Can't Touch This (Web): Part 2
- Published: May 01, 2003, By Timothy J. Walker, TJWalker & Assoc. Inc.
Last month we covered the web and process motives for touch-free web handling. This month we ll begin to review web line options for “Can t Touch This” webs.
Often, a touch-free requirement applies only to one side of the web or may not include the uncoated web edges. These no-touch qualifiers will make a dramatic difference in how to approach the “can t touch this” challenge. Let s start with the toughest case: true touch-free handling, where the web cannot be touched whatsoever.
Touch-free handling over a short range occurs on every web line. Once the web leaves a roller, it qualifies as “untouched” until it reaches the next roller. But how long can we keep this up? The answer can t be calculated from a simple equation, but all long spans — whether vertical or horizontal — have practical limits.
Long spans are subject to gravity. In long horizontal spans, gravity creates catenary sag, deflecting the web from the roller-to-roller tangent line similar to a suspended clothesline or chain (catena is Latin for chain). Catenary sag is a direct function of tension, web weight, and length. With a moving web, the sag will oscillate due to web density or tension variations.
Long vertical spans are real estate savers and avoid catenary sag, but they still are subject to gravity. The top portion of a long vertical span carries the rest of the span s weight (and tension). With wimpy webs, an overly long span may yield or break the web. When web weight is small relative to web tension, vertical spans are preferred over horizontal spans. Alas, the bigger headache in long vertical spans is not web handling but the operations hassles of working on a multi-level web line.
Tracking and sailing problems are more likely to limit span length than is gravity. Increased web flexibility in long spans magnifies tracking errors, leading to misalignment and wrinkling defects. A long web span is an effective sail, fluttering in any ambient airflow. Due to tracking, wrinkling, and airflow sensitivity, I usually recommend keeping unsupported span lengths to less than three web widths.
With unsupported vertical or horizontal long spans, we must pull on the web from afar like we would on a marionette. Air flotation systems provide mid-span forces needed to maintain control of a web without roller contact. Using directed, pressurized air streams, air flotation systems shape the moving web into planar, sinusoidal, cylindrical, or helical forms.
Air foils, the gentlest air floating system, blow air parallel to the web, using the Coanda effect to control web position and planar shape. Air impingement, the more aggressive method, blows perpendicular to the web from both sides. Alternating top and bottom air nozzles force the web into a sinusoidal shape, creating a down-web curvature to oppose cross-web buckling. Air foil and impingement nozzles are effective answers for controlled, touch-free handling over long distances; however, they can t replace the roller function of turning or reversing a web.
Cylindrical air turns (a.k.a. reversers) are roller substitutes. To be touch-free guaranteed, an air turn must be more than a simple air-lubricated cylinder. True touch-free air turns maintain a calculated float height with sufficient air volume and internal cushion pressure to fend off tension spikes. As an added benefit, air turns also can support a web in a helical shape for 90-deg turns or as part of a web flip. Air turns are an effective and overly feared web handling tool.
All air flotation systems will always have less stability than rollers. They will have more problems with tracking, web flutter, noise, and maintenance, but they are the first choice for long touch-free handling of low-porosity webs.
Next month, we will address the qualified “can t touch this” that applies to one-sided or edge-only contact.
Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 404/373-3771; This email address is being protected from spambots. You need JavaScript enabled to view it.; tjwa.com
