Small Flexing, Big Benefits

Published: October 31, 2003, By Timothy J. Walker TJWalker & Assoc. Inc.

Spreading and antiwrinkle rollers are great tools for web handlers.

Like any good tool, having it in your toolbox is great, but what makes it valuable is understanding how it works and when to use it.

Over the past three columns, I have been laying out the knowledge a web handler needs to understand spreading rollers. Last month I described the primary mechanism of bowed and expanding-surface rollers. This month let's take on their similarly misunderstood cousin, the flexible spiral roller.

All spiral rollers, whether rigid or flexible, are con artists. Their turning roller's barber pole optical illusion tricks your eye. The rotating spirals, starting at the roller center, create a sense of outward motion. As the spiral turns relative to your fixed perspective, you almost can feel the plowing action. Don't be conned by this illusion.

A wedge spreads only if there is relative motion. A field plow spreads dirt as it's driven through the field. In web handling, the web and roller are not moving relative to each other — they turn together. Saying the wedge of a spiral roller spreads the web is like saying a plow sitting unattended in a cornfield moves dirt as the Earth rotates. If there is no relative motion, a plow's wedge does not spread.

The key mechanism of the flexible spiral roller is not in the wedge-shaped groove but in the subtle deflections of the biased flutes. If you push on the flexible flutes with your thumb, you will notice they deflect laterally. The grooves are machined intentionally at an angle so the tensioned web will deflect the flutes away from the machine centerline.

The force you apply with your thumb is many times the force exerted by the web, so the web-induced deflection will be much smaller, in tenths of mils. How can so little motion have a significant effect?

Last month I described how the dominant spreading mechanism for bowed and expanding-surface rollers is the parallel entry principle. When the roller's surface motion is angled relative to the machine centerline, the roller rotation will displace the web laterally as it rotates. Greater surface vector angle or longer entry span will increase the spreading. For both roller types, the web spreading motion was greater than the roller's surface motion. The same angle surface motion is at the heart of how a flexible spiral roller works.

What determines a flexible spiral roller surface vector? The micro-deflection occurs quickly as the tensioned web contacts the roller. Dividing the lateral deflection by the similarly small length over which it occurs produces a significant angle surface motion, the heart of the spreading effect. This may sound shocking, but the flexible spiral works on the same mechanism as bowed and expander rollers.

Let's put some numbers on this effect. Again I will refer to Ron Swanson's landmark paper from the 1997 International Conference on Web Handling. For the given case, the flexible spiral roller's flutes deflected only 0.2 mils but were able to spread a short span of 10-30 mils, more than 50-150x the mechanical deflection. It wouldn't seem 10-30 mils of spreading would be significant, but compared to a standard idler, this roller was 5x less sensitive to wrinkling from misalignment.

Will the web re-gather on the downstream side of the flexible spiral roller when the tension and flute deflection is removed? No, remember the spreading is 50+ times the mechanical deflection. Without an entry span, the flute's elastic recovery is small compared to the overall spreading. Also, since the web is taut and cylindrical, it will gain shape stiffness to resist this small load on the downstream side.

How much wrap is needed? The surface vector angle is set within a degree or two of contact. Additional wrap is needed to have sufficient traction forces to bend the web. Thin, long-span webs will see benefits with 5-10 deg of wrap. Webs that are more difficult to bend will need more wrap.

A flexible spiral roller is a great web handling tool, especially if you understand the roles flute deflection, entry span, and traction play in its application.