WO2001098583A1

WO2001098583A1 - A method of ensuring flatness of a vane in a headbox by means of a mounting arrangement, headbox with such a mounting arrangement and mounting arrangement therefor

Info

Publication number: WO2001098583A1
Application number: PCT/SE2001/001368
Authority: WO
Inventors: Jan Anders Erikson; Tord Gustav Gustavsson; Ingvar Berndt Erik Klerelid; Joakim Norrman
Original assignee: Metso Paper Karlstad Ab
Priority date: 2000-06-22
Filing date: 2001-06-18
Publication date: 2001-12-27
Also published as: ATE372411T1; EP1313911B1; DE60130343T2; BR0111649A; DE60130343D1; EP1313911A1; CA2407913A1; AU2001274758A1; JP2004501294A; CA2407913C

Abstract

A method of ensuring the flatness of a vane (14) that is mounted in a headbox by means of a mounting arrangement including engagement dowels (22) for cooperation with a downstream support wall (25) of a transverse groove (20), said vane being affected during operation by shearing forces from the stock and by retaining forces from the mounting arrangement. In accordance with the invention outer engagement dowels are mounted at the side edges (31, 32) of the vane to cooperate during a specific period of time, as the only engagement dowels (22a), with the downstream support wall in order to take up said shearing forces, whereby tensile stresses will arise in the downstream end portion (33) of the vane in the cross machine direction. The invention also relates to a headbox having such a mounting arrangement and the mounting arrangement per se in which the vane within and downstream of an inner area (35) of the upstream end portion (21) of the vane is arranged to move freely in the machine direction in relation to said downstream support wall during said period of time.

Description

A method of ensuring flatness of a vane in a headbox by means of a mounting arrangement, headbox with such a mounting arrangement and mounting arrangement therefor

The present invention relates to a method of ensuring the flatness of a vane that is detachably mounted in a headbox by means of a mounting arrangement which includes a plurality of engagement members that are connected to the vane at its upstream end portion, and a longitudinal groove for receiving the engagement members of the vane, said groove having inner, downstream and upstream support walls, that face towards said engagement members for cooperation therewith, said vane being affected during operation by shearing forces caused by stock flowing along the vane, and by retaining forces exerted on the vane by the mounting arrangement.

The invention also relates to a headbox for delivering a jet of stock to a forming zone in a former for wet forming of a fiber web, including

- a slice, having a chamber,

- a turbulence generator including

- turbulence channels opening into the slice chamber, and - at least one anchoring element that separates the turbulence channels,

- at least one vane arranged in the slice chamber,

- and an arrangement for detachable mounting of the vane to said anchoring element, said mounting arrangement including

- a plurality of engagement members that are connected to the vane at its upstream end portion, and

- an elongate structural element having a longitudinal groove for receiving the engagement members of the vane, said groove having inner, parallel downstream and upstream support walls that face towards said engagement members for cooperation therewith. The invention also relates to an arrangement for detachably mounting a vane to an anchoring element of a turbulence generator of a headbox for delivering a jet of stock to a forming zone in a former for wet forming a fiber web, including

- a slice, having a chamber,

- said turbulence generator including

- turbulence channels opening into the slice chamber, and

- said anchoring element that separates the turbulence channels,

- at least one vane arranged in the slice chamber, said mounting arrangement including - a plurality of engagement members that are connected to the vane at its upstream end portion, and

- an elongate structural element having a longitudinal groove for receiving the engagement members of the vane, said groove having inner, parallel, downstream and upstream support walls that face towards said engagement members for cooperation therewith.

