EP0301989B1 - Web unwinding machine with reel-carrying towers - Google Patents

Description

The present invention relates to the field of machines for unwinding strip materials from reels, for feeding, in particular, in particular continuously, printing machines, cutters, folder-gluers, or other machines for shaping, and relates to a machine to unroll.

The existing unrolling machines, intended for supplying the aforementioned machines, are generally ill-suited to supplying said machines due to their size and their relatively large weight, which do not allow installation in multi-storey buildings. fitted with relatively low freight elevators and whose ceiling heights are also often relatively low.

In known unwinding machines, the reels are almost always loaded by moving the centering heads or pins in an arc, because these heads or pins are mounted, either at the end of a pivoting arm , or on a turntable. However, the axis of the coil moves in a horizontal and / or vertical plane, which requires, for the introduction of the heads or spindles, tedious successive displacements of each of the two elements, alternatively, these displacements having to be controlled by the machine operator, until the axes of the heads or spindles and the spool coincide.

In addition, certain known loading methods have the drawback of forcing the operator to push the spool in the direction of its axis in order to achieve the penetration of the spindle in overhang, or centering head, into the reel core, either by using its own force, or by using a more or less practical motorized handling device and a high cost price.

In addition, certain existing unwinding machines require, during the entire unwinding operation of the reel, that said reel is partially surrounded by a belt intended for launching it and for controlling its unwinding speed. This embodiment has the drawback of bringing into play, at the level of the belt applying to a part of the periphery of the coil, a power which is detrimental to the surface state of the strip of material to be unwound, or even to the very structure of said strip, in particular in the case of unwinding of a strip of fragile or perforated material, in particular in the case of rapid acceleration or emergency stop. In addition, with these known machines, the driver is obliged to operate near the unwinding strip, so that he is exposed to the risk of accidents, in particular due to the presence of the belt (re-entering point). which is contrary to generally accepted safety instructions.

Unwinding machines are also known in which the connection of the finishing strip of a reel with the beginning of the strip of the following reel is carried out during a temporary stop, the finishing strip passing through a reserve of strip disposed between the outlet of the unrolling machine and the processing machine located downstream.

In these known machines, however, there is no secure means of guaranteeing that the strip connection will not fall into a later critical area, for example under the reciprocating tool of a folder or cutter or d '' a transverse perforator, the thickness of the fitting with its adhesive tape may cause a jam and the entire production line to stop.

To overcome this drawback, it has been proposed to launch the new reel at the precise moment when the tape reserve corresponds to a multiple of the format increased by a certain fraction depending on the position. desired connection.

But, taking into account the elasticity of the strip, on a route, inside the reserve, which can reach several tens of meters, it is not possible to count on very reliable strip lengths, where an imprecision of the positioning of the tape connection.

Also known from FR-A-2207496 is a device for feeding a strip material with automatic cutting and connection, which comprises reel carriers which can be moved laterally and a strip fixing bar which can be moved with the reel holders. This document does not, however, describe a spool holder allowing automated loading by lateral displacement.

Furthermore, US-A-3753833 discloses a connection system provided with two completely independent cutting means. Such a system does not allow the creation of butt joints and cuts the strips by means of two saw blades, which does not allow obtaining a clean connection, nor performing an oblique cut. . Finally, this document does not describe a means for fulfilling the loading and lateral adjustment functions.

The present invention aims to overcome these drawbacks.

It has, in fact, for its object an unwinding machine essentially constituted by two reel-carrying towers capped at their upper part by a bridge structure for receiving a tape accumulator and by two tape connection boxes when stopped. , each assigned to a reel holder tower and mounted movable in translation, transversely to the strip on the bridge structure, characterized in that each reel holder tower is constituted by a connection frame to the bridge structure and by a frame mobile movably mounted on the frame, perpendicular to the bridge structure, this chassis carrying, in addition, a means of vertical displacement of the spindle, mounted in overhang, to be inserted in the reel to be unwound, and being connected in translation to the corresponding box.

• la figure 1 est une vue en élévation latérale du dispositif conforme à l'invention;
• la figure 2 est une vue en plan du dispositif suivant la figure 1;
• la figure 3 est une vue en élévation frontale et en coupe, à plus grande échelle, suivant A-A de la figure 1;
• la figure 4 est une vue à plus grande échelle, en élévation et partiellement en coupe, d'un caisson de raccordement;
• la figure 5 est une vue à plus grande échelle, en élévation frontale et en coupe d'une tour porte-bobines, et
• la figure 6 est une vue en plan et en coupe de la tour porte-bobines suivant la figure 5.

