DWH Technical Topic: Web-path length: Layon rollers

By Clarence Klassen, P. Eng.

To perform a winder index or cutover, the layon roller must be moved away from the winding roll. This permits the turret rotation to proceed without interference. The layon roller must then be returned to contact the incoming core for the cut.

Moving the layon roller inevitably changes the web path length. Changing the web length indirectly affects tension control in two ways. Firstly, the change in web length must result in a speed change in the spindle RPM. Secondly, the spindle RPM change will affect the spindle’s calculated diameter. An incorrect diameter will result in incorrect torque for producing tension.

As the layon roller moves away from the spindle, whether up or down, the web path must increase. This results in the spindle RPM decreasing. Tension tends to increase at this time. The reduced RPM, but constant line speed will result in the spindle diameter calculator producing a larger diameter. This slows the spindle further, resulting in a larger diameter… The larger diameter results in additional torque, increasing the tension for the spindle…

As the layon roller moves back into contact with the core, the web path shortens. This results in the outgoing winding spindle speeding up. Tension tends to decrease at this time. The increased RPM, but constant line speed will result in a smaller diameter. This increases the spindle speed further, resulting in a smaller diameter… The smaller diameter results in less torque, decreasing tension for the spindle…

We have to break these cascading feedback loops during an index. That is typically done by freezing the diameter at the last known good value and using a technique called torque hold.