The article examines the influence of several factors (load, moisture, velocity) on the value of the friction force. The research was conducted on three different granular materials of biological origin (peas, maize, triticale). In addition, attention was given to the geometrical surface structure of grains on contact with the structural material. The proposed mathematical model took account of four independent factors. Statistical analysis showed their significant influence on the friction force. It was also found that velocity, in the tested range (30-60 mm/min), had a negligible impact on the value of the friction force. (C) 2018 Elsevier B.V. All rights reserved.