"Assuming good traction ..." is a phrase that is often seen at the beginning of papers on web handling. Authors, including yours truly, use it to dismiss a mare's nest of problems. The biggest of them is the effect of air lubrication and its interaction with surface roughness of both the roller and the web. Because of it, the web begins to lose traction as soon as it starts moving. A lot is known about air lubrication. There are even models available for precisely predicting the coefficient of friction of a particular web on a particular roller at a particular speed. But, they usually require numerical data that isn't available or, alternatively, require a good deal of educated judgment. However, there is still great value in understanding the physical principles of this subject. It can help you troubleshoot problems and predict which direction things will go when you change something.
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