METHOD FOR CROSS-DIRECTION PROFILING OF A PAPER OR BOARD WEB IN A NIP

Abstract: A nip is formed between a backing roll (10) and a loading roll (20). The loading roll (20) is comprised of a stationary support structure (21) and, adapted to run about the same, a flexible mantle (22) supported by its inner surface (22a) on said support structure (21) by means of at least one supporting member (23) serving to press said flexible mantle (22) against said backing roll (10). The loading roll (20) includes further at least one essentially axially aligned row of discrete pressure medium feed points (25, 26), whereby a pressure and/or flow rate controlled flow of pressure medium can be fed through the feed points (25, 26) of said at least one row against said inner surface (22a) of said mantle (22). According to the method, the pressure medium fed through said feed points (26) is used cross-machine direction (CD) profiling of the paper or board web passing through the nip (N).

Description: Method for cross-direction profiling of a paper or board web in a nip The invention relates to a method according to the preamble of claim 1 for cross- direction profiling of a paper or board web in a nip. The method according to the invention can be employed in a nip formed between a backing roll and a loading roll. The loading roll comprises a stationary support structure framework and a flexible mantle running about said structure, as well as a loading member adapted between the inner surface of said flexible mantle and said support structure so as to permit the flexible mantle to be loaded against said backing roll. The loading member itself may serve as a piston being adapted into a cylinder in which case a pressure medium acts directly on said loading member. Alternatively, the loading member may be actuated by separate loading cylinders adapted between the loading member and the stationary support structure. The loading member is generally actuated by means of two rows of loading cylinders extending over the cross-machine direction and being displaced at a distance from each other in the machine direction. With two loading cylinders displaced at a distance from each other in the machine direction it is possible to make machine direction profiling of the web. In the latter situation, the cross-machine direction profiling of a paper or board web in the nip is implemented in the state-of-the-art embodiments so that the loading member is actuated at different pressure levels applied to the pairs of loading cylinders displaced at a distance from each other in the cross-machine direction. When a higher pressure is applied on the loading member at one of the cylinder pairs, the loading member will slightly bend at said cylinder pair consequently loading the running web with a higher nip pressure at said point. EE patent publication 0 777 012 discloses a pressure roll comprising a stationary support structure and an hose mantle running thereabout supported on the support structure by means of at least one loading member which is adapted to be compressible against the inner surface of said hose mantle and is at least partially hydrodynamically lubricated. The loading member itself serves as a piston provided with a pressure chamber which piston is adapted into a cylinder. The load-bearing surface of the loading member, is provided with at least one row of discrete oil inlets parallel to the roll axis, of which oil inlets at least a portion are fed with a pressure medium not communicating with the pressure medium that is passed to the pressure chamber of the loading member. Each of the oil inlets is provided with a flow-restricting bore through which oil enters onto the load- bearing surface of the loading member. The goal of the invention disclosed in the cited EP patent publication 0 777012 is to provide in all circumstances the load-bearing surface of the loading member with a supply of fresh and cooled lubricating oil, whereby a maximally uniform machine direction temperature profile is attained on the load-bearing surface of the loading member. The hydraulic oil passed into the pressure chamber of the loading member that urges the loading member toward the backing roll will warm up due to the energy input of the loading action and, as the same oil is also passed via a capillary duct onto the surface of the loading member, its lubrication qualities are already degraded. When the lubricating oil is passed onto the load- bearing surface of the loading member, it forms an oil film between the load- bearing surface of the loading member and the inner surface of the hose mantle, thus passing with the moving hose mantle to the trailing side of the load-bearing surface. As the lubricating oil is subjected to continuous warming-up toward the trailing side of the load-bearing surface, the temperature profile of the load- bearing surface will become nonuniform. This problem is solved according to the cited EP publication by arranging