- May 31, 2006, Timothy J. Walker, TJWalker & Assoc. Inc.
Why float a web? If you can float your web over a stationary plate or cylinder, you’ve got something simpler than a roller. But if floating your web requires a complex air flow control system, you’re getting into something not just more complex than a roller but more expensive, noisier, and less stable. So again, why float a web?
Because air floating can do things a roller can’t. The most common reason to float a web is to change its direction without touching it, such as turning the wet or sticky side in a drying or curing oven. The next most common reason is to make a right-angle turn of your machine centerline. You also can stabilize your web from flutter with air or induce lateral shape stiffening to prevent curl or wrinkles in long spans.
A right-angle turn bar usually is a perforated cylinder set at a 45-deg angle to the machine centerline. The web enters the turn bar, wraps the cylinder through 180 deg, and exits on a parallel plane, one diameter higher or lower than it entered, now traveling at a right angle from whence it came.
To visualize this, fold a strip of paper to form a right-angle “L” and stick your pen in at the 45-deg fold point. Right-angle turns are used to flip a web (using two turn bars), to make a U-turn in your process, or to bring a web in or out from a winder set perpendicular to a line’s main centerline. (Don’t try this with a roller. Webs don’t like to travel helically around rollers.) The problem with a nonlubricated turn bar is the tension increase from friction. In some applications, you can use natural lubrication, something you get when the entrained air by the moving web and roller is greater than the roughness or textures of their surfaces (see “Web Lines,” February 2006).
To float your web, do the opposite of all my nonlubricating advice: Think smooth, large radius, and low tension. In web handling, usually we calculate air lubrication on a spinning roller, but the web alone can entrain enough air to float on a nonmoving cylinder or curved plate.
If self-lubricating isn’t enough air, you can add more with a forced air system. Simply pump some air into a perforated cylinder or plate, run the web over it, and immediately you will notice a drop in sliding friction. When the air flow is small, I like to call these air-greased surfaces. The intention is to reduce friction, but air-greased surfaces still may have some contact and are not appropriate for a scratch-sensitive product.
If you want to ensure noncontact, you must work a little harder. To establish a controlled float height, think about the web’s opposing tension and air leakage. The pressure to float a web is equal to the tension (in force per width) divided by the radius of curvature. Several calculations later—considering the air escape velocity, total flow, supply pressure, pressure drop—you will have an engineered, guaranteed-to-almost-never-contact air turn.
To reduce out-of-plane web flutter, mount a flat plate parallel to a web span. Why does this work? Put two plates of glass together, then try to pull them apart. It’s not necessarily adhesion fighting you; it’s the low-pressure vacuum you create by trying to expand the low volume of air between the glass plates. This same effect inhibits web motion from a close proximity plate.
The Coanda effect is another form of air float stabilizing. If you blow air parallel to the web, it will create pressure that opposes the web if it tries to move perpendicular to the air flow. Air foil nozzles use the Coanda effect.
Lastly, you can induce crossweb stiffness with air. Staggered air nozzles in an over-under-over configuration commonly are used in long, contact-free spans of air floatation dryers. The air-induced shape increases lateral stiffness, preventing the web from curling or wrinkling.
Except for nonvacuum processes, your web is going to interact with the air around it. Use these tricks, and float your web to your advantage.
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; email@example.com; tjwa.com.