Abstract:
A device which is used to guide at least two flow media having different pressures with a shaft or similar force-transmitting element, and a pressure insulating element such as a housing surrounding the shaft or similar. Areas arranged next to each other in the direction of the axis are determined between the force-transmitting element and the pressure-insulating element by sealing elements; at least one of the preferably magnetofluidic sealing elements is leakage-free, and two areas for fluids (A, B) having different pressures flank an area for an auxiliary liquid (H), whereby said area is subdivided by a device into two partial areas for two different pressure areas. A conveying medium is allocated to the area at high pressure and ambient air is allocated to the area at low pressure. The auxiliary liquid (H) is a carrier oil of the magnetofluid, optionally a silicon oil, allocated to the sealing element.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a device and method for guiding at least two flow media having different pressures.  
         [0002]     The transmission of movements and forces through pressure-retaining boundary walls between two fluid systems such as gases and liquids having different pressures is conventionally achieved essentially by means of shaft seals and rod seals such as gland seals, sealing rings and sliding ring seals. Ambient air at ambient pressure is usually found on the low-pressure side. In vacuum systems, the ambient air is on the high-pressure side. In order to function in a trouble-free manner, the aforementioned types of seal require a certain leakage flow from the higher-pressure side to the low-pressure side, since these are contact seals which require a lubricant in order not to be damaged during operation.  
         [0003]     In many applications, however, such a leakage is not desirable or is even forbidden because the fluid is toxic, has a bad odor or is explosive for example, or because a high vacuum has to be maintained. Dual systems using blocking media—for example dual-action sliding ring seals—make it possible to reduce the leakage or substitute the leakage of the pressurized fluid with the leakage of a less harmful blocking fluid.  
         [0004]     Leakage-free systems are at present achieved essentially in accordance with three technical principles: canned motor, magnetic coupling and magnetofluidic seal.  
         [0005]     In the case of a canned motor, the motor is part of the machine, apparatus or device, for example often used in a pump. The stator is positioned on the low-pressure side of the pump and is isolated from the high-pressure side by means of a non-magnetizable can. The rotor is located within the high-pressure side of the pump. The torque is transmitted in a contactless manner from the stator to the rotor via electromagnetic forces through the can.  
         [0006]     The magnetic coupling which is also customary in pump engineering operates according to a similar principle, but instead of a stator winding on the low-pressure side of the pump there is an external rotor with an arrangement of permanent magnets, opposite which there is a corresponding arrangement of permanent magnets or an induction cage or ring on the rotor side. The external rotor is connected to a conventional motor which generates the torque, said torque being transmitted to the rotor—again in a contactless manner—via magnetic field lines. The two coupling elements are usually insulated from one another in pressure terms by means of a cup-shaped housing element, a containment shroud.  
         [0007]     In the design based on magnetofluid, a magnetizable liquid—usually a dispersion of very fine ferromagnetic particles using an auxiliary material in a carrier oil—forms an extremely flexible and adaptable impermeable sealing element, e.g. in the form of a “liquid O-ring” between shaft and housing, which is fixed at the location of the gap to be sealed by means of a suitably configured magnetic field. This type of seal is used commercially for example in hard drives and vacuum systems in surface technology.  
         [0008]     Said leakage-free types of seal have a number of disadvantages in particular for pump engineering; both canned motors and magnetic couplings require bearing elements for the rotor bearing, which bearing elements have to be lubricated by the conveying medium of the pump itself and thus are very susceptible to faults. The advantage of magnetic coupling, namely the ability to use standard motors, is not obtained in the case of a canned motor. By contrast, the magnetic coupling has the disadvantage that, if different powers have to be transmitted, not only is it necessary to use different motors, but also different-sized couplings also have to be used in order for it not to be necessary to take into account any price disadvantage in the case of small powers. At high powers, both principles are limited by the type of torque transmission and the bearing system in terms of their ability to be used to transmit power, due to the overproportionally increasing cost. High eddy current losses which are induced in cans and containment shrouds of conventional type made of non-magnetic metal alloys are particularly disadvantageous.  
         [0009]     The usability of magnetofluidic seals is limited to small pressure differences. By way of example, in order to seal 1 bar with respect to vacuum, six sealing elements connected one behind the other are required. However, the customary pressure range for single-stage centrifugal pumps extends up to 25 bar, and goes far beyond this for special applications and other pump systems. Moreover, the chemical compatibility and mixing processes between the fluids involved and the magnetofluid have to be taken into account.  
         [0010]     Knowing these conditions, the inventor set himself the aim of providing a leakage-free system in a device of the type mentioned above, which eliminates the aforementioned disadvantages and also permits the transmission of very high powers between areas with high pressure differences—preferably at least 25 bar—without requiring any lubrication of the bearings by one of the fluids involved. Moreover, the invention is also intended to be more cost-effective and easier to use than devices according to the prior art.  
       SUMMARY OF THE INVENTION  
       [0011]     According to the invention, sealing means or sealing elements are arranged between a force-transmitting member, for example a shaft, and a pressure-insulating element, such as a housing or similar, in such a way that three areas—which in particular lie next to one another in the direction of the axis—are formed: one area with a first fluid having a certain pressure (for example a conveying medium at 25 bar), one area for a second fluid having a pressure different to that of the first fluid (for example ambient air at 1 bar absolute), and a third area arranged between said areas for an auxiliary medium or auxiliary liquid. This latter area is subdivided by means of a device into two partial areas for two different pressure regions.  
