Application
To predict the tendency for a pair of misaligned rollers to form wrinkles and related misbehavior. This app can be used to determine options to reduce this misbehavior, such as changing tension, as well as to guide the maintenance department on how close rollers need to be aligned.
Background
A web will tend to bend to enter the next roller at a right angle. This is known as the Normal Entry Law because it is the normal way webs and rollers interact and because normal is a synonym for right angle. This bending will always result in four risks: slackness on the inside of the curve, over-tensioning on the outside of the curve, an offset in the path and a diagonal wrinkle pointing to the narrow side of the pair of rollers that are not parallel. The challenge is not to predict risks; those are already known and listed above. Instead, it is to predict the actual occurrence of a slack edge and wrinkle and the amount of change in web path called TD offset. Note that this app focuses on in-plane (parallel or tram) misalignment that is at least one order of magnitude more critical than out-of-plane (skew or twist) misalignment. The occurrence and prediction of wrinkles, and their cures, are fully covered in
The Web Handling Handbook by Roisum, Walker and Jones.
How To Use
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 be 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. In general, all inputs are required and most are easy to determine. As general guidelines when specific information is not available, tension is usually in the 10-25% of MD tensile strength, the friction coefficient is on the order of 0.3 and a separate table gives you modulus.
What to Do With The Calculations
The first thing any good web handler will try for many problems is to see if tension makes a difference. If tension is low enough, you might get a slack edge (considered by some as a fail), but no hard wrinkles crossing the rollers (considered a fail on most every product). If the tension is high enough, you might be able to muscle your way through a problem.
However, using tension to solve misalignment wrinkle problems is a very risky and limiting strategy in the long run. Instead, the maintenance department should find the threshold of in-plane roller misalignment that will cause wrinkling at any tension (from either trials or this app) and then set maintenance standards accordingly. A safety factor of 2-4 might be applied to allow for other troubles, such as baggy webs, and the inevitable degradation of alignment with time. It is quite possible that the required alignment tolerances might be beyond what can be done with hand tools (attempts with simple tape-measures are likely to make matters worse) so that optical, laser or the gyroscopic tooling that is specialized for roller alignment might need to be employed. The gyroscopic technique is especially attractive as it can be done with much less down-time.
Hard and Soft wrinkles
Sometimes when you play with settings nothing happens to the prediction about Soft wrinkles (those that pass over the roller doing no real harm). For example, friction coefficient and roller diameter are both important factors influencing whether a hard wrinkle (which permanently creases the web) will form, but sometimes you can alter them over a wide range and nothing seems to happen. That's because large areas of "wrinkle space" are relatively safe. But you might be close to the transition between soft and hard and be unaware of the danger. So when you have your current parameters set up to your satisfaction, deliberately change some of them within the range that is likely to occur from time to time in production to check whether you remain safe or if it really is possible to get into trouble.