EP0384817A1 - Plant for making prestressed concrete articles, in particular elongate construction elements - Google Patents

Abstract

Plant for making prestressed concrete articles, comprising an elongate production area (10), means (20, 22) for tensioning prestressing armatures (24) and a moulding apparatus comprising a mobile unit (18) supporting a multiplicity of mould elements (34, 36, 38) parallel to the longitudinal direction of the production area. Each mould element comprises, at its rear end, an end-shaping device (42, 44, 46) making it possible to shape to precise dimensions the transverse cross-section of the moulded elements (P1, P2). <IMAGE>

Description

The invention relates to an installation for manufacturing prestressed concrete elements, in particular elongated building elements, such as posts, beams, beams or the like.

It relates more particularly to such an installation which comprises an elongated manufacturing area, means for tensioning prestressing reinforcements above the manufacturing area and a molding apparatus comprising a movable unit movable in translation on the manufacturing area and supporting a multiplicity of mold elements parallel to the longitudinal direction of the manufacturing area to form the side walls of molds, the bottom of which is formed by the manufacturing area, which makes it possible to manufacture the elements, one after the other, from one end to the other of the manufacturing area.

Installations of this kind are already known from the French Patents 72 31148 and 79 07427 in the name of the Applicant, published respectively under the numbers 2 152 604 and 2 421 040.

These known installations are used for the manufacture, by spinning, on large manufacturing areas, typically of a hundred meters in length for a width of 2.5 m, of prestressed concrete construction elements, in particular of T-shaped general section beams

With these known installations, prestressing reinforcements are placed under tension above the manufacturing area, first fractions of elements are produced by pouring concrete in a single operation around said reinforcements in the molds formed by the molding apparatus and, after pouring the concrete, the elements of the molds forming the side walls are made to slide in order to manufacture other fractions of elements.

Also, there are provided, for molding, end end walls capable of being moved over the manufacturing area as and when the elements are molded.

After the molding operation, the hardening of the concrete is allowed to take place, advantageously by accelerating the latter by steaming, and the tension of the reinforcements is then released after sufficient hardening of the concrete.

Also known from document DE 2 225 510 is an installation of the aforementioned kind which has a much more complex structure and which performs the concrete pouring in two operations.

All known installations of this kind have the drawback of leading to more or less rapid wear of the mold elements, which results in a modification of the transverse dimensions of the molded elements. The mold elements must then be completely replaced in order to produce molded elements with precise dimensions again.

The object of the invention is in particular to improve the known installations of this kind so as to obtain molded elements having reduced dimensional tolerances, a more precise cross-sectional outline, and having increased performance and mechanical characteristics.

It is also an object of the invention to provide such an installation which makes it possible to manufacture a greater number of elements, and this for the same width of the manufacturing area.

The invention provides an installation of the type defined in the introduction, in which each mold element has a current cross section of constant dimensions and comprises, at its rear end relative to the direction of movement of the moving assembly, a shaping device d end which is removably mounted on said rear end and which has a substantially wider cross section, in at least one direction, than the current cross section of the corresponding mold element, which makes it possible to define between two shaping devices adjacent, a precise dimension section and to conform, to said dimensions, after pouring the concrete, the cross section of the molded elements on the manufacturing area.

These shaping devices make it possible, in combination with the manufacturing area, to conform to precise dimensions the products molded on three faces, namely the two lateral faces and the underside.

As a result, the products obtained have better dimensional tolerances.

According to another characteristic of the invention, each mold element comprises a core suitable for being arranged parallel to the longitudinal direction of the manufacturing area and in a vertical arrangement, said core extending over the entire length of the element mold, and said core being provided, on at least one of its vertical faces, with a resistant shaping element removably mounted at the rear end of the core and with a hollow half-shell mounted so fixed over the entire remaining length of the core.

Thus, the shaping elements, which in fact constitute wearing parts, can be replaced easily, while that the hollow shells which wear much less than the shaping elements remain permanently attached to the core of the mold element.

Advantageously, the mold elements comprise two lateral mold elements framing intermediate mold elements, the core of each lateral mold element comprising a shaping element and a half-shell on one of its vertical faces, the core of each element. of an intermediate mold comprising a shaping element and a half-shell on each of its two vertical faces.

Advantageously, each shaping element is a solid block provided with internal milling for its removable fixing on the core, by means of screws driven into said milling.

