DESCRIPTION

TECHNICAL FIELD

[0001] The present invention relates to components of machines used in plants for producing cellulose webs, in particular in paper-making plants. Some embodiments disclosed herein especially refer to cylinders for machines producing cellulose plies or webs, such as paper and tissue paper. In particular, hollow cylinders are disclosed that are adapted to be pressurized by means of steam or other heat transfer fluid and intended to dry cellulose plies or webs driven around the cylinders.

BACKGROUND ART

[0002] Wet methods are generally used for producing cellulose webs, sheets, and plies, such as paper plies. According to these methods, cellulose fibers are suspended in water; the obtained suspension is distributed on forming fabrics by means of headboxes so as to form a layer of cellulose pulp. The water content in the pulp is decreased by draining systems, until a cellulose fibers layer is formed, the solid content whereof is sufficient to transfer the layer onto one or more drying cylinder that remove further humidity from the cellulose layer by making it evaporate thanks to heat transferred to the cellulose layer from the inside of the cylinder through the cylindrical wall of the cylinder. Heat is usually supplied by a heat transfer fluid flowing in the drying cylinder. To this end, the drying cylinder is hollow.

[0003] The heat transfer fluid is usually high-pressure steam. High pressure values are used to allow operating temperatures that are high enough to remove efficiently humidity from the cellulose layer until to have the desired residual humidity level before removing the obtained cellulose ply from the drying cylinder.

[0004] Therefore, the drying cylinders shall be so manufactured as to withstand high pressures under dynamic load conditions and high operating temperatures.

[0005] Yankee cylinders, which are generally used for the production of paper, especially tissue paper, as well as drying rollers work under particularly heavy operating conditions.. These machine members are bulky and critical from a safety viewpoint, because of the operating pressures and temperatures and the consequent thermal and mechanical stresses. High speed rotation of Yankee cylinders and other drying cylinders used in paper-making machines generate fatigue stresses, which are particularly critical due to the risk of breakage in case of cracks or defects in the metal, of which the cylinders are made.

[0006] Originally, drying rollers and Yankee cylinders were mainly made of cast iron. Recently, several systems have been studied for manufacturing drying rollers and Yankee cylinders made of steel. This material is particularly suitable for drying rollers and Yankee cylinders as it allows a better transfer of heat to the cellulose ply driven around the outer surface of the cylinder. The steel cylinders are comprised of parts that are welded together, typically a cylindrical shell with two axial ends, to which respective end walls are welded, these walls carrying the pins for supporting and rotating the cylinder.

[0007] Examples of welded Yankee cylinders are disclosed in US, 438, 752, EP1838922, WO2020/194358, DE202020106010, WO2019/209164, EP3556936, US2016/0228969.

[0008] AT-11146U1 discloses a steel Yankee cylinder including a cylindrical shell, the axial ends whereof form respective annular projections facing towards the axis of the cylindrical shell. Along each annular projection a respective end wall is connected. The perimeter edge of each end wall and the radially inward edge of the respective annular projection are machined so as to connect the end wall and the annular projection by means of an inner welding bead and an outer welding bead. The maximum diameter of each end wall is equal to or smaller than the inner minimum diameter of the respective annular projection of the axial end of the cylindrical shell. In this way the end wall can be positioned with its respective peripheral edge aligned with the annular projection of the axial end of the cylindrical shell and the two members can be welded to one another by the inner and outer welding beads. Mechanical stresses in axial direction generated on each end wall by the steam pressure inside the Yankee cylinder are entirely supported by the welding beads, which represent critical points from the viewpoint of the mechanical resistance of the Yankee cylinder. [0009] The welds joining together the end walls and the cylindrical shell in Yankee cylinders and drying rollers for paper-making machines shall meet very high quality and safety standards, as these cylinders are in practice pressure tanks. The welds shall withstand the stresses generated by pressure, the weight of the cylinder and the thermal expansion. These stresses vary as the temperature changes and, due to the rotation of the cylinders and the change in operating conditions, they may be cyclical, thus inducing fatigue stresses. Therefore, the welds shall not have defects that could cause dangerous breakages. Therefore, only highly specialized personnel can make these welds, and the welds shall be checked through X-rays, both at the beginning of operation and periodically over time. This significantly affects both the production costs and times and the operating cost in the course of the useful life of the cylinder.

