Abstract:
A magnetically driven agitator which includes a member adapted to be mounted through a wall of a receptacle and which has a sleeve in which is housed a rotor which supports a first magnetic coupling structure. A propelling screw is disposed around the sleeve and supports a second magnetic coupling structure which cooperates with the first magnetic coupling structure in order to rotate the screw about an axis of rotation. The rotor is moveable parallel to the axis of rotation inside the sleeve between a first position where the first and second coupling structures are generally opposite to provide a maximum magnetic force therebetween to selected second positions where the magnetic force between the first and second coupling structures is varied such that the magnetic force may be reduced or substantially eliminated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a magnetically driven agitator and to a process for adjusting the couple limiting transmission of effort of such an agitator. 
     2. Brief Description of the Related Art 
     Agitators are conventionally used for stirring a mixture contained inside a recipient in order to avoid decantation or any other alteration of the mixture in the course of time. A magnetically driven agitator has the advantage that the propelling screw which it comprises is set in motion by a magnetic coupling which occurs without physical contact between two rotating parts of which one is driven by the driven shaft of an electric motor while the other is constituted by a propelling screw. This makes it possible to arrange the part associated with the shaft of the electric motor outside the recipient while the propelling screw is installed inside the recipient. Any danger of leakage at the level of the agitator may thus be set aside. This is particularly useful when the mixture is toxic or when pollution thereof by outside agents is to be avoided, such as for example in the case of a medicinal composition. 
     The magnetic coupling used for an industrial agitator must be intense in order to drive the propelling screw of the agitator with a sufficient force. Now, it is sometimes necessary to proceed with dismantling of the propelling screw, in particular for reasons of maintenance or inspection of the recipient in which the mixture is formed. It is thus standard to provide cleaning the propelling screw of an agitator and/or sterilizing it outside the recipient at the end of each production batch. It is sometimes necessary to dismantle the propelling screw in order to proceed with the standard exchange of wear pieces such as bearings. When a propelling screw has been dismantled, it must be returned into position on its support with the greatest precautions, avoiding as much as possible knocks that might damage the blades, the bearings and/or the surface of the recipient. 
     The magnetic forces necessary for driving the propelling screw of an industrial agitator are such that the effort that an operator must exert to remove the propelling screw is considerable, as this effort must overcome both the weight of the propelling screw and the force of magnetic coupling necessary for the drive. This is even more critical when the propelling screw is replaced in position, insofar as, when it is being installed offered, it may happen that the magnetic effort is so intense that the propelling screw escapes the operator&#39;s grip and is violently applied against its support, consequently damaging the bearings and even injuring the operator. 
     In addition, it is particularly delicate for an operator to place the propelling screw in perfect alignment with the axis of rotation of its rotor, with the result that, if the propelling screw escapes the operator&#39;s grip due to the magnetic force that it undergoes, it tends to be applied aslant on its support, which may lead to the destruction of one of its blades, to the marking of the inner surface of the tank and/or to one of the bearings being damaged. In order to overcome this drawback, it may be envisaged systematically to dismantle the drive assembly of the propelling screw located outside the recipient, i.e. it drive motor and possibly the reduction gear which is associated therewith, in order to eliminate the magnetic forces exerted on the propelling screw in the course of assembly or dismantling. Such an approach requires that an operator manipulate heavy and cumbersome parts, these parts generally being located under the manufacturing tanks or recipients and being difficult to access. In addition, such a dismantling of these drive systems must be followed by re-assembly during which the axes of the rotating parts must be very precisely aligned, which is not always possible taking into account the difficult access to the zones of re-assembly of the motor and its possible reduction gear. Moreover, dismantling of the outer part of the agitator involves exposing the magnetic drive rotor to the open air, this rotor being provided with permanent magnets of which the outer surfaces may be covered with magnetic impurities. Taking into account the small clearance present around the rotor, these impurities may lead to a machining of the magnets and to a blockage of the agitator. 
     It is a particular object of the present invention to overcome these drawbacks by proposing a magnetically driven agitator of which the propelling screw may be easily dismantled and returned into place, without the risk of the magnetic forces disturbing these operations and without requiring the complete dismantling of the part of the agitator outside the recipient on which it is mounted. 
     SUMMARY OF THE INVENTION 
     To that end, the invention relates to a magnetically driven agitator which comprises a flange adapted to be tightly mounted in a wall of a recipient and provided with a blind sleeve inside which is housed a rotor supporting a first magnetic coupling means, while a propelling screw disposed around this sleeve is equipped with a second magnetic coupling means for driving this propelling screw about an axis of rotation. This agitator is characterized in that the rotor is movable in translation, parallel to this axis inside the sleeve, between a first position where the first and second coupling means are opposite, so that they cooperate for driving the propelling screw in rotation, and a second position where they do not interact, or only little, so that this propelling screw may be displaced with respect to the sleeve without noteworthy interaction of the first and second coupling means. 
