Guidelines for rolls used in web handling

By R. Duane Smith, product manager, specialty winding, Davis-Standard Converting Systems tension differential.

Editor’s Note: The author’s web-handling technical paper, “Guidelines for selecting proper idler rolls,” appeared in the 2012 Q2 issue of Converting Quarterly.

Today’s web processing and converting lines must handle a wide variety of substrates, resulting in wide tension ranges at ever-increasing processing speeds. All web handling requires conveying a web straight through a process without distortion or defects such as wrinkles. A converting process or film producing line requires the use of idler rolls, pull rolls, spreaders and edge guides to provide control of a web as it is conveyed. Without the correct size and application of these devices, the web might not track properly, may wrinkle, and web breaks could happen. This paper provides general guidelines for selecting these items for various conditions. It covers tension-isolating pull rolls, web spreaders and web edge guides used to provide control of a web as it is conveyed.

Spreader rolls

most powerful spreader rolls – are reverse crown spreader, flex

The paramount rule in proper web handling is to provide good web-tension isolation between the unwind, the various processes and the winding tension. This isolation normally is provided by driven pull rolls between each of the zones. The pull roll must have sufficient grip on the web to provide the required tension isolation between the tension zones.

Three types of pull rolls commonly are used. First is the nip type, where a rubber-covered roll is air-loaded against a driven steel isolation as the positive nip prevents slippage over the driven nip roll as long as sufficient nip loading is provided. A general rule of thumb for the amount of nip required is three times the web

Second is the “S Wrap” type, which consists of two or more driven rolls, usually rubber covered, with sufficient web wrap and web-to-roll coefficient of friction to isolate the tension differential (see Figure 1). The drawback of an “S Wrap” pull-roll system is that it will not isolate tension waves that can be experienced when unwinding a badly out-of-round roll.

Third is the vacuum or suction roll, which is used when only one side of the web can be contacted. The design of a vaccum-roll pull roll needs to have adjustable deckles for applications with varying web widths, and vacuum blowers should be remotely mounted.

Spreader rolls are used to laterally spread the web for the purpose of eliminating wrinkles and distributing web tension in the crossmachine direction for the separation of slit webs. Typically, spreader rolls are used prior to nip rolls, coating, laminating, slitting and winding. A spreader roll may be required after a long horizontal web lead. Common types of web spreaders used on web forming and converting lines – from the most passive to roll (see Figure 1). The nip pull roll provides the greatest tension

spreader roll, bowed spreader roll and expanding surface spreader roll.

The reverse crown spreader roll is sometimes called a negative crown or concave roll (see Figure 2). This is an idler roll machined where the center of the roll is smaller in diameter than the outside edges.

“S Wrap” Pull Roll

The theory of this roll’s spreading action is the reverse of a crowned pulley. The crowned pulley forces the belt to run in the center. The reverse crown causes each side of the web to move toward the roll’s edges, creating a spreading action. Traction between the web and the reverse crown spreader must be maintained as slippage makes this roll a “buncher-upper” instead of a spreader. Applying tape around the ends of an continued on page 60 u 2012 Quarter 3 • www.convertingquarterly.com 59

t continued from page 59

FIGURE 2. Reverse crown spreader roll

idler roll is commonly referred to as the “poor man’s reverse crown spreader.”

The flex spreader roll is a resilient, covered, grooved roll with opposing center spirals cut on a bias toward the center of the roll (see Figure 3a). As the roll turns, the web’s tension tries to collapse these spirals and exerts a continuous stretching from the center to the edges of the web (see Figure 3b).

The spreading action on this type of roll requires sufficient web tension to be applied so as to collapse the cover. Traction is not normally a problem on this type of spreader. The flex spreader is designed for the product or products run on that particular machine and has no adjustability. FIGURE 3a. Flex spreader roll FIGURE 3b. Effect of web on flex spreader roll The bowed spreader roll has a non-rotating shaft that is curved along its longitudinal axis. This shaft supports individual, bearing-mounted rotating segments. The segments usually are covered with a flexible sleeve that is vent-air grooved. As the web passes over a bowed roll, the web will tend to travel a path perpendicular to the roll’s axis of rotation causing the center of the web to travel straight over the roll and the rest of the web to turn and travel outward. The spreading action is affected by the amount of bow, web wrap and web tension (see Figure 4).

Bowed rolls require proper web lead-in and lead-out distances and a proper amount of web wrap to be effective as spreading devices. The spreading action takes place downstream of the bowed roll. The lead-in distance should be as long as possible to allow the spreading action to be as gentle as possible. The lead-out distance should be as short as possible to lock in the spreading of the web right after it leaves the bowed roll. The traction between the web and the bowed roll is critical for the spreading action. When running light tensions, it is suggested that

the bowed spreader roll be driven.

FIGURE 4. The bowed spreader roll’s spreading action

The expanding surface spreader roll is a very powerful web spreader having a series of rubber cords that are stretched as the roll rotates (see Figure 5). The web needs to wrap this roll in the area where the expanding elements start to expand and leave the roll’s surface before the elements start to retract. The drawback of this type of spreader is that the expanding rubber cords can mark the sheet. An improvement to reduce the possibility of marking is the addition of a continuous rubber-sleeve spreader. When running light tensions, it is suggested that this roll be driven. Due to the constant expansion and contraction, the application of this

type of spreader is for slower line speeds.

