Full-scale dynamometer tests of composite railway brake shoes including latxa sheep wool fibers

Abstract

The main target of the present work is to characterize the effect of the inclusion of natural sheep wool (SW) into a railway brake block composition and then to compare it to that of a set of three organic fibers commonly used in the friction material industry: aramid fiber (AF), cellulose fiber (CF) and polyacrylonitrile fiber (PAN). In order to achieve this, 4 versions of the same friction material with a fixed amount of each organic fiber were produced and one more sample was manufactured including no organic fibers. The characterization work consisted of friction tests on a full-scale railway test rig. Then, the samples were SEM analyzed in order to characterize the tested surface microstructure. It was found that all organic fibers helped achieve a more stable bedding, and showed lower friction in wet conditions. They also affected the recovery %. Pictures of the blocks were taken after certain phases of the test and, although the failure sequence remained the same for all samples, the organic fibers very much influenced the magnitude of the wear rates. Sheep wool led to better results than cellulose. No final conclusions could be drawn with respect to metal pick-up. SEM analysis evidenced primary and secondary plateaus, but no significant differences were observed depending on the fiber nature. Finally, a Life Cycle Assessment with a “from cradle to gate” perspective was carried out. Ecoinvent v3.5 database and CML and ReCiPe Endpoint methodologies were used to evaluate the environmental impact create by the five brake block materials. Overall, cellulose, PAN and sheep wool brake blocks show slightly lower environmental impacts that the base material or than aramid fibers. Therefore, Latxa sheep wool offers a good balance between low cost, adequate wear rates and environmental impact, making it a compelling substitute for cellulose fibers.