Low inertia apparatus for accumulating and applying tension to webs

Abstract: A web (44) passes over an air bar (40) or metering drum, defines a loop (48, 50, 52) within an open space (46) defined between two pneumatic tensioning wall members (14, 16), and passes over an air bar (42) or metering drum. Pressurized air issues through apertures (36) arranged in rows along web control surfaces (32, 34) to establish curtains of air flowing between the surfaces and the web, thereby applying tension to the web due to the Bernoulli and viscous drag effects.

Description: Field of the Invention The invention concerns apparatus and methods for applying tension to moving or stationary webs. More particularly, the invention is related to an improved, low inertia technique for accumulating and applying tension to webs using flowing gas. Background of the Invention In various industrial equipment for making or treating web materials, such as indexing apparatus used to convey and process fragile webs like photographic film, sections of the web in one part of the apparatus may be stopped; others may be moving at essentially constant speed; and still others may be accelerating or decelerating at high rates of speed. In such equipment, there is a need to be able to isolate from one another the sections of the web experiencing different velocities and accelerations. Thus, there is a need to accumulate and release web at high acceleration rates while maintaining a reasonably constant level of tension in the web. Traditional accumulating devices have created a serpentine path for the web using one or more fixed idler rolls and one or more moving idler rolls mounted to a common frame. The distance between the fixed and moving idler rolls is increased or decreased to accumulate or pay out web. These devices however are not suitable for use at high indexing rates of delicate webs because the forces necessary to overcome the rotational and translational inertia of the moving rollers and their support frame must be supplied by the web and can cause unacceptable variations in web tension. Another solution to this problem is the conventional vacuum box accumulator, which has no moving parts other than the web and thus solves the inertia force problems of devices using fixed and moveable idler rolls. A loop of web is suspended in a close-fitting box. When a partial vacuum is applied below the loop, with the loop acting as a seal between atmospheric pressure and the partial vacuum, the differential air pressure across the loop tensions the web. Web thus may be added to or removed from the vacuum box at very high acceleration rates while maintaining a relatively constant tension level. Such vacuum boxes are used extensively in magnetic tape transport systems. The abstract of JP-A-53 057805 published in Patent abstracts of Japan, Vol. 2, No 94 (E-047) of August 5, 1978 describes a tape buffering device. This device comprises an elongated chamber at atmospheric pressure having opposed first and second spaced lateral surfaces. One end of the chamber is under a negative pressure in order to trap a flexible tape inside the chamber. The opposed lateral surfaces are provided with nozzles in order to flow air currents into the chamber so that air bearings between the lateral wall and the tape are provided. Unfortunately, vacuum boxes are not adapted to applications which must accommodate webs of various widths or webs of irregular or varying width. This is because a very close clearance must be maintained between the edges of the web and the inside walls of the vacuum box in order to maintain the partial vacuum. Additionally, there is an associated risk of contact between the face and edges of the web and the walls of the vacuum box, which can scratch the web face or damage the edges. So, a need has existed for a low inertia web tensioner and accumulator which, without disassembly or modification, can readily accommodate changes in width of the web and which minimizes face or edge contact by the web. Summary of the Invention The problem mentioned above is solved by an apparatus according to claim 1. The apparatus comprises principally two facing curtains of high velocity air which issue from small passages or apertures through two opposed control surfaces; said surfaces delimiting a space which is open to ambient pressure. The control surfaces may be continuous, single surfaces or may be comprised of a plurality of smaller surfaces separated by grooves, channels or open spaces. The curtains of air act together with their respective control surfaces to tension web spans on both sides of a free loop of film by virtue of the Bernoulli and viscous drag effects. The curtains are such that they longitudinally and transversely tension the web and hold it in a stable, vibration free and spaced apart relation with the control surfaces. This arrangement allows for accumulating and tensioning of webs, particularly very thin webs requiring low tension levels. There are no moving parts other than the web; so, there are no inertia forces to overcome, other than the small inertia of the web itself The apparatus accepts webs of widely varying widths, thus overcoming the width-specific limitations of vacuum boxes. The apparatus of the invention offer unique, truly non-contact ways to accumulate and tension webs. The air or other fluid used to provide the curtains also may be used for heating, drying, cooling, oxidizing or otherwise treating the web. In one