EP0483000B1 - Device for the blocking of lens blanks with a lockable arm - Google Patents

Description

The present invention relates generally to the fitting, on a glass of working glasses, and for example to overflow, of a block, said here for simple convenience gripping block, which, adapted to not adhere to a such glasses that temporarily, for example through a suction cup, is intended to allow the establishment, and the blocking in position, on a machine suitable for the work to be performed.

It relates more precisely to the case where this installation is carried out using a device, commonly called a gliding device.

In document FR-A-2 608 492, this gliding device comprises, for the installation of a gripping block on a spectacle lens arranged in standby on a plate carried by a frame, a gliding arm, which is adapted to to receive, in a removable manner, the gripping block to be placed, a block, or carriage, on which the sliding arm is pivotally mounted between two extreme positions, one raised, of loading, the other, tilted, of laying , and which is itself movable in a rectilinear manner on the frame between two extreme positions, one, remote, of rest, the other, advanced, end of race, and drive means, with, for the passage of the sliding arm from one of its positions to the other, as well as for that of the block which carries it, cam means carried by another block mounted mobile in synchronism with the previous one but in the opposite direction to this one.

In document EP-A-420 753, which was not yet published on the priority date from which the present invention benefits, the gliding device comprises, in substitution for such cam means, and therefore in a generally very configuration different, a transmission, which intervenes between the drive means and any one at least of the elements that constitute the gliding arm and the carriage, and from which it follows that, at least when the carriage is in the vicinity of its position of end of travel, the gliding arm is in the tilted installation position.

Thus, the pivoting of the sliding arm and the movement of the carriage advantageously result from the same movement, induced by the drive means, the sliding arm passing to its tilted laying position before the carriage moves in the direction of its advanced end-of-travel position or during this movement, and returning normally to the raised loading position at the end of the return of the carriage to its retracted rest position.

This arrangement is satisfactory, and the present invention relates to a sliding device of the same type.

But it turns out that, at least in some cases, there may be some indecision in the actual sequence of execution of the pivoting of the gluing arm relative to the movement of the carriage.

However, if, for example, the sliding arm comes to rise prematurely, for example from the start of the carriage return movement, there is the risk of untimely interference between this sliding arm and the spectacle lens on which he has just placed a gripping block.

This may result in a fall of this spectacle lens, and thereby its breakage.

The present invention generally relates to a provision making it possible to avoid this drawback and leading to other advantages.

More specifically, it relates to a sliding device of the kind comprising, for the installation of a gripping block on a spectacle lens arranged in standby on a plate carried by a frame, a sliding arm, which is suitable to receive, in a removable manner, the gripping block to be placed, a carriage, on which the sliding arm is pivotally mounted between two extreme positions, one, raised, of loading, the other, tilted, of laying, and which is itself mounted movable in a rectilinear manner on the frame between two extreme positions, one, remote, at rest, the other, advanced, end of stroke, and drive means, this gliding device being generally characterized in that, between said drive means and at least any one of the elements which constitute the gliding arm and the carriage, there is a transmission which results in that, at least when the carriage is at in the vicinity of its end-of-travel position, the sliding arm is in the tilted installation position, and in that, between the sliding arm and the carriage, there are latching means specific to a disengageable locking of the sliding arm on carriage when in the tilted installation position.

In practice, these latching means extend their effects throughout the movement, in one direction or the other, of the carriage.

In any case, they advantageously prevent the sliding arm from raising up prematurely after the installation of the gripping block.

Thus, any interference with the spectacle lens on which this gripping block has been placed is prevented.

In addition, according to a development of the invention, advantage is taken of these latching means to ensure precise positioning of the gloving arm when it is in the laying position.

Thus, the installation of the gripping block can advantageously be done in a particularly rigorous manner, in a position duly centered relative to the spectacle lens to be fitted.

