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The measurement of forces in chip refiners Siadat, Seyed Mohammad Ali


The purpose of the chip refining process in the pulp and paper industry is to produce wood pulps and to enhance certain desirable properties of the fibres in the pulp suspension, by subjecting the wood material to cycles of shear and compressive stress. This process has commonly been quantified in terms of energy-based parameters such as specific energy and refining intensity, but such methods, although useful for establishing energy-pulp quality relationships, do not describe the underlying fundamental mechanisms of the process. It has been suggested that a knowledge of the stress-strain history of individual fibres can yield a deeper understanding of the process [Page, Fundamental Research Symposium (1989)]. With the long-term goal of such an understanding, the forces experienced by pulp in the refining zone are measured in a laboratory refiner operating at 700 rpm with softwood TMP at 16% consistency. This is done using a two-axis force sensor, originally designed by A . Bankes, P. Wild and D. Ouellet. The design, in its original form, did not perform well enough to provide a reliable force measurement under the high excitation frequency of passing refiner bars, as the sensor's resonant frequency was too low. A modified design with a much higher natural frequency is presented here, along with force measurements in the refining zone for various plate clearances, and varying dilution flow rate. From these measurements, we see that both the normal and shear forces contain a component due to the ploughing action of the corner of the bar through the floe (termed the comer force), a phenomenon previously only seen in the shear force [Batchelor et al. J. Pulp Pap. Sci. 23(1) (1997), Senger et al., Proceedings of the International Mechanical Pulping Conference, (2001)]. The peak normal force increased with decreasing plate clearance, while the peak shear force measured by the sensor did not exceed UN, corresponding to a maximum shear force per unit bar length of 2.2 kN/m. The equivalent tangential coefficient of friction decreased with decreasing plate clearance, and also decreased upon the addition of dilution water. The sensor design needs further refinement for testing at higher refiner rotational speeds, as signals acquired at high speeds are distorted by the sensor's resonant vibrations.

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