A method for manufacturing a yankee drying cylinder

Abstract

The invention relates to a method for manufacturing a Yankee drying cylinder (1). The method may comprise providing (S1) a design of an external surface (7) of a portion (2a) of a shell (2) of the cylinder (1), wherein, in a cross-section which coincides with a rotational axis (RA) of the cylinder (1), a contour (C7) of the external surface (7) is a crown contour. The method may comprise providing (S2) a design of an internal surface of said portion (2a), which presents a plurality of circumferential grooves (3). In said cross-section, a maximum difference of the radial distance (DB) from the external surface (7) to the bottoms of the grooves (3), is less than a maximum difference (MD) of the radial distance from the rotational axis to said external surface (7). The shell may be machined (S3) internally, using the design of the internal surface (6a) as a model.

Description

[0001] A METHOD FOR MANUFACTURING A YANKEE DRYING CYLINDER

[0002] TECHNICAL FIELD

[0003] The invention relates to a method for manufacturing a Yankee drying cylinder for a machine for manufacturing a fibrous web.

[0004] BACKGROUND

[0005] Yankee drying cylinders are used in the manufacturing of paper, in particular tissue paper. Such a cylinder can be designed to be supplied with hot steam to the interior of the cylinder such that the cylinder is heated. When installed in a paper making machine such as a tissue machine, the Yankee drying cylinder is arranged to be rotated. In operation, a wet fibrous web coming from a forming section of the paper making machine is transferred to the outer surface of the Yankee drying cylinder and follows the surface of the Yankee drying cylinder as the cylinder rotates, whereby the hot outer surface serves to dry the web.

[0006] The shell and end walls of a Yankee drying cylinder may be manufactured by casting. Thereby, the end walls may be secured to the shell by means of fastening elements such as bolts. Alternatively, the shell and end walls may be manufactured from steel plates. Thereby, the end walls may be secured to the shell by means of welding.

[0007] To improve heat transfer to the outer surface of the shell while retaining an adequate stiffness of the shell, and also to facilitate the collection of condensed steam, a plurality of circumferential grooves are machined in the internal surface of the shell.

[0008] There is a problem in that a Yankee drying cylinder may present an uneven distribution of heat over its external surface. This means that the drying capacity differs from one area of the external surface to another. This means that the production speed has to be adapted to the area with the lowest temperature, and this reduces the efficiency of the process.

[0009] WO2019219267A1 discusses uneven heat distribution caused by a wall thickness variation introduced by welding, and suggests a final machining step of the outer cylindrical surface to reduce the wall thickness variation effect. Nevertheless, there is a desire to further reduce the unevenness in the distribution of heat over an external surface of a Yankee drying cylinder.

[0010] SUMMARY

[0011] An object of the invention is to reduce the unevenness in the distribution of heat over an external surface of a Yankee drying cylinder.

[0012] The object is reached with a method for manufacturing a Yankee drying cylinder for a machine for manufacturing a fibrous web, according to claim 1.

[0013] The method comprises providing a design of an external surface of a portion of a shell of the Yankee drying cylinder, which portion starts and ends, in the axial direction of the shell, no more than 300 mm, preferably no more than 200 mm, from axial ends of the shell, such that, in a cross-section which coincides with a rotational axis of the Yankee drying cylinder, there is at two different locations along the rotational axis, in said portion of the shell, a difference in the radial distance from the rotational axis to the external surface. Said locations along the rotational axis are preferably at different distances from a center of the shell. The center of the shell may be located halfway between the axial ends. Thereby, the diameter of the external surface may decrease from the center of the shell towards the axial ends of the shell. The method further comprises providing a design of a machined internal surface of said portion of the shell, and machining the shell internally, using the design of the machined internal surface as a model for the machined internal surface of said portion of the shell. The design of the machined internal surface is such that, in said cross-section and along the axial extension of said portion of the shell, a maximum difference of the radial distance from the external surface to a final internal surface of said portion of the shell, or, where the final internal surface of said portion of the shell presents a plurality of circumferential grooves, to the bottoms of the grooves, is less than a maximum difference of the radial distance from the rotational axis to the external surface of said portion of the shell.

[0014] The design of the external surface of the portion of the shell may be a design of the final external surface of the portion of the shell. In some embodiments, the design of the machined internal surface is a design of the final internal surface of said portion of the shell. Thus, in some embodiments, the method comprises providing a design of a final internal surface of said portion of the shell, such that, in said cross-section and along the axial extension of said portion of the shell, a maximum difference of the radial distance from the designed external surface to the final internal surface of said portion of the shell, or, where the final internal surface of said portion of the shell presents a plurality of circumferential grooves, to the bottoms of the grooves, is less than a maximum difference of the radial distance from the rotational axis to the external surface of said portion of the shell, and machining the shell internally, using the design of the final internal surface as a model for the final internal surface of said portion of the shell.