• la figure 1 est une vue en élévation, suivant la flèche I de la figure 4, d'un dispositif de glantage suivant l'invention, pour la position reculée de repos de son chariot et la position relevée de chargement de son bras de glantage ;
• la figure 2 en est une vue de dessus, suivant la flèche II de la figure 1 ;
• la figure 3 en est une vue en coupe transversale, suivant la ligne brisée III-III de la figure 1 ;
• la figure 4 en est une vue latérale, suivant la flèche IV de la figure 1 ;
• la figure 5 reprend, à échelle supérieure, le détail de la figure 3 repéré par un encart V sur cette figure 3 ;
• la figure 6 reprend, à échelle supérieure, le détail de la figure 4 repéré par un encart VI sur cette figure 4 ;
• la figure 7 est une vue schématique en perspective d'une partie de la transmission mise en oeuvre dans le dispositif de glantage suivant l'invention ;
• la figure 8 est une vue partielle en perspective relative à l'un des coulisseaux constitutifs du chariot que comporte ce dispositif de glantage ;
• la figure 9 est une vue latérale, qui, reprenant pour partie celle de la figure 4, illustre la position avancée de fin de course de son chariot et la position basculée de pose de son bras de glantage.

The characteristics and advantages of the invention will become apparent from the description which follows, by way of example, with reference to the appended schematic drawings in which:

• Figure 1 is an elevational view, along arrow I of Figure 4, of a gliding device according to the invention, for the retracted position of rest of its carriage and the raised loading position of its gliding arm;
• Figure 2 is a top view, along arrow II of Figure 1;
• Figure 3 is a cross-sectional view along the broken line III-III of Figure 1;
• Figure 4 is a side view, along arrow IV of Figure 1;
• Figure 5 shows, on a larger scale, the detail of Figure 3 identified by an insert V in this Figure 3;
• Figure 6 shows, on a larger scale, the detail of Figure 4 identified by an insert VI in this Figure 4;
• Figure 7 is a schematic perspective view of part of the transmission implemented in the gliding device according to the invention;
• FIG. 8 is a partial perspective view relating to one of the slides constituting the carriage that this gliding device comprises;
• Figure 9 is a side view, which, partly taking that of Figure 4, illustrates the advanced position of the end of travel of its carriage and the tilted position of installation of its gloving arm.

As illustrated by these figures, the gliding device 10 according to the invention comprises, for the installation of a gripping block, not shown, on a spectacle lens 11 placed waiting on a plate 12, as shown schematically in broken lines in Figure 4, a gliding arm 13, which is adapted to receive, removably, this gripping block, presenting for this purpose a receptacle 14, Figures 1 and 3, a carriage 15, on which the arm glantage 13 is pivotally mounted between two extreme positions, one, raised, loading, Figures 1 to 4, the other, tilted, laying, Figure 9, and which is itself mounted movable rectilinearly on a frame 16, between two extreme positions, one, moved back, at rest, FIGS. 1 to 4, the other, advanced, at the end of the race, FIG. 9, drive means, detailed later, and intervening between these drive means and at least any one of the elements that constitute the arm d e glantage 13 and the carriage 15, a transmission 17, which will also be detailed later, and from which it follows that, at least when the carriage 15 is in the vicinity of its advanced end-of-travel position, the gliding arm 13 is in position tilted installation, as shown in Figure 9.

In the embodiment shown, and according to one aspect of the invention, the frame 16 is made, in a particularly rigid manner, of two shells 18, which, duly facing one another, along a plane P1 containing l the laying axis A1, that is to say the axis along which the gripping block must be laid, are from place to place duly coupled to one another by tie rods 20.

Overall, FIG. 4, this frame 16 comprises a base 21, by which it can be attached directly, or indirectly, by means of spacers on any support, for example the frame of an outline reading device, a head 22, on which the sliding arm 13 and the carriage 15 intervene, and, connecting one another this base 21 and this head 22, a body 23.

In the embodiment shown, the gliding arm 13 is in the form of a stirrup.

It thus has a middle part 25, which is equipped, on the front, with the receptacle 14, and two lateral arms 26, by which it frames the head 22 of the frame 16.

According to arrangements which, not falling within the scope of the present invention, will not be described here, the sliding arm 13 also carries, in its middle part 25, two ejectors 27, which, each arranged respectively on either side of the receptacle 14, are intended to come to bear on the spectacle lens 11 for the maintenance of the latter on the plate 12 during the return movement of the carriage 15 from its advanced end-of-travel position to its retracted position of rest.