A known headbox of the type described above has engagement members in the form of oblong engagement bodies or engagement dowels arranged in a row at the upstream end portion of the vane, and extending in the cross machine direction. The engagement dowels have parts protruding from the vane to cooperate with the support walls of the connection bar. The vane is influenced during operation both by a shearing force in the machine direction, caused by stock flows along the vane, as well as a retaining force directed against the machine direction, exerted on the engagement dowels by the support wall situated downstream, the retaining force being intended during operation to be distributed uniformly between the engagement dowels. In practice, however, the retaining force may be distributed non-uniformly between the engagement dowels so that the shearing force on the vane gives rise to local compressive stresses in the cross machine direction in the downstream end portion of the vane. Where compressive stresses arise the vane buckles, making its downstream end portion uneven, which is not desirable, particularly at a separating-vane that separates two layers of stock, since good layering of stock is dependent on a flat separating-vane. If the separating-vane is not flat, streaks having a grammage different from the rest of the paper web may appear, for instance.

The above-mentioned compressive stresses may arise as a result of variations in the placing of the engagement dowels within a predetermined tolerance interval. The placing of the engagement dowels within the tolerance interval may, for instance, deviate from an ideal placing in such a way that certain engagement dowels are downstream of the other engagement dowels, in which case the retaining force will be distributed in an uncontrolled manner between the engagement dowels, with the risk of compressive stresses appearing in the downstream end portion of the vane, resulting in buckling.

Compressive stresses may also appear in a vane consisting of a plastic material, e.g. glassfiber-reinforced epoxy resin, and having reduced thickness in the machine direction so that the downstream end portion of the vane is relatively thin in relation to the upstream end portion. A vane of plastic material absorbs water from the surroundings both during storage prior to mounting, and also after mounting in the headbox, when the vane absorbs liquid from the stocks. As a result of the differences in thickness the thinner downstream end portion of the vane will become saturated earlier than the thicker upstream end portion of the vane. As the downstream end portion becomes saturated in the direction away from the downstream edge, the downstream end portion lengthens in the cross machine direction, whereas the thicker, unsaturated upstream end portion of the vane retains its dimensions. The extension of the vane in the downstream edge results in the downstream edge of the vane endeavouring to assume a convex form and its upstream edge a concave form. When such a partially saturated vane is influenced during operation by said shearing force from the stocks, the retaining force will be distributed non-uniformly between the engagement dowels so that the downstream end portion of the vane becomes buckled.

The object of the present invention is to essentially reduce the problems mentioned above and to provide a method which will efficiently ensure the flatness of a vane .

It is also an object of the invention to provide a mounting arrangement and a headbox with such a mounting arrangement for each of the vanes which is designed so as to ensure flatness of the vane during operation.

The method in accordance with the invention is characterized by mounting at least one outer engagement member or an outer group of two or more engagement members in the proximity of each side edge of the vane, and said two outer engagement members or said two outer groups of engagement members cooperating during operation for at least one specific period of time, as the only engagement members, with the downstream support wall to take up said shearing forces, whereby tensile stresses are arising in a downstream end portion of the vane in the cross machine direction. The headbox and the mounting arrangement in accordance with the invention are characterized in that said plurality of engagement members include at least one outer engagement member or an outer group of two or more engagement members in the proximity of each side edge of the vane, said two outer engagement members or said two outer groups of engagement members are arranged during operation for at least one specific period of time, as the only engagement members, to cooperate with the downstream support wall to take up the shearing forces generated in the vane by the flowing stocks, and in that an inner area of the upstream end portion defined by the two outer engagement members and the two outer groups of engagement members, respectively, is free from engagement members or has inner engagement members which, at least in the unloaded state of the vane are located upstream of said downstream support wall so that the vane within and downstream of said inner area is arranged to be able to move freely in the machine direction in relation to said downstream support wall during said period of time or part thereof .

The invention is described in more detail in the following with reference to the drawings.

Figure 1 is a sectional view in machine direction of a part of a multilayer headbox mounted to deliver a multilayer jet of stock into a gap leading to a forming zone in a twin wire former of roll type.

Figure 2 is a sectional view of an arrangement for mounting one of the vanes in the slice chamber of the headbox in connection with a group of turbulence channels in the headbox according to Figure 1. Figure 3 is a view from above of an unloaded vane of metal, and shows parts of a conventional mounting arrangement .

Figure 4 is a view from above of a vane in accordance with Figure 3 during operation.

