Analysis of brake pad friction characteristic obtained by polygon testing of vehicle

Abstract

Friction material of brake pad is one of the most important, but also the most delicate elements of the brake because it has great influence on performance of basic tasks of braking system in different operation conditions. The development and formulation of such friction materials is a balance of functional, physical-chemical and abstract properties, constantly fraught with contradiction. However, the following friction material properties are generally desirable: high static and dynamic coefficient of friction under all permissible environmental circumstances, stable and predictable dynamic friction coefficient within the requisite range of operating conditions (flash-surface rather than bulk temperature), minimum wear characteristics combined with frictional properties that inhibit counter surface wear, i.e. kind to the rotating counter member, adequate shear strength to resist rupture or catastrophic failure, adequate corrosion resistance to retain the preferred property spectrum in the presence of water, salt, sand and mud, sufficient thermal insulation to avoid brake fluid overheating and vapor-lock, conformance to the counter surface to facilitate uniform heat generation, zero judder generation or excitation characteristics, acceptable costs of raw materials, processing and manufacturing technologies. Final selection of brake pad friction material for particular brake, i.e. for certain vehicle, is based on numerous analysis, mostly on experimental research, and from the aspect of all important requests. Testing of material friction characteristics demands delicate procedure, because their features can not be estimated based on their chemical structure, configuration or on other data used for estimation of metals and alloys, but exclusively based on experimental methods. There is a large number of testing methodologies, while some procedures are subjects to international and national standards. The most significant tests include determination of material properties regarding friction and wear. There are two basic ways for testing of friction properties: within the brake itself or on samples, i.e. friction materials test tubes. Frequently, laboratory tests of friction characteristics are not enough, but it is necessary to test the brake plates and pads in real operation conditions. Measuring equipment necessary for polygon tests of passenger and heavy vehicles includes calibrated transducers for speed, deceleration, pressure in hydraulic brake installation and noise, as well as thermo-couples for monitoring of plate and pad temperatures. Analysis of brake pad friction characteristic obtained by the polygon testing of vehicle is presented in this paper. The influence of variations of pressure in brake cylinder and vehicle speed on friction coefficient during tests is analyzed based on acquired signals of pressure in brake cylinder, vehicle speed, brake torque and angular velocity of wheels. Also, variation of friction coefficient during braking (real time) is monitored, starting with the assumption of cosine distribution of surface pressure along the brake pad. Obtained results confirm the importance of testing the friction characteristics of brake pad materials on vehicle in real operation conditions.