EP0173601B1 - Perfectly dry and closed vacuum pump with rectilinear movement for alternating compression - Google Patents

Abstract

A completely dry fluid-tight reciprocating vacuum pump comprises one or more pumping chambers defined between two rigid parts which face one another in the axial direction. One is reciprocated in a straight line relative to the other. They are connected to one another by at least one axially flexible member of appropriate radial stiffness. Each of these members comprises an outside part fixed to a first of these rigid parts. The side of the first rigid part facing the flexible member is shaped to have a profile in diametral cross-section which is substantially the same as the profile in diametral cross-section that this member tends to assume, by virtue of its stiffness, in the configuration with the aforementioned rigid parts moved towards one another. The pump can be used for pumping corrosive and radioactive gases, such as tritium, for example.

Description

The present invention relates to a fully dry and sealed vacuum pump.

Fully dry pumps of the spiral type and with metal bellows sealing are already known. Such pumps are described in particular in French patents FR-2 141 402 and FR-2 153 129 by Mr Paul VULLIEZ and their correspondents DE-2 225 327 and US-3 802 809. These pumps are capable of delivering at atmospheric pressure while attaining a residual ultimate vacuum of 10 Pa (10 ^-1 millibars) around.

These patents propose, among other things, to combine these dry spiral pumps with synthetic diaphragm pumps, which makes it possible to reach a vacuum limit of approximately 10-1 Pa (10-3 millibars). This solution is however limited to pumping gases compatible with the nature of the synthetic membrane; it is not applicable to the pumping of corrosive or radioactive gases, such as those used, for example, in uranium enrichment or thermonuclear fusion processes where only the use of entirely metallic pumping members is admissible .

Dry vacuum pumps are also known which involve alternative compression of metal bellows which, in combination with spiral pumps of the aforementioned type, allow the pumping of corrosive or radioactive gases up to a limit vacuum of the same order (10- ¹ Pa). However, the performance of these bellows pumps remains limited due to the residual volumes which inevitably remain in the vicinity of these bellows.

Finally, other types of diaphragm pumps or compressors are known, described for example in French patent No. 1,457,688 or American patent No. 2,021,156 or in German patent No. 802,642.

The devices described in the first two patents are provided with one or more rigid rings mounted floating in the internal fold or folds of the membrane. In addition to the drawbacks, mentioned above, resulting in certain cases from the use of synthetic membranes, the presence of the rings results in rapid wear of the membrane making this type of device unusable for the uses more particularly envisaged in the context of invention.

Furthermore, German patent No. 802 642 describes a bellows compressor cooperating with planar end walls of the chamber. Such a structure does not make it possible to obtain a high compression ratio without contact wear.

The present invention relates to a new type of vacuum pump whose pumping members are metallic and which has better performance. The invention thus relates to a new dry pump with rectilinear movement of alternative compression allowing, with a simple and robust construction and an operation free of hazards, to obtain good performances in limit vacuum thanks for each compression cycle to an almost expulsion. full of pumped gas.

The invention also relates to an alternative compression dry pump which, in combination with a dry spiral pump of the aforementioned type, allows the pumping of all gases, including corrosive and radioactive, with a significant improvement in the vacuum limit and the discharge pressure with respect to known dry pumps.

In this spirit, the invention relates to a completely dry and sealed vacuum pump comprising at least one variable volume pumping chamber connected to a suction pipe and to a delivery pipe and non-return means arranged to ensure a flow one-way fluid from said suction manifold to said discharge manifold, said chamber having a fixed axial end wall and a movable axial end wall, characterized in that said pumping chamber has at least one axially flexible circular metallic member having a certain radial stiffness, in that this member has its external part fixed to a rigid part of said pumping chamber and is made integral with a mobile assembly subjected to an alternating axial movement between a spaced apart configuration and a close configuration of said axial end walls and in that said metal member and the axial end wall against the watercolour it comes to apply have substantially the same curved diametral profile, in said close configuration, so that the residual space between said flexible metallic member and this axial end wall facing it is as small as possible.

The adequacy of the profile of the rigid parts of the pumping chamber with the profile taken by the flexible metal member in close configuration allows an almost total expulsion of the gas located in the pumping chamber, the residual volume thereof being thus reduced at least.

