Editor's Note: This is last in a series of articles on web guiding. Part 1 covered understanding web-guide accuracy. Part 2 explained how to properly size actuators. Part 3 described how to find the right web-guide sensor.

If converting was like a good night of poker, every input roll would be flush, every web would be straight, and every two rollers would be a parallel pair. But in the real world, whether Texas Hold 'Em or converting, you will be dealt many bad hands for every nice one. Rolls are telescoped or offset, webs are cambered or baggy, and rollers are out of alignment or deflecting. These imperfections will all contribute to off-tracking of the web.

A little off-tracking is okay, but too much lateral shifting can create big problems. Automatic web guides are the Ace up your sleeve that you need to turn a bad hand into a winner.

To find a winning hand in web guiding, there are three easy steps. One—determine where you need lateral position accuracy. Two—choose which web guide will best meet your needs at each guiding location. Three—Install the web guides in each location following the application guidelines. Once you decided which type of guides you will use in your process, the next step is to ensure each is installed properly.

There are four types of web lateral-position control systems (sometimes called web guides or edge position control/EPC): unwind sidelay guide, displacement guide, steering guide and winder sidelay guide. Three of these are truly web-guiding systems (unwind sidelay, displacement, and steering), redirecting the web to the correct lateral position. The last option, winder sidelay, doesn't actually guide the web, but instead moves the equipment to meet or chase the web.

The correct web guide, set up properly, at the right location will give you a winning hand every run for every product. Keep reading to understand how to determine the right type of guide by process location and some rules to set up each type of guide. But don't worry. You don't have to play all these cards by yourself. You can trust that the professionals who have been working on web guides for years will ensure that any guide you install will give you a winning hand.

Let's break down Steps 1 and 2 into three possible web guide locations: At the input or unwind of your process, at any intermediate location between unwinding and winding, and at the winding process (See Table 1). Next are application guidelines for the four types of web guides along with five basic questions you should answer about each.

The unwind sidelay guide positions the web so the web enters the machine at the desired lateral position. The unwind guide corrects for incorrect placement of the roll of web on the unwind shaft, and roll defects such as telescoped wraps in the roll.

Where should the web sensor be? The sensor is a fixed reference to where you want the web to be. It should be placed immediately downstream of the unwind sidelay guide between the last shifting roller and the first non-shifting roller. As the web unwinds, at least the first roller it contacts should be sidelay shifted with the unwinding roll. This establishes a fixed web path to detect the web that doesn't change as the roll diameter decreases. Also, this roller should have sufficient traction to prevent the shifting span to send any lateral forces back to the unwinding roll.

What are recommended web span lengths? The shifting span is usually 0.5 to 1.0 web widths in length, but since there is no intentional twisting or bending, these spans can be as short as 25 percent of the web's width.

What are the recommended wrap angles? Since sidelay guiding has no significant twisting or bending, there are no critical wrap angles.

How important is good web-to-roller traction? To prevent upsetting the unwinding roll, there should be enough traction on a roller or rollers to isolate the unwinding roll from any tension variations in the correction span.

What other factors are important to this type of web guide? The sidelay guide needs to have sufficient actuator size and good structural rigidity between the actuator and the winding roll to prevent undamped oscillations.

An intermediate displacement guide is used in areas where span lengths are shorter and a long, free span does not exist. Most displacement guides have two rollers, but the location of two additional rollers that determine the entry and exit span are critical to their proper setup.

Where should the web sensor be? The sensor is a fixed reference to where you want the web to be. It should be placed immediately downstream of the displacement guide between the last shifting roller and the first non-shifting roller. Depending on the displacement angle and the sensor jaw gap, the sensor may need to be one-third or more down the exit span to avoid web contact with extreme displacement twisting.

What are recommended web span lengths? The most common layout is to have entry, displacement, and exit spans all greater than one web width. Longer displacement spans reduce the angle required for a given correction. Entry and exit span minimum should avoid excessive high-edge stress and not create slack in the center of the web.

What are the recommended wrap angles? Displacement guides have two general layouts, either a U-shaped web path where the web exits parallel but in the reverse direction to how it enters, or a Z-shaped path where the web exits parallel and continuing in the same direction it entered. In either U or Z layouts, the web enters and exits perpendicular to the displacement plane.

How important is good web-to-roller traction? Displacement guides are relatively insensitive to web-to-roller traction. However, some small variations in guiding, especially relative to the exit span asymmetrical twisting, can be eliminated by using rollers designed to have good traction at all running conditions.