The invention will be better understood from the description below, which relates to a preferred embodiment, given by way of nonlimiting example and explained with reference to the appended schematic drawings, in which:

• Figure 1 is a side elevational view of the device according to the invention;
• Figure 2 is a plan view of the device according to Figure 1;
• Figure 3 is a front elevational view in section, on a larger scale, along AA of Figure 1;
• Figure 4 is an enlarged view, in elevation and partially in section, of a connection box;
• FIG. 5 is a view on a larger scale, in front elevation and in section of a spool holder tower, and
• FIG. 6 is a plan view and in section of the reel holder tower according to FIG. 5.

In accordance with the invention, and as shown more particularly, by way of example, Figures 1 and 2 of the accompanying drawings, the unrolling machine is essentially constituted by two reel-holder towers 1 capped at their upper part by a structure in bridge 2 for receiving an accumulator 3 of bands and by two boxes 4 for connecting bands when stopped. The boxes 4 are each assigned to a reel holder tower 1 and are mounted movable in translation, transversely to the strip under the bridge structure 2.

Each reel holder tower 1 is constituted by a frame 5 for connection to the bridge structure 2, and by a movable frame 6 movably mounted on the frame 5 perpendicular to the bridge structure 2, this frame 6 carrying, in addition , a means 7 for vertical displacement of the spindle 8, mounted in cantilever, to be inserted in the coil 9 to be unwound, and being connected in translation to the corresponding box 4 (Figures 1, 4, 5 and 6). The mobile frame 6 is guided on the frame 5 is produced either by means of rails or by rolling on the foundation plate of said frame 5, and its transverse movement is controlled by a jack, by a pinion and rack assembly, or similar.

The mounting of the movable frame 6 on the frame 5 allows said frame 6 to be displaced from its normal position for unwinding the reels to a loading position, so as to introduce the spindle 8 into the mandrel of the reel 9 and loading the latter.

The means 7 for vertical displacement of the spindle 8 mounted in cantilever is constituted in the form of a support cage for the spindle 8, housing the axial drive mechanism of the latter by a current motor regulated by a variator of the four quadrant type, and guided on rails 10 of the front face of the movable frame 6 by means of pads 11, or by means of guide slides, the spindle 8 passing through a vertical slot 12 of said front face of the chassis 6, and by a mechanism 13 for vertical displacement of the means 7, in the form of a screw, rack, chain, or other lifting mechanism, actuated by a motor, by a jack, or manually.

In Figures 5 and 6 of the accompanying drawings, the mechanism 13 is only shown, for reasons of simplification of the drawing, by an axis line.

The machine according to the invention is, moreover, equipped, in front of the frames 5, with a guide rail 14 for a motorized carriage (not shown) for handling coils whose stop positions in front of the vertical axis of the pins 8 of the reel-holder turns 1 are provided automatically, in a known manner, by latches and position detectors.

According to another characteristic of the invention, each movable frame 6 of the reel-holder towers 1 is provided with a device for detecting the diameter of the reels, mechanical, of the probe type, with electromagnetic waves, of the radar type, or in the form of a photoelectric detector, which is connected, in a known manner, to a computer controlling the mechanism 13 for vertical displacement of the means 7. Such detection devices combined with computers are known and are not described in more detail.

This embodiment allows complete automation of the loading of the coils, because the said coils can be stopped precisely in front of the axis of the spindle 8, then loaded by automatic movement of the mobile chassis 6.

According to another characteristic of the invention, the mechanism 13 is advantageously connected, at its anchoring point on the chassis 6 or on the means 7, to a strain gauge (not shown), this gauge being able to advantageously be connected to a stock management computer. This embodiment makes it possible to carry out a very precise weighing of the reel after its loading on the spindle 8, so that the acquisition a special weighing scale, bulky and expensive, generally housed in the ground in front of the unrolling machine, is no longer necessary. Indeed, the forecast of a strain gauge, allows to carry out successive weight records which can be recorded by a paper consumption management computer and taking into account the weight of the raw reel arriving from stock, then that of the coil removed from the outer turns damaged during transport and, finally, the weight of the coil at the end of the unwinding operation.