     Thanks to the fact that the rotor is movable in translation, it may be retracted during the operations of assembly and dismantling of the propelling screw, so that the magnetic coupling means that it supports, such as permanent magnets, is spaced apart from the coupling means which equip the propelling screw by a distance sufficient to avoid the magnetic force between these coupling means disturbing the assembly or dismantling of the propelling screw. Displacement of the rotor inside the sleeve occurs in a direction corresponding to the shear of the air gap between the magnetic coupling means, i.e in practice between the permanent magnets respectively associated with the rotor and the propelling screw. Such shear perpendicular to the magnetic force created between these magnets does not require exerting a considerable effort insofar as this magnetic force does not oppose this shear. In other words, the magnetic coupling between the magnets of the rotor and of the propelling screw is nullified without this magnetic coupling greatly opposing the translation of the rotor. During assembly or re-assembly of the propelling screw, it suffices to place the latter on the sleeve, then to move the rotor in translation inside the sleeve until the magnets that it carries are opposite the magnets carries by the propelling screw. The invention therefore allows the propelling screw to be placed in position without interference with the magnets of the rotor, then to displace the rotor until it can drive the propelling screw, such displacement being effected in a direction such that it is unnecessary to overcome an intense magnetic force. 
     According to a first advantageous aspect of the invention, the rotor is displaced in rotation about its axis and in translation parallel to this axis by a drive shaft itself movable in rotation about this axis and in translation parallel to this axis, between two positions corresponding to the first and second positions of the rotor. The rotor can be provided to present a central recess for receiving a screw for mounting the rotor on the shaft, this screw being disposed substantially along the axis. 
     According to another advantageous aspect of the invention, the rotor and/or the drive shaft are adapted to be immobilized, in their movement of translation parallel to the axis, in an intermediate position between the first and second positions. This aspect of the invention makes it possible to use the agitator while the magnetic coupling effort between the rotor and the propelling screw is reduced with respect to the position where the respective coupling means are opposite, with the result that the drive couple of the propelling screw is limited. This is advantageous when it is desired to use the agitator with a mixture whose viscosity is variable, for example due to a chemical reaction. In that case, the agitator is blocked when the viscosity of the mixture is such that the couple which can be transmitted to the propelling screw is less than that which would be necessary to set it in motion in the mixture. 
     According to another advantageous aspect of the invention, the drive shaft is mounted to slide inside a hollow driven shaft of a reduction gear, these drive shaft and hollow shaft being corrected in rotation. Reduction gear is understood to mean any device for transmitting a movement of rotation, whatever its reduction ratio. In particular, it may be a bevel gear of ratio 1/1. The hollow shaft may be provided to bear a nut whose inner thread is adapted to cooperate with an outer thread of a threaded rod secured to the drive shaft. This makes it possible to create a nut and endless screw system for adjusting the position of the drive shaft and of the rotor in translation. The threaded rod is advantageously provided, at one of its ends projecting outside the hollow shaft, with means for controlling its rotation about its axis. In that case, a cap for protecting this end of the threaded rod and these control means may be provided, such cap being removably mounted on the reduction gear. 
     According to a variant embodiment of the invention, the rotor may be controlled in translation along the axis by a pneumatic or hydraulic jack. 
     The invention also relates to a process for adjusting the couple limiting transmission of force of an agitator as described hereinbefore, which consists in adjusting the position of the rotor along an axis, so that the first and second magnetic coupling means are more or less opposite. Thanks to the process of the invention, the couple transmissible between the rotor and the propelling screw is more or less great, as indicated hereinabove. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood on reading the following description of two embodiments of a magnetically driven agitator in accordance with its principle and of their process of adjustment, given solely by way of example and made with reference to the accompanying drawings, in which: 
     FIG. 1 schematically shows a recipient for a mixture equipped with an agitator according to the invention; 
     FIG. 2 is a view on a larger scale of the detail II of FIG. 1, the agitator being shown in section, in a first position; 
     FIG. 3 is a view similar to FIG. 2, with the agitator in a second position; and 
     FIG. 4 is a partial view of the lower part of an agitator in accordance with a second embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 shows a recipient  1  formed by a tank  2  and containing a mixture  3 , while a lid  4  is provided to obturate an upper opening of the tank. 
