- September 01, 2001, William E. Hawkins, Film Handling Solutions Ltd.
Many wide-belt processes, whether they are made of metal, fabric, or plastic, may experience tracking problems. Some will have tracking problems even though guiding equipment is present.
Usually, there are three primary reasons for wide-belt tracking problems: the presence of nonaligned rollers; the presence of non-uniform stresses when the belts are elongated; and belts that are constructed with diagonal seams.
Careful attention to alignment of rollers with 90-180 deg of wrap is critical for good tracking when belts are running under high tension and tracking friction with the belt surface is high. Self-centering rollers, rollers with flat surfaces in the center and tapered surfaces at the edges, do not work well on wider belts. The figure below is an example of a narrow flat belt that has no guide rollers except one with a flat surface and one with tapered edge surfaces. When there is sufficient span between these end rollers, the belt will oscillate across the belt centerline of the roller with the tapered surfaces and will therefore transmit power satisfactorily for many processes without additional guiding.
However, belts that are used for conveying materials in a programmed manner do not have the liberty of oscillating very far from the centerline. This kind of belt must be guided by an end roller that pivots in the plane of the belt about a point upstream of the pivoting roller. An edge sensor is needed to detect the belt variance from the centerline. The variance data allows the pivoting roller control panel to pivot the roller so the belt will track back to the desired position. The sensitivity of the system depends on how much variance may be tolerated.
Non-uniform internal stresses can prevent a belt from elongating uniformly as it is put under load, and the belt may experience greater narrowing on the tighter edge. The tracking vectors of the roller and the belt resistance tension members at the belt/roller interface do not line up parallel to the machine centerline in this case. The belt will tend to track off the machine centerline toward the side that is less tight.
In most cases, the guiding equipment described above is useful for steering belts with non-uniform internal stresses.
Wide metal belts with welded diagonal seams often experience poor tracking when they are put under load. This is especially true if the seam is thicker than the rest of the belt material or if the belt does not elongate as much in the seams under tension as the rest of the belt material. When one span of the belt is tighter than the opposite span, there is a certain amount of creep or slip between the belt and both roller surfaces. Slip or creep is relative motion. Torque is produced in the plane of the belt in the creep zones of each end roller by the relative motion and is not opposed by the rollers. This torque tends to move the belt away from the machine centerline.
Sometimes the guiding equipment described above will not stop the tracking migration toward the edge limits, especially when the belt is under high tension. Also, physical (barrier) guides usually will not stop this kind of belt movement. And attempting to track the belt by angling the other support rollers generally does not work.
One solution is to cut the diagonal seam out of the belt and install a section so the belt has two seams. Another solution, more risky but possibly more practical, is to cut the belt on an opposing diagonal through the original seam, so when the new cut is welded together, the new and old seam form an X at the exact center of the belt.
William E. Hawkins has 30-plus years of process and equipment development in web handling, including experience on all types of converting equipment. He specializes in thin web applications. Contact him at 740/474-5840; fax: 740/474-3148; e-mail: firstname.lastname@example.org.