The mounting of a strain gauge is known per se and is not explained in more detail in the description.

The following operations for bringing in new coils and mounting these coils on the spindles are advantageously controlled and controlled by a programmable automaton, so that each coil change can be carried out by simply sending an order to move the turn 1 to provide a reel to its loading position, the machine then monitoring operations until the reel 9 is brought to the spindle 8 in its unwinding position, the transverse movement device for each movable frame 6 allowing, moreover, to ensure lateral position corrections of the unwinding reel 9.

In accordance with another characteristic of the invention, the mobile chassis 6 is advantageously provided, at the vertical slot 12 for passage of the spindle 8, with a means for blocking in the vertical position the guide sleeve of the spindle 8, in the form of locks cooperating with notches of said sleeve, clamping jaws, acting laterally on said sleeve, or other locking devices in position.

The connection boxes 4 are mounted in opposition, each under the bridge structure 2 movable transversely relative to the latter, by means of horizontal slides 15 integral with said structure 2 and projecting forward relative to the latter, and are secured in translation each with the corresponding tower 1 (Figure 3). Due to this arrangement, each box 4 follows all the movements of the corresponding tower 1 and protrudes towards the side of the conductor to facilitate access to it, each box 4 being linked to its corresponding coil holder tower 1 in such a way that 'they always move together.

On the face facing the opposite box, each box 4 has a plate 16 provided, on the one hand, with a transverse slot 17 inclined relative to the horizontal in which is mounted, movable by means of a jack or '' a linear motor, a cutting device 18, and, on the other hand, several rows of air suction holes 19, or pliers, these holes 19 or pliers being arranged, on the one hand, on the other and on the other of the slot 17 and, on the other hand, parallel to the lower edge of the plate 16 which is, on the other hand, constituted by three planes forming an obtuse angle between them, the central plane containing the inclined transverse slot 17, and, on the other hand, mounted along one of its horizontal edges, preferably along its lower edge, pivotally, on the box 4, the pivot axis 20 of the plate 16 simultaneously forming the support for return cylinders 21 of the strip to be unwound.

The boxes 4 advantageously have a width greater than the greatest possible width of strips to be unrolled and their plate 16 has, on either side of its useful surface corresponding to the maximum width of strip to be unrolled, a recessed surface allowing arrange the command and control instruments 22, as well as a means 23 for protecting the cutting device 18 in the rest position, in the form of a cover.

In addition, the plates 16 of the boxes 4 are, on the one hand, symmetrical with respect to a vertical plane passing between the boxes 4 and, on the other hand, each loaded on its inner face by a pivoting device about the axis 20 such as a jack, a cam, or the like.

Thus, the simultaneous actuation of the movement device of each plate 16 has the effect of causing said plates 16 to pivot about their axis 20, and of producing an intimate application of the central planes containing the transverse slots 17 of said plates 16 with the interposition of finishing and new strips, as well as their adhesive butt-joint bonding.

In addition, according to another characteristic of the invention each plate 16 is provided, on its central plane containing the inclined transverse slot 17, with a groove 24 extending parallel to said slot 17 at a distance at least equal to the width a double-sided adhesive joint connecting the finishing and new strips. This embodiment allows, without any modification or adjustment of the machine, to make a connection between bands by overlap.

After loading a new reel on a spindle 8 of a tower 1, placing said reel in the high position with the means 7 for vertical displacement and locking in position of said means 7, the reel is freed of turns which may be damaged by transport , and the machine operator brings the start of the strip to the plate 16 of the corresponding box 4. This work is carried out in the loading position of the reel 8, that is to say when it is, together with the tower 1 and the corresponding box 4, projecting relative to the bridge structure 2, so that it can be carried out in good ergonomic conditions. The driver then performs, at eye level, the attachment of the end of the strip by suction to the plate 16 of the casing 4, the actuation of the suction device, connected to the plate 16 behind the suction holes 19, being carried out in a known manner by means of a pushbutton. The start of the strip is then cut at an angle, either manually by guiding a knife into the transverse slot 17, or automatically by means of the cutting device 18, and the excess strip is removed. The suction at the holes 19 surrounding the slot 17 is then cut so as to allow the partial removal of the strip end of the plate 16 in order to carry out the laying along the axis of the slot 17 of an adhesive seal. which is held on the plate 16 by re-aspiration of the holes 19 surrounding the slot 17, then the strip end is applied to the plate and partially to the joint for the future butt connection. The strip of the new coil is thus ready to be connected, and the order to return to the unwinding position of the tower 1 and its box 4 can be given. During this waiting period, the coil will be kept locked in rotation by a small locking brake known per se.