Figure 5 is a view from above of a vane of moisture-absorbing plastic material and shows parts of a conventional mounting arrangement.

Figure 6 is a sectional view along the line VI-VI in Figure 5.

Figure 7 is a view from above of an unloaded vane, and shows parts of a mounting arrangement in accordance with a first embodiment of the invention.

Figure 8 is a view from above of the vane in accordance with Figure 7 during operation.

Figure 9 is a view from above of an unloaded vane and shows parts of a mounting arrangement in accordance with a second embodiment of the invention.

Figure 10 is a view from above of the vane in accordance with Figure 9 during operation.

Figure 11 is a view from above of an unloaded vane and shows parts of a mounting arrangement in accordance with a third embodiment of the invention.

Figure 12 is a view from above of the vane in accordance with Figure 11 during operation. Figure 13 is a view from above of an unloaded vane and shows parts of a mounting arrangement in accordance with a fourth embodiment of the invention.

Figure 14 is a view from above of the vane in accordance with Figure 13 during operation.

Figure 1 shows schematically a headbox designed to deliver a three-layer jet of stock into a gap 1 leading to a forming zone in a twin wire former of roll type. The twin wire former has an inner forming wire 2 , a rotatable forming roll 3, an outer forming wire 4 and a rotatable breast roll 5.

The headbox has a turbulence generator including a group of turbulence channels 6 and a slice 7 arranged downstream of the turbulence channels 6 and containing a chamber 8 that converges from its upstream end in the direction of the flow of stock and terminates in a slice opening 9 at its downstream end.

The turbulence channels 6 are arranged in three sections for supplying three different stocks, for instance, into the slice chamber 8. The lower section and the middle section each have two rows of turbulence channels 6 arranged close together, while the upper section has three such rows of turbulence channels 6. The rows of turbulence channels 6 extend in the cross machine direction and adjacent rows of turbulence channels 6 are separated by elongate stable anchoring elements 10 extending in the cross machine direction. The anchoring element 10 has an elongate, through engagement groove 11 (see Figure 2) , with a side opening 12 facing the slice chamber 8. The group of turbulence channels 6 is connected at its upstream end to a feeding system (not shown) comprising three stores of stock and suitable flow spreaders for uniform distribution of each stock to the rows of turbulence channels 6 in the associated section and uniform distribution of the stock within each row of turbulence channels 6.

In the embodiment shown the headbox has six vanes 14 which divide the slice chamber 8 into seven converging channels 15 communicating with the rows of turbulence channels 6. Two of the vanes 14 constitute stock-separating vanes 14a that are arranged to separate the three stocks from each other and extend through the slice opening 9 a predetermined distance to form a jet which thus consists of three layers. The stock-separating vanes 14a also have turbulence-generating function. The other vanes are only turbulence vanes 14b having their free ends situated inside the slice chamber at a predetermined distance from the slice opening 9. The vanes 14 are relatively rigid and may consist of a metal material, usually titanium, or a plastic material, usually glassfiber-reinforced or carbonfiber-reinforced epoxy resin. The vanes 14 are sufficiently stiff to support various pressures and velocities of the flows of stock. Each vane 14 is arranged to be detachably mounted to said anchoring element 10 by means of an mounting arrangement comprising an elongate structural element 16 and engagement members 22 arranged in the upstream end portion 21 of the vane 14. In the embodiment shown the structural element 16 consists of a connection bar and the engagement members 22 of cylindrical engagement dowels (see Figure 2) disposed at right angles to the plane of the vane 14. The connection bar 16, consisting of metal, e.g. bronze, is the same length as the width of the vane 14 and includes in the following order an engagement part 17 situated downstream, a flexible waist part 18, and an engagement part 19 situated upstream and forming a pivot. The engagement part 17 is provided with an elongate, through groove 20 to receive the upstream end portion 21 of the vane 14 and its engagement dowels

