- June 30, 2004, Timothy J. Walker TJWalker & Assoc. Inc.
Steering and displacement guides are the two most popular forms of intermediate guiding.
Steering guides appear simpler, since they are often a single actuating roller. Don't be fooled. Thinking a single-roller steering guide is simpler than a two-roller displacement guide is like thinking a unicycle is easier to ride than a bicycle. Simple in design does not always mean simple in use.
Steering guides are web benders. If the web is off target laterally, the steering guide will shift a roller to bring the web back to the desired position. Steering guides use the parallel entry principle, which I've covered in many of my past columns.
If you want to apply the “unicycle of web guiding,” let's see if we can keep you from crashing with a few practical guidelines.
Use steering guides only for long-span corrections. When a steering roller is angled out of parallelism, it will displace the web laterally proportional to the misalignment angle multiplied by the upstream span length.
To create a significant lateral shift, a steering roller must have either a large misalignment angle or a long multiplying span length. (A steering span is considered long when the distance from the steering roller to the upstream roller is more than three web widths in length.) Attempting to bend a short span with a steering guide creates several problems, including lateral slippage, wrinkling, and tension variations.
Good traction is critical to good steering. Web bending isn't free. Just like bending a beam, we need to apply a lateral force to the web. The web bending force comes from web-to-roller traction (a function of traction coefficient, wrap angle, and tension). Remember, air lubrication can reduce traction, so strongly consider a rough or textured roller for your steering guide.
Long-span steering requires less traction. A shorter span is stiffer and requires more force to bend, so a short-span steering guide will fail from traction loss sooner.
Short-span steering often will create wrinkles. For most rollers, we take great effort to ensure parallelism, since we know misaligned rollers create large crossweb tension variations and wrinkles. To keep your steering guide from being a wrinkle factory, use a long entry span.
Install steering rollers with a 90-deg wrap with the pivot plane perpendicular to the exit span. We want to steer the web in the entry span, not the exit span. These wrap and pivot plan guidelines will set up the steering guide to bend upstream and twist downstream of the steering roller, creating the least post-guide error.
Occasionally, steering guides are installed with two pivoting rollers and an exit span parallel to the pivot plane. This almost always leads to trouble. The exit span bending will create wrinkles (if the span is short) or post-guide error (if the span is long).
Avoid under- or over-steering by tuning your steering guide to the span geometry. A steering guide roller's actuation includes both lateral translation and misaligning rotation. The translation carries the web to the new position as fast as the actuator will move.
The rotation is coordinated with the translation to make the web happy in the new position (satisfying the parallel entry principle). The wrong ratio of translation to rotation will cause a poor dynamic response.
Test your steering guide for under- or over-steering. This is a hard to explain without a picture but easy to test for. Draw a line where the web runs on your steering roller. In manual mode, shift the roller 1 in. to the left. If the web moves more than 1 in., you are over-steering; less than 1 in. is under-steering. Adjust the pivoting geometry until the web and roller move together.
With a little more space, I'd tell you about the benefits of good traction on the roller upstream of a steering roller, that steering guides are great as the first roller at the exit of an air flotation oven, and that in most cases you'd be better off using a displacement guide, but that may be too much backseat advice.
Contact me if you'd like to learn more, I'll try not to steer you wrong.
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 404/373-3771; email@example.com; tjwa.com.