1. Field of the Invention
The present invention pertains to advancing material of indeterminate length through an arcuate guide on a cushion of pressurized air. The material is floatingly advanced through the guide without flutter or vibration.
2. Description of the Prior Art
The invention pertains to contactless turning guides for a running web and for guiding and controlling a running web through a change of direction. These turning guides are also commonly referred to as air rolls or air turns and are formed as a partial drum-like member having an arcuately curved surface which can be variable as to it transverse length and also variable as to the length of its arc. These prior art turning guides have a transverse nozzle slot at both the beginning and end of the arcuate path for supplying pressurized air to the air cushion located between the slots over the arcuately curved surface. Prior art contactless turning guides for running webs are disclosed in U.S. Pat. No. 4,182,472 issued on Jan. 8, 1980 and U.S. Pat. No. 4,282,998 issued on Aug. 11, 1981, both to Peekna. Other prior art contactless turning guides are shown in U.S. Pat. No. 4,197,972 which issued Apr. 15, 1980 to Robert A. Daane; U.S. Pat. No. 4,938,404 which issued Jul. 3, 1990 to Daane and Randall Helms, and U.S. Pat. No. 4,925,077 which issued to Daane and Helms on May 15, 1990.
As the web of indeterminate length travels over and beyond the leading or trailing edges of the arcuate surface in these prior art devices, air is spilled back off the arcuate path in a direction generally opposite that of the pressurized air emerging from the nozzle slots. The velocity of the air spilling back off the arcuate path decreases in velocity as it moves away from the edge of the arcuate path and causes an increase in pressure on the web at the edge of and slightly beyond the edge of the arcuate surface. The air is thus destabilized and tends to push the web away from the arcuately curved surface and from its intended path beyond the edge of the arcuate surface having generally the same radius as that surface. This destabilization may cause the web to begin vibrating at high frequency in the air turn which may cause the web to tear and fail. This problem is exacerbated in air turns approaching 90.degree. and with decreasing web tension.