[0015] In other embodiments, the design of the machined internal surface is a design of an intermediate internal surface of said portion of the shell, which intermediate internal surface is made in dependence on an anticipated deformation of the shell at a deformation causing manufacturing step following the step of machining of the shell internally whereby the intermediate internal surface becomes the final internal surface. Thereby, the method may comprise providing a design of an intermediate internal surface of said portion of the shell, based on the design of the external surface and an anticipated deformation of the shell at a deformation causing manufacturing step following a step of machining of the shell internally, such that with the anticipated deformation, in said cross-section and along the axial extension of said portion of the shell, a maximum difference of the radial distance from the designed external surface to a final internal surface of said portion of the shell, or, where the final internal surface of said portion of the shell presents a plurality of circumferential grooves, to the bottoms of the grooves, is less than a maximum difference of the radial distance from the rotational axis to the external surface of said portion of the shell, and performing the step of machining the shell internally, using the design of the intermediate internal surface as a model for the intermediate internal surface of said portion of the shell.

[0016] The design of the external surface, the final internal surface, or the intermediate internal surface may be provided with any suitable design software, or with suitable physical drawing tools. Using the design of the internal surface as a model for the internal surface may be done in any suitable manner. For example, the machining of the shell internally may be done with a robotic milling tool which is controlled by a controller which is programmed based on the design of the internal surface.

[0017] In cases where a plurality of circumferential grooves are machined in the internal surface, the Yankee drying cylinder may be adapted for steam heating as exemplified below.

[0018] By the difference in the radial distance from the rotational axis to the external surface, the external surface deviates from cylindrical. This may be due to crowning or end shrinking at welding, as exemplified below. In some embodiments, the radial distance from the rotational axis to the external surface is non-constant in the axial direction of the shell. In other embodiments, in one or more sections said radial distance may be constant while in one or more other sections it is non-constant, e.g. sloping. It is even conceivable that there are, in the shell axial direction, one or more steps of the radial distance from the rotational axis to the external surface.

[0019] By the maximum difference of the radial distance from the designed external surface to the bottoms of the grooves being less than a maximum difference of the radial distance from the rotational axis to the external surface of said portion of the shell, the heat transfer from the interior of the Yankee drying cylinder to the external surface thereof, via the shell material between the bottoms of the grooves and the external surface, may be substantially the same from one groove to another. Thereby, the distribution of heat over the external surface may be even.

[0020] In cases where the maximum difference, of the radial distance from the designed external surface to the final internal surface, is less than the maximum difference of the radial distance from the rotational axis to the external surface, the shell may have a thickness that is substantially the same along said portion of the shell. Thereby, when the manufacturing method is completed, the shell may have an internal surface that is smooth along said portion of the shell. Thereby, the Yankee drying cylinder may be adapted for induction heating as exemplified below. Thereby, since the thickness is substantially the same along said portion of the shell, heating created by electric currents in the shell caused by one or more inductors interacting electromagnetically with the shell, will be distributed equally over the external shell surface. The even heat distribution over the external shell surface provided by embodiments of the invention means that a reduction of production speed to adapt to surface areas with relatively low temperatures can be avoided. Machining the shell internally, using the design of the final internal surface, or the design of the intermediate internal surface, as a model for the final internal surface, or for the intermediate internal surface, respectively, contributes to a Yankee drying cylinder with a reduced unevenness of the degree of heating over the external shell surface.

[0021] It should be noted that in cases where a plurality of circumferential grooves are machined in the internal surface of the shell, the Yankee drying cylinder may be adapted for induction heating. Vice versa, on cases where said portion of the shell has an internal surface that is smooth, the Yankee drying cylinder may be adapted for steam heating. Nevertheless, the grooves may increase the heat absorption capacity of the shell, and may therefore be particularly suitable for steam heating.

[0022] It should be noted that also in portions of the shell that are outside of said centered portion of the shell, e.g. in portions at the axial ends of the shell, a maximum difference, here called a first difference, of the radial distance from a designed external surface to a final internal surface, or, where the final internal surface presents a plurality of circumferential grooves, to the bottoms of the grooves, may be less than a maximum difference, here called a second difference, of the radial distance from the rotational axis to the external surface of such portions. However, in some embodiments, in such outer shell portions, the first difference my be equal to or larger than the second difference.

[0023] The external surface of said portion of the shell may be a final external surface, e.g. an external surface of the Yankee drying cylinder when ready for use. In some embodiments, the final internal surface is designed within said restrictions in relation to the external surface, and the shell is machined internally with an aim to obtain an internal surface which is as close to the design of the final internal surface as possible.

[0024] However, in other embodiments, the shell is machined internally with an aim to obtain an internal surface which is as close to the design of the intermediate internal surface as possible, which intermediate internal surface is designed in dependence on the anticipated deformation of the shell at the deformation causing manufacturing step following the internal machining of the shell. Below, examples of such embodiments are provided.