• 1. Le contrôle de toute fuite accidentelle à l'un des organes métalliques flexibles par la mise en communication de ce volume étanche avec un pressostat de contrôle et d'alarme.
• 2. La réduction des effets de fatigue supportés par l'organe métallique flexible de pompage lorsque ses parties externes sont soumises directement à la pression atmosphérique.

The invention also relates to a dry pump with double metallic insulation with respect to the atmospheric atmosphere produced by the installation of a safety bellows placed around the flexible metallic member and connected to the rigid parts. The establishment in the sealed annular volume thus created of a judiciously chosen pressure and notably lower than atmospheric pressure makes it possible to obtain in a simple way the following two remarkable results:

• 1. The control of any accidental leak to one of the flexible metal members by the communication of this sealed volume with a pressure and control pressure switch.
• 2. Reduction of the fatigue effects borne by the flexible metallic pumping member when its external parts are subjected directly to atmospheric pressure.

The flexible metallic pumping member can be in the annular form with a substantially toric cross section, it can in this case be manufactured in one piece, such as a bellows element, or in two pieces by the assembly and following their internal edges of two metal diaphragms preformed along a substantially demitoric cross section, that is to say open radially outward (Figure 1 and Figure 2). This kind of flexible member of substantially toroidal shape is better suited to short strokes of the mobile pumping member and to obtaining relatively high discharge pressures.

Advantageously, this type of dry pump can be equipped with another model of flexible metallic member obtained from a circular disc preformed in the free position, according to a diametral profile giving the best performance of axial flexibility for a minimum of fatigue effort. . The back-to-back meeting of two of these discs in their central part allows, while retaining for the same stroke of each of them the advantages of flexibility, to double the volume of each pumping chamber. In this case, the central assembly of these two discs must include a communication hole between the two parts of the same pumping chamber.

As already mentioned, the use of the vacuum pump according to the invention is of particular interest when it is combined with the above-mentioned spiral pumps.

In this case the dry vacuum strictly limit obtained can reach 10- ² 10-3 Pa (10- ⁴ to 10-5 millibar) and this together with obtaining a notable discharge pressure according to the type of flexible body employee.

• la figure 1 est une vue schématique en coupe axiale d'une partie d'une pompe suivant l'invention dans laquelle l'organe métallique flexible de pompage constitué par un élément annulaire de section droite sensiblement torique réalisé par l'assemblage de deux diaphragmes est disposé entre, d'une part, une paroi rigide fixe et, d'autre part, une paroi rigide mobile de manière à assurer un pompage de l'espace compris entre l'élément annulaire et les deux pièces rigides ;
• la figure 2 est une vue analogue à la figure 1, dans laquelle la partie de pompe représentée à la figure 1 est doublée de telle façon que l'une des chambres de pompage se trouve au volume minimal, tandis que l'autre est au volume maximal, ce qui permet un pompage en série ;
• la figure 3 est une vue en coupe diamétrale d'un organe métallique flexible de pompage obtenu par le préformage d'un seul disque métallique ;
• la figure 4 est également une vue en coupe diamétrale d'un organe métallique flexible constitué par l'assemblage de deux organes identiques à celui de la figure 3 ;
• la figure 5 est une vue schématique en coupe axiale d'une pompe analogue à celle représentée figure 2 mais où chacune des deux chambres de pompage est équipée avec un organe métallique flexible simple tel que représenté figure 3. Sur cette figure est également représenté un agencement de soupapes différent de celui de la figure 2 ;
• la figure 6 est une vue analogue à la figure 5 mais avec un organe métallique flexible double tel que représenté figure 4 ;
• la figure 7 est une vue en coupe selon la ligne VII-VII de la figure 8 d'un groupe de pompage intégralement sec et etanche réalisé par couplage d'une pompe selon la figure 5 avec une pompe à spirale à mouvement de translation circulaire ; et
• la figure 8 est une vue en coupe suivant la lire brisée VIII-VIII de la figure 7.

