A spinning-tube device for dynamic friction coefficient measurement of granular fertiliser particles

This paper describes the development of a device capable of measuring the dynamic friction coefficient of particles accelerated in a tube spinning in a horizontal plane. The dynamic friction coefficient is not the classical Coulomb friction coefficient, but a lumped parameter influenced by the Coulomb friction coefficient between particle and tube, aerodynamic effects, bouncing effects, as well as particle size, shape and texture. The principle consists of measuring the time interval associated with particles travelling through a known distance along a tube section. This time interval served as an input for a model that infers the dynamic friction coefficient. The results for plastic spheres showed that (1) the mean dynamic friction coefficient was higher than expected (0.25), and (2) there was significant variability in the data (standard deviation (SD) 0.03) even though the particles were inserted using a precise and reproducible method. The variability in dynamic friction coefficient was inversely proportional to the rotational velocity of the tube. This led to the conjecture that owing to the higher Coriolis forces, the particles more closely follow a straight trajectory along the inner sidewall of the tube. Owing to the lower variability, the dynamic friction coefficient measured at the highest rotational velocity (800 min^-1) was assumed most reliable. The mean dynamic friction coefficient of irregularly shaped potassium chloride fertiliser was measured as 0.44, SD 0.05, and for the quasi-spherical ammonium nitrate fertiliser a mean value of 0.31, with an SD of 0.03 was obtained.