- August 01, 2003, Timothy J. Walker, TJWalker & Assoc. Inc.
Parallel flow usually is a good thing. In driving your car down the road, if you and the other drivers agree to keep parallel to the lines in the road, things go smoothly. If you have a small error in the angle of your front tires relative to the road, you will begin to track laterally, increasing your error with every revolution. If you observe your error, you can shift your tire angle in the opposite direction and track back to your lane.
From this we learn a good rule of driving: Keep your tires turning parallel to the direction you wish to travel. Parallel tracking is a good thing.
What causes problems with parallel tracking in driving? The first condition may seem obvious, but the car must be moving. The lateral shift of parallel tracking requires the tires to rotate to move the car relative to the road.
Second, to move the car in a new direction requires external force acting on the car. The force required to overcome momentum comes from the traction between the tires and the road. Traction forces are available proportional to the car's weight and tire-to-road traction coefficient. If the traction is too low, such as under icy conditions, the tires may point in a new direction, but the force required to redirect the car is unavailable.
In web lines, parallel flow still is a good thing. In tracking a web through a web line, if we keep the web parallel to the machine centerline, things go smoothly. If there is a small error in the angle of a roller relative to the web angle, every revolution will attempt to track the web laterally. If we observe this error, we can realign our rollers or install an automatic web guide. In either case, we would redirect the web back to the intended lateral position.
From this we learn a good rule of web tracking: Keep your rollers turning parallel to the machine centerline. Parallel tracking is a good thing.
Why does the moving web tend toward parallel flow? The web reacts to the direction of the roller's surface. Imagine pouring water onto a spinning roller. Which direction will the water fly off? It will fly off parallel to the direction of the roller's rotation, perpendicular to the axis of rotation. The force vectors of a roller always will be parallel to its rotation.
How does a web interact with a spinning roller? You can learn a little about web-roller tracking while sitting at your desk. Take a tape dispenser and a pencil. Pull out some tape from the roll without cutting it off, and start winding the leading edge around your pencil. Rotate your pencil, winding on the pencil “core” while the tape dispenser provides back tension. If you hold the pencil perpendicular (a.k.a. normal) to the dispenser, you should be winding a relatively well-aligned roll of tape on your pencil.
(Note: This effect commonly is described as normal entry to the roller's axis of rotation. However, as we apply this rule to other situations, I believe there will be less confusion if we describe the web's entry as parallel to direction of the roller's surface motion.)
What happens if you tilt the pencil away from perpendicular entry by a small degree? As you continue to wind, you will notice the web will track laterally, attempting to align parallel to the pencil's rotation.
As with driving, parallel flow in web handling depends on both motion and traction. If you stop rotating your pencil, nothing much happens. Traction is important since redirecting with the parallel entry rule bends the web.
Bending a web, like bending a cantilevered beam, requires an external force to create this mechanical deformation. If a roller doesn't have enough grip on the web, it will not be able to exert the force needed to bend the web to parallel entry.
How much force is required for the web to obey the parallel entry rule? I'll try to answer that question next month as we continue to go with the parallel flow of web and rollers.
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