What Is the Right Tension?

Of all the questions about web handling, the most-asked question has to be: “What is the right web handling tension for my product?” This is a great question and one of the first things that needs to be determined to design tensioning, rollers, and structural elements of a converting process.

If you conduct a poll on tension across the converting industry, the leading vote-getter among tensions likely will be 1 PLI or 1 lb/linear in. of width. (This means a 50-in.-wide web runs at 50 lb total tension, a 12-in. web runs at 12 lb, and so on).

If you tally up this imaginary poll, you will find 80% of webs run between 0.3 and 3.0 PLI, and 95% of processes are happy between 0.1 and 10 PLI. But survey results or rules of thumb will only get you so far and must have some origin in engineering. Why would 1 PLI be right for you? When should you consider going up or down from the starting point?

The engineering root of the “right tension” for your web starts with the mechanical properties of your web. Most webs can be considered elastic, meaning they will snap back to their original dimensions when tension is removed.

Find the yield or break points in terms of stress and strain from tensile-elongation testing. Find the yield point (or break point in brittle webs) of your product. Define this “bad” tension in PLI/product, psi, or strain, and stay below it.

Most tensile-elongation tests are completed under reasonably controlled temperature and humidity with well-cut samples. Beware of process factors that can change your product's yield or break point dramatically.

Yield stress drops when heating thermoplastic webs, when super-heating foils, and when paper moisture content is high. Break points drop with poor slit edge quality or when edges are nicked or damaged. Poor splices may break or fail much sooner than the web itself.

Once you find the damage tension or elongation, where should you set your process tension? A good starting point is 10%-20% of damaging condition or a 10:1 or 5:1 safety factor.

Why is such a large safety factor needed? It's not that your tension control system will have wild variations from the tension set point (though a 20% swing from set point isn't unusual). The problem is variations from average tension in both the machine and crossweb direction.

Tensions in a web line vary like temperatures in a home. Tension will vary crossweb from roller misalignment, roller diameter variations, web bagginess, and misaligned splices. Tension will vary in the machine direction from gravity or roller drag and inertia losses. Feel free to live closer to the danger point with a lower safety factor if you have a narrow web, low bagginess, well-aligned/low-drag/low-mass rollers, or a limited number of rollers in a tension zone.

There are many cases where your process or problem solution means going to higher or lower than normal tension. Use low tension to let wrinkles slide out and reduce excessive wound roll tightness. Use high tension to avoid scratching and slip, pull out wrinkles, and increase roll tightness. Go lower or higher tension to match strains at laminating. Look out for processes like nipping or coating that can put extreme stresses or drags on your web.

For laminate webs comprised of many layers, you should consider the yield or break point of each layer. It doesn't matter if the polyester film survives tensioning if the metallized coating on it cracks and crazes into an esthetic or nonfunctional mess. Like many web handling problems, think about the strain or stretch that causes problems and set tensions to avoid over-stretching.

So what is the right tension? As with most simple questions, the answer is complicated, but there is an answer. Start with tensile-elongation testing; be sure to look at not just the break data but also yield points. A proper tension safety factor will ensure the web will get through your process no worse for wear (or tension).

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; tjwalker@tjwa.com; www.webhandling.com.

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