Concave Rollers Pros & Cons

Web Lines

I’m always amazed when a wrinkle problem can be solved with a few feet of masking tape. Got some nasty wrinkles on a roller? Some tape here, some tape there—wrinkles be gone. Adding tape collars or bands to a cylindrical roller just under each of the web’s edges turns a cylindrical roller into a wrinkle-stopping concave roller.

Concave rollers go by several names but all feature a changing roller diameter, larger at the edges than the center. The opposite of a concave roller is a crowned roller; thus concave rollers also are known as reverse-crowned rollers. Depending on the exact diameter profile, concave rollers may have an hourglass, bowtie, dumbbell, or parabolic profile.

The mechanism: The web tracking over the larger diameters lanes will have increased strain and tension. The laterally different tensions induce a moment (or torque) on the web. The moment creates a slight inward bend in the web. The web’s bent shape conflicts with the parallel entry rule (see “Going With the Parallel Flow,” August 2003).

The result: a slight lateral displacement, enough to remove the wrinkle.

You can change a standard cylindrical roller into a concave roller quickly by adding tape collars to either side. Use a tape that has reasonably good friction relative to your product. Look for a tape that has some roughness if you would like this trick to work at high speeds and low tensions. Apply the tape to build up the roller diameter on a percent basis, targeting a buildup equal to 1–3x the stretch of your product.

If you need to build up the tape more than one–two layers, apply the tape in a tapering stepped profile. Too severe of a step from the tape collar to the untaped roller will create a high shear and a wrinkle-inducing roller profile.

If this trick is so great, why isn’t everyone doing it? Good question. Let’s review the pros and cons of concave rollers.

Pro #1: Concave rollers are inexpensive. Just pennies of masking tape can save thousands of dollars in waste. Even machining a concave profile into a metal roller is inexpensive compared to other anti-wrinkle roller options.

Pro #2: Concave rollers are low drag and can be low inertia. Other anti-wrinkle rollers have significant drag from rubber-stretching hysteresis and higher bearing drag. Concave rollers should have the same low mass shell and low-drag bearings as your other idlers.

Pro #3: Concave rollers can work at high speeds. It is easy to add air-lubrication-preventing texture to a concave roller. Concave rollers are not dependent on rubber deformation or rubber sleeve stiffness that may have changing or detrimental effects as line speed increases.

Pro #4: Concave rollers can have a hard-coated metal surface to resist wear or harsh chemicals.

Pro # 5: Concave rollers are a speedy solution. It can take as little as 2 min to have a taped concave roller up and running.

Con #1: Concave rollers are difficult to design to be effective with big width changes. The most effective concave roller profiles create a big torque on the web by shifting the web tension to the extreme edges. However, this effect may disappear or is greatly reduced when a narrow web runs on the same roller.

Con #2: Concave rollers will create wrinkles if they slip. Concave rollers need to have enough of a combination of tension, traction coefficient, and wrap angle to prevent slip and support diameter-induced tension differential from upstream to downstream sides.

Con #3: Concave rollers made from tape may cause contamination. If you look at tape collars that have been on too long, you’ll see missing pieces that have picked off and gone downstream, possibly all the way to your customer.

Con #4: Concave rollers are less effective for low-modulus materials and pretty much ineffective for viscoelastic webs. The concave roller mechanism requires the web to act like a solid beam, creating a torque that induces a bending in the web. A viscoelastic web will flow instead of curving, defeating this mechanism. Low-modulus webs may need a more dramatic spreading mechanism to induce enough movement into the web to be effective.

When you look at the low cost, speed, and reduced application restrictions, I think the pros of concave rollers outweigh the cons.



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.


To read more of Timothy J. Walker’s Web Lines columns, visit our Web Lines Archives.


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