JPH0510215B2 - - Google Patents

Description

TECHNICAL FIELD The present invention relates in particular to a device for obtaining different variable differential velocities at different locations of an elastically or plastically deformable flat film or plate. BACKGROUND OF THE INVENTION In many industries, particularly in the textile and metallurgical industries, thinning and calendering of flat materials is common practice. Sometimes it is necessary to pretension plastic, metal or textile products. In the packaging industry, the plastic film is often stretched before it is wrapped around the package and then allowed to shrink naturally or artificially. Generally, to accomplish these tasks it is necessary to create different velocities between two different locations on the film. To do this, two rollers are used that rotate at different rotational speeds about mutually parallel axes. The roller rotation speed on the downstream side is greater than that on the upstream side. In particular, the angular velocity of the rollers can be varied by mechanical, electronic, hydraulic means, and by gearboxes, gear trains, etc. However, with the devices of the prior art it is not possible to obtain changes in the speed ratio of values much greater than 1, for example even up to 3. Furthermore, during the start-up phase of tape winding, it is not possible to obtain a differential velocity ratio equal to 1 without a coupling device. Object and Arrangement of the Invention The main object of the invention is to overcome these drawbacks. The present invention is an apparatus for obtaining variable tangential differential velocities at different locations of a deformable flat film or plate moving along at least one stretching roller rotating about an axis, Or it relates to a device in which the plate moves in a direction perpendicular to the axis of rotation of the roller and lies on the generatrix of the stretching roller. This device is characterized in that the roller has a rotational speed that does not change over time, but the distance from the generatrix that supports the film to the axis of rotation of the roller changes over time. According to a first embodiment of the present invention, the stretching roller is formed by a plurality of adjacent generatrixes such that the distance from the downstream generatrix, from which the stretched film is discharged, to the rotating shaft increases continuously over time. has been done. According to the second embodiment, the stretching roller is a deformable cylinder, in particular, the axis of rotation of the cylinder is eccentric with respect to the axis of rotation, and the axis of rotation of the cylinder is eccentric with respect to the axis of rotation, from the upstream generatrix into which the unstretched film enters. The distance to the downstream generatrix from which the stretched film exits increases continuously with time. The film is brought into contact on the generatrix by at least one pressure member exerting pressure on the roller, in particular the support roller, which member is displaced as it rotates about its axis by the displacement of the support generatrix, the film being displaced by the displacement of the support generatrix. It is positioned between the support member and the roller. The ratio between the initial distance R ₀ from the upstream generatrix to the rotation axis and the distance R ₁ from the downstream generatrix to the rotation axis is adjustable. According to the third embodiment, the film is conveyed by the rotation of two cylindrical rollers having circular cross sections and parallel rotation axes, the upstream radius R ₀ and the downstream radius R ₁ of which change over time. . The stretching ratio, more specifically, the ratio of the differential tangential velocity V ₁ at a point on the downstream film after stretching to the differential tangential velocity V ₀ at a point on the upstream film before stretching is the radius R ₁ of the downstream roller. is proportional to the ratio of R to the radius R ₀ of the upstream roller. It will be appreciated that a device embodying the principles of the present invention has significant advantages over prior art devices. In fact, either the position of the rotation axis is changed relative to the rotation axis, that is, the distance between the generatrix in contact with the film and the rotation axis of the roller is adjusted.
