Web Lines: Pitch & Catch Guiding

If you are like me, you find yourself thinking about spring as a way to cope with long winter days. For many, spring means baseball.

A good way to combine your baseball daydreams with your converting operation is to think about web guiding. Unwind and winder guiding systems, often called sidelay guides, are like baseball pitchers and catchers.

An unwind sidelay guide is like a baseball pitcher. A pitcher's goal is to throw a strike, to brush the batter, or to pitch the ball to a target.

An unwind sidelay guide has the same goal: to unwind the web in a lateral position relative to the machine centerline, usually centered on the machine centerline, but sometimes intentionally off-center. For a pitcher, the reference is home plate, a fixed target. For an unwind sidelay guide, the target is a fixed web sensor, either a single edge sensor or a set of web centering sensors.

It is a misnomer to call a rewind sidelay guiding system a web guide, since it doesn't guide the web at all. The motion of a rewind sidelay guide instead chases the web, guiding the winder to the correct position to catch the web.

A rewind sidelay guide is like a baseball catcher. The catcher doesn't care whether the pitch is a strike, inside, or outside. A catcher has to go find the ball where it is and be in position to catch the ball in his glove.

A rewind sidelay guide doesn't care where the web is. It simply detects the web's position and shifts so the web lands on the core in the right position.

The components of unwind and rewind sidelay guiding systems are nearly identical. The motion of sidelay unwind and rewind web guides is similar, a simple left-right sliding on linear rails. Both mount the winder/unwinder on linear slides and shift the unwinding or winding roll laterally with a linear actuator, either a motor or hydraulic cylinder. Both have a web edge or center sensor. The difference between unwind and rewind sidelay guiding is in the sensor motion and location.

An unwind sidelay sensor is stationary, serving as a target to where the web should run versus machine centerline. The sensor should be placed at the upstream end of the span, between the last shifting roller and the first fixed roller.

A rewind sidelay sensor moves with the shifting of the rewinding core. The sensor usually is mounted on an arm that moves with the sidelay platform and detects the web position at the downstream end of the span between the last two fixed rollers. This position isn't always critical, but it attempts to isolate the incoming web's position from the sidelay motion.

Sidelay guides at both unwinds and rewinds are nearly foolproof once installed properly. Unlike the other common web guide (steering and displacement), the motion of sidelay guides does not involve any misalignment of rollers. There is no twisting of the web and bending is minimized. Since twisting and bending are minimized, they don't need long span length in the correction spans (many times less than a web width).

The biggest potential drawbacks to sidelay guides when compared to displacement and steering style guides are inertial or structural limitations. To be able to shift an entire winder and massive roll, sidelay guides will need much larger actuators.

Since sidelay systems typically move an entire winder stand and a massive roll, they inherently will have more backlash and flexure. Even with these potential limitations, sidelay web guides are the key to starting and ending most converting processes on the right track.

I highly recommend you ask your boss if you can take a field trip to study the fundamentals of sidelay guiding. Maybe the peanuts and Cracker Jack can go on your expense account.

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.


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