Drawing Conclusions: Part II
- Published: May 31, 2005, By Timothy J. Walker TJWalker & Assoc. Inc.
Web Lines
The principles of draw control usually create a good deal of confusion. Let’s pick up where we left off in my last column and see if we can move from drawing confusions to draw control conclusions.
I set out a scenario where I asked you to predict the web’s strain in a draw zone. I eventually gave out three data points: the web speed at roller #1, the web speed at roller #2, and the strain of the web at roller #1. This info is enough to determine the system’s eventual steady-state condition, but without knowing the initial strain in the draw zone and the time the system has been running, this still isn’t enough information to know the draw zone web strain.
How are draw zones time-dependent? The draw zone’s time constant is equal to the web length between the first and last draw-controlled rollers divided by the web speed. When any condition in the draw zone changes, it takes 3x the time constant to move 95% of the way to the new steady-state condition.
For example, a printing press with 50 ft of web from the first to last print station and running at 200 fpm has a the time constant of 15 sec (50 ft/200 fpm). If the input web tension (and strain) is changed, it will take 45 sec for the change to feed through the system.
I’ve seen operators chasing their tails after changing the infeed tension. They try to keep the multi-station press in registration before the draw zone reaches a steady state. At 45 sec, this isn’t a long wait, but if the press is running at low speed, say 50 fpm, then you have an agonizing 3-min wait before you see the registration return.
This time delay is the biggest negative of draw zones. Where closed loop tension control and open loop torque control will get to their steady state quickly, draw zones take time. This is like waiting for the hot water to get to the shower head in the morning. Your shower would continue to act like a draw zone if the hot and cold knobs adjusted the flow 30 ft. away. Each adjustment would require purging the entire pipeline before you felt the new temperature.
Draw zones also are poor in handling slack webs. Since web strains and draw zone percentages often are less than 1%, if the slackness in a draw zone is 5% of the zone length, it will be some time before the excess material is purged.
Why is draw control used? Draw is a great way to limit or control the stretch of your web. For nonwoven, crepe paper, and low-modulus films, applying too much tension will stretch the product beyond its elastic limit. Even high-modulus materials such as polyester or steel will be best handled in draw control when their temperature weakens their mechanical properties.
If you have a series of driven rollers, think about the draw of each roller relative to the first driven roller—what I would call the total draw. Having a small 0.5% stretch between driven rollers doesn’t seem like much until you do it ten times for a 5% stretch that yields or breaks your web.
What creates tension variations in a draw zone? Asking this question may be missing the point, since draw control usually is intended to control strain, not tension. But since draw control is used in tension control, it’s worth talking about how strain becomes tension.
All the variables reviewed so far (speeds, initial strains, time) will lead you to the draw zone’s web strain. To find tension, multiply the strain by the web’s modulus. Is the modulus constant? It would be nice to say "yes," but material properties change from moisture, temperature, cross-linking, strain rates, and the big wildcard—viscoelasticity. In many processes, the web’s response to strain will be predictably elastic, but be on the lookout for unusual mechanical properties and the resulting drawing confusion.
What are the most common applications of draw control? Small draws are common for any multi-roller driven sections (presses, slitters, small-wrap over rollers, and unnipped pull rollers) and for low-modulus or easily yielding webs. Large draws are used for film orienting and for separation after sheeting.
I hope I’ve cleared up some of the most common misconceptions and confusion of draw control. Draw your own conclusions on whether draw control is right for you.
To read Drawing Conclusions: Part 1, click here.
Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 651/686-5400; This email address is being protected from spambots. You need JavaScript enabled to view it.; tjwa.com.
