Paper360 - March/April 2015 - (Page 64)
the bottom line | KNOWLEDGE BUILDER
solving guide Problems on
A forming fabric that doesn't run straight creates problems, from
excessive fabric wear to complete destruction of the fabric.
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
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.
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,
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
Table of Contents for the Digital Edition of Paper360 - March/April 2015
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
Microfibrils to Transform Paper Furnish
New Energy Windfall
Power from Waste
Paper360 - March/April 2015