HYDROSTATIC FILM SUPPORT

Abstract: A device is disclosed for handling webs of material by means of a hydrostatic support including parallel bearing bars and a supporting fluid supplied between the bars. The device is useful, for example, to move, flatten, treat, and wind film webs for a variety of purposes.

Description: BACKGROUND OF THE INVENTION Field of the Invention In many industrial operations involving the handling of webs, productivity is affected by the difficulty and expense of the handling, itself. web materials must often be transported without being wrinkled and scratched. Web materials must often be flattened or held or wound at a certain predetermined tension. Web materials must often be treated by heat or by particular fluids. The device of this invention facilitates such web material handling. Description of the Prior Art United States Patent No. 3,201,985 issued August 24, 1965, discloses a device for determining the tension on a moving web of material. In use of that device, a web is moved over a single arcuate surface with holes at areas of web proximity. Pressure of a gas cushion between the arcuate surface and the web is known to equal the ratio of web tension to arcuate surface radius. The web tension can, therefore, be calculated by determination of the pressure. United Kingdom Patent No. 2,029,358 published March 19,1980, discloses a web transport system wherein an air cushion is generated on an arcuate surface for the purpose of providing so-called contactless support of the web. US-A-3 807 056 also discloses a web supporting device, for contact-free support and drying of a web of material. The device comprises a box-like structure which has an open side facing the web. Within the box are arranged a pair of tubular members, the edges of which are spaced from the sides of the box to provide a slot between each tubular member and the neighbouring box wall. A plate extends between the tubular members and closes the space between the tubular members, to form an air baffle facing towards the web. In use, air is passed from the slots over the curvilinear surfaces of the tubular members. The air flows from the two slots and combines to provide an air cushion between the web and the baffle. The rapidly flowing air contacts the web over a considerable distance and leads to an effective drying action. The air over the baffle flows laterally out of the device or is drawn through slots in the plate. US-A-3 498 515 relates to a fluid cushion turning roll for supporting and guiding a longitudinally moving web through a turn. The turning roll comprises a plurality of rows of nozzles, the rows being angularly displaced from each other around the turn. Each row of nozzles is formed by a plurality of openings formed in an elongate metal bar. Air is supplied under pressure to the nozzles, to support the web. Air exhaust passages are located between the nozzle rows. US-A-3678599 (equivalent to DE-A-1951002), discloses a web treating device wherein a pair of arcuate spaced-apart web support surfaces are fitted with holes at web proximate areas to withdraw web treatment gases supplied under pressure against a moving web from between the arcuate surfaces. Pneumatic means for supporting and transporting webs are well known. Single nozzle devices have been made which can be used alone or in tandem or parallel with additional devices as disclosed in United States Patent No. 3763571 issued October 9, 1973. Similar single nozzle devices used in multiple series have been disclosed in United States Patent No. 3498515 issued March 3, 1970. DE-C-975243 describes a device for the contact-free transport of webs to be dried. The device has nozzles arranged above and below the film, the nozzles being in the form of a slit in the web width direction. Exhaust air chambers are arranged between the nozzles. The nozzles direct half of the air which enters the device in the machine direction and half against the machine direction of the web. The specification states that this double nozzle design holds the web freely floating and tension-free, even when the dynamic pressure loading of the web amounts to several times the web's weight. US-A-3738018 describes a device for drying coated webs wherein the coated web is supported over a pair of closely spaced rotatable cylinders. A suction box is disposed between the peripheries of the cylinders. US-A-3,540,671 (equivalent to FR-A-1570933) discloses in Figure 4 an arrangement for providing an air cushion between a paper web and a pressure guide. A notch extends across the width of the guide and a compressed air conduit communicates with the notch. SUMMARY OF THE INVENTION According to a first aspect of this invention, there is provided a fluid cushion web handling device for providing hydrostatic support to a moving web of material, comprising (a) adjacent bearing bars, across which the supported web moves, in operation, the bars having arcuate, smooth surfaces that are holefree in the areas of web proximity in operation, (b) a pressure chamber between the bearing bars, and (c) fluid supply means to the pressure chamber, characterised in that the bearing bars are cylindrical and rotatable. The invention, in another aspect, also provides a method of handling a web of material with a device of the invention. In case of operation at low speed or at high web tension, having both bearing bars rotatable in the direction of the web travel may act to increase the thickness of the boundary fluid layer where the film is in close proximity to the bearing bar surfaces and to reduce the relative velocity difference between the web and the bearing bars. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a web handling device of this invention. DESCRIPTION OF PREFERRED EMBODIMENTS The device of this invention is useful in handling webs of a variety of sizes and materials and for a variety of operations. The web handler of this invention can be used with webs of paper or textiles or of film material such as cellophane or thermoplastics or the like. There appears to be no limit to the width of web which can be used, the web handling device, for example, being directly substitutable for presently-used web transfer roller devices for simple transfer applications. The device of this invention is useful for assuring that a web of thin material, such as a thermoplastic film, is completely flat and wrinkle-free immediately prior to winding the film onto a roll or subjecting the film to a further operation or treatment. The device is also useful in the area of web tension adjustment and equalization. Because the web being handled is supported away from the surfaces of the device of this invention by fluid forces, a variation of tensions on the web can be absorbed by means of this device. Such variation of tensions can be caused by eccentricities in transport rolls, changes in drive roll speeds, and the like. Looking to Fig. 1, a device of this invention is shown, in cross-section. Bearing bars 1 are cylindrical and extend, in substantially parallel, spaced-apart relation, beyond the width of web 2 being moved across the bars. Bearing bars 1 have surfaces 3 that are entirely holefree. The space between bearing bars 1 serves as a pressure chamber 4 and a fluid supply 5 serves as a distributor for fluid under pressure to pressure chamber 4. Fluid supply 5 is by holes or a nozzle in plenum 8 shaped to fit closely to the contour of bearing bars 1 and used to direct fluid from plenum cavity 9 through fluid supply 5 into pressure chamber 4. The arcuate surfaces in the proximity of the supported web are convex in substantially the same direction and away from the pressure chamber and segments of the arcuate surfaces in the proximity of the supported web have radii which are located on the same side of the supported web, in operation. Bearing bars 1 are rotatably mounted. In operation rotatably mounted bearing bars provide reduced relative motion, with moving webs; and such reduced relative motion reduces, even further, any effects of incidental contact between the web and the bearing bars 1. The arcuate surfaces 3 should be smooth and, in any event, any roughnesses should be lower than the thickness of the boundary fluid layer to achieve scratch-free operation. The pressure chamber 4 can be blocked at the ends, if desired, and such blocking will reduce fluid requirements. It has been found that the web handling device will be more effective in web flattening and wrinkle removing if the ends are not blocked or not blocked completely. Fluid is required in a volume and at a force which is adequate to generate a pressure in pressure chamber 4 to support web 2 as the web is moved across bearing bars 1. Fluid requirements will vary, of course, with several factors such as the length of bearing bars 1, the spaces existing between the edge of web 2 and the end of bearing bars 1, the means used, if any, for sealing or blocking the ends of pressure chamber 4, the space required or desired between the web 2 and the arcuate surface 3 of bars 1, in operation, the temperature and viscosity of the fluid, the kind of fluid, and the like. The fluid can be liquid or gas, is usually gas, and is most often air. Of course, the fluid can be a liquid such as water or an organic carrier or solvent for some web treatment material. The fluid can be used as a heat transfer medium to heat or cool the web either as a liquid or as a gas. The web 2 can be introduced to the film handling device or taken away from the device at any angle so long as the film contacts or is proximate to the arcuate surfaces 3 of the bearing bars 1. Also the angle of web introduction can be different from the angle of web take away. Bearing bars 1 are usually parallel with one another but need not be so. When the bars 1 are not parallel, the angle of web introduction and take away may vary along the width of the web. There is no need for both of a pair of bars 1 to have the same radius. The surfaces 3 must, however, be holefree in active areas to avoid fluid flow upsets caused by localized pressure variations. (The so-called active areas of arcuate surfaces 3 are the areas which are near to a web during operation of the web handling device.) Holes in active areas of the arcuate surfaces 3 can cause web flutter due to high or low pressure at the hole and can cause diminished control of the web and supporting fluid movement. Bearing bars 1 are adjacent and spaced apart a distance appropriate to accomplish the desired objective. For example, to flatten and unwrinkle a film web, the arcuate surfaces 3 should be spaced from about 1 to about 10 centimeters apart. The spacing most often used is the sum of the radii of each of the bearing bars 1. To heat or cool or treat the surface of a web, the spacing of the bearing bars can be increased to accommodate increased contact times which might be required. The fluid pressure necessary to operate the device of this invention decreases with an increase in spacing between the bearing bars. The fluid pressure required to support a web in this device is equal to the tension on the web divided by the radius made by the web as it is supported between the lines of closest web proximity to the bearing bars during operation (which lines along the length of bearing bars 1 actually include the area where the web and the surface would come ino contact if the web were under tension but not supported): wherein P is the pressure of the fluid supporting the web; T is the tension exerted on the web; and R is the radius of the circular segment formed by the supported web. As the spacing between bearing bars increases, the radius of a supported web naturally increases and, for the same tension, the required supporting pressure decreases inversely. Contact between the web and the active areas of each of the two arcuate surfaces 3 is prevented, in operation, by the formation of a boundary fluid layer the thickness of which depends on the web speed, the web tension, the curvatures of surfaces 3 and the viscosity of the fluid in the convergent gap which is defined, on one side, by the web and on the other side by the arcuate surfaces 3.

