- January 01, 2011, By Timothy J. Walker Contributing Editor
What can reverse the direction of your web without touching it? What can redirect a web's running angle or inclination with the tension losses of roller drag or inertia?
What can turn your web process centerline at a right angle? How can you take a running web and flip the bottom side up while maintaining the same process center line?
How can you unwind and wind horizontally, yet run your process with the web running on end vertically? How can you measure the tension in a web — even the crossweb tension variations — without touching it?
There are two answers to these questions.
- Travel to an alternate universe and break the traditional laws of physics.
- Use an air turn, also known as an air turn bar or air reverser.
An air turn is a cylindrical element with either a slotted, perforated, or porous surface combined with an air supply. In design, the air turn has much in common with air hockey tables and hovercraft.
All three supply a constant velocity of air to create enough cushion pressure over a working area to offset a load. The air hockey table lifts the light plastic puck; the hovercraft lifts the ship and its cargo; and an air turn offsets the pressure of web tension over a radius.
The pressure needed to offset tension is easy to calculate; it's the tension in force per width divided by the air turn's effective radius (P=T/R). If you are running a web at 1 PLI (lb/in.) over a 10-in. dia air turn, the pressure under the web will be 0.2 psi (1 PLI/5-in. radius).
You can see that this is a low pressure that should be fairly easy to create with blowing air. From this equation, you can see that it will take more pressure to offset higher tension or air turns with a smaller radius.
In designing air turns, there are two approaches:
- Partial lubrication for reduced friction contact
- Full lubrication to a target non-contacting float height
The air hockey table is an example of a partially lubricating design. The goal is low friction, but occasional contact between the puck and table will happen.
I've seen many partially lubricated air turns — what I considered an air-greased bar. They are usually smaller in diameter (less than 6 in.) and feature a sparse hole pattern (e.g., ⅛-in. hole for every 4 sq in.), and the air supply is via a constant speed fan (or more expensively, supplied with plant air!). These work great for many cases, but don't expect them to handle your optical film scratch-free or float a PSA product sticky side in.
A fully lubricated air turn requires a bit more engineering. Besides calculating the cushion pressure to offset tension, a fully lubricated air turn will consider the following:
- The height the web will float above the curved surface without contact
- Supplying sufficient volume of air to make up for the escape of air from the entry, exit, and side leakage
- Performing the first two functions with acceptable insensitivity to crossweb tension variations from misalignment or web bagginess
- Accounting for the diameter and float height in the elevation change with the entry to exit rollers
- Supplying the air from an independent blower and motor system
Using a 180-deg wrapped air turn will reverse the web's direction from north to south, east to west, or up to down without the contact, drag, and inertia of a roller. By far the most common application of air reversing air turns is in drying ovens, in which an air turn can reduce the footprint of an oven and avoid the problems of a hot roller.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; email@example.com; www.webhandling.com.