Alternatively, the stretching ratio can be easily adjusted either by adjusting the mutual roller radii. Additionally, several rollers can be used with varying radii to obtain different differential velocities, if desired. The invention will now be described in more detail with reference to the accompanying drawings, in which embodiments of the invention are shown. Embodiments The device of the invention is capable of obtaining variable tangential differential velocities at different locations of a deformable film or plate, more particularly of an elastically or plastically deformable film. Elastic deformation is reversible, and when stretched the film recovers to its initial length, whereas plastic deformation is partially irreversible, and when stretched, the film has a final length greater than its initial length. It is well known that the length Tangential differential velocity is the speed at which the film moves along at least one roller or is conveyed along by the rotation of at least one roller rotating about an axis, the film moving along at least one roller or rotating about an axis. and is tangential to the generatrix of the roller. According to the invention, the roller has a rotational speed N that does not change over time, while the distance from the generatrix supporting the film to the axis of rotation of the roller changes over time. FIG. 1 shows a first embodiment of the invention. The film 1 enters the stretching device 2 upstream and emerges from there downstream. The tangential velocity of the film 1 on the upstream side is V ₀ and the tangential velocity of the film 1 on the downstream side is equal to V _N . The device 2 makes it possible to obtain a tangential differential velocity V _N that varies at different locations on the film 1 . The film has a flat shape and is shown perpendicular to the plane of the paper in the figure. Preferably, the film is elastically or plastically deformable and made of a synthetic material. The length L ₀ between two points AB on the upstream film is smaller than the length L _N between the same points AB on the downstream film. According to the invention, the device 2 stretches the film, ie the section AB having a length L gradually reaches the length _LN . (Figure 1). This first embodiment has a large number of adjacent busbars _G0 ...
..., G _N. These generatrixes G are connected to the drive mechanism 4 with the axis C as the center.
It is preferably rotated by at least two spiders formed by flat circular sections at each end of the generatrix G _N. The rotation center C ₁ through which the spider's rotation axis passes moves relative to the rotation center C of the stretching roller 3 . The rotation direction of the drive mechanism 4 is the same as the rotation direction of the stretching roller 3. The film 1 is transported by the rotation of at least one roller 3 and is oriented around an axis of rotation at a center C perpendicular to the plane of the figure. The film 1 moves in a direction perpendicular to the rotational axis of the roller 3 and is placed on the generatrix G _N of the roller 3 at time t _N . At time t ₀ , film 1 is on bus line G ₀ and its velocity is V ₀ ; at time t ₁ = t ₀ +△t, film 1 is on bus line G ₁ and its velocity is V ₁ =V ₀ +△V ₀ ; lies on the film bus line G _N at time t _N ,
Its speed is V _N =V+Σ△V _N-1 . The velocity increment ΔV _N-1 is preferably constant, but may vary. The stretching roller 3 rotates in the direction of the arrow F ₁ , and the distance from the support generatrix G _N , which has a rotational angular velocity N that does not change over time, to the rotation axis of the stretching roller changes. The distance C ₁ C ₀ ,..., C ₁ G _N is C ₁ C ₀
＞……＞C ₁ G _N and the rotation axis of the drive mechanism 4 C ₁
The distance from the rotating shaft to the downstream generating line G ₀ is preferably smaller than the distance from the rotating shaft to the downstream generating line G _N. This distance G _N C ₁ increases continuously from upstream to downstream. The initial upstream distance G _N C ₁ is equal to above, while the final upstream distance is equal to R. The distance e from the rotation center C of the stretching roller 3 to the rotation center C ₁ of the drive means 4 is adjustable. This allows the final tangential velocity V _N to be adjusted with respect to the initial tangential velocity V ₀ and thereby the draw ratio
It becomes possible to adjust V _N /V. This draw ratio can be increased to much greater than 1, such as 2 or 3, or it can be reduced to less than 1. The stretching ratio is made equal to 1 when stretching plastic films used for pallet tapes. This stretch ratio equal to 1 is necessary at the start of winding to prevent the film from being stretched. In this case the distance CG ₁ is zero. By moving the center C relative to the center _C1 in this manner, the present invention can gradually adjust the drawing ratio. In a second embodiment of the device according to the invention (FIG. 2), the drawing roller is a rotating cylinder 5, preferably of circular cross section, whose axis of rotation at center C is eccentric from the axis of rotation at center C. There is. Preferably, this stretching cylinder is deformable and made of an elastic material. Adjacent bus lines G ₀ , C ₁ , ...,
The distance between G _N increases continuously during rotation (in the direction of arrow F ₂ ) as the center of rotation C ₂ is displaced upstream with respect to the center of rotation C ₃ . The film 1 rests on the generatrix G _N at time t _N , causing deformation of the deformable support cylinder 5 as the loop pulley 6 rotates about the center C ₃ . The film is introduced on the upstream side where the distance r = C ₂ G ₀ from the rotation center C ₂ to the generatrix G ₀ is the shortest, and the distance R from the rotation center C ₂ to the generatrix G _N
=C ₂ G _N comes out on the longest downstream side. By adjusting the distance C ₂ C ₃ with the help of rollers 6, the stretching ratio can be adjusted.
