Dynamic Behaviour of the Doctor Blades Used in Paper Industry

The dynamical behaviour of doctor blades used in paper industry for cleaning press-rolls is studied numerically and semi-analytically. A multiscale approach combining the dynamic effects of uni-directional roughness of the rolls on the lubrication/cleaning process of the blade is established and a parametric study regarding the geometrical and elastic properties of the blade carried out. The ultimate goal is the theoretical prediction of the cleaning efficiency and the estimated frictional energy loss. INTRODUCTION With a share of 5.7% of the total industrial energy consumption in 2004, the paper industry is one of the largest energy consumer worldwide [1]. In this study, we focus on the tribological behaviour of the doctor blades, alone responsible for the 3% of electricity losses [2]. In this contribution, we extend the steady-state approach in [3] by introducing in the description of the mechanical behaviour of the blades their design and the impact of irregularities of the roll surface on the lubrication mechanism. THEORETICAL APROACH The blade holder is a rigid body constrained to the frame of the machine by a hinge and its position is controlled by a pressurised hose. The doctor blade is taken as a cantilever beam, subject to small linear elastic deformations and fixed to the blade holder by a fixed joint. The blade deformations in the axial and transversal direction are modelled in terms of two coupled spring-mass systems. The lubrication process is described in conventional manner by the Reynolds equation. Let x and t denote the streamwise direction along the roll surface and the time, respectively, suitably normalised. As a key feature of our analysis, then the accordingly rescaled film thickness h(x,t) depends on the movement of the blade tip in contact, expressed by h0, and the resultant waviness of the surface on a macroscopic and microscopic scale, expressed by r(x,t), h(x, t) = h0(t) + (a−x) tan α − r(x,t). (1) Here the blade tip is considered as a pad being in contact at a length a, in the current setting inclined by an angle α. Specifically, a given particle height s defines the cleaning efficiency Cs as the percentage of time where min(h) < s: Fig. 1. Fig. 1: Cleaning efficiency C10μm for various values of the contact lengths a and the stiffness, normalised by its maximum value (legend on the right). ACKNOWLEDGMENTSSpecial thanks go to the Austrian Research Funding Agency(FFG, Project Nr.: 848334) and the Austrian Association of Pulp andPaper Chemists and Technicians (ÖZEPA). REFERENCES[1] International Energy Agency, 2003, “Tracking industrial energyefficiency and CO2 emissions in support of the G8 Plan ofAction”. [2] Holmberg k., Siilasto R., Laitinen T., Andersson P. and A.Jasberg, 2003, “Global energy consumption due to friction inpaper machines”, Tribology International, 62, pp. 58-77. [3] Rodriguez Ripoll M., Scheichl B., Bianchi D., Jakab B. andFranek F., 2014, “Development of a mechanical model ofdoctor blade press-roll tribosystem with aim to optimisecleaning performance: numerical predictions and firstexperimental verification”, Tribology Materials Surfaces &Interface, 8(1), pp. 41-47.