The Signs of Shear Wrinkle
- Published: August 01, 2004, By by Timothy J. Walker TJWalker & Assoc. Inc.
Your web is talking to you. Not out loud but in a form of sign language. It's saying, “Oh, I'm shifting to the right (or left) in this span,” or “Ouch, I'm shifting so much I'm wrinkling on this roller.”
What does the sign language of the web look like? It's not spoken with finger and hand gestures but with troughs and wrinkles. Since troughs and wrinkles aren't standardized terms, a quick review of their definitions will help me translate the web's sign language.
A trough is the buckled, out-of-plane web in a span between rollers. Troughs usually are spaced in wavelengths of 1-3 in. crossweb, running mostly in the machine direction but may be angled off 5-20 deg. Some people refer to troughs as soft wrinkles, since they usually don't leave a permanent mark in the web.
A wrinkle is formed when a web buckles while wrapping a roller. Wrinkles usually form one or two at a time with the cross-sectional shape of a bunched-up inchworm. They may hold a steady position on a roller or form on one side and travel laterally like a moving wave. Wrinkles commonly will leave a permanent mark in the web (known as a crease or hard wrinkle) or may even create small rips in paper webs. Wrinkles come in many forms (I covered tracking-type wrinkles earlier this year, see May '04, p26).
Of the many wrinkle causes, shear stress wrinkles are the most studied, due to their widespread waste and the ease of creating them in a lab.
Shearing probably creates images of cutting hair or slitting webs, but it also should make you think about troughing, buckled webs, and wrinkles.
The term “shear wrinkles” implies a shearing or scissoring action. The shearing action of scissors or shear knives develops when one blade pushes down while the other pushes up. Unlike compressive stresses, the up and down forces don't oppose each other directly but are exerted along different planes of the material. The unfortunate matter caught in the middle will be sheared.
Shear wrinkles are caused by web bending and the resulting stresses, whether the bending is created by misaligned rollers, one-side diameter variations, or a lateral shift from an abrupt web guide.
Getting back to translating the web's sign language, let's read the sign language of a single sheet of paper and scale that up to translating our moving web's signs.
Place a sheet of paper on your desktop running away from you (imagine the web centerline runs from 6:00 to 12:00 on an imaginary clock face. Holding the near side of the web fixed, slide the far side slightly toward 1:00. What happens? With even the slightest shift toward 1:00, you should see troughs and buckles forming in the sheet. These troughs will run from 7:00 to 1:00. If you slide the web back, the troughs should go away. If you slide the far web edge in the other direction, toward 11:00, you will see troughs form with an orientation of 5:00 to 11:00.
To create a shear wrinkle in a moving web, twist a roller out of alignment in the plane of a web span. A little misalignment will create a small amount of bending and shear, creating subtle troughing. More misalignment will create larger troughs. With even more misalignment and web bending, the buckled, trough-shaped web will travel over the downstream roller, forming a wrinkle and crease defect. Note: A clockwise misalignment in the downstream roller will create a clockwise troughing or creasing.
The combination of conditions that creates a wrinkle (tension, span length and width, traction, and web thickness, modulus, and uniformity) is a complex engineering problem, but it is predictable with advanced web wrinkling models.
If you see clockwise-angled troughs or wrinkles in your process, understand the web is telling you, “Oh, I'm shifting to the right in this span.” Learn these signs. By combining these web signals with an understanding of what shifts the web, you can determine the correct course of action to correct the problem. The web's sign language will not provide the quantitative feedback of lasers, levels, Pi tapes, or transits, but these signs should be your first diagnostic signal when facing shear wrinkles.
With a little practice, you should see more of the best web signal, “Ah, that's better; I'm right on track.”
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; This email address is being protected from spambots. You need JavaScript enabled to view it.; tjwa.com
