A Slippery Answer to Web Scratching

What percent slippage in idler roller speed relative to web speed is required to create scratches?

Answer #1: Zero percent slip.

Scratches are a form of abrasive wear. Abrasive wear occurs any time two surfaces are pressed together and then slid relative to each other. Each material will give up something.

The softer material, usually the web, will have the more obvious loss of material. Over time, the web abrading the rollers, especially rubber-covered rollers, also will be apparent. Scratching is promoted by surface irregularities and debris — both acting as stress concentrators — increasing the local abrasion event (the scratch).

For scratch-sensitive products, every effort should be made to ensure idler rollers are not slipping. I advise scratch-sensitive converters to implement an idler roller maintenance plan.

Recently I completed a project for a client in which we analyzed the performance of more than 300 idlers on a coater. Using the “spin-down” test and timing the deceleration rate of a freely spinning roller, we could find the “bad actors” and calculate a Traction Safety Factor (TSF), an indicator of how close each roller is to slipping.

The TSF is the ratio of driving torque (wrap*tension*COF*radius) to drag torque (bearing drag torque and inertial torque). The result: Bad idlers are repaired, and marginally driven rollers are modified with more wrap, higher friction, or better bearings. Also, an idler roller minimum performance is identified to ensure new rollers meet an appropriate standard. This approach nearly can eliminate scratch defects.

For answer #2, we need to look at things on a micro-scale.

A tensioned web is elongated a small amount (web strain = tensile stress/modulus). As the web goes around a driven pull roller, moving from one tension to another, the web's strain will change. The moving web will contact the roller with the strain proportional to the upstream tension. But the web strain needs to make a transition to the downstream tension, and it does this while on the roller. For many webs, the change in strain from tension extremes is less than 1%, but the roller does not move with the web, creating a slip and abrasion condition.

The belt equation (a.k.a. the band-brake or capstan equation) defines the critical wrap angle where the strain transition will begin. During the entire critical angle wrap on the downstream side of the roller, there will be “micro-slip.” Does this cause scratching? On a microscopic scale, this abrasion probably is detectable, but to the naked eye, this “micro-slip” is not a problem.

Micro-slip happens on every roller, not just tension-isolating pull rollers. The drag of driving an idler creates a small tension change in the web, so there always is a small web strain change in the final wrap of the roller and micro-slip.

Answer #2: Slip less than web strain is okay.

Lastly, when surfaces in an engine or bearing need to slip, but wear is undesirable, what do we do? We lubricate them.

You can't pour oil on every roller (unless you're in the steel industry), but high-speed web handling has a natural lubricant — air. It is possible to have massive slip, even a stopped roller, and not scratch the web. Air lubrication is promoted by large diameter, high speeds, low tensions, and smooth surfaces (both roller and web).

Answer #3: Up to 100% slip may be okay (if properly lubricated).

In summary, my slippery answers to acceptable roller slip — zero, a percent less than web strain, and 100% slip — all may be okay. Those are my answers and I'm sticking to them. I hope I haven't left you scratching your head on this one.


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 or at tjwalker@tjwa.com. Visit tjwa.com



Subscribe to PFFC's EClips Newsletter