Traction in web handling: A review

Abstract

In web processes, at least one moving machine component must drive the material through a friction force at the interface. This force, or traction, inevitably falls as speed increases, and is accompanied by a degree of slip. As a result, traction often limits productivity through constraints on product output and quality. Although there are high traction elements available for web lines, such as nips, edge grippers and vacuum pull rollers; the majority of machines still rely on driven rollers wrapped by the web. Machinery manufacturers seek to optimise the design of the traction elements by specifying layout, drive and roller surfaces, whereas material manufacturers seek web surfaces that will perform well irrespective of machine. In other situations, low traction may be desirable. A roller imparts stability to the web if traction is high enough to ensure speed matching, or if the traction is low at all times so that roller has little influence on the web. It is important to avoid intermediate situations where intermittent slip occurs.

An understanding of traction, and design tools based on validated models, are clearly desirable. The reduction in traction with speed is dependent on roughness of web and roller. Current models tend to be based on statistical descriptions of the surfaces, rather than parameters suggested from the physics of the interaction. However, the models do permit a number of subtle effects, such as web permeability and the constriction at the exit point, to be included.

When applying traction theory to a web line, it is important to know where the web and roller speeds are matched, and this cannot be designated arbitrarily. Furthermore, adjustments of one roller speed can result in a remote roller moving from speed matched to a slip situation. A model, validated at low speeds, will be used to demonstrate these effects.

Traction also provides force perpendicular to the direction of web travel. Loss of traction may occur if the vector sum of lateral force and tension change is greater than the available friction force, causing web movement sideways. Also, lateral traction and slip are important in determining wrinkling and scratching on a roller.

Deviations from elastic web behaviour reduce available traction. The areas of speed matching no longer have constant tension, and extra zones of slip may appear. As an example, a model of a thermal vapour deposition on a film on a cooled drum will be described. Toe heat load causes thermal expansion, which tends to reduce tension in the machine direction and generate lateral compression. If the tension is too low, wrinkles form, and are set in as the material lifts off the drum and rapidly heats up.