The characteristics and advantages of the invention will emerge from the following description of embodiments described by way of example with reference to the accompanying drawings in which:

• FIG. 1 is a schematic view in axial section of a part of a pump according to the invention in which the flexible metallic pumping member constituted by an annular element of substantially toric cross section produced by the assembly of two diaphragms is disposed between, on the one hand, a fixed rigid wall and, on the other hand, a movable rigid wall so as to ensure pumping of the space between the annular element and the two rigid parts;
• Figure 2 is a view similar to Figure 1, in which the pump part shown in Figure 1 is doubled so that one of the pumping chambers is at minimum volume, while the other is at volume maximum, which allows series pumping;
• Figure 3 is a diametrical sectional view of a flexible metal pumping member obtained by preforming a single metal disc;
• Figure 4 is also a diametral sectional view of a flexible metal member formed by the assembly of two identical members to that of Figure 3;
• Figure 5 is a schematic view in axial section of a pump similar to that shown in Figure 2 but where each of the two pumping chambers is equipped with a simple flexible metal member as shown in Figure 3. An arrangement is also shown in this figure valves different from that of Figure 2;
• Figure 6 is a view similar to Figure 5 but with a flexible double metal member as shown in Figure 4;
• Figure 7 is a sectional view along line VII-VII of Figure 8 of a completely dry and sealed pumping unit produced by coupling a pump according to Figure 5 with a spiral pump with circular translation movement; and
• FIG. 8 is a sectional view along the broken reading VIII-VIII of FIG. 7.

FIG. 1 shows, by way of nonlimiting example, a part of an entirely dry and sealed vacuum pump with rectilinear movement of alternative compression, adapted according to the invention to pump all kinds of media, even corrosive or radioactive.

This pump part comprises a pumping chamber 1 whose variable volume is included inside radially of a circular metallic member more particularly here annular with a substantially toric cross section, open externally, axially flexible and formed here of two annular metal diaphragms flexible 2 and 3 connected back to back, by their internal circular edges 4. This chamber is also delimited by two rigid parts 5 and 6 axially opposite defining the walls of axial end of the pumping chamber and animated, one with respect to the other, of a rectilinear alternating axial movement, along an axis AA, between a spaced configuration and a close configuration. The annular element 2-3 is connected along its external circular edges 7 and 8, for example by welding, to each of the two rigid parts, respectively. These rigid parts have axially opposite, central surfaces 9 and 10 of complementary profiles, and, between them and the fixing lines of the annular element, recessed annular zones 9A and 10A. The profile of these zones 9A and 10A must be extremely precise so that in close configuration it can perfectly correspond to a small clearance close to that which then takes the flexible annular element 2, 3, the central surfaces 9 and 10, represented planar in the example of Figure 1, also coming to marry the same little game. Thanks to these provisions, the residual volume of chamber 1 is thus reduced to a minimum.

In the pump part shown by way of example in FIG. 1, the upper rigid part 5 is fixed to a frame B.

The lower rigid part 6 belongs to a mobile assembly M driven in movement with the aid of an eccentric device 11 of the "frame" type, the axis X-X of which crosses the axis A-A at right angles. As shown, the device 11 comprises a drive shaft 11A, aligned along the axis XX, integral with an eccentric disc 11B engaged in a ball bearing 11 C. The movable rigid part 6 has two support areas 6a and 6b parallel to the central surface 10, between which is engaged with clearance the outer track of the ball bearing 11 C.

During the rotation of the eccentric 11 from its low position shown in FIG. 1, the latter exerts on the crew M an axial thrust upwards, any radial friction component being eliminated by means of rolling cooperation of the ball bearing 11 C with the bearing surface 6a. This upward movement continues until the position corresponding to the close configuration, then the rotation of the eccentric 11 produces a downward movement of the movable assembly M and this continues cyclically. The axial movement of the crew M is defined by the eccentricity "e" of the eccentric 11.

In the rigid parts 5 and 6 are arranged non-return valves of the ball type 12 and 13 ensuring a one-way fluid flow in a tube 14 comprising a suction section 14A and a discharge section 14B.

In operation, during the upward movement of the mobile assembly M, the flexible annular element 2-3 is gradually brought to conform to a very small clearance near the surfaces of the zones 9A and 10A of the pumping chamber at the same time as the central surfaces 9 and 10 of this same chamber also conform to a very small play. The gas is thus almost entirely expelled through the discharge orifice.

FIG. 2 represents a pump according to the invention which comprises two pumping chambers 1 and 1 ′ of the aforementioned type, but arranged head to tail. The same reference signs have been used to designate, with respect to the upper pumping chamber 1, the same elements as in FIG. 1, and similar reference signs, but followed by a "prime" index to designate the corresponding elements of the lower pumping chamber 1 '.

