PFFC - November 2008 - (Page 12)

WEB LINES Shifty Answers to Nip-Induced Tracking N By Timothy J. Walker Contributing Editor ipping rollers are used in many valueload than the other, that side pulls the web faster adding processes in web converting— than the other side, creating more tension on the roll coating, laminating, embossing, high nip load side. calendering, high-speed winding, and u A2 | The high tension side torques the entering more. Imperfect nips, where the load is uneven, can web span and may cause it to bend away from create process variations, but they also may induce the high tension side. The web’s first response a lateral shifting of your web, leading to misalignis to move away from the high nip load side. ment or wrinkling. Before I can tell you about how uneven left-right nip loads will induce lateral tracking of your web, I In my experience, must get on my anti-nip soapbox. The nip that isn’t there will not shift your web (or wrinkle it). In my experience, nips and webs are a volatile combination (baggy webs, in particular, and all are a webs are baggy to some degree). Nips should be avoided whenever possible. Nips are required for combination. many processes (listed above), but nips are the last choice for creating friction. Please choose large wrap angles on high-friction rollers or vacuum-suction rollers over nips if you u A3 | The web bending from the tension need simple high friction. (Okay, I’m getting off variations presents a web at an angle that violates the parallel entry rule, so if there is good my anti-nip soapbox now.) traction, the web will spiral back toward the high How does a nip roller induce lateral shifts in nip load side, similar to how webs with good your web? If you have left-right nip load variations, tracking will track to the large diameter side of a higher on one side than the other, which way will tapered roller. the web go? Do we know the answer? u A4 | Traction? You bet. This is a nip. Nips Some work completed by the Web Handling usually have an abundance of traction. Putting Research Center (WHRC) at Oklahoma State it all together, in a controlled experiment, the Univ. sheds some light on these questions. The nipweb will move to the high load side of a set of induced lateral tracking phenomenon needs to be nipping rollers. assessed in several steps. nips & webs volatile How does the nip create cross-web tension variations? u Q2 | Which way will the torque, created by the tension variations, bend the web? u Q3 | Will the web bending create a curvature as it enters the nip roller, violating the parallel entry rule (see “Web Lines,” August 2003, p20)? u Q4 | Does the nip have sufficient traction to bend the web to obey the parallel entry rule? u Q1 | u Bonus question | Will the web always track to the high load side of a nipped roller system? u Answer | Not necessarily. A slipping nip, such as a nip grabbing an oiled metal web, may shift the web away from the high load side. In the most predictable setup, the WHRC experiments showed, for a rubber-rubber nip, the web will track to the high nip load side. In these tests, the answers to the four questions above are as follows: u A1 | The COMING NEXT MONTH rubber in the nip speeds up proportional to the nip load. If one side of a nip roller set has more Real-world nips include many complications to the simplified WHRC experiments. One or both rollers may be steel. High nip loads may create web-rubber slippage when the nip’s rubber cover stretches many times more than a stiff web can. If the web wraps either roller prior to the nip point, what effects will friction and deflection have? What if there is a big tension change across DRIVING NIP the nip that induces slippage? ROLLERS As usual, questions about web shifting may get answers that are equally shifty. 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; tjwalker@tjwa.com; www.webhandling.com. 12 | NOVEMBER 2008 WWW.PFFC-ONLINE.COM http://www.webhandling.com http://WWW.PFFC-ONLINE.COM

Table of Contents Feed for the Digital Edition of PFFC - November 2008

PFFC - November 2008 Contents First Glance Calendar of Events From the Editor Web Lines Process Management News Clips Narrow Web & Label Reporter Narrow Web & Label: What’s New Products TLMI Awards Card Making Web Coating Ink Dispensing Lab/Pilot/Technical Facilities Directory Testing & Measuring Products What’s New Products Services Directory Classified Marketplace Advertisers Index Experience Speaks

PFFC - November 2008

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