PFFC - October 2007 - (Page 14)

WEB LINES B By Timothy J. Walker Contributing Editor In Search of Tension Isolation igfoot, the Loch Ness Monster, the Yeti. of the web entering the zone plus or minus the These are but a few of the world’s myths. change in strain in the zone. If a web enters a zone In web handling, one of the myths told controlled at either end by driven rollers in a 1:1 most frequently is that of the independent ratio, the tension zone doesn’t alter the strain of the tension zone, protected from other tension zones by web, and the tension in the zone is the same as the the protective powers of the tension isolation. Like entering web. most myths, we want to believe, but alas, scientific If the tension zone has a positive or negative reason is the myth buster. speed ratio (or draw), it would increase or decrease What do people imagine when they use the the tension, respectively. But the tension in any term “tension isolation”? It is the belief that two zone is not independent of the upstream tension, tension zones are independent of each other if the since tensioning of elastic webs always will start friction of the web on the “isolating” driven roller with what is the upstream tension and go up or is greater than the tension differential between the down from there. two zones. The myth of tension isolation is busted when I totally agree with the goal that each pull roller the upstream tension is altered, and no matter should have traction greater than the anticipated how high the pull roller friction may be, this new tension differential. It’s good to avoid pull roller slip baseline tension will enter the tension zone as the since slipping pacers lead web moves through to unknown web speed, the system. and slipping followers lead This concept is to uncontrolled tensions. known as strain Though the sufficient drive transport. The strain roller friction prevents slip, of the web is a property it is incorrect to say the that moves with the non-slipping pull roller web as it enters and provides tension isolation exits a tension zone. where the tension in one For draw controlled zone is totally independent processes, this of another. concept of strain The myth of tension transport is critical to isolation stems from our understanding your understanding of statics. process (see “Drawing Imagine a large brick Conclusions,” my sitting on a table with two columns of May and ropes tied to it. The brick’s weight and the COF of June 2005). In closed-loop or torque-controlled the two materials create an available frictional force zones, the system will adjust quickly for any between the brick and the table. upstream tension changes, making it seem as if To slide the brick, you have to pull on one rope tension isolation is true, but any monitor of process with enough force to overcome this friction. If speeds would show how upstream tensions feed you pull on both ropes in opposing directions, the through the system. frictional force will isolate the tensions of the two With the tension isolation myth busted, the ropes as long as the rope tension differential list of usual suspects for some defects, is lower than the available friction. such as coating variations, must grow The myth-buster lies in the to include anything that changes difference between static and upstream tension. dynamic cases. In a web line, Since strain travels in the web’s the moving web carries tension direction, tension isolation is nearly information with it as it passes from true in protecting a zone from DIFFERENTIAL one tension zone to another, even downstream tensions, but I think I saw WINDING LIMITS a National Enquirer story on how it’s all without slippage. Part 1 The tension in any zone is controlled by aliens held in Area 51, so determined by the strain or stretch I’m not sure what to believe now. In a web line, the moving web carries tension information with it as it passes from one tension zone to another. COMING NEXT MONTH Web handling expert Tim Walker, president of TJWalker+Assoc., has 20+ years of experience in web processes, education, development, and production problem solving. Contact him at 651/686-5400; tjwalker@tjwa.com; www.webhandling.com. 14 | OCTOBER 2007 WWW.PFFC-ONLINE.COM http://www.webhandling.com http://WWW.PFFC-ONLINE.COM

Table of Contents Feed for the Digital Edition of PFFC - October 2007

PFFC - October 2007 Contents First Glance From the Editor Calendar of Events Web Lines Process Management Package Converting News Clips Narrow Web & Label Reporter Narrow Web & Label What’s New Products Bagmaking Slitting/Rewinding Digital Printing EB Flexo Web Handling Paperboard Packaging Security Packaging Business Transactions What’s New Products Focus: Unwinds & Rewinds Trade Show in Print Services Directory Classified Marketplace Equipment Buyers & Sellers Advertisers Index thePLACE Experience Speaks

PFFC - October 2007

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