- April 01, 2010, By Timothy J. Walker Contributing Editor
Where is your web? In the machine direction, hopefully, it is continuously running between your upstream and downstream processes. But where is it laterally?
Last month, we reviewed all the needs for lateral positioning of the web, including requirements ranging from fairly loose (staying on the rollers) to quite tight (winding a roll with the side as nice as a phonograph record).
Why isn't the web in the right lateral position? There are two strong forces that promote centerline tracking. First, tension will tend to pull a bad web straight. Second, the web tracking rule promotes centerline tracking on well aligned rollers in traction with the web (see “Going with the Parallel Flow,” PFFC, August 2003, p20, or visit www.pffc-online.com/web_lines). Even with these two strong effects, our webs wander.
If you've been reading “Web Lines” for some time, you will notice my usual modus operandi of problem solving here. Attempted remedies should be based on at least a guess of what causes a problem.
Having a list of common causes makes this step easier. I group lateral errors in web position into four categories, each with several common causes.
Lateral Errors in Initial Positioning | Many webs start off in the wrong place. The original sin of lateral error is unwinding. Is the web centered on the core? Is the core centered on the unwinder? Is the unwinder centered on the machine centerline? An automatic unwind guide can put much of these concerns to rest.
The next biggest sin is threading. When you thread up a line from scratch, invariably, you put the web slightly off center of where it wants to run. In many paper processes, the transition from threaded to running position will break the web. Avoid this problem by threading with less breakable webs (thicker webs or films) and avoid losing your tensioned, threaded web position at reasonable costs.
Lateral Errors in Web Transport | The five big causes that shift the web are misalignment, diameter variations, web camber/bagginess, uneven nipping, and interaction of the web to air or fluid flow. All of these are minimized by reducing variations.
Align your rollers (to less than 200 micro-radians). Machine rollers to cylindricity. Reject overly baggy or cambered webs, and use tension to pull out the rest. Ensure nip loading is uniform side to side, size nip rollers for low deflection, and approach nip points from a low angle relative to loading or deflecting plane. Ensure fluid flow is perpendicular to the web and return flow is uniformly distributed.
Lateral Errors in Automatic Guiding | Automatic web guides are meant to reduce lateral errors, but like a broken car or drunk driver, they can be the cause of a problem as well. Ensure automatic guides have the appropriate range, gain, rigidity, and correction rate. Get the geometry right, especially in steering-type guides. Ensure edge, center, or line guides can detect your web; are close to the guide they work with; and work to reduce, not increase, the work of downstream guides within the same process.
Lateral Errors in Winding | Before blaming the winding process, always check to see if the source of wound roll shifting is from upstream handling. Winding is susceptible to all the problems of web transport (see above), but also: width recovery from in-roll tension losses, core shifting from equipment misalignment, deflection, or loss of grip between cores and shafts.
Beyond this list, perfect handling and perfect winders will still see shifted layers if care is not taken to control air lubrication (usually prevented with nipped winding) and cinching (see “Belt Tightening Gone Bad,” PFFC, February 2003, p24, and March 2003, p20, or www.pffc-online.com/web_lines).
Resolving or reducing lateral errors has the secondary (maybe primary) benefit of eliminating bending-induced wrinkles. Next month we will address an important aspect of bending, guiding, steering, and wrinkling: How much force is required to bend a web?
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; firstname.lastname@example.org; www.webhandling.com.