Why Tension?

Web Lines

What is the first step of most converting processes? Tensioning the web.

When a slitter operator splices in a new jumbo roll, the smart ones will back tension the newly spliced web, taking out the slack. For more automated lines, before a line goes to "run" mode, I like to see a brief "tension" or "stall" mode, where the follower drives will pull the slack out of the web from unwind to rewind.

Why is tensioning such a crucial first step? What are the benefits of a well-tensioned web? What is the right amount of tension for your product? Don’t slack off now...keep reading for these answers and more.

Why tension the web? It’s mostly a matter of control. Letting the web loose is like taking your dog off leash. Some dogs will behave themselves and some will chase a rabbit into the woods. It’s the same with webs. Some webs will behave themselves, especially if they have enough stiffness through modulus, thickness, width, and small curvature. Too many webs react like rabbit-frenzied dogs or sheets hanging in the wind to be left untensioned.

Tensioning the web in spans and over rollers gives stiffness and straightness not necessarily inherent in the web. Tension-induced stiffness will reduce lateral bending and shifting and improve a web’s ability to resist wrinkling temptations.

How much tension is right?

Too much tension is easy to define. Don’t pull so hard you will break or yield the web. Don’t run at 90% of a web’s break point and figure a 10% safety factor is enough. Roller misalignment, roller diameter variations, and web bagginess all are capable of create extreme crossweb tension variations.

Since these cross-web factors easily can create lanes of tension 3x–5x higher than the average tension, normally I recommend using a tension setpoint of 10–20% of the web’s yield or break point. Think about how a web’s break or yield stress may vary in your process. Heating polymer films will depress their elastic modulus and yield points. Drying paper and poor edge quality will mean breaks at lower tensions.

Too little tension is more difficult to define; it varies by your process needs. If your web needs to go through nipped rollers or lie flat through slitting, you should tension the web enough to pull out the web bagginess. In long horizontal spans, tension is needed to reduce excessive catenary sag. Tension is needed to overcome idler roller force demands, such as roller drag, inertia, and any web-roller bonding forces.

These roller factors really start adding up (or subtracting) when you have a zone with ten or more rollers. There is a minimum tension needed at every undriven roller to keep it turning. Having more than enough driving force from tension, traction coefficient, and wrap angle to overcome a roller’s opposing forces is what I call the Traction Safety Factor.

How will uniform tension benefit coating and laminating? Most coating processes are a precise metering of another material onto the surface of the moving web. It’s obvious that good speed control is required to get the right ratio of web to coating, but the importance of uniform tensioning often is missed. A web can have perfect, slip-free bond to a coating backup roller but still have web control coating variations. If the incoming web is tensioned to 1% elongation, a 20% tension swing equals to a 0.2% speed change and coating variation in the final product.

Proper tension control at laminating is hard to see while the web is running, but curly, scroll-forming samples will show quickly when the proper laminating strain matching is messed up.

How important is tensioning and winding?

Tensioning each wrap as it goes onto the roll creates the internal pressure and friction that is just right to hold the roll together. Too little tension and the roll package easily falls apart or sags excessively. Too much tension and the core or roll’s inner layers are crushed. Too much torque applied to the center of the roll, and the whole thing may begin to shift tangentially like a giant clock spring, leading to crepe buckles or telescoping.

Whether center or surface winding, tension doesn’t have to do all the work in creating roll tightness. If you have a loaded nip roller, a significant amount of roll tightness can be created by the nip-induced tension independent of handling tension. Why tension? To control lateral position and keep the web flat; to avoid breaking, yielding, wrinkling, and scratching; to have quality coating and laminating; and to make good rolls.

My advice to all converters: Don’t be slackers—keep your web tensioned and don’t let go.

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.

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

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