Web tension variations caused by temperature changes and slip on rollers
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Date
2011-06Author
Jones, D. P.
McCann, M. J.
Abbott, S . J.
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Show full item recordAbstract
Webs are frequently heated or cooled on rollers, where thermal expansion or contraction attempts to reduce or increase tension respectively. In the case of vacuum coating, the web first cools on the chilled drum, then is heated by coating deposition and radiation, and finally cools before exit. Friction between the web and drum surface may be unable to sustain the tension gradient that the temperature profile would produce in the web moving at constant speed. In that case, zones of microslip exist over at least part of the wrap, possibly including the entry region. A method of calculating the tension profile around the roller or drum for an elastic web in steady state will be presented, using simple friction laws. The speed difference between web and drum surface is determined by iteration, working backwards from the exit tension until the correct entry tension is attained. However, forward integration is necessary if the web is not elastic, or the heat transfer is affected by the tension or the amount of slip. The calculation shows that the web may be slipping over a large proportion of the contact area. In some cases, the speed matching is only momentary. Practically, this may lead to difficulties in controlling tension or speed. The model also allows the cross-web direction stress to be estimated. In a region of heating, this often becomes compressive, and can be compared with a critical value for buckling. Exceeding this level would lead to wrinkle formation in the hot, soft web, usually a serious quality problem. The model can therefore be used to explore process design and conditions to reduce the likelihood of wrinkling and improve the ability to control tension and speed.
Citation
Jones, D. P., McCann, M. J., & Abbott, S . J. (2011, June). Web tension variations caused by temperature changes and slip on rollers. Paper presented at the Eleventh International Conference on Web Handling (IWEB), Stillwater, OK.