THE EFFECT OF PRESSURE AND OVER-CONSOLIDATION ON THE GRIP OF THE ROLL SURFACE ON PREPARED CANE AND BAGASSE

Mill feeding is acknowledged to be an important part of the crushing process. For good feeding of prepared cane or bagasse between a pair of rolls, the roll surface needs to grip the material and the material must be strong enough to prevent internal shear failure. Sugar mills spend a great deal of effort preparing and maintaining the surface roughness of roll grooves to ensure good feeding. The magnitude of the coefficient of friction on the roll surface is a necessary input for modelling of the crushing process, with the maximum achievable value being the coefficient of internal shear of the prepared cane or bagasse. Previous investigations have shown that the magnitude of the coefficient of shear (the maximum coefficient of friction) of normally consolidated prepared cane on rough surfaces is strongly pressure dependent. This paper presents measurements on normally consolidated prepared cane, first bagasse, and final bagasse at pressures spanning most of those present in milling that support that conclusion. The magnitudes of the coefficient of shear for the three materials are quite similar. Also, values of coefficients for over-consolidated prepared cane and bagasse are presented which suggest that, at least at relatively low pressures, the coefficient of shear of bagasse may be history dependent. That is, bagasse which has been previously loaded to a higher pressure than its current state (over-consolidated) may exhibit a higher coefficient of shear than bagasse which has simply been compressed to the same pressure. Over-consolidation is expected to occur after the underfeed nip, the pressure feeder nip, the feed nip, and the delivery nip. The over-consolidated bagasse may then achieve a high coefficient of friction when the bagasse contacts the arced surface on the next roll, implying that it may feed better. The magnitudes of the coefficients are of use as coefficients of friction for feeding calculations and as boundary condition inputs to a milling model.