         [0012]     The auxiliary liquid may be for example a silicone oil, which is also used as the carrier oil of a magnetofluid; this is because it has proven advantageous to use magnetofluidic sealing means, in particular to delimit the area for the auxiliary liquid. This magnetofluidic seal hermetically seals the area.  
         [0013]     Located in the area comprising the auxiliary liquid or auxiliary fluid are means which generate a pressure difference within this area, wherein the higher pressure is generated on the side toward the fluid having a higher pressure and vice versa. The pressure difference which can be generated must correspond at least to the maximum pressure difference which occurs between the first and second fluids.  
         [0014]     Advantageously, a conveying medium should be assigned to the higher-pressure area and ambient air should be assigned to the low-pressure area. The auxiliary liquid should be a carrier oil of the magnetofluid assigned to the sealing element, optionally a silicone oil.  
         [0015]     According to the invention, the area for the auxiliary liquid has two connections, one of which is designed to generate a vacuum and the other of which is designed as a passage for the auxiliary liquid. Moreover, the partial area for the higher pressure of the auxiliary liquid should be assigned to the area for the fluid having a higher pressure.  
         [0016]     The subject matter of the invention is also characterized by geometric parts which can be moved relative to one another and are assigned to the pressure-insulating element and to the force-transmitting member, said parts forming a conveying device for the auxiliary liquid so as to generate a pressure difference. The device which divides the area for the auxiliary liquid is preferably a conveying device.  
         [0017]     The pressure difference within the auxiliary liquid is advantageously generated by relative movements of geometric parts which are statically assigned to the force-transmitting member and to the pressure-insulating element (the housing), and form a conveyinq device, for example a pump, for the auxiliary liquid. Suitable measures, for example the provision of a non-return valve, in this case ensure that no pressure compensation between the high-pressure region and the low-pressure region of the auxiliary liquid takes place when the system is idle.  
         [0018]     According to another feature of the invention, the pressure difference which can be generated corresponds at least to the maximum pressure difference which occurs between the fluids.  
         [0019]     According to the invention, means are furthermore provided which react to the pressure difference between the fluid having the high pressure and the maximum pressure of the auxiliary liquid. According to the invention, the reaction is used to adjust said pressure difference to a value close to zero by suitable means. This may be effected for example by adjusting the power of the means which generate the pressure difference or by adjusting a return flow from the region of high pressure of the auxiliary liquid to the region of low pressure. There are members for adjusting the power of the means which generate the pressure difference or members for adjusting a return flow from the higher-pressure partial area of the auxiliary liquid to the low-pressure partial area.  
         [0020]     Advantageously, a line with a valve-type overflow device is provided between the partial areas for the auxiliary liquid.  
         [0021]     If, according to the invention, the volume of at least the area for the auxiliary liquid is designed to be variable, then in particular the partial area for the low pressure region of the auxiliary liquid may be configured with a variable volume. The ability to vary the volume of the area for the auxiliary liquid compensates for changes in the density and thus the volume of the auxiliary fluid—caused by changes in temperature or even pressure.  
         [0022]     By configuring the area assigned to the auxiliary liquid such that it has a variable volume, it is possible to ensure according to the invention that the pressure difference between the minimum pressure of the auxiliary liquid and the pressure of the fluid having the lower pressure is also almost zero. This can be achieved for example by means of a flexible membrane between one side of the area for the auxiliary liquid and the fluid having the corresponding pressure, or—in a particularly advantageous manner—by arranging at least one magnetofluidic seal such that it can be moved. In an arrangement with ambient air at normal pressure (1 bar) on the low-pressure side, it is most advantageous to configure the area with a variable volume on this side.  
         [0023]     Said means ensure that the magnetofluidic seals are subjected only to low pressure differences even in the event of high pressure differences between the first and second fluid, and thus their hermetic sealing effect is ensured. Force transmission takes place mechanically via the force-transmitting element, for example a shaft, so that high transmission powers are possible.  
         [0024]     The magnetofluidic seal for the high-pressure side preferably consists of three sealing elements, represented by three permanent magnets magnetized in the direction of the axis, with associated ferromagnetic pole shoes which each generate a concentric magnetic field that fixes a ferrofluid as sealing medium. These are provided in a non-magnetic carrier ring. According to the invention, the carrier ring is fixed to the housing via a—preferably metallic—bellows. Said bellows is intended to bear against the carrier ring or lock ring and on the other side to bear against the pressure-carrying element. Easy assembly of the device is achieved by fixing the bellows to a bushing, which is sealed by an O-ring with respect to the housing and is fixed to the housing bushing by a threaded ring.  