     In the bottom wall  2   a  of the tank  2  is disposed a flange  5  made of a magnetic material supporting a reduction gear  6  and an electric motor  7 , located outside the tank  2 . A propelling screw  8  is arranged inside the tank  2 , in the mixture  3 . The rotation of the propelling screw  8  by the motor  7  results in a displacement of its blades  8   a , of which two are visible in the Figures, around an axis of rotation X-X′, which has the effect of stirring the mixture  3 . 
     The flange  5  forms a blind or hollow sleeve  10  which extends inside the tank  2 . A bearing  11  is mounted on the end  10   a  of the sleeve  10  thanks to a pin  12  whose threaded end  12   a  is received in a tapping  10   b  of the end  10   a  of the sleeve  10 . The bearing  11  supports a bush  13  of which the outer surface  13   a  constitutes a bearing surface. The propelling screw  8  comprises a head  14  formed by a ring  15  whose inner circular surface  15   a  is intended to fit around the surface  13   a  of the bush  13 . A smooth bearing is thus produced by contact, for example metal metal, between surfaces  13   a  and  15   a.    
     Three branches, of which two, referenced  15   b , are visible in the Figures, and which are regularly distributed on the periphery of the ring  15 , extend outwardly therefrom and support a sheath  16  disposed around the blind sleeve  10 . The blades  8   a  of the propelling screw  8  are welded on the outside of the sheath  16 . This sheath  16  bears, on its inner surface  16   a  directed toward the sleeve  10 , two rows of permanent magnets  17  whose north-south polarity is directed in directions A-A′ and B-B′ substantially perpendicular to axis X-X′. 
     A rotor  20  is disposed inside the sleeve  10  and comprises a first bush-shaped part  20   a  on the outer surface  20   b  of which are mounted permanent magnets  21  aligned with the magnets  17 , in the configuration of FIG.  2 . In this configuration, the directions C-C′ and D-D′ of polarity of the magnets  21  are substantially aligned with directions A-A′ and B-B′. The rotor  20  is provided to rotate about axis X-X′ and, thanks to the magnetic coupling produced between the magnets  21  and  17 , to drive the propelling screw  8  in rotation about this axis. 
     The rotor  20  is secured to with a drive shaft  22  thanks to a screw  23  disposed in the central recess  20   c  of the bush  20   a  along axis X-X′ and penetrating in an end tapping  22   a  of the shaft  22 . The rotor  20  is provided with an axial bore  20   d  for receiving the end  22   b  of the shaft  22  in which the tapping  22   a  is made. A screw  24  is disposed in a radial bore in that part of the rotor  20  in which the bore  20   d  is made, so as to abut against the outer radial surface of the end  22   b  of the shaft  22 , so as to immobilize the elements  20  and  22  in rotation. 
     The flange  5  is secured to a distance piece or spacer  25  on which is mounted the reduction gear  6 . The distance piece  25  defines a cylindrical housing  26  for receiving the shaft  22 . 
     A driven shaft  27  of the reduction gear  6  is driven by pinions  28  and  29 , pinion  29  being in mesh, thanks to a shaft  30 , with the driven shaft  31  of the motor  7 . The shaft  27  is hollow and defines an inner volume in which the shaft  22  may slide, shafts  22  and  27  being fast in rotation thanks to a key  32 . In this way, the shaft  22  may slide inside the shaft  27  and be driven in rotation thereby, thanks to the key  32 . Reference  27   a  designates a longitudinal groove in the shaft  27  in which the key  32  may slide. 
     The end  22   c  of the shaft  22  opposite the rotor  20  is secured to a threaded rod  33  of which a first end  33   a  is provided to penetrate in a housing  22   d  of the end  22   c . Two screws  34  serve to immobilize the end  33   a  of the rod  33  axially inside the housing  22   d.    
     The second end  33   b  of the rod  33  cooperates with a nut  35  fixed to the end of the shaft  27  opposite the distance piece  25 . A nut  36  is immobilized on the end  32   b  of the rod  33  thanks to a pin  37 . It is thus possible to drive the threaded rod  33  in rotation inside the nut  35  thanks to nut  36 . A screw-nut system has thus been constituted which, due to the fixed position in translation of the nut  35  with respect to the axis X-X′, makes it possible to control the displacement in translation of the rod  33 , the shaft  22  and the rotor  20  between the two positions respectively shown in FIGS. 2 and 3. The nut  36  therefore constitutes a means for controlling the displacement of these elements along axis X-X′. 
     A cap  38  is mounted on the reduction gear  6  around the end  33   b  of the threaded rod  33  in order to protect the elements  35  to  37 . The cap  38 , made of metal or plastics material, may be screwed or clipped on the reduction gear  6 . 
