In the above scenario, I asked you to predict the web’s strain in a draw zone, given 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.
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.