In the case of a strip connection by overlapping, the cutting of the end of the new strip is carried out manually by guiding a knife in the groove 24, the additional length of strip remaining maintained on the plate 16 of the box 4, then being provided with a double-sided adhesive strip. All other operations are identical to those of an end-to-end connection.

When returning to the starting position, the lateral positioning of tower 1 with its coil can be adjusted as indicated above, and synchronous lateral movements of the two towers can be performed automatically while maintaining lateral alignment of the coils and perfect coincidence of caisson slots 4.

According to an alternative embodiment of the invention, not shown in the accompanying drawings, the rails 15 for guiding the boxes 4 can also extend parallel to the transverse slots 17 inclined relative to the horizontal. So the two opposite slots 17 of the boxes 4 always slide one opposite the other and in alignment, whatever the respective positions of the towers 1, therefore even when the boxes 4 move separately.

When the predetermined minimum diameter of the unwinding reel is detected, in a known manner, by optical means or by mechanical probe or by electronic calculation of the diameter, the order of connection of the new reel is given, either by an automaton, or by an audible and / or luminous alarm warning the operator. At this instant, the strip accumulator 3 is normally filled, so that the finishing reel can be braked until it comes to a complete stop by means of the brake motor actuating its support pin 8. The suction device, connected to the holes 19 of the plate 16 of the corresponding box 4, is then placed under vacuum and the cutting device 18 of this box 4 is actuated simultaneously, so that the finishing strip is cut at an angle, then the rest of the strip connected to the finishing reel is automatically removed from the plate 16, after cutting off the suction at the holes in the lower part of the said plate, by reverse rotation of the axial unwinding motor.

According to another characteristic of the invention, the machine can be provided with at least one removable cutting counter-bar (not shown) applying the strip on the plate 16 along the cutting slot 17 of each box 4 , indifferently, this cutting counterpart bar being maneuvered into the release position by means of a pair of pneumatic cylinders after the passage of the knife 18, one of these jacks serving to move the bar in front of the box 4 and the other to its application against the plate 16 of the box with tightening of the strip to be cut, this second jack can be a rotary jack or a double-acting jack pivoting the bar towards one or the other box 4. This embodiment allows maintenance perfect tape, flat on the plate 16 and obtaining a clean cut, even with slightly rigid materials.

The plates 16 of the boxes 4 are then pushed towards one another by pivoting about their axis 20, until the tail of the ending strip comes to stick on the adhesive seal at the end of the strip of the new reel . This movement of the plates 16 takes place very quickly, and in fact only corresponds to a pulse producing the bonding pressure of the ends of the strip, triggers, at the end of the pulse, the cut off of the suction through the holes 19 and, simultaneously, the starting of the axial unwinding motor of the new coil. The latter is accelerated in rotation until its normal working speed is obtained, and the paper accumulator 3 can be recharged.

The motors fitted to the towers 1, intended for the axial drive of the spindles 8, are advantageously, in known manner, DC motors sufficiently powerful to allow rapid acceleration and deceleration of the coils in order to avoid oversizing of the reserve belt variation 3. The power variation of these motors is subject to the permanent control of two variables, namely the belt speed of the machine fed downstream by the unwinding machine, and the prescribed belt tension is continuously measured at various places. All of these variables and parameters can be managed, in a known manner, by a computer which harmonizes and ensures a band flow at constant voltage despite possible changes in production speed or the operations of connecting the reel when stopped, which use the back-and-forth of the tape accumulator 3.

The position of the mobile trolley of the accumulator 3 is continuously regulated by means of a special motor, the anchoring of the mechanical socket of which is connected to a force measuring element placed on the mobile trolley. This force measuring element can also be mounted on a guide roller placed at the exit of the carriage so as to continuously monitor the tension of the belt, which must remain constant thanks to the combined action of the mobile carriage and the spindle unwinding motor.

