## In Search of Tension Isolation

Web Lines

Bigfoot, the Loch Ness Monster, the Yeti. These are but a few of the world’s myths. In web handling, one of the myths told most frequently is that of the independent tension zone, protected from other tension zones by the protective powers of the tension isolation. Like most myths, we want to believe, but alas, scientific reason is the myth buster.

What do people imagine when they use the term “tension isolation”? It is the belief that two tension zones are independent of each other if the friction of the web on the “isolating” driven roller is greater than the tension differential between the two zones.

I totally agree with the goal that each pull roller should have traction greater than the anticipated tension differential. It’s good to avoid pull roller slip since slipping pacers lead to unknown web speed, and slipping followers lead to uncontrolled tensions. Though the sufficient drive roller friction prevents slip, it is incorrect to say the non-slipping pull roller provides tension isolation where the tension in one zone is totally independent of another.

The myth of tension isolation stems from our understanding of statics. Imagine a large brick sitting on a table with two ropes tied to it. The brick’s weight and the COF of the two materials create an available frictional force between the brick and the table.

To slide the brick, you have to pull on one rope with enough force to overcome this friction. If you pull on both ropes in opposing directions, the frictional force will isolate the tensions of the two ropes as long as the rope tension differential is lower than the available friction.

The myth-buster lies in the difference between static and dynamic cases. In a web line, the moving web carries tension information with it as it passes from one tension zone to another, even without slippage.

The tension in any zone is determined by the strain or stretch of the web entering the zone plus or minus the change in strain in the zone. If a web enters a zone controlled at either end by driven rollers in a 1:1 ratio, the tension zone doesn’t alter the strain of the web, and the tension in the zone is the same as the entering web.

If the tension zone has a positive or negative speed ratio (or draw), it would increase or decrease the tension, respectively. But the tension in any zone is not independent of the upstream tension, since tensioning of elastic webs always will start with what is the upstream tension and go up or down from there.

The myth of tension isolation is busted when the upstream tension is altered, and no matter how high the pull roller friction may be, this new baseline tension will enter the tension zone as the web moves through the system.

This concept is known as strain transport. The strain of the web is a property that moves with the web as it enters and exits a tension zone.

For draw controlled processes, this concept of strain transport is critical to understanding your process (see “Drawing Conclusions,” my columns of May and June 2005). In closed-loop or torque-controlled zones, the system will adjust quickly for any upstream tension changes, making it seem as if tension isolation is true, but any monitor of process speeds would show how upstream tensions feed through the system.

With the tension isolation myth busted, the list of usual suspects for some defects, such as coating variations, must grow to include anything that changes upstream tension.