[0010] In any case, welds still remain potential cause of even catastrophic breakages of the cylinder, with the risk of accidents, which may be fatal due to the operating temperatures and pressures of these machine members.

[0011] In addition to being less thermally efficient, the cylinders made of cast iron also require that the end walls and the cylindrical shell are joined together through screw couplings. These couplings greatly affect the cylinder production costs, as they require a lot of boring and threading processing, as well as long assembly times.

[0012] It would be therefore beneficial to provide Yankee cylinders and other drying rollers to be used in paper-making machines that partially or completely overcome the drawbacks of the prior art cylinders mentioned above.

SUMMARY OF THE INVENTION

[0013] According to an aspect, a cylinder of a paper-making machine is disclosed, comprising a cylindrical shell that defines an axis of the cylinder and has opposite axial ends, to which two end walls are fastened, closing the hollow inner space of the cylinder. Respective pins for supporting and rotating the cylinder may be attached to the two end walls. At least one of the axial ends of the cylindrical shell comprises an annular inner projection that extends radially towards the cylinder axis and has a minimum diameter that is smaller than the maximum diameter of the respective end wall. The end wall is so mounted that a portion of the end wall having the maximum diameter is arranged in an axially inboard position with respect to a portion of the annular inner projection having the minimum diameter. In this way, in use, the pressure in the cylinder pushes the end wall to abut against the annular inner projection of the respective axial end of the cylinder, thus discharging the stress caused by the pressure onto the axial end.

[0014] Differently from the prior art Yankee cylinders, in particular the Yankee cylinder of ATI 1146U1, the axial thrust generated by the steam pressure in the Yankee cylinder is not applied on the weld.

[0015] There is therefore no need for expensive welds or screw connections for joining together the end wall and the shell. A less expensive and safer cylinder is thus provided, that can also require less manufacturing time.

[0016] Preferably, the same mounting criterion is applied to both the end walls of the cylinder, using radial projections inside the two axial ends of the cylindrical shell.

[0017] Essentially, according to an aspect the invention provides for a cylinder with an outer cylindrical shell and two opposite end walls, wherein at least one axial end, and preferably both axial ends, of the shell define an annular projection forming an undercut, against which the respective end wall is fastened from the inside. In practice, the end walls have a maximum diameter that is greater than the minimum inner diameter of the respective projection.

[0018] The undercut may be formed by providing an end wall having two portions of different diameter, and preferably of cylindrical shape, put over each other along the axis of the cylinder. The portion of greater diameter is arranged inside a portion of smaller diameter of the annular projection. In further embodiments, the undercut is formed by providing an annular projection having a radially inner surface of truncated- conical shape and a corresponding end wall having a truncated-conical perimeter surface, whose shape is substantially identical to that of the truncated-conical inner surface of the annular projection. The two truncated-conical surfaces are oriented such that the larger base faces the inside of the cylinder whilst the smaller base faces the outside of the cylinder.

[0019] To allow mounting, the projection and/or the end wall have at least a notch or a flattened part or a recess, allowing the end wall to be inserted beyond the annular proj ection.

[0020] According to a further aspect, a method is provided for manufacturing a cylinder for paper-making machines, comprising the following steps: providing a cylindrical shell defining an axis of the cylinder and having a first axial end and a second axial end; wherein the first axial end comprises an annular inner projection that extends radially towards the axis of the cylinder; inserting, in the cylindrical shell, at least a first end wall having a maximum diameter greater than a minimum diameter of the first annular projection; wherein the insertion of the first end wall through the first annular projection is allowed by at least one of the following: a radial notch in the first annular projection, a radial perimeter recess of the first end wall.

[0021] Further advantageous features and embodiments of the cylinder and of the method for manufacturing will be described below and are set forth in the attached claims, which form an integral part of the present description.