Claims: A fluid cushion web handling device for providing hydrostatic support to a moving web of material comprising (a) adjacent bearing bars (1), across which the supported web (2) moves, in operation, the bars (1) having arcuate, smooth surfaces (3) that are holefree in the areas of web proximity in operation, (b) a pressure chamber (4) between the bearing bars (1), and (c) fluid supply means (5) to the pressure chamber (4), characterised in that the bearing bars (1) are cylindrical and rotatable. The web handling device of claim 1 wherein the surfaces (3) of the bearing bars (1) are entirely holefree. The web handling device of claim 1 or claim 2 wherein the bearing bars (1) are substantially parallel. A method of handling a web (2) of material, in which the web (2) is moved across the bearing bars (1) of a device as defined in claim 1, 2 or 3 while fluid under pressure is supplied to the pressure chamber (4) of the device by the fluid supply means (5) so as to generate a pressure in the pressure chamber (4) to support the web (2). A method according to claim 4, in which the fluid is gaseous. A method according to claim 5, in which the fluid is air. A method according to claim 4, 5 or 6, in which the fluid is used as a heat transfer medium. A method according to claim 4, in which the fluid is water or an organic carrier or solvent for web-treatment material.

Inventors: PETITJEAN, GILLES
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1986
Main classification: B65H23
IPC codes: B65H23, B65H20