According to another characteristic of the invention, the installation comprises a compacting device which is arranged transversely to the direction of movement of the moving element and which is capable of shaping and compacting the top face of the elements molded in the region of end shaping devices.

This compacting device, which is concerned with the fourth face, or upper face, of the molded elements has a double function. It acts as a height adjuster to calibrate each molded element at its correct height just before demolding and it then acts as a compactor by exerting pressure on the upper face to force the concrete to compress in the rear part of the die, which corresponds to the end compactor.

Advantageously, the compacting device comprises a lifting frame pivotally mounted relative to the movable assembly between a raised position of rest and a lowered working position.

In a preferred embodiment of the invention, the frame carries two parallel tubes, arranged horizontally and transversely to the direction of movement of the moving element, these tubes being adapted to be driven in an alternating movement by means of 'an eccentric motor to come both to bear in the working position, on top of the mold elements in line with the end shaping devices.

• - la figure 1 est une vue de dessus d'une installation selon l'invention ;
• - la figure 2 est une vue latérale d'un élément de moule intercalaire ;
• - la figure 3 est une vue en coupe selon la ligne III-III de la figure 2 ;
• - la figure 4 est une vue en coupe selon la ligne IV-IV de la figure 2 ;
• - la figure 5 est une vue en coupe d'un élément de moule laté­ral selon la ligne V-V de la figure 1 ;
• - la figure 6 est une vue en coupe du même élément de moule latéral suivant la ligne VI-VI de la figure 1 ;
• - la figure 7 est une vue en coupe de deux éléments de moules intercalaires au niveau de leur section courante et faisant apparaître la section transversale d'un élément moulé ;
• - la figure 8 est une vue en coupe de deux éléments de moules intercalaires, au niveau de leur dispositif conformateur d'extrémité faisant apparaître la forme de la section de l'élément moulé ;
• - la figure 9 est une vue latérale de l'équipage mobile ;
• - la figure 10 est une vue partielle, à échelle agrandie, de l'équipage mobile de la figure 9 ;
• - la figure 11 est une vue partielle de dessus selon la flèche XI de la figure 10 ; et
• - la figure 12 est une vue frontale selon la flèche XII de la figure 10.

In the description which follows, given only by way of example, reference is made to the appended drawings in which:

• - Figure 1 is a top view of an installation according to the invention;
• - Figure 2 is a side view of an intermediate mold element;
• - Figure 3 is a sectional view along line III-III of Figure 2;
• - Figure 4 is a sectional view along the line IV-IV of Figure 2;
• - Figure 5 is a sectional view of a side mold element along the line VV of Figure 1;
• - Figure 6 is a sectional view of the same side mold element along the line VI-VI of Figure 1;
• - Figure 7 is a sectional view of two intermediate mold elements at their current section and showing the cross section of a molded element;
• - Figure 8 is a sectional view of two intermediate mold elements, at their shaping device end showing the shape of the section of the molded element;
• - Figure 9 is a side view of the moving assembly;
• - Figure 10 is a partial view, on an enlarged scale, of the mobile assembly of Figure 9;
• - Figure 11 is a partial top view along arrow XI of Figure 10; and
• FIG. 12 is a front view according to arrow XII in FIG. 10.

First of all, reference is made to FIG. 1 which shows a manufacturing area 10 of generally rectangular shape comprising, in a manner known per se, a metal deck supported by a masonry body and capable of being heated to cause steaming concrete. This manufacturing area which may have, for example, a length of about 100 m, for a width of about 2.5 m, is limited by two end edges 12 and 14 and by two longitudinal edges carrying two parallel rails 16 used for the translational movement of a movable assembly 18 over the entire length of the area 10, from one to the other of its ends. The installation further comprises two end heads 20 and 22 used for anchoring and tensioning a multiplicity of prestressing reinforcements 24 arranged above the manufacturing area and parallel to the direction longitudinal of the latter.

The movable element 18 comprises a frame formed by a front transverse block 26 and a rear transverse block 28 joined together by two lateral beams 30 and 32. The frame of the movable element 18 is intended to receive, in a removable manner, a molding apparatus comprising a multiplicity of elements molds parallel to the longitudinal direction of the bench and assembled together in a manner which will be described in more detail with reference to FIG. 9.

In the example, these mold elements, which are intended to form the side walls of the molds, comprise seven elements: two lateral elements 34 and 36 and five identical intermediate elements 38. Each mold element is fixed by its front end to the front block 26 and by its rear end to the rear block 28.