     Functioning is as follows: 
     From the position of FIG. 2, and when it is necessary to dismantle the propelling screw  8 , the cap  38  is withdrawn and the nut  36  is maneuvered with a spanner in the sense of unscrewing the rod  33  with respect to the nut  35 , so that this rod is progressively extracted outside the driven shaft tube  27 , which consequently drives the shaft  22  in the direction of the nut  35  up to the position shown in FIG.  3 . In this position, the magnets  17  and  21  are no longer opposite, since the bush  20   a  of the rotor  20  has arrived in a part of the inner volume of the sleeve  10  located outside the the tank  2 . In this position, the directions of polarity C-C′ and D-D′ of the magnets  21  are offset with respect to the directions of polarity A-A′ and B-B′ of the magnets  17  by a distance d such that the force of magnetic coupling between these magnets is virtually zero. In other words, there is in that case no noteworthy magnetic interaction between the magnets  17  and  21 . The propelling screw  8  may therefore be removed, without the magnetic forces exerted between the magnets  17  and  21  substantially opposing this. 
     At the end of the cleaning and/or maintenance operations, the propelling screw  8  may be returned in place without too great a force of attraction due to magnets  21  being exerted on magnets  17 . 
     It is then possible to maneuver the nut  36  in the direction opposite the one mentioned above, so that the threaded rod  33  penetrates again inside the driven shaft tube  27  and the rotor  20  is pushed by the shaft  22  up to the position of FIG. 2 in which the magnets  17  and  21  are again opposite. 
     The displacement of the magnets  21  with respect to the magnets  17  occurs perpendicularly to the efforts of magnetic attraction which are exerted between these magnetic coupling elements, with the result that these forces do not have to be overcome while they may be particularly great if the air gap between the magnets is reduced. An effect of shear of these forces is provoked here and the movement of the rotor  20  between the positions of FIGS. 2 and 3 is relatively easy. Moreover, a considerable gear reduction may be obtained as a function of the pitch of the thread of the rod  33  and of the nut  35 . 
     While the rotor  20  is being re-mounted inside the sleeve  10 , it is possible to interrupt rotation of the threaded rod  33  in an intermediate position between those of FIGS. 2 and 3, only a fraction of the magnets  21  arriving opposite magnets  17  nearest the bottom of the tank  2 . Under these conditions, the maximum couple which may be transmitted between the rotor  20  and the sheath  16  of the propelling screw  8  is less than in the position of FIG. 2, which proves useful when it is desired to limit the couple that may be transmitteed to the propelling screw  8 , particularly in the case of a mixture  3  whose viscosity develops as a function of time, for example due to a chemical reaction modifying this viscosity. It is thus possible to avoid agitation of the mixture at the end of a chemical reaction, i.e. when this viscosity has attained a predetermined value. The value of the maximum couple transmissible depends on the proportion of the magnets  17  and  21  which are opposite and create a partial magnetic coupling force. It is possible to calibrate the agitator of the invention to determine the maximum couple transmissible as a function of the position of the rotor  20 . After such calibration, the rod  33  may be graduated, which allows a user to move it towards the inside of the tube  27  as a function of the limiting or maximum couple desired. 
     A device (not shown) for detecting the effective movement of the propelling screw  8  may be associated with the agitator of the invention in order to warn the operator when the propelling screw  8  is no longer rotating whereas the motor  6  is operating, such a situation corresponding to a predetermined value of viscosity for the mixture  3 . Stopping of the motor  7  may be programmed under these conditions. 
     When the rotor is positioned in its position of drive of the propelling screw  8 , whether it be question of the position of FIG. 2 or an intermediate position, the cap  38  is re-mounted on the reduction gear  6 . 
     In the embodiment of the invention shown in FIG. 4, elements similar to those of the embodiment of FIGS. 1 to  3  bear identical references. The agitator of this second embodiment differs from the preceding one essentially in that the end  33   b  of a rod  33 ′, which is not threaded, is secured to a piston  50  mobile in translation along axis X-X′ inside a cylinder  51 . Elements  50  and  51  belonging to a pneumatic jack  52  supplied with air via two conduits  53  and  54 . As a function of the pressures respectively prevailing in the chambers defined inside the cylinder  51  on either side of the piston  50 , the rod  33  is displaced along axis X-X′, which makes it possible to displace the magnets  21  of the rotor  20  with respect to the magnets  17  of the sheath  16 , as in the first embodiment. 
     Jack  52  might, of course, equally well be a hydraulic jack. 
     The use of a jack allows a rapid movement of the rotor  20  inside the sleeve  10 , which movement may be automated. 
     Other systems for controlling the displacement of the rotor  20  inside the sleeve  10  may be envisaged, in particular a system incorporating springs or cams. 
     In any case, the process described hereinbefore for adjusting the couple limiting transmission of force remains applicable.