In the embodiment shown, and according to one aspect of the invention, the carriage 15 is reduced to two slides 28, one per lateral arm 26 of the glider arm 13, and these slides 28, which each have a generally rectangular outline, are each individually mounted mobile in two grooves 30 provided for this purpose on the frame 16.

In practice, these grooves 30 each affect respectively the shells 18 constituting this frame 16, on the outer surface thereof, and for the retention of the slides 28, it is reported, on these shells 18, on either side grooves 30, plates 31 which partially close them.

Let D be the direction of movement of the slides 28.

Returned to the carriage 15 which they form, it merges with the laying axis A1.

For the pivoting mounting of the sliding arm 13 on the carriage 15, each of the lateral arms 26 of this sliding arm 13 forms, locally, a hub 32 by which it is engaged in journalling, with the interposition of a bearing 33, on a cylindrical boss 34 provided locally projecting for this purpose on the corresponding slide 28.

For each lateral arm 26 of the sliding arm 13, a screw 36 ensures the security of the corresponding assembly.

For one of the bearings 33 in question, the internal cage of this bearing 33 is blocked by this screw 36, while for the other, it is free, any mounting clearances between the various constituents in question thus stacking all from the first of these cages.

Let A2 be the pivot axis of the gliding arm 13 on the carriage 15.

It is orthogonal to the direction of movement D of this carriage 15.

Let P2, or laying plane, be the plane it forms with the latter.

According to the invention, between the sliding arm 13 and the carriage 15, there are latching means specific to a disengageable locking of this sliding arm 13 on this carriage 15 when it is in the tilted installation position.

In the embodiment shown, these latching means comprise at least one pawl 38, which, carried by any of the elements that constitute the gliding arm 13 and the carriage 15, is pivotally mounted on the latter, a lug 40, which, projecting from the other of these elements, is interposed on the path of the pawl 38, elastic return means, explained later, which permanently urge the pawl 38 towards an active position for which, as shown in Figure 9, it is capable of hooking on the lug 40, and, for retaining the pawl 38 in the inactive position, away from the lug 40, when, as shown in Figures 1 to 4 , the carriage 15 is in the retracted rest position, a stop 41, which, carried by the frame 16, is also interposed on the path of the pawl 38, on the other side of it relative to the lug 40.

In practice, in this embodiment, and according to methods described in more detail later, the pawl 38 is carried by the carriage 15, the associated lug 40 protrudes from the glider arm 13, and there are thus two pawls 38, one for each lateral arm 26 of the sliding arm 13, and, in correspondence, two pins 40.

Let A3 be the pivot axis of the pawls 38 on the carriage 15.

It is parallel to the pivot axis A2 of the sliding arm 13 on the carriage 15, and, like the latter, it extends in the laying plane P2.

In practice, and for reasons which will appear later, the pawls 38 are each mounted adjustable in position on the carriage 15, perpendicular to the laying plane P2, that is to say to the plane containing the direction of movement D of this carriage 15 and the pivot axis A2 of the sliding arm 13.

In practice, too, the two pawls 38 are each respectively carried by the two slides 28 constituting the carriage 15.

They are in the form of plates which extend parallel to these slides 28, and they each comprise a lever arm 42, which, at its end, has a square return 43, with a substantially straight flank, by which they are capable of being hooked onto the associated lug 40, a central part 44, by which they are pivotally engaged on a hub 45, duly shouldered, carried by the corresponding slide 28, and a lever arm 46, which, aligned with the lever lever 42 above, extends on the other side of the middle portion 44 relative thereto, and by which they are adapted to be actuated by the corresponding stop 41.

Their hub 45 crosses the concerned slide 28 of the carriage 15, by means of a boss 47 locally thickening this slide 28.

For this crossing, this boss 47 has a hole 48 elongated in a buttonhole perpendicular to the laying plane P2, FIG. 5.

The possibility of adjusting the pawls 38 depends on the lengthening of this bore 48.