Edge-guiding systems are needed on almost every webprocessing line. These are necessary to compensate for dished and/or telescoping rolls of material being unwound to guide webs out of sections with long web leads, such as dryers, and/or into critical alignment processes such as printing, laminating or corrugation operations. Web guides also are used to provide web oscillation to help randomize web-profile defects on most slitting/ rewinding operations.

The keys to success of web guiding are: • The guiding system should be located as close as possible to the point in the process that precedes where accurate web positioning is required. • The guiding system needs to be designed to accommodate the maximum lateral web displacment required. • On roller-type guides, the lead-in web length needs to be as long as possible because the steering action takes place upstream to the first shifting roller. • The sensor needs to be the proper type for the web that is being guided and located as close to the shifting roll as possible. • The shifting idler just before the sensor needs to have a tractionized surface for steering without slippage.

Web sidelay and edge guiding at unwind is very common to correct for dished rolls or rolls that have been wound off-center on a core. It also can be used to compensate for roll-telescoping problems when unwinding rolls of material that were not wound properly on the previous process. Two different concepts can be used for edge guiding. The first method is by a two-roll displacement guide commonly know as an offset pivot guide (OPG) (see Figure 6). FIGURE 6. Offset pivot guide The second method is where the unwind/splicer is mounted on a moveable base, and the whole unwinding system is shifted to guide the web into the process. Both systems use a shifting mechanism controlled by a sensor fixed to the floor and can be powered hydraulically or electronically.

The two-roll displacement guide (OPG) method has two possible advantages over the shifting unwind: • The two-roll guide is normally less expensive. • It allows the unwind/splicer to be fixed to the floor, which may be desireable for roll loading.

Some of the disadvantages of the two-roll displacement guide: • It requires more idler rolls and generally more floor space. • The amount of correction is limited without excessive web stress and wrinkles. • It is not recommended for foil due to the above.

The shifting-base system is more expensive because the complete unwinding/splicing system needs to be mounted on precision linear rails and bearings. In addition, the actuator has to be much larger than on an OPG system. However, from a web-handling perspective, it is preferred because the travel is not limited, it can continued on page 62 u

handle offset rolls without web stress and it takes up less space.

t continued from page 61 Intermediate guiding – Webs can shift on a process line due to uneven tension, idler-roll alignment or loss of traction. Intermediate guides need to be provided just before edgesensitive processes on the line and after long web leads. An OPG can be used as an intermediate guide as long as parallel-web entry and exit leads can be provided.

At the exit of a floating dryer, a 90-deg steering-type guide is provided where only one side of the web can be contacted (see Figure 7). This guide requires a long entering span, which the dryer provides.

FIGURE 8a. Unwind guiding system sensor location

FIGURE 7. 90-deg steering-type guide at dryer exit

Web-chasing systems – Sometimes on a lamination or special application process, such as applying a glue line at a specific distance from a printed line on a web, a hunt-and-chase guiding system can be incorporated. This will guide the other web or application device to the edge or line of the incoming web.

Winder edge guiding is the last chance to ensure the winding of a straight-sided roll when edge slitting is not incorporated into the winder. The guiding system senses an incoming web’s position and guides the winder to that position (see Figures 8a-b). For this application, the incoming web needs to be stable, and the sensor position needs to be as close to the fixed incoming roll as possible (preferably just upstream). The sensor is fixed to the shifting winder and is cantilevered to the web coming onto the fixed roll. A very rigid mounting of the sensor off the shifting winder is critical for a successful winder edge-guiding operation.

Winding oscillating is provided to randomize variations of web thickness (gauge bands) across the web. This is especially true when winding thin films into large-diameter rolls. A very small fractional percentage of thickness variation can result in major hard spots and soft area defects known as rope marks or corrugation wrinkles in the finished rolls of web material.

To minimize these defects, the web or winder must be randomized or oscillated back and forth slowly before or during winding. For web randomization, either an OPG is used to oscillate the web just before winding, or the winder and slitter FIGURE 8b. Winder guiding system sensor location

section can be located on a sidelay base and oscillated. Winder oscillation is best for minimizing the possibility of web wrinkling but makes roll unloading and core loading more challenging. n

R. Duane Smith, product mgr., specialty winding, at DavisStandard Converting Systems / Black Clawson Converting Machinery (Fulton, NY), has been associated with Black Clawson for more than 38 years. He has been awarded two patents in the winding field, has made over 50 technical presentations and published 18 articles in major international trade journals and magazines. An active member of AIMCAL since 1991, Duane has given 24 technical presentations at CEMA and AIMCAL conferences. In 2002, he received the SPE Certificate of Recognition for his “significant contributions made to the Society and the plastics industry,” and earlier this year, he was named a TAPPI Fellow. Duane can be reached at 315-598-7121, ext. 312, email: smithd@bc-egan.com, www.bc-egan.com.