in addition to said row of capillary ducts communicating with the pressure chamber of the loading member at least one other row of capillary ducts, whereby the latter capillary ducts are arranged to communicate with a separate feed channel which is formed into the loading member aligned parallel to the axis of the hose mantle roll and which feed channel is fed from a separate pressure medium source. Thus, the second row of capillary ducts allows the load-bearing surface of the loading member to be flushed in all circumstances with fresh and cooled lubricating oil, whereby the lubricating qualities of the oil film are improved and the machine direction temperature profile over the load-bearing surface of the loading member can be made more uniform. The cited EP patent publication 0 777 012 also teaches that these capillary ducts, which form a row in the cross-machine direction and are connected to a separate pressure medium source, may be arranged into groups of desired size, whereby the lubricating oil feed ducts of the groups may be equipped with control valves. Thus, the pressure and/or flow of the lubricating oil fed to each group can be controlled separately. In this fashion, the pressure roll may be adjusted to meet the needs of the actual operating conditions. Obviously, the teaching must be understood to mean that when the lubricating oil channels communicating with a pressurized fluid chamber of the loading member fail to provide a uniform and sufficiently thick lubricating oil film between the loading member and the hose mantle, the lubrication may be intensified by feeding additional fresh lubricating oil between the loading member and the envelope through separate lubricating oil channels. WO patent publication 98/04844 discloses a roll for pressing a web. The roll has a stationary axial shaft with a mantle rotating about the same and having loading elements adapted thereto that support the mantle on the axial shaft and serve to load the mantle against a backing roll. Normally, each loading element loads the mantle via an individual back-up element which is connected to the loading element and has a lubricating oil pocket thereon. However, a small gap remains between the adjacent back-up elements through which lubricating oil can escape from the nip formed between the inner surface of the mantle and the back-up elements. At these gaps, the pressure of the lubricating oil may drop to zero, which can be seen as a deformation of the mantle of the roll. To avoid this problem, there is adapted a sealing list between the back-up elements and the inner surface of the mantle, the list extending over a plurality of loading elements in the axial direction of the roll and, preferably, over the entire axial length of the roll. The lubricating oil pockets are formed on the sealing list rather than on the back-up elements. This arrangement makes it possible to eliminate the gaps between individual back-up elements and to apply a uniform lubricating oil pressure distribution on the inner surface of the mantle over the entire axial length of the roll. The roll has a fiber-reinforced plastic covering, that is, a composite mantle, with a thickness of about 15-20 mm, which means that also such a mantle may deformate due to lubricating oil pressure. The main characteristics of the method according to the invention are disclosed in the characterizing part of claim 1. A loading roll implemented by virtue of the method according to the invention requires at least one row of pressure medium feed points that are aligned essentially parallel to the axis of the roll and are displaced apart from each other so that each feed point can be used for feeding a pressure and/or flow-rate controlled flow of pressure medium against the inner surface of the mantle of the loading roll. This row of pressure medium feed points may be located either on the loading member of the loading roll or, alternatively, just in front of the loading member relative to the rotation direction of the mantle of the loading roll. The invention is based on the idea that a prior-art construction is controlled in an unorthodox manner contrary to the teachings of the prior art. According to the prior-art teaching, a lubrication medium arrangement comprising discrete feed points is controlled so that a maximally uniform lubrication medium film is formed between the roll mantle and the loading member. In contrast, the goal of the present invention is to particularly establish between the mantle of the roll and the loading member a nonuniform profile of the lubrication medium film in the cross-machine direction. The thickness variations of the cross-machine direction profile of the lubrication medium film in turn lead to a nonuniform cross-machine direction nip pressure profile, whereby the paper or board web is subjected to a nip pressure profile which is different at different cross-machine direction points of the nip. The invention utilizes actively a