Get a Grip: Driving Your Web
- Published: January 01, 2008, By By Timothy J. Walker Contributing Editor
In my October 2007 column, “In Search of Tension Isolation,” I myth-busted the belief that a high traction driven roller can completely isolate one tension zone from another. However, the subtle effect of web transport should not sway you from designing all driven rollers with traction sufficient to support your apparent or actual tension differential between tension zones.
If your driven pacer roller slips, you lose control of line speed. If your driven follower rollers slip, you lose tension control. As if losing speed and tension control isn't bad enough, slipping rollers of any kind lead to loss of lateral control and scratched webs.
Most roll-to-roll web processes have at least one driven roller, commonly called the pacer or master roller, that controls process speed. A basic web line of unwind, pacer, and winder creates two tension zones: one controlled by the unwind and one by the winder. If the unwinding and winding tensions are about the same, it doesn't require much pacer roller traction to keep the web from slipping and to maintain speed control and tension separation.
As processes get more complicated, additional intermediate driven rollers may be required to allow each process step to have its own unique optimized tension. As a web process gets more rollers or moves to lower tensions, more driven rollers may be added to reduce the percentage of tension lost to roller drag and inertial torques. Each added driven roller creates another tension zone and another point where traction capacity needs to be greater than the apparent or actual tension differential.
Many coating lines have four tension zones controlling unwinding tension, tension into coating, tension in the drying process, and winding tension, respectively. The four zones are separated by (see how I avoid the verb “isolated” here) three intermediate driven sections.
The options for driving a web between tension zones include the following:
Unnipped rollers
Simple, usually with a larger wrap angle and high traction surface (rubber with a roughness or groove pattern). These are my first choice, since they are highly tolerant of baggy webs. The belt equation defines their traction capacity, stating that the tension ratio (not absolute differential) must be less than the natural log e to the power of the wrap angle (in radians) times the coefficient of traction. Unnipped rollers aren't helpful when input or output tension goes to zero, since that is an unsupportable infinite tension ratio.
Vacuum-assisted unnipped rollers (or belts)
A great option for driving with limited wrap angle. A high traction surface, like rubber, gives vacuum rollers especially strong traction. Surprisingly, these rollers can partially air lubricate at high speeds if they are too smooth and the grooves or holes are too far apart.
Nipped rollers
My third choice, since they are notorious for wrinkling baggy webs. Nipping rollers should be larger than other rollers to minimize deflection-associated problems. They do provide a reliable traction capacity directly proportional to nipping force and traction coefficient, and they are good at preventing air lubrication.
Tenters (or stenters)
Edge-only gripping systems using clips or needles. These usually are reserved for film or textile heated processes.
Each driven section should have sufficient traction capacity to handle the apparent or actual tension differential. In many applications, actual tension differentials are unknown either because tension isn't measured or is measured so many rollers away from the driven roller that the actual tension differential varies greatly from the measured valued due to drag and inertial losses or additions.
In any case, get a grip and choose an option that will have more than the traction capacity you think you will need.
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; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.webhandling.com.