When C ₂ matches C ₃ , the draw ratio is 1. In the third embodiment of the present invention (FIGS. 3 to 8), the film has a rotation axis XX parallel to the circular cross section,
moving along two cylindrical rollers 7, 8 with YY, the radius of the cylinder 7 upstream of said rollers R ₀
and the radius R ₁ of the downstream cylinder 8 may change over time. Film 1 moves in a direction perpendicular to the axes XX and YY. According to the invention, the tangential velocity V ₀ of the film 1 on the upstream side is different from the tangential velocity V ₁ on the downstream side. The film 1 drives the set of moving rollers 7, 8 at a speed _V1 . In one variant, the film is moved at a speed V ₁ by a roller 8 driven by a motor 9.
is driven by. In the present invention, the rollers 7 and 8 are connected to the cylinder 8.
The cylinder 7 is rotated such that the ratio K of the rotation speed N of the cylinder 7 to the rotation speed N ₀ of the cylinder 7 is constant. Cylinder 7 works in the direction of arrow F ₃ , while cylinder 8 rotates in the direction of arrow F ₄ (in the opposite direction) and is driven in mutually opposite directions. Therefore, if the stretching ratio of the film is E, the length of the portion AB on the downstream side is _L1 , and the length _L0 of the portion AB of the film 1 on the upstream side, the following equation holds. E=L ₁ /L _0If L ₀ =V ₀ t, L ₁ =V ₁ t at time t, then L ₁ /L ₀ =E=V ₁ /V _0However , the tangential velocity is the radius of the cylinder due to the rotation of the cylinder. V ₁ = N ₁ R ₁ V ₀ = N ₀ R ₀ E = V ₁ /V ₀ = N ₁ /N ₀ R ₁ /R ₀ = KR ₁ /R _0According to the present invention If K=N ₁ /N ₀ Radius of upstream cylinder 7 and downstream cylinder 8
By changing R ₀ , R ₁ the tangential velocity V ₀ ,
The ratio of V ₁ can be changed. Since the radii R ₀ and R ₁ are thus variable, the distances from the generatrix G _N to the rotation centers C ₀ and C ₁ change. If R ₀ , R ₁ vary between the limit values r and R when R ₁ = r and R ₀ = Ra, the stretching ratio E is at a minimum (Fig. 4A), whereas if R ₁ = R, If Ra= r , the drawing ratio E becomes maximum (Figure 4B). Therefore, E _nax = R/r and E _nio = r/R, E _nax /E _nio
= ^R2 / ^r2 . If we try to change the stretching ratio from 0 to 100%, E _nax /E _nio = 2 and R/r = V ₂ , and for example, R = ±113 mm, r = ±80 mm, and average ±97 mm. If we try to change the stretching ratio from 0 to 200%, we will get E _nax /E _nio = 3, r = ±70 mm R = ±123 mm, average radius ±97 mm. If we make the draw ratio equal to 1, then K will be equal to ±1.4 in the first case and ±1.7 in the second case. The rotational speed ratio K is adjusted by a gear 10. According to the embodiment of the invention shown in Figures 7A, 7B, 7C and 7D, the device for variable pre-stretching or stretching consists of a plurality of long curved sections 11 forming a cylindrical roller surface. It is equipped with two stretching rollers 7 and 8. The rotational axes XX and YY of these two rollers are parallel. A member 12 guiding the parts 11 of the rotation axes XX, YY in radial movement moves them away from and towards each other. A drive mechanism 13 that rotates the part 11 rotates the part along the axis XX,
