Drawing Conclusions: Part 1

Web Lines

In planning the tension control for any web line, there are only three options: tension control, torque control, and draw control. Between any two rollers or rolls controlled by a motor, brake, or clutch, you have to select one of these three choices.

The most advanced of these options is closed loop tension control. Torque control is the easiest option when tension is created by a brake or clutch. The third option, draw control, is the open loop alternative to tensioning the web between two speed-controlled rollers.

What is draw control?
Draw control is simple. Any time you drive the web with two or more rollers, you have draw. Each driven roller can have its own motor or share a motor using a timing belt, chain, or line shaft. Driven rollers in draw control work as a team. They speed up together, slow down together, and they don’t necessarily care what the web is doing. Remember, draw is a machine property. The web goes along for the ride.

What determines draw in converting equipment?
In many machines, the draw is a fixed value. The machine designers determine the draw by selecting gear ratios and roller diameters. Many slitter/rewinders and line-shafted presses drive two or more rollers with one motor. Each driven roller’s rpm is determined by motor speed and the gear ratio of the motor to the rollers. The rpm turns in to a surface speed depending on the roller’s circumference.

Some machines have a programmable draw, allowing you to dial in the ratio or percent draw between driven sections. Closed loop tension control using pacer and follower driven rollers also will use draw, but the draw will be moving up and down to satisfy the tension trim control loop.

What is the relationship between machine draw and web strain?
Draw is a machine characteristic. Often it is confused with strain, which is a web property. Draw and strain both can be described in percent, but where strain is always relative to zero strain; draw can be relative to any initial speed. An untensioned 10-in. web sample stretched 0.1 in. has a strain of 1%. Given two driven rollers, if the first is driven at 100 fpm and the second at 101 fpm, this is a draw of 1%. For larger draws, it is more common to talk about a draw ration. If two sections are driven at 100 fpm and 300 fpm, such as in length orienting film, the draw ratio is 3:1.

How does draw control create strain?
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 roller 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.


Next month: Draw zone time constants, common draw control pitfalls, and top converting draw applications.



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; jwalker@tjwa.com; tjwa.com.


To read more of Timothy J. Walker’s Web Lines columns, visit our Web Lines Archives.


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