WEBCON Inc.
Negative Crowning of Felt Roll








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Improve CD Uniformity of Tissue
Reduce Wear on Felt

PURPOSE

Unless it is corrected, the trade line (seam) in a felt will run with a "smile". In order to straighten this smile, a negative (concave) crown is ground onto a strategically selected felt roll. This will
  • Minimize shear deformation in the felt to prevent porosity variation in CD. This helps in sheet formation, pick-up, pressing and drying.
  • Reduce shear stresses in the felt/fabric to minimize slip across the felt. This will ensure uniform felt wear in CD.

AREAS OF APPLICATION

  1. Forming section wires: sheet formation, pick-up.
  2. Pressing and drying sections.
  3. Fabric in TAD machines.

METHOD OF SOLUTION


The distance traveled by the felt in the middle is not the same as that at the edges due to:
  • Roll deflections on account of the roll weight, felt tension and vacuum, which reduces the path length travelled by the felt in the middle.
  • The crowning of the pressure roll drives the felt faster in the middle than at the edges.
A felt roll which will provide the best correction for the least amount of crown is selected for the purpose of crowning. A set of equations is now developed and solved iteratively to compute the negative crown.

INPUT REQUIREMENT

  1. Schematic drawing of the felt section showing x, y coords of the rolls.
  2. Drawing of the rolls showing i.d., o.d., face, bearing span. journals.
  3. Roll material, and any crowning.
  4. Vacuum on any roll, and its geometry,
  5. Felt material (sample or specification) and felt tension.
OUTPUT RESULT
  1. Identification of the roll selected for negative crown.
  2. Amount of negative crown, and degree of cosine curve.
CASE HISTORIES
  • In a 200+" wide LDC paper machine running at 5500 fpm. the negative crown dropped the smile of the trade line in the felt  from 14" to almost zero; and the felt life increased by weeks.
  • Negative crown was prescribed for a number of other machines (TAD, HWC) ; but no feedback from the papermakers, except that they were happy.
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