         [0025]     Within the context of the invention, the lock ring or carrier ring furthermore contains a sealing disk (advantageously molded from silicon carbide) which forms part of a mechanical sealing system consisting of two similar SiC disks. One of the disks has depressions in the contact face, said depressions having a depth of a few μm and running in a spiral manner from the outside toward the center of the disk in a manner corresponding to an axial spiral groove bearing which acts from the outside toward the inside; these depressions advantageously start from the disk edge and end at a distance from a central opening of the annular sealing disk. One function of said bellows is to movably mount the sealing disk in a manner assigned to the housing bushing, and thus to limit its conveying effect caused by the pressure difference.  
         [0026]     If, during operation, the sealing disks generate a higher pressure than the pressure to be sealed off within the pump, the carrier ring with the associated sealing disk is moved in the direction of the pressure to be sealed off; the distance between the sealing disks becomes greater and consequently decreases the conveying effect. On the other hand, too low a pressure generated by the sealing disks leads to a reduction of the gap between the sealing disks and thus to an increase in the conveying effect.  
         [0027]     It is within the scope of the invention that the means for achieving the sealing effect are in this case assigned to a shaft sleeve and a housing bushing. Shaft sleeve and housing bushing and also all parts in contact with the conveying fluid of the pump are made of non-magnetic materials which are sufficiently strong and chemically resistant to the conveying fluid. O-rings provide static sealing of the shaft sleeve with respect to the shaft and of the housing bushing with respect to the housing. The housing bushing can be fixed to the housing by means of screws. The hermetic seal is in this case formed in such a way that it can be installed and removed as a unit.  
         [0028]     According to another feature of the invention, shaft sleeve and housing bushing are held at a defined axial spacing and such that they can rotate concentrically with respect to one another by means of roller bearings—for example by means of a double angular contact ball bearing. If necessary, the bearing is also suitable for absorbing axial forces acting on the shaft. To this end, the shaft sleeve must be fixed to the shaft for example by means of a securing ring or a shaft nut.  
         [0029]     It has proven to be advantageous to fix the roller bearing in an annular space delimited by the shaft sleeve and the housing bushing. This roller bearing should be fixed in said annular space by means of securing rings of the housing bushing or shaft bushing and/or by means of a flange-like radial outer ring.  
         [0030]     According to another feature of the invention, the roller bearing bears against an outer ring of the shaft sleeve, with one of the sealing disks made of silicon carbide being assigned to the other side thereof. Advantageously, one of the sealing disks is mounted in a section of the annular space which widens in steps in the axial direction away from the outer ring, with the lock ring comprising the other sealing disk being arranged in front of said section.  
         [0031]     According to the invention, a radial gap runs between the outer face of the sealing disk and the adjacent lock ring, said radial gap optionally being adjoined on one side by an axial annular gap between the shaft and the sealing elements and on the other side by a further axial annular gap which passes below the adjacent sealing disk.  
         [0032]     For the sake of better fixing, the sealing disk should moreover be connected to the center wall of the lock ring by means of at least one axis-parallel drive pin.  
         [0033]     It is also important to the invention that a chamber which is partially filled with a gas, for example air or inert gas, may be arranged in front of the side of the device which is acted upon by a liquid as fluid, for example in front of the magnetofluidic sealing element on the carrier ring or lock ring, said chamber moreover being sealed off from the shaft on the side facing away from the device by means of a sealing gap of approximately 0.1 mm; the diameter of said sealing gap is selected to be greater than the diameter of the sealing gap of the magnetofluidic sealing element on the carrier ring but smaller than the diameter of the outer chamber wall.  
         [0034]     According to the invention, the volume of the chamber and the widths of the sealing gaps are configured such that, when the arrangement is horizontal and the system is idle, and at ambient pressure inside the chamber, a certain residual gas volume is always present in the upper region of the chamber above the sealing gap of the chamber. During operation, this gas volume collects concentrically around the shaft in the region of smallest diameter of the shaft—in this case in the region of the sealing gap of the magnetofluidic seal, and is compressed to a smaller volume by means of the operating pressure. Even if the two volumes are of equal size, no gas should escape from the sealing gap of the chamber by suitably selecting the width of the latter. On the other hand, the second volume should be large enough to completely cover the sealing gap of the magnetofluidic seal during operation, even at maximum pressure. According to another feature of the invention, an advantageous width or diameter ratio between the sealing gap of the magnetofluidic seal, the sealing gap of the chamber and also the internal outer diameter of the latter is 1 to 1.2 to 1.5.  
         [0035]     The arrangement ensures that the magnetofluidic seal, during operation, always comes into contact only with gas. Mixing of the magnetofluid with a liquid to be sealed off is thus effectively prevented.  
         [0036]     In the case of liquids to be sealed off whereby no chemical reaction with air is to be expected or any reaction is harmless, the residual volume of air within the chamber can be used during filling of the pump. Otherwise, an auxiliary connection to the chamber is required, in order to fill it with an inert gas before the pump is started.  
         [0037]     The invention thus encompasses a number of functional complexes which are associated with one another, namely firstly the areas with the hermetic seals and the auxiliary fluid, also means for generating the pressure difference, then the adjustment of the pressure difference by means of high pressure. It also encompasses the pressure compensation in the auxiliary fluid—the pressure difference with respect to low pressure—and also the described additional device for introducing gas.  