BRIEF DESCRIPTION OF THE DRAWING

[0022] The invention shall be better understood by following the description and the accompanying drawing, which show non-limiting exemplary embodiments of the invention. More specifically, in the drawing:

Fig. l is a schematic cross-section of a cylinder in an embodiment;

Fig.2 shows an enlargement of the detail I of Fig.l;

Fig.3 shows an enlargement of the detail of Fig.2 in a further embodiment;

Fig.4 shows an enlargement of the detail of Fig.2 in a further embodiment;

Figs. 5 A, 6A, 7A, 8A and 9A are front views of a sequence for mounting an end wall on a cylindrical shell;

Figs. 5B, 6B, 7B, 8B, 9B and 10 are side views of the same mounting sequence;

Figs. 11 to 17 show a sequence for mounting the rod and the support and rotation pins on the two end walls of the cylinder;

Figs. 18A and 18B show respectively a front and a side view of an equipment for mounting the end walls;

Fig.19 shows a detail of an embodiment of a cylinder with inner grooves for condensate collection;

RECTIFIED SHEET (RULE 91) ISA/EP Figs. 20A, 21A, 22A, and 23A are front views of a sequence for mounting an end wall on a cylindrical shell in a further embodiment;

Figs. 20B, 21B, 22B and 23B are side views of the same mounting sequence of Figs.20A-23A;

Figs. 25A, 26A, and 27A are front views of a further sequence for mounting an end wall on a cylindrical shell in a further embodiment;

Figs. 25B, 26B, and 27B are side views of the same mounting sequence of Figs.25A-27A;

Fig.28 is a schematic front view of an end wall in a further embodiment, before being mounted on the cylinder;

Fig.29 shows a side view according to XXIX-XXIX of Fig.28;

Fig.30 is a side view of an end of the cylindrical shell to be combined with the end wall of Figs. 28, 29;

Fig.31 shows a cross-section according to XXXI-XXXI of Fig.39;

Fig.32 is a side view of the end wall of Fig.28, 29 aligned with the cylindrical shell for being mounted thereon;

Fig.33 shows a cross-section according to XXXIII-XXXIII of Fig. 32;

Fig.34 shows a view analogous to that of Fig.32 in a subsequent mounting step;

Fig.35 shows a cross-section according to XXXV-XXXV of Fig.34;

Fig.36 shows a view analogous to that of Fig.34 in a subsequent mounting step;

Fig.37 shows a cross-section according to XXXVII-XXXVII of Fig.36;

Fig.38 shows a view analogous to that of Fig.36 in a subsequent mounting step;

Fig.39 shows a cross-section according to XXXTX-XXXTX of Fig.38;

Fig.40 shows a cross-section according to the line XL-XL of Fig.34;

Fig.41 shows a cross-section according to the line XLI-XLI of Fig.36; and

Fig.42 shows a cross-section according to the line XLII-XLII of Fig.38.

DETAILED DESCRIPTION

[0023] As it will be clearly apparent from the detailed description below of some embodiments, in order to have a simpler and safer structure of the cylinder it is envisaged to manufacture a cylindrical shell with axial ends that are provided with annular inner projections forming respective shoulders, against which respective end walls rest and to which are fastened and welded. In this way, the pressure inside the cylinder, generated for example by heating steam, generates a thrust on the end wall, which is

RECTIFIED SHEET (RULE 91) ISA/EP discharged onto the annular projection rather than being integrally discharged onto the welds or screw couplings joining the end wall and the cylindrical shell. Even if, in principle, this technique may be applied to only one of the two end walls of the cylinder, it is however advantageous to adopt it for both the end walls.

[0024] For allowing insertion of an end wall in the cylindrical shell beyond the annular projection having a diameter that is smaller than the outer diameter of the end wall, the end wall and/or the annular projection may be provided with one or more notches or recesses, i.e. flattened parts, so as to have points of the end wall where the diameter is smaller than the maximum diameter and/or points of the annular projection where the inner diameter is greater than the minimum diameter.

[0025] As it will be detailed below, this arrangement is preferably provided for both end walls. In other words, both the end walls, with the respective annular inner projections, are so shaped as to allow the end wall to be inserted in the cylindrical shell beyond the annular projection. However, in currently less advantageous embodiments, it is possible to use this technique for mounting only one of the two end walls, making it pass beyond an annular projection of smaller inner diameter, whilst mounting the other one in a different way.

[0026] With reference to the drawing, Figs. l and 2 show a cross-section according to a plane containing the rotation axis, of a simplified embodiment of a cylinder according to the invention. The cylinder is indicated with the reference number 1 and the rotation axis thereof is labeled A-A.

