AU3125399A

AU3125399A - Positioning device of a flexible printing plate on a plate cylinder

Info

Publication number: AU3125399A
Application number: AU31253/99A
Authority: AU
Inventors: Roger Saladin; Charles Stark
Original assignee: Bobst SA
Current assignee: Bobst Mex SA
Priority date: 1998-06-02
Filing date: 1999-05-25
Publication date: 1999-12-09
Anticipated expiration: 2019-05-25
Also published as: EP0972639A1; CA2273388C; BR9902635A; ES2195469T3; AU748606B2; EP0972639B1; CA2273388A1; JP2000033688A; DE69906426D1; CH689717A5; KR20000005811A; KR100334476B1; DK0972639T3; CN1237513A; DE69906426T2; CN1085147C; ATE236016T1; JP3192640B2

Abstract

The positioner, for a flexible plate on a carrier cylinder (3) with a lengthwise slit (1) in its shaft, is actuated by a lateral movement of the carrier cylinder beyond a set course by an exterior component fixed to the printing machine frame at one end of the cylinder and a crown cam (14) at the other. The inner edges (12, 13) of the crown cam act as guides for a roller (11) on a mobile base plate. Two other cams are located at the base of the roller shaft slit to control the movement of rollers (15) linked to the base plate.

Description

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AUSTRALIA

Patents Act 1990 Bobst S A

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT

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Invention Title: Positioning device of a flexible printing plate on a plate cylinder The following statement is a full description of this invention including the best method of performing it known to us:- The invention relates to a device for positioning a flexible printing plate on a plate-bearing cylinder, the device being adapted inter alia to bias the flexible printing plate on the plate-bearing cylinder of a rotary printing machine.

Machines of this kind, which are frequently used in the packaging industry for printing flat articles such as sheets of paper or cardboard, are usually called flexographic machines and comprise a number of component stations, i.e. (in a logical progression from upstream to downstream with reference to the direction of motion of the flat articles) an insertion station where sheets are successively gripped from the bottom l of a stack, a series of one or more printing groups with one or more interposed dryers if required, and a final receiving station where the printed articles are collected. The sheets are transferred from one station or group to another above the stations and by means of A3 a train of rollers or a suction system, so that the printed bottom surfaces of the sheet are left entirely free.

The printing-plate positioning device is of use in the said printing groups. Each printing group, which is generally adapted to print in a particular colour, inter alia comprises a plate-bearing or plate cylinder around which the flexible printing plate is rolled and B(IB~9~PPII~-iU~ tensioned. The printing plate, which is coated with ink by a screen cylinder, prints one or more patterns, depending on the number of printing formes which it contains, during each rotation of the plate cylinder on to the sheet moving between the cylinder and a pressure roller, both being disposed so that the bottom surface of the sheets is printed.

The printing forme, which is made of rubber or plastic, is usually stuck to a flexible support base made of polyester such as Mylar and forming an assembly which hereinafter will be called the "printing plate". After the printing forme has been made, the step of sticking it to the polyester base is critical. This step, which is performed by the manufacturer, directly determines the accuracy of register between the printing in different colours. The upstream and downstream ends of the flexible support base are equipped with a plastic strip, one longitudinal edge of which has a U-section for fastening the printing plate in a claw secured to the print cylinder. The claw in turn is disposed parallel to the axis of rotation of the cylinder. A thin metal plate riveted to the plastic strip and formed with a slot having a longitudinal o upstream/downstream axis is for rapid, accurate, lateral positioning of the printing plate on its cylinder by fitting on to a centring pin secured to the claw.

Any fault in the lateral or longitudinal positioning of the printing forme, during sticking on to the flexible polyester support base, can easily be made up by lateral movement or by rotating the plate cylinder around its axis, but the situation is different when B~ L~C~l~e~L l~ln~ 4 the fault is due to biasing of the printing forme. In that case its edges are no longer oriented in a direction parallel or perpendicular to the upstream and downstream edges of the polyester base, and the result is irremediably biased positioning of the printing forme relative to the direction determined by the axis of rotation of the plate cylinder.

In the case where a number of printing formes are stuck 1 to the flexible polyester base and are all affected by the same faulty biasing, it will be assumed, as is usually the case, that the top edges of each printing forme are all aligned and can thus be treated like a single large printing forme which has been faultily positioned inter alia through biased sticking.

