8.3 Draw Control

Here’s the crazy part about draw control. As simple as it is to design, how it creates strain and the corresponding tension is confounding. Let’s see if I can demystify it.

Imagine a machine section with two driven rollers, the first at 100 fpm and the second at 101 fpm. The draw is 1%. What will be the steady-state web strain in this draw zone?

If you run an elastic web through these two rollers, you would be correct to assume the web will be stretched 1%. Yes, but stretching 1% doesn’t mean the web has 1% strain.

The initial condition is critical to knowing the final strain. If I stretch a relaxed rubber band 10%, the strain is 10%. But if I stretch it another 10%, the strain will be 20%. Draw control is a similarly additive process. The draw or stretching will modify the entering condition.

Asking you to predict strain from draw is a trick question. I can’t estimate the draw zone strain unless I give you three values: the speeds of both rollers and the strain of the entering web. So let me ask the fair question. Let’s say the entering web is strained 0.5%. Now can you tell me the tension? Hmmm. It starts at 0.5%, we stretch it 1% more for total of 1.5% strain. Correct? Yes, 1.5% strain is the anticipated steady strain in the draw zone.

That was a qualified “yes.” I chose my words carefully. To truly know draw zone strain, we need even more information. Why? Because draw zones have a time constant that determines how quickly the draw conditions will get to steady state or respond to changes in upstream strain or roller speeds.