Influence analysis of brake linings roughness on static friction coefficient between brake linings and shoes

The static coefficient of friction between lining and shoe plays a fundamental role in the lining fixing project, which is the most important parameter for the riveted joint calculation. For the lining riveting, the rivet needs to ensure that friction material and shoe remain in contact through the normal force applied on the surfaces, but the rivet should not be exposed to shear forces. Thus, the brake torque transmission must occur through the static coefficient of friction between lining and shoe, not allowing relative slips or movements between the pair in contact. Therefore, the present study aims to understand the influence of the static friction coefficient between lining and shoe as a function of the lining internal superficial roughness, from the evaluation of different roughness conditions - contact area with shoe -. The static coefficient of friction between lining and shoe is a complex measurement to be performed, due to the cylindrical geometry of the drum brake system, so for the present study, a measuring device based on plane geometry will be proposed. Thus, three different friction materials were evaluated, and each one of them had a different formulation and mechanical properties, such as hardness, shear strength, impact resistance, dynamic coefficient of friction, and stiffness. Plane samples were made for each of these friction materials, with three different surface finishing obtained by changing the machining parameters - speed and feed -, generating different surface roughness profiles. For each friction material sample, three sequential tests were performed in order to measure the static coefficient of friction, using a universal testing machine and a flat counter piece with the same material and painting of the shoe. For the test, a specific device was developed, allowing the application of tangential force only on the friction material, keeping the counter piece fixed. From this study, it was possible to conclude that each friction material tested has its level of static coefficient of friction, nevertheless, the superficial roughness did not show influence over static CoF. The static coefficient of friction shows a variation up to 14% for the same friction material and surface condition, being higher than the variation of the same material under different surface conditions.