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Crowned Rolls






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Crowned and Skewed Rolls for CD Uniformity

PURPOSE

Nipped rolls must be ground with proper crown to exhibit a uniform nip pli across the rolls. This will result in uniform paper bulk in CD, and the converted products, rolls or folded, will  be equally dense from one side to the other.

AREAS OF APPLICATION

  • Press and Dewatering Rolls,
  • Calendering and Laminating Rolls,
  • Embossing and Texturizing Rolls.

METHOD OF SOLUTION

  • Analyze the current crown and nip loading (pli), and relate it to the current bulk profile.
  • Take nip impressions in the middle and the ends.
  • Determine the desired nip loading, and compute the new crown.
  • Determine the degree of cosine curve for best fit.
  • In case of skewed rolls, develop the skew vs. nip loading curve.
COMPUTER PROGRAMS
  • "CROWN" program computes the radial crown required on one of the rolls, the mating roll is uncrowned. Alternatively, crown can be split in any desired ratio between the rolls.
  • "ROLSKEW" program computes the offset at the support bearings on each end of one of the rolls.
  • "ZEROCR" program calculates the optimal bearing location for rolls with internal bearings.
  • "ROLAYR" program computes the effect of the varying thickness of rubber cover for a crowned roll, and provides a crown correction equation.*
  • "CRNCOR" program develops correction for existing crown when the nip impression is not uniform across.
  • "ROLDEF1" computes the crown curve of rolls with a uniform wall thickness and varying cover thickness as a result of crowning.*
  • "ROLDEF2" is same as above but with rolls of varying wall thickness.*
  • "ROLDEF3" solves the case for a 3-roll stack.*

INPUT REQUIREMENT

  • Dimensioned drawing of the rolls.
  • Loading geometry

MEASURE OF SUCCESS

  • Nip impression taken in the middle should match that at the ends.
  • The bulk profile of the paper should be fairly even in CD.
  • The converted product density should be uniform.

CASE HISTORIES

Crowns have been calculated for a number of installations with satisfactory results. 
Usually the desired result is obtained on the first try.

* For details on these items please refer to "Deformation of Contacting Rollers with Compliant Coatings", by Balbir Singh, Ph.D. Thesis, University of Pennsylvania, 1979.

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