Goldilocks and the Three Bears Deal with Traction Problems

Rollers are the hands of web handling — the hands we use to grip the web to meet our goals for web speed, position, and flatness. To be able to position the web, we need to “get a grip,” creating the frictional force between the web and each roller. How much grip or traction do we need?

There's a famous children's story about a girl named Goldilocks and three bears. As the girl explores the vacant bears' home, everything seems to fall into one of three categories: “too much,” “too little,” or “just right.” I'd like you to apply this same approach to determining the optimal “grip” for your web line.

Before we can talk about too much or too little grip, we need to review the factors that determine total web-to-roller traction. A good first estimate of web-to-roller traction is:

Friction = Tension × Wrap Angle × Friction Coefficient

In this equation, friction and tension are in units of force (lbf, N, or kgf), wrap angle is in radians, and coefficient of friction (COF) is dimensionless.

It can be surprising there are no width or radius factors in this equation. The normal force is found by multiplying web pressure by roller wrap area. Increasing area by radius or width is offset by a proportional decrease in pressure (P=T/RW), canceling each other out.

This is not to say width and radius are not important to web traction optimization. Friction will increase with width, since tension typically is set proportional to web width. Radius can be an important variable, especially with high-speed processes where air lubrication increases proportional to radius.

The four main factors that determine total traction are tension per unit width, width, wrap, or COF. We could adjust any of these in our effort to find “just right” grip; however, width usually is fixed for the given product and tension may be fixed for a process requirement. This leaves wrap angle and COF as the main optimizing variables to adjust in our goal of finding a grip that is “just right.”

The most common sign of “too little” grip is web scratching, from an idler roller not turning at web speed or from a driven roller unable to isolate a zone-to-zone tension differential. Too little grip also will be observed as poor tracking, guiding, or wound roll alignment. Increasing traction variables prevents these defects (such as more wrap angle or more tension).

How will we know when we have “too much” traction? The top hazard associated with “too much” grip is web wrinkling. High traction by itself does not cause wrinkles, but enough friction to hold the web in a buckled form is a prerequisite for wrinkling. Wrinkles first appear on high-wrap angle, high-friction rollers.

Also, wider, higher-friction webs are more prone to wrinkles. Reducing wrap angles and lowering web-to-roller friction often can prevent wrinkles. However, if traction is lowered too far, we can be back to the “too little” traction problems.

“Just right” traction is between “too little” and “too much,” between web slip and web wrinkling. Sometimes this window is wide open, especially for wrinkle-insensitive materials. Other times, this window seems to be closed. Reducing roller drag and inertial torques will lower the “too little” traction criteria. Optimizing spans, tension, and product design will open up the “too much” traction wrinkle sensitivity.

By understanding the optimized window between these two extremes we, too, can enjoy the pleasures of “just right.”

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 314/323-6256; tjwalker@tjwa.com; tjwa.com


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