Measurement and control of the tension distribution across the web in a newspaper printing press

Abstract

A bending moment in a newsprint web caused by unevenly distributed tension across the web leads to disturbances in the production. Measurements in a printing press during newspaper production showed large variations in the bending moment. The cause was found to be variations in mechanical properties of the paper, variations induced by the printing process e.g. reel changes, variable press speed and different amounts of fountain solution in different parts of the web. It has been found that the moment can be eliminated by an adjustment of the angle of the axis of a roller in the press.

A control system has been developed to eliminate the moment of the web in the printing press. The bending moment and the forces in the machine and cross-machine directions are measured by a roller mounted on load cells. The same roller can be used both as a sensor and as the controlling adjustable roller.

In the first section of this paper, the manner in which adjustments of a control roller will influence the stresses in the paper and the stability of the web has been theoretically studied. Based on the Timoshenko beam model, a mathematical model of the web has been established. Numerical calculations of forces, moments and lateral displacements in the web have to be made. The model is formulated in such a way that the stress characteristics in a web span are governed by only two dimensionless parameters, KsL and g. These parameters depend on the tension in the web, the free length and width of the web span and the bending and shear stiffness of the web.

The model can also be used to draw conclusions about the optimum design of the web lead when a control roller is to be installed in a printing press. The lengths of the spans on both sides of the control roller and the wrap angle can be changed to diminish the lateral displacement downstream of the control roller to negligible values. The design of the web lead must also fulfil the condition that the load on the upstream span from an adjustment of the control roller is smaller than the load on the downstream span. Measurements in the laboratory with 300 mm wide webs confirmed these theoretical results. It was also experimentally found possible to change the moment in the web without any permanent lateral displacement of the web at the rollers downstream of the control roller.

In the second section of this paper, the development of a closed loop control of the moment in a newspaper printing press based on measured values of the bending moment, the web tension and the web speed is described. The largest disturbances occurred during the automatic reel changes, especially when two papers subjected to a flying splice had different elastic properties. The time constant of the control-roller response was chosen to be of the same order as the rise time of the change of the moment from the automatic reel change. Recorded data showed that the control system was able to suppress the fast variations of the moment which occur during the automatic reel change at full press speed.

Analysis of data from a large number of printed reels also showed that the correction signal from the control system reflected inherent properties of the reels. The device can therefore be used to map paper and reel properties.

Several hundreds of reels have been printed in a production press with the automatic control system in operation. A considerable improvement in runnability has been observed.