These chambers 1 and 1 'are centered and aligned on the same axis AA and the moving parts 6 and 6' which contribute, with the rigid fixed parts 5 and 5 'and the annular elements 2-3 and 2'-3', to defining them are both integral with the movable assembly M whose axial guidance relative to the frame B is ensured by the annular elements 2-3 and 2'-3 'thanks in particular to their radial stiffness. In this way, these chambers follow compression cycles in phase opposition, resulting in a pumping in series from chamber 1 'to chamber 1.

The tubing 14A which passes through the movable assembly M constitutes an intake tubing for the pumping chamber 1, but a discharge tubing for the pumping chamber 1 ', which receives a suction tubing 14C which is advantageously in the extension tubing 14A and 14B which constitutes the delivery tubing of chamber 1.

For safety measures, each of the annular elements or flexible metallic pumping elements 2-3 and 2'-3 'is preferably surrounded by a metal bellows 15 or 15', fixed to the rigid fixed and mobile parts 5, 6 to which said annular element is fixed. These annular elements and these bellows define annular safety chambers 15A and 15'A which provide isolation of the pumping chambers from the outside. These annular chambers are advantageously connected, by tubes 17A and 17B connecting to a pressure switch 16 adapted to control that the pressure of the control gas contained in these chambers remains between two limit values, and to trigger any control and safety system when this pressure leaves this range of admissible values. The pressure switch thus makes it possible to detect any rupture either of an annular element or of a bellows, without any direct communication having been established between the interior of the pumping chambers and the exterior. This results in good safety in the case of pumping dangerous gases.

Advantageously, this same arrangement makes it possible, by maintaining the control gas in the chambers 15A and 15'A at a pressure value significantly lower than the atmospheric pressure, to reduce the fatigue forces borne by the flexible metallic pumping bodies 2-3 and 2'-3 'when their external faces are directly subjected to atmospheric pressure. This appreciable advantage results in an increase in the service life of the flexible metallic pumping member, and therefore in the reliability of the entire pump.

FIG. 3 shows a flexible metallic pump member 18 obtained from a solid disc and preformed according to a diametrical profile allowing optimal axial flexibility, that is to say the best axial displacement for the corresponding minimum axial force .

More precisely, the desired shape of the metallic member 18 is determined by subjecting one of the faces of a solid metallic disc to a predetermined hydrostatic pressure, the disc being held at its periphery and on a circular central part 33. The simple forming process makes it possible to obtain the diametral profile giving optimum axial flexibility, but it is not necessarily used industrially to produce the flexible members 18. The latter can for example be obtained by stamping similar discs using tools. presses designed to reproduce the desired profile, determined as indicated above. This pumping member 18 is intended to be connected along its outer edge 7 to the fixed rigid part 5 and to the movable assembly M (FIG. 5) by a central fixing at 33, by any suitable means.

FIG. 4 shows a flexible metallic member constituted by two members 18 identical to that of FIG. 3, and joined by their respective central parts 33, so that their diametrical profiles are substantially symmetrical with respect to a plane transverse to the axial direction of displacement. The outer edges 7 and 8 are intended to be tightly connected, for example by welding to each of the rigid parts 5 and 6 respectively and the central part 33 has a passage 34 for the communication between the two parts of the pumping chamber thus created.

Figure 5 shows a dry pump assembly similar to that shown in Figure 2, but where the two pumping chambers 1 and 1 'are between the rigid parts 5 and 5' and the flexible pumping members constituted by two preformed metal discs such as shown in FIG. 3. These unitary members 18 and 18 ′ are joined by their outer edges 7 and 7 ′ to the rigid parts 5 and 5 ′, and their alternating axial movement is transmitted to them by the action of the mobile assembly M from which they are united by their connection in 33 and 33 '. In this arrangement, the flexible member 18 somehow forms the movable axial end wall of the variable volume chamber. This example of construction includes the details already described in FIG. 2 and relating in particular to the establishment of the double envelope by the bellows 15 and 15 ′ to the control of the spaces 15A and 15 ′ A by the pressure switch 16, and to the parts such l 'eccentric 11 constituting the whole of the cage eccentric. Unlike FIG. 2, the gas suction and delivery pipes 14 are located outside the pump body and they are fitted with non-return valves 14a of the type with flaps or lamellae according to the conventional solutions specific to a alternative pumping.