[0027] The cylinder 1 may be a Yankee cylinder for a machine for paper wet manufacturing. The cylinder 1 may be also a different drying cylinder, for instance a dryer roller for a paper-making machine.

[0028] The cylinder comprises a cylindrical shell 3 having a first axial end and a second axial end, both labeled 3 A. Each axial end is closed by a respective end wall 5. Inside the end walls 5 and the cylindrical shell 3 a hollow inner space 7 is defined, where a heat transfer fluid can be supplied and/or flow. If the cylinder 1 is a drying cylinder or a Yankee cylinder for paper-making machines, the hollow inner space 7 may be used for making hot steam circulate, thus transferring heat to the paper driven around the cylindrical surface of the cylindrical shell 3. The end walls 5 are integral with respective pins 9 for supporting and rotating the cylinder 1.

[0029] At least one of the axial ends 3A of the cylindrical shell 3 comprises an annular inner projection. In the illustrated example, each of the two axial ends comprises a respective annular inner projection 11 that extends radially towards the axis A-A of the cylinder 1. Fig.2 shows an enlargement of a detail of one of the two annular projections 11 in an embodiment. The enlarged portion corresponds to the area indicated with I in Fig.1.

[0030] In Figs.l and 2, the minimum inner diameter of the annular inner projection 11 is labeled DI, and the maximum diameter of the respective end wall 5 is labeled D2. The minimum inner diameter DI of the inner projection 11 is smaller than the maximum diameter D2 of the end wall 5. In this way, the force generated on each end wall 5 by the pressure of the fluid filling the inner space 7 of the cylinder 1 is discharged on the respective annular projection 11.

[0031] Each end wall 5 and the respective annular projection 11 may be welded together through a first welding bead 13 provided on the outer surface of the end wall. The welding bead 13 does not serve as a structural resistant element, since the thrust generated by the pressurized fluid inside the cylinder 1 is discharged, as mentioned above, from the end wall 5 to the shell 3 through the annular projection 11. The main function of the welding bead 13 is to provide a seal preventing leakage of pressurized fluid from the inner space 7 of the cylinder 1.

[0032] For preventing oxidation in the interface area between the end wall 5 and the projection 11, a second welding bead 15 may be advantageously provided, applied to the inner surface of the end wall. Usually, the dimensions of the cylinder 1 are such to allow a person to enter inside the cylinder 1 through a manhole (not shown) provided in one of the two end walls 5, for making the inner welding bead 15.

[0033] In the embodiment of Fig.2, the end wall 5 comprises a first portion 5 A and a second portion 5B. In this embodiment, the portions 5 A and 5B have cylindrical shape. The first portion 5A has a diameter smaller than that of the second portion 5B. More in particular, the portion 5 A has a diameter approximately equal to the minimum inner diameter DI of the annular projection 11 and is surrounded by the annular projection 11. In the illustrated embodiment, the portion 5 A has a thickness, i.e. an axial dimension (dimension parallel to the axis A-A of the cylinder 1) smaller than the axial dimension of the annular projection 11, so that the welding bead 13 is inside the annular projection 11.

[0034] The second portion 5B is the portion of end wall having the maximum diameter D2. In the embodiment illustrated in Fig.2, the diameter D2 is smaller than the inner diameter of the cylindrical shell 3, so as to leave enough space for the inner welding bead 15.

[0035] Fig.3 shows an enlargement of the detail I of Fig. 1 in a second embodiment. The same reference numbers indicate parts that are the same or equivalent to those described with reference to Figs.l and 2 and that will not be described again. In Fig.3, close to each axial end 3 A the cylindrical shell 3 comprises a groove 3B for optimizing the distribution of stresses generated inside the cylindrical shell 3 by the pressure in the inner space 7 of the cylinder 1.

[0036] Fig.4 shows an enlargement of the detail I of Fig. 1 in a third embodiment. The same reference numbers indicate the same or equivalent parts to those described with reference to Figs.l and 2, that will not be described again. In Fig.4 the end wall 5 has a first portion 5A of truncated-conical shape and an inner second portion 5B of cylindrical shape. The maximum diameter D2 (diameter of the larger base of the truncated cone) of the truncated-conical portion 5A is greater than the minimum inner diameter DI represented by the diameter of the smaller base of the truncated cone, corresponding to the smaller diameter of the annular projection 11, that is smaller than the maximum diameter D2. This conical coupling between the annular projection 11 and the end wall 5 has the same result of discharging onto the annular projection 11 the forces applied to the end wall 5, in particular the stress in axial direction.

