User's Manual for the Web Tension Calculator

Application

To recommend tensions for your web.

How To

Choose the units of measure as either Metric or US. As you enter the key values you get instant feedback on the key outputs in the blue boxes. Some users prefer to use Text entry (it's more precise). Others tend to prefer Slider entry (especially on smaller devices). Feel free to choose whichever is the most useful for any occasion.

While you can choose either Metric or US units, you cannot specify which type of units within those systems are used. The most common usage is fixed. For example, thickness will given in µm (0.000,001 meters) and mils (0.001 inches). Pay special attention to make sure that your input/output values are in the units specified or convert to/from as needed.

Tension Guidelines - Overview

Setting tension guidelines is like recommending body weight in several ways. It largely depends on thickness in the case of webs and height in the case of body weight. It also depends on the chemistry/structure of the web and the build of one�s body. Finally, there is a range for any situation that could be considered acceptable.

In an ideal world, an individual would know better than a medical doctor what body weight would be best for himself. It is the same in the web world. An experienced and attentive process engineer or operator would know what tension was about best for any given web and machine section and condition. This presumes, of course, that some experimentation had already been done to observe tension related issues at both the low and high end of a runnable range to find a happy medium. This app may not be for those situations unless you are curious what general recommendations could be found.

Rather, this is for those who have new materials, new machines or have not sufficient prior experimentation to know where to begin with tension settings. All you need to know is web thickness (caliper, gage) for most situations. It would be prudent to also read over the exceptions and indications sections below for further refinements from this basic starting point. Full guidelines on setting tension are provided in Chapter 5 of The Web Handling Handbook by Roisum, Walker and Jones.

The basis of this recommendation is the compilation of six published sources for a variety of materials. These original sources have widely varying perspective, yet come up with very similar results. The sources are typically machine builders who tabulate what seems to work best as reported by experience process and service engineers as well as operating personnel. The medium or starting point tension was obtained by taking the average of most of these published recommendations except those indicated below. The low and high values reflect the range that might be runnable. The user should explore both ends of the tension range as described in the indications section.

Exceptions

The web world is full of exceptions. This section will list only the most common.

Low Modulus Materials would be run lower or much lower than the above recommendations. Pulling hard on these materials can result in MD trough wrinkles in a curtain like pattern, severe necking (width reduction) and even web damage. Low modulus could be considered those whose MD modulus is less than around 75 kpsi or 500 MPa, though of course there is no sharp boundary between low and medium modulus. Materials such as this include polyethylene, polypropylene, Teflon, many food wraps, some nonwovens and some elastomeric compounds. You can recognize a material like this because you can see the stretch when you pull on the web with your hands.

Ultra-High Modulus Materials include metals and a few exotics like carbon fiber. They are usually not tensioned as high as given above for reasons such as wrinkling (especially foils) and equipment cost.

Thick (and/or Strong) Materials can usually take tensions as defined above without risk of damage. However, machines are often not sized to do this for two practical reasons. The first is that using tension to flatten and stabilize the web, as is common with lighter materials, is not so effective with heavy materials. The second is that designing and building a machine to be able to handle large tensions is expensive. In other words, it may be a high cost for little gain. Still, there are strong webs that are pulled to a notable fraction of yield so again we have exceptions. Again there is no strong cutoff, but materials thicker than say 0.040� or 1 mm are usually not tensioned as much as given above even though they could be.

Lamination where Curl is a concern means we often must break tension guidelines on the ingoing side to minimize curl. The low modulus leg is tensioned at the very low edge of runnability. While the high modulus leg is tensioned at maximum (the web or machine can tolerate), this knob is not anywhere near as strong for curl control as the low modulus side. Note that the outgoing leg or composite is tensioned as calculated above or via the 10-20% of MD tensile strength guideline.

Winding is one of the most tension sensitive of web handling processes. While the above guidelines may give you a starting point, you will quickly adjust the tensions based on the mix of high and low tension defects that are observed on the wound rolls.

Indications

Your customer, the web, will tell you whether the tension is too high, too low, both or just right or that the process is not sensitive to tension. Of course, this presumes you have the ability to change tension and you carefully observe the results over a long enough time to be sure of the tendency and variation.

Low Tension indications include: web flutter, web floppiness, web path instabilities and loose wound roll defects to name a few.

High Tension indications include: web breaks, web damage, necking, MD trough wrinkles in a curtain-like pattern (only low modulus webs) and tight wound roll defects to name a few.

Tension Insensitivity over a wide range is common with tolerant products and processes. Apparent insensitivity may occur when the mechanical and/or electrical tension capacity of the machine is limited.