Web and roll length: How do you measure it?
David Roisum, Ph.D. Consulting Technical Editor -- Converting Magazine, 1/1/2009 12:00:00 AM
Roll length is such a common measure that few even think about it until they get customer complaints for discrepancies. A common prophylactic response is to “pad” the roll with extra material. However, whether you supply more or less material than the contract demands, the result will be the same: increased costs.
The first thought may be that there is some problem with the footage counter. Most commonly, we count revolutions of a roller or, much more primitively, a footage counter running on the product. Then, length is defined as the number of revolutions of the roller or wheel multiplied by its circumference. I've seen machines that had three or more counters on them, presumably because the owner thinks that the more you measure, the closer the answer will be to reality.
Yes, there are many potential problems with footage counters of all types. It's possible that the web could slip on a roller. Many people are so afraid of that possibility, even though slippage is rare and can be avoided by design calculation (band-brake equation) and can be detected by measurement (think about the anti-lock braking systems on your car). Analogous and even more complicated issues arise on footage wheels. We presume to know the diameter when the plastic wheels can and do wear (detectable), and the wheel is not perfectly aligned (skew and tilt). Worse yet, the effective diameter of the wheel for measurement purposes is not actually the wheel's diameter. The deformation under nip makes the wheel appear larger than it is as measured by a caliper.
Roll length truly has no definition
Then, there are operational issues. Do the counters start and stop precisely at the beginning and the end of a wound roll? Does the supplier or the customer take a few wraps off the top of the roll to clean it up? Did they start the counter while threading or after thread-up? Did the customer consider the wraps left on the core? And so on... Finally, there are higher-order phenomena that cause the web's length to change with time, such as crystallization of some film polymers and changes in length due to product moisture.
However, the biggest problem of all has nothing to do with measurement, operation or exotic mechanics. It is that roll length has no definition. No standards body has told us what length actually is. Consider that the supplier almost always defines/measures web length when the web is under tension. The customer, on the other hand, will often define/measure length as the number of units produced multiplied by the unit's length measured under no tension in the test lab. The customer will thus measure a shorter roll. Who is right?
There are other ways to define length that are prone to even more discrepancy. Some people use volumetric or weight methods. For example, you can define length as the area of a wound roll. What is the length of a roll? It depends on whom you ask and how it is measured. It's never, however, going to be as simple as reading a calibrated footage counter.
920/312-8466drroisum@aol.com, www.roisum.com
As I say, not as I do
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