Where Web Tension Exists, How to Measure Tension and a Word About Taper Tension

Most converting applications include three types of tension zones – unwind, internal and rewind. Each zone must be controlled independently. Multiple zones of each type are common in many converting applications; however, they normally are categorized as unwind, internal or rewind.  

Each tension zone is very unique and must be controlled independently. Since independent tension controls are used in each zone, each zone may have its own tension level. This means, for example, tension in the unwind zone may be 1 PLI then increase to 2 PLI in the internal zone then decrease to 1.5 PLI in the rewind zone.

Torque required to provide a certain level of tension to a web is total tension measured across the web times roll radius. This means the torque driving the unwind shaft must decrease at a linear ratio, relative to roll diameter, as an unwind roll decreases in size, through a machine run, to keep tension constant. Conversely, the torque driving the rewind shaft must increase at a linear ratio, relative to roll diameter, as a rewind roll increases in size, through a machine run, to keep tension constant. Tension in the unwind and rewind zones is very dynamic. Roll diameters are constantly changing, so torque and speed must be constantly adjusted relative to changing roll diameters. Tension and speed in the internal zones is much more stable since roll diameters in these zones does not change. However, some control is required to set and maintain desired levels. Web defects, splices, desired machine speeds, machine defects and other variables will effect tension in the internal zones.

Remember the equation of torque = tension x radius is linear, so torque must be decreased (for unwinds) and increased (for rewinds) at a linear rate relative to roll radius. For example, if you start with a 17.5 inch roll radius (35 inch diameter) and unwind down to a 1.75 inch core radius (3.5 inch diameter); 17.5 divided by 1.75 is 10:1 ratio. That means, if you start at 100 inch pounds of torque at the beginning of the unwind roll for proper tension, the torque must be linearly decreased to 10 inch pounds of torque at the core to maintain constant tension.

The machine designer must determine required tension levels for each zone. Often times required tension levels can only be determined after actually running the web through the machine, since all webs and all processes are somewhat unique. TAPPI (Technical association of the Pulp and Paper Industry), as well as many other industry organizations, publish estimated proper tension levels for several different types of webs and laminations. However, keep in mind these values are only guidelines and “best estimates” based on many years of combined industry experience. The actual best tension to run your specific web and process will, most likely, vary from the guideline. Another very general rule of thumb is proper web tension is usually between 10-25% of the tensile strength of your web.

You can measure the tension at which you currently run your process. There are several methods for doing this.

• If you already have a load cell tension control, it normally has the ability to display actual tension in total pounds across the web. You can also purchase load cells with only a display (no control) if you would like to measure tension within any zone in your machine. 

• If your machine has dancers, you can calculate the loading of the dancer on the web if you know the web geometry and the loading force on the dancer. To do this, you will need a drawing of the dancer and web path through the dancer. You also need to know the type of loading and the force applied. Another simpler method would be to place a scale of some sort to measure the force the dancer is loading on the web. Do this without web threaded through the dancer. Remember, as long as the dancer remains somewhere within its travel (between its physical limits of completely full or completely empty) the tension on the web is equal to the loading in the dancer.  

•  Another crude but very effective way to measure tension in the unwind zone is the “fish scale method”. This method works for the unwind zone only. It can only be utilized if there is a brake on the unwind station. Unwrap a small amount of material off the unwind roll. Set the unwind brake to the torque output normally set for a machine run. Wrap the leading edge of the web around a bar and hook the “fish scale” through the bar. Pull on the fish scale until the unwind starts to turn. Record the weight reading on the fish scale. This is the actual tension on the web, at this point. Divide the total weight by web width in inches to get PLI (Pounds Per Linear Inch). Do not attempt this method of measuring tension if there is a drive motor on the unwind or for a rewind with a drive motor. Serious injury could result if this method of measuring tension is attempted when there is a drive motor at the unwind or rewind zone.

• Tension can also be calculated if you know the model of a brake or clutch, the output level to that brake or clutch at a given roll diameter. By knowing the torque output of the brake or clutch we can plug the values into torque = roll radius x tension and extrapolate the actual tension the brake or clutch is delivering to the web.

Taper tension:

Everything we have discussed so far is based on constant tension applications. Do not confuse constant tension by varying (or tapering) torque to achieve constant tension as taper tension. Taper tension is NEVER desired for the unwind or internal zones. As a web is unwound and is processed, constant web tension is ALWAYS desired. Taper tension is normally desired in the rewind zone. 

As a roll is rewound each layer builds up compressive forces which can cause inner layers, toward the core, to buckle. Wrinkling, starring and crushed cores will occur if these compressive forces become to great. Taper tension relieves these compressive forces by actually decreasing web tension as the roll builds. The amount of taper tension is normally measured in percentage of set tension. For example, using the same 3.5 inch core diameter and 35 inch outside roll diameter, if taper tension was set to 50%, the tension will be linearly decreased from 3.5 inch core to 35 inch roll diameter so the tension will be half at the outside of the roll compared to the core.

We will discuss how tension controls work in the next installment.