A threaded tie rod 50, with screw and nut, ensures the safety of the corresponding assembly.

It suffices to tighten the screw so that the hub 45 carrying a pawl 38 is duly locked in position on the slide 28 which carries it.

In practice, the elastic return means associated with such a pawl 38 are constituted by a torsion spring 52, the median torsion part of which is engaged on the hub 45 and one arm of which is hooked onto the pawl 38 while the other is hooked on the corresponding slide 28.

Jointly, the lug 40 associated with each of the pawls 38 is in the form of a boss projecting from the corresponding lateral arm 26 of the sliding arm 13, and, more precisely, on an extension 54 thereof.

In practice, this lug 40 has a rectangular cross section, with two faces parallel to the laying plane P2 and two faces perpendicular thereto.

It is by one of its faces parallel to the laying plane P2 that such a lug 40 is able to cooperate in hooking with the associated pawl 38.

To support the gliding arm 13 in its tilted installation position, the carriage 15 carries a stop 56 adjustable in position perpendicular to the installation plane P2.

In practice, this stop 56 is formed by the end of a screw passing through, perpendicularly to this laying plane P2, the boss 47 of one of the slides 28 constituting the carriage 15.

During assembly, the gliding arm 13 is brought into abutment against this stop 56, as shown in FIG. 9.

There is then an adjustment thereof so that, in the tilted position of laying which is then hers, the gliding arm 13 extends very exactly along the laying axis A1, for fitting the gripping block to be deposited suitably centered.

The pawls 38 are then hooked on the lugs 40, and, according to the methods described above, their adjustment is made in position so that the return square 43 of their lever arm 42 is very exactly in contact, by its substantially straight flank of the corresponding face of these lugs 40.

Thus, in the tilted installation position, the sliding arm 13 is firmly locked between two supports, one provided by the stop 56, the other provided by the pawls 38.

Finally, in the embodiment shown, the stop 41 associated with each of the pawls 38 is simply constituted by a pin carried by the corresponding shell 18 of the frame 16.

The drive means used for tilting the sliding arm 13 and moving the carriage 15 comprise an electric motor 60, housed in the frame 16.

The transmission 17 intervening between this motor 60 and any one of the elements that constitute the gliding arm 13 and the carriage 15 is, overall, of the type with belt (s) and pinions.

Of course, the word belt must be understood in the broad sense, covering both cables and actual belts, and, likewise, the pinions can be constituted by simple pulleys.

In fact, the transmission 17 implemented in the embodiment shown comprises, on the one hand, at least one belt 61, which is in practice a toothed belt, and which is passed in endless loop on two pinions 62, 63 , one wedged on the output shaft of the motor 60, the other rotatably mounted on the frame 16, and, on the other hand, at least one cable 64, which, generally forming an endless loop, is locally passed looped on a pulley 65 fixed in rotation on the preceding pinion 63, making in practice several turns on this pulley 65.

This acts as a drive pulley for him.

The assembly extends over one of the sides of the frame 16.

The cable 64 is also passed locally in a loop on a deflection member 66 carried by the sliding arm 13, and, more precisely, by the corresponding lateral arm 26 thereof.

In practice, this return member 66 is wedged on the sliding arm 13.

For example, and as shown, it is very simply formed by a tubular extension of the hub 32 of the corresponding side arm 26 thereof.

This return member 66 is therefore coaxial with the pivot axis A2 of the sliding arm 13.

In practice, the two ends 68, 69 of the cable 64 are both coupled to the sliding arm 16, for example using a pin 67, after passing over the return member 66.

According to the invention, the cable 64 extends parallel to the direction of movement D of the carriage 15 on either side of the return member 66.

In practice, the corresponding strands extend on the same side of the deflection member 66, being both in a plane parallel to the laying plane P2, in the immediate vicinity of this laying plane P2 .

The force to which the carriage 15 is subjected, by means of the sliding arm 13, due to the cable 64, therefore extends as close as possible to the direction of movement D of this carriage 15, parallel to the latter, for the benefit of the overall performance.

