Digital Magazine

Differential Rewinding: Part One

Don't try to do ten things at once. While this is good advice, sometimes ten things come flying at you at once. If you focus on one item, you risk dropping the other nine.

If you operate a slitter/rewinder, you know what I'm talking about. It would be nice to wind one roll at a time, taking care to start each roll smoothly and give it dedicated tension control. On a slitter/rewinder, however, it's common to rewind ten slit rolls (or more) at once. Losing nine of the rolls is not an option.

The simplest approach is to wind ten rolls abutted side by side on a single shaft. Winding rolls on a common shaft without roll-to-roll separation usually leads to shuffling, where adjacent winding rolls interleave with each other. Shuffling is prevented by using a spreader device to create a gap between neighboring strands or splitting the strands between two rewind bars (duplex rewinding). Spreading can degrade the natural accuracy of the post-slit edge, but duplex winding preserves tracking accuracy of slitting.

What is lock-bar winding?
Lock-bar winding couples multiple rolls directly to a common shaft. Each roll turns once per shaft revolution. This approach often is plagued with web slackness and roll structure variability. Slackness in lock-bar winding comes from either web bagginess (from cross-web length variations) or roll-to-roll diameter variations (from cross-web thickness variations). Even before the onset of slackness, these variations are causing roll tightness variations. Longer strands or small-diameter rolls need to rotate faster than average to prevent slackness or soft roll defects. Shorter strands or larger-diameter rolls must rotate slower than average to prevent hard roll defects. Roll tightness uniformity from lock-bar winding is dependent on product length and diameter variability.

What is differential winding?

Ideally, each winding roll would take up its strand with the proper winding tension independent of length or diameter variations. With differential winding, each winding roll turns at the rpm appropriate for roll diameter and entering web length. Differential winding bars have a variety of designs, but all feature a clutching mechanism for each roll. In general, differential bars have an internal shaft driven faster than the winding rolls. A clutching mechanism limits the torque transmitted from the shaft to each roll. Roll tightness uniformity (and yield) from differential winding is independent of product variations, now a function of roll-to-roll clutching variations.

When is differential winding needed?
Since all products have some degree of dimensional variations, it would seem differential winding is always needed, but in fact, this is not the case.

Differential winding is needed if baggy web length variations are significantly large relative to web strain. Higher-modulus baggy products, where it is difficult to simply pull out the bagginess, are more likely to need differential winding for baggy webs.

Differential winding also is needed when caliper variations create significant diameter variations. Surprisingly, thicker-than-average slit strands do not always make larger rolls. The relationship of caliper to common shaft winding roll diameter is dependent on, to paraphrase a famous toilet paper commercial, how “squeeze-ably soft” the roll is. More squeeze-able rolls, like many paper or thick adhesive-coated products, may compensate for caliper variations through changing roll density. Less squeeze-able rolls, like most film and foil products, cannot compress radially, so caliper directly creates roll diameter. Roll squeeze-ability (a.k.a. radial or stack modulus) is dependent on thickness direction modulus, roughness, coatings, air entrainment, and winding tension.

Do you need differential winding?
Try lock-bar winding with your worst input material. If you have slack web or significant roll hardness variations, you need differential winding.

Armed with differential winding, you are well prepared to handle ten things at once — at least ten rolls.

Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 404/373-3771; This email address is being protected from spambots. You need JavaScript enabled to view it.; tjwa.com.

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