embodiment, the means for directing air flows each comprise a plurality of apertures at acute angles through the respective control surface and means for passing pressurized gas through the apertures. The control surfaces may be elongated parallel to the web, transverse to the web, or at an acute angle to the web. The control surfaces may be broken by axially or transversely extending grooves, or by other patterns of grooves or open channels, for exhausting or venting gas from between the web and the control surfaces. The control surfaces may be comprised from a plurality of smaller, neighboring surfaces having apertures. The neighboring surfaces need not be coplanar. A portion of the apertures may be directed alternately toward opposite edges of the surfaces to apply transverse force to flatten the web toward the surfaces. When exhaust grooves or channels are used, the apertures may be located centrally on the control surfaces between the grooves, or within the grooves. With the apertures located within the grooves, the grooves may each comprise a curved side adjacent the apertures to reduce flow disturbances between said web and said surfaces. The streams of gas may issue from the apertures at angles tangential to the curved side. With reference to a longitudinal axis between the opposite edges of the control surface, a portion of the grooves may be on each side of the axis and the curved side of each groove may be a side of the groove further from the axis. Brief Description of the Drawings The foregoing features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings. Figure 1 illustrates schematically a perspective view of the apparatus of the invention. Figure 2 illustrates schematically a perspective view of the angles of the apertures provided through the surfaces of the plates and the axial grooves for exhausting gas from between the web and the plates. Figure 3 illustrates schematically a perspective view of an alternative geometry for the apertures and grooves. Figure 4 illustrates schematically and fragmentarily a perspective view of yet another embodiment of the invention in which the control surface is comprised of smaller units separated by open longitudinal channels. Figure 5 illustrates schematically a perspective view of a further embodiment of the invention comprised of smaller units separated by open transverse channels. Figure 6 illustrates schematically a plan view of a tensioning plate including chevron-shaped rows of apertures separated by similarly shaped exhaust grooves. Figure 7 illustrates schematically a plan view of a tensioning plate including a rectangular array of smaller control surfaces separated by a rectangular grid of exhaust grooves. Detailed Description of the Invention The following is a detailed description of the preferred embodiments of the invention, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several Figures. One embodiment of an apparatus 10 according to the invention is illustrated in Figure 1. A face plate 12 supports upstream and downstream pneumatic tensioning plates or wall members 14, 16, having longitudinally extending axes 18, 20. Wall member 14 comprises an upper first end 22 and an opposite, lower second end 24; and wall member 16, an upper third end 26 and an opposite, lower fourth end 28. Opposite edges 23, 25 extend between ends 22, 24; and opposite edges 27, 29, between ends 26, 28. Typically, the upper and lower ends are positioned opposite one another, as illustrated. Within each wall member, as indicated fragmentarily in Figure 1, an interior plenum 30 is provided. The wall members comprise respective web control surfaces 32, 34 which extend between the upper and lower ends. The control surfaces face each other and preferably are essentially flat, though a moderate amount of waviness is acceptable. The control surfaces may be arranged vertically or at any convenient angle depending on the weight of the web being tensioned. Also, the control surfaces may be parallel. Plastic webs 152 mm wide and 0.0015 to 0.0064 mm thick and copier paper have been transported successfully at a wide variety of orientations of the control surfaces. The planes of the control surfaces, as extended, may intersect without departing from the scope of my invention. Thus, control surfaces 32, 34 may converge toward or diverge from each other between ends 22, 26 and ends 24, 28. Each of surfaces 32, 34 is pierced by several longitudinally extended, preferably parallel rows or patterns of passages or apertures 36 which extend from interior plenum 30. For ease of illustration, apertures 36 are shown only through surface 32. A source 38 of pressurized gas, such as air, is connected to the plenums 30; so that, flows of air are emitted from the apertures. As shown in Figure 2, the axis of each passage or aperture 36, illustrated by an arrow, extends generally toward the second ends 24, 28 at an acute angle a above the associated web control surface. For example, angle α may be approximately 35 to 40°. As a result, flows of air issuing from apertures 36 have substantial components toward ends 24, 28 and tend to establish curtains of air flowing along the web control surfaces. Conventional cylindrical air bars 40, 42 or conventional metering drums are mounted to face plate 12 just above upper ends 22, 26, respectively, to provide low friction web support and guidance into the apparatus. Very low friction and inertia rollers also could be used. Gas such as air is supplied