[0037] The cylindrical shell 3 with the respective annular projections 11 may be manufactured using any known technique. For example, the axial ends 3 A of the cylindrical shell 3 may be welded to a central portion of constant annular cross-section; or they may be realized from a raw cylindrical piece through chip removal of a layer of material of thickness equal to the radial thickness of the inner surface along the whole axial extension of the cylindrical piece. In further embodiments, the cylindrical shell 3 may be forged. These two techniques (chip removal and forging) avoid the need for circumferential welds on the cylindrical shell.

[0038] The sequence of Figs.5A, 5B to 9A, 9B shows how to mount the end walls 5. In the illustrated embodiment, the end walls 5 are symmetrical and are mounted in the same way. The sequence of Figs. 5A-9B shows the mounting of only one of the end walls 5. In this sequence, the figures indicated with the letter A show the mounting in a cross-section according to a plane containing the axis of the cylinder 1, whilst the figures indicated with the letter B show the mounting from a leading view, i.e., observing the cylinder 1 according to a visual line parallel to the axis A-A of the cylinder 1.

[0039] In the sequence of Figs.5A-9B the embodiment of Figs.1 and 2 is shown. The same mounting sequence can be used for mounting the end walls of the embodiment of Figs.3 and 4.

[0040] Moreover, whilst in Figs. l and 2 a simplified cylinder is shown, in the sequence of Figs.5A-9B the mounting is shown of a cylinder 1 adapted to be provided with a central rod joining the two end walls 5 together. Figs.10 to 17 show the steps for mounting the rod and the support and rotation pins that, in this case, are applied to the rod, which is in turn connected to the end walls 5. If a rod is not provided, the support and rotation pins may be pre-mounted on the end walls or, preferably, mounted on the end walls once these latter have been fastened to the cylindrical shell.

[0041] Coming back to the sequence of Figs.5 A- 9B, in Figs.5 A, 5B the step is shown when an end wall 5 is moved towards the respective axial end 3A of the cylindrical shell 3. The end wall is kept rotated by 90° with respect to the final position, i.e. it is kept with the median plane thereof on a plane containing the axis A-A of the cylinder 1.

[0042] In this embodiment, for inserting the end wall 5 inside the cylindrical shell 3 beyond the annular inner projection 11 of minimum inner diameter DI smaller than the diameter D2 of the end wall 5, the annular projection 11 comprises two diametrically opposite notches 11A of such a depth that the end wall 5 can be moved beyond the annular inner projection 11, aligned with the notches 11 A, as shown in Figs.5A, 5B, 6A, 6B. [0043] In the illustrated embodiment, each end wall 5 has a central hole 5C for anchoring the axial rod, as described below.

[0044] In Figs.6A, 6B the end wall 5 has been inserted beyond the annular inner projection 11 and can be rotated around a diameter axis parallel to the alignment of the notches 11 A, as shown in Figs.7A, 7B, until it takes the position orthogonal to the axis A-A, as shown in Figs.8A, 8B. Now, each end wall can be moved parallel to the axis A-A until to abut against the annular inner projection 11, as shown in Figs.9A, 9B. In this position one or both welds 13, 15 can be performed.

[0045] Fig.10 shows the partially mounted cylinder 1, with the end walls 5 welded to the cylindrical shell 3. Welds may be done at the notches 11 A for sealing the inner space 7 of the cylinder 1.

[0046] In a modified embodiment, a single notch 11 A of greater depth may be provided in an annular inner projection 11 that may have a radial extension greater than that shown in the figures. This modified of embodiment can be less preferred, as the opening that remains at the notch after having mounted the end wall 5 is of greater dimensions and therefore requires a more accurate welding for sealing the inner space 7 of the cylinder 1.

[0047] Figs.11 to 17 show a sequence of mounting of the axial rod and of the support and rotations pins 9.

