- April 01, 2008, By Timothy J. Walker Contributing Editor
Webs are blind — they don't know where they are going. With a well-aligned roller, sufficient tension, and good traction, the web has a tendency to track down the centerline of a machine. However, web and machine are not perfect and many processes occasionally lose tension or traction, so centerline tracking can't be assumed. Enter the automatic web guide, the seeing-eye dog of the converting industry.
All converting processes have some left-right tolerance of web position. The first goal in web guiding is to get the unwinding roll aligned to the roller or centerline of the machine. This usually can be done manually by eyeballing where the unwinding roll is placed on the unwind chuck or shaft.
After hitting the first roller, the left-right tolerance of web position may get more challenging. Does the web align to the coating head? To trim knives? To the laminator? To the winder core? To the other layers in a winding roll? If you worry about your blind and imperfect web going astray, select an automatic guide to meet your needs.
There are three main points where guiding is needed:
- at unwinding;
- at an intermediate point in your process;
- at winding.
There are four options for automatic web guides:
A. unwind sidelay;
D. rewind sidelay.
For each guiding need, there are at least two good options to meet your needs. For unwind guiding, first consider E, none of the above.
If your input roll alignment is good and you manually align to a centerline reference mark, there is no need for an automatic unwind guide. However, if you have telescoped input rolls or want to start your process with accurate lateral position, choose either A or B. Both sidelay unwinding and displacement guides are nearly foolproof options. Once set up properly, it is easy to forget these guides are even there because they usually perform their function without fail.
An unwind sidelay guide has the simplest web path but requires a larger actuator to move the unwind and unwinder. A displacement guide immediately downstream of the unwind adds slightly to the web path but guides with a smaller actuator and without shifting the unwinding roll or unwinder. For intermediate guiding, the first choice is a displacement guide.
Steering guides should be reserved for one application: to guide a long span, such as the web exiting an oven. Steering guides are a tempting choice, since their design appears to take up less space than a displacement guide, requiring only one moving roller. But don't do it. Steering guides, by far, are the source of many more guiding headaches than the other options.
Steering guides with short entry spans are wrinkle generators. Steering guides need to be designed carefully for the correct spans, wrap angles, traction, and pivot radius or they will fail to perform as anticipated. For long spans, they are the best. If you have a short span, consider yourself warned.
For winder guiding, choose either a displacement guide immediately upstream of the winder or sidelay. A winder sidelay guide isn't really a web guide but a web chaser or winder guide, placing your winder in front of the web wherever it may wander, much like a baseball catcher chases wild pitches.
The best winder guiding is a sixth option: slitting. No automatic web guide will ever beat the exactness of the web's edge position immediately after slitting. When the side of a wound roll looks like a phonograph record, it is almost certainly from winding immediately after slitting.
The right automatic guide installed properly is a wonderful thing. It is a tireless, faithful, often forgotten key to your operation.
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; firstname.lastname@example.org; www.webhandling.com.