PFFC - March 2009 - (Page 12) WEB LINES The Great Span Length Debate | Part 1 W By Timothy J. Walker Contributing Editor hat is the best span length? How far apart should rollers be? Is there a good rule of thumb on span length, such as “span lengths should be less than one (or one and a half) web widths?” Whether this rule sounds good to you will depend on the webs with which you work. For a 60-in.-wide web, one roller every 60 in. or one every web width seems reasonable. For paper or polypropylene film makers working with widths that can be more than 20 ft, having a roller every 20 ft is not reasonable. For narrow web handling, such as 2-in. wide tapes, it would be ridiculous to require a roller every 2 in. There are at least half a dozen things that come to mind in the great span length debate. Let’s review the checklist to figure out what span length is right for you. horizontal, when it causes catenary sag (the same as the sag of high tension power lines between poles). In a 20-ft span, a 1-mil polyester web at 1 PLI will sag 0.75 in. A 10-mil polyester web would sag ten times more or require 10 PLI tension to get back to 0.75-in. of sag. Sag will increase directly with span length squared and inversely with tension. If you start adding loads onto your web, such as the mass of a wet coating or the force of air impingement, then the deflections will increase proportionally. Rollers in u Costs (since rollers don’t grow on trees) | A 3-in.-dia roller can cost from $5–$10/in. of width. A 6-in.-dia roller will run closer to be considered a $20/in. ($40 if you want stainless steel). If you need to transport the web 100 ft through an oven, ten 70-in. rollers will cost $14,000. The debate whether span lengths should be every 10 ft or every 2.5 ft (equal to 2x or one-half of a 60-in. web width) is a In vertical spans, the effects of gravity are often $42,000 question. negligible. The tension in a web rising vertically will Roller costs, just like any piece of equipment, increase by the web span’s weight as it rises from don’t stop at the purchase price. Rollers have to the bottom to the top of a vertical span (or drop the be aligned. Their bearings and surface need to same amount in falling vertical spans). For a 10-mil be maintained. When the webs break, each roller thick polyester rising 20 ft, the tension change will needs to be rethreaded. When slimed, each roller be only 0.12 PLI—hardly worth talking about. has to be cleaned. Suddenly this simple question is more complicated. u Bagginess and Misalignment | If either of these u Ergonomics and Safety | Rollers in close imperfections drives the web to slackness—look proximity always should be considered a safety out. In a 50-in. span, just 0.040 in. of excess hazard. In some cases, a guardable nip point can length or misalignment will allow the web to be two rollers as far as 6 in. apart. deviate from the web’s plane by 1-in. That’s less For ease of threading, roller spacing should than a 0.1% error. Close spacing of rollers is start at 4 in. between roller–roller and required to prevent contact when passing roller–machine surfaces, unless process through slots, a tunnel, or to avoid other or space limitations trump this goal. non-moving elements. u Gravity | Most CAD drawings of web lines show rollers and Span length debate so far: Short webs as circles with tangential spans are expensive; extremely short connecting lines. Real webs will spans are hazardous; and long spans THE GREAT sag and flutter from gravity, web lead to flutter and out-of-plane SPAN DEBATE bagginess, and roller misalignment. problems. Next month we’ll cover Part 2 Gravity is most obvious and traction, tension, guiding, wrinkling, significant as spans are closer to and spreading. always should close proximity safety hazard. COMING NEXT MONTH 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; tjwalker@tjwa.com; www.webhandling.com. 12 | MARCH 2009 WWW.PFFC-ONLINE.COM http://www.webhandling.com http://WWW.PFFC-ONLINE.COM
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