Winding Education and Information
Step 2 – Select the Winding Process to Meet Your Needs
Any winding process, no matter the size of the roll or specifics of the design, is a system that controls stress or strain of each layer as it is added of the winding roll. Winding should be considered a building process, similar to erecting a skyscraper. Just like the land and basement of a building supports the rising structure, a winding roll’s core and initial layers are the foundation for building a good roll. In addition to a roll’s structure and tightness, a winder design will also determine the alignment of the layers, prevent wrinkling, and control the entering the wound roll. In choosing a winding process, consider both the active process of winding (such as preventing wrinkles or telescoping) and the long-term structure of the wound roll.
All winders can be categorized by three factors:
1) how the winder controls the last span and initial contact with the winding roll.
2) how the torque is delivered to drive or brake the turning roll.
3) how the web tension is controlled as a function of roll radius.
Controlling the Last Span and Initial Contact:
In free span winding, the distance to the last upstream roller before entering the winding roll is fixed and equal to the core diameter or longer.
In gap winding, the distance to the last upstream roller is controlled to stay near the winding roll, holding the last span between the gap roller and winding roll to a fixed span distance, typically 0.25 to 2 inches.
In nipped winding, the last roller before winding is pressed against the winding roll with a controlled load. Surface winding, where the roll is driven from the surface, is always nipped winding.
Torque Delivery:
In center winding, the all or most of the desired torque is applied to the roll’s center via a connection to the roll’s core. Almost all unwinding processes are controlled by center-applied torque.
In surface winding, the all or most of the desired torque is applied to the roll’s surface, through a nipping driven roller (or alternately using a nipping driven belt).
In combination winding, the winding roll is driven both from its center and its surface.
Tension Profiling:
In constant tension winding, the web handling tension is held constant as the roll grows from core to its final diameter.
In constant torque winding, the torque applied to the roll is held constant as the roll grows from core to its final diameter.
Taper tension is a programmable control system that changes the tension set point as a function of roll diameter or length. There is no industry standard for how taper tension is calculated, but two methods (linear and hyperbolic) are commonly used.
Choosing between Free Span, Gap, and Nipped Winding
| Options |
When to Select This Option |
| Free Span Winding |
Free span winding is quite common, but it the least forgiving for challenging products or processes.
In handling webs, usually great care is taken to design rollers with uniform diameter, low runout, and good alignment to avoid web wrinkling and wander. Many winding rolls, by the nature of imprecise cores and crossweb thickness variations, will have relatively poor diameter uniformity, runout, and alignment. Allowing the winding roll to control a long free span as it approaches the non-uniform winding roll may be acceptable for robust webs, but will likely to increase wrinkles and wander delicate products.
To avoid the wrinkling and wander as the web enters the winding roll, spread or flatten the web as close as possible to entering the winding roll. The best option to transfer a flat, non-wandering web onto the winding roll is to avoid free span winding in favor or gap or nipped winding.
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| Gap Winding |
Gap winding has the two strong benefits of reducing wrinkles and wandering relative to free span winding.
In gap winding, the roll tightness is entirely controlled by the web tension as it first contacts the winding roll. The roll structure or tightness can be varied as the roll grows by using taper tension.
Winding usually moves from gap to nipped winding when the entrained air layer entering a winding roll creates problems such as shifted layer or rolls loosening too much in storage as the air bleeds out. Air entrainment and lubrication will increase with increasing diameter, speed, and width or decreasing tension, roughness, and thickness variations. |
| Nipped Winding |
Nipped winding has the two strong benefits of gap winding, plus the added effects of reducing entrained air, increasing roll tightness, and reducing roll diameter variations. Another benefit that is often overlooked is how a nipped roller that engages the winding roll or rolls from below can offset deflection due to gravity.
These added effects should be considered with some caution. It may seem that entrained air only causes problems (lubrication, web wander, looseness over time), but some amount of entrained air is also a benefit to winding, supporting the web layers in below average thickness lanes. Try winding in a vacuum chamber to learn the benefits of entrained air.
