Vacuum box and method of operation for non-contact web transport therein

In a vacuum box of the type including a front wall;

• a back wall, the front and back walls having inner surfaces facing each other;
• a pair of side walls extended between the front and back walls to define a cross section with at least four interior wall corners;
• means for guiding a length of web into and out of one end of the box, said box further having an inlet slot and outlet slot through which said of web is guided, the length having an incoming span, a central curved loop and an outgoing span, said central curved loop having a central portion and transverse outer ends opposite said central portion;
• and means for applying vacuum near an opposite end of the box, the improvement comprising:
• means for substantially equalizing pressure along opposite front and back walls, said equalizing pressure permitting local equalization of pressure on opposite front and rear faces of the incoming and outgoing spans of the web proximate to recesses along the lengths of the spans, whereby more downward force is exerted on the central portion of the central curved loop than on the transverse outer ends of the central portion of the central curved loop which passes over said recesses thereby causing incoming and outgoing spans to remain substantially spaced from and out of contact with the side walls, said means for equalizing comprising a plurality of recesses formed in the inner surface of at least one of the front and back walls, the recesses being spaced at discrete locations along substantially the length of the incoming and outgoing spans near the interior wall corners of the at least one wall;
• and said means for equalizing further comprising a corresponding plurality of bores, one bore extending from each recess into an adjacent side wall and a port extending from each bore into the box to facilitate equalizing the pressure on opposite front and rear faces of webs having a transverse curl toward a centerline of the box.

In a vacuum box of the type including a front wall;

• a back wall, the front and back walls having inner surfaces facing each other;
• a pair of side walls extended between the front and back walls to define a cross section with at least four interior wall corners;
• means for guiding a length of web into and out of one end of the box, said box further having an inlet slot and outlet slot through which said of web is guided, the length having an incoming span, a central curved loop and an outgoing span, said central curved loop having a central portion and transverse outer ends opposite said central portion;
• and means for applying vacuum near an opposite end of the box, the improvement comprising:
• means for substantially equalizing pressure along opposite front and back walls, said equalizing pressure permitting local equalization of pressure on opposite front and rear faces of the incoming and outgoing spans of the web proximate to recesses along the lengths of the spans, whereby more downward force is exerted on the central portion of the central curved loop than on the transverse outer ends of the central portion of the central curved loop which passes over said recesses thereby causing incoming and outgoing spans to remain substantially spaced from and out of contact with the side walls, said means for equalizing comprising a plurality of recesses formed in the inner surface of at least one of the front and back walls, the recesses being spaced at discrete locations along substantially the length of the incoming and outgoing spans near the interior wall corners of the at least one wall;
• and said means for equalizing further comprising a bore extending from each recess through an adjacent side wall to a recess in an opposite front or back wall and a plurality of ports extending from each bore into the box to facilitate equalizing the pressure on opposite front and rear faces of webs having a transverse curl toward a centerline of the box.