The reliability and the performances of this type of fully dry pump are due to the great axial flexibility of the flexible members such as 18 and to the reduction of the effects of fatigue which they support by putting under low pressure in the sealed annular spaces 15A and 15'A, from their exterior surfaces.

Figure 6 is a dry pump construction identical to that shown in Figure 5 but with the adaptation of a pumping member consisting of two members 18 joined together by their central part 33 and as shown in Figure 4. These double members 18 and 18 'are connected by their outer edges 7-7' and 8-8 'respectively to the rigid fixed parts 5 and 5' and to the part 6 which is part of the mobile assembly M, which under the action of the Eccentric cage transmits to these flexible members their alternating axial movement.

It will be noted that for the same work for each of the 2 elements of the same member 18 thus formed, the volume of the pumping chamber 1 is doubled compared to the volume of a chamber equipped as in FIG. 5 with a single element 18 unlined. Between the discarded configuration and the close configuration of the member 181st interconnection hole 34 between the two parts of the chamber 1 allows the expulsion of all of the gases. The present construction benefits from the various advantageous arrangements already mentioned for the construction according to FIGS. 2 and 5.

Although an entirely dry and sealed pump of the aforementioned type can be used alone, it is advantageous to associate it with another pump, for example of the spiral type previously mentioned. Figures 7 and 8 show such a scroll pump 19A associated with a pump 19B of the type described in connection with Figure 5 above.

The 19A scroll pump, the principle of which is now well known from the aforementioned patents of Mr. Paul VULLIEZ (Fr-2 141 402 and FR-2 153 159; DE-2 225 327 or US-3 802 809, in particular) will only be described below in summary.

It comprises, in an enclosure closed by a bell 26 and a double bellows assembly 27, a fixed spiral 24 secured by a plate 24A of a gas suction barrel 25 and, nested therein, a movable spiral 22 driven in an exact circular translation movement adapted to make it extend radially and axially, respectively, with a very small constant clearance, the fixed spiral 24 and the associated base plate 24A. The circular translational movement is controlled by three eccentrics 23 of the same eccentricity "e", integral with three parallel shafts 20 and engaged by means of bearings in cavities arranged axially in the part 22A carrying the movable spiral 22.

One, 20A, of the three shafts 20 is rotated by a motor 21, the other shafts only serving to ensure rigorous guidance of the movable spiral 22. The assembly is arranged in a frame B1.

The gases to be pumped are sucked, through a connection tube to the enclosure to be emptied (not shown) in the tube 28 of the barrel 25, penetrate between the spirals at their periphery and by compression between these spirals, are discharged towards the axis in an annular chamber 29 between the barrel 25 and the assembly 27 with double bellows.

These gases are then sucked through a pipe 30 to the pumping chambers 1 and 1 ′ of the pump 19B.

This pump 19B is analogous to the pump of FIG. 5 but shown in FIG. 7 along a section plane perpendicular to that of this FIG. 5; the corresponding elements bear the same figures as in this figure 5. This pump, in the manner previously described, delivers said gases, either to the atmosphere, or in a capacity or in a circuit where they can, according to the invention, and according to the type of flexible member used be transferred to a pressure higher than atmospheric pressure.

It will be noted that the drive of the eccentric 11 of the pump 19B (eccentricity "e _l ") is ensured by the rotation of a shaft 32 arranged in the extension of the shaft 20A driven by the motor 21. The connection between shafts 20 and 32 can be made either directly or through any speed reduction system, the alternative compression pump 19B retaining sufficient efficiency at reduced speeds, taking into account the low mass flow rate of the gases to be pumped at pressures where most of the pumping is done by the spiral pump.

Advantageously, the volume situated between the two bellows of the double bellows assembly 27 is placed in communication (by a tube 17C), like the volumes 15A and 15'A situated between the flexible members 18 and 18 'and the bellows 15 and 15 ', with a pressure switch 16 suitable for controlling any leakage to the flexible members 27, 18-18', 15-15 '.

The establishment for this control of a pressure limited to a few tens of millibars in the sealed annular spaces 15A and 15'A also makes it possible to significantly reduce the effects of fatigue on the flexible members 18 whose external faces are thus removed from the action of atmospheric pressure.