The guide member 12 is rotated about YY, and finally the guide member 12 is actuated by the control unit 14. The drive mechanism 13 for rotating the part 11 preferably consists of a hub 15 capable of rotating the part 11 about the axes XX, YY, which are grooved axes. These threaded shafts are pivoted on bearings 16. The member 12 for guiding said part 11 in a radial sliding manner consists of a ramp 17 provided in a groove 18 provided in the hub 15 . Each cylinder 11 has at least two hubs 15. The hub 15 of the cylinder 7 is a ribbed shaft XX or
Slide along YY at the same time. The ramp is slidable radially within the groove 18. Thereby, the radius of the cylinders 7, 8 can be changed by sliding the ramp 17 radially in the groove 18 and at the same time sliding the hub 15 axially. Each cylindrical roller 7, 8 has at least one flange 19 for reliable guidance of the end of the part 11. A stop 20 slides radially relative to the flange 19. The device also comprises a control unit 14 for actuating the guide means 12 for sliding the part 11 radially and axially. The control unit 14 controls the rotation axes XX and YY of the rollers 7 and 8.
It consists of a double fork 21 pivoted about an axis 22 orthogonal to the fork. The double fork 21 is actuated by a support rim 22 mounted on a thrust bearing 23 at one end of the axis of rotation XX, YY of the roller. When the double fork 21 rests against the upstream roller 7 in this way (FIG. 7A), it acts on the stop 23 and causes the hub 15 of the upstream cylinder 7 to slide axially. The hub 15 is moved towards the end of the shaft XX which does not have the stop 23. As the hub 15 moves, it compresses the return spring 24. The portion 11 and the inclined portion 17 are connected to a circumferential spring 25 provided at the height of the inclined portion 17.
It makes a tight contact on the sliding hub 15. As hub 15 moves along axis XX, portion 1
1 radially outwards thereby ensuring an increase in the radius of the roller 7 corresponding to R. On the other hand, the fork 21 has one end connected to the downstream roller 8.
The hub 15 is located at a position facing the hub 15 of the downstream cylinder 7. The return spring 24 presses the sliding hub and control unit into a position corresponding to the minimum radius of the downstream roller 8 corresponding to r. A gear 26 adjusts the rotational speed of the upstream roller 7 relative to the rotational speed of the downstream roller. In FIG. 7A, the radius on the upstream side is R and the radius on the downstream side is r, and the stretching ratio is the minimum. On the other hand, by swinging the fork 21 (FIG. 7B) in the opposite direction, the radius on the upstream side is set to R, and the radius on the downstream side is set to R, so that the stretching ratio becomes maximum. The rotation of fork 21 is controlled by control unit 2.
7. As shown, this unit may be a hydraulically, pneumatically or electrically operated cylinder or a manual nut-and-bolt system. In this way, the value of the radius of the rollers 7, 8, that is, the value of the stretching coefficient, is also determined in accordance with the position of the control unit 27 for controlling the fork 21. The position of the controller unit 27 may be controlled by electrical means based on predetermined or programmed settings. In yet another embodiment of the invention (Section 8A)
(FIGS. 8B and 8B) A gear 26 is provided on the side of the control unit 14. The ramp 17 is integrated with a shaft 18 eccentric to the axis of rotation XX, which axis XX has a stop 29 on which a spring 24 rests. As the fork 21 pivots, it compresses the spring 24, thereby causing the hub 15 to move in the direction of arrow _F4 , i.e., fork 2.