These seven mold elements thus form the side walls of six molds used for the manufacture of six elongated elements produced one after the other, from the end 12 of the manufacturing area 10 to its end 14. In the example shown in Figure 1, a first series of six elements P1 has been manufactured and a second series of six elements P2 is being manufactured.

The molding process is essentially that described in the two French Patents 72 31148 and 79 07427 mentioned above, which may be referred to for further explanation.

Briefly, the molding process consists first of all in putting in place the prestressed reinforcements under tension, in manufacturing a first fraction of elements (in the example the elements P1) by pouring concrete, in a single operation, around said reinforcements in the molds bounded by the manufacturing area and by the mold elements 34, 36, 38, while fitting end end walls 40 (FIG. 1). The concrete is then vibrated, the movable element 18 is then moved in the direction of the arrow F (FIG. 1) to slide the elements 34, 36 and 38 forming the side walls of the molds, the front wall of the molds is again moved molds and the operation is repeated to pour a new quantity of concrete.

According to the invention, each mold element 34, 36 or 38 comprises, at its rear end with respect to the direction of movement of the movable element 18, an end shaping device 42, 44 respectively for the elements 34 and 36, and 46 for the elements 38. The shaping devices 42, 44 and 46 are removably mounted on the mold elements.

These conforming devices 42, 44, 46 define between them, two by two, a precise dimension section, which makes it possible to conform, after pouring the concrete, the cross section of the elements molded on the manufacturing area.

It will be understood with reference to FIG. 1, that the elements P2 which are in the process of being manufactured, see their respective cross section shaped by the devices 42, 44 and 46 as the moving element 18 moves in the direction of arrow F.

Referring now to Figure 2 which shows an intermediate mold element 38. This element comprises a core 48 adapted to be arranged parallel to the longitudinal direction of the manufacturing area and in a vertical arrangement. This core 48 has the general shape of an L and comprises a horizontal branch 50 extending over the entire length of the mold and connecting, at its rear end, to a vertical branch 52. The branch 50 supports, in its rear part , a shaping device 46 and, over its entire remaining length, two shells 54a and 54b which are connected to the shaping device 46 by a transition section 56.

As shown in Figure 3, the core 48 of the mold element 38 is a flat iron arranged vertically and having two opposite faces 58a and 58b. The half-shell 54a is a sheet folded at 60a according to a rounded shape and has an end 62a welded to the face 58a and another end 64a welded to the edge of a horizontal flat iron 66, itself welded to the core 48. The half-shell 54b constitutes the symmetric of the half-shell 54a and also comprises a folding rounded 60b and two end edges 62b and 64b welded respectively to the core 48 and to the flat iron 66. The flat iron 66 determines a flat and horizontal upper face 68, the total height of the element between this face 68 and the horizontal lower edge 70 of core 48 being equal to H1.

Referring now to Figure 4. The shaping device 46 comprises two shaping elements 46a and 46b constituted by solid blocks fixed on either side of the core 48 respectively against the faces 58a and 58b. These blocks have, in cross section, a shape very close to that of the half-shells 54a and 54b. In the example, the block 46a comprises a lower face 72a connected by a sharp edge 74a to a lateral face 76a, the latter also being connected by a sharp edge 78a to an upper face 80a. The element 46b comprises, in a corresponding manner, faces 72b, 76b and 80b connected together by sharp edges 74b and 78b. The faces 80a and 80b are flat and horizontal and distant from the lower edge 70 of the core 48 by a height H2 slightly less than the height H1 (FIG. 3). The block 46b comprises four holes 82 used for the introduction of four screws 84 (FIG. 2 and FIG. 4) capable of passing through corresponding holes 86 of the core 48 and of being screwed into threads 88 formed in the element. 46a. After the screws 84 have been put in place and screwed in, it is possible to put putty in the holes 82, so as not to cause discontinuities in the face 76b.

