Bowed rolls are effective but relatively expensive web spreading devices. A bowed roll is a flexible roll of consistent diameter that is mounted on a central shaft by way of a series of bearings or sleeves along the shaft. When a curvature is induced into the shaft, the result is a bow in the roll face. Bowed rolls remove wrinkles or spread the web because the web will attempt to orient itself at a right angle along the face of the roll, thus pulling each side of the web in different directions. Also, as the web wraps the roll, it has to travel a longer distance at the center and is therefore tighter. Bowed rolls are commonly used to separate two webs just after slitting. Bowed rolls are very aggressive and may cause stretching or distortion in many webs. Furthermore, bowed rollers are very high friction devices and for many web materials, if web driven, may cause more problems than they solve. These rolls often require the installation of separate drive to compensate for their high friction.

Back to Top

Expanding surface rollers (rubber band rolls) use polymer bands that expand and contract when the roll is turned. The bands are attached to cams on each end of the roll so that during half of each rotation of the roll the bands are expanding and the other half they are contracting. As the web travels over the roll, it is riding on the polymer bands while they are expanding, thus the web is encouraged to expand as well. The expanding roll is an effective web spreader but like the bowed roller, it is expensive and a high friction device that may require a separate drive. Furthermore, this roll has more moving parts that will require maintenance or replacement.

Back to Top

Rubber spreader rollers are idler rolls covered with a very soft polymer and grooved with angled, outward spiraling grooves cut into the surface of the covering. Conventional wisdom is that the force of web tension causes the soft polymer to deflect downward and outward, thus spreading the web. The main problem with this theory is that as the web exits the roll, the tension induced compression force is reduced to zero and the rubber lands will go right back to where they started. It remains debatable whether this flexing action actually induces spreading or if the deflection of the covering simply causes an angle on the roller's surface creating the spreading action. Nevertheless, the rubber spreader remains one of the most favorite, cost effective anti wrinkle or web spreading devices.

Back to Top

Grooved metal rolls are also often applied as wrinkle removal devices. Because there is no scientific reason that these should be effective, Webex engineers do not actively recommend the use of grooved metal rollers. However, we have many users in real-world applications that swear by the effectiveness of grooved metal rolls as wrinkle removal devices. We believe that these rolls may appear to be effective anti wrinkle rolls for a variety of reasons.

1. The grooves will alleviate entrapped air, which may be a source of wrinkles.
2. The grooves may simply be allowing the web to flatten out to its full width.
3. The roll and the web may be in a slight state of slip where the grooves may have a minor plowing affect on the web.
4. The installation of a precision grooved roll may simply be replacing a roll of inferior geometry, thus correcting for misalignment in the process.

Back to Top

Although widely applied and very effective, the reverse taper roll is the least understood of all spreader rolls. The reverse taper (RT) is also referred to as a concave roll, reverse crown or bow tie roll. The reverse taper roll has a larger diameter at the outer edges and a smaller diameter at the roll centerline. The roll, as a result of its profile, creates a surface speed differential at each different roll diameter. The resulting tension differential, or higher edge stress, ultimately results in the material climbing outward toward the larger diameter edges of the roll. To be effective the web and the roll must be in a state of traction and therefore, web tension and wrap angle are very important components to the effectiveness of reverse taper rolls. The amount of taper or crown depends on the application but generally is .0005" to .001" per inch, diametrical taper from the roll centerline to the edge. Reverse taper rolls are the definitely the most cost effective web spreading devices. They have an advantage in that, even if they do not achieve the desired action, they can still be effectively applied anywhere in the line as general idler rollers.

Back to Top