What other factors are important to this type of web guide? The displacement guide should be a twist-displace-twist geometry. The displacement-guide pivot frame should be installed so the pivot point is centered over the entry span. Though displacement is usually completed between two parallel rollers, alternate designs will displace the web over a single larger roller or any number of rollers mounted on the displacement framework. Displacement guides, with proper setups, are relatively trouble-free compared to the other intermediate guiding option—the steering guide.

An intermediate steering guide is used where you need to control the position of a long span. Most steering guides have one roller, but the location of three additional rollers that determine the entry, pre-entry, and exit span are critical to their proper setup.

Where should the web sensor be? See Displacement Guides. Sensor position and minimum exit span rules are the same for steering and displacement guides.

What are recommended web span lengths? There are three web spans important to a steering guide's performance: the entry span, the exit span, and the pre-entry span. Controlling the web in the entry span should be viewed as the primary function of any steering guide. Steering guides should be used only when a displacement guide won't meet your needs.

Steering guides work by bending the web in the entry span. If spans are too short, this creates high stress on one edge and slack web on the other. The minimum entry span equations are used to avoid excessive edge stress or a slack edge based on web strain and the guide's correction range.

The classic application of a steering guide is at the exit of a long, unsupported span in a drying or curing oven. Long spans are flexible and prone to large lateral web shifts from web bagginess, roller misalignments or diameter variations, and external forces, such as oven air flow. A steering guide at the end of a long span will actively control the position of the long oven span, preventing the web from tracking off the roller face or crashing into the oven walls.

The minimum exit span is the same calculation as minimum displacement guide entry and exit spans.

What are the recommended wrap angles? The most stable steering guides have a 90-deg turn between the entry and exit spans. Usually, this is a 90-deg wrap on a single roller, but for cases requiring more traction or other functions, a steering guide may have two or more rollers. The pivot plan should be perpendicular to the exit span, creating a twisting action in the exit span (similar to the exit span of a displacement guide). For 90-deg wrapped steering guides, this will make the pivot plane parallel to the entry span.

How important is good web-to-roller traction? Good traction is critical to steering guides, more critical than for the other guiding options. To be able to bend the web in the entry span, a steering guide needs to apply a lateral force to the web. Tensioned webs are like beams and require force to bend. If there is insufficient force to bend the web, the lateral shift will be less than expected, leading to poor control and unstable web guiding.

What factors are important to this type of web guide? A steering guide has a unique motion combining lateral translation and rotation of the pivoting roller(s). The ideal steering unit is setup up to have a pivoting motion perpendicular to the exit span, parallel to the entry span, with a phantom pivot point with a radial arc length equal to roughly two-thirds into the upstream entry span. If the radial pivot radius is too long or too short, the steering guide will have poor control, either under-steering or over-steering the web.

The pre-entry span length should be shorter than the steering guide's entry span length. Steering guides may become unstable if the entry span is short relative to the pre-entry span. The mechanism of this instability is complex, but involves the entry span's tension variations crossing over the upstream roller and creating a web shift in the pre-entry span.

This instability is easily avoided by following the recommended minimum entry spans, ensuring good traction on the roller upstream of the steering guide, and making the pre-entry span length relatively short. Since steering guides typically have long entry spans, this is rarely a problem but should be noted.

Where should the web sensor be? As a chasing system, the sensor serves a different function that the other three web-guiding options. In chase guiding, the sensor seeks the web and is a reference to where the chasing element should be. For a winder sidelay, the sensor moves with the sidelay actuating winder. For more stable control, the sensor should detect the web upstream of the last stationary roller so the winder actuation doesn't interfere with measuring the incoming web's position.

What are recommended web span lengths? The corrections span, between the last stationary roller and first roller on the actuating winder is usually 0.5 to 1.0 web widths in length, but since there is no intentional twisting or bending, these spans can be as short as 25 percent of the web's width.

What are the recommended wrap angles? Since sidelay guiding has no significant twisting or bending, there are not critical wrap angles.

How important is good web-to-roller traction? To prevent upsetting the winding roll, there should be enough traction on a roller or rollers to isolate the winding roll from any tension variations in the correction span.

What other factors are important to this type of web guide? See Unwind Sidelay Guides.

For an expanded version of this article, go to www.convertingmagazine.com