In Part 5 of the Web Guiding Systems Series we discussed Guide Accuracy and Response in Roll Winding Systems
Figure 6 – Web Paths and Traction Changes
One of the most troublesome of the edge position excursions is known variously as the acceleration or ramp offset. Here, the web moves sideways when machine speed is increased or decreased. Usually the rate of movement is most severe at the top and bottom of the speed change rather than on the speed ramp itself. Unfortunately, the term “acceleration offset” belies the true nature of the cause of the edge movement. Indeed, the web does not even know how fast it is moving. The cause of most acceleration offsets is illustrated by the various web tracks through a machine given in Figure 6. Here we show how the web moves through a machine where one of the rollers is misaligned. In the case of absolute traction, the web will conform to the Normal Entry law on all rollers, including the one that is misaligned. Note how the web moves over as the result of this misalignment. In the case of pure flotation, however, the web is not steered by the ‘roller’ and, thus, passes straight through the machine. The case of sliding is intermediate in that there is a small offsetting of the web. Thus, every roller or element that touches the web also steers the web. However, if the roller is stationary and the state of traction constant, the path of the web will remain constant. That is not to imply straight. Obviously the web will snake through the machine in conformance to web handling laws. What we are saying is that the path will remain, for the most part, consistent. However, if the web changes from full to partial tracking the path of the web will change slightly in response. This change in traction will be subtle and not easily picked up by conventional observations and measurements. Nonetheless, it will cause the web to move in response. While there are many ways the state of traction can change, the most common is due to a tension change on a lightly wrapped roller. This means that if our drive allows tension variations, the web might shift slightly on some of the rollers. Furthermore, the condition which is most difficult to hold tensions is during a speed change. Thus, as we not see, the acceleration offset is not due to the speed or speed change itself, but rather due to tension variations that can and will accompany the speed change. Our first efforts should then be to tune the drive so that tension is held well at sensors (load cells) as well as elsewhere where there are no sensors. Sometimes tension is held well only at the drive points or sensors, but not elsewhere because the web might be pulling against excessive roller inertia or drag. However, we can also expect to reduce the severity of the offset if we reduce roller misalignment or other geometrical problems. The surest way to do this is through optical alignment of every roller in the line because even the lowly idler is as capable of shifting the web as any of major process rollers. However, sometimes it is not the roller, but rather an airfloat oven, that is steering the web. Here, the first step is to balance the ovens by clearances and airflow measurements. However, the web will be the ultimate judge of balance as its position should not vary objectionably during tension changes or even dryer engagement/disengagement (for those that can open). Finally, you want to keep your web as uniform, flat, and baggy free as possible. This is because the web with profile troubles will merely exaggerate the difficulties discussed here. Only when material and machine are made true will edges run consistent.
Written for Faustel by David R. Roisum, Ph.D.