Web Handling Q&A on Lateral Web Movement

Recently, I received a question from a long time PFFC and “Web Lines” column reader dealing with lateral movement in a web. My response follows.

Q: I have been visiting the Paper, Film & Foil Converter website for many years now, and I have read many of your past articles on web handling, but my colleagues and I still have problems understanding lateral movement in a web. I am to believe a uniform web (i.e., no shape defects) is expected to travel parallel to machine centerline if it has sufficient tension and the rollers are well-aligned.

My question is related to non-uniform webs (i.e., baggy or skewed). Will a skewed web track on centerline, towards its loose side, or towards its tight side? Some of my colleagues say the web will have a tendency to track towards its tightest edge, but they offer no reason or formula as to why. Any insight or reference on this subject would be greatly appreciated.

A: First, Thank you for being a loyal reader. Your question is a good one and one that has led to many debates over the years, even among web handling 'experts.'

Does a baggy web track to its tight or loose side?

My old colleagues at 3M (Ron Swanson, Dan Carlson, and Jim Dobbs) presented a paper at the International Conference on Web Handling, "Lateral Dynamics of Non-Uniform Webs," that did a great job at addressing this question. They constructed a lab system to create a non-uniform web of known dimensions by cutting a programmed curved web. After guiding this web to a starting point, they could measure lateral tracking variations of straight, left-curving (baggy on the right), and right-curving (baggy on the left) webs.

Here’s a quick summary of their results:

  • On aluminum rollers, they could not measure a significant tracking variation in baggy versus straight webs.
  • On a high traction roller (covered with rubber tape), the baggy webs tracked toward their loose, baggy side, though this was not a strong tracking mechanism.

The mechanics of this is not clearly defined, but it is hypothesized to be related to how a web tracks if its angle of approach is not parallel to the roller's surface vectors. The asymmetric baggy web presents a slight curvature at the point of contact leading to the next portion of web making contact at a shifted point on the roller, continuing until the web is arranged to present the web parallel to the roller direction. When a baggy web is under sufficient tension to elongate the short side of the web equal to the long side of the web, there should not be any curvature, and the web should track straight.

In past works, Ron Swanson has documented lateral tracking of baggy webs in air flotation ovens. This is a strong effect. The baggy side of the web floats higher off an air bar or air turn, presenting the web at an angle to the impinging air, and creates strong lateral forces−much like a sail boat in the wind. In this case, the web tracks to the non-baggy, tight side.

Other 'rules of thumb' on tight side tracking are sometimes associated with roller misalignment, roller diameter variations, and belt−not web−tracking. Most prove not to be viable.

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