Shear stresses and material slip in high pressure roller mills

Abstract

The compaction of the particle bed in a high pressure roller mill causes pressure and shear forces on the rollers, which have been measured simultaneously with a sensor. As known, the pressure peak arises short above the gap neck, the shear is firstly orientated against and then in the rotation direction. However, the zero point does not coincides with the pressure peak as often said but lies before it. In the entire compression zone and in the first part of the relaxation zone, the shear–pressure ratio is smaller than the coefficient of external friction μ, slip, therefore, does not exist in this region. This ratio exceeds μ only near to the outlet and indicates slip there. Based on a force balance on a differential strip and with the assumption of proportionality between the transversal pressure in the bed and the normal pressure on the roller, an equation arises relating shear to pressure. Only one property of the granular material remains in this equation, the stress ratio coefficient λ defined as in powder mechanics. The calculated shear agrees well with the measured value in the compression zone until to the shear minimum but is clearly smaller further down. The analysis of the measurements in view of the theoretical considerations leads to the conclusions that slip only exists in the region near to the outlet. Here, the material is extruded, which causes a tensile force acting on the material above and increases by that the shear over the calculation. This effect ceases further up, therefore, the calculation approaches the measured values.