Shear stresses and material slip in high pressure roller mills

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.