[0025] The shell may have an extension of 2-10 meters, for example, 4-8 meters, e.g. around 5 meters. As understood, the portion of the shell, for which the design of the machined internal surface is provided, may start and end, in the axial direction of the shell, no more than 300 mm from the axial ends of the shell. Thereby, features of the invention may be presented from an axial center of the shell and at least up to 300 mm from the axial ends of the shell. In some embodiments, features of the invention are provided up to the axial ends of the shell. In other embodiments, the shell presents at the axial end geometries diverting from features of the invention. For example, the geometries of the axial ends of the shell may be adapted to mounting of end walls at the axial ends. Also, the shell may be thicker at the axial ends than elsewhere, e.g. due to reasons of structural integrity. In any case, the portion of the shell, for which the design of the machined internal surface is provided, starts and ends no more than 300 mm from the axial ends of the shell. In some embodiments, said portion of the shell, for which the design of the machined internal surface is provided, starts and ends no more than 200 mm, or even no more than 100 mm, from the axial ends of the shell. Said portion is preferably centered in the axial direction of the shell. Said portion may form at least 80%, preferably at least 85%, more preferably at least 90%, of the axial extension of the shell. Said portion may form less than 98% of the axial extension of the shell.

[0026] Preferably, the thickness of said centered portion of the shell varies no more than 4%, preferably no more than 3%, more preferably no more than 2.5%, along the axial extension of said portion of the shell. In cases where machining the shell internally comprises machining a plurality of circumferential grooves in the internal surface, the thickness of the shell is defined by the distances from the bottoms of the grooves to the external surface. Thereby, the thickness could be referred to as a nominal thickness.

[0027] The grooves may be provided to improve heat transfer to the external surface, e.g. in case the Yankee drying cylinder is steam heated. The grooves may be referred to as heat transfer enhancing grooves.

[0028] Said groves may be referred to as first grooves. It should be noted that in some embodiments, there could be additional, second grooves, with distances from the groove bottoms to the external surface, that differ from with distances from the bottoms of the first grooves to the external surface. In some embodiments, at each groove in the internal surface of said portion of the shell, or in a sub-group of grooves including at least 50%, 70%, or 90% of all machined grooves in the internal surface of said portion of the shell, the distance from the external surface with the final contour to the bottom of the groove deviates no more than 4%, preferably no more than 3%, more preferably no more than 2.5%, from the distance from the external surface to the bottom of any other of the grooves, or any other of the grooves in the sub-group.

[0029] Preferably the internal surface of the shell is machined so that, when the manufacturing method is completed, the shell has a capacity (k=X / t) to transfer heat from the internal surface to the external surface that varies no more than 4%, preferably no more than 3%, more preferably no more than 2.5%, along the axial extension of said portion of the shell.

[0030] In some embodiments, where machining the shell internally comprises machining a plurality of circumferential grooves in the internal surface, machining the grooves comprises machining the grooves so that the bottoms of one or more of the grooves in a region at a center of said portion of the shell of the Yankee drying cylinder are at a larger radial distance from the rotational axis than the bottoms of one or more of the grooves closer to axial ends of said portion of the shell. Thereby, the grooves may be adapted to a crown of the external surface.

[0031] In embodiments with the anticipated deformation, as exemplified above, the deformation causing manufacturing step may comprise welding end walls at axial ends of the shell. This welding may be done subsequently to machining the shell internally. Thereby, the method may comprise, before welding the end walls to the shell, determining the anticipated deformation as an anticipated deformation of the shell caused by the welding. Further, the method may comprise determining, based on the anticipated deformation, the shape of a virtual contour of the external surface of said portion of the shell, which virtual contour defines the location of the external surface in said cross-section before welding the end walls to the shell. Based on the virtual contour of the external surface, the intermediate internal surface may be designed so that in said cross-section and along the axial extension of said portion of the shell, a maximum difference of the radial distance from the virtual contour to the intermediate internal surface, or, where the intermediate internal surface presents a plurality of circumferential grooves, to the bottoms of the grooves, is less than a maximum difference of the radial distance from the rotational axis to the virtual contour.

[0032] For example, where a plurality of circumferential grooves are machined in the internal surface, the grooves may be machined so that the bottoms of the grooves are at substantially the same distance from the virtual contour.

[0033] Thereby, when the shell deforms due to the welding of the end walls to the shell, the virtual contour may move so as to coincide with the desired external surface according to the design of the external surface. If the intermediate internal surface is, or the bottoms of the grooves are, along the shell rotational axis, at substantially the same distance from the virtual contour, and the intermediate internal surface will at the welding follow the movement of the virtual contour, the intermediate internal surface, or the bottoms of the grooves, may be at the same distance from the external surface, along the shell rotational axis.

[0034] It should be noted that in some embodiments with the anticipated deformation, the method comprises providing a design of a final internal surface. Based on the design of the final internal surface and the anticipated deformation, the shape of the intermediate internal surface is determined. Thereby, no virtual contour of the external surface is needed.