In addition to the return member 66 carried by the gliding arm 13 and the drive pulley 65, the cable 64 passes over other return members 70, 71, 72, all carried by the frame 16.

The two return members 70, 71 are aligned with the return member 66 carried by the gliding arm 13, being each respectively disposed on either side thereof.

The deflection member 70, which is disposed opposite the drive pulley 65, is in practice carried by an arm 74 attached to the frame 16, on the one hand, to take account of the transverse offset induced by this pulley d 'drive 65 due to the multiple turns on it of the cable 64, and, on the other hand, to be spaced from the frame 16 and thus allow the passage between it and him of the glider arm 13 when the latter is in position tilted installation.

The return member 71 is itself directly carried by the frame 16.

As for the return member 72, it is carried by a slide 75 mounted adjustable in position on the frame 16 to form a tensioner.

It follows from the above that the cable 64 generally forms a rectangular loop, at the angles of which extend, in addition to the drive pulley 65, the return members 70, 71, 72.

In practice, these return members 70, 71, 72 each consist of a rotary pulley.

Like the drive pulley 65, they are all bypassed from the outside by the cable 64, all extending inside the loop formed by the latter.

As a corollary, no return member is provided on the carriage 15.

As is more apparent in FIGS. 1 and 7, the cable 64 thus used extends in two planes P3, P4, which, both perpendicular to the laying plane P2, are offset l 'relative to each other along the pivot axis A2 of the gliding arm 13, the passage from one to the other of these planes P3, P4 taking place, on the one hand, on the member of return 66 carried by the gliding arm 13, and, on the other hand, on the drive pulley 65 on which the cable 64 makes several turns.

Thus, for each of the return members 70, 71, 72, the two strands of the cable 64 bypassing such a return member 70, 71, 72 advantageously extend one and the other in a plane perpendicular to the axis of rotation of this organ of reference 70, 71, 72, for the benefit of a minimization of the corresponding friction, and therefore of the longevity of the cable 64.

In practice, two cables 64 are used on either side of the frame 16, one per lateral arm 26 of the glider arm 13, and, likewise, in the embodiment shown, it is put using two toothed belts 61.

However, as a variant, only one may suffice, the pulley 65 of the cable 64 located on the opposite side then no longer being for the latter a drive pulley, but a simple return member.

In the embodiment shown, the drive means associated with the sliding arm 13 and the carriage 15 comprise, in addition to the motor 60, elastic means intended to overcome, for the rest position of the carriage 15, the elastic force due to the springs 52 constituting the elastic return means of the pawls 38, and thus allowing the effective release of the sliding arm 13.

It is, in the embodiment shown, at least one spring 80, which, coupled, at one of its ends, to a lug 81 provided for this purpose projecting from the frame 16, of a side thereof, is coupled, at its other end, to the corresponding slide 28 of the carriage 15, by means of the screw forming the stop 56 carried by this slide 28, after passing over two return members 83, 84, which are both rotatably mounted on the frame 16, and one of which, the return member 84, is coaxial with the return member 71 of the corresponding cable 64.

Of course, two springs 80 can be provided, being arranged, each respectively, on either side of the frame 16.

To define the retracted rest position of the carriage 15, a stop 85 is provided on the frame 16.

In the embodiment shown, it is a pin which, projecting from the frame 16, is interposed on the path of one of the slides 28 constituting the carriage 15.

Similarly, for the definition of the raised position of the gliding arm 13, a stop 86 is provided on the frame 16.

In the embodiment shown, it is the head of a screw attached for this purpose on one of the arms 74 carrying a return member 70.

When the sliding arm 13 is thus in the raised loading position, it acts on a microswitch 88 to which the power supply to the electric motor 60 is controlled.

Finally, in addition to this motor 60, there are provided, in the frame 16, lighting means comprising a lamp 89 with which it is associated, to direct its beam on the plate 12, a deflection mirror, not visible in the figures , and a lens 90.

At rest, the carriage 15 is in the retracted position of rest, bearing against the stop 85, and, jointly, the sliding arm 13 is in the raised loading position, bearing against the stop 86, FIG. 4.

