technische Hochschule Wildau, Germany; Ritter Jörg, RTE Akustik + Prüftechnik GmbH, Germany.