Preventing Loss of Focus
In line or contrast guiding, optical sensors or cameras are used to track a printed feature or line. These sensors require a stable focal distance to operate accurately. However, as the web guide mechanism actuates (moves back and forth), it creates a "twisting" effect in the web span, causing the web plane to shift closer to or further away from the sensor. This movement, known as pass line variation, can cause the sensor to lose focus, affecting the amount of light reflected back and disrupting the "teaching" or tracking of the feature.
Web Stabilization
The dead bar acts as a stabilizer. By wrapping the web over this bar, the web is forced to maintain a fixed distance relative to the sensor, regardless of the guide's movement or the twisting happening in the span. The sensor is then mounted specifically to "look" at the web directly over this dead bar.
Installation Specifications
- Mounting: The sensor should be installed to view the web while it is in contact with the dead bar.
- Wrap Angle: To prevent the dead bar from influencing the lateral dynamics of the web too heavily (which could cause drag or steering issues), the wrap angle should be kept small.
- Dead Bar (Stationary): Maximum of 5 degrees wrap.
- Idler/Backup Roller (Rotating): Up to 15 degrees wrap.
Application in Terminal Guides
For unwind or rewind applications where line guiding is required, the setup is slightly different to accommodate the moving stands:
- Unwind: A shifting idler or dead bar must move with the unwind stand to support the web, but the sensor must be fixed to the machine frame,.
- Rewind: The sensor moves with the rewind stand (chasing the web), and a fixed idler or dead bar is placed in the span just before the rewind.