Preventing air film between web and roller

In apparatus wherein a web is confined to lengthwise motion in one direction along a defined path and wherein said path has one portion in which the web extends substantially straight and has another portion which begins at the termination of said one portion and in which the web is curved in partial wrapping engagement with a cylindrical surface of a roller that rotates to have the peripheral speed of its said surface match the speed of lengthwise motion of the web, air jet web control means for substantially preventing the intrusion of air between said roller and the web, said web control means comprising a nozzle at the side of the web opposite said roller, to which pressure air is fed and which has an outlet for pressure air that opens towards the web, said web control means being characterized by:

• A. said outlet in the nozzle being elongated transversely to the length of the web and extending substantially across the full width of the web;
• B. said outlet being within a small distance in said web motion direction from the termination of said one portion of said path;
• C. said outlet having a width not substantially greater than is adequate for issuance of air therefrom at a substantially uniform rate all along its length;
• and
• D. said nozzle being spaced from said surface of the roller by a distance large enough to accommodate the thickness of the web and permit air to issue from the outlet and impact the web but small enough to prevent substantial divergence in said web motion direction of the stream of air issuing from the outlet.

A method of ensuring intimate contact between a web that moves lengthwise in one direction along a defined path and a roller with a cylindrical surface around which the web has partial wrapping engagement and which rotates to maintain its said surface at a peripheral speed matching the lengthwise speed of the web, said method being characterized by:

• directing towards the web, from the side thereof opposite the roller, a jet of air which
• A. extends substantially entirely across the web along a line transverse to the length of the web;
• B. impacts the web within a short distance in said direction of web motion from the line of boundary between a straight portion of the web that extends towards the roller and the curved portion of the web that is wrapped around the roller;
• and
• C. impacts the web in a zone which has an extension in said web motion direction that is on the order of a few hundredths of an inch, so as to impose upon the web a pressure gradient in said direction that is high enough to substantially prevent the intrusion of an air film between the web and the roller notwithstanding that the impact pressure of said jet against the web may be relatively low.

In apparatus wherein a web moves lengthwise in one direction along a defined path and wherein said path has a straight portion in which the web extends in said direction towards a rotatable roller and has a curved portion in which the web is in partial wrapping engagement with said surface of the roller, the boundary between said portions of the path being at a line of tangency, nozzle means at the side of the web opposite the roller, having an outlet from which pressurized air is emitted as a jet directed towards the web to prevent intrusion of an air film between the web and said surface of the roller, the outlet of said nozzle means being in the form of a slot elongated transversely to the length of the web and extending entirely across the web, said outlet being characterized by:

• A. a width not substantially greater than is adequate to ensure issuance of pressure air therefrom at a substantially uniform rate all along its length;
• and
• B. a location
• (1) close enough to the web to avoid substantial divergence and dispersion of said jet before it impacts the web and
• (2) within a small distance in said direction of web motion from said line of tangency.