[0035] The anticipated deformation at the deformation causing manufacturing step can be determined by an iterative process. For example, an assumed anticipated deformation can be used in a design and manufacturing process of a Yankee drying cylinder. When the cylinder has been finalized, measurements can be made, e.g. by means of a laser measuring tool, to establish the extent to which the thickness of the shell differs along the axial extension of said portion of the shell. Based on an established difference of the thickness along the axial extension, an adjusted anticipated deformation can be determined and used in a design and manufacturing process of another Yankee drying cylinder.

[0036] In some embodiments, the virtual contour may be closer to the Yankee rotational axis at the center of the shell than at the ends of the shell. Thereby, in embodiments where a plurality of circumferential grooves are machined in the internal surface, before welding, the grooves may be machined so that the bottoms of one or more of the grooves in a region at the center of the shell are at a smaller radial distance from the rotational axis than the bottoms of one or more of the grooves further away from the center of the shell.

[0037] Preferably, the method comprises machining the shell externally using the design of the external surface as a model for the external surface of said portion of the shell. Thereby, the machining the shell externally may be done subsequently to the machining of the shell internally. Where end walls are welded to the shell, the external machining of the shell may be done after the welding of the end walls to the shell.

[0038] In some embodiments, providing the design of the external surface of said portion of the shell comprises providing a design of a crown of the external surface. Thereby, the maximum crown may be at least 1.0 mm, preferably at least 1.5 mm. The maximum crown may be a difference in the distance from the external surface to the rotational axis, at the center of the shell, to the distance from the external surface to the rotational axis, at the axial ends of the shell. In some embodiments, the size of the final crown may differ from the size of the crown according to the design of the crown, due to minor manufacturing deviations not made on purpose.

[0039] The object is also reached with a Yankee drying cylinder according to any one of claims 14- 17, or use of a Yankee drying cylinder according to any one of claims 18-21.

[0040] DESCRIPTION OF THE DRAWINGS

[0041] Below embodiments of the invention will be described with reference to the drawings, in which fig. 1 shows schematically a part of a machine for producing a fibrous web, fig. 2 shows schematically an exploded cross-sectional view of a Yankee drying cylinder in the machine in fig. 1, with the section coinciding with a rotational axis of the cylinder, fig. 3 shows schematically a cross-sectional view of the Yankee drying cylinder in fig. 2 in an assembled state, fig. 4 shows a cross-sectional view of a part of a portion of a shell in the Yankee drying cylinder in fig. 3, at a step in a method for manufacturing the cylinder, according to an embodiment of the invention, with the section coinciding with a rotational axis of the cylinder,

[0042] - Fig. 4a depicts tools used in the manufacturing method described with reference to fig. 4, fig. 5 shows a flow diagram depicting steps in the manufacturing method described with reference to fig. 4, fig. 6 is a diagram showing a crown of an external surface of the cylinder shell with the axial position on the horizontal axis and the radial position on the vertical axis, fig. 7 shows the view of fig. 4 at a subsequent step of the manufacturing method, fig. 8 - fig. 10 show cross-sectional views of a part of a shell in a Yankee drying cylinder, at respective steps in a method for manufacturing the cylinder, according to another embodiment of the invention, with the section coinciding with a rotational axis of the cylinder, fig. 11 shows schematically a cross-sectional view of a Yankee drying cylinder manufactured according to a further embodiment of the invention, and fig. 12 - fig. 14 show cross-sectional views of a part of a shell in the Yankee drying cylinder in fig. 11, at respective steps in a method according to the further embodiment, with the section coinciding with a rotational axis of the cylinder.

[0043] DETAILED DESCRIPTION OF EMBODIMENTS

[0044] With reference to fig. 1, a Yankee drying cylinder 1 is shown in operation. The Yankee drying cylinder may be a part of a machine for making a fibrous web, such as a tissue web. A fibrous web W is carried by a fabric 50 to a nip between a nip roll 51 and the Yankee drying cylinder 1. The fabric 50 may be a felt which is permeable to air and water and capable of receiving water. The nip roll 51 may be, for example, a suction roll or a shoe press roll but it may also be some other kind of roll such as a deflection-compensated roll.

[0045] In the nip between the nip roll 51 and the Yankee drying cylinder 1, the fibrous web W is transferred to the external surface 7 of the Yankee drying cylinder 1. The nip may be a dewatering nip. Thereby, the dryness of the web when the web is carried by the fabric 50 to the nip may be 10-35% fiber content by weight, preferably 15-25% fiber content by weight. I.e. in the web, the mass of dry fibers is 10-35%, preferably 15-25%, of the mass of water and fibers. Preferably, the linear load in the nip is at least 70 kN / m, preferably at least 80 kN / m. Thereby, the web making machine could be without a Through Air Drying (TAD) cylinder upstream of the Yankee drying cylinder 1. Nevertheless, embodiments of the invention can also be applied to web making machines with one or more TAD cylinders.

[0046] The Yankee drying cylinder 1 has a smooth outer surface 7. The fibrous web W follows the surface of the Yankee drying cylinder 1 as the Yankee drying cylinder rotates in the direction of arrow R around a rotational axis RA.

