A: Some rewinders can apply torque in either a locked- or slip-core mode. In locked-core mode, every daughter roll across the width must turn at the same RPM because the expandable shaft connects them all together. Locked-core works fine if the gage profile is quite level or the material is compressible. The problem is that gage variations are a way of life for certain incompressible grades such as film.

As seen in the accompanying figures, the individual rolls tend to build to reflect the incoming gage variations. Diameter is not, as commonly believed, proportional to local gage. (The actual gage variation could be an order of magnitude greater than the resulting diameter variation because the winding process tends to be self-leveling.) In any case, thick lanes wind bigger and thin lanes wind smaller. As we look at the side view, (bottom left) we see that the bigger roll will turn at a faster surface speed, thus higher draw, and will necessarily pull a higher tension than the neighboring small roll.

However, the real problem may be on the small roll. It will turn at a slower surface speed and won't be able to take up as much material. The small roll then winds looser than the big roll. How loose? Quite possibly loose enough to make a gathering puddle on the floor, if you could run that way.

What's the operator to do? The first thing to do is to crank the only knob available; winding tightness. If you pull really hard, you might be able to muscle your way through. If this doesn't do it for whatever reason, you will next look at the root cause—namely, gage variation. Unfortunately, the pleas of the winder operator are always ignored, even if they assemble a bullet-proof case such as:

The smallest diameter rolls wind loose,

The small diameter rolls are softer by a hardness measurement,

The small diameter rolls weigh less,

The small diameter rolls can be temporarily fixed by throwing scrap into the roll (to make up for the shortage of material from manufacturing).

There are many reasons that manufacturing will ignore the protests of the winder operator:

They do not understand the physics,

They do not want to acknowledge the fact that they are the root of the problem,

They hide behind the flimsy shields of “The scanner/test lab gage measurements do not show troubled rolls.” “We are in spec.” “It's the best we can do.”

Thus, getting absolutely no satisfaction and probably no sympathy either from extrusion, coating or whoever is responsible, the winder operator is back where he started. Now, he has only one card left to play to reduce waste levels: differential winding. With differential winding, also called slip-core winding, each roll can turn at its own RPM, is independently torqued and thus independently tensioned.

A: Slip-core, a.k.a, differential winding, is one way to widen the window of gage variations that can be handled. While it is a way of life for many, however, it is not a cure-all for gage variation. There are situations where it has no application whatsoever.

First, it is seldom suitable for surface winders.

Second, it can only be applied to multiple rolls wound on the same shaft.

Third, it has no utility on compressible materials, because such materials, like nonwovens or textiles, have enough cushion to absorb gage variations. To check for compressibility, merely hit the roll with a stick. If it thuds, it is compressible. If the stick cracks or rings, then the web is incompressible and could be a candidate for differential winding.

By this definitive test you may get a different answer for the same material. Very loosely-wound film is compressible, while very tightly-wound film is incompressible.

Not all gage-variation patterns are helped by slip-core winding. For example, an abrupt change in caliper in the middle of a single roll would not be a good candidate because you can't have one end of a roll turn at a different RPM than the other end of the same roll. Surface speeds would be inevitably different. The classic gage pattern of medium-low-high across a narrow lane in a roll, which causes the corrugation defect, is not a good candidate for slip-core winding.

Even if you do have a good application for slip-core winding, however, the equipment is quite expensive. A really good shaft on a wide machine could cost $100,000. Not only is the shaft expensive to buy, it's also expensive to maintain. The shaft needs to be frequently disassembled to check condition, clean and replace worn pads as a set.

Some of the cheaper shaft designs do not have clutch pads but rather brake directly on the core itself. On these styles you can generate fiber debris which may contaminate your product.

Then there is the confusion of tension and taper-tension settings. Without going into details, taper tension is already so cryptically implemented by builders that even more straightforward locked-core winders are not straightforward.

