The short answer is you can't measure something unless you first define it. This lack of definition causes more trouble between customers and suppliers than does the measurement itself. "Wait a minute," you say! "Length is easy to measure, all you need is to lay a tape measure alongside the web." In concept it is simple, but in practice it can be quite difficult, because many rolls exceed 1,000 ft and some are approaching 100,000 ft in length. So tape measures won't work to verify lengths of any but the shortest of wound rolls.

"How about a footage counter?" you counter? Better yet, you can count the revolutions of an idler roller using an optical or magnetic target. Even better still, borrow the motor tach output of a driven roller. Each of these is an improvement over the previous one because there are fewer moving parts, and the ones that remain are easier to work around. Wheel mounted footage tachs are a Fred Flintstone way to do business because they are in the way, wheel diameter can vary due to wear, effective diameter will vary due to misalignment and they are short lived compared to the more elegant roller solutions.

In any of these cases, however, we have two issues to consider. The first is the diameter of the roller or wheel. The wheel's "effective" diameter is not the diameter of the roller/wheel as measured by a micrometer. It is the diameter plus the web thickness in the case of the roller. The wheel is much more complicated because of the nip, but it also is not the same as the micrometer indicates. The second issue is possible slippage, especially with rollers.

You could avoid contact altogether and still not avoid complications. One way is to use a commercial laser to measure footage or speed. Though expensive, it does not have diameter or slippage issues and can resolve length to 1 part in 1,000. Even with the laser, however, we have all sorts of ways that the customer and the supplier won't agree on footage measurement.

Consider simple web tension. What happens if the supplier measures footage at 2 lb/in and the customer runs only 1 lb/in? The customer will read a length that indicates they were "shorted" because the web will retract elastically due to the release of tension. Worse yet, consider a supplier "measuring" 1,000 feet in the roll, where the customer sheets foot-long products. The sheet, under no tension, will retract even more. The parts count will be less, even far less for low modulus materials, than the expected 1,000.

Other complications: paper made wet will dry out in storage and thus shrink. Similar behavior happens when a film or foil web is measured hot, these materials will contract when they cool. Many polymers exhibit yet another behavior—shrinkage due to crystallization, which occurs well after the roll is wound and packed for shipping.

Even with no instrumentation or material issues, we still have operational chances for discrepancy. When is the footage counter enabled and disabled during the winding cycle? Are wraps taken off the top of the roll during manufacture? Are wraps taken off the top of the roll prior to converting at the customer? Does the customer unwind right down to the last wrap on the core?

I hope this gives you some things to think about when resolving length issues with your customer or supplier. Padding the length does not necessarily solve the problem either. If you pad too much, you give away material. If you pad too little, the customer will notice short rolls. Synchronized systems, such as multiple rolls feeding into a single machine, present an even greater challenge. When the first runs out, all the remaining material will be discarded as waste.

It does no good to look for standards for length measurement to agree on, because there are none. We can only do our best to negotiate a common method or use conversion factors.

920/725-7671, DRroisum@aol.com, www.roisum.com