nonuniform nip pressure profile generated by means of a nonuniform lubrication medium film thickness profile for cross-machine direction profiling of a paper or board web in a nip. The minimum zone spacing in the cross-machine direction is determined by the width of the discharge points of the individual lubrication medium feed points, the spacing between the feed points and the stiffness of the mantle of the roll. In the method according to the invention, the cross-machine direction profiling of the paper or board web may thus be carried out using a very dense zone spacing set at, e.g., about 20 mm. A preferred application of the method according to the invention is a calender in which the nip is formed between a heated backing roll and a loading roll. The method according to the invention is also applicable to a press in which the nip is formed between a backing roll and a loading roll. In a calender, the web passes through the nip between the outer surface of the mantle of the backing roll and the outer surface of the mantle of the loading roll. In a press, the web generally passes through the nip between a first fabric travelling on the outer surface of the mantle of the backing roll and a second fabric travelling on the outer surface of the mantle of the loading roll. A calender and a press may also differ as to the shape of the loading member forming the nip and the length of the nip. The SymBelt™ type of hose rolls produced by applicant have a hydrodynamic lubricating oil pocket made on the sliding face of the loading member. In a calender application, the lubricating oil pocket generally comprises only one pocket extending over the entire width in the cross-machine direction. In a press, the sliding face of the loading member generally has an individual lubricating oil pocket formed separately at each lubricating oil feed point, thus requiring the loading member to have a plurality of lubricating oil pockets made thereto over the entire length in the cross-machine direction. In the following the invention will be described by making reference to the appended drawings, whereby the details thereof are not understood to limit the scope of the invention, in which diagrams FIG. 1A shows a schematic cross-sectional view of a nip in which the method according to the invention can be utilized; FIG. IB shows an axonometric view of one loading member of the nip; and FIG. 2 shows a cross-sectional view in the cross-machine direction at the lubricating oil ducts of the loading member illustrated in FIG. IB. FIG. 1A shows schematically a cross sectional view of a nip formed between a backing roll 10 and loading roll 20, while FIG. IB shows an axonometric view of a loading member, that is, a loading shoe 23 adapted to the interior space of the loading roll 20. The mantle of the loading roll 20 is formed by a so-called hose mantle 22 and, accordingly, the loading roll 20 will later in the text be called a hose mantle roll 20. In FIG. 1A, the direction of rotation of the backing roll 10 is denoted by arrow S i and the direction of rotation of the mantle 22 of the loading roll is denoted by arrow S 2 . Furthermore, the directions in FIG. IB are denoted as the machine direction MD, that is, the travel direction of the web, the cross- machine direction CD and the web thickness direction Z. The hose mantle roll 20 comprises a stationary support structure 21 and a hose mantle 22 adapted to run about the same. The flexible hose mantle 22 is supported by its inner surface 22a on a loading shoe 23 extending over the width of the machine in the cross-machine direction CD. The loading shoe 23 in turn is supported on the support structure 21 of the hose mantle roll 20 by two loading cylinder rows 27,28 aligned at a distance from each other in the machine direction MD and extending over the width of the machine in the cross-machine direction CD. The loading shoe 23 and, hence, hose mantle 22 may be loaded against the backing roll 10 by adjusting the loading force imposed by the cylinders 27, 28. The load-bearing surface 24 of the loading shoe 23 is formed so as to form an extended nip N with the backing roll 10. The load-bearing surface 24 of the loading shoe 23 is comprised of three areas 24a,24b,24c. The first area 24a forms the leading edge of a hydrodynamic web support area, the second area 24b forms a hydrostatic lubricating oil pocket and the third area 24c forms the trailing edge of a hydrodynamic web support area. The lubricating oil pocket 24b is formed into a recess between the leading edge area 24a and the trailing edge area 24c adjoining the recess. The radiuses of curvature in the portions of the leading edge area 24a and the trailing edge area 24c adjoining the lubricating oil pocket 24b are made compliant with the radius of curvature of the backing roll 10. The flexible hose mantle 22 will pass between the load-bearing surface 24 of the loading shoe 23 and the outer surface 10b of the mantle of the backing