The purpose of this paper is to demonstrate the complexity and the importance of the wall friction coefficient in the design of solids handling equipment. In such operations the particles are often subject to appreciable normal loads which may be sufficient to cause breakage. This may, in turn, cause a considerable increase in the wall friction coefficient. The consequence may be that it is necessary to design, for example, a silo on the basis of the increased value if mass flow is to be ensured. In this paper, we report a study, using a new reciprocating shear cell, of particle breakage and its influence on the wall friction. This allows a sample to shear relative to a stainless steel wall at various angles of inclination. The new share cell is an improved design of the horizontally reciprocating shear cell described previously. With this new shear cell, the inclination of the wall can be varied. The paper confirms the rapid increase in effective wall friction which occurs after a displacement of a few meters. In the case of brittle materials, this is mainly due to breakage of the particles and migration of fines. The wall inclination is thus a relevant factor.