[0048] As shown in Fig.11, in this embodiment the support and rotation pins 9 are integral with two portions 21 of an axial rod 23 (Fig.17) connecting the two end walls 5 to each other. Each portion of rod 21 is inserted through the hole 5C of the respective end wall 5, as shown in Fig.12, until the two rod portions 21 abut against each other (Fig.16). More in particular, in the illustrated embodiment the two rod portions 21 are inserted in the holes 5C (Figs. 11, 12) by a bridge crane, not shown, supporting the portions through aerial support members 31. Then, a lower support 33 is inserted for each rod portion 21. By supporting each rod portion 21 through the respective end wall 5 and the respective lower support 33, it is possible to move the aerial support members 31 so as to engage the support and rotation pins 9 (Fig.14). The lower supports 33 are removed (Fig.15) and the two rod portions 21 are completely inserted in the cylinder 1 until the two inner ends 21 A of the rod portions 21 abut against each other. At the end 21A each rod portion 21 comprises an inner flange 21B. The two inner flanges 21B are used to join the two rod portions 21 together, for example through screw-bolt or welding coupling, not shown. The inner dimension of the rod 23 is such as to allow a person to enter, through a manhole (not shown), inside at least one of the two rod portions 21 for coupling the two portions 21 together.

[0049] The coupling between the two rod portions 21 may be such as put the rod 23 under tensile stress. To this end, the mutual coupling between the two rod portions 21 is advantageously realized through screws and bolts.

[0050] Each rod portion 21 comprises, at the end opposite to the end 21A, a flange 21C that is inserted in a seat provided in the outer face of the respective end wall 5. The end wall 5 and the respective flange 21C of the rod portion 21 may be joined through a screw-bolt coupling, not shown.

[0051] Fig. 17 shows the fully assembled cylinder 1. In this figure, also the bolts connecting the rod 23 to the end walls 5 are schematically shown, as well as the bolts connecting the two rod portions 21 together.

[0052] In Figs.l8A and 18B an equipment 34 is schematically shown, which may be used for mounting a flange 5 in the cylindrical shell 3 of the cylinder 1. The equipment 34 may be supported by a bridge crane (not shown).

[0053] In some embodiments, the cylindrical shell 3 of the cylinder 1 can be provided with annular grooves 3S for collecting the condensate that is formed during machine operation due to the transfer of vaporization heat from the steam inside the inner space 7 of the cylinder 1 through the cylindrical shell 3 towards a cellulose ply driven around the outer cylindrical surface of the cylindrical shell 3.

[0054] Similarly to the sequence of Figs.5A to 9B, Figs.20A, 20B, 21 A, 21B, 22A, 22B, 23 A, 23B show a sequence of mounting of an end wall 5 in the cylindrical shell 3 in a further embodiment. The same numbers indicate the same or equivalent parts to those of the previous figures, which will not be described again.

[0055] The main difference between the embodiment of Figs.5A-9B and the embodiment of Figs.20A-23B is that, in this case, for inserting the end wall 5 in the cylindrical shell 3 beyond the annular inner projection 11, the end wall 5 comprises two radial penmeter recesses 5D in the shape of chord sectors removed from the circular edge of the end wall 5. The two chord sectors are arranged in two diametrically opposite peripheral areas and are identified by two respective chords ideally parallel to each other. The distance between the two flattened parts forming the recesses 5D is such that the end wall 5 can be inserted beyond the annular inner projection 11, whose minimum inner diameter DI is smaller than the maximum diameter D2 of the end wall.

[0056] The subsequent operations for assembling the cylinder 1 are essentially the same already described above. The two openings that remain along the annular inner projection or edge 11 in correspondence of the flattened parts or recesses 5D may be closed through welding.

[0057] In a modified embodiment, only a single deeper radial perimeter recess 5D is provided instead of two diametrically opposite recesses. However, in this case the opening to be welded between the end wall 5 and the annular inner projection 11 is larger.

[0058] Figs.24A, 24B to 27A, 27B show a sequence of mounting an end wall 5 on the cylindrical shell 3 in an improved embodiment. The same numbers indicate the same or equivalent parts to those of the previous embodiments, which will not be described again.

