If an exact method for determining the measure of work losses in flowing bulk materials was defined (i.e. determination of the angle of internal friction as well), models of bulk material flow, models of internal processes in these materials and models of their behavior would become essentially exact and precise. Such concept would bring a stabilization and optimization of many technological operations in processing bulk materials. In this article, there is a proposed model for the angle of internal friction in the bulk material within the interval of angles phi(1) is an element of (5.66 degrees, 30.36 degrees) based on the main types of structures of particle arrangement in a volume of monodisperse particle size which corresponds to the assumed unit size R=1. This paper is the first of a set of contributions attempting to make models of bulk materials more exact by using the concept of the angle of internal friction. The models of piston and shell flow of bulk material form a background for this concept. The paper shows that a balance interpretation of the values of the angle of internal friction is principle possible. The relation of the geometry of voids and geometry of particles can be described and defined (within the frame of the theory of piston and shell flow mechanism). This concept is directly or indirectly supported by a great number of independent publications. It can be assumed that this interpretation will be commonly accepted. (C) 2012 Elsevier B.V. All rights reserved.