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- January 01, 2010, By Timothy J. Walker Contributing Editor
Tension and speed control are the two most fundamental purposes of any web handling system. Speed control is easy for most people to understand, since they practice speed control every time they press the accelerator or brake in their car, but tension control is not as intuitive.
To make things more complicated, where a web line typically has one speed set point, there may be two, three, or even 15 tension set points in a line. For each of these tension set points, it may not be obvious how or where they are controlling tension.
When talking about multiple tension set points, the term “tension zone” usually is used to answer the “where?” question. But what exactly is a tension zone?
- A tension zone is a section of a web line between any two torque or speed controlling devices where the tension is set by design or by control. Torque or speed controlling devices typically include motors, clutches, and brakes.
To count the number of tension zones in your process, start at the unwind. Follow the web path through the system, and count the number of times the web wraps a roller (winding cores count as a roller here) that is connected to a motor, clutch, or brake.
The number of tension zones will be the total number of speed and torque contact points minus one. If you have a braked unwind, one drive roller, and a clutched windup, you have three speed or torque controlling devices and two tension zones (three minus one is two). If you have a driven unwind, five driven rollers, and a driven winder, you have six tension zones.
Why is there one less tension zone than speed and torque controllers? It takes two devices to pull on the web and create tension.
The Zen koan asks, “What is the sound of one hand clapping?” The Zen converter asks, “What is the tension of one drive pulling?” The answer to both is nothing. Just as you can't clap without one hand opposing another, you can't tension a web without one device pulling against another.
Each tension zone will have one device controlling tension. In a braked unwind, the brake torque divided by the roll's radius sets the unwinding tension. (We are ignoring mechanical torque losses and inertial effects here.)
Question | If we have X number of devices and (X-1) of tension zones, what is the function of the extra device?
Answer | It is the process pacer or master speed controller and therefore doesn't care about tension.
If you use this definition to count the number of tension zones in your process, don't be surprised to find you have more tension zones than the equipment control panel or operator's manual says there are. The most common “undocumented” tension zones are the short section of web between two speed controlled rollers, rollers that are considered in speed ratio or draw control and may or may not be adjustable by an operator.
Some examples of undocumented tension zones include the following:
- In a printing press, the backup rollers are all speed controlled and set a nearly 1:1 speed ratio.
- In a section where multiple rollers are driven with one motor, such as an S-wrap pull roller, or the driven rollers in the middle of a slitter/rewinder.
When driven properly, these sections are easily ignored, but when the surface speed ratios are off by improper gearing or unintended diameter changes, understanding that these are tension zones can help diagnose and solve a problem.
Since many webs are sensitive to speed changes as small as one tenth of one percent, documenting each tension zone, no matter how small, and keeping them in control is as important as the known and obvious tension zones.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; firstname.lastname@example.org; www.webhandling.com.