Webs of material (including but not limited to tissue paper, towel paper, other papers, board, plastics, and polymers) are transported through spans that typically have web stabilizers, such as shown in U.S. Pat. No. 4,321,107. The webs move at a relatively high speed through the spans and across the stabilizers.
Stabilizers traditionally have a generally flat or planar surface against which the web moves as the web traverses a span. The stabilizer is positioned adjacent the web such that the web is a short distance from the flat surface of the web. The web moves at a high speed, such as 4,000 to 7,000 feet per minute (1,200 to 2,100 meters per minute). The movement of the web induces air flows on both the top and bottom sides of the web. The air flow tends to move at the same speed as the web.
The web may flutter due to disturbances in the air flows on either or both sides of the web. Disturbances may be caused by the laminar air stream immediately adjacent the web, e.g. the air flow boundary layer, to separate from the web such that a disturbed airflow is adjacent the web.
A web stabilizer having a surface immediately adjacent the web reduces the tendency of the web to flutter. U.S. Pat. Nos. 4,321,107 and 4,906,333 disclose examples of web stabilizers. As the web moves across the surface of the stabilizer, the stabilizer provides a physical barrier to web flutter in the direction the stabilizer and tends to smooth the air flow between the stabilizer and web. By smoothing the air flow, a laminar boundary layer air flow may be maintained adjacent the web, which reduces flutter of the web.
A difficulty with conventional stabilizers is that the web tends to fall away from the surface of the stabilizer, especially if the surface is long in the direction of web travel and the web travels below the stabilizer. Bump bars have been added to the leading edges of stabilizers to reduce flutter. A bump bar is a pipe or bar (circular in cross-section) welded to the leading edge of the stabilizer and extending below (in the direction of the web) the stabilizer such that the web first moves over the bar before moving over the bottom surface of the stabilizer.
Another approach to overcome the difficulty of web flutter below a stabilizer is to inject a high velocity air stream in the gap between the stabilizer surface and web, such as disclosed in U.S. Pat. No. 6,325,896. The high velocity air reduces the air pressure between the web and stabilizer. The reduced air pressure draws the web towards the stabilizer. However, injecting a high velocity air stream requires an air supply, air ducts and air jets or slots, which increase the cost to make and operate a stabilizer. Further, the air injection nozzles and slots are subject to clogging.
Another approach is to shape the stabilizer as an airfoil such that a low pressure is formed between the stabilizer and the web, as disclosed in U.S. Pat. No. 6,325,896. However, an airfoil shaped stabilizer, that is long relative to the direction of web travel, has difficulty in reducing flutter in the downstream region of the stabilizer. There is a need for web stabilizers that suppress web flutter over long stabilizer surfaces, have low manufacturing and operating costs, and are not susceptible to clogging of air injection nozzles and slots.