- April 01, 2009, By Timothy J. Walker Contributing Editor
Here a roller, there a roller, everywhere a roller-roller. That may make an entertaining toddler's song, but it isn't a good strategy for web line design.
Last month, in The Great Span Length Debate | Part 1 we debated roller spacing (a.k.a. web span length) relative to cost, safety, out-of-plane deviations from gravity, bagginess, and misalignment. Let's continue the debate.
The length of span has no significant effect on web-to-roller traction. Longer spans don't increase the friction coefficient, tension, wrap angle, lubrication, or roughness/grooving. If sequential rollers are wrapped in over-under-over-under, more rollers will create larger wrap angles per roller. In arched or flat dryers where a series of rollers are all on one side of the web, more rollers means less wrap angle per roller. For rollers with less than two degrees of wrap, a one degree change can be the difference between stick and slip, but roller performance is the more likely solution to slip problems.
- Air Turns
It is rarely a good idea to have an air turn controlling a long span, especially a long entering span. A web is perfectly happy to wrap helically around an air turn (which we like in a web flip) or shift back and forth violently (which we rarely enjoy). Prevent this with a roller (usually on the non-air turn side) as close as possible to the air turn's entrance and good air turn alignment.
- Air Flotation
Whether using Coanda or impingement nozzles, the longest spans in converting are in well-designed air flotation ovens, but this special case falls outside the great span debate.
- Tension Control
Span length is usually a non-factor in tension control. Total length increases a draw system's response time, and more rollers will increase MD tension variations within the zone, but neither specifically changes if any individual span is long or short. Short spans in and out of dancer or load cell rollers will increase the system mass-spring harmonic frequency and reduce the likelihood of problems in below 1,000 fpm speeds.
Long spans are better. Long spans get more correction for a given angle change. Long spans have greatly reduced tension variations from bending and twisting, especially if a longer translation span means misalignment angles are smaller.
This is the Catch-22 of span lengths. Longer spans are less sensitive to shear wrinkles from misalignment or diameter variations. Short spans are less sensitive to tracking wrinkles from deflection, baggy-center webs, or crowned winding rolls. I lean toward short spans in wrinkle-sensitive webs since good equipment design can prevent misalignment and diameter variations but can't stop baggy webs.
Spreaders can spread more with less force if entry spans are long. Exit spans should be short to reduce post-spreader tracking and immediately upstream of your critical process if the spreading is for anti-wrinkle benefits.
Short spans in and out of slitting are best to minimize slit quality problems of a web that flutters or sags away from the ideal cut point.
Of all the span problems I see over and over, lack of span control at winding is the most common. Old paper winders insensitive to wrinkling are ill-prepared to winding film product. Old turret winders that lose their optimized entry span control during indexing are a major cause of waste worldwide.
It's cheating, but true, to say the answer to the great span debate is “it depends,” but clearly it does. Here's my educated gut feeling on spacing transport rollers. I am comfortable with 1-6-in. web spans up to 6 ft, 24-80-in. up to 8 ft, and ultra-wide 100-200-in. webs with spans to 15 ft. I think you can double these lengths for purely vertical spans, since gravity doesn't pull the web out of plane.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; firstname.lastname@example.org; www.webhandling.com.