INDEPENDENT VARIABLES
The independent variables were defined as follows. Four rotor designs were tested in the experiment: the traditional design with rectangular hammers and three new designs
with hammers in the shape of a circle section. A characteristic feature of the tested designs was the hammer angle (α) measured between the opposite faces of the hammer. For the traditional rotor design with rectangular hammers, the hammer angle is 0°.
The hammer gap (s) is the space between the ends of the rotating hammers and the inner surface of the screen. In the experiments, the hammer gap was adjusted by using hammers of different lengths, as measured along the axis of symmetry from the hammer mounting hole to the hammer end.
The diameter of the holes in the screen (d) determines the degree of fragmentation. In the experiments, smooth screens, 3 mm thick, with hole diameters of 3 or 5 mm were used. The wrapping angle of the grinding chamber was 360°.
The hammer end peripheral speed (v) was defined as the product of the angular velocity of the mill shaft and the radius formed by the ends of the rotating hammers.
The experimental design included four values of hammer angle, three values of hammer gap, two values of screen hole diameter, and five values of hammer end peripheral speed. These values are listed in table 2.
The independent variables and their values were selected on the basis of extensive literature studies regarding the theory and design of agricultural machinery, as well as research carried out by the authors. The following criteria were used for selecting the values of the independent variables.
The traditional rotor design with rectangular hammers (α = 0°) is widely used in currently manufactured hammer mills. The new design was equipped with hammers in the shape of a circle section. The hammer angles of the new hammers were determined based on the geometric features of the rotor, the number of attached hammers, and their distribution on the periphery of the rotor.
The size of the hammer gap was determined on the basis of empirical studies, which resulted in a range of values (10, 15, and 20 mm).
Depending on the type of material and the expected degree of fragmentation, screens with holes of different diameters are commonly used in industrial hammer mills. The
diameter of the screen holes determines the degree of fragmentation and the mill throughput. In this study, smooth screens with holes of 3 or 5 mm diameter were used, as they are widely used for grinding barley and triticale.