1 in the axial direction of movement. Inclined part 27
slides in groove 18, thereby causing the downstream cylinder to expand. On the other hand, the upstream cylinder 8 is not compressed. Part 11 preferably has an adhesion-promoting lining on its outer circumferential surface. 5 and 6 show other types of rollers. A small-diameter half-roller 30 is shown on the left side of each figure, and a large-diameter half-roller 31 is shown on the right side. In FIG. 5, half roller 30 is half roller 31.
It consists of a part 32 which is likewise integrated with a radially compressible element 33. Preferably they are made of elastomeric material. These radial elements 33 are placed between a radially fixed partition 34 and a radially movable partition 35 which is integrated with the shaft and an axially movable shaft 36 . As the shaft 36 moves axially, the movable partition 35 approaches the fixed partition 34 and the elastic elements are compressed axially and deformed radially, pressing the sections 32 radially outward. The radius of the roller increases. When the movable partition 35 moves away from the fixed partition 34, the elastic element returns to its initial shape. The elastic element is part 11
provided at both ends of the In FIG. 6, part 11 is moved by hydraulic means. The displacement of the piston shaft 37 is controlled by the portion 11 by the fluid transmitting pressure between the piston shaft 37 and the support 38.
This results in a displacement of the support portion 38. This liquid is contained in a chamber formed by a hollow shaft 39, so that when the shaft 37 slides axially, the support 38 slides radially. In some applications, the devices described above, particularly those shown in Figures 1, 5, 6, 7A-7D, 8A and 8B, may be noisy due to the rolling of the roller section or metal bar over the taut film. It has the disadvantage of emitting Furthermore, the stretched film undergoes transverse shrinkage. That is, the final width of the stretched film is smaller than its initial width. This is a drawback for tape quality in the packaging industry. Moreover, the part consisting of the metal bar forming the rotating cylinder may cause the taut film to break more or less at its metal edges. In addition, the cylinder part or the metal extension bar is restored by a return spring in order to reduce the diameter of the extension cylinder, if it is originally large. Now, since these return springs are provided inside an elongated cylinder made of various parts, the device structure becomes complicated. The invention therefore also aims to remedy these drawbacks by providing a roller with external elastic means for returning the cylinder part towards the axis of rotation. These external return devices preferably consist of at least one elastic band around and within the part. For example, the elastic bands are circular rings placed at a certain distance from each other. Preferably, the ring is integrated with the part, for example by adhesive. By providing external elastic return means on the part, a return spring for reducing the radius of the cylinder can be eliminated. Moreover, this elastic band can considerably reduce the noise emitted by the metal bar or the part striking the taut film. Furthermore, this elastic device also serves as a means of protection for the film. This is because the elastic device covers the edges of the metal bars etc. and thereby prevents these bars etc. from coming into contact with the stretched film. The roller 40 shown in FIG. 9 is of the type designated 3 in FIG. 1 or designated 7 and 8 in FIGS. 7A-7D, FIGS. It may be of any type. This roller 40 consists of a plurality of longitudinal sections 41 ₁ , 41 ₂ , . . . 41 _N each having a cylindrical surface. According to the invention, external means are provided for returning the roller 40 portion 41 towards the XX axis. Therefore, when the parts 41 ₁ , 41 ₂ , 41 _N are moved with the aid of the above-mentioned separate means to increase the radius R ₀ or r ₁ , the external elastic device _{42 2} , 41 _N is returned to the rotation axis XX of the cylinder 40. These external elastic means 42 have sections 41 ₁ , 41
It consists of an elastic band placed around and outside the _{2,41 N.} According to a first embodiment of the invention shown in FIG. 9, the elastic band consists of several circular rings 43 ₁ , 43 2 , 43 2 , 43 ₂ around the axis XX, which are arranged at a certain distance from each other.
