Bowed Rollers for Web Converting
Curved Axis (Bowed) Spreader Roller?
Description:
A Curved Axis (Bowed) Roller spreader roller is manufactured exactly as the name states – the center axle of the roller is bowed (not linear). A series of internal ball bearings, supported by the axle, in turn support a rubber sleeve which is continuous across the face. The amount of curve (bow) in the roller face is application dependent and is available in both an adjustable and non-adjustable version.
Theory of operation:
This roller works based on two different spreading principals.
First, as we know, a web will seek to align itself perpendicular to this roller (as long as it is in traction with the roller). This causes the web to spread at any given interval across its width as it tries to maintain the 90° tangent points across the web width.
Second, the rubber sleeve, which spans the roller, is actually narrower on the entry side than the exit side, so as it rotates, the rubber sleeve stretches and the material, laying on top of the rubber surface, stretches with it.
Because this roller normally operates best with minimal wrap angle, the first spreading principal does most of the work. Less surface contact between the web and the rubber sleeve means less stretching principal will effect overall spreading. However, the spreading based on web handling principal is hardly affected by wrap angle.
Recommended wrap angle is usually less than 30°. Many times, when it appears a curved axis (bowed) roller is not operating properly, this is due to wrap angles to great for the application. More important than wrap angle, lead-in and lead-out distances are critical. Typical lead-in (entry span) to the curved axis (bowed) roller should be approximately double the lead-out distance (exit span). Most of the spreading caused by this roller is done in the entry span to this roller, so the longer the entry span is, the more spreading will be realized. The exit span should be kept to a minimum so the web stays spread. The longer the exit span is, the more chance there is for the spreading affect to be lost (or the web may return to its wrinkled or natural state).
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Curved Axis (Bow) Bar (stationary, does not rotate)
Description:
A Curved Axis (bow) bar spreader bar can be manufactured from steel or aluminum. Many times this spreading device is nothing more than a pipe, curved, using a pipe bender.
Theory of operation:
This spreading device works differently than a curved axis (bowed) roller. The web handling principals that effect web behavior for a curved axis (bowed) roller are not the same as the effects of a curved axis (bow) bar. The web must slip over the face of this bar because it is stationary, it does not rotate. Therefore, the web is not in traction with the bar so the web handling principal that affects the web where a bowed roller is used does not apply here. Instead, the curved axis (bow) bar works based on a tension differential generated by the raised center of the bar. The center, being raised, has the highest tension. The tension decreases toward the bar ends. This tension differential causes wrinkles to be pushed out from the center of the web. The web is attempting to flow through the path of least resistance, which is closest toward the ends of the bar.
Recommended wrap angle for this spreading device can be from 15° to 90°. Most applications require minimal wrap. More wrap angle usually means more potential web distortion.
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Expanding surface anti-wrinkle and spreader rollers in the next installment of this newsletter!!! In two weeks - don't miss it!