To solve this problem, various known devices can rotate the front and rear claws relative to the axis of rotation of the plate cylinder so as to bias the .O printing plate by an equal and opposite amount to the discovered fault.

•coo To this end, the patent US 5 488 904 describes a device comprising two parallel bars gripping the ends of the printing plate. A mechanism controlled by manually rotating a graduated eccentric imposes lateral movement (parallel to the axis of rotation of the cylinder) on the rear bar for fastening the printing plate and pivots the front bar via a set of bent parts ;0 articulated on a number of pivots disposed at their ends. The rear bar, actuated by a spring at one of its ends, is automatically pivoted into a position parallel to that occupied by the front bar when the printing plate is tensioned via its two ends in the device. The tBS~B-e~_~aC~~ entire mechanism is disposed in the body of the plate cylinder at the bottom of a wide deep groove machined parallel to the axis of rotation of the cylinder. The eccentric controlling the mechanism is mounted on one side of the plate cylinder.

Another printing-plate biasing device is disclosed in EP-232 730 where the inventor proposes a mechanism based on pivoting of two tension bars in which the ends I of the printing plate are clamped. The tension bars are interconnected at their ends by two triangular articulated plates and mounted on two pivots secured to the plate cylinder. The pivots are rectangular so that the printing-plate tensioning bars pivot around a I virtual point P along the central axis of the plate cylinder. The control means is disposed on one side of the plate cylinder and comprises a complex mechanism made up of articulated levers which can be actuated by a jack when the cylinder is at rest in a given dO position.

Patent JP 62-56146 solves the printing-plate biasing problem by means of a device comprising two bars each oo: actuated by two hydraulic jacks. When the position of the printing plate has to be diagonally adjusted, the oil pressure in the four jacks is reduced so that four springs, previously compressed by the four jacks, can expand and press the printing-plate fastening bars so that they open. The amount of the adjustment is Sdigitally fed into a controller which converts it into a corresponding oil pressure which is applied to two additional jacks disposed on the same side at one end of the fastening bars. Under the action of the two additional jacks, two inclined planes can be ~-~dCl~s~l~BIILI~8~L(D slP independently moved in a straight line so as to force one end of each printing-plate fastening bar, when the bars close, in a perpendicular movement owing to the triangular geometrical shape of the inclined plane.

The inclined planes may also, in a preferred embodiment, be actuated by a motorised screw mechanism.

These devices have a number of drawbacks, such as: the presence of a bulky "on-board" mechanism which requires the body of the plate cylinder to be formed with a groove which is large compared with the cylinder diameter, an undesirable imbalance is produced by the size of the means actuating the said mechanism, resulting in constraints in the cylinder bearings when the cylinder reaches a certain speed of rotation, 0 the circumferential work surface around the cylinder is reduced owing to the considerable width of the groove required by the biasing device, in some cases adjustment is manual, requiring complete stoppage of the machine and prolonging the adjustment time, adjustment of the printing plate cannot be completely automated in cases where it is necessary to re-start old jobs for which the adjustments could have been digitally saved and stored, and in the event of non-manual adjustment, the printing-plate biasing mechanism must be controlled by BB~e~ZZ L~, an additional actuator independently of that used for lateral or circumferential movement of the plate cylinder.

The printing-plate biasing device according to the invention is designed to obviate these disadvantages by providing a simple automatic device which takes up minimum space in the body of the plate cylinder and can be used without stopping the machine. The desire to Soccupy a minimum volume has been an important factor in constructing a device offering the advantage of not being dependent on a printing-plate biasing mechanism actuated by an actuator disposed in the body of the plate cylinder. For example, the device described in 5 the invention drives the printing-plate biasing mechanism by means of an electric motor which is secured to the machine frame and is the same as the motor already used for moving the entire plate cylinder when laterally adjusted. This solution permits more •O efficient use of the actuator already available for lateral adjustment of the printing plate, and also S.provides a biasing mechanism which is cheaper, less bulky, and rugged and reliable, and can easily be adapted to those plate cylinders which do not have such a mechanism. Finally, as a result of the mechanism, an external electronic controller connected to a number of position sensors disposed outside the cylinder can be used to measure and store the various adjustment positions which are suitable for respective jobs which 0 can easily be restarted subsequently without the need for a registering operation. This latter alternative provides an additional advantage which is particularly suitable for the mechanism according to the invention.