[0047] The Yankee drying cylinder 1 is heated. It is normally heated from within by a heating medium such as hot steam. A source of hot steam is symbolically indicated by the reference numeral 16. As the fibrous web W is in contact with the outer surface 7 of the Yankee drying cylinder 1, the hot surface 7 of the Yankee drying cylinder 1 will cause water in the fibrous web W to evaporate such that the fibrous web W is dried.

[0048] As is known per se, the Yankee drying cylinder 1 may be partially enclosed by a hood (not shown) with hot air to further assist in the drying process.

[0049] The fibrous web W is removed from the surface 7 of the Yankee drying cylinder 1. For example, it may be creped away from the outer surface 7 by means of a doctor 53 as schematically indicated in fig. 1. It should be understood that the Yankee drying cylinder 1 is journalled for rotation and the reference numeral 52 indicates a journal for the Yankee drying cylinder 1. The Yankee drying cylinder may be followed by other equipment such as a reel- up.

[0050] As can be seen in fig. 2 and fig. 3, the Yankee drying cylinder 1 comprises a shell 2 that has an axial extension E (fig. 2), a first axial end 4 and a second axial end 5. Halfway between the axial ends 4, 5 is what is herein referred to as the center CS of the shell 2 or the center of the Yankee drying cylinder 1.

[0051] The shell 2 may have an external diameter between 2.0 m and 12.0 m, preferably between 4.0 m and 10.0 m, and an axial length between 1.0 m and 12.0 m, preferably between 2.0 m and The shell 2 has an internal surface 6 and an external surface 7. The shell is close to cylindrical, but the external surface 7 diverges from being cylindrical by a crown (not shown in fig. 2 and fig. 3), as described below.

[0052] For each axial end 4, 5 of the shell, an end wall 8, 9 is joined to the shell 2 such that the shell 2 and the end walls 8, 9 define an enclosed space 13 (see fig. 3). A central tie member 14 extends between the end walls 8, 9 and is joined to the end walls 8, 9. The central tie member 14 may be joined to the end walls 8, 9 by weld joints 19, 24, 25, 26 (see fig. 3). The end walls 8, 9 may be connected to the shell 2 by weld joints 20, 21, 22, 23 such that the entire structure comprising the shell 2, the end walls 8, 9 and the central tie member 14 is held together by weld joints 19, 20, 21, 22, 23, 24, 25, 26 (see fig. 3).

[0053] Preferably, the central tie member 14 is formed by or comprises a tubular structure that is connectable to a source 16 of hot steam outside the Yankee drying cylinder 1 (see fig. 1). As can be seen in for example fig. 3, the tubular structure is provided with openings 17 such that hot steam can enter the Yankee drying cylinder 1 via the tubular structure of the tie member 14, pass through the openings 17 in the tubular structure and reach the inner surface 6 of the shell 2.

[0054] The internal surface 6 of the shell 2 is provided with internal circumferential grooves 3. The circumferential grooves 3 serve to improve heat transfer to the outer surface 7 of the shell 2 when the Yankee drying cylinder 1 is being used. When a heating medium such as hot steam is sent into the enclosed space 13, the steam will condensate against the internal surface 6 and heat energy will be transferred to the shell 2.

[0055] The grooves may accommodate small pipes, so called "straw pipes" or "straws" (not shown), through which the mixture of steam and condensate is sucked. The straws are combined in racks (not shown) communicating with collectors which take the condensate-steam mixture towards the center of the cylinder from which it is brought to a separator.

[0056] With reference to fig. 4 - fig. 7 an embodiment of a method for manufacturing the Yankee drying cylinder 1 will be described. Design steps of the method may be done with a suitable design software on a computer 31, schematically depicted in fig. 4a. As a result of the design steps a software design file may be provided. A robotic milling machine 32 may be used for machining steps of the method. A control unit 33 may be arranged to control the milling machine 32. The control may be done with a software control file which is created based on the design file.

[0057] Fig. 4 shows a part of a centered portion 2a of the shell 2, which forms at least 80% of the axial extension of the shell. The axial extent of the shell portion 2a is indicated in fig. 2.

[0058] As an intermediate unit in the manufacturing process, the shell 2 may be provided with a cylindrical external surface 7a. The internal surface 6 may also be cylindrical.

[0059] The method comprises designing SI the external surface of the shell 2, or at least of the shell portion 2a. Thereby, a desired shape of a contour C7 of the external surface 7 of the shell 2, or at least of the shell portion 2a, on the Yankee drying cylinder 1 after completion of the manufacturing method, is determined. The contour C7 runs along a cross-section which coincides with a rotational axis RA (fig. 1) of the Yankee drying cylinder 1.

[0060] The contour C7 could be a crown contour. Thereby, the diameter of the external surface 7 may be continuously decreasing, non-linearly, from the center CS towards both ends 4, 5 (fig. 2). It is understood that thereby, there is at two different locations LI, L2 (fig. 4) along the rotational axis RA a difference in the radial distance RD1, RD2 from the rotational axis RA to the designed external surface 7.