[0059] In the sequence of Figs.24A to 27B an end wall 5 is provided, comprising two diametrically opposite radial flattened parts or perimeter recesses 5D and an annular inner projection 11 with two diametrically opposite notches 11 A. With this configuration, the depth of the notches 11A and of the radial perimeter recesses 5D may be halved with respect to the depth of the notches 11 A and the radial perimeter recesses 5D of the previous embodiments. For instance, the depth of the notches 11A and the consequent depth of the recesses 5D may be such that the recesses 5D are provided only in the portion 5 A of greater diameter of the end wall 5. The portion 5B of smaller diameter, essentially equal to the minimum inner diameter DI of the annular inner projection 11, is not recessed and keeps a continuous circular profile.

[0060] In this way, once the end wall 5 has been inserted in the inner space 7 of the cylindrical shell 3, has been rotated around the axis orthogonal to the axis A-A of the cylindrical shell (Figs.25 A, 26A) and has abutted against the annular inner projection 11 (Figs.26A, 26B), it is rotated around the axis A-A so as to bring the recesses 5D in an angularly offset position with respect to the notches 11 A. In this way there is no opening along the annular edge of the end wall 5, and this latter is sealed by continuous welding beads 13, 15 without the need for further material for closing the openings that, in the other embodiments, remain in correspondence of the notches 11A or the recesses 5D. In fact, the radial perimeter recesses 5D are sealed by integer (not notched) portions of the annular projection 11.

[0061] In a modified embodiment, a single radial notch 11 A is provided in the annular projection 11 and a single recess 5D is provided in the end wall 5, that are then angularly offset with respect to each other in order to seal mechanically the end wall 5 through the integer portion of the annular projection 11.

[0062] In all embodiments of Figs.20A and following, an axial rod 23 is mounted, as shown in Figs.5A-19.

[0063] In other embodiments, for example for cylinders of smaller dimensions, the axial rod 23 may be omitted, as schematically indicated in Fig.1.

[0064] In the illustrated embodiments, in which notches are provided in the radial projections for allowing or facilitating the passage of the end wall, each annular inner projection is provided with at least one radial notch, beyond which the respective end wall passes. This configuration is particularly advantageous as it allows insertion of each end wall directly in the axial end of the cylindrical shell in the position where the end wall shall be then fastened. In further embodiments, only one annular inner projection is provided with a notch or with two diametrically opposite radial notches. In this case, both end walls are inserted beyond the same annular inner projection. One of the two end walls is translated along the whole axial extension of the cylindrical shell, until it reaches the axial end opposite to that beyond which it has been inserted. The other end wall is inserted through the axial end, to which it is then fastened.

[0065] It is also possible that one annular projection has one or two radial notches whilst the other projection has no notches, and that the end wall associated with the annular inner projection without notches is inserted through one or two opposite perimeter recesses provided on the end wall. [0066] Figs.28 to 42 schematically show a further embodiment of a cylinder and the related assembly method. Only one axial end of the cylindrical shell and of the assembled cylinder is shown in these figures for the sake of simplicity.

[0067] More in particular, Figs.28, 29 show a front view and a side view of an end wall 5, and Figs.30 and 31 show a front view and a side view of an axial end 3 A of the cylindrical shell 3 so configured as to be coupled to the end wall of Figs.28, 29.

[0068] The axial end 3 A of the cylindrical shell 3 comprises an annular inner projection 11 that, in this embodiment, comprises, or is formed by, a plurality of first radial teeth 1 IB preferably arranged at constant pitch around the axis A-A of the cylindrical shell 3. The first radial teeth 1 IB radially project towards the inside of the cylindrical shell, i.e. towards the axis A-A of the cylindrical shell, and extend in tangential direction, i.e. they have an angular extension around the axis A-A of the cylindrical shell 3. The teeth of each pair of first consecutive radial teeth 1 IB are spaced from each another by a radial recess or notch 11 A. Therefore, differently from the above-described embodiments, the annular inner projection 11 comprises a number of radial notches greater than two. In the illustrated embodiment, the radial notches 11A and the first radial teeth 11B have approximately the same angular extension, i.e. approximately equal length in tangential direction, but this is not mandatory.

