PFFC - January 2009 - (Page 12)

WEB LINES How Much Misalignment Is Trouble? ost converting equipment suppliers and manufacturers take care to ensure the rollers of their machines are aligned. Designing for good alignment, machining and assembling to close tolerances, measuring to ensure alignment, and periodically rechecking roller parallelism all cost money, so we ought to think about what we are getting for our money. Some products gain more from precise alignment, while other products quite likely are insensitive to misalignment. How do we know what the alignment specification goal should be for a specific product? My general rule of thumb on converting equipment alignment calls for targeting a parallelism of 2 mils/ft. This converts to about 0.2 mils/in. or mm/m, 0.01 degrees, or 0.7 seconds of a degree. It’s a small angle, usually imperceptible to the naked eye. You will find equipment aligned to this specification is achievable, measurable, and will eliminate unwanted roller-induced wrinkles, lateral shifting, and edge flutter in most webs. What problems will occur if roller alignment is only accurate to 10 mils/ft? Three big problems result from roller misalignment: M age tension and strain. Since each span has two rollers that potentially can be misaligned, the critical alignment of any one roller is equal to the strain of the web. By Timothy J. Walker Contributing Editor Whether a roller misalignment the short side of the web is span to go dependent on the strain or stretch in the web. causes slack Low modulus materials, such as most nonwoven and softer films, will stretch more that 1% under web tension, so a roller needs greater misalignment to relax this elongation out of the web to a point where looseness would occur. A 1% misalignment in a 2-ft span is about 0.25-in. or 50 mils/ft for a 60-in. web width. u Lateral shifting; Higher modulus materials (such as polyester films, u Loose edges; many papers, and all foils) will have a much lower u Shear wrinkles. tolerance for how much misalignment causes edge looseness and associated problems of out-of-plane In my previous column of April 2004, “When sagging or flutter. A 2-ft span of 1-mil polyester tenRollers Fight, Webs Lose,” I provided more details sioned at 1 PLI (lbs/in. of width) is stretched about on the definitions of level and tram and gave some 0.2% or 100 mils. In longer spans, it will take more examples of lateral shifts from misalignment. (Find misalignment to create a slack edge. this column on the web at www.pffc-online.com/ This absolute misalignment to induce a slack mag/paper_rollers_fight_webs/index.html). edge is independent of width, but viewed on a mils In short spans, the tracking from roller misalign- per foot basis, wider products are more sensitive. As ment will be as imperceptible as the roller misalign- web width reaches 50–100 in., the 100-mil error ment. In long spans, a misalignment of 10 mils/ft converts to 25 or 10 mils/ft. will shift by 7–10 mils/ft of web span length. In a In aluminum foil, elongation from tension is 10-ft span, the web will shift 70–100 mils, a nolower yet, 0.01%. A 2-ft span needs only 5 mils of ticeable but not earth-shattering amount. misalignment to loosen an edge, just 1 mil/ft for a Whether a roller misalignment causes the 60-in. wide product. Unless you are running short side of the web span to go slack is aluminum foil or extremely wide, it looks dependent on the strain or stretch in like 10 mils/ft should be just fine for the web. To drive an edge of the web many processes. into slackness, the misalignment of Next month we’ll continue to two rollers in a web span needs to be drill down into the complexity of two times the stretch in the web. what is an unacceptable roller misROLLER In this case, the tension in the cenalignment. We’ll provide more real MISALIGNMENT terline of the web will not change, but numbers of how much misalignment Part 2 one edge will lose all its tension and the will wrinkle a web based on stiffness, other edge will stretch to double the averwidth, and span lengths. COMING NEXT MONTH 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 | JANUARY 2009 WWW.PFFC-ONLINE.COM http://www.pffc-online.com/mag/paper_rollers_fight_webs/index.html http://www.pffc-online.com/mag/paper_rollers_fight_webs/index.html http://www.webhandling.com http://WWW.PFFC-ONLINE.COM

Table of Contents Feed for the Digital Edition of PFFC - January 2009

PFFC - January 2009 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 In-Mold Labels Cover: Ink Trends Economic Outlook Brand Security Show Preview What’s New Products Services Directory Classified Marketplace Equipment Buyers & Sellers Advertisers Index Experience Speaks

PFFC - January 2009

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.