[0061] The crown may be provided to compensate for one or more of temperature deformations during use, impact of nip press lead, internal steam pressure. Thereby, the Yankee drying cylinder and the nip roll may bend away from each other with the greatest amount of bending occurring at the centers of the cylinder and the roll. This manifests itself in the form of a pressure gradient between the center of the cylinder and its ends. To compensate for this bending and the resultant pressure gradient, a crown is used. The crown may be provided by lathing and / or grinding the external surface of the shell.

[0062] The crown may coincide with a curve, as exemplified in fig. 6 which shows a schematic crown contour within the interval called “crowned face”. The axial extension of the crowned face may be slightly smaller than the axial extension E of the shell (fig. 2). The crown contour will then be affected by two factors: the maximum crown in the center of the roll (indicated in fig. 6), and the crowned face on the cylinder. The maximum crown in the center of the roll may be for example between 1 mm and 3 mm.

[0063] In fig. 4 the maximum crown is exaggerated for this presentation.

[0064] The method further comprises designing S2 the internal surface 6 of the shell 2, so that the bottoms 3B of the grooves 3 are at substantially the same distance DB from the designed external surface 7 with the contour with the desired shape. As understood from fig. 4, in this example, this involves designing the grooves 3 so that the bottoms 3B of the grooves 3 in a region at a center CS of the shell 2 are at a larger radial distance from the rotational axis than the bottoms of grooves 3 further away from the center CS.

[0065] More generally, the internal surface 6a is designed such that, along the rotational axis RA, a maximum difference of the radial distance DB from the designed external surface 7 to the bottoms of the grooves 3, is less than a maximum difference MD (fig. 4) of the radial distance from the rotational axis RA to the external surface 7.

[0066] The method further comprises making S3 the grooves 3 using the design of the internal surface 6a as a model for the internal surface 6a. This may be done with the milling machine 32 sketched in fig. 4a, and a software control file which is created based on a design file including the internal surface design.

[0067] Subsequently to machining the grooves 3, the end walls 8, 9 are welded S4 to the at axial ends 4, 5 of the shell 2 (fig. 2).

[0068] As illustrated in fig. 7, subsequently, the shell 2 is machined S5 so as to obtain the determined desired shape of the external surface 7 contour. As a result, the bottoms 3B of the grooves 3 are at substantially the same distance from the external surface 7 with the contour with the desired shape.

[0069] As illustrated with the broken line SM, in some embodiments, the internal surface 6 is machined so that the distance between the internal and external surfaces 6, 7 are substantially the same along the rotational axis of the Yankee drying cylinder 1. For this, the ridges of between the grooves 3 may be machined. On other embodiments, such machining of the internal surface 6 may be omitted.

[0070] Thus, by taking the crown into consideration for the design of the inside of the shell, it is possible to reduce differences in the shell thickness.

[0071] With reference to fig. 8 - fig. 10 another embodiment of the invention will be described. In fig. 8 and 9-10 contour shapes have been exaggerated for clarity of this presentation.

[0072] It could happen that the shell 2 is deformed by the step of welding the end walls 8, 9 to it. If the geometrical deformation could be anticipated before welding, the machining of the inside of the shell could be made in such a way to compensate for the deformation.

[0073] As a first intermediate unit in the manufacturing process, the shell 2 may be provided with a cylindrical external surface 7a. The internal surface may also be cylindrical.

[0074] The method comprises designing S21 the external surface of the shell 2, or at least of a shell portion 2a similar to the one indicated in fig. 2. Thereby, a desired shape of a contour C7 of the external surface 7 of the shell 2 on the Yankee drying cylinder 1 after completion of the manufacturing method, is determined. The contour C7 could be a crown contour. It is understood that thereby, there is at two different locations LI, L2 (fig. 8) along the rotational axis RA a difference in the radial distance RD1, RD2 from the rotational axis RA to the designed external surface 7.

[0075] The method further comprises determining S22 an anticipated deformation before welding the end walls 8, 9 to the shell 2. The method further comprises determining S23, based on the anticipated deformation, the shape of a virtual contour V7 of the external surface 7 of the shell 2, which virtual contour V7 defines the location of the desired contour C7 before welding the end walls 8, 9 to the shell 2.

[0076] The method further comprises designing S24 the internal surface 6a with the plurality of circumferential grooves 3, so that the bottoms of the grooves 3 are at substantially the same distance DB from the virtual contour V7. In this example, as a result, the bottoms of one or more of the grooves 3 in a region at the center of the shell 2 are at a smaller radial distance from the Yankee rotational axis than the bottoms of one or more of the grooves 3 further away from the center of the shell.

[0077] The method further comprises making S25 the grooves 3 using the design of the internal surface 6a as a model for the internal surface 6a.