It is made of _43N . For example, a circular ring of height h has a radius r substantially equal to the radius r of the stretching roller. That is, the radius r of the band 43 _N is slightly smaller than the minimum radius that the roller can have. In this way, when the bands 43 ₁ , 43 ₂ , 43 _N are not stretched, the substantially adjacent portions 41 ₁ , 41 ₂ ,
Maintain 41 _N. The parts 41 ₁ , 41 _N are the parts 41 ₁ , 41 ₂ , 41 _N
When the elastic bands 43 _{1 , 43 2 , 43 N are displaced in the axial direction of the outward pointing arrow F so as to separate them from the axis of rotation XX of the roller 40, a radial force is applied to the elastic bands 43 1} , 43 ₂ , 43 _N , causing the bands to move away from their initial radius. is also large and has a radius substantially equal to the radius desired for the stretching roller. This band is then subjected to a stretching force. When the means for separating the sections 43 ₁ -41 _N are not activated, the band is no longer subjected to the force F ₃ and by its elasticity returns to its initial shape and tends to return to the initial radius r. This results in part 41 ₁ ,
41 ₂ and 41 _N are the forces needed to return this to its original position.
Receives a force in the opposite direction to F ₅ . The circular rings 43 ₁ , 43 ₂ , 43 _N are preferably separated from each other by a distance e. The distance e between two consecutive rings 43 ₁ , 43 ₂ is preferably several mm, for example 2 to 3 mm. FIG. 10 shows an enlarged view of the two rings 43 ₁ , 43 ₂ and the film 1 placed thereon. The film 1 rests on the rings 43 ₁ and 43 ₂ , while when the film is between the rings 43 ₁ and 43 ₂ , i.e. in the space formed between the rings 43 ₁ and 43 ₂ . Do not touch external parts. FIG. 11 shows a second embodiment of the present invention. The band forming the elastic means 42 consists of two parts 44 and 45. The first portion 44 is a helically wound band in one direction, while the second portion 45 is a helically wound band in the opposite direction. The first portion 44 is wound in the direction opposite to the direction of rotation of the roller 40, while the second upper portion is wound in the direction of rotation of the roller 40. Similarly, the vertices of bands 44 and 45 are not adjacent to each other, but are separated by a distance e. This distance e is a number
mm, particularly preferably 2 to 3 mm
mm. The vertices of the helix are therefore not facing each other and are symmetrically distributed with respect to the longitudinal axis Z--Z of the stretched film. Thus, the helical winding direction of the bands 44, 45 is such that the film is at the outer edge 100 of the stretched film.
and 110, the direction is such that it is acted upon by the reaction component of the apex of the helix applied to the film. As a result, the film 1 undergoes a transverse stretching in the direction of the reaction component of the helical winding applied to the film (in the direction of the arrow r), and the film tends to contract transversely as it is stretched, or at least has an elastic helical band. There will be less lateral shrinkage than when stretched by rollers without rollers. The plane of the apex forms an acute angle α with the direction of displacement of the stretched film. Preferably α is smaller than 46°. A further advantage of this elastic means according to the invention is that the parts 41 ₁ , 41 _N made of metal bars do not strike the film. This is because the elastic means are provided between the film and these metal bars. This results in a reduction in the noise emitted by the device. In addition, the tendency of the film to be cut by the edges of the portions 41 ₁ , 41 _N is reduced because the film does not come into direct contact with the portions 41 ₁ , 41 _N. Finally, this circular ring 43 ₁ , 43 ₂ , 43 _N or helically wound band 41 , 45 is preferably integrated with the cylinder part 41 ₁ , 41 _N , for example by means of adhesive. Preferably, this ring or spiral band is made of a vulcanized rubber sheet.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of the first embodiment of the present invention, Fig. 2 is a schematic diagram of the second embodiment of the invention, Fig. 3 is a perspective view of the third embodiment, and Figs. 4A and 4B are Schematic representations of a variant with roller radii for minimum and maximum stretching, respectively, FIG. 5 shows a first embodiment of the roller, FIG. 6 shows a second embodiment of the roller, FIGS. 7A and 7B respectively. A schematic view of the first modification of the third embodiment in the case of minimum stretching and maximum stretching; FIGS. 7C and 7D are respectively a plan view of the roller portion and a plan view of the means for controlling expansion and contraction of the roller; FIGS. 8A and 8B are schematic diagrams of the second modification of the third embodiment shown in FIGS. 7A and 7B in the case of minimum stretching and maximum stretching, respectively, and FIG. 9 is a schematic diagram of the roller of the present invention. 10 is an enlarged view showing details of the roller of FIG. 9, FIG. 11 is a side view of another modification similar to FIG. 9, and FIG. 12 is a diagram of FIG. 11. FIG. 3 is a schematic perspective view showing the operation of the device of the present invention equipped with the roller shown in FIG. 1... Film, 3, 5... Stretching roller, 7...
