J. K. Good, PhD, PE
Professor & Noble Foundation Chair, Oklahoma State University, USA
INTRODUCTION
A web is a material whose length is much in excess of the width and the width is much larger than the thickness. Various grades of paper, plastic films, metal foils and laminates can all be broadly categorized as webs. Web media are often stored for periods of time in the form of a wound roll. This form of storage is chosen because it is simply the most convenient and often the only form for storing vast lengths of web with minimal damage and loss. Winding can damage the web and has become a topic which has received considerable attention in the literature. Amongst operations in a web process line, the winder and the unwinder are often sites at which web defects appear that may result in a loss of quality or may lead to a break or burst of the web resulting in lost productivity. The purpose of this paper is to review the state of the science of winding and unwinding rolls. The threads of pertinent information in the literature focus on: the stresses that are wound into rolls, how those stresses are affected by the type of winder employed, how those stresses vary during roll storage, how the stresses predicted by models can be used to predict wound roll defects, and finally the measurements that are available to verify modeling efforts or help solve production problems. Each thread will be examined in this review from the perspective of what references exist but also giving enough detail that an appreciation for the significance of a topic can be developed .
WINDING & UNWINDING MACHINES
The winding methods which are used in the industry today can be categorized into the following types and are shown in Figure 1:• Center Winder – The center winder is classified as such as the torque towind the roll is provided to the center or core of the winding roll. Thetorque provided to the core is not necessarily constant and is often taperedto produce optimal internal residual stresses in the wound roll.• Center Winder with Rider Roller – This winder is similar to the centerwinder except an idler roller, sometimes called a rider or nip roller, ispressed into contact with the outer surface of the winding roll. In somecases the idler roller may be tendency driven to ensure the roller is movingat web velocity. The web may contact the wound roll first or wrap the niproller first. The torque required to wind the roll is provided through thecore.• Surface Winder – Physically this winder can appear similar to the centerwinder with a rider roller. The difference is that the torque to wind the rollis now provided through the rider or nip roller and the core is not driven.The nip roller is typically driven at a programmed velocity. Surface windersare prevalent in situations where web thickness may vary significantlyacross the web width where center winding would result in a winding rollwith non-uniform radius that could eventually result in a burst or qualityloss due to corrugations and fold overs.
Differential Torque Winders – A differential torque winder mechanicallyappears similar to a center winder with a rider roll or a surface winder. Theonly difference is that now substantial torque is provided to both the coreand the rider roller. Typically the rider or nip roller is driven in velocitycontrol and the core is driven in torque control. Both the rider and the coremay be driven in torque control but it is more common to provide velocitycontrol to the rider to prevent runaways. Sometimes this type of winding iscalled surface winding with center torque assist. The assist torque to thecore may not be large; in fact it is often just sufficient to overcome bearingdrag on the center shaft, the rolling resistance between the rider and thewinding roll, and other loss factors. This type of winder is often the onlysolution when winding webs whose surfaces have low coefficient of friction. This type of winder is common in slitting operations. The incomingweb is slit on the roller in velocity control and then independent torque canbe provided to the cores of each slit.• Two Drum Winders – These winders are popular in the paper and textileindustries and are known for their high productivity. In this winder the webwraps a first drum (a drum is a large diameter roller) which is in velocitycontrol, wraps about the winding roll, proceeds beneath a rider roller whichis impinged into the winding roll, wraps the winding roll further and passesbeneath a second drum which is in torque control, and finally proceeds andpasses beneath the first drum again where the outer layer of web becomesthe second layer in the winding roll. This method of control describes amajority of two drum winders but there are certainly cases in which thesecond drum is controlled in a draw mode (differential velocity) withrespect to the first drum and other cases where both drums could berunning in torque control. The productivity of this winder can be quitehigh as the core is handled minimally. Two drum winders are often used asrewinders following the paper making process. One of the difficulties oftwo drum winders is the uncontrollable nip load due to the dead weight ofthe winding roll resting upon the drums which could lead to high webtensions that could burst the web. This effectively limited the weight andthus the diameter of the rolls which could be wound on this type ofwinder. It has been recognized that the nip load between the drums andwound roll could be reduced by pressurizing the air between the twodrums, resulting in lift on the wound roll, and thus allowing larger rolls tobe wound.• Belted Winders – Some webs such as tissues and others cannot be subjectedto rider rolls and high nip loads without the loss of loft. Fragile webs suchas these are sometimes wound on belted winders. The benefit of this typewinder is minimal nip load and thus minimal compaction of a high loft .
web. The majority of the energy required to wind the roll is provided to the rollers upon which the belt is driven. In some cases some center assist torque may be input to the core.There are other types of winders which exist which do not fall precisely into the categories listed, but the groups presented here comprise a large majority of the winders used commercially. In other cases you will find subgroups of winders that fit amongst one of the larger groups mentioned here. Gap winders are an example. Physically gap winders appear very much as a center winder with a rider roller. Upon close observation it is found the rider roller is not impinged into the winding roll, in fact there is an air gap between the rider and wound roll. Thus a gap winder is a center winder and can be very useful when wrinkling is a problem on the wound roll. The web wraps the idler prior to contacting the wound roll and the short span prevents wrinkles and fold-overs from occurring which can be useful on webs with thickness variation. A second example would be a winder in which the configuration is largely that of the two drum winder where the second drum is replaced with a belt support. The winding torque which ordinarily would have been supplied by the second drum would now be applied via the belt drive. The benefit of the belt support is a reduction in nip-induced slippage and hence a decrease in the maximum tension in the web which allows rolls of greater diameter to be wound prior to web burst failures.There are fewer types of unwinding machines. The unwinders fall into two main categories including:• Center Braked Unwinds – The center braking may be applied via a drive/motor or through mechanical brakes which range from pneumatic discs, toeddy current or magnetic particle devices, and in some cases magnetichysteretic brakes. All of these braking devices are capable of working witha closed loop tension controller with varied frequency response dependingon the circumstances.• Belted Unwinds – These unwinders are common in paper printingmachines. A belt is wrapped about a sector of the unwinding roll and aload is applied to the belt. A normal load or pressure results between thebelt and the outside of the roll which in conjunction with the kinetic friction provides control of the unwind tension. Many look upon the beltedunwind as archaic and outdated but this device can be superior to centerbraked or driven core shafts as the unwinding roll is not subjected to thetorque required for tension control. Although the belted unwinder mayperform well for steady state tension conditions, transient conditions suchas emergency stops and high decelerations can result in runaheads at theunwind which can also result in web loss and waste (1) .