roll 10 so as to form an extended nip N. To the lubricating oil pocket 24b of the loading shoe 23, from the underside 23a of the loading shoe, there is adapted a first row of ducts 25 disposed at a distance from each other and extending over the cross-machine direction CD, the ducts permitting lubricating oil to be pumped into the lubricating oil pocket 24b. Into the lubricating oil pocket 24b, from the underside of the loading shoe 23, is also adapted to pass a second row of ducts 26 disposed at a distance from each other, the ducts permitting a pressure medium for the purpose of cross-machine direction profiling of the web to be pumped into the lubricating oil pocket 24b independently of the lubrication medium. This second row of channels 26 is disposed after the first row of channels 25 in the machine direction MD at a location where the depth of the lubricating oil pocket 24b is shallower than at the first row of channels 25. When the pressure medium for the cross-machine direction profiling of the web is introduced onto the load-bearing surface 24 of the loading shoe 23 via the hydrostatic lubricating oil pocket 24b, the wear caused by the profile-controlling pressure medium on the hose mantle 22 is minimized. The individual ducts 26 of the second row are advantageously divided into groups so that in each group there is at least one duct 26, whereby the flow rate and/or pressure of the pressure medium flow into each group can be controlled individually. This can be accomplished so that each group is fed from a separate pressure medium source having a controllable flow rate and/or pressure. Another alternative embodiment is shown in FIG. 2 illustrating a sectional view in the cross-machine direction CD at the lubricating oil ducts 26 of the loading shoe 23 illustrated in FIG. IB. The embodiment shown in FIG. 2 uses a common-rail feed channel 30, wherefrom a manifold of channels with valves 31 are branched into each duct 26, thus permitting the flow rate and/or pressure of the pressure medium being fed into the ducts 26 to be individually controlled by means of the valves 31. In both arrangements, the pressure medium can be continually fed into each group at a desired flow rate and/or pressure for the purpose of cross-machine direction CD profiling of the paper or board web being passed through the nip. When the profile control is active, at least one of the profile control ducts 26 is used for feeding a pressure medium into the lubricating oil pocket 24b at a pressure Pi and flow rate higher than the respective pressure P 0 at which the lubricating fluid is being fed into the lubricating oil pocket 24b. Hence, the excess pressure medium discharged from the profile control ducts 26 cannot immediately spread out into the lubricating oil pocket 24b, but rather, the excess pressure medium flow presses with a locally higher force the readily deformable hose mantle 22 against the backing roll 10. Resultingly, the web at that point in the cross-machine direction CD is subjected to a pressing force higher than the average pressure along the nip, thus facilitating the web profile control in the cross-machine direction CD. In a calender equipped with the applicant's SymBelt™ roll embodiment, the pressure medium is pumped into the lubricating oil ducts 25 normally at a pressure of about 100 bar. The profile control ducts 26 are fed with the pressure medium at a pressure of about 100-120 bar. In the above-described embodiment, the loading shoe 23 is actuated by two hydraulic cylinder rows 27,28. The invention is as well suited for use in an arrangement, where the loading shoe 23 itself acts as the piston moving in a cylinder. The loading shoe 23 is formed so as to form a pressure chamber into which the pressure medium can be fed in order to press the loading shoe 23 against the backing roll 10. Then, the row of lubricating oil ducts 25 exiting in the lubricating oil pocket 24b may be directly communicating with said pressure chamber or, alternatively, the row of lubricating oil ducts 25 can be fed from a separate pressure medium source. The ducts 26 used for profiling and arranged in suitable groups are in all circumstances fed from a separate pressure medium source. While the rows of ducts 25 and 26 are in FIG. 1 drawn so as to be aligned in two parallel rows in the machine direction MD, they may as well be merged into a single row without departing from the scope of the invention. The ducts 26 used for profiling in the cross-machine direction CD are located interlaced between the ducts 25 used for the lubrication of the load-bearing surface 24 of the loading shoe 23. The ducts may be ordered so that every first duct is a lubricating oil duct 25 and every other duct is a profile control duct 26, but also other arrangements are possible. In FIG. 1 is shown only one row of ducts 26 for cross-machine direction profiling of the web, but the invention may as well use a plurality of rows of ducts 26 arranged adjacent