EB2014-ST-006 CALIPER-INDEPENDENT INVESTIGATION OF BRAKE PADS 1Fecher, Norbert*, 2Thiesing, Jochen*, 1Winner, Hermann 1Technische Universität Darmstadt, Institute of Automotive Engineering, Germany, 2Federal-Mogul Friction Products GmbH, Germany KEYWORDS brake pad testing, basic research, methods development, validation ABSTRACT

The examination of the characteristics of brake pads is typically done on fly wheel dyna-mometer 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 is-sues 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 non-braking 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 over-all 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 fric-tion 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 cali-per on a fly wheel dynamometer.

INTRODUCTION For economic reasons manufacturers of technical systems are always anxious to reduce costs and/or to increase the technical maturity of their products. A purposeful optimization suc-ceeds only with sufficiently deep system understanding of characteristics and internal recipro-cal effects of the own product. Within the scope of the here introduced, new research method for the caliper-independent investigation of brake pads the questions of a brake pad manufacturer are in focus. He would like to extend his system understanding under use of proven investigation tools such as the fly wheel dynamometer. Therefore the following main questions are given as examples: What are the impacts of different pad ingredients and mixtures as well as manufacturing

process on the major pad characteristics such as friction coefficient, temperature stability and wear?

What are the wear portions in the braked and in the non-braked operation (e.g. caused by residual brake torque) and by which measures can these effects be positively affected?

The tests for the investigation of service brakes are usually performed on a fly wheel dyna-mometer (1). This offers a realistic view on the system characteristics. With focus on the iso-lated items concerning the brake pad, the following effects that occur in operation are affect-ing the results: Caliper deflection: skews the brake pads and leads to a non-homogeneous surface pressure

distribution with taper wear Radial pad movement: can disturb the assignment of the direction of forces and movement

(3) Residual torque: causes wear and heat input in the cooling phases (4) Furthermore some essential variables such as the clamp force are not measured on a dyna-mometer. Although this measurement is needed to calculate the friction coefficient of the in-vestigated brake pad (5). The goal of the development of the new research method is to design a new actuation device in such a way that the disadvantages of a total brake system are avoided as environment for the brake pad investigation. Nevertheless the advantages of the proven investigation tool "fly wheel dynamometer" should be used. REQUIREMENTS To describe the processes in the investigated brake pad, the axial force (clamp force), pad deformation and the tangential brake force have to be measured. For this reason the first re-quirement is: the clamp force has to be measured as near to the pad as possible As the tangential force could not be measured easily, it is calculated by the brake torque and the effective friction radius. The last could be approximated by the distance between the rota-

tion axis and the center of the pad (3). The precondition for this simplification is a homogene-ous surface pressure (6). Hence, the next requirements are: the pad guidance design has to provide a parallel motion, the clamp force has to be applied homogenously and symmetrically to the pad. For investigation of wear characteristics it is necessary to assemble a defined clearance in the non-braked phases to clearly avoid wear effects. In comparison to real brake system tests the influence of residual torques on wear could be analyzed. For the automated assessment of clearance and the automated wear evaluation it is necessary to detect the contact between pad and disk as well as to measure the displacement of the brake pad during the braking, which results in the following requirements: need of automatic clearance adjustment clear detection of pad to disc contact, also for rotating disc measurement of pad displacement during brake application The requirements of the actuation device, shown above, only represent an exemplary excerpt from the extensive requirement catalog in the context of the system development. DESIGN AND FUNCTION Based on the numerous requirements the actuation device was realized during a complex sys-tem development process as follows.

Figure 1: Actuation device for the caliper-independent investigation of brake pads

Figure 1 shows the realized actuation device for the caliper-independent investigation of brake pads on the mock-up table. With the back panel and the knuckle fixture it can be mounted directly on the dynamometer. The actuation device can be divided up into three function levels (Figure 2), which are ar-ranged one above the other for reasons of building limitation of space between torque measur-ing and drive shaft flange at the dynamometer.

Figure 2: The three functional levels of the actuation device

In level 1 two axial mobile carriages are guided within a large, very rigidly designed basic frame. The brake pads are fixed in the carriages that retain both the clamp as well as the tan-gential brake force. Also the two measurement devices the piezo elements and the laser sen-sors are placed in level 1. The piezo elements measure the clamp force and the contact force between pad and disk. The laser sensors detect the displacement and the wear of the brake pads on the backside of the carriages. Furthermore one thermo element is mounted in each carriage for measuring the temperature next to the piezo elements to avoid thermical damages. A more detailed view is given in figure 3.

Figure 3: Detailed view of level 1 without frame

The bearing of the carriages is implemented as flat cage guidance, which is nearly free of fric-tion.

In Level 3, the hydraulic force is generated (Figure 4). The hydraulic diameter of the cylinders is 63mm, so that typical testing programs on fly wheel dynamometers can drive the actuator device without special adaptation.