Referring now to FIG. 5 which shows a lateral mold element 34, this comprises a core 90 of the same shape as the core 48 described above and having two faces vertical 92a and 92b. This core is surmounted, as in the case of FIG. 3, by a flat iron 66 and it also comprises a half-shell 54b welded on the one hand to the face 92b and on the other hand to the flat iron 66. On the on the other side of the core 90 is provided a hollow stiffening section 94 which is welded to the face 92a with the interposition of a flat iron 96. In the example, the section 14 is a hollow section of square section. Furthermore, the profile 94 is connected to the flat iron 66 by a folded sheet 98 whose one edge 100 is welded to the profile 94 and whose other edge 102 is welded to one edge of the iron 66. The sheet 98 has a flat upper face and horizontal 104 in the same plane as the upper face 68 of the flat iron 66. The height between this upper face and the lower edge 106 of the core 90 is also equal to H1.

As shown in Figure 6, the profile 94 and the sheet 98 extend over the entire length of the core 90. In the rear part of the core, the shaping element 42 comprises a single element 42b similar to the element 46b shown in FIG. 4. This element is fixed by screws 82 directly to the core 90. Over the entire length of the shaping element 42, the flat iron 66 is extended by a square iron 108 which is welded against the face 92a and which serves as a support for the end 102 of the folded sheet 98. The height of the assembly is equal to H2 as in the case of FIG. 4.

It will thus be understood that the end shaping devices (FIG. 4 and FIG. 6) have a height slightly lower than that of the mold element in the rest of its length, that is to say on its current cross section. (Figures 3 and 5).

Reference is now made to FIGS. 7 and 8. The end shaping device 46 (FIG. 8) has a substantially wider cross section, in at least one dimension (in the example the horizontal dimension), than that of the current cross section of the corresponding mold element. As a result, the distance D1 which separates the half-shells facing two adjacent mold elements (FIG. 7) is slightly greater than the distance D2 which separates the two adjacent shaping devices (FIG. 8) and this for a given height relative to the plane of the manufacturing area 10.

In the example shown in FIGS. 7 and 8, the installation is used for the manufacture of prestressed concrete P2 beams each comprising three prestressing reinforcements 24. These beams have a section in the general shape of T arranged in an inverted position. Each beam includes in particular a lower face 110 which is in contact with the manufacturing area 10 and an upper face 112 which is, as will be seen below, leveled and compacted during its passage between the shaping devices 46. Thus when a building element P2 passes between two adjacent shaping devices, for example between two devices 46, the cross section of this building element is gradually narrowed and brought to precise dimensions by the shaping devices. In addition, the section of the element obtained has sharp edges and not protruding edges, as was the case with the installations of the prior art.

In the part of the mold elements, corresponding to their current cross-section, the particular assembly of the half-shells gives high lateral inertia to these shells, the assembly thus having a lateral inertia, 30% greater than that of the elements of existing molds.

It should also be noted that the mold elements of the invention define both in the region of their current cross section (FIG. 7) and in the region of the cones. end formers (Figure 8), a flat horizontal reference surface. This horizontal reference surface is connected, by sharp edges 78a and 78b (FIG. 8) with the lateral faces of the shaping elements. As a result, the mold elements have better lateral rigidity, which limits the deformation during the introduction of the concrete and gives, on demolding, more straight molded elements.

Furthermore, because the upper face of the mold elements is flat and defines sharp edges with the shaping elements, the molded element can be leveled more perfectly on its upper surface 112, as will be seen below, compared with the elements obtained in the prior art. In fact, previously, the mold elements had rounded edges, even on the upper face, which led to the formation of burrs when removed from the mold.

The shaping elements 46a and 46b, which form wear elements, can be easily replaced so that the molded elements always have a cross section corresponding to the desired dimensions.

Advantageously, each end shaping device is formed of a material or comprises a coating of a material which facilitates the flow of concrete upon demolding and which thus reduces the skin tearing of the product. For example, elements can be made of chromed steel, stainless steel, ceramic, or even elements comprising a coating of polytetrafluoroethylene or rigid nylon.

Reference is now made to FIG. 9 in which the mobile assembly 18 is represented with its front unit 26, its rear unit 28 and one of its longitudinal members 32. This equipment moves on the rails 16 by means of wheels 114. The installation is in operation and ensures the molding of a series of beams P2, a first series of beams P1 having just been molded and still being connected to the series P1 by the prestressing reinforcements 24.

The two lateral mold elements 34 and 36 and the five intermediate mold elements 38 are assembled together using an anterior cross member 116 and a rear cross member 118 to form an interchangeable "grid". The crosspieces 116 and 118 are fixed respectively to the front block 26 and to the rear block 28. In a manner known per se, the side members 30 and 32 can be used for rolling a comb beam (not shown) used for the movement of the transverse molds 40 .

