New Technique for Measuring Clearance in Low-consistency Refiners

Refiner action depends on forces applied to fibres, and stresses on fibres are functions of strains reached, which in turn are related to the actual gap clearance. Hence, bar gap clearance has a major impact on fibre treatment, and this has been pointed out in previous studies. However, gap clearance measurements reported in the literature are questionable and in this study we obtained measured values that are significantly larger. The experimental values obtained in this study, carried out with a linear variable differential transformer (LVDT) displacement gauge to measure gap clearance, indicate some new features of the mechanical behaviour of refiner. Firstly, an 'out-of-tram' effect producing a lack of parallelism has been shown to be a feature on low consistency refiners. Secondly, the disc closing force when pulp is flowing cannot be achieved when only water is flowing. We discuss the consequences of this new experimental evidence and clarify some previous observations reported in literature, in terms of our gap clearance measurement procedure. Finally, a new protocol to measure the absolute value of the gap clearance is proposed.