The examination of the characteristics of brake pads is typically done on fly wheel dynamometer test benches. For the actuation of the pads, standard or near-standard wheel brake systems are used which consist of wheel bearing, brake disc, brake caliper and brake pad. In this configuration, the system can be operated realistically to its later usage in the vehicle and also can be investigated. In the brake pad development there are often more fundamental issues in focus, such as the determination of dependencies between the brake pad material and its production on the one hand and physical properties (eg. friction and wear behavior) on the other. Many effects that occur in operation, such as caliper deflection or radial pad movement are depending on properties of the brake system and therefore the brake system is gravely affecting the results of the investigation regarding the basic relationships.
To overcome these disadvantages, the Institute of Automotive Engineering (Fahrzeugtechnik Darmstadt - FZD) of the Technische Universität Darmstadt developed an actuating device for brake pads for use on the fly wheel dynamometer test bench in cooperation with the Federal Mogul Friction Products GmbH. With this device it is possible to actuate brake pads without a brake caliper on the dynamometer. Essential requirements for the development of the device have been especially the parallel guidance of the pads as well as the ability to adjust a defined clearance and the measurement of the clamping force as close as possible to the brake pads. After a successful commissioning of the device at the FZD dynamometer the basic operation of the actuator could be verified in an extensive fundamental test series.
As part of the validation tests two different types of brake linings (Low Met, Cu-free NAO) were tested in the Main-Taunus-Kurs (MTK)-wear test on the dynamometer and the test was conducted additionally with the braking actuator as well as with the real caliper. In the nonbraking phases a defined clearance between pad and disk was set, to prevent any non-braking wear. The result shows that the wear on the pads with active clearance adjustment is around 25% below the measured values with the real caliper. Thus, it could already be shown in this first phase that the non-braking wear in the brake caliper has a significant impact on the overall wear behavior. Moreover, it was shown that the inner and outer pad wear is almost similar when used in the braking actuator, as well as the radial and tangential wear. This is seen as an evidence of the successfully realized parallel guidance of the pads and for a homogeneous force distribution.
With regards to the friction coefficient, a value of about 18% higher compared to the caliper was measured, for whom friction conditions are not ideal due to the caliper deformation under braking conditions and the associated inclination of the brake pads. As these results show, with the braking actuator the real friction and wear behavior of the friction material can be tested independently of the vehicle braking system.
Subject of this publication is a new method in the area of "Scale Testing for Research and Development" of brake pads, the description of the design of the braking actuator as well as the actual results recorded in comparison tests between the braking actuator and a brake caliper on a fly wheel dynamometer.
Norbert Fecher, Hermann Winner - Technische Universität Darmstadt; Jochen Thiesing - Federal-Mogul Friction Products GmbH