I have often heard that web tension is “one of the most misunderstood concepts” in the web converting and packaging industry. Often articles about web tension focus on the different types of tension control (open versus closed loop, etc) and describe the different tension zones on a machine. This is critical information to understand because it will help you know your machines and be better equipped to maintain them, troubleshoot them and keep them running the best possible product. But when I first started learning about the web handling industry, I found the control concepts a bit heady until I got a feel for what tension was all about and really understood the mechanism of tension itself.
The Concept of TensionTension is the amount of pulling force transmitted through an object (like your web or a rope or a fishing line) when pulled tightly by forces acting on opposite ends. It is not the pulling force itself; it’s just how that force exists in the object under tension. If you tie a weight to one end of a rope and fix the other end of the rope to a rigid surface and let the weight hang, the force of gravity pulls the weight down and the rigid surface resists the force equally and opposite. This is Newton’s 3rd law of Physics. Also, that same force exists as tension in every point along that rope, which makes it stretch evenly. If you load more weight on the rope it will start to stretch and eventually break. That break point is the tension limit of the rope.
When you are out at the lake, your fishing line has a pound test rating. The maximum force a fish can pull (if your fishing reel is locked and rod won’t bend anymore) or the maximum you can reel the line in (if you hooked a boat anchor thinking it was a big one) before you snap the fishing line is the pound test rating of the line. In either case one end of the line is pulling against a fixed object at the other end of the line. A web behaves the same way, except the force is spread across the web width instead of being concentrated in a small fishing line diameter. This is lineal tension and the unit of measurement is PLI, “pounds per linear inch” or “lb/in.” You divide the total force pulling on the web in pounds by the total width of the web. We use PLI instead of total tension (lbs) to describe materials that come in many different roll widths. Instead of hanging a weight from a rope attached to a tree branch, think of lineal tension as attaching a weight to the bottom dowel of a vinyl curtain that is hanging from the ceiling.
What Are You Pulling Against?The rope and fishing line examples are good for understanding tension, but in a web printing press, slitter/rewinder or other converting machine, web tension is never really created by pulling material from a fixed object like all of these examples. If you completely locked up the brake on your unwind stand and start pulling the web with your nip or winder, this would happen until either the brake finally gave way and started to slip or the web snapped – but the machines aren’t designed to run that way. Instead, the web is always being pulled off of a rotating object (unwind roll, s-wrap rollers, nip, etc.) that is not locked up, but is giving way and rotating. Similar to when a monster fish grabs the end of the line, starts darting away with it, and you choose how to resist it.
If the reel is allowed to freewheel, the crank would spin away along with the fish – but there is very little tension in the line when this happens; just the resistance the line sees from the bearings in the spinning reel and the drag of the rod eyelets. This is like having a freewheeling brake or motor (completely unengaged) on your unwind roll or upstream nip roll. If either of these were just spinning with impossible “frictionless bearings,” there would never be tension in the web no matter how hard or fast it was pulled, and if the upstream pull slows down, the unwind web would just go slack.
Regulating the Tension Using SpeedBack to the monster fish example, the crank could be held and the line actively let out by rotating the handle in the unspool direction at a slower speed. If this is done and the fish is still pulling harder or trying to swim faster, the tension in the line is being regulated by how fast or slow it is unspooled. This is like regulating web tension between two sets of nip rollers. When the downstream nip is pulling fast, web tension is created by slowing down the upstream nip. More speed difference creates more tension (sometimes called “draw tension”) and less speed difference makes less tension.
Regulating the Tension Using TorqueWhen the line is slowly let out while the fish is speeding away, the pulling force can be felt from the fish as the crank handle presses; the torque of the crank handle is what’s being felt. If the amount of torque placed on that crank handle is actively adjusted, the tension in the fishing line can be regulated. On a braked unwind web machine this is how tension is regulated.
But What is Torque?Torque can be described as force applied at a distance away from where the force is being resisted (similar to a wrench handle). If torque is applied and it creates motion, the motion will be rotating rather than straight. With a torque wrench, the force is applied at the end of the handle to break away a bolt that is screwed tightly into a threaded hole. Unscrewing that bolt without a wrench would be nearly impossible! If it can’t be broken away using a standard wrench, the next best thing is a “breaker bar” – the longer the arm, the less force needed to oppose the torque of the bolt.
In an unwinding roll of web material, the tension pulls at the outside diameter of the roll, but it is most commonly resisted at the center by a brake, creating torque. In order to keep the material tension constant as the roll diameter unwinds, the pull needs to be resisted with more torque when the diameter is large, and resisted with less torque as that diameter changes down.
How Do I Know My Web Tension?At times during set-up, troubleshooting, or running a machine, it may be easier to recognize a preferred tension setting by feel, rather than actual values. If the machine’s load cells are adjusted properly, it’s as simple as reading the tension setting and knowing what measurement units are being read (lbs, kg, etc.). But many machines do not have this built-in feature. In those cases, here are some ways to find the value:
• Refer to TAPPI and other industry material charts for known tension values referenced by different material types and thicknesses. These tend to be a guideline as everyone has different specific needs.
• Purchase and install a set of load cells and amplifier/indicator. These are then calibrated to known weights and a reading will always occur.
• For a braked unwind or torque-controlled clutch rewind, the tension can be estimated by back calculating. If the max torque output of the brake/clutch is known and the air pressure used at a given roll diameter and web width when the web is at a good “feeling” tension spot, some math can be used to solve for the PLI value.
Defining the tension “sweet spot” for any given machine, material or process is like defining the best “sweet spot” at your local fishing lake. Depending on the time of day, the season and what fish you are trying to catch will determine your approach. There are similarly many variables in your web process that factor in. To be very generic, a machine should be run with light enough tension to avoid creating wrinkles, while still making a quality product.
The purpose of upgrading a machine with modern automated tension control devices is to take the human element (and the variables) out of the equation and have something that is repeatable. It allows for fewer unknowns and more focus on the task at hand, along with peace of mind and confidently completed jobs.