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a result of one or both of said tolerance intervals the positions of the engagement dowels 22 in relation to the downstream support wall 25 may vary. This is illustrated in Figure 3 in that the engagement dowel 22e is situated downstream, i.e. closer to the downstream support wall 25 than the other engagement dowels 22. Figure 4 shows the vane 14, made of metal, during operation where shearing forces caused by the stocks flowing along the vane 14 press the engagement dowels 22 towards the downstream support wall 25. The shearing forces act along the surfaces of the vane 14 and are illustrated in Figure 4 by downwardly directed force arrows designated F_s. The retaining forces exerted by the downstream support wall 25 on the engagement dowels 22 are illustrated by upwardly directed force arrows designated F_r. Since, as can be seen in Figure 4, the initial position of the engagement dowel 22e is downstream of the other engagement dowels 22, the retaining force F_r, acting on the engagement dowel 22e, is greater than the retaining forces F_r acting on the adjacent engagement dowels 22. As a result of the loading that then arises, the vane 14 is subjected to a bending moment in machine direction, which is illustrated in Figure 4 by moment arrows denoted M^ at both side edges 31, 32 of the vane 14. The bending moment causes compressive stresses in the downstream end portion 33 of the vane 14, in the cross machine direction, illustrated in Figure 4 by tension arrows designated S^ . The compressive stresses Sμ buckle the vane 14, as illustrated in Figure 4 by the wave-shaped lines in the downstream end portion 33.

As mentioned earlier, buckling may arise in a vane made of a moisture-absorbing plastic material and having narrowing thickness in the machine direction, as a result of the thinner, downstream end portion of the vane becoming saturated earlier than the thicker upstream end portion of the vane. Such a vane 14 is described in the following with reference to Figures 5 and 6 where the vane 14 is shown in unloaded state after, for instance, a certain operating period when it has been in contact with the flowing stocks. As the downstream end portion 33 of the vane 14 becomes saturated in the direction away from the downstream edge 30, the downstream end portion 33 becomes stretched in the cross machine direction, while the thicker, unsaturated upstream end portion 21 of the vane 14 retains its dimensions. For that reason tensions arise in the vane 14 causing the vane to bend in its plane so that the downstream edge 30 of the vane endeavour to assume a convex form and its upstream edge 29 a concave form, as shown in Figure 5. During operation the load distribution between the engagement dowels 22 becomes non-uniform since the intermediate engagement dowels 22 take up a larger part of the retaining force than the engagement dowels 22 situated closer to the side edges 31, 32 of the vane 14, in the same way as for the vane shown in Figure 4. In this case the resultant loading also leads to a bending moment in machine direction, compressive stresses in the cross machine direction in the downstream end part 33 of the vane 14 and buckling of the downstream end portion 33 of the vane 14. As will be understood, the tolerance-dependent buckling described in connection with Figures 3 and 4 also can arise in such a vane made of plastic material and therefore reinforce the buckling described above, caused by swelling.

Figure 7 shows an unloaded vane 14 with parts of a mounting arrangement in accordance with a first embodiment of the invention. Figure 8 shows the same vane 14 during operation. The vane 14 is symmetrical with respect to its centre line 34, which coincides with the machine direction. An outer engagement dowel 22a is arranged in the proximity of each side edge 31, 32 of the vane 14, for cooperation with the downstream support wall

other restrictions than the strength of the vane at the attachment locations for the outer engagement dowels 22a and the position of the downstream edge 30 which must not be such that the stock layering is affected unfavourably. In such an embodiment no compressive stresses can arise in the downstream end portion 33 of the vane.

Figure 9 shows an unloaded vane 14 with parts of a mounting arrangement in accordance with a second embodiment of the invention where three engagement dowels 22b, forming an outer group 36, are arranged in the proximity of each side edge 31, 32 of the vane 14. The engagement dowels 22b are arranged adjacent each other in a row in the cross machine direction. Here too, the inner area 35 of the upstream end portion 21 of the vane, extending between the two outer groups 36 is free from engagement dowels so that said inner area ^'35 of the vane 14, as well as the area downstream of this, are arranged to be able to move freely in the machine direction in relation to the downstream support wall 25. The retaining forces F_r and the shearing forces F_s together create a bending moment M]-, as shown in Figure 10. The bending moment M]-, bends the vane 14 in its plane, stretches the downstream edge 30 of the vane 14 and generates tensile stresses S^ in the cross machine direction in the downstream end portion 33 of the vane 14. The displacement arises under the same circumstances as those described for the vane in accordance with Figure 7.