...Upstream roller, 8...Downstream roller, 40...
Stretching roller, 41... Longitudinal portion, 42... Elastic means, 43... Circular ring.

Claims (1)

Claims: 1 comprising first and second rollers, each roller comprising a plurality of longitudinally extending surface members, the surface members having a radius on each roller corresponding to each area of the film; a first means for stretching a deformable film forming a substantially cylindrical film support surface defined in accordance with the invention; a first rotary shaft extending longitudinally;
a second rotating shaft rotatably supporting the second roller supported substantially parallel to the first rotating shaft; a second means for radially moving the surface member; third means coupled to the rotating shaft for rotationally driving the shaft and the surface member; a diameter of the support surface of the first roller for stretching the film; is varied relative to the diameter of the support surface of the second roller to create a speed difference between the film area supported by the first roller and the film area supported by the second roller. a control unit connected to said second means for controlling the movement of said surface member to adjust the diameter of said cylindrical support surface. 2. The ratio of the tangential velocity V ₁ at the downstream location of the film after stretching to the tangential velocity V ₀ at the upstream location of the film before stretching, that is, the stretching ratio E, is the radius R of the support surface of the first roller. 2. The device according to claim ₁ , wherein the ratio of the support surface of the second roller of 1 to the radius R ₀ is proportional to the ratio of the support surface of the second roller of 1 to the radius R 0 . 3. Apparatus according to claim 2, in which the control unit varies the drawing ratio E by simultaneously varying the radii R ₁ and R ₀ between two values r and R. 4. The device according to claim 3, in which the radii R ₁ and R ₀ vary in the range r=±80 mm and R=±113 mm at a stretching ratio in the range 0 to 100%. 5. The device according to claim 3, in which the radii R ₁ and R ₀ vary in the range r=±70 mm and R=±123 mm at a stretching ratio in the range 0 to 200%. 6 The range in which the stretching ratio changes is the same as that of the first and second
4. A device as claimed in claim 3, adjustable by changing the ratio of the rotational speeds of the rollers. 7. The second means includes a plurality of guide members connected to the surface members of the first and second rollers, and a plurality of guide members connected to the surface members of the first and second rollers;
a plurality of hubs coupled to each of the second rotating shafts and the control unit, each hub defining at least one groove for slidably receiving a respective guide member; and configured to slide substantially axially relative to each said shaft to radially move said guide member and each surface member, thereby adjusting the diameter of said each support surface. An apparatus according to claim 1. 8. The control unit is coupled to a fork member pivoted on an axis substantially perpendicular to the first and second shafts, and a first end of the fork member and a hub of the first shaft. a first rim member configured to move the hub relative to the first shaft when the fork member pivots about its axis; when the fork member rotates about its axis, the first end of the fork member moves the hub relative to the second shaft in a direction opposite to the direction in which the first end of the fork member moves the hub of the first shaft; and a second rim member configured to vary the diameter of the support surface of the second roller with respect to the diameter of the support surface of the first roller. equipment.