[0069] As shown in Figs.28 and 29, the end wall 5 has a shape complementary to that of the axial end 3 A of the cylindrical shell 3. More in particular, the end wall 5 comprises a perimeter edge defining a plurality of perimeter recesses 5D, whose sequence around the perimeter edge of the end wall 5 defines a plurality of second radial teeth 5E, extending opposite the axis A-A of the end wall. The shape of the radial teeth 5E and the respective notches 5D is complementary to the shape of the first radial teeth 1 IB and the respective notches 11 A, so that the end wall 5 is inserted in the cylindrical shell 3 with a reciprocal movement between the cylindrical shell 3 and the end wall 5 in a direction parallel to the axis A-A of the cylindrical shell, having arranged the cylindrical shell 3 and the end wall 5 coaxial with each other, as shown in Figs.32 and 33.

[0070] In Figs.32 and 33, the mutual angular position of the end wall 5 and the cylindrical shell 3 is such that the second radial teeth 5E are aligned, i.e. are angularly phased, with the notches 11 A of the annular projection 11. In this way it is possible to insert the end wall 5 inside the cylindrical shell 3, even if the maximum diameter D2 of the end wall, i.e. the diameter of the cylindrical surface of envelope of the second teeth 5E, is greater than the minimum diameter DI of the axial end 3 A of the cylindrical shell 3. The minimum diameter D 1 is the diameter of the cylindrical surface tangent to the first radial teeth 1 IB.

[0071] An annular abutment 50 is associated with each axial end 3 A of the cylindrical shell 3, which abutment has an inner diameter D3 and projects radially inwardly, i.e., towards the axis A-A of the cylindrical shell 3.

[0072] The annular abutment 50 is arranged at such a distance from the inner projection 11 as to form an annular seat for the end wall 5. The annular seat is indicated with the reference number 53. The axial dimension of the seat 53 is substantially equal to the thickness of the end wall 5, or more precisely of the outer perimeter portion thereof.

[0073] The mounting of the end wall 5 in the cylindrical shell 3, that have been described with reference to Figs.28 to 31, is performed as shown in the sequence of Figs.32 to 38. In Figs.32 and 33, the end wall 5 has been positioned in axial alignment with the cylindrical shell 3 and in front of the end 3 A of the shell, by mutually aligning the second teeth 5E and the notches 11 A. In the subsequent step, shown in Figs.34 and 35, the end wall 5 has been inserted in the seat 53 making the second teeth 5E pass through the notches 11 A.

[0074] Once the end wall 5 has been inserted in the seat 53, preferably into contact with the annular abutment 50, the end wall 5 and the cylindrical shell 3 are rotated with respect to each other around the axis A-A so as to bring the second teeth 5E in an angular position corresponding to the first radial teeth 11B, as shown in Figs.36, 37 (intermediate angular position) and in Figs.38, 39, these latter showing the final mutual angular position between end wall 5 and cylindrical shell 3.

[0075] Once the position of Figs.38, 39 has been achieved, the end wall 5 may be welded to the cylindrical shell 3.

[0076] In some embodiments, to have a more reliable and precise positioning of the end wall 5 with respect to the cylindrical shell 3, the first radial teeth 11B and the second teeth 5E have a wedge-shaped cross-section. More in particular, the first radial teeth 1 IB and the second teeth 5E have a wedge-shaped cross section along a surface of cylindrical cross-section coaxial with the cylindrical shell 3. Figs.40, 41 and 42 show a cross-section according to a cylindrical surface, indicated with a dash-dot line in Figs.34, 36 and 38, in three distinct mutual angular positions of the end wall 5 and the cylindrical shell 3. S5 and Si l indicate preferably flat surfaces of the first radial teeth 1 IB and the second radial teeth 5E facing each other and adapted to be in mutual contact when the end wall 5 is mounted in the cylindrical shell 3, as shown in Figs.38, 39 and 42.

[0077] As the mutual contact surfaces S5 and SI 1 are inclined with respect to a flat surface orthogonal to the axis A-A, the mutual rotation between the end wall 5 and the cylindrical shell 3 generates an axial thrust of the end wall 5 against the annular abutment 50, thus defining a unique final position of the end wall 5 with respect to the cylindrical shell 3. Once this position has been achieved (Fig.42), the end wall 5 is welded to the cylindrical shell 3 by an inner welding bead, or by an outer welding bead, or by an inner and an outer welding bead. In the drawing, the welding bead is not shown for the sake of simplicity.

[0078] The described mounting criterion may be used also for mounting the second axial end (not shown) of the cylindrical shell 3.