8 To this end the invention relates to a device for positioning a flexible printing plate according to claim i.

In order to define a few terms introduced into the description and describing the position of certain components in the printing machine, we shall use the expressions "operator's side" and "opposite operator's side" these terms being used by general agreement to denote a particular side relative to the longitudinal central axis of the machine. This choice avoids any confusion regarding the usual terms "left" and "right", which depend on the observer's point of view. Similarly, the orientation of some axes or objects will be described by the usual terms "longitudinal" and "lateral", always with reference to the central axis of the machine, the orientation of which depends on the direction of travel of the cardboard elements. Finally, note that the terms O "upstream" and "downstream" refer to the direction of motion of the flat articles in the printing machine.

The invention will be more clearly understood from an

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embodiment given by way of non-limitative example and .illustrated by the accompanying drawings in which: Figs. la and lb are plan views respectively of the C.O.C. third and the C.C. third of the biasing device mounted on the plate cylinder; Figs. 2 is a partial view in section of the C.C. end of the device for positioning a flexible printing plate on the plate cylinder; q;--f~P Figs. 3 and 4 are views in section of the biasing device mounted on the plate cylinder, along lines A-A and B-B in Figs. la and Ib respectively; Figs. 5a, 5b are plan views of the C.O.C. and C.C.

halves respectively of the baseplate and of the claw support fitted thereon; Fig. 6 is a partial view in section, in a direction parallel to the axis of rotation of the plate cylinder, of the said cylinder near the C.C. cam; Fig. 7 is a plan view of the C.C. cam and the corresponding stirrup; Fig. 8a is a very diagrammatic plan view of the printing-plate biasing device, more particularly when the mechanism is in an intermediate position which does not initially bias the printing plate; 0 IFig. 8b differs from Fig. 8a in that the printing-plate biasing mechanism is in action in a first end position after clockwise rotation, and 5 Fig. 8c differs from Fig. 8b in that the printing-plate biasing mechanism is action in a second and opposite end position after anticlockwise rotation.

Figs. la and Ib are general plan views of a device for positioning a printing plate disposed in a groove 1 machined parallel to the axis of rotation 2 of a plate cylinder 3. As shown more clearly in the corresponding sections illustrated in Figs. 3 and 4, the biasing device is supported by a baseplate 4 on which an immovable front claw 5 is mounted and grips the front edge of a printing plate 19 in a notch 7, the printing plate being disposed around the plate cylinder 3. A movable rear claw 6 facing the front claw 5 grips the rear edge of the plate 19 in a notch 8 and at its C.O.C. end has a slide 9 engaging a roller 10 screwed to a block 100 secured to the plate cylinder 3 by two screws. At the opposite end of the positioning device (Fig. 2) the basepla te 4 is attached to a roller 11 ICplaced between the sides 12, 13 of a circular or annular cam 14 mounted on the machine frame (not 9. shown) Figs. la and lb show two other rollers 15, 16 which are secured to the baseplate 4 and slide in two respective cams 17, 18 rigidly secured to the bottom of Sthe groove 1 in the plate holder 3. A means 101, inter alia comprising an electric motor, is mechanically connected to the plate cylinder 3 and moves it laterally along its axis of rotation 2.

99.. 2QAs shown in Figs. la, lb and 3, the baseplate 4 comprises a number of supports 20 in the form of rectangular blocks held at their base by screws 21 and above which a longitudinally bent protective metal plate 22 is secured by screws 23, followed by the front ~Sclaw 5, which is held by screws 24 which also grip the protective plate 22. Opposite the front claw 5, the rear claw 6 is secured by fastening means 103 (Fig. 4) to a support 25 comprising a metal plate formed with a number of slots 26 (Figs 5a, 5b) through which journals Z0 27 extend by means of which the rear claw 6 is fastened on the support 25 by screws 28. The slots 26 are oriented so that the claw support 25 can move laterally as required relative to the rear claw 6 and also so as to drive the rear claw 6 in a longitudinal movement 11 perpendicular to the axis of rotation 2 of the plate cylinder. The thus-mentioned movement comprises closing and opening the rear claw 6, so as respectively to tension or release the printing plate 19 from the plate cylinder 3. The claw support 25 is rigidly secured by screws 29 on to two blocks 30 in the form of rectangular cuboids and sliding along two axes or shafts 31. The shafts are held at each end by a support 32 secured to the baseplate 4 by screws 33.

10 Above each support, a clamping screw 34 locks the shaft .31 in its support 32. The claw support 25 can thus be longitudinally moved along the axes or shafts 31 and entrain the rear support 6, owing to the perpendicular ee.

orientation of the slots 26.

