Converting Rx for Bowed Rollers

Web Lines

In pharmaceuticals, certain drugs are controlled substances: They are available to the public but require a doctor’s prescription and strict instructions for proper use. In converting, there are some solutions that should be controlled, available to anyone but requiring detailed instructions for use. At the top of my list of converting controlled substances: bowed rollers.

For those not familiar with bowed rollers, they are a uniform-diameter but bow-shaped roller. The outside of a bowed roller is a rubber sleeve, internally supported by a series of narrow idler rollers mounted on a bent shaft.

For some thicker web applications, the rubber sleeve is left out, but only if the web is stiff enough not to fall into the gaps between segments. Many bowed rollers have a fixed bow (the depth of the chord), but some are adjustable, using a split shaft that can be dialed in from straight to slightly bowed to strongly bowed.

Why should bowed rollers require a prescription? Bowed rollers are strong medicine and thus should be prescribed for specific maladies and used only as directed. Also like strong medicine, it takes only a small amount to have a big effect. Thinking more is better will lead to the bowed roller equivalent of an overdose.

Let’s read the “Directions for Use” fine print on the bowed roller bottle.

1. Bowed rollers have two active spreading mechanisms. The first and stronger one attempts to displace the web laterally, so all entering lanes follow the parallel entry principle (see “Going With the Parallel Flow,” August 2003). The second, during the contact with the bowed roller, is from the expanding rubber surface attempting to pull the web out as it rotates on the expanding side of its rotation.

2. Bowed rollers require good web-to-roller traction to pull out or bend the web to a new lateral position. Webs with high lateral stiffness (length, width, thickness, modulus, tension) will need more traction forces to redirect the web. The total web expansion will fall short of theoretical spreading if the traction forces are too low. Air lubrication should be avoided (for cases with high speed, low tension, large radius, smooth roller, smooth web, nonporous web) by increasing roughness or grooving the bowed roller.

3. Bowed rollers can be used to spread a single web to tautness or create a gap between a set of slit strands. See special instruction for slit strand spreading (coming next month).

4. For most applications, bowed rollers should be wrapped between 30 and 60 deg, always wrapping the roller on the expanding side of its rotation. For a web path neutral setup, the wrap angle should be centered between the most contracted and most expanded positions on the bowed roller.

5. A bowed roller can tighten up symmetrical web bagginess, whether baggy edges or a baggy center. If the wrap is shifted toward the most contracted side (also considered a “nose” down wrap), the bowed roller will tighten the web edges. If the wrap is more toward the fully expanded side (considered a “nose” up wrap), the bowed roller will tighten a baggy center.

Like any prescription, the directions for use need to be followed by a list of known side effects:

1. Bowed rollers, like most spreading rollers, can increase lateral web shifting or destabilize the web, especially if the web is off-center.

2. Excessive bowing, which is smaller than you think, will lead to web-to-roller slip and bowed roller wear. Most bowed rollers wear more quickly than other rubber-covered rollers.

3. Bowed rollers have a local anti-wrinkle benefit but little or no downstream effects (except for not passing on a wrinkled web or creating a gap between slit strands).

4. Bowed rollers can exert large stresses on the web, enough to yield or break delicate webs.

5. Bowed rollers will create higher tension losses than most standard idler rollers due to the rubber hysteresis and internal bearing drag.

This concludes the side of the bottle instructions for the bowed roller “patient.” Next month, we talk with bowed roller prescribing “doctors” to help them determine what dosage will be right for a particular application.



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 651/686-5400; jwalker@tjwa.com; tjwa.com.


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