Web Tension
The center wind tension on a rewind is generated by the motor’s torque. In the case of multiple motors being used, the total tension is the sum of the motors’ torques. If torque is held constant, the tension on a roll will depreciate as its diameter increases, until the tension drops below what is needed to actually continue winding. Because of this, it is important to use a slitter/rewinder option which can vary tension as needed.
According to slitter/rewinder manufacturer and FTA member Atlas Titan, “Experience has shown that winding with constant tension rarely provides the ideal winding conditions. Usually, the rewind tension needs to be decreased as the diameter of the roll builds up, but not by as much as is achieved with constant torque. In practice, the tension should decrease between 5 percent and 30 percent with diameter.”
Unwind Tension
As its name implies, unwind tension is that which is applied to the web by the unwind motor/brake.
On a center surface machine, unwind tension is a factor until the web contacts the winding drum. At that point, the unwind tension is transferred into the material. Because of this, unwind tension can affect the finished product. If the unwind tension causes the material to stretch, the deformed web will be rewound, causing damage to it and potentially causing a core to collapse.
On a center winding machine, unwind tension is involved only until the point at which the web passes over the final pull roll. Once this happens, it is only the rewind motors that contribute tension. While this is notably different than a center surface winder, there still exists the potential for the material to be stretched or warped. Because the unwind tension dissipates past the last pull roll, there is time for the web to return to normal. A center winder typically has unwind tension set as low as possible.
Unwind tension is controlled with the assistance of a dancer roller, a pivoted roller over which the web passes after leaving the unwind.
Machine Rollers
A rewinder’s rolls serve two main purposes: To transfer the material from the unwind to the rewind, and to keep the material smooth and flat for slitting. Motors are not used to propel these rolls on all machines. On some, specifically narrow units, the rollers are lightweight and of low inertia. This allows them to be guided by the web itself, speeding up and slowing down with the machine. On a wide machine or one tasked with moving a thin web, the rolls must be driven.
In the case of a rewinder with driven rolls, it is extremely important the speed is properly controlled. Failure to do so can lead to several potential issues, including:
- Material damage: If the rolls move at a speed faster than that of the material, they can scratch it
- Slack material: If the rolls move at a speed slower than that of the material, they will not smooth it out and the material will instead begin to slack
In order to avoid rolls that move too fast or too slow, they can be powered with direct drive or linked together with a belt (common drive). Moving from unwind to rewind, speed should be increased progressively and in small increments—Atlas Titan recommends less than 0.1 percent per roll. This can be accomplished with several drive options:
- Individual roll drive: Here, a constant rotational speed is supplied to each roll
- Timing belt drive: A gearbox is used to generate differences in speed, enabling their adjustment while the rewinder is running
- Flat belt drive: The rolls are the same size but, moving from the back of the machine to the front, the pulleys become progressively smaller
Atlas Titan covers winding and web handling in its three part series titled A Guide to Rewinding, along with a number of additional articles and resources, available for download from its website at www.atlasconverting.com.