As indicated above, the mold elements 34, 36 and 38 ensure, in combination with manufacturing area 10, the molding of the elements on three faces, namely the two lateral faces and the bottom face. The invention also provides for the molding of the top face (face 112 in FIGS. 7 and 8) by means of a compacting device designated as a whole by the reference 120 in FIG. 9. The device 120 comprises a frame liftable 122 pivotally mounted relative to the movable member 18 between a raised position of rest (not shown) and a lowered working position (Figure 9). The frame 122 is articulated, at one of its ends, by a horizontal axis 124 on a support 126 integral with the rear block 28. The frame 122, which has the general shape of a bracket, carries at its free end, two Parallel tubes 128 disposed horizontally and transversely to the direction of movement of the moving element 18. These two tubes are capable of being reciprocated and both come to bear, in the working position, on the top of the mold elements, in line with end shaping devices 42, 44, 46.

The two tubes 128 (figure 11) are articulated with two levers of ends 134 and 136 around four hinge axes 138 to form a deformable parallelogram. The levers 134 and 136 are also articulated on the frame by means of respective axes 140 and 142. In addition, the lever 134, which is substantially longer than the lever 136, is articulated at 144 on a link 146 itself articulated on the eccentric 132. The tubes 128 are articulated to the levers 134 and 136 by the axes 138, which include springs 148. In addition, the articulation axes 140 and 142 of the two levers are mounted in two yokes end 150 and are also provided with springs 152 (FIG. 10). The two yokes 150 are provided at the two ends of a transverse beam 154 mounted at the ends of two uprights 156 forming part of the frame 122. In addition, the end levers 134 and 136 are articulated on their respective axes 140 and 142 by ball joints 158 (Figure 10). The calibration of the spring 152 is equal to the sum of the respective calibration of the two springs 148.

The frame 122, which is pivotally mounted directly on the mobile assembly 118 and which, as a variant, could be pivotally mounted on a carriage coupled to said crew, ensures that the tubes 128 have no longitudinal relative movement by with respect to the frame of the moving assembly 18. It follows that the tubes 128 are, in the working position, always in line with the end shaping devices. These tubes move only in a transverse relative movement, under the action of the eccentric movement transmitted by the rod 146.

The compactor device 120 has a double function: firstly it calibrates the molded elements at the desired height (H2) just before demolding and it exerts pressure on the fourth face (top face 112) to force the concrete to compress in the rear part of the mold elements (playing the role of die). In any case, the second tube (the one located behind the direction of movement of arrow F) completes the work done by the first tube.

As a result, the molded elements are compacted laterally by the end conformers 42, 44 and 46 and vertically by the compacting device 120.

As a result, the mechanical resistance of the molded element is increased by about 30% to bending, since such an element, under bending stress, needs excellent concrete in the region of its upper face 112. This is extremely important for molded elements of elongated shape, such as beams or the like, which are precisely stressed in bending, which results in compression at their upper face.

It also follows that the resistance dispersions of the molded elements are reduced because the concrete is very homogeneous.

Consequently, with equal concrete characteristics (same dosage of cement), the molded element is more resistant in bending by around 30%. To obtain an equal flexural strength, the concrete may have a lower cement dosage. This results in savings on the final cost.

The invention is particularly applicable for the manufacture of elongate molded elements, such as beams, etc. Taking into account that the installation makes it possible to manufacture molded elements with cross sections with very precise dimensions, and with a much more homogeneous concrete, it is possible to manufacture, on an area of given width, many more molded elements than with prior art facilities.

For example, over an area 2.50 meters wide, it is possible to manufacture series of twenty-six beams.

In addition, the installation makes it possible to obtain molded elements of precise section, the end shaping devices being able to be easily replaced in the event of wear.