Figure 11 shows an unloaded vane 14 with parts of a mounting arrangement in accordance with a third embodiment of the invention, which is more suitable for high stock-flow velocities_, than the embodiments described previously. The vane 14 is provided with engagement dowels 22b, arranged in outer groups 36, as in the second embodiment described in connection with Figures 9 and 10, as well as engagement dowels 22c arranged in two inner groups 37 with three engagement dowels 22c in each group 37. The inner groups 37 of engagement dowels 22c are arranged at a predetermined distance from the outer groups 36. Each inner group 37 of engagement dowels 22c is arranged at a predetermined distance from the downstream support wall 25, e.g. about 5 mm. The distance to the outer group 36 of engagement dowels 22b can then be about 2000 mm. A first period of time commences with the stocks starting to flow through the headbox and finishes, e.g. when the inner groups 37 of engagement dowels 22c come into contact with the downstream support wall 25 in that the inner area 35 has been displaced in the machine direction under the influence of the shearing forces F_s from the stocks, whereupon the downstream edge 30 of the vane 14 is stretched and a tensile stress S^ in the cross machine direction is built up in the downstream end portion 33 of the vane 14. At the end of the period of time the machine speed has a predetermined value. It will thus be understood that the position of each inner group 37 of engagement dowels 22c in relation to the downstream support wall 25 and to the outer group 36 of engagement dowels 22b is decisive for each stock flow rate. During a second period of time, extending up to a moment when an increased machine speed has been set, the inner part-area 35a, defined by the inner groups 37 of engagement dowels 22c, moves forwards in machine direction, the movement being limited by the displaced position when there is a risk of compressive stresses appearing in the downstream end portion 33 of the vane 14. When the vane consists of a plastic material and is narrowing, the swelling phenomenon must also be taken into account in choosing maximum stock flow rate or machine speed and determining the positions of the inner groups 37 of engagement dowels 22c. Instead of increasing the machine speed from the existing value when the inner groups 37 of engagement dowels 22c are in contact with the downstream support wall 25, the tensile stress that still exists in the downstream end portion 33 of the vane can be utilized to compensate the compressive stresses deriving from the swelling.

In a vane 14 consisting of plastic material and having a length of 800 mm, a width of 5500 mm, a thickness of the upstream end portion 21 of 4 mm, a thickness of the downstream end portion 33 of 0.5 mm, and which is intended to be subjected to a maximum stock flow rate of 2000 m/min, for instance, a suitable distance between two adjacent outer and inner groups 36, 37 may be about 2000 mm. In this case the inner groups 37 of engagement dowels 22c may be situated about 5 mm from the downstream support wall 25, seen in unloaded state of the vane 14. The engagement dowels in each group 36, 37 are preferably placed about 50 mm from each other. It is preferable to arrange the engagement dowels 22b and 22c within each group 36, 37 so that the distance to the downstream support wall 25 increases in two adjacent engagement dowels in the direction from the closest side edge

31, 32, respectively, of the vane 14. A suitable increase in this distance is about 0.1 mm.

It will be understood that the invention is not limited to three engagement dowels 22 in each group. More or fewer, e.g. two or four engagement dowels 22, may be used in each group. Neither is the invention limited to two inner groups 37 of engagement dowels 22. It is thus possible, for instance, to place additional inner groups of engagement dowels 22, spaced from the support wall 25, between the outer and inner groups 36, 37.