         [0038]     Also within the scope of the invention is a method in which—particularly using the above-described device—between the force-transmitting member and the pressure-insulating element, fluids having different pressures are held in areas which are in each case delimited by a sealing element, and between said areas an auxiliary liquid or auxiliary fluid is held in an area; two different pressure regions are established in the latter, and moreover the partial area for the higher pressure of said auxiliary liquid is intended to be assigned to the area for the fluid having a higher pressure. The area for the auxiliary liquid is intended to be thermally sealed by means of magnetofluidic sealing elements on either side with respect to the areas for the fluids.  
         [0039]     A further method step provides that the area for the auxiliary liquid is acted upon by a vacuum prior to the introduction of said liquid; the auxiliary liquid can thus fill all the hollow spaces within the device.  
         [0040]     Moreover, a return flow from the higher-pressure partial area of the auxiliary liquid to the low-pressure partial area is to be adjusted.  
         [0041]     The method according to the invention also comprises the fact that the pressure difference within the auxiliary liquid is generated by the relative movement of geometric elements which are assigned to the shaft on the one hand and to the pressure-insulating element on the other hand and form a conveying device.  
         [0042]     According to another feature of the method, a conveying effect for the auxiliary liquid is created by means of sealing disks which between them delimit spiral grooves or depressions. The conveying effect of the sealing disks should be increased by increasing the pressure thereof and also the section with respect to one another.  
         [0043]     Another feature of the method according to the invention provides that, in a chamber which is arranged in front of the sealing element and contains a gas, the gas volume during operation collects concentrically around the shaft in the region of the sealing gap between the sealing element and said shaft, and is compressed by means of the operating pressure.  
         [0044]     In particular, the following details can be regarded as advantages of the system according to the invention: 
        can be produced at low cost;     no eddy current losses;     can be installed as a cartridge;     simple replacement possible;     takes up a small amount of space;     no sliding bearing required within the pump;     axial force can be absorbed by the integrated roller bearing;     use of cost-effective ferrite magnets is possible;     can be used even for very high-power pumps;     can easily be integrated in existing pump models.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0055]     Further advantages, features and details of the invention emerge from the following description of preferred examples of embodiments and with reference to the drawing, in which:  
         [0056]      FIG. 1  shows a sealing region of a pump shaft, in longitudinal section, with a seal according to the invention, prior to assembly;  
         [0057]      FIG. 2  shows the sealing region of  FIG. 1  in the assembled state;  
         [0058]      FIG. 3  shows the sealing region on a somewhat enlarged scale compared to  FIG. 2 , without the pump shaft;  
         [0059]      FIG. 4  shows an enlarged detail from  FIGS. 2, 3 ;  
         [0060]      FIG. 5  shows an enlarged detail from  FIG. 4  in a different embodiment;  
         [0061]      FIG. 6  shows a housing bushing of the sealing region, in longitudinal section;  
         [0062]      FIG. 7  shows a shaft sleeve of the sealing region, in longitudinal section;  
         [0063]     FIGS.  8  to  10  show diametral sections through different members of the sealing region which surround the shaft bushing;  
         [0064]      FIG. 11  shows an enlarged detail of  FIG. 10 ;  
         [0065]      FIG. 12  shows a plan view of an annular sealing disk intended for the sealing region;  
         [0066]      FIGS. 13, 14  show two diametral sections through a pair of sealing disks along line D in  FIG. 12 ;  
         [0067]      FIG. 15  shows a schematic cross section through part of the device;  
         [0068]      FIG. 16  shows a schematic diagram of a magnetofluidic seal;  
         [0069]      FIG. 17  shows a schematic assignment of cross sections with an additional device at different method stages;  
         [0070]     FIGS.  18  to  20  show three different sealing situations on the pump shaft, the latter being shown in side view. 
     
    
     DETAILED DESCRIPTION  
       [0071]     A sealing region Q of the pump shaft  10  of a centrifugal pump (not shown in any greater detail) comprises a shaft sleeve  12  having a length a of 60 mm and an inner diameter d of in this case 30 mm, said shaft sleeve being coaxial with its longitudinal axis M 1  in relation to the longitudinal axis M of the pump shaft  10 ; the wall thickness b of the shaft sleeve  12  is 5 mm. At a central distance a 1  of approximately 25 mm from the front edge  14  of the shaft sleeve  12 , there protrudes from the latter an integrally formed outer ring  16 , as shown in  FIG. 7 , said outer ring having an identical wall thickness b and a collar length e of approximately 7 mm. An outer groove  18  for an O-ring  20  can be seen close to the outer ring  16 ; a further O-ring  20  is mounted in an inner groove  19  close to the front edge  14 . A second outer groove  22  is located close to the illustrated rear edge  15  of the shaft sleeve  12 , as a recess for a securing ring which will be described below.  
         [0072]     The shaft sleeve  12  is surrounded by a coaxial housing bushing  26  of said length a, the inner diameter d 1  of which is in this case 68 mm with a wall thickness b 1  of again 5 mm. The O-rings  20  provide static sealing of the shaft sleeve  12  with respect to the pump shaft  10  and of the housing bushing  26  with respect to the pump housing. Moreover, the housing bushing  26  can be fixed to the housing by means of screws.  