to the air bars from source 38. Preferably, the planes of the control surfaces, when extended, are approximately tangent to the surfaces of the air bars. A web 44 to be accumulated and tensioned is threaded over air bar 40; into a space 46 between the web control surfaces, space 46 being open at both ends to ambient pressure; and over air bar 42. Thus, an incoming span 48 of the web faces web control surface 32; an outgoing span 50 faces web control surface 34; and a central curved portion 52 of the web joins the two spans to complete a loop between the wall members. As illustrated, wall members 14, 16; the pattern of apertures 36; and air bars 40, 42 all have widths or lengths substantially exceeding the width of web 44. This excess width facilitates threading of the web into the apparatus, enables the apparatus to be used with webs of various widths and even permits simultaneous conveying of two or more webs, side by side. In operation of the apparatus shown in Figure 1, web 44 could move intermittently or continuously. Sometimes, the portions of the web upstream of air bar 40 and downstream of air bar 42 could move continuously; other times, one of them could stop; and other times, both of them could stop, depending on the cycle of operation of the associated web transport equipment, not illustrated. During such movement, the flows of air from apertures 36 establish along web control surfaces 32, 34 curtains of high velocity air which impinge on the outer surfaces of the loop to support the loop out of contact with the web control surfaces due to the Bernoulli effect; and, at least, to apply a downward or axial tension to the incoming and outgoing spans of the web due to the viscous drag effect. Depending on the length of the loop and wall members 14, 16, more or less web can be accumulated in the apparatus. Depending on the amount of viscous drag established by the curtains of fluid such as air or other gas, more or less axial tension can be applied to the web. The lengths of the passages from interior plenum 30 to form apertures 36 should be substantially greater than their diameter in order for the gas streams issuing from the passages to have well-developed flow patterns. Passages 3.81 mm long and 0.46 to 0.71 mm in diameter have been found effective with gas pressure in the range of 1 to 5 kPa, for webs from copier paper to 0.006 mm Mylar. In the embodiments of Figures 1 and 2, the passages also may be arranged at a compound angle β of, for example, 20° in an inward, outward or alternating inward, then outward direction, relative to the longitudinal axis of each row of apertures 36. As a result of such an alternating arrangement of the passages, gas emitted from apertures 36 will tend simultaneously to tension the web longitudinally in the direction of curved portion 52; and to tension the web transversely to longitudinal axes 18, 20 and thus flatten the web to surfaces 32, 34. Apertures 36 preferably are arranged in parallel rows extended longitudinally of each plate between ends 22, 24 and 26, 28. For the aperture size, operating pressure and materials mentioned previously, the longitudinal spacing δ between apertures 36 may be approximately 8.38 mm; and the transverse spacing γ between rows of apertures may be approximately 31.88 mm. In the embodiments of Figures 1 and 2, the alternately directed passages may be in rows having a transverse spacing ε of approximately 1.9 mm. As shown in Figure 2, control surfaces 32, 34 may be discontinuous with one or more venting grooves 54 provided between the rows of apertures, the grooves being parallel to the rows. The transverse width and depth of grooves 54 may be approximately 3.18 mm and the transverse spacing between the grooves and a line of holes may be approximately 16 mm with the rows of apertures centrally located between the grooves. The separate surfaces between grooves 54 comprise control surfaces 32, 34 and need not be coplanar, as much as 1.02 mm variation from coplanarity being acceptable for the arrangements and web materials previously described. Alternatively, as shown in Figure 3, the apertures advantageously may be situated within grooves 54 and directed to tangentially engage a radiused or curved side 56 of the groove to use the Coanda effect to distribute the localized disturbance force of the gas stream over a greater area. The Coanda effect at the curved edges causes the flows from the apertures to attach themselves to and follow curved side 56 in a laminar type flow and then to exhaust into the next adjacent exhaust groove. The apertures may be located next to the base of curved side 56, as illustrated, or more centrally on the bottom surface of the groove. For a groove sized and spaced as previously described, curved side 56 may have a radius of curvature of approximately 1.59 mm. On each side of the longitudinal axis 18, 20 of the control surface, curved side 56 should be the side of the groove further from the axis; so that, the streams of gas are directed toward opposite edges of the surface on opposite sides of the axis. Figure 4 shows how control surface 32, 34 may be comprised of surfaces of a plurality of smaller tensioning plates 14', 16' separated by open channels 58. Plates 14', 16' may be arranged essentially vertically and parallel as in Figure 4 or horizontally and parallel as in Figure 5 or at any suitable intermediate angle. In the embodiment of Figure 4, the central rows of passages 36 extend along each plate 14', 