Quantitative analysis of nip-induced tension by use of digital image processing

Abstract

Winding paper in contact with a drum leads to the production of nip-induced tension which, to a high degree, influences the structure of the roll. Thus, comprehending the processes in the nip, including each parameter involved, is desirable to avoid roll defects.

This paper deals with a new measurement technique that is able to observe the process of building up nip-induced tension in the outer layers of a roll. With the use of digital image processing, this method ensures the registration of the two-dimensional displacement field of the outer layers with fairly high precision. Based on the first principal of the equations of mechanics, it is possible to derive the stress and strain in the layers from the displacement field. Thus, it succeeds to calculate the tangential stress in the outer layers of a roll taking the compression as well as the slippage of the layers into account.

In all experimental test series the tangential stress increases within the first 4 to 15 layers to a maximum value. Afterwards, there is still a considerable displacement of the layers relative to the core, but the layer-to-layer slippage is so small that the change in strain is negligible. Furthermore, it is shown that there is a reduced increase of nip-induced tension in the first approximate 50 layers near the core. Comparing the nip-induced displacement of different type of papers, there are clearly differences noticeable concerning the amount of displacement, as well as the rapidness of increase to the final displacement.