It goes without saying that the foregoing description has been offered for illustrative purposes only and that numerous variants can be proposed by those skilled in the art, in particular by combining the various aforementioned provisions, without going beyond the ambit of the invention. invention, in particular with regard to the particular structure of the flexible metallic pumping members, the arrangement of the various pumping chambers within the same pump or the method of coupling a pump according to the invention to another pump, in particular of the spiral type but not necessarily.

Claims (11)

1. A perfectly dry and closed vacuum pump comprising at least one variable volume pumping chamber (1) connected to a suction pipe and a discharge pipe, and anti-return means arranged to ensure a uni-directional fluid circulation from the said suction pipe to the said discharge pipe, the said chamber comprising a stationary axial end wall (5) and a movable axial end wall (6; 18), characterized in that the said pumping chamber comprises at least one circular metallic element (2, 3; 18) flexible axially and having a certain radial stiffness, in that this element has its outer part fixed to a rigid part of the said pumping chamber and is permanently attached to a movable assembly (6) subjected to an alternating movement between a separated configuration and an adjacent configuration of the said axial end walls, and in that the said circular metallic element (2, 3; 18) and the axial end wall (5, 6) against which it comes to be applied have substantially the same diametrically curved profile, in the said adjacent configuration, so that the residual volume between the said flexible metal element and that axial end wall which faces it is the smallest possible.

2. A vacuum pump according to claim 1, characterized in that the, or each, pumping chamber comprises a single circular metallic element (18) previously mentioned, the outer part of which is fixed to the said stationary axial end wall (5) to form a seal and a central part of which (33) is fixed to the said movable assembly (6).

3. A vacuum pump according to claim 1, characterized in that the, or each, pumping chamber comprises two circular metallic elements (18) previously mentioned, connected by their similar central parts (33) in such a way that their respective diametrical profiles are substantially symmetrical with respect to a transverse plane, in that the outer parts of these two circular metal elements (18) are respectively joined to form a seal with the said stationary axial end wall and with the movable axial end wall, and in that a passage (34) is defined in the said assembled central parts.

4. A vacuum pump according to claim 1, characterized in that the circular metallic element previously mentioned has an annular configuration with a substantially toroidal cross- section, opened outwardly to define two outer circular edges (7, 8) joined respectively to rigid parts of the stationary axial end wall (5) and the movable axial end wall (6), these walls comprising hollowed out annular zones (9A, 10A) with a diametrical profile corresponding substantially to that which an adjacent part of the neighbouring metallic element forms in the previously mentioned adjacent configuration.

5. A vacuum pump according to claim 4, characterized in that the said circular metal element is formed from two flexible annular metallic diaphragms (2, 3) joined back to back by their internal circular edges (4).

6. A vacuum pump according to any one of the preceding claims, characterized in that the circular metallic element or elements (18) of the, or each, pumping chamber is, or are, surrounded by a metallic bellows (15) joined axially to form a seal, on the one hand to a stationary rigid part (5), and on the other hand to a movable rigid part (6) of the pump, in such a way as to define an annular safety chamber (15A) separating the said pumping chamber from the outside environment.

7. A vacuum pump according to claim 6, characterized in that the aforementioned annular safety chamber or chambers is connected to means for measuring pressure (16).

8. A vacuum pump according to claim 6 or 7, characterized in that the aforementioned annular safety chamber or chambers (15A) is brought to a pressure considerably lower than the pressure of the said outside environment.

9. A vacuum pump according to any one of claims 1 to 3, characterized in that the diametrical profile of the aforementioned circular metallic element results from the application of a predetermined hydrostatic pressure on a metallic disc held at its periphery and at a central part, to obtain an optimal axial flexiblity.

10. A vacuum pump according to any one of the preceding claims, characterized in that the aforementioned movable assembly (6) is entrained in a rectilinear alternating movement by an arrangement of the encased eccentric kind (11 ).

11. A vacuum pump according to any one of the preceding claims, characterized in that it is mounted in association with a perfectly dry and closed vacuum pump of the kind comprising a movable spiral (22) imbricated in a stationary spiral (24) and arranged to describe, in this stationary spiral, a circular translational movement controlled by three eccentrics (23) having the same eccentricity and integral with three parallel shafts (20) one of which (20A) is attached to a motor (21), and in that the said eccentric device of the encased eccentric kind (11) is mounted on a shaft (32) situated on the prolongation of that (20A) shaft of the three which is driven by the said motor, the said eccentric device (11) being driven in rotation by the said shaft (20A).

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