[0078] Reference is made to fig. 9. Subsequently, the end walls 8, 9 are welded S26 to the shell 2. Thereby the external surface 7a of the first intermediate unit is deformed to form a deformed external surface 7b of a second intermediate unit. However, as a result of the deformation, the virtual contour V7 coincides with the desired contour C7 of the external surface 7.

[0079] As illustrated in fig. 10, subsequently, the shell 2 is machined S27 so as to obtain the determined desired shape of the external surface 7 contour. As a result, the bottoms 3B of the grooves 3 are at substantially the same distance from the external surface 7 with the contour with the desired shape.

[0080] Thus, the internal surface is designed so that after the deformation caused by the welding, the bottoms 3B of the grooves 3 are at substantially the same distance from the external surface 7 with the contour with the desired shape. More generally, the internal surface 6a is designed such that, with the anticipated deformation, along the rotational axis RA, a maximum difference of the radial distance DB from the designed external surface 7 to the bottoms of the grooves 3, is less than a maximum difference MD (fig. 8) of the radial distance from the rotational axis RA to the external surface 7.

[0081] With reference to fig. 11, a Yankee drying cylinder 1 manufactured according to a further embodiment of the invention, will be described.

[0082] Inside the shell 2 a fixed electromagnetic induction heating system comprising an inductor 11 is arranged. The inductor 11 interacts electromagnetically with the shell 2 to produce induced electric currents in the shell. The inductor is arranged close to the internal surface 6 and does not rotate together with the shell 2. It may be supported by a suitable structure (not shown). In this embodiment, the internal surface 6 of the shell 2 is not provided with internal circumferential grooves 3.

[0083] Reference is made to fig. 12. In a method for manufacturing the Yankee drying cylinder 1 in fig. 11, as an intermediate unit, the shell 2 may be provided with a cylindrical external surface 7a, and the internal surface 6 may also be cylindrical.

[0084] The method comprises designing the external surface of the shell 2, or at least of a shell portion 2a similar to the one indicated in fig. 2. Thereby, a desired shape of a contour C7 of the external surface 7 of the shell 2 on the Yankee drying cylinder 1 after completion of the manufacturing method, is determined. The contour C7 could be a crown contour. In fig. 12 the maximum crown is exaggerated for this presentation.

[0085] Reference is made also to fig. 13. The method further comprises designing the internal surface of the shell 2, so as to provide an internal surface 6b which is, along the rotational axis of the Yankee, at substantially the same distance DB from the external surface 7 with the contour with the desired shape. As understood from fig. 13, in this example, this involves designing the internal surface so that the internal surface 6b is in a region at a center CS of the shell 2 at a larger radial distance from the rotational axis than it is further away from the center.

[0086] The method further comprises machining the internal surface of the shell 2 according to the design of the internal surface. Subsequently to machining the internal surface 6, the end walls 8, 9 (fig. 11) are welded to the at axial ends of the shell 2.

[0087] As illustrated in fig. 14, subsequently, the shell 2 is machined so as to obtain the determined desired shape of the external surface 7 contour. As a result, the machined internal surface 6b is, along the rotational axis of the Yankee, at the same distance DB from the external surface 7 with the contour with the desired shape.

[0088] Embodiments of the invention can also be used in other types of Yankee heating technologies, for example where one or more heating element, such an electrical resistive heating element, are attached to the internal surface of the shell. It is to be understood that the embodiments described above and shown in the drawings are only to be regarded as non-limiting examples of the invention and that it may be modified in many ways within the scope of the patent claims.

Claims

1. CLAIMS1. A method for manufacturing a Yankee drying cylinder (1) for a machine for manufacturing a fibrous web, comprising providing (SI) a design of an external surface (7) of a portion (2a) of a shell (2) of the Yankee drying cylinder (1), which portion starts and ends, in the axial direction of the shell, no more than 300 mm, preferably no more than 200 mm, from axial ends (4, 5) of the shell, such that, in a cross-section which coincides with a rotational axis (RA) of the Yankee drying cylinder (1), there is at two different locations (LI, L2) along the rotational axis (RA), in said portion of the shell, a difference in the radial distance (RD1, RD2) from the rotational axis (RA) to the external surface (7), providing (S2) a design of a machined internal surface of said portion (2a) of the shell (2), and machining (S3) the shell internally, using the design of the machined internal surface (6a) as a model for the machined internal surface (6a) of said portion of the shell (2), characterized in that- the design of the machined internal surface (6a) is such that, in said cross-section and along the axial extension of said portion of the shell, a maximum difference of the radial distance (DB) from the external surface (7) to a final internal surface (6a) of said portion of the shell, or, where the final internal surface (6a) of said portion of the shell presents a plurality of circumferential grooves (3), to the bottoms of the grooves (3), is less than a maximum difference (MD) of the radial distance from the rotational axis to the external surface (7) of said portion of the shell.

2. A method according to claim 1, wherein the thickness of said portion (2a) of the shell varies no more than 4%, preferably no more than 3%, more preferably no more than 2.5%, along the axial extension of said portion of the shell.

