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Crowned and Concave Rollers for Combating Wrinkles


In past articles, we have discussed causes of wrinkles. The next few articles will discuss how to combat web wrinkles.

Principal of operation

Description:
A crowned spreader roll can be manufactured from a variety of materials, such as steel, aluminum, stainless steel or plated aluminum or steel and can be rubber covered. Although crowned rolls with a rubber surface are the most common, it is not necessary for every application that the surface to be rubber. A crowned spreader roll has an arced surface across its' face with the arc sweeping from the ends out toward the center of the roll face. The ends of the roll face are the smallest diameter while the roll diameter increases in equal increments across its' face to the center. The roll face diameter variance is strictly application dependent. However, it can be generally stated that the bigger the roll diameter, the less the variance. The longer the roll face the more the variance. Extreme variances may be .001" for a large diameter, short face roll and a .100" variance for a small diameter long face roll.

Theory of operation:
By varying the roll diameter across the face of the roll (roll is arced outward across its' face), tension will vary across the roll face. The increased tension toward the center of the roll will cause tensile stress in the center of the web forcing wrinkles to be pushed out from the center toward the roll face ends. See figure 15.

Crowned Roll diagram

Recommended wrap angle varies greatly with this type of spreader roll. Typical wrap angles may range between 30 deg. - 180deg. Greater degrees of wrap angle will cause greater effect on the web. Spreading may increase with greater amounts of wrap, but the occurrence of web distortion will also increase.

Advantages:

  • This type of spreader roll has a smooth surface, so it will not mark the surface of a web.
  • Most any machine shop, rubber roll manufacturer or idler roll manufacturer can machine a crowned roll. This makes this type of spreader roll one of the least expensive, most readily available types of spreader roll.
  • Simplicity of design, with no special parts makes this spreader roll extremely easy to maintain.
  • Crowned spreader rolls can be used with all types of materials. The amount of crown depends greatly on the type of web. More extensible materials, like films, will require a minimal amount of crown, where more rigid materials, like paper, will require more amount of crown.

Disadvantages:

  • The theory of operation for this type of spreader roll is not completely sound. While the tension variance through roll diameter variance is true, the idea of this theory actually spreading the material is unfounded, especially for preventing or removing wrinkles under varying circumstances (see Causes of Web Wrinkles section). An example of this is that while this type of spreader roll may help with wrinkles caused by a baggy center by adding tension to the loose center area of the web, it may worsen the wrinkles caused by baggy edges because the center of the web is already tight. The tightness in the center of the web will only be increased with the use of this roll.
  • Further reason why this type of spreader may do more harm than good is surface speed variance across its' face. As this roll rotates, the surface speed at the smaller diameter roll face off center is actually less than in the center (this is the case because the diameter is smaller at the ends than in the center). Slower surface speed toward each end of the roll face can actually drive the web at a slower speed than the center, which will cause the material edges to move in the cross machine direction toward the center of the web (exactly opposite what a spreader roll is supposed to do).
  • Because this roll is not linear across its' face (while the web is or should be) it can stretch, distort and/or tear the center of the web.
  • There is no set formula for the amount of crown for every application, so the amount of crown can only be determined through trial and error or past experience.

Concave Spreader Roll

Description:
A concave spreader roll can be manufactured from a variety of materials, such as steel, aluminum, stainless steel or plated aluminum or steel and can be rubber covered. A concave spreader roll has an arced surface across its face with the arc sweeping from the ends in toward the center of the roll face. The ends of the roll face are the largest diameter while the roll diameter decreases in equal increments across its' face to the center. The roll face diameter variance is strictly application dependent. However, it can be generally stated that the bigger the roll diameter, the less the variance. The longer the roll face the more the variance. Extreme variances may be .001" for a large diameter, short face roll and a .100" variance for a small diameter long face roll.

Theory of operation:
By varying the roll diameter across the face of the roll (roll is arced inward across its face), surface speed will vary across the roll face. The increased surface speed toward the edges of the roll will cause wrinkles to "walk" out from the center toward the roll face ends. See figure 16.

Concave Roll diagram

Recommended wrap angle varies greatly with this type of spreader roll. Typical wrap angles may range between 30 deg - 180 deg. Greater degrees of wrap angle will cause greater effect on the web. Spreading may increase with greater amounts of wrap, but the occurrence of web distortion will also increase.

Advantages:

  • This type of spreader roll has a smooth surface, so it will not mark the surface of a web.
  • Most any machine shop, rubber roll manufacturer or idler roll manufacturer can machine a concave roll. This makes this type of spreader roll one of the least expensive, most readily available types of spreader roll.
  • Simplicity of design, with no special parts makes this spreader roll extremely easy to maintain.
  • Concave spreader rolls can be used with all types of materials. The amount of concave profile depends greatly on the type of web. More extensible materials, like films, will require a minimal amount of concave profile, where more rigid materials, like paper, will require more amount of concave profile.

Disadvantages:

  • While the theory of operation for this type of spreader roll is sound. It may not work or it may even compound certain wrinkling conditions. An example of this is that even though this spreader roll relies on surface speed differential across its' face (not tension variation, as in the case of the crowned roll) to spread the web, tension variations will occur across the web because the roll face is not linear. This roll will probably perform well for baggy edges type wrinkles – the roll diameter being larger toward the roll face ends adding tension to the loose edges. It could compound the problem for baggy center type wrinkles - the roll face profile worsening the looseness in tension, in the center of the web.
  • Because this roll is not linear across its' face (while the web is or should be) it can stretch, distort and/or tear the edges of the web.
  • There is no set formula for the amount of concave profile requirement for every application, so the amount of concave profile can only be determined through trial and error or past experience.

We will discuss grooved rollers in the next article.


Jeff Damour, Converter Accessory Corporation, 800-433-2413

**DISCLAIMER - A great deal of time has been invested in the development of our weekly tech tips. To the best of our knowledge, they are accurate. It is up to the user to verify all results.

THE AUTHOR ASSUMES NO LIABILITY CONNECTED WITH THE USE OF THIS INFORMATION OR THE RESULTS OBTAINED FROM IT.



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