After the gripping block to be placed has been placed in the receptacle 14, the engine 60 is started.

In the corresponding control device, and in accordance with arrangements which, not falling within the scope of the present invention, will not be described here, a concealment of the cut-off signal is then given by the microswitch 88.

As soon as the engine 60 is started, the cables 64, duly entrained by the toothed belts 61, cause a rotation of the gliding arm 13 by a quarter of a turn, which results in the passage in the tilted position of laying it , with the support of its lateral arms 26 on the corresponding stops 56 provided for them on the carriage 15.

Continuing their action, the cables 64 then cause the carriage 15 to move in the direction of its end-of-travel position.

From the start of this movement, the pawls 38, duly released, are hooked onto the lugs 40 of the gliding arm 13.

As will be noted, the barycenter of the faces in contact between the square return 43 of these pawls 38 and the lugs 40 of the gliding arm 13 is then located beyond the pivot axis A3 of these pawls 38 by compared to their engagement movement on these lugs 40.

Any untimely detachment of these pawls 38 is thus avoided.

The movement of the carriage 15 continues until the gripping block carried by the gliding arm 13 is effectively applied to the spectacle lens 11 to be fitted.

On the one hand, due to the precise positioning of the gliding arm 13 relative to the carriage 15 and to its locking thereon, and, on the other hand, of an application of the corresponding forces substantially in the direction of movement D of this carriage 15, the positioning of the gripping block is advantageously carried out under strictly controlled and safe conditions.

After this installation, which marks the arrival of the carriage 15 in the advanced end-of-travel position, a reversal of the direction of rotation is carried out, according to arrangements which, not falling within the scope of the present invention, motor 60.

This results in a movement in the opposite direction of the carriage 15, from its previous end-of-travel position to its retracted initial rest position.

Throughout this return movement, the sliding arm 13 advantageously remains in all certainty in the tilted laying position, without the risk of a premature return to the raised loading position.

Shortly before the arrival of the carriage 15 bearing against the stop 85 defining its retracted rest position, the stops 41 carried by the frame 16 cause the pawls 38 to tilt.

Once released, the gliding arm 13 subjected to the stress of the cables 64 passes from its tilted laying position to its raised loading position, until its lateral arms 26 press against the corresponding stops 86.

At the end of this movement, the sliding arm 13 actuates the microswitch 88, thereby causing the motor 60 to stop.

A new work cycle can then begin.

Of course, the present invention is not limited to the embodiment described and shown, but encompasses any variant.

In particular, if in the embodiment more particularly described and shown, there are provided ejectors which hold the spectacle lens on the plate after a gripping block has been placed there, it is not always necessarily so. .

In the case, therefore, where such ejectors are not provided, the sliding arm carries with it the spectacle lens when the carriage returns from its end position to its rest position, and the initial position raised for loading. of this gliding arm then becomes for this spectacle lens an unloading position.

Claims (16)

