The Great Span Length Debate | Part 1

What 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.

  • 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 $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 2× or one-half of a 60-in. web width) is a $42,000 question.

    Roller costs, just like any piece of equipment, don't stop at the purchase price. Rollers have to be aligned. Their bearings and surface need to be maintained. When the webs break, each roller needs to be rethreaded. When slimed, each roller has to be cleaned. Suddenly this simple question is more complicated.

  • Ergonomics and Safety

    Rollers in close proximity always should be considered a safety hazard. In some cases, a guardable nip point can be two rollers as far as 6 in. apart.

    For ease of threading, roller spacing should start at 4 in. between roller-roller and roller-machine surfaces, unless process or space limitations trump this goal.

  • Gravity

    Most CAD drawings of web lines show rollers and webs as circles with tangential connecting lines. Real webs will sag and flutter from gravity, web bagginess, and roller misalignment.

    Gravity is most obvious and significant as spans are closer to 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.

    In vertical spans, the effects of gravity are often negligible. The tension in a web rising vertically will increase by the web span's weight as it rises from the bottom to the top of a vertical span (or drop the same amount in falling vertical spans). For a 10-mil thick polyester rising 20 ft, the tension change will be only 0.12 PLI — hardly worth talking about.

  • Bagginess and Misalignment

    If either of these imperfections drives the web to slackness — look out. In a 50-in. span, just 0.040 in. of excess length or misalignment will allow the web to deviate from the web's plane by 1-in. That's less than a 0.1% error. Close spacing of rollers is required to prevent contact when passing through slots, a tunnel, or to avoid other non-moving elements.

    Span length debate so far: Short spans are expensive; extremely short spans are hazardous; and long spans lead to flutter and out-of-plane problems. Next month we'll cover traction, tension, guiding, wrinkling, and spreading.

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.


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