The longitudinal movement for opening the rear claw 6 ~is controlled by seven pneumatic jacks 40 (Figs. operating simultaneously and supplied by ducts 41 secured on the baseplate 4 by attachment flanges or .0 clamps 42 and interconnected by connections 43. The jacks 40 are secured by screws 45 on to L-shaped supports 44 which can also serve as attachment points for fastening the protective plate 22 and the front claw 7. The supports 44 are regularly spaced and each ch positioned by two screws on the baseplate 4. Metal plates 46, bent at right angles at their ends, are positioned facing the jacks 40 and are welded, standing on one of their two main edges, on the claw support When the pneumatic circuit supplying the jacks 40 is !O pressurised, the shafts 47 of the jacks (Figs. la, ib) push the claw support 25 in front of them while resting on the plates 46. The rear claw 6 associated with the support 25 is then open.

~-"L~Li~-~~"IUIP64afilk~~ Once the pressure in the jacks has been relaxed, the front claw 6 is closed by a number of compression springs 48 (Figs. 4, la, Ib) which push the claw support 25 in a longitudinal closing movement. Each spring 48 is guided by the rod of a screw 49 which extends through the bent plate 46 on which one end of the springs rests. The second end of the springs 48 rests on a plug 50 screwed to a retaining block 51 which in turn is secured to the baseplate 4 by two screws 52. The retaining blocks 51 also serve as a support for fastening to a second bent protective plate which partly covers the top of the claw support The protective plate 53 is secured at its base by a number of screws 54 (Figs. la, Ib) disposed in the retaining blocks 51, and is formed with circular openings giving free access to the plugs The printing plate 19 is laterally positioned on the front claw 5 by a centring pin 55 (Fig. 4) driven into O the upper part of the front claw 5 and accurately disposed on the longitudinal axis of the printing machine.

Fig. 5b shows a fastening 60 attached to the C.C. end a of the baseplate 4 by screws 61. The roller 11 is positioned by a screw 62 on the longitudinal axis of the said fastening. Two holes 63 situated approximately at the first and last third of the baseplate 4 have a substantially greater diameter than 3O that of the two bushes 64 extending through them (Fig.

The overdimensioning of the holes 63 provides the space necessary for movement of the baseplate 4 while holding it pressed against the bottom of the groove 1 by the holding means 102, thus counteracting the centrifugal force exerted on the baseplate 4 when the plate cylinder rotates. The holding means 102 comprise bushes 64 around which two bearings 65 made of synthetic material having a low coefficient of friction Sgrip the baseplate 4 in a sandwich and enable it to move easily. A compression spring 66 disposed between two other washers 67, 68 and a screw 69 resting on the top washer 68 and screwed into the bottom of the groove 1 after extending through the bush 64 hold the O baseplate 4 clamped in a plane parallel to the direction of the plate cylinder. The fastening 60 is also formed with a hole 70 through which a screw 71 extends, the hole and the screw serving the same purpose as the holes 63 and screws 69 described Shereinbefore.

As shown in Figs. 5a and 5b, two rectangular openings 72 cut in the baseplate 4 are for receiving the C.C.

cam 17 and the C.O.C. cam l8 secured to the plate .Ocylinder 3. The openings 72 are bigger than the bases 80 of the curve 17 and 18, so that the baseplate 4 can move freely around the cams without butting against either of them. Three aligned holes are situated at each narrow end of the openings 72, the central holes ~673 being of use for accurately positioning a stirrup 74 via two cylindrical pins 75, whereas the end holes 76 are for fastening the stirrup 74 to the baseplate 4 via screws 77. As illustrated 'in Figs. 6 and 7, the stirrups 74 constitute two br.idges secured to the baseplate and disposed above the cams 16 and 17, the rollers 15 and 16 being screwed from underneath into the largest surfaces of the respective cams 17 and 18 and sliding therein. Screws 78 are forsecuring the _iiij~r_:__ 14 protective plate 22 and the front claw 5 above the stirrup 74.

Cams 16 and 17 are symmetrical with one another and comprise a stepped base 80 formed with a hole for receiving a centring pin 81 and also with two securing holes 83 in which two screws 82 slide and are screwed into the plate-holding cylinder 3. Above the base two small triangular projecting parts 84 form the core of the cam, between which the roller 15 slides in the C.C. cam or the roller 16 in the C.O.C. cam.

