Understanding composition, construction is key to optimizing performance
April 10, 2007
Coil processors have several choices in the rolls they use to put tension on the coil. One of these choices is a pair of nonwoven rolls. They act like sponges in that they remove lubricants, dirt, and metal fines from the metal. Understanding how nonwoven rolls are constructed and how they work can help coil processors extend their service life.
The composition and construction of a roll, its fabric, and how it is finished are critical in understanding how a roll works. Knowing how a roll works, in turn, is the key to optimizing its performance and longevity.
A nonwoven fabric is a combination of natural or synthetic fibers and molten plastics intertwined to form a base or web. Because the roll fabric comprises fibers that vary in diameter and length, the fabric's strength and flexibility exceed that of any of the individual fibers. Adding a polymer-binding agent to the fabric enhances its mechanical properties.
The fabric has a large number of voids, or open spaces. This open structure allows the fabric to pick up and hold dirt and fluid much like a sponge does. As the fabric is compressed onto a roll core, some of the voids get compressed. Thus, a roll with 93 Shore A hardness potentially has less open structure than the same roll compressed to a net hardness of 85 Shore A.
To understand how nonwoven fabric works, it's important first to learn how it is made. Of the many processes used to produce nonwoven fabrics, all share three main steps: web forming, bonding, and finishing.
Forming is the process of bringing together individual fibers of various diameters and lengths and weaving them together to form a random pattern or web. Bonding sometimes is applied to provide specific mechanical enhancements, such as added resiliency against cuts, tears, and abrasion. Finishing may involve sealers or repellents and even include printing or embossing.
The roll production process has five steps: testing and engineering, die cutting, pressing, finishing, and validating and documenting.
Nonwoven fabrics work like a sponge that pumps fluid off and onto the steel as the rolls rotate. To achieve optimal tension, three elements work together during roll operation:
Void Volume. Void volume is an indication or measure of the amount of open space between the combined fibers and binder material web in a nonwoven fabric. Void volume has a direct impact on the ability of a nonwoven roll to pick up dirt and wring oil off steel.
PLI. Pounds per linear inch, or PLI, is used to indicate the amount of pressure, in pounds, per linear inch across the face of two rolls as they come together. PLI is used to calculate roll force. It is a common term used by the roll producer, machine builder, and end user to describe the force exerted on and through the rolls.
Roll Footprint. Proper PLI ensures that the rolls' contact points create an adequate footprint. This, in turn, allows you to establish the proper roll loading (or footprint) to achieve desired results. Understanding PLI and the footprint lead to proper machine setup.
Following inspection, installation, and setup procedures that protect the roll covers can help you to extend their useful service life.
Installation and Setup
Proper operation extends the roll service life and reduces the amount of reconditioning needed when service is necessary.
Verify Roll Diameters. Rolls for common drive systems are manufactured in pairs with identical diameters. Rolls with matching diameters rotate at the same speed, which helps ensure that they perform correctly. If rolls are not matched, they will operate at different speeds, leading to loose coils, premature roll wear, roll skidding, part damage (including binding, tearing, and wrinkling), unpredictable steering, oil lines or streaks on the coil, uneven strip tension, and premature bearing wear.[image4]
Slitter Head Setup. Set up the slitter head at the center of the slitter mandrel whenever possible. Repeatedly setting up the slitter head to the left or the right of center causes uneven wear on the tension rolls. Uneven wear causes steering and tracking problems and reduces the rolls' service life.
Setting the Force. Use just enough roll force and brake force to provide equal tension on each slit coil section. The force and brake amounts vary; they depend on the metal gauge, coil width, and the number of slit sections. Use additional roll and brake force for the first five to 10 wraps on the recoiler, then reduce the force.
Inline Maintenance. When the rolls are not in use (between slitter head setups, for instance), run them under normal roll force in jog mode to wring excess oil and dirt from the roll surface and void areas. This helps to maintain the coefficient of friction and provides equal tensioning across the roll face.
This operation also levels the roll surface. For example, when slitting a narrow coil, the entire roll face does not engage the metal, resulting in minor waves or edge marks in the area of the metal path. Applying roll force to the entire face reduces or removes the waves and edge marks, thus increasing the tension consistency across subsequent wide coils.
Check for the following when you notice steering problems:
Check the following points when you notice slipping:
When the rolls have accumulated too much oil, dirt, or metal particles to grip coil, return them to the manufacturer for service. This is a matter of cutting them down to remove the excess dirt and oils that have loaded the surface voids. The normal service cycle is six to 12 months, depending on the metal type and the number of operational hours.
Jim Rusczyk is managing director and Doug Goetz is general manager for FKM USA LLC, 400 S. LaGrange Road, Unit A, Frankfort, IL 60423, 815-469-2473