         [0073]     At a central distance a 2  of in this case approximately 20 mm from the front edge  28  of the housing bushing  26 , there protrudes from the wall  30  thereof an integrally formed flange ring  32  having a diameter f of 100 mm and a width g of 10 mm, which contains (for example two) radial threaded bores  34  for plug screws  35  and also four axis-parallel openings  36  for connecting screws  38 .  
         [0074]     At an axial distance i (approximately 10 mm) from said front edge  28 , the wall  30  of the housing bushing  36  has two steps in the inward and axial direction. These two steps  40 ,  40   a , each having a small radial height, are necessary since the inner diameter d 2  of the front edge  28 , at 73 mm, is greater than the diameter d 1  of 68 mm on the other side; the front edge  28  is offered by a wall section  30   a  which adjoins said flange ring  32 . In the region of this flange ring  32 , an inner molded ring  42  having a small radial height and a width i 2  of 10 mm is molded out of the wall  30  (see  FIG. 6 ).  
         [0075]     An inner groove  23  runs close to the rear edge  44  of the housing bushing  26 , said inner groove lying opposite the abovementioned outer groove  22  of the shaft sleeve  12  and jointly holding with the latter a pair of securing rings  46 ,  46   i  which run in the cylindrical annular space  50  formed by the shaft sleeve  12  and the housing bushing  26 ; as shown in  FIG. 1 , said cylindrical annular space merges at the molded ring  42  into a stepped section  51  of the intermediate space between shaft housing  12  and housing bushing  26 .  
         [0076]     Between the securing rings  46 ,  46   i  and the outer ring  16  of the shaft housing  12 , a roller bearing  52  is seated in the cylindrical annular space  50 , for example a double angular contact ball bearing, which keeps the shaft sleeve  12  and the housing bushing  26  at a defined axial and radial spacing and such that they can rotate concentrically. To this end, the shaft sleeve  12  must be fixed on the shaft  10 , for example by means of the inner securing ring  46   i  or a shaft nut.  
         [0077]      FIGS. 1, 4 ,  5  in particular show that the abovementioned steps  40 ,  40   a  serve as a stop for a retaining ring  56 , which is L-shaped in cross section, and an O-ring  20  which is held by said retaining ring; these rings are pushed axially into the stepped section  51  as shown in  FIG. 1 . The other step  40 , which has an integrally formed outer ring  57  having a height n 3  of approximately 5 mm, lies at a distance opposite the retaining ring  56 , which is pressed against the step  40   a  by a front ring  54  surrounded by the front edge  28  and has an inner diameter n of 64 mm, an outer diameter n 1  of 74 mm and a width k of 7 mm,  
         [0078]     A carrier ring or lock ring  60  which has an axial width k 1  of 15 mm and two steps in the radial direction is fitted within the front ring  54  and the retaining ring  56 , said carrier ring or lock ring having an axis-parallel outer wall  61  with an inner diameter z of 65 mm, as can clearly be seen from  FIG. 8 . Approximately in the center between the outer edge  62  of this outer wall  61  and a radial annular front wall  65  of the lock ring  60 , the latter is stepped by means of a (likewise annular) radial central wall  63 ; integrally formed on the latter is an axis-parallel wall ring  64  having an outer diameter z 1  of 51 mm, and said front wall  65  is integrally formed on the latter. The diameter z 2  of the central opening  66  of the front wall  65  is 35 mm. The cross section of the retaining ring  56  thus consists of two angled sections, the outer section containing the outer wall  61  and the central wall  63 ; the latter is adjoined by the wall ring  64  of the inner angled section, which also comprises the front wall  65  and ends at the central opening  66 .  
         [0079]     Between the central wall  63  of the non-magnetic carrier ring or lock ring  60  and the aforementioned front ring  54 , an annular, preferably metallic, bellows  68  can be seen, which is connected to the outer ring  57  and on the inside to the central wall  63  of the carrier ring  60 . The latter is fixed in the housing bushing  26 . Arranged within the wall ring  64  or the carrier ring  60  are three respectively annular magnetic seals  70 , the structure of which can be seen in particular from  FIGS. 10, 11 . Their width q is approximately 3 mm, the inner diameter y of the ring opening  72  is approximately 35 mm and the outer diameter y 1  is in this case 50 mm. Reference  74  denotes a permanent magnet for a ferrofluid, which contains two pole shoes N, S as shown in  FIG. 16 , for example a ring which is U-shaped in cross section as shown in  FIG. 11  at  76  and consists of at least two parts, as an iron limiter with a gap  78  having a width q 1  of approximately 0.1 mm which opens toward the ring opening  72 .  
         [0080]     The three sealing elements  70  form a magnetofluidic seal with respect to the high-pressure side and are three permanent magnets magnetized in the direction of the axis with associated ferromagnetic pole shoes N, S which each generate a concentrated magnetic field that fixes a ferrofluid as sealing medium. In order to make the device easier to assemble, the bellows  68  bears against the front ring  54  and with the retaining ring  56  is sealed with respect to the housing bushing  26  by means of an O-ring  20 , which is fixed to the housing bushing  26  by means of the front ring  54  provided with an outer thread.  
         [0081]     Two further magnetic seals  70  of the above-described type are arranged at the rear side of the securing rings  46 . These magnetic seals  70  are surrounded by two corresponding magnetic seals  70   a  of different diameter, with a spacer ring  79  being arranged therebetween.  