1. A blocking device of the kind comprising, for fitting a gripping block to a spectacles lens (11) disposed in a waiting position on a plate (12), a blocking arm (13) which is adapted to receive removably the gripping block to be fitted, a carriage (15) on which the blocking arm (13) is mounted pivotably between two limit positions, one being a raised loading position and the other being a pivoted fitting position, and which is itself mounted movably rectilinearly on a frame (16) between two limit positions, one being a retracted rest position and the other being an advanced end-of-travel position, and entrainment means (60), characterised in tht disposed between the entrainment means (60) and any one at least of the elements constituted by the blocking arm (13) and the carriage (15) is a transmission (17) which provides that at least when the carriage (15) is in the vicinity of its advanced end-of-travel position the blocking arm (13) is in the pivoted fitting position, and that disposed between the blocking arm (13) and the carriage (15) are latching means (38, 40) for releasably locking said blocking arm (13) to said carriage (15) when it is in the pivoted fitting position.
2. A blocking device according to claim 1 characterised in that said latching means (38, 40) comprise at least one catch (30) which, carried by any one of the elements constituted by the blocking arm (13) and the carriage (15), is mounted pivotably on same, a lug (40) which, in projecting relationship on the other of said elements, is interposed in the path of movement of said catch (38), resilient means (52) which constantly urge said catch (38) towards an active position in which it is capable of latching on to said lug (40) and, for retaining said catch (38) in an inactive position, at a spacing from said lug (40), when the carriage (15) is in the retracted rest position, an abutment (41) which, carried by the frame (16), is also interposed in the path of movement of said catch (38) on the other side of same with respect to said lug (40).
3. A blocking device according to claim 2 characterised in that the catch (38) is carried by the carriage (15) and the associated lug (40) is in projecting relationship on the blocking arm (13).
4. A blocking device according to claim 3 characterised in that, for supporting the blocking arm (13) in its pivoted fitting position, the carriage (15) carries an abutment (56) which is adjustable in position perpendicularly to the plane (P2) containing the axis (A2) of pivotal movement of the blocking arm (13) and the direction (D) of displacement of the carriage (15).
5. A blocking device according to any one of claims 2 to 4 characterised in that the catch (38) is mounted adjustably in position on the carriage (15) perpendicularly to the plane (P2) containing the axis (A2) of pivotal movement of the blocking arm (13) and the direction (D) of displacement of the carriage (15).
6. A blocking device according to any one of claims 2 to 5 characterised in that, in the pivoted fitting position of the blocking arm (13), the barycentre of the faces which are in contact as between the catch (38) and the lug (40) is disposed beyond the axis of pivotal movement of said catch (38) with respect to its movement of engagement on to said lug (40).
7. A blocking device according to any one of claims 2 to 6 characterised in that, the blocking arm (13) being of a stirrup shape, with a central portion (25) and two lateral arms (26), there are two catches (38), with one for each lateral arm (26) of the blocking arm (13).
8. A blocking device according to any one of claims 1 to 7 characterised in that the transmission (17) comprises at least one cable (64) which, passed in a loop around a direction-changing member (66) carried by the blocking arm (13), extends parallel to the direction (D) of displacement of the carriage (15) on respective sides thereof.
9. A blocking device according to claim 8 characterised in that said cable (64) extends in two planes (P3, P4) which, both being perpendicular to the plane (P2) containing the direction (D) of displacement of the carriage (15), and the axis (A2) of pivotal movement of the blocking arm (13), are displaced relative to each other along said axis (A2) of pivotal movement, the transfer from one of said planes (P3, P4) to the other occurring on the one hand around the direction-changing member (66) carried by the blocking arm (13) and on the other hand a drive pulley (65) around which the cable (64) forms a plurality of turns.
10. A blocking device according to any one of claims 5 to 8 characterised in that the direction-changing member (66) carried by the blocking arm (13) is fixed on same and the two ends (68, 69) of the cable (64) are both coupled to the blocking arm (13) after passing around said direction-changing member (66).
11. A blocking device according to any one of claims 8 to 10 characterised in that, besides the direction-changing member (66) carried by the blocking arm (13), the cable (64) passes around other direction-changing members (65, 70, 71, 72) of which one forms a drive pulley and which are all carried by the frame (16).
12. A blocking device according to claim 11 characterised in that two of said other direction-changing members (70, 71) are aligned with that (66) carried by the blocking arm (13), each being disposed on respective sides thereof.
13. A blocking device according to any one of claims 8 to 12 characterised in that the cable (64) overall forms a rectangular loop.
14. A blocking device according to any one of claims 7 to 13 characterised in that, the blocking arm (13) being of a stirrup shape, with a central portion (25) and two lateral arms (26), the transmission (17) comprises two cables (64), with one for each lateral arm (26) of the blocking arm (13).
15. A blocking device according to any one claims 1 to 14 characterised in that, the blocking arm (13) being of a stirrup shape, with a central portion (25) and two lateral arms (26), the carriage (15) is reduced to two slide members (28), with one for each lateral arm (26) of the blocking arm (13), said slide members (28) each being individually mounted movably in two grooves (30) provided for that purpose on the frame (16).
16. A blocking device according to any one of claims 1 to 15 characterised in that the frame (16) is formed by two shell portions (18).

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