With differential winders, any semblance of fundamental units such as starting and ending tensions in PLI (lbs. per linear inch) is completely lost. This is not inherent in the application; it is just the practice of the industry where craft still rules over science long past any sensible reason to do so.

Finally, differential shafts can actually increase tension variations. It is true that since the rolls are allowed to turn independently, it is possible to even out the tensions. There is, however, another bit of physics in play; friction is quite variable. Even with identical settings, such as side load, individual clutches will not develop identical torques. At its best with a brand new shaft of superb design, you might see a 2:1 tension variation from the loosest to tightest roll in the set due solely to friction variations. It gets worse with cheaper shafts, poor maintenance (mixing old and new parts) and poor operator habits (such as spraying WD-40 onto clutches which wind tighter than their neighbors). Here, we might see as much as an 8:1 tension variation. Thus, we merely trade one source of tension variation, gage, for another, namely clutch-friction variability.

Just because you own a slip-core shaft does not necessarily mean you should always use it in the slip-core mode. To do so will, at the very least, wear out parts faster. It could also increase tension variation. The only way to know for sure which mode is best for a given grade or situation is to do a brief study. Run one set in locked-core, the next in slip-core mode, and so on, for a half dozen sets. Measure and compare the hardness variability of the rolls wound in the two different modes. Whichever mode has the least variability is the one that should be used.

A: Center-wind torque assist is the third knob in the TNT's of winding: Tension, Nip, Torque and speed. It is the least common, most expensive and is easily the most misunderstood. Center-wind torque assist always requires two motors. One motor is attached to the winding roll. The other motor is attached to the roller. The roll and roller must be nipped together.

The sum of the two motors (minus drag) creates simple ingoing web tension. The difference between the two motors creates the torque (center-wind torque assist.) This motor application is unusual in that the motors may be opposing each other. In other words, one motor may be turning +20 amps and the other may be regenerating at -10 amps. Perhaps the only other common application for motors fighting each other is in curl control on dual-drive laminators. The mechanics are quite similar.

To make the roll tighter, put more power into the winding roll and drag with the roller. To make the roll looser, put more power into the roller and drag with the winding roll. For this reason, center winds with an undriven layon roller will wind slightly tighter than a surface wind even for the same tension and power input. On two-drum winders, the roll is made tighter by shifting torque to the front drum and looser by shifting torque to the back drum.

What does all of this do? Quite simple. Tension makes the roll tighter. Nip makes the roll tighter. Torque makes the roll tighter (and looser). In other words, at the end of the day when all is said and done, all of the knobs do just one thing: affect roll tightness. Thus, while the winding MACHINE may have as many as four knobs (TNT's), the winding PROCESS has only one knob (wound-roll tightness).

Why would you need all of those knobs if they all do the same thing? After all, these knobs cost money and greatly increase the complexity of the process. The answer is to extend the range. What if the nip is giving you trouble, such as damaging material at a gage band? You can back off on nip and increase tension in its place and the roll will end up equally tight. What happens if tension is giving you trouble, such as necking or web breaks? You can back off on tension and increase nip, and the roll will end up equally tight. What if you need a really tight roll? Increase both tension and nip.

So why a third knob (torque) when you already have two (tension and nip)? Perhaps your first two knobs are maxed and you still need more tightness. Perhaps your first two knobs are in the mud and the roll is still too tight. Thus, the third knob extends the range to super loose or super tight. An equally valid answer is that the nip knob is not effective on compressible materials over an incompressible surface (core). Thus, some materials will wind looser for the first inch or two because the nip is not effective. Thus, torque can take up the slack (excuse the pun) at the start of winding.

Is there a moral to all of this? Perhaps it is caveat emptor. Know what you are buying, why you are buying it and how you are going to use it. More knobs don't necessarily increase efficiency. They may just increase confusion.

For more information on the consulting supplier: Finishing Technologies 920/725-7671, e-mail: drroisum@aol.com www. roisum.com