Troughs at the Exit of a Roller

It ‘s common knowledge that troughs can form at the exit of driven rollers. The web wants to expand laterally when moving from an area of higher to lower MD tension (reduction of Poisson contraction). While it’s on the entry side of a roller, the web is locked to it by friction. The MD stress doesn’t change and the lateral stress conditions remain the same as they were in the entering span (zero at an aligned roller with steady tension). But, when the web nears the exit of the roller, it comes under the influence of the reduced MD tension and lateral compressive stress develops. As the web enters the free span, it tries to expand but is still restrained at the roller and buckles. You can see it happening in this photo from my lab machine. A stretchy latex web was used so that the deformation would be more visible.

The web is 26 mils thick and the angle of wrap is 155 degrees. The coefficient of friction for this combination of materials ranges from 0.75 to 1.0 depending on cleanliness (wiping the web and rollers with isopropyl alcohol will produce the higher value for a while. In this instance, it was probably close to the 0.75 value.

The web was being pulled at constant speed and the input tension was held constant as the roller drive torque was varied. At low values of torque (entry and exit tensions close to the same value) the troughs didn’t form. As the drive torque was increased, there would be a value where the troughs were deepest. Then, as the torque was increased further, the exit tension (and the normal force that creates friction) would become low enough that the web would do all its spreading while still on the roller surface and the troughs disappeared.

It seems unlikely that troughs like this could turn into fully formed wrinkles, because the material where they could develop is always being moved off the roller. However, wrinkles might develop on the next roller if the span in between were very short.