         [0082]     Said lock ring or carrier ring  60  furthermore contains a disk  80  made of silicon carbide which is shown schematically in  FIGS. 12, 13 , said disk forming part of a mechanical sealing system consisting of two similar SiC disks  80 ,  80   a  having a width g 1  of approximately 7 mm, with a central opening  82  having a diameter t of approximately 39 mm. The outer diameter t 1  of the disk  80 ,  80   a  is assumed to be approximately 65 mm. Here, sixteen spiral grooves  86 , which start from the disk edge  81  and are curved in the shape of a segment of a circle when seen in plan view, are etched or ground into the front face or contact face  84  of the right-hand disk  80   a  shown in FIGS.  1  to  5  and  13 , in accordance with an axial spiral groove bearing acting from the outside toward the inside, said grooves  86  having a depth of 10 μm to 20 μm. These spiral grooves  86  end at a radial distance from the central opening  66  and are separated by correspondingly curved insulating ribs  88 . The pump direction and the spiral grooves  86  are defined on the disk  80   a  from the outside toward the center in  FIG. 12 .  
         [0083]     The spiral grooves  86  may be formed both in the stationary and in the moving disk  80 ,  80   a . The important thing is that the machined front face  84  of the other disk  80 ,  80   a  lies directly opposite, so that the conveying effect is produced during operation.  
         [0084]     The sealing elements  70  and the disk  80  in the carrier ring  60  are sealed with respect to the latter, e.g. tightly shrunk on. The second disk  80   a  is arranged opposite the first disk on the shaft sleeve  12 .  FIG. 5  clearly shows an annular gap  13  between the disk  80  and the shaft sleeve  12 . In the selected example of embodiment, the SiC disk  80   a  is fixed by the outer ring  16  as a lateral stop and by an O-ring  20  which at the same time forms a seal with respect to the shaft sleeve  12  and causes it to be driven in rotation therewith. If necessary, rotation therewith can be assisted for example by means of a drive pin between stop  16  and SiC disk  80   a . The opposite faces of the disks  80 ,  80   a  are machined flat in the micrometer range and have a suitably fine depth of surface roughness. The bellows  68  of the carrier ring  60  ensures a mobility of the contact faces of the disks  80 ,  80   a  with respect to one another in the axial direction at a distance of from zero to a few tenths of a millimeter. When the system is idle, the disks  80 ,  80   a  are pressed together by means of the pressure difference to be sealed off, and thus the high-pressure side of the device is sealed off from the low-pressure side by means of the disks  80 ,  80   a . As mentioned above, sealing elements  70  and sealing disk  80  on the carrier disk  60  are kept at a defined concentric distance of approximately 0.1 mm from the shaft sleeve  12  by means of the annular gap  13  ( FIG. 5 ).  
         [0085]      FIG. 14  is intended to illustrate the build-up of pressure due to the conveying effect between the two disks  80 ,  80   a . The top diagram shows the build-up of pressure when only the left-hand disk  80  is subjected to a force and the pressure level on the outside and inside of the disk is the same (function as spiral groove axial bearing). The two diagrams therebelow show possible pressure gradients when the force is generated by a medium pressure on the left-hand disk  80  and a correspondingly higher pressure level on the inside of the disk, as is the case according to the invention. Depending on the pressure gradient, an additional measure for pressure adjustment as shown in  FIG. 5  may be necessary, as will be explained below.  
         [0086]     The magnetofluidic seal toward the atmosphere side consists of the four above-described sealing elements  70 ,  70   a  which, as already mentioned, are arranged at the securing rings  46  in such a way that two elements  70  are directed toward the shaft sleeve  12  and two elements  70   a  are directed toward the housing bushing  26 . In this case, the magnetofluid not only has a sealing effect but also has a centering effect, so that the disk  80  with the sealing elements is freely movable in the axial direction between the shaft sleeve  12  and the housing bushing  26 , which in this region lie concentrically and cylindrically with respect to one another. As a result, the volume in the region between the magnetofluidic seals is variable on the low-pressure side as required, and thus ensures a pressure difference heading toward zero between the low-pressure side of the auxiliary fluid and the environment.  
         [0087]      FIG. 15  shows how the space between the magnetofluidic sealing elements  70  is advantageously filled with an auxiliary liquid by means of two connections  33 —or the two threaded bores  35 . While one connection  33  is used for the operation of filling with the auxiliary liquid, the other serves to subject the device to a vacuum beforehand, so that the auxiliary liquid fills all the hollow spaces within the device Q. By suitably arranging the connections  33  at the opposite sides of the annular space  27  in the housing bushing  26  which surrounds the sealing disk  80   a  assigned to the shaft sleeve  12 , it is possible for a pressure difference to be generated between the connections  33 , which can be used to cause the device to be flowed through by auxiliary liquid from an external container during operation, e.g. for cooling purposes. This is achieved for example in that the annular space  27  has two different sides, and one of the sides of the annular space  27  is at a very small radial distance of in this case 0.1 mm from the disk  80  while the other side is at a greater distance of approximately 1 mm from the disk  80 .  