3. A method according to any one of the preceding claims, wherein machining the shell internally comprises machining a plurality of circumferential grooves (3) in the internal surface (6), wherein machining the grooves (3) comprises machining the grooves (3) so that the bottoms of one or more of the grooves (3) in a region at acenter of said portion (2a) of the shell of the Yankee drying cylinder (1) are at a larger radial distance from the rotational axis than the bottoms of one or more of the grooves (3) closer to the axial ends of said portion of the shell.

4. A method according to any one of the preceding claims, wherein the design of the machined internal surface (6a) is a design of the final internal surface (6a) of said portion (2a) of the shell (2).

5. A method according to any one of claims 1-3, wherein the design of the machined internal surface (6a) is a design of an intermediate internal surface of said portion (2a) of the shell (2), which intermediate internal surface is made in dependence on an anticipated deformation of the shell at a deformation causing manufacturing step following the step of machining of the shell internally whereby the intermediate internal surface becomes the final internal surface.

6. A method according to claim 5, wherein the deformation causing manufacturing step comprises welding end walls (8, 9) at axial ends of the shell (2).

7. A method according to claim 6, comprising, before welding the end walls (8, 9) to the shell (2), determining the anticipated deformation as an anticipated deformation of the shell caused by the welding.

8. A method according to claim 7, comprising determining, based on the anticipated deformation, the shape of a virtual contour (V7) of the external surface (7) of said portion (2a) of the shell (2), which virtual contour (V7) defines the location of the external surface (7) in said cross-section before welding the end walls (8, 9) to the shell (2).

9. A method according to claim 8, comprising providing the design of an intermediate internal surface of said portion (2a) of the shell (2), such that, in said cross-section and along the axial extension of said portion of the shell, a maximum difference of the radial distance (DB) from the virtual contour (V7) of the external surface (7) to the intermediate internal surface, or, where the intermediate internal surface presents a plurality of circumferential grooves (3), to the bottoms of the grooves (3), is less thana maximum difference of the radial distance from the rotational axis to the external surface (7) of said portion of the shell with the anticipated deformation.

10. A method according to any one of the preceding claims, comprising machining the shell externally using the design of the external surface (7) as a model for the external surface (7) of said portion (2a) of the shell (2).

11. A method according to claim 10, wherein the machining the shell (2) externally is done subsequently to the machining of the shell internally.

12. A method according any one of the preceding claims, wherein providing the design of the external surface (7) of said portion (2a) of the shell (2) comprises providing a design of a crown of the external surface.

13. A method according to claim 12, wherein the maximum crown is at least 1.0 mm, preferably at least 1.5 mm.

14. A Yankee drying cylinder (1) comprising a shell (2), wherein a portion (2a) of the shell (2) forms 80% of the axial extension of the shell and is centered in the axial direction of the shell, wherein, in a cross-section which coincides with a rotational axis (RA) of the Yankee drying cylinder (1) and along the axial extension of said portion of the shell, there is, at two different locations (LI, L2) along the rotational axis, a difference in the radial distance, from the rotational axis to an external surface (7) of said portion, characterized in that, in said cross-section and along the axial extension of said portion of the shell, a maximum difference of the radial distance (DB) from the external surface (7) to an internal surface (6a) of said portion (2a) of the shell (2), or, where the internal surface (6a) presents a plurality of circumferential grooves (3), to the bottoms of the grooves (3), is less than a maximum difference (MD) of the radial distance from the rotational axis to the external surface (7).

15. A Yankee drying cylinder (1) according to claim 14, wherein the thickness of said portion (2a) of the shell varies no more than 4%, preferably no more than 3%, more preferably no more than 2.5%, along the axial extension of said portion of the shell.2216. A Yankee drying cylinder (1) according to any one of claims 14-15, wherein the external surface (7) of the shell (2) is crowned.

17. A Yankee drying cylinder (1) according to any one of claims 14-16, wherein a plurality of circumferential grooves (3) are provided in the internal surface (6) of the shell (2), wherein the bottoms of one or more of the grooves (3) in a region at a center of the Yankee drying cylinder (1) are at a larger radial distance from the rotational axis (RA) of the Yankee drying cylinder (1) than the bottoms of one or more of the grooves (3) closer to axial ends of said portion (2a) of the shell of the Yankee drying cylinder (1).

18. Use of a Yankee drying cylinder according to any one of claims 14-17, or of a Yankee drying cylinder manufactured according to any one of claims 1-13, wherein a fibrous web (W) is carried by a fabric (50) to a nip between the Yankee drying cylinder (1) and a nip roll (51).

19. Use according to claim 18, wherein the linear load in the nip is at least 70 kN / m, preferably at least 80 kN / m.

20. Use according to any one of claims 18-19, wherein the nip is a dewatering nip.

21. Use according to any one of claims 18-20, wherein the dryness of the web when the web is carried by the fabric (50) to the nip is 10-35% fiber content by weight, preferably 15-25% fiber content by weight.

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