In addition, according to another characteristic of the invention, the unrolling machine can be provided with a marking device (not shown) advantageously placed at the level of the casing 4 close to the fixed part of the accumulator 3, above of the inclined transverse slot 17 on the plate 16, or on the upper face of this box, or even at the entrance to the accumulator 3, this marking device being a mechanical perforation or other device, or a deposit device or inkjet controlled with the same periodicity as the strip shaping located downstream, and being disposed at a distance from the plane containing the inclined slot 17 equal to a fraction of a step of the processing machine located downstream. In the aforementioned case, a step corresponds to a fixed length of material treated by the processing machine, that is to say, for example, in the case of an envelope manufacturing machine, to the developed developed length to make an envelope. Thus, a fraction of a step will advantageously correspond to a distance such that the joint between strips is automatically located on a part of the envelope having no marking or cutting. This process makes it possible to neutralize the elasticity phenomena of the strip which are harmful to precise location of the connector.

Finally, according to another characteristic of the invention, not shown in the accompanying drawings, the unrolling machine can be provided with support arms for the cantilevered end of the pins 8, these arms being either mounted on the chassis 6 of the towers 1, retractable, and with the possibility of telescopic extension and retraction in correspondence with the downward and upward movement of the vertical displacement means 7, or mounted on the frames 5 so retractable, and movable in translation together with the chassis 6 or by means of its own control means actuated simultaneously and in an identical manner to the translation device of said chassis 6. This embodiment allows, in particular, the loading of coils of large dimension, in particular large width, and of a heavy weight.

Thanks to the invention, it is possible to produce a machine for unrolling continuous strips from reels, which can be loaded automatically and whose strip connections can also be connected directly with possible identification of pre-prints, as well as of the fitting with respect to subsequent strip shaping operations, so that deterioration of the shaping tools of downstream machines can be avoided. In addition, the bias cut, as well as the possibility of carrying out, depending on the characteristics of the material to be unrolled, an end-to-end or overlap connection makes it possible to minimize the risks of jamming on the main machine and to obtain better productivity.

Claims (16)

1. Unwinding machine, essentially constituted by two reel stands (1) covered at their upper part by a bridge structure (2) for receiving a web accumulator (3) and by two boxes (4) for joining webs at a standstill, each assigned to a reel stand (1) and mounted so as to be movable in translation transversely to the web on the bridge structure (2), characterised in that each reel stand (1) is constituted by a frame (5) for connection to the bridge structure (2) and by a movable chassis (6) mounted in a manner displaceable on the frame (5) perpendicularly to the bridge structure (2), this chassis (6) furthermore carrying a means (7) for vertical displacement of the spindle (8), mounted as a cantilever, for insertion into the reel (9) to be unwound, and being linked in translation to the corresponding box (4).

2. Machine according to Claim 1, characterised in that the guiding of the movable chassis (6) on the frame (5) is achieved either by means of rails or by rolling on the baseplate of the said frame (5), and its transverse displacement is controlled by an actuator, by a rack and pinion assembly or similar.

3. Machine according to either of Claims 1 and 2, characterised in that the means (7) for vertical displacement of the spindle (8) is in the form of a support cage for the spindle (8) housing the mechanism for driving the latter by means of a d.c. motor regulated by a four-quadrant variator, and guided on rails (10) on the front face of the movable chassis (6) by means of shoes (11) or by means of guiding slides, the spindle (8), mounted as a cantilever, passing through a vertical slot (12) of the said front face of the chassis (6), and by a mechanism (13) for vertical displacement of the means (7), in the form of a screw mechanism, rack mechanism, chain mechanism or other means of raising, actuated by a motor, by an actuator or manually.

4. Machine according to any one of Claims 1 to 3, characterised in that it is furthermore equipped, in front of the frames (5), with a guide rail (14) for a motorised reel-handling carriage, the stopping positions of which in front of the vertical axis of the spindles (8) of the reel stands (1) are assured in a known manner by latches and position detectors.

5. Machine according to Claim 3, characterised in that each movable chassis (6) of the reel stands (1) is equipped with a device for detecting the diameter of the reels, either mechanical, of the feeler type, of the type which employs electromagnetic waves, of the radar type or in the form of a photoelectric detector, which is linked in a known manner to a computer controlling the mechanism (13) for vertical displacement of the means (7), such that the loading of the reels can be effected automatically.

6. Machine according to Claim 5, characterised in that the mechanism (13) is advantageously linked, at its point of anchorage on the chassis (6) or on the means (7), to a strain gauge, it being advantageously possible for this gauge to be linked to a stock-control computer.