         [0088]     During operation, the SiC sealing disks  80 ,  80   a  with the spiral grooves  86  confer a conveying effect with respect to one another on the auxiliary liquid, which creates between the low-pressure side and the high-pressure side of the device Q a pressure difference which corresponds to the conveying effect. The auxiliary liquid is selected in such a way that on the one hand good lubrication of the roller bearing  52  is ensured and the highest possible pressure difference can be produced via the sealing disks  80 ,  80   a  (advantageously: high viscosity), and on the other hand the heating of the auxiliary liquid remains within controllable limits (max. approximately 80° C., advantageously: low viscosity). The auxiliary liquid is moreover selected in such a way that it is compatible with the magnetofluid of the seals  70 ,  70   a ; use may advantageously be made of the carrier oil of the magnetofluid (e.g. a silicone oil).  
         [0089]     In order to prevent a “breakthrough” of the magnetofluidic seal on the high-pressure side due to overpressure (three rings withstand a pressure difference of max. approximately 0.5 bar), the conveying effect of the sealing disks  80 ,  80   a  must be limited by the pressure difference bearing against the seal on the high-pressure side. This is achieved by the aforementioned mobility of the sealing disk  80  assigned to the housing bushing  26 , by means of the bellows  68 . If, during operation, the sealing disks  80 ,  80   a  generate a higher pressure than the pressure to be sealed off within the pump, the carrier disk  60  with the associated sealing disk  80  is moved in the direction of the pressure to be sealed off: the distance between the sealing disks  80 ,  80   a  becomes greater and consequently decreases the conveying effect. On the other hand, too low a pressure generated by the sealing disks  80 ,  80   a  leads to a reduction of the gap between the sealing disks  80 ,  80   a  and thus to an increase in the conveying effect.  
         [0090]     In cases where the above-described self-adjustment effect between the sealing disks  80 ,  80   a  is not sufficient, the adjustment can be assisted by means of an overcurrent function between the high-pressure and low-pressure region of the auxiliary liquid. In this case, the sealing disk  80  on the high-pressure side can be displaced axially within the carrier ring  60  and is arranged with radial air toward the outside—radial gap  17  between carrier ring  60  and sealing disk  80  of 0.1 mm in  FIG. 5 . In order to fix it to and drive it in rotation with the carrier ring  60 , use is made of at least two drive pins  67 , as shown in  FIG. 5 . At the outer end of the sealing disk  80 , a radial stop face  69  delimits a sealing gap. The arrangement of the stop face  69  is selected in such a way that the sealing disk  80  lifts away from the carrier ring  60  and thus opens the sealing gap when the pressure between the sealing disk  80  and the carrier ring  60  is higher than the pressure of the fluid to be sealed off on the high-pressure side. An annular gap  21  runs in an axis-parallel manner from the stop face  69 , said annular gap being delimited on one side by the outer wall  61  of the carrier ring  60  and on the other side by the circumference of the sealing disk  80  assigned to the housing bushing  26 .  
         [0091]     Particularly in applications where no chemically aggressive media are to be sealed off, there are various possibilities for reducing the costs of the design. For example, the functions of the shaft sleeve  12  and of the housing bushing  26  can be performed by shaft  10  and housing. The magnetofluidic seals can be produced in a cost-effective manner if the shaft  10  is made of ferromagnetic material, so that the magnetic field lines are guided through the shaft  10 . As a result, arrangements are possible in which the magnetic field of a single permanent magnet is guided across a number of sealing gaps. However, the centering effect required on the low-pressure side is then no longer provided. By contrast, an instability exists, so that adaptation of the volume of the area for the auxiliary liquid must be achieved in some way other than that described. For simple applications, said sealing disks  80 ,  80   a  made of SiC may be produced from more cost-effective materials and integrated in other components.  
         [0092]     The illustrated principle for generating a pressure difference by means of sealing disks  80 ,  80   a  with spiral grooves  86  is merely one possible embodiment. Other principles—such as conveying threads for example—are conceivable and possible.  
         [0093]     The basic structure of a magnetofluidic seal can be seen in  FIG. 16 . The magnetic field of an annular permanent magnet  74  with axial magnetization is concentrated on an annular gap  77  around the shaft  10  by means of two pole shoes  73 . The concentrated field keeps a magnetofluid  75  stationary in said annular gap  77 , which thus gives rise to a sealing effect between the two sides of the structure.  
         [0094]     In order to prevent any mixing between the liquid to be sealed off and the magnetofluid of the seal  70 , the above-described device is supplemented as follows, as shown in  FIG. 17 .  
         [0095]     A region, an area or a chamber  90  is arranged in front of the magnetofluidic seal  70  on the carrier ring  60 , said chamber being partially filled with a gas G, for example air or an inert gas. On the side facing away from the device, the chamber  90  is sealed off with respect to the shaft  10  by means of an annular gap or sealing gap  92  having a width q 3  of approximately 0.1 mm, the diameter f 1  of which is greater than the diameter of the sealing gap  78  of the magnetofluidic seal  70  on the carrier ring  60  but smaller than the diameter f 2  of the outer chamber wall  94 .  