Figure 13 shows an unloaded vane 14 with parts of a mounting arrangement in accordance with a fourth embodiment of the invention, the engagement dowels 22 being arranged in a row along a curved line extending between the side edges 31, 32 and symmetrical about the centre line 34. In the embodiment shown the engagement dowels 22 are arranged with uniform spacing but in accordance with an alternative embodiment (not shown) the spaces may be different and distributed in a regular pattern, e.g. groups of engagement dowels with equal distance between them within the group and equal but greater distance between the groups. In the embodiment shown in Figure 13 a certain number, e.g. 3-5, of the engagement dowels situated nearest a side edge 31, 32 may be considered to be included in an outer group of engagement dowels, whereas the other engagement dowels may be considered to constitute separate inner engagement dowels situated one after the other, or to form inner groups of engagement dowels, depending on the shape of the curved line and the distance between the engagement dowels as mentioned above. If the highest machine speed is to be used immediately for such a vane, a period of time commences at the moment when the stocks start flowing through the headbox and extends to the moment when the engagement dowels 22 closest to the centre line 34 also come into contact with the downstream support wall 25 as a result of the influence of the shearing forces F_s from the stocks, whereupon the downstream edge 30 is stretched and a tensile force S^ in the cross machine direction is built up in the downstream end portion during this period of time, as illustrated in Figure 14. If the vane consists of a plastic material, is narrowing and can no longer be moved forwards within the central area, there may be such a large excess of tensile stress in the downstream end portion at the end of said period of time that remaining swelling gives compressive stresses that are balanced by said excess of tensile stress. If the tensile stress decreases to zero and the vane is still not saturated, i.e. the swelling is going on, said maximum machine speed must be reduced in a corresponding degree. It will be understood that periods of time shorter than that described exist which thus terminate at a moment when a lower machine speed than the maximum is set and corresponds to a specific displacement of the inner area so that at least two inner engagement dowels or two inner groups of engagement dowels situated at a distance from the centre line 34 of the vane, are in contact with the downstream support wall 25.

In the vane shown in Figure 13 the engagement dowels are arranged in a row along a curved line which, when the vane is unloaded, has a certain extension in the machine direction. By mounting such a vane in a connection bar where the distance between the previously mentioned support walls is less than the extension of the curved line in the machine direction, tensile stresses in the downstream end portion of the vane can be provided already when the vane is mounted in the groove of the connection bar. Through the narrow groove recess, in relation to the curved line, forming said support walls, the outer engagement dowels situated closest to the side edges of the vane will be caused, upon insertion of the vane into the groove, to cooperate with the support wall situated downstream of the groove and will absorb support forces therefrom. In corresponding manner, the inner engagement dowels situated nearest the centre line of the vane will cooperate with the support wall situated upstream of the groove and will absorb support forces therefrom. In the same way as the above-mentioned shearing and retaining forces, the support forces will bend the vane in its plane, stretch the downstream edge of the vane and generate tensile stresses in the cross machine direction in the downstream end portion of the vane. These tensile stresses ensure that the vane is flat right at the start-up phase of the headbox, i.e. before the stocks have had time to influence the vane.

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14 can move freely, i.e. without influence from outer retaining forces from engagement dowels, in the machine direction in relation to the downstream support wall 25 during said period of time or part thereof. By arranging the engagement dowels 22 in the manner described above they create, during operation, shearing forces F_s acting on the vane 14, together with the retaining forces F_r acting on the engagement dowels 22 a bending moment M^, which under normal operating conditions always bends the vane 14 in its plane and generates tensile stresses S^ in the cross machine direction in the downstream end portion 33 of the vane 14. The placing of the engagement dowels 22 in accordance with the principle of the invention prevents that the compressive stresses described previously will arise in the downstream end portion 33 of the vane 14. A characteristic feature of the invention is thus that compressive stresses are prevented in the vane, which compressive stresses may cause the vane to buckle so that the stock layering may be affected in an unfavourable manner.

The invention has been described above in connection with engagement members in the form of engagement dowels 22. However, it will be understood that the invention can be realized with other types of engagement members. Besides the engagement members being designed as a plurality of discrete elements such as engagement dowels, they may consist of a continuous engagement element cooperating with said downstream support wall in accordance with the principles of the invention.

