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.

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