Paper360 - March/April 2015 - (Page 64)

the bottom line | KNOWLEDGE BUILDER solving guide Problems on Forming Fabrics A forming fabric that doesn't run straight creates problems, from excessive fabric wear to complete destruction of the fabric. Figure 1 The guiding principle. A pivoting guide roll keeps the fabric running straight. The fabric will always travel toward the side of the guide roll that it contacts first (Figure 1). Friction between the guide roll and the forming fabric is key to the guiding process. When one edge of the fabric encounters friction, it will be slowed down a little. This again will generate a cross direction force in the fabric, moving it toward the side with more friction. This applies to all items contacting the fabric, such as a misaligned suction box, which will also cause guiding problems. Increasing friction on the guide roll can be achieved by: increasing fabric tension, increasing the angle of wrap on the guide roll, increasing diameter of the guide roll thereby increasing the contact area, and increasing roll softness. Water reduces friction. Water trapped between the roll and the fabric can act as a lubricant and drastically reduce the guide system's effectiveness. Since fabric showering is critical to the total paper making operation, any showering changes are normally a last resort to improve fabric guiding. Wire tension. The higher the fabric tension, the more responsive the guiding system. Always check for the correct operating tension on the fabric on both sides of the machine (TS and DS), and at the same position, for example before the guiding roll. 64 Paper360º MARCH/APRIL 2015 Tension differences between the two sides are usually related to either misalignments or the forming fabric itself. Increasing angle of wrap on the guide roll. The larger the angle of wrap around the guide roll, the higher the effectiveness of the guiding system. The minimum wrap angle varies a bit with the length of the fabric. For longer fabrics, such as on Fourdrinier machines, a wrap angle of at least 25° (Figure 2) is needed to adequately guide a fabric. When a relatively short fabric is installed, the wrap angle will decrease and guiding problems may occur. Location of the guide roll. The location of the guide roll on some (older) machines can contribute to guiding problems. When the guide roll is adjacent to the stretch roll, either on the ingoing or outgoing side, the wrap angle will change when the stretch roll moves. Ideally the guide roll is positioned between two stationary rolls. Guide roll cover. Roll cover material and its hardness are critical in maintaining adequate friction. Rubber-covered rolls with a hardness of 20 P&J are recommended for guiding today's fabrics. However, because rubber rolls tend to harden over time, periodic roll grinds, every 2-3 years, are necessary. Polyamide yarns in the fabric. The yarns in the CD direction of a forming fabric, are usually made of polyester and/or polyamide. While polyamide is more wear resistant, the friction coefficient between polyamide and the guiding roll is about 30 percent less than with polyester yarns. Thus, tension, wrap angle and roll cover are even more important for forming fabrics with a high polyamide content. Figure 2 www.tappi.org http://www.tappi.org

Table of Contents for the Digital Edition of Paper360 - March/April 2015

Guest Editorial
Over the Wire
TAPPI's Centennial: A Celebration 100 Years in the Making
Precision Alignment of Winders
Single-stream Waste Processing
iRoll at Irving
Fully Automated Continuous Digester
Twin Roll Press Upgrade
TAPPI Journal Summaries
Effluent Treatment
Microfibrils to Transform Paper Furnish
Consolidation Watch
Knowledge Builder
New Energy Windfall
Power from Waste
Safety Survey
Association News
ASPI News
Online Exclusives
Advertisers Index

Paper360 - March/April 2015

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