It will also be understood that the invention can be realized using other mounting arrangements than those described above. The vane 14 may be attached directly to the anchoring element 10, for instance, which then has the same function as the elongate connection bar 16 and has a groove with support walls similar to that in the connection bar.

Claims

C A I M S

1. A method of ensuring the flatness of a vane (14) that is detachably mounted in a headbox by means of a mounting arrangement which includes a plurality of engagement members (22) that are connected to the vane (14) at its upstream end portion (21) , and a longitudinal groove (20) for receiving the engagement members (22) of the vane (14) , said groove (20) having inner, downstream and upstream support walls (25, 26) , that face towards said engagement members (22) for cooperation therewith, said vane (14) being affected during operation by shearing forces caused by stock flowing along the vane, and by retaining forces exerted on the vane (14) by the mounting arrangement, characterized by mounting at least one outer engagement member (22a) or an outer group (36) of two or more engagement members (22b) in the proximity of each side edge (31, 32) of the vane (14) , and said two outer engagement members (22a) or said two outer groups (36) of engagement members (22b) cooperating during operation for at least one specific period of time, as the only engagement members (22a or 22b, resp.), with the downstream support wall (25) to take up said shearing forces, whereby tensile stresses are arising in a downstream end portion (33) of the vane (14) in the cross machine direction.

2. A method as claimed in claim 1, characterized by acting of said retaining forces on side edge portions of the vane (14) and by free moving of the portion (35) of the vane (14) between said side edge portions in the machine direction under the. influence of said shearing forces.

3. A method as claimed in claim 1 or 2, characterized by bending the vane (14) , under the influence of said shearing forces and retaining forces, so as to stretch the downstream edge (30) of the vane (14) and generate said tensile stresses in the cross machine direction in the downstream end portion (33) of the vane (14) .

4. A headbox for delivering a jet of stock to a forming zone in a former for wet forming of a fiber web, including a slice (7) , having a chamber (8) , a turbulence generator including - turbulence channels (6) opening into the slice chamber (8) , and at least one anchoring element (10) that separates the turbulence channels (6) , at least one vane (14) arranged in the slice chamber (8) , and an arrangement for detachable mounting of the vane (14) to said anchoring element (10) , said mounting arrangement including a plurality of engagement members (22) that are connected to the vane (14) at its upstream end portion (21) , and an elongate structural element (16) having a longitudinal groove (20) for receiving the engagement members (22) of the vane (14) , said groove (20) having inner, parallel, downstream and upstream support walls (25, 26), that face towards said engagement members (22) for cooperation therewith, characterized in that said plurality of engagement members (22) include at least one outer engagement member

(22a) or an outer group (36) of two or more engagement members (22b) in the proximity of each side edge (31, 32) of the vane (14) , said two outer engagement members (22a) or said two outer groups (36) of engagement members (22b) are arranged during operation for at least one specific period of time, as the only engagement members (22a or 22b, resp.), to cooperate with the downstream support wall (25) to take up the shearing forces generated in the vane (14) by the flowing stocks, and in that an inner area (35) of the upstream end portion (21) defined by the two outer engagement members (22a) and the two outer groups (36) of engagement members (22b) , respectively, is free from engagement members or has inner engagement members (22c) which, at least in the unloaded state of the vane (14) are located upstream of said downstream support wall (25) so that the vane (14) within and downstream of said inner area (35) is arranged to be able to move freely in the machine direction in relation to said downstream support wall (25) during said period of time or part thereof.

5. A headbox as claimed in claim 4, characterized in that said inner engagement members (22c) form at least two inner groups (37) arranged at a predetermined distance from the outer engagement members (22a) or the outer groups (36) of engagement members (22b) , and in that each inner engagement member (22c) is arranged at a predetermined distance from the downstream support wall (25) .

6. A headbox as claimed in claim 5, characterized in that said distance to the downstream support wall (25) increases for two adjacent engagement members (22b; 22c) within each group (36; 37) in the direction from the side edge (31, 32) of the vane (14) .

