The Harms of Harmony

Web Lines

On Bravo network’s "Inside the Actor’s Studio," James Lipton asks each guest, "What sound or noise do you love?" followed by "What sound or noise do you hate?" The "loved" answers include harmonious sounds such as a child’s voice, falling rain, or the ocean waves. The hated sounds are less harmonious, such as a baby crying, traffic noise, or car alarms. The ear and brain enjoy harmony, so it may surprise you when harmony is a bad thing. In many web processes, however, harmony creates waste.

A harmonic is a wavelength or vibration. Web processes are filled with rotating elements. If any rotating element is out of balance or eccentric, it will create a one-per-revolution upset into the web motion or harmonic.

From a simple handling viewpoint, these harmonic motion hiccups may be no problem, but combined with coating, extrusion, slitting, or winding operations, they may lead to waste.

Harmonics in web speed or tension are one source of chatter (crossweb stripes in coating or extrusion thickness). Speed variations are usually a bigger problem than tension variations. For roll coating methods, a roller’s speed is a key variable to control coating thickness. For extrusion coating, precision in coating flow and web speed combine to create thickness uniformity.

For many coating methods, tension variations won’t cause a significant problem until the web slips on drive rollers and fails to hold the line speed setpoint. Some thin film coating methods called free span coating use the web’s tension to create back pressure that controls coating thickness. With free span coating, tension variations directly create coating thickness variations.

If you suspect a web harmonic is responsible for your chatter, what are the top suspects? The harmonic could come from mechanical or electronic components. The problem could be poor tuning of the motor’s speed control loop or motor cogging, but the mechanical suspects usually are to blame.

Assuming we have perfect rotation from motor output, what harmonic trouble can happen on the way to the web? Every mechanical component is suspect, including couplings, drive shafts, pulleys, timing belts, bearings, gearboxes, roller shafts, and roller shells. If any component is misaligned, bent, or eccentric, harmonics are produced.

Reducing speed variations is straightforward—improve or simplify your drive train. Based on harmonic frequency, you can divide the harmonics between the low- and high-speed side of the gearbox. Reduce speed harmonics of poor shaft alignment, shaft deflection, and coupling backlash by minimizing your drive train. Change from a line shaft to individual servo motors. Change from a chain or timing belt to a flat belt. Eliminate your gearbox by changing to an AC motor that can create full torque at low speed.

Reducing tension variations is a little trickier. Now drive and idler rollers both are suspects. Initially, look at the local tension zone, but also look at the immediate upstream tension zones. Don’t be fooled into thinking a high-friction drive roller completely isolates tension upsets from passing from zone to zone. Every tension zone works off the baseline of the incoming web strain. As tension and web strain change upstream, downstream tension zones will respond. Web strain travels with the moving web, independent of how much frictional "isolation" you have.

To diagnose a tension harmonic, do a Fourier analysis on the raw, unfiltered signal from a tension roller. Divide the harmonic wavelengths by line speed to get the effective circumference and diameter of the source. Look for unbalanced rollers, eccentricity, bent shafts, and rollers moving from stick to slip conditions. For high-speed processing, watch for rollers or shafts reaching their critical speed, the rotation rate at which the roller’s mass and deflection hit its natural frequency.

While taming the harmonics of web speed and tension, don’t forget to look at the fluid delivery side of things. Coating or extrusion chatter also can come from uneven flow control and elastic vibrations in fluids. Beyond speed and tension variations, look out for harmonic problems in slitting and web guiding.

What sound or noise do you hate? How about the sound of the plant manager crying over the product waste created by web speed and tension harmonics?



Subscribe to PFFC's EClips Newsletter