7. Machine according to Claim 3, characterised in that the movable chassis (6) is advantageously provided, in the region of the vertical slot (12) for the passage of the spindle (8), with a means for locking the guide sleeve of the spindle (8) in a vertical position, in the form of latches cooperating with notches of the said sleeve, clamping jaws acting laterally on the said sleeve, or other devices for fixing in position.

8. Machine according to Claim 1, characterised in that the joining boxes (4) are mounted in opposition, each under the bridge structure (2) in a manner displaceable transversely in relation to the latter by means of horizontal slides (15) rigidly connected to the said structure (2) and protruding forwards in relation to the latter, and are each rigidly connected in translation to the corresponding stand (1) and protrude towards the operator side to facilitate access for him, each box (4) being linked to its corresponding reel stand (1) in such a way that they are always displaced as one.

9. Machine according to either of Claims 1 and 8, characterised in that, on its face turned towards the opposite box, each box (4) has a plate (16) provided, on the one hand, with a transverse slot (17) inclined relative to the horizontal in which a cutting device (18) is mounted in a manner displaceable by means of an actuator or a linear motor, and, on the other hand, with a plurality of rows of holes (19) for sucking in air, or of grips, these holes (19) or grips being arranged, on the one hand, on each side of the slot (17) and, on the other hand, parallel at least to the lower edge of the plate (16), which is furthermore, on the one hand, constituted by three planes forming between them an obtuse angle, the central plane containing the inclined transverse slot (17), and, on the other hand, mounted pivotably along one of its horizontal edges, preferably along its lower edge, on the box (4), the pivoting spindle (20) of the plate (16) simultaneously forming the support of return cylinders (21) for the web to be unwound.

10. Machine according to Claim 9, characterised in that the boxes (4) advantageously have a width greater than the greatest possible width of webs to be unwound and, on each side of its effective surface corresponding to the maximum width of web to be unwound, their plate (16) has a recessed surface permitting the arrangement of the monitoring and control instruments (22), and a means (23) for protecting the cutting device (18) in the rest position in the form of a cover.

11. Machine according to either of Claims 9 and 10, characterised in that the plates (16) of the boxes (4) are, on the one hand, symmetrical in relation to a vertical plane passing between the boxes (4) and, on the other hand, each loaded on its inner face by a device for pivoting about the spindle (20), such as an actuator, a cam or other device.

12. Machine according to any one of Claims 9 to 11, characterised in that each plate (16) is provided on its central plane containing the inclined transverse slot (17) with a groove (24) extending parallel to the said slot (17) at a distance at least equal to the width of a connection joint with a double adhesive face between the web which is finishing and the new web.

13. Machine according to any one of Claims 1, 8 and 9, characterised in that the guide rails (15) of the boxes (4) can also extend parallel to the transverse slots (17) inclined in relation to the horizontal.

14. Machine according to any one of Claims 1 and 8 to 13, characterised in that it is provided with at least one movable cutting buffer bar which applies the web equally to the plate (16) along the cutting slot (17) of each box (4), this cutting buffer bar being manoeuvred into the release position by means of a pair of pneumatic actuators after the passage of the knife (18), one of these actuators serving for the displacement of the bar in front of the box (4) and the other for applying it to the plate (16) of the box with clamping of the web to be cut, it being possible for this second actuator to be a rotary actuator or a double-action actuator pivoting the bar towards one or other box (4).

15. Machine according to Claim 9, characterised in that it is provided with a marking device advantageously placed in the region of the box (4), near to the fixed part of the accumulator (3), above the inclined transverse slot (17), on the plate (16) or on the upper face of this box or at the input of the accumulator (3), this marking device being a mechanical perforating device or other mechanical device or a deposition or ink-jet device controlled with the same periodicity as the web conversion situated downstream and being arranged at a distance from the plane containing the inclined slot (17) which is equal to a fraction of a step of the conversion machine situated downstream.

16. Machine according to Claim 1, characterised in that it is provided with arms for supporting the overhanging end of the spindles (8), these arms either being mounted on the chassis (6) of the stands (1) in a retractable manner and with the possibility of telescopic extension and retraction in correspondence with the downward and upward movement of the vertical displacement means (7) or mounted on the frames (5) in a manner retractable and displaceable in translation together with the chassis (6) or via their own control means actuated simultaneously with and in a manner identical to the translation device of the said chassis (6).

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