In order clearly to show the basic operation of the mechanism according to the invention, Figs. 8a, 8b, 8b are very diagrammatic views of three very different positions in which the printing-plate positioning device may be. When the printing plate is not being biased, the mechanism is in an intermediate position as diagrammatically shown in Fig. 8a.

When the printing plate needs to be biased in one or the other direction, the plate-biasing mechanism can be actuate by simple lateral displacement of the plate cylinder in the direction of its axis of rotation 2.

The lateral movement of the plate cylinder 3 is controlled by the means 101, which inter alia comprises an electric motor coupled to toothed wheels engaging an endless screw. This conventional mechanism can convert circular motion of the motor shaft into :he rectilinear motion necessary for lateral displacement of the plate cylinder. A displacement of the plate cylinder brought about by the motor and the mechanism is also automated by sensors which detect the lateral posi:ion of the plate cylinder. The sensors are connected to an external electric controller (not shown) which actuates the motor of the means 101. In the case of a given job, therefore, the biasing of a printing plate can easily be controlled and stored by the electronic controller via the position sensors, by simply storing the lateral displacement of the plate cylinder.

As shown in Fig. 8b, when the cylinder is displaced from the to the C.C. the roller 11 after 0 traveling a certain distance will rest against the side 12 of the annular cam 13. This is followed by a S"combined movement of the baseplate 4 guided by rollers ooele and 16 sliding in cams 17, 18, in which the S"baseplate 4 pivots around the roller 11 and simultaneously moves in translation as shown by upward sliding of the roller 11 along the side 12 of the annular cam 14. Since the slide 9 of the rear claw 6 *...only slides longitudinally on the roller 10, it permits only motion in translation and in rotation around the ao fixed pivot point comprising the roller 10 secured to the plate cylinder 3. In view of the longitudinal orientation of the slide, no movement along the axis 2 can be transmitted to the rear claw 6. Owing therefore to the claw, which can only move parallel to the axis 2 of the plate cylinder, and owing to the combined movement to which the front claw 5 is also restricted, the front and rear edges of the printing plate can be biased, in a manner as subtle as it is simple, in a scissors movement while remaining parallel to their ?0 original position.

Fig. 8c shows the same combined movement, in an opposite direction, resulting from displacement of the plate cylinder 3 from C.C. to C.O.C. In that case the roller rests on and slides along the opposite side 13 of the annular cam 13.

When the operator wishes to adjust the biased position Sof a printing plate 19, he will actuate a control which slightly releases the pressure in the jacks 40, so as to relax the printing plate and detach it from the plate cylinder, and so as to actuate the motor of the means 101 and move the plate cylinder 3 in one or the 10 other direction. Once the roller 11, after making :contact with one side 12 or 13 of the annular cam 14, has moved along the said cam by a distance corresponding to the desired biasing, the plate S* cylinder 3 will move in the opposite direction so as to re-position it at the lateral register required for "printing. The last-mentioned movement in no way alters the biasing of the printing plate 19 since, during the reverse movement, the roller 11 is immediately released from the side of the annular cam 14. Since the A'O tensioned printing plate adheres to the plate cylinder, the biased position of the printing plate is maintained. If the position of the lateral register of the plate cylinder is known, the biasing of the printing plate can then be automated at a chosen angular value simply by inserting the value into the electronic controller.

Accordingly the operations for positioning and biasing a printing plate on a plate cylinder equipped with a device according to the invention can be summarised in the following steps: a) Opening the rear claw 6 by jacks 40 which compress the springs 48; 17 Fastening the front edge of the printing plate 19 in the notch 7 in the front claw 5 and in the lateral centring pin S Slowly rotating the plate cylinder 3 around its axis 2 so that the plate 19 winds around the cylinder; Fastening the rear edge of the printing plate 19 in the notch 8 in the rear claw 6; Closing the rear claw 6 by releasing the pressure in the jacks Rapidly rotating the plate cylinder 3 and I5 actuating the jacks 40 so as slightly to detach the printing plate 19 from the cylinder so that the plate can be biased; Biasing the printing plate 19 by laterally moving SO the plate cylinder 3 through a certain set distance corresponding to the desired inclination; Opposite lateral movement of the plate cylinder 3 so as to restore the lateral register required for printing, and Releasing the pressure in the jacks In addition to fastening the printing plate around its b0 cylinder, all the other operations described hereinbefore can easily be automated owing to the design and the specific mechanism for biasing the printing plate according to the invention. When an old I QIF~ iilh9IIlIII]] printing job is restarted, the device has the advantage of not requiring any further manual adjustment of the printing plate if the adjustment values, previously known for the said plate, have been stored. Finally the device can also be used for biasing the printing plate when the plate cylinder is rotating at full speed. It is therefore not necessary to stop the machine completely when the slope of the printing plate is adjusted, and this considerably reduces the 10 operating costs when preparing a new printing job.

