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(see Truism #3).
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 are usually 0.5 to 1 web widths in length, but since there is no intentional twisting or bending, these spans can be as short as 25% of the web’s width.

What are the recommended wrap angles?
How important is good web-to-roller traction?

Since sidelay guiding has no significant twisting or bending, there are not critical wrap angles. 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 (see Truism #5).

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.

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 (see Truism #3).

The sensor should be no more than one third down the exit span length. Closer is better, but make sure the exit span’s out-of-plane twisting doesn’t cause the web to contact the sensor, especially for sensors with smaller gaps.

What are recommended web span lengths?

The most common layout is to have entry and exit spans of one-half to one web width in length and a displacement span equal to the roller face length. Longer displacement spans reduce the angle required for a given correction length. Entry and exit span minimum should avoid excessive high edge stress and not create slack or zero tension in the center of the web (see Truism #7).

If the web is stiff, the span length equations should be used to calculate the minimum entry and exit span length.

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 layout, the web enters and exits perpendicular to the displacement plane (see Truism #9).

How important is good web-to-roller traction?

Displacement guides require good traction to ensure the web doesn’t slip as the guide translates laterally. 

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 the 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.

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 steering guide between the last shifting roller and the first non-shifting roller.

The sensor should be no more than one third down the exit span length. Closer is better, but make sure the exit span out-of-plane twisting doesn’t cause contact with the sensor, especially for sensors with smaller gaps.

(Sensor position and minimum exit span rules are the same for steering and displacement guides. See Truism #3).

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 only be used where a displacement guide won’t meet your needs.

Steering guides work by laterally bending the web in the entry span (see Truism #2). If spans are too short, this will create 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 correction range (see Truism #8). The web in the exit span of a 90 degree wrap steering guide is twisted, the same as a displacment guide.

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-degree turn between the entry and exit spans. Usually, this is a 90 degree 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 plane 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-degree 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 (see Truism #5). 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 other factors are important to this type of web guide?

The ideal steering unit is set 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.

Unlike displacement guides where the pivot point is fixed in the design phase, a steering guide’s pivot point must be tuned to the installation geometry. 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 have reduced steering effect or even a reverse steering effect if the entry span is short relative to the pre-entry span. The mechanism of how the these span lengths create a reduced or negative effect is complex, but involves the entry span’s tension variations crossing over the upstream roller and creating a reverse direction web shift in the pre-entry span.

This potential problem 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 to prevent this easily avoided problem.

Where should the web sensor be?

As a chasing system, the sensor serves a different function that the other three web guiding options (see Truism #6). 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 web widths in length, but since there is no intentional twisting or bending, these spans can be as short as 25% of the web’s width.

What are the recommended wrap angles?
How important is good web-to-roller traction?

Since sidelay guiding has no significant twisting or bending, there are not critical wrap angles. 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 (see Truism #5). Usually a wrap angle of 15 degrees of more will satisfy this condition, but many combinations of tension, wrap angle, and traction coefficient will eliminate this potential problem.

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.