Nipped rollers also increase roll tightness. If a product is moved from gap to nipped winding, the nip-induced tension may create an overly tight roll and associated high pressure or high stress defects. Overly tight rolls may be avoided by reducing the tension or minimizing the nip force.
Nipped winding reduces diameter variations by pressing harder on the larger diameter portions of the winding roll. This has the negative effect of magnifying the pressure and stress differences between thick and thin lanes within the roll.
Nipped winding’s affect on wrinkling may be a double-edged sword. A nipping roller can be an excellent way to transfer a flat web onto the winding roll without wrinkles, but if the entering web is baggy, as with any nip, a loose edge may buildup ahead of the nip, forming an accumulation wrinkle.
Some winding processes will use both gap and nipped winding. Winders with automatic roll transfers may need a winding nip to press the leading tail onto the new core and tighten up the initial layers if there are any core start wrinkles. After transfer the winder may shift from nipped to gap winding. Turret winders may also shift from nipped to gap winding during the index cycle. Less advance turret winders designs are unable to maintain nipping contact with the winding roll as it indexes to the unloading position, not only shifting to gap winding, but to free span winding and exhibit the wrinkling and wander during the unnipped index process.
In much of web handling, gravity is a small force relative to web tension, but in winding, especially in rolls of high slenderness ratios (length / diameter), long journal lengths, or winding several large slit rolls on a single shaft, the deflections from gravity can cause misalignment, wrinkling, and roll collapse. A non-deflection nip roller pressing up from the bottom of a winding roll or rolls can provide the support needed to minimize deflection and the associated problems.
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Choosing Between Center, Surface, or Combination Winding
The choice between center and surface winding is dependent on many considerations, including nipping, roll transfer, roll buildup ratio, tendency to wind too tight or too loose, and largely on what is most common within your section of the converting industry.
Many products can be successfully wound with either center or surface winding, as long as the tension, torque, and nip load ranges are appropriate for the product. The tension created by any winder and controlling the tightness of the winding roll has potentially two components: center torque induced tension and nip-induced tension.
In free span of gap winding, all the tension is created by center torque. In surface winding, almost all the tension is created by the nip load (the negative torque or drag at the core of a surface winder may have an effect of reducing the winding tension). In center winding with a nip roller, the outer layer’s tension is created by the additive effects of the center torque and nip load. In combination winding, similar to nipped center winding, the winding tension is created by the additive effects of the nip load and center-assist torque.
Combination winders are usually controlled similar to a surface winder, but with added torque at the core’s center.
Some of the factors used to choose between center and surface winding include:
| Factors |
Selecting Center or Surface Winding |
| Nip Sensitivity |
Nipping is the first factor in choosing between center and surface winding. Some product cannot handle even relatively low nip pressure or are too baggy to pass through a nip with out wrinkles, therefore, surface winding, which requires nipped contact is not an option. Center winders can run in either gap or nip mode, but gap winding requires position control, a feature not necessarily included in nip control systems.
Choose Surface Winding
If your products are not nip sensitive.
Choose Center Winding
If your products are damaged by a nip.
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| Porosity, Roughness, and Air Entrainment |
Surface winding is the preferred winder design for winding rough or porous materials such as paper, non-wovens, and textiles.
In smooth and non-porous products, even at low speeds a surface winder may be prone air bubbles forming (sometimes causing wrinkles) between the web and surface roller or the outside layer of the winding roll. Special designs of grooved, textured, or profiled surface driving rollers or torque assist from center assist or secondary surface rollers may alleviate the bubble problems.
The bubbles and bubble-wrinkles don’t form in porous and rougher products, which is why surface winding is the more common in paper, non-woven, and textile winding.
Choose Surface Winding
If your product is porous or rough. For non-porous products, consider center torque assist to pull out unwanted pre-nip bubbles.