Claims (16)

1. Installation for the production of prestressed concrete elements, in particular elongated construction elements, such as posts, beams or the like, comprising an elongated manufacturing area (10), means (20, 22) for placing under tension of prestressing reinforcements (24) above the manufacturing area and a molding apparatus comprising a mobile assembly (18) displaceable in translation on the manufacturing area and supporting a multiplicity of mold elements ( 34, 36, 38) parallel to the longitudinal direction of the manufacturing area to form the side walls of molds, the bottom of which is formed by the manufacturing area, which makes it possible to manufacture the elements (P1, P2) the one after the other from one end to the other of the manufacturing area, characterized in that each mold element (34, 36, 38) has a current cross section of constant dimensions and in that it comprises, at its end in. rear, with respect to the direction of movement (F) of the mobile assembly, an end shaping device (42, 44, 46) which is removably mounted on said rear end and which has a substantially wider cross section , in at least one dimension, than the current cross section of the corresponding mold element, which makes it possible to define between two adjacent shaping devices a section of precise dimensions and to conform to these dimensions, after pouring the concrete, the cross section molded elements. 2. Installation according to claim 1, characterized in that each mold element (34, 36, 38) comprises a core (48, 90) suitable for being arranged parallel to the longitudinal direction of the manufacturing area (10) and in a vertical arrangement, said core extending over the entire length of the mold element, and said core being provided on one at less of its vertical faces (58a, 58b; 92a, 92b) of a resistant shaping element (46a, 46b) removably mounted at the rear end of the core and of a half-shell (54a, 54b) hollow fixedly mounted over the entire remaining length of the core. 3. Installation according to claim 2, characterized in that the mold elements comprise two lateral mold elements (34, 36) framing intermediate mold elements (38) and in that the core (90) of each element lateral mold (34, 36) comprises a shaping element (42b) and a half-shell (54b) on one of its vertical faces (92b), while the core (48) of each intermediate mold element (38 ) comprises a shaping element (46a, 46b) and a half-shell (54a; 54b) on each of its vertical faces (58a, 58b). 4. Installation according to one of claims 2 and 3, characterized in that each shaping element (46a, 46b) is a solid block provided with internal millings (82) for its removable attachment to the core (48, 90) thanks to screws (84) driven into said millings. 5. Installation according to one of claims 2 to 4, characterized in that each shaping element (46a, 46b) has sharp edges (74a, 78a; 74b, 78b). 6. Installation according to one of claims 2 to 5, characterized in that each half-shell (54a, 54b) has a lower free edge (62a, 62b) connected directly to the core (48, 90), for example by welding, and an upper free edge (64a, 64b) connected to the core by means of a horizontal flat iron (66), itself welded to the core (48, 90). 7. Installation according to claim 3, characterized in that that the core (90) of each lateral mold element (34, 36) has on its outer face (92a), opposite to that which receives the shaping element (46b) and the half-shell (54b), a profile stiffening hollow (94) extending over the entire length of the core (90). 8. Installation according to one of claims 1 to 7, characterized in that the cross section of the end shaper (42, 44, 46) is connected to the current cross section of the mold element by a transition zone (56). 9. Installation according to one of claims 1 to 8, characterized in that the end shaping device (42, 44, 46) is formed of a material or comprises a coating of a material facilitating the flow of concrete. 10. Installation according to one of claims 1 to 9, characterized in that the end shaping device (42, 44, 46) has a cross section of height (H2) smaller than the height (H1) of the mold element over the rest of its length. 11. Installation according to one of claims 1 to 10, characterized in that it comprises a compacting device (120) which is arranged transversely with respect to the direction of movement of the mobile assembly and which is capable of conforming and compact the top face (112) of the molded elements (P1, P2) in the region of the end shaping devices (42, 44, 46). 12. Installation according to claim 11, characterized in that the compacting device (120) comprises a lifting frame (122) pivotally mounted relative to the movable assembly (18) between a raised position of rest and a lowered position of job. 13. Installation according to claim 12, characterized in that the lifting frame (122) carries two parallel tubes (128), arranged horizontally and transversely to the direction of movement of the moving element, these tubes being adapted to be animated by reciprocating by means of an eccentric motor (130), said tubes both coming to bear, in the working position, on top of the mold elements, in line with the end shaping devices (42, 44, 46). 14. Device according to one of claims 12 and 13, characterized in that the lifting frame (122) is pivotally mounted (124) either directly on the movable assembly (18), or on a carriage coupled to said equipping. 15. Device according to claim 13, characterized in that the two tubes (128) are articulated with two end levers (134, 136) to form a deformable parallelogram, each lever being articulated on the frame and one (134 ) of the two levers being further connected to the eccentric (132) by a link (146). 16. Device according to claim 14, characterized in that the tubes (128) are articulated to the end levers (134, 136) by pins (138) comprising springs (148) and in that the end levers (134 and 136) are themselves articulated to the lifting frame (122) by ball joints (158) on axes (140, 142) provided with springs (152).

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