PFFC - January 2008 - (Page 12) WEB LINES Get a Grip: Driving Your Web I By Timothy J. Walker Contributing Editor n my October 2007 column, “In Search of Unnipped rollers | Simple, usually with a larger Tension Isolation,” I myth-busted the belief wrap angle and high traction surface (rubber with that a high traction driven roller can coma roughness or groove pattern). These are my first pletely isolate one tension zone from another. choice, since they are highly tolerant of baggy However, the subtle effect of web transport should webs. The belt equation defines their traction canot sway you from designing all driven rollers with pacity, stating that the tension ratio (not absolute traction sufficient to support your apparent or acdifferential) must be less than the natural log e to tual tension differential between tension zones. the power of the wrap angle (in radians) times the If your driven pacer roller slips, you lose control of coefficient of traction. Unnipped rollers aren’t helpline speed. If your driven follower rollers slip, you lose ful when input or output tension goes to zero, since tension control. As if losing speed and tension control that is an unsupportable infinite tension ratio. isn’t bad enough, slipping rollers of any kind lead to Vacuum-assisted unnipped rollers (or belts) | loss of lateral control and A great option for driving scratched webs. with limited wrap angle. Most roll-to-roll web A high traction surface, processes have at least one like rubber, gives vacuum driven roller, commonly rollers especially strong called the pacer or master traction. Surprisingly, roller, that controls process these rollers can parspeed. A basic web line tially air lubricate at high of unwind, pacer, and speeds if they are too smooth and the grooves winder creates two tension or holes are too far apart. zones: one controlled by Nipped rollers | the unwind and one by the My third choice, since winder. If the unwinding they are notorious for and winding tensions are wrinkling baggy webs. about the same, it doesn’t Nipping rollers should be require much pacer roller larger than other rollers traction to keep the web to minimize deflectionfrom slipping and to mainassociated problems. tain speed control and tenThey do provide a relision separation. able traction capacity As processes get more directly proportional to complicated, additional intermediate driven rollers may be required to allow nipping force and traction coefficient, and they are good at preventing air lubrication. each process step to have its own unique optimized Tenters (or stenters) | Edge-only gripping tension. As a web process gets more rollers or moves systems using clips or needles. These usually are to lower tensions, more driven rollers may be added to reduce the percentage of tension lost to roller drag reserved for film or textile heated processes. Each driven section should have sufficient tracand inertial torques. Each added driven roller creates tion capacity to handle the apparent or actual another tension zone and another point where tractension differential. In many applications, tion capacity needs to be greater than the apactual tension differentials are unknown parent or actual tension differential. either because tension isn’t measured Many coating lines have four tension or is measured so many rollers away zones controlling unwinding tension, from the driven roller that the actutension into coating, tension in the dryal tension differential varies greatly ing process, and winding tension, refrom the measured valued due to spectively. The four zones are separated SPECIFYING drag and inertial losses or additions. by (see how I avoid the verb “isolated” EQUIPMENT In any case, get a grip and choose an here) three intermediate driven sections. option that will have more than the tracThe options for driving a web betion capacity you think you will need. tween tension zones include the following: additional intermediate driven rollers may be required to allow each process step to have its own unique optimized tension. As processes get more complicated, 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. 12 | JANUARY 2008 WWW.PFFC-ONLINE.COM http://www.webhandling.com http://WWW.PFFC-ONLINE.COM
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