In Part 1 of this series we introduced web guiding systems in roll-to-roll winding.
Web Steering Guide
Figure 2 – Web Steering Guide
The web steering guide, as shown in Figure 2, is a roller that has ends which are mounted on a pair of angled linear bearings. As the actuator pushes the roller sideways, the raceways cause the guide roller to pivot about an imaginary point called the instant center. The principles of operation are via a controlled misalignment in the plane of the incoming web and the Normal Entry law. In other words, if the roller is angled, the web must bend sideways in order to enter the roller’s axis at a right angle. Since the Normal Entry Law is valid only for traction, any slippage of the web and roller will compromise or destroy guiding.
Web steering guides are common whenever there is a long entry span, such as following an air flotation oven. Indeed, this type of guide requires the entry span length to be at least 3-5 web widths long to minimize in-plane bending stresses at the exit of the upstream roller. Similarly, the exit span should be at least 1/2 web width long to minimize stresses on the edges of the web due to twisting about it’s center line. Finally, the length of the pre-entry span must be shorter than the entry span. A common problem to avoid is to place a spreader or other roller between the oven and the steering guide. This shortens the entering span and makes pre-entering span longer than the entering span.
There are two basic types of guide movement. The easiest is to simply pivot the roller from one end. While this end pivoted is adequate for some endless forming belts, it is not optimum. A better response can be achieved by locating the instant center about 1/2-2/3 of the entry span upstream. This arrangement causes the roller to also move sideways, thus carrying the web with it. The location of the instant center is determined by adjusting the angles of the linear bearings upon which the guide roller ends sit. The location is where the perpendiculars of the two slides intersect.
In Part 3 of the Web Guiding Series we discuss Web Displacement Guides.
Written for Faustel by David R. Roisum, Ph.D.