7. A headbox as claimed in claim 6, characterized in that said increase in distance is about 0.1 mm.

8. A headbox as claimed in claim 4, characterized in that the engagement members (22) are arranged along a curved line extending between the side edges (31, 32) with uniform or non-uniform spacing.

9. A headbox as claimed in any one of claims 4-8, characterized in that in its unloaded state the vane (14) has a pre-shaped concave downstream edge (30) which is stretched during operation to substantially straight form through said displacement of the vane (14) in relation to the outer engagement members (22a) or the outer groups (36) of engagement members (22b) .

10. A headbox as claimed in any one of claims 4-9, characterized in that even at the end of said period of time the vane is free from compressive stresses.

11. A headbox as claimed in any one of claims 4-10, characterized in that said structural element (16) comprises a connection bar with an engagement part (17) containing said groove (20) , a flexible waist part (18) and an engagement part (19) for mounting in a groove of said anchoring element (10) .

12. An arrangement for detachably mounting a vane (14) to an anchoring element (10) of a turbulence generator of a headbox for delivering a jet of stock -,to a forming zone in a former for wet forming a fiber web, including a slice (7) , having a chamber (8) , - said turbulence generator including turbulence channels (6) opening into the slice chamber (8) , and said anchoring element (10) that separates the turbulence channels (6) , - at least one vane (14) arranged in the slice chamber (8) , said mounting arrangement including a plurality of engagement members (22) that are connected to the vane (14) at its upstream end portion (21) , and - an elongate structural element (16) having a longitudinal groove (20) for receiving the engagement members (22) of the vane (14) , said groove (20) having inner, parallel, downstream and upstream support walls (25, 26) that face towards said engagement members (22) for cooperation therewith, characterized in that said plurality of engagement members (22) include at least one outer engagement member (22a) or an outer group (36) of two or more engagement members (22b) in the proximity of each side edge (31, 32) of the vane (14) , said two outer engagement members (22a) or said two outer groups (36) of engagement members (22b) are arranged during operation for at least one specific period of time, as the only engagement members (22a or 22b, resp.), to cooperate with the downstream support wall (25) to take up the shearing forces generated in the vane (14) by the flowing stocks, and in that an inner area (35) of the upstream end portion (21) defined by the two outer engagement members (22a) and the two outer groups (36) of engagement members (22b) , respectively, is free from engagement members or has inner engagement members (22c) which, at least in the unloaded state of the vane (14) are located upstream of said downstream support wall (25) so that the vane (14) within and downstream of said inner area (35) is arranged to be able to move freely in the machine direction in relation to said downstream support wall (25) during said period of time or part thereof.

13. An arrangement as claimed in claim 12, characterized in that said inner engagement members (22c). form at least two inner groups (37) arranged at a predetermined distance from the outer engagement members (22a) or the outer groups (36) of engagement members (22b) , and in that each inner engagement member (22c) is arranged at a predetermined distance from the downstream support wall

14. An arrangement as claimed in claim 13 , characterized in that said distance to the downstream support wall (25) increases for two adjacent engagement members (22b; 22c) •within each group (36; 37) in the direction from the side edge (31, 32) of the vane (14) .

15. An arrangement as claimed in claim 14, characterized in that said increase in distance is about 0.1 mm.

16. An arrangement as claimed in claim 12, characterized in that the engagement members (22) are arranged along a curved line extending between the side edges (31, 32) with uniform or non-uniform spacing.

17. An arrangement as claimed in any one of claims

12-16, characterized in that in its unloaded state the vane (14) has a pre-shaped concave downstream edge (30) which is stretched during operation to substantially straight form through said displacement of the vane (14) in relation to the outer engagement members (22a) or the outer groups (36) of engagement members (22b) .

18. An arrangement as claimed in any one of claims 12-17, characterized in .that even at the end of said period of time the vane is free from compressive stresses.

19. An arrangement as claimed in any one of claims 12-18, characterized in that said structural element (16) comprises a connection bar with an engagement part (17) containing said groove (20) , a flexible waist part (18) and an engagement part (19) for mounting in a groove of said anchoring element (10) .