         [0096]     The volume of the chamber  90  and the diameters of the sealing gaps are configured such that, when the arrangement is horizontal and the system is idle, and at ambient pressure inside the chamber  90 , a certain residual gas volume V 0  is always present in the upper region of the chamber  90 , above the sealing gap  92  thereof. During operation, this gas volume collects concentrically around the shaft  10  in the region of smallest diameter of the rotor—this is in the present case the sealing gap  77  of the magnetofluidic seal  70 —and is compressed to a volume V 1  by means of the operating pressure. Even if V 1  is equal to V 0 , no gas should escape from the sealing gap  92  of the chamber  90  by suitably selecting the diameter f 1  of said sealing gap  92 . On the other hand, V 1  should be large enough to completely cover the sealing gap  77  of the magnetofluidic seal  70  during operation, even at maximum pressure. An advantageous diameter ratio between the sealing gap  77  of the magnetofluidic seal  70 , the sealing gap  92  of the chamber  90  and the internal outer diameter of the chamber is 1 to 1.2 to 1.5. In  FIG. 17 , V 1 * denotes the gas volume at maximum pressure.  
         [0097]     As already mentioned, the arrangement ensures that the magnetofluidic seal, during operation, always comes into contact only with gas. Mixing of the magnetofluid with a liquid to be sealed off is thus effectively prevented.  
         [0098]     FIGS.  18  to  20  show in an abstract manner one principle of the invention concerning two magnetofluidic seals  70  which run at an axial distance s from one another, said seals being arranged on a shaft  10  and on a housing wall  24  (as pressure-insulating element) which runs parallel thereto, such that three regions or areas are formed: one area  90   a  with a fluid A having a certain pressure that is to be sealed off (for example conveying medium at 25 bar), one area  96  with an auxiliary liquid H between the seals  70 , and also an area  98  with a fluid B having a different pressure from fluid A (e.g. ambient air at 1 bar absolute). The middle area  96  is divided into two halves or sections  96   a ,  96   b  by means of a conveying device  100 , which is schematically shown as a pump symbol in the form of a circle with an inner triangle, for the means which generate a conveying effect and thus a pressure difference. The connection  71  of the circle to the housing side and the connection  71   a  of the triangle to the shaft side symbolizes the assignment of the components of the conveying device to moving and stationary parts of the device.  
         [0099]     The areas  90   a ,  96   a  shown by dots illustrate regions of high pressure; the pressure difference between said areas is detected by suitable means (symbolized by the “measurement line”  95  and the symbol “deltaP=0!”) and a signal (symbolized by the arrow line  95   a ) is generated for adjusting the conveying device  100  as a function of the pressure difference. Low pressure prevails in the dot-free areas  96   b ,  98 .  
         [0100]     In  FIG. 18 , the pressure adjustment takes place solely by adjusting the conveying device via the pressure difference (preferred solution). In addition to this, reference may be made to  FIG. 4 .  FIG. 19  shows the pressure adjustment by means of an overcurrent device  97  (connected to said measurement line  95  by an arrow line  95   b  and symbolized by an overcurrent valve), which is activated by the pressure difference and is located in a line  99  which connects the areas  96   b  and  98 .  FIG. 20  illustrates the combination of the two adjustment variants according to  FIG. 5  of the specific embodiment.  
         [0101]     Located in the region  96  containing the auxiliary liquid H are means which generate a pressure difference within this region  96 , wherein the higher pressure is generated on the side of fluid A having the higher pressure and vice versa. The pressure difference which can be generated must correspond at least to the maximum pressure difference which occurs between fluid A and fluid B. There are also means which react to the pressure difference between fluid A having the higher pressure and the maximum pressure of the auxiliary liquid H. The reaction is used to adjust said pressure difference to a value close to zero, using suitable means. This may be effected for example by adjusting the power of the means which generate the pressure difference, or by adjusting a return flow from the high-pressure area  90   a  of the auxiliary liquid H to the low-pressure area  96   b .  
         [0102]     By configuring the area assigned to the auxiliary liquid H such that it has a variable volume, it is possible to ensure that the pressure difference between the minimum pressure of the auxiliary liquid H and the pressure of the fluid B having the lower pressure is also almost zero. This can be achieved for example by means of a flexible membrane between one side of the area for the auxiliary liquid H and the fluid having the corresponding pressure, or by arranging one of the magnetofluidic seals  70  such that it can be moved. In an arrangement with ambient air at normal pressure (1 bar) on the low-pressure side, it is most advantageous to configure the area  96  with a variable volume on this side.  
         [0103]     Said means ensure that the magnetofluidic seals  70  are subjected only to low pressure differences even in the event of high pressure differences between the fluids A, B, and thus their hermetic sealing function is ensured. Force transmission takes place mechanically via the force-transmitting element, for example the shaft  10 , so that high transmission powers are possible.  
         [0104]     The pressure difference within the auxiliary liquid H is generated for example by relative movement of geometric elements which are statically assigned to the shaft  10  and to the housing, and form a conveying device for the auxiliary liquid H. Suitable measures, for example the provision of said non-return valve, in this case ensure that no pressure compensation between the high-pressure area and the low-pressure area  96   a  and  96   b  of the auxiliary liquid H takes place when the system is idle.