Web Guiding and Tension
Tension Control & Web Guidance
Nicholas D. Datson, sales manager I
n a modern sheeting operation, web conditioning systems will often include tension control and web guidance arrangements. When used in conjunction with a close tolerance drive, these systems provide the basis for a precision sheet. This article will explore some tension control systems and web steering for optimising sheeter effi ciency.
TENSION CONTROL SYSTEMS Web tension plays a major role in sheet length accuracy. Too much roll stand brake pressure creates web slippage in the cutter’s draw roll section. If brake pressure is not reduced as the roll’s diameter decreases, slippage occurs, resulting in short sheets. In contrast, insuffi cient brake pressure will cause the sheeter to pull wrinkles in the web.
When sheeting at speeds beyond 300 fpm (90 mpm), an operator may not have the reaction time necessary to reduce brake pressure as the roll rapidly unwinds. This inability to compensate quickly can have an impact on sheet length accuracy, usually resulting in short sheets. Therefore, for high-speed sheeting, automatic systems are recommended to maintain constant tension as the roll unwinds. Load cell, dancer roll and proportional type systems are available to maintain constant tension. In the load cell design, an idler roll is mounted on a tension sensing device. The idler roll is located between two web carrier rolls. The web is threaded under the fi rst carrier roll, over the idler roll and under the second carrier roll. As the tension of the
roll and reduces the brake pressure at a rate relative to a preset tension at the core’s diameter. Another version of the proportional design uses the change in weight of the roll to vary the brake tension, maintaining constant tension as the roll gets smaller.
PROPORTIONAL TENSION CONTROL OPERATOR STATION.
Web Steering System combined with Heavy Duty Penetrating De-Curl System.
web increases, the idler roll is pulled down and the load cell, which, through an electro-pneumatic signal, controls the roll stand’s brake pressure, senses the defl ection.
The dancer roll method utilises an air loaded dancer roll that sends an impulse to a transducer that governs the roll stand’s brake pressure. In this design, the web is threaded around the dancer roll. As the web tension varies, the dancer roll is pulled out of its “dead center” position. The transducer senses this movement and the brake pressure is adjusted until the dancer roll moves back to its neutral position.
The proportional type tension control monitors the lineal footage of the web unwound and the number of revolutions of the roll at the unwind. The controller calculates the diameter of the
Maintaining proper brake pressure for each roll is essential for precision sheeting. Well-designed installations use independent tension control systems for each web. In contrast, less expensive designs use one tension system to govern the brake pressure of all rolls. This “slave” design is less desirable because each roll has its own tension characteristics.
TROUBLESHOOTING WEB TENSION PROBLEMS
While too much brake pressure at the unwind will result in short sheets, too little brake pressure will cause wrinkles. Operators may have to experiment to fi nd the proper setting that works best. Roll stands with automatic tension control can
be outfi tted with too many brake pads, which require little brake pressure. Low brake pressures are diffi cult to control. This problem can be solved by reducing the number of pads or by using pads with a lower coeffi cient of friction. This arrangement requires higher air pressure and off ers greater control.
WEB GUIDANCE SYSTEMS
Web guidance systems evenly align webs as they feed into the cutter. These systems are recommended for poorly wound or telescoped rolls because they eliminate the need to take an edge trim. Additionally, when slitting and sheeting multiple piles at high speeds, a web steering system can negate the need for the taking of side trim, thereby reducing waste.
In these high-speed sheeting applications, the most suitable system allows for the steering of the web after the unwind and prior to the web infeed of the sheeter. In this design, the web steering unit is separated from the roll stand and a series of rolls aligns the web. Web steering off ers greater responsiveness at higher speeds than other methods.
46 September 2024
www.convertermag.com
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