to each other in the machine direction MD. The ducts 26 may form a desired pattern on the load-bearing surface 24 of the loading shoe 23. In FIG. 1, the rows of the lubricating oil ducts 25 and the profile control ducts 26, respectively, are shown to discharge into the lubricating oil pocket 24b formed on the load-bearing surface 24 of the loading shoe 23. but this is not necessary for the invention. The lubricating oil ducts 25 and/or the profile control ducts 26 may as well be arranged to discharge e.g. into recesses made on the load-bearing surface 24 of the loading shoe 23. In FIG. 1 the loading shoe 23 is drawn so as to extend uniformly over the entire width of the machine in the cross-machine direction CD, but without departing from the spirit of the invention, a plurality of shoe sections adjoining in an end-to- end fashion in the cross-machine direction CD can be used to form the loading shoe 23. In FIG. 1A is shown an embodiment in which the hose mantle roll 20 is equipped with a flexible hose mantle 22, but the method according to the invention is also applicable to roll embodiments having their mantle 22 made from a less resilient material. For instance, the cited WO patent publication 98/04844 uses a composite mantle having a thickness of about 15-20 mm. Also this type of mantle may be made deformable under the pressure of the lubricating oil, thus allowing the use of a composite-material mantle in lieu of the hose mantle as the roll mantle. In all circumstances, the mantle of the roll must have so much resilience as to be deformable under the pressure of the oil fed into the profile control ducts. The invention is specified in the appended claims, whereby the details of the invention may be varied within the scope and inventive spirit disclosed therein from those of the exemplifying embodiment described above.

Claims: Claims 1. Method for cross-direction profiling of a paper or board web in a nip formed between a backing roll (10) and a loading roll (20), which loading roll is comprised of a stationary support structure (21) and, adapted to run about the same, a flexible mantle (22) supported by its inner surface (22a) on said support structure (21) by means of at least one supporting member (23) serving to press said flexible mantle (22) against said backing roll (10), and which loading roll (20) further includes at least one essentially axially aligned row of discrete pressure medium feed points (25,26), whereby a pressure and/or flow rate controlled flow of pressure medium is fed through the feed points (25,26) of said at least one row against said inner surface (22a) of said mantle (22), characterized in that the pressure medium fed through said feed points (26) is used for controlling the cross-machine direction (CD) profiling of the paper or board web passing through said nip (N). 2. Method according to claim 1, characterized in that said supporting member (23) is provided with two rows of pressure medium feed points (25,26), whereby the feed points (25) of the first row are used for feeding a uniform film of pressure medium as a lubricant between said inner surface (22a) of said mantle (22) and the load-bearing surface (24) of said supporting member (23), while the feed points (26) of the second row are used for feeding pressure medium for cross- machine direction (CD) profiling of the paper or board web passing through said nip (N). 3. Method according to claim 1, characterized in that said supporting member (23) is provided with one row of pressure medium feed points (25) and, in front of the supporting member (23) relative to the direction of rotation (S 2 ) of said mantle (22), there is provided another row of pressure medium feed points (26). whereby the feed points (25) of the first row are used for feeding a uniform film of pressure medium as a lubricant between said inner surface (22a) of said mantle (22) and the load-bearing surface (24) of said supporting member (23), while the feed points (26) of the second row are used for feeding pressure medium for cross- machine direction (CD) profiling of the paper or board web passing through said nip (N). 4. Method according to claim 1, characterized in that on said load-bearing surface (24) of said supporting member (23), there is formed a lubricating medium pocket (24b) into which two rows of pressure medium feed points are arranged to exit, whereby the feed points (25) of the first row are used for feeding a uniform film of pressure medium as a lubricant between said inner surface (22a) of said mantle (22) and the load-bearing surface (24) of said supporting member (23), while the feed points (26) of the second row are used for feeding pressure medium for cross-machine direction (CD) profiling of the paper or board web passing through said nip (N).

Inventors: TORVI TIMO, TORVI, TIMO
Applicants: TORVI TIMO, VALMET CORP, TORVI, TIMO, VALMET CORPORATION
Application year: 2000
Main classification: D21F3
IPC codes: D21F3, D21G1