Figure 4: Sectional view of the entire actuation device The function of level 2 is to mount the two levers that transport the hydraulic force from level 3 to the carriages in level 1. While braking the bearing points are fixed. During the off-brake phases they can be moved by electric motors to generate the clearance (Figure 4). For moving the carriages in level 1 by the electric motors in level 2, the volumes of the hy-draulic cylinders are locked-in by electric valves. While braking, the valves are opened and the cylinders are hydraulically connected, so that a possible disk runout can be compensated hydraulically. The proper function of the actuation device was shown by an extensive verification and vali-dation phase (7). RESULTS For validation testing two well-known test procedures were used. For performance testing the SAE J2522 Dynamometer Global Brake Effectivness Test (AK-Master) was chosen. It is well known as an industry standard for performance evaluation. To evaluate the wear behavior the Main Taunus wear test was performed. This test is also widely accepted in the automotive industry and is based on vehicle data that was taken on a mountain road in the Frankfurt area. The tests of the actuation device and a front axle caliper with 57mm piston diameter were performed on standard inertia dynamometer. Ferodo 4273 a Low Metal material which is in series production was used to compare the brake caliper versus the actuation device. As the actuation device is much larger and heavier than a caliper it had to be supported by a frame in the dynamometer to avoid any influences on the torque cell. The actuation device is mounted with steel cables in the frame. The cables are guided by wheel bearings to allow tan-gential movement for torque measurement (Figure 5).

Figure 5: Actuation device mounted on dynamometer The original parameters of the vehicle which was equipped with the tested brake pad was used for testing. For the actuation device the air gap between each pad and disk was set to 0.3mm in the off-brake sections. The pad wear achieved with the actuation device is significantly lower than the wear achieved with the standard front axle brake caliper. This is shown clearly and repeatable by the Main Taunus wear test (Figure 6). The results can be explained with the air gap between disk and pad during the off-brake sec-tion. This air gap prevents any wear through residual torque.

Figure 6: Wear result MTK wear test

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A wear reduction of about 25% was achieved. This clearly shows that a significant part of the brake pad wear in standard brake calipers is generated under off-brake conditions due to re-sidual torque. Test T032515 with a standard brake caliper showed similar low wear results as the actuation device, it can be assumed that this caliper provided a higher air gap than the oth-er calipers. That also shows that wear results with standard calipers are somehow volatile, as they depend on the roll back behavior of the piston. The taper wear of the actuation device was minimal due to the parallel guidance of the pads (Figure 7)

Figure 7: Taper wear after MTK wear test with brakeing device During the AK-Master test the general characteristics of the friction coefficient was similar. Differences can be seen in the pressure sensitivity and the friction level. The actuation device has a higher pressure sensitivity than the brake caliper. This phenome-non can be explained with differences in the friction film due to different off brake conditions. The level of the friction values was significantly higher when using the actuation device (Fig-ure 8, 9).

Figure 8: Friction result AKM performance test

Figure 9: Friction result AKM performance test The µnom increased from 0.38 to 0.45 (+17%) at the same time the wear values were reduced by 18%. The tangential taper wear was significantly higher in the conventional brake caliper, it can be assumed that this is due to deflection of the caliper. ( Figure 10,11).

Figure 10: Radial taper wear of pads in brake caliper during AKM

Figure 11: Radial taper wear of pads in actuation device during AKM This evidently shows that the brake caliper itself has an important influence on the achievable friction value. Reasons for that could be the deflection of the housing , influence of tangential forces to the pad guidance and as a consequence uneven pressure distribution on the pad sur-face. This work proved that the actuation device is a valuable tool to evaluate friction materials without influences of the surrounding brake system. The values given by the actuation system are the real µ and the real wear values of the friction material, as they are not disturbed from brake caliper efficiency and influences of uncontrolled off brake behavior . OUTLOOK Based on the above results, the actuator device shows great potential for all questions: in which the influence of the brake caliper is to eliminate or to investigate, where the capability of friction materials have to be evaluated. Typical investigations could be: Influence of friction material compressibility or design on hot judder, as the pressure dis-

tribution and energy input to the rotor is homogeneous. DTV creation by adjusting the air gap of each pad with defined disk run out and meas-

urement of touching forces. The co-operation partners will use the actuation device as a standard tool for friction material development in the future. Materials can be evaluated on their real behavior and then be se-lected for specific projects depending on their real capabilities. It also is a helpful tool to evaluate the efficiency of brake systems in total. REFERENCES (1) Schäuffele, Jörg; Zurawka, Thomas: Automotive Software Engineering; 3. Auflage,

Vieweg, Wiesbaden, 2006 (2) Degenstein et al.: Messung der Kräfte in der Reibfläche einer Scheibenbremse wäh-

rend des Bremsvorgangs, Brake-Tech, 2006

(3) Degenstein, Th.: Kraftmessung in Scheibenbremsen, Fortschritt-Berichte VDI Reihe 12, Nr. 655, Düsseldorf: VDI-Verlag 2007

(4) Tao, Jason J.; Chang, H. T.: A System Approach to the Drag Performance of Disc Brake Caliper, SAE-Paper 2003-01-3300, 2000

(5) Breuer, Bert; Bill, Karlheinz H.; Hrsg.: Bremsenhandbuch; 4. Auflage, Springer-Vieweg, Wiesbaden, 2011

(6) Haag, M.: Modellierung der Radbremse für virtuelle Prüfstandsversuche im frühen Auslegungsstadium, Fortschritt-Berichte VDI Reihe 12, Nr. 758, Düsseldorf: VDI-Verlag 2012

(7) Fecher, N., Maschmeyer, H., Winner, H., Paul, H.-G., Dlugosch, F.-J.: Bremssystem-unabhängige Untersuchung von Bremsbelägen, SIMVEC 2012, VDI-Wissensforum, 20.-21.11.2012, Baden-Baden

(8) SAE J2522 Dynamometer Global Brake Effectivness Test