The importance of moisture in the methodology of measuring the friction of plant granular materials (PGM)

• The problem of measuring the friction force of granular materials was discussed. • The pressure value of the granular material on the contact surface was measured • A method of calculating the actual pressure including moisture content was proposed. The methodology for measuring friction of plant granular materials PGM should take into account the behaviour of the material and the interactions of the individual grains. PGM placed in a bottomless supported box transfers part of its vertical load onto the side walls. This effect means that the actual pressure on the contact surface is lower that what would be expected from the material mass and the load. This effect is well-known and has been frequently described in the literature, however this has been mainly for large containers (silos). The authors of this paper studied the influence of this phenomenon on a small box. The study considered five plant granular materials (wheat, maize, beans, peas, and oats), eight load values ranging from 5.35 to 74.49 N, three heights of the PGM layer and several moisture levels ranging from 4.8 % to 32.7 %. The proposed model based on a modification of the well-known Janssen model is a good fit for the measurement results obtained. The modification involved taking into account k ’ coefficient (determined in a previous publication by the authors) and above all the moisture content ( Mc ) of the material. The study is significant in the context of developing a cohesive methodology of friction measurement of PGM .