16'; whereas, in the embodiment of Figure 5, the rows of passages extend from plate to plate. As shown in Figure 6, passages 36 may be arranged in a chevron-shaped pattern between chevron-shaped exhaust grooves 54', in much the same manner as in commonly assigned U.S. Patent 4,493,548, the contents of which are incorporated by reference into this specification. Figure 7 illustrates yet another embodiment of tensioning plates 14, 16 in which a plurality of smaller rectangular control surfaces 32, 34 are spaced from each other by a rectangular grid of exhaust grooves 60. Those skilled in the art will appreciate that a variety of shapes of smaller control surfaces and channels may be used without departing from the invention. Thus the flows of gas from apertures 36 establish curtains of air along surfaces 32, 34 which, due to the Bernoulli effect, exert both a lifting force normal to the web and holding force to support the web a predetermined distance above the surface. This distance is a function of the rate of gas flow from the apertures, the inclination angle α, the orientation angle β, the diameter of the apertures, the distance γ between the rows of apertures, and the distance between the exhaust grooves. These same variables also govern the magnitude of the axial and transverse tensioning forces applied to the web due to viscous drag effects. Advantageous Effect of the Invention The apparatus of the invention shares with conventional vacuum box accumulators the advantage of accumulating webs without any moving parts and thus minimizes tension variations in the web during web-indexing movements. Unlike vacuum boxes, however, the apparatus of the invention will accept webs of various widths to be accumulated successively or concurrently side by side without modification of the apparatus. The apparatus has several additional advantages relative to vacuum boxes when used with very thin (less than 20 µm) and relatively wide (greater than 16 mm) webs, which are very fragile and prone to developing static charges. Firstly, the apparatus is much easier to use in a production environment since its very open construction facilitates either manual or automatic threading means. Secondly, since the apparatus accumulates web completely without contact, including edge contact, the ever-present concern of edge damage on such thin webs is eliminated, which otherwise might cause a catastrophic tear.

Claims: Apparatus for applying tension to a web (44), comprising: at least one first control surface (32) having first and second opposite ends (22, 24); first means (38, 36) for directing flows of gas along said first control surface in a direction from said first end (22) toward said second end (24); at least one second control surface (34) having third and fourth opposite ends (26, 28), said second control surface facing said first control surface with said first and second ends respectively opposite said third and fourth ends, whereby the space (46) between said control surfaces (32, 34) is open to ambient pressure; second means (38, 36) for directing flows of gas along said second control surface in a direction from said third end (26) toward said fourth end (28); and means (40, 42) near said first and third ends for guiding a loop of web into and out of said space so that said flows of gas support said web spaced from said control surfaces and apply tension to said web in the direction of said second and fourth ends. Apparatus according to Claim 1, wherein there are a first plurality of said first control surfaces and a second plurality of said second control surfaces; and said first and second means direct flows of gas along said first and second pluralities, respectively. Apparatus according to Claim 1 or 2, wherein said first means comprises a first plurality of apertures angled through said first control surface; said second means comprises a second plurality of apertures angled through said second control surface; further comprising means (38) for passing pressurized gas through said first and second pluralities of apertures. Apparatus according to Claim 3, wherein said first and second control surfaces each comprise opposite edges extending between said opposite ends; and a portion of said apertures are directed toward opposite edges of each of said control surfaces to apply transverse force to flatten said web toward said surfaces. Apparatus according to Claim 3, further comprising axially extending grooves (54, 58) in said control surfaces. Apparatus according to Claim 5, wherein said apertures are located in said grooves and each of said grooves comprise a curved side (56) adjacent said apertures between said web and said surfaces. Apparatus according to Claim 6, wherein streams of gas issuing from said apertures are angled tangentially to said curved side. Apparatus according to Claim 6, wherein each of said control surfaces comprises a longitudinal axis between said opposite ends, a portion of said grooves being on each side of said axis and said curved side of each groove being a side of the groove further from said axis. Apparatus according to Claim 2, wherein said first control surfaces are spaced from each other in said first plurality and said second control surfaces are spaced from each other in said second plurality.

Inventors: LONG MICHAEL, LONG, MICHAEL, LONG, MICHAEL, C/O EASTMAN KODAK CO.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1995
Main classification: B65H23
IPC codes: B65H23, B65H20