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EuroBrake 2022: Testing requirements and friction couples session overview

The Testing requirements and friction couples session will take place on Thursday May 19th and will be chaired by Georg-Peter Ostermeyer of TU Brunschweig and Neomy Zaquen of Lapinus.

Topics and speakers for the session include:

Wear measurements with copper-modified brake pads

Jacek Kijanski, TU Brunschweig

Copper plays an important role in brake pads. For example, copper is frequently said to have a wear-inhibiting effect. At the same time, copper particles released from brake pads have a negative impact on the environment if they enter the groundwater. For this reason, efforts are being made to substitute copper in brake linings as far as possible. In cooperation with Volkswagen AG, brake pads with different copper contents (powder) are investigated in this work, focusing specifically on their wear behaviour on a pin-on-disc tribometer. The tribometer, which is available at the Institute for Dynamics and Vibrations at the Technical University of Braunschweig, is fully automated and has high-precision laser measurement technology that is specially optimized for wear analysis. In principle, this allows high-precision surface measurement after every friction application. In addition, the tribometer is equipped with a heating box, which allows measurements with disc and pad temperatures of up to 200°C.

This paper systematically investigates how the copper content in brake pads controls wear in the short and long term. Like observations in the literature, the results show that no general statements can be made regarding the influence of the copper powder content on wear behaviour and that the shape of the copper as well as the interactions of copper with other brake pad components seem to play a significant role.

Effect of humidity levels on the compressibility of automotive brake friction materials

Charles Greening, Greening Testing Labs

The load-deflection relationship of automotive brake friction materials (also known as “compressibility”) is a characteristic which may both influence and advise as to brake performance, including driver experience, noise and vibration behaviour, and manufacturer quality control. It has been reported that changes in environmental humidity may significantly alter friction material compressibility in a formulation-dependent fashion – effects which are important considerations for testing protocols and real-world applicability.

Greening’s presentation includes a rigorous investigation of humidity-dependent properties across differing friction materials and manufacturers, taking advantage of the high resolution and excellent repeatability of the Greening Model 1140 Compressibility Test Machine. Sets of brake pads from (at least) two different manufacturers and of varied material type (non-asbestos organic, semi-metallic, low-metallic, etc.) will be weighed to assess changes to moisture content after climate-controlled storage for at least 24 hours at Ambient, Low, and High relative humidity levels. Pad compressibility at ambient temperature and 130 bar simulated brake line pressure will be measured a total of 8 times (24-hoursoak before each test) at [2 x] Ambient, [2 x] Low, [2 x High], and [2 x Ambient] humidity levels. This testing protocol will allow for confirmation of repeatability at comparable moisture levels. Humidity-dependent differences relative to material types, manufacturers, and individual pad variation will be presented, as will the measurement of

rebound/recovery after repeated rounds of testing. Results will provide a valuable resource in the awareness and understanding of how humidity effects may play a role in the design, testing, and manufacture of automotive brake friction materials.

An investigation of changing properties of copper-free brake pads and disc during consecutive wear test cycles of SAE J2707B procedure

Meechai Sriwiboon, Compact International Co. Ltd

For quality control purposes as well as for brake friction and noise modeling purposes, there is urgent need to establish good database if one is to establish correlations or trends between material properties and brake friction/noise. This investigation was undertaken to find out how pad properties change when the pads are subjected to repeated SAE J2707B wear cycles, wearing out roughly 40% of the available pad thickness. Pad natural frequencies continued to decrease while disc natural frequencies decreased initially and then slowly increased. The pad dynamic modulus continued to decrease from Wear Cycle #1 to Wear Cycle #5. Within a Wear Cycle, the pad dynamic modulus changed from one Wear Block to another Wear Block. A good relation was found between the friction coefficient and dynamic modulus, which was adjusted for pressure dependence and temperature dependence. Pad thickness loss measurement was always smaller than the thickness loss calculated from the pad weight loss due to expansion of the pad contact surface region (heat-affected layer) caused by exposure to high braking temperatures. The pad dynamic modulus increased with time while waiting for the next test and the increase was linear with waiting time.

Brake dyno with minimized flywheel mass - what can modern AC motors achieve?

Johannes Leopold, RENK Test System GmbH

The new and further development of vehicle brake systems today mainly takes place on flywheel brake test rigs. In these test benches, the brake disc is accelerated to the desired test speed by means of an electric motor. The vehicle mass is simulated using rotating flywheel masses that are integrated in the drive train of the test rig and are thus mechanically coupled to the brake disc to be tested. Different vehicle masses are usually implemented with several inertia masses, which can be coupled and decoupled as required. An exact setting to the vehicle mass to be simulated can then be made via the drive motor, which acts as a generator/motor during the test run and thus can represent an additional positive or negative inertia. In principle, a very simple and realistic simulation of the vehicle masses can be achieved using the flywheels, but the structurally complex implementation of such a solution ultimately means a not inconsiderable cost item in the total price of such a test system.

The great advances that have been made in recent years with high-torque motors and their control accuracy make it possible today to ask whether the standard centrifugal masses can be replaced by a mass inertia simulation of large motors. For this purpose, extensive investigations and calculations are currently being carried out at RENK Test System GmbH using the example of a full-scale flywheel brake test bench for brake systems in rail traffic. The results of this work will be presented with the proposed technical paper:

  • Estimation of the limits and control accuracy of the electrical inertia simulation by testing it on the real test bench

  • Examination of the essential assemblies (base frame, basic flywheel, floating bearing (test specimen holder), test station) with regard to the necessary function, design and costs

  • Presentation of the advantages and disadvantages of the "classic flywheel brake test bench" compared to the "brake test bench with electrical mass simulation"

We would expressly welcome a following discussion of the presented study results (also regarding the partly personal preferences of the operators).

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