S: Numerous improvements can be made to the device within S. the scope of the claims.

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Claims

1. A device for positioning a flexible printing plate on a plate-bearing cylinder the device being adapted to bias a printing plate in a rotary printing machine, the plate cylinder having an axis of rotation and formed with a groove disposed parallel to the axis of rotation and containing the said printing-plate positioning device, characterised in that the device is actuated by lateral displacement, outside a certain travel distance, of the plate .""cylinder which is moved by an external means *99*99 secured to the frame of the printing machine and by means of an annular cam disposed outside the Scylinder and facing the means on the inner edges of which a roller bears and is secured to a baseplate which is movable and is held free from the plate cylinder by holding means and by **.two cams rigidly secured to the bottom of the O groove in the plate cylinder and controlling the movement of two rollers connected to the baseplate in an opposed sloping movement.

2. A device according to claim i, characterised in that the outer means for moving the printing- plate positioning device comprises a motor and a mechanism for conventional lateral movement of the plate cylinder along its axis of rotation and a digital controller for measuring and storing the value and the direction of the lateral movements of the plate cylinder by use of conventional sensors disposed in the printing machine frame.

3. A device according to claim 1, characterised in that each cam comprises a rectangular base fixed and accurately positioned in the bottom of the groove in the plate cylinder by screws and by at least one centring pin and in that two triangular projecting parts above the base are disposed so that the rollers can slide without clearance between the longest surfaces of the triangular projecting parts, which are oriented at relative to the axis of rotation of the plate cylinder

4. A device according to claim 1, characterised in that the baseplate has two rectangular openings IS larger than the bases and at the positions occupied by the cams so that the cams can extend through them, a number of holes adjoining each opening for the purpose of rigidly and accurately securing two stirrups holding the 3 0 rollers other regularly-spaced openings being adapted to receive the means for holding a the baseplate on the plate cylinder the baseplate also supporting a front claw substantially equal in length to the groove and rigidly secured thereto via blocks and supports a rear claw is disposed facing the first claw and is held by fastening means on a claw support which cannot move independently of =he rear claw except parallel to the longer sides of the j0 baseplate A device according to claim 1, characterised in that the holding means each comprise a bush substantially smaller in diameter than the opening through which it extends, and round which the following are stacked in succession: a washer resting on a bearing disposed at the said place under the baseplate a compression spring and an upper washer which bears the head of a screw which extends through the bush and is screwed into the bottom of the groove in the plate cylinder

6. A device according to claim 4, characterised in that the stirrups form bridges, the largest o surfaces of which are screwed below the rollers the stirrups being disposed and fastened to the baseplate above each respective cam

7. A device according to claim 4, characterised in that the claw support comprises a metal plate, substantially equal in length to the front claw and formed with a number of regularly spaced slots oriented parallel to its longest sides and through P0 which the fastening means extend, each comprising a journal into which a screw is screwed and holds the rear claw on the claw support the support being mounted on at least two blocks each sliding on an axis disposed perpendicular to the length of the baseplate and held at its ends by two supports screwed into the baseplate so that the rear claw and the claw support can slide along the axes in a movement comprising opening and closing the rear claw

8. A device according to claim 7, characterised in that the rear claw is closed by a number of compression springs each resting on a retaining block secured to the baseplate and pushing the BE(le*n~-j-tii 9ne~i~aE~g~q(ase~~~~~ rear claw towards the front claw via a number of metal plates disposed upright and perpendicular to the front claw support and secured to the lower surface thereof.

9. A device according to claims 7 and 8, characterised in that the rear claw is opened by a number of pneumatic jacks fastened to the supports and on a shaft which pushes the plates so as 10 to compress the springs

10. A device according to claim 8, characterised in that each retaining block is formed with a Sthreaded hole for inserting each compression spring and in which a plug is screwed and serves as a rest for the said springs DATED THIS 25 DAY OF MAY 1999 BOBST SA Patent Attorneys for the Applicant:- F.B.RICE CO S So•