Choose Center Winding
If your product is non-porous and smooth. |
| Torque Capacity |
Center winding transmits torque from the roll’s center to the winding roll’s surface to create web tension. Therefore, all roll layers must have a sufficient torque capacity (the combination of pressure, area, and friction coefficient) to be greater than the applied torque. This is especially challenging for low friction material, loose core starts, and large buildup ratios. Surface winding does not required torque to be transmitted through the roll’s layers, except enough to overcome any rolling resistance of the roll shaft or chuck bearings or torque generated during acceleration or deceleration.
Choose Surface Winding
If your product is wound to large roll buildup ratios, 5:1 and higher.
If your product is slippery or shear sensitive and would be damage by torque transmission from the roll center to the outside layers.
Choose Center Winding
If your product has low or moderate buildup ratios.
If your product has little tourniquet effect and good friction. |
Wrinkle-Free Roll Transfers
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Historically, for full-speed, automatic roll transfer, there have been more equipment designs of center winders that start the web onto a new core with minimum wrinkling. Surface winding transfers have often involved tearing the web and long foldover or wrinkles at the core start.
For compressible product like papers, nonwovens, and fabrics, any core start foldovers or wrinkles can be quickly masked by the roll’s initial layers. However, for less compressible products, such as films and foils, any wrinkle or foldover at the core start will imprint through hundreds of layers, creating unacceptable waste levels. Recently, more equipment builders have inventive solutions to good core starts on surface winders, so this center vs. surface selection criteria is less of a factor.
Choose Surface Winding
If your product is insensitive to core start wrinkles.
If an automatic surface winder can handle your product.
Option: Center torque assist on surface winders (the most common form of combination winding) can help tighten otherwise loose core starts.
Choose Center Winding
If your product would benefit from the many proven automatic transfer center winder designs.
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Choosing between tension profiling options:
Options
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When to Select This Option |
| Constant Torque |
Constant Torque In constant torque winding, the torque applied to the roll is held constant as the roll grows from core to its final diameter. Constant torque is the simplest winding method, applying an unchanging torque to the roll’s center using a clutch or torque-controlled motor.
If the roll buildup ratio is small, the small decreasing tension of constant torque winding may not be significant, but if the buildup ratio is over 4:1, the final tension will be less than 25% of the starting tension, often too low to maintain good control of the entering web.
For center winding, constant torque winding creates a tension that will change inversely with roll diameter, decreasing tension as a winding roll grows or increasing as an unwinding roll gets smaller. For a surface winder, since the diameter of the surface driving roller is constant, constant torque and constant tension winding are the same. |
Constant Tension
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In constant tension winding, the web handling tension is held constant as the roll grows from core to its final diameter. In constant tension center winding, a winder needs either diameter or tension feedback to calculate the correct torque increase proportional to increasing diameter. |
| Linear Taper Tension |
Linear taper tension changes the tension linearly with roll diameter and is most common on winders with tension measurement or dancer roller position feedback. The winding tension profile is defined by two variables, initial tension and taper percent, where the percent entered is how much the tension will decrease as the roll grows to final or maximum diameter. (Some linear taper programs define the percent taper as the amount of tension remaining at the end of the roll.)
A linear taper equation needs to know the roll’s final diameter. Since the final length and diameter of a roll may vary, linear taper programs often
use a winder maximum diameter as the final diameter. In using maximum diameter, if a winding roll is stopped before the maximum, the actual tension change will be less than the tension taper percent setting. |
| Hyperbolic (Torque-Based) Taper Tension |
Hyperbolic or torque-based taper changes the applied torque linearly with roll diameter. Torque-based taper does not require tension measurement or feedback, only a torque generator (a torque motor or clutch) and a diameter sensor. In hyperbolic taper, constant tension winding is usually consider a 0 and constant torque winding is consider 100. A hyperbolic taper of 50 will increase torque with increasing roll diameter at half the rate of constant tension winding. |
| Specialized Taper Tension |
Specialized taper tension is the most advanced tapering options and allows any function (linear, hyperbolic, polynomial, stepped, or other) to control tension over the roll’s buildup. The most common specialized option is the ‘hard start’ used to increase firmness at the core start or increase tension during a cut and transfer process. Nipped winders many include an option to taper nip load as the roll diameter or process speed changes. |
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