Abstract:
A magnetic fluid seal is provided, capable of producing a stable scaling performance by stably holding magnetic fluid in place between two members, even in cases when the two members become eccentric. The magnetic fluid seal includes: an annular magnetic circuit forming member that is disposed on a housing; an annular member that is disposed on a shaft; and a magnetic fluid that is magnetically held between axially opposing surfaces of the magnetic circuit forming member and the annular member. In addition, the annular member comprises a flexible member that is swayable so that a portion of the annular member that opposes the magnetic circuit forming member follows the magnetic circuit forming member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a magnetic fluid seal to seal an annular gap between two members rotating relative to each other. 
     2. Description of Related Art 
     In the past, magnetic fluid seals have been known, which seal an annular gap between two members that rotate relative to each other. As for a magnetic fluid seal, there is an advantage in that friction torque can be extremely decreased, in comparison with a seal composed of a rubber, a resin, or the like. Meanwhile, in the case of a magnetic fluid seal, there is a disadvantage in that a structure for stably holding a magnetic fluid in place between two members rotating relative to each other is difficult in comparison with a solid material such as a rubber, a resin, or the like. 
     In order to stably hold a magnetic fluid in place, there is a need to stably form a magnetic circuit and to decrease the variation in a region (space) where a magnetic fluid is held. Accordingly, in cases when the two relatively rotating members become eccentric to each other, it has been difficult to stably hold the magnetic fluid in place. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2010-058254 
         Patent Literature 2: JP-A No. 2003-254445 
         Patent Literature 3: JP-A No. 2002-349718 
         Patent Literature 4: JP-A No. 07-111026 
         Patent Literature 5: Japanese Utility Model Application Laid-Open (JP-U) No. 06-071969 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide a magnetic fluid seal that is capable of producing a stable sealing performance by stably holding the magnetic fluid in place, even in cases when the two members become eccentric. 
     Solution to Problem 
     The present invention has employed the following means, in order to solve the above problem. 
     Specifically, a magnetic fluid seal of the present invention to seal an annular gap between two members rotating relative to each other, comprises: an annular magnetic circuit forming member being disposed on one of the two members; an annular member being disposed on the other of the two members; and a magnetic fluid being magnetically held between axially opposing surfaces of the magnetic circuit forming member and the annular member, wherein the annular member comprises a flexible member being swayable so that a portion of the annular member opposing the magnetic circuit forming member follows the magnetic circuit forming member. 
     According to the present invention, the magnetic fluid is magnetically held between the axially opposing surfaces of the magnetic circuit forming member and the annular member. In addition, the portion of the annular member opposing the magnetic circuit forming member follows the magnetic circuit forming member. 
     Accordingly, even when the interval of the annular gap between the two members is varied due to the eccentricity of the two members or the like, or even when the two members move relatively in the axial direction, the distance between the opposing surfaces where the magnetic fluid is magnetically held can be kept constant. Therefore, the magnetic fluid is stably held in place (magnetically held). 
     It is preferable that at least a portion of the annular member which is in contact with the magnetic fluid has a structure that is capable of absorbing and retaining the magnetic fluid therein. 
     In doing so, the magnetic fluid can be held in place more reliably and be supplied between the opposing surfaces even when the amount of the magnetic fluid is decreased due to the dispersion or the like. Furthermore, even when the material itself of the annular member is a non-magnetic material, the portion thereof which absorbs and retains the magnetic fluid performs an equivalent function as that of a magnetic material, so that the magnetic circuit can be stably formed. 
     It is preferable that an annular dispersion preventing member which prevents the magnetic fluid from being dispersed be disposed radially outward of a portion on which the magnetic fluid being magnetically held. 
     In doing so, the dispersion of the magnetic fluid to the exterior can be suppressed, even when part of the magnetic fluid is separated from the portion where the magnetic fluid is held against the magnetic attraction force due to a centrifugal force applied to the magnetic fluid. 
     It is preferable that an annular labyrinth seal forming member to form a labyrinth seal structure axially outside of the portion on which the magnetic fluid being magnetically held is disposed on any of the two members. 
     In doing so, it is possible to suppress the leak of the magnetic fluid to the exterior, and the entry of foreign debris (dust, etc.) into the interior of the magnetic fluid seal. 
     Note that the above individual configurations may be employed in combination when possible. 
     As described above, the present invention is capable of producing the stable sealing performance by stably holding a magnetic fluid in place, even in cases when the two members become eccentric. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating a magnetic fluid seal according to Example 1 of the present invention. 
         FIG. 2  is a schematic cross-sectional view illustrating the magnetic fluid seal according to Example 1 of the present invention. 
         FIGS. 3A-3G  are schematic cross-sectional views illustrating various Modification Examples of a magnetic circuit forming member according to Example 1 of the present invention. 
         FIGS. 4A-4D  are schematic cross-sectional views illustrating a magnetic fluid seal according to Example 2 of the present invention. 
         FIG. 5  is a schematic cross-sectional view illustrating a magnetic fluid seal according to Example 3 of the present invention. 
         FIG. 6  is a schematic cross-sectional view illustrating a magnetic fluid seal according to Example 4 of the present invention. 
         FIG. 7A  is a schematic cross-sectional view illustrating a magnetic fluid seal according to Example of the present invention, and  FIG. 7B  is a view illustrating an inner wall surface of a labyrinth seal forming member. 
         FIG. 8  is a schematic cross-sectional view illustrating a magnetic fluid seal according to Example 6 of the present invention. 
         FIGS. 9A-9C  are schematic cross-sectional views illustrating a magnetic fluid seal according to Example 7 of the present invention. 
         FIGS. 10A-10F  are views illustrating various Modification Examples of a magnet. 
         FIGS. 11A-11G  are views illustrating various Modification Examples of an annular member. 
         FIGS. 12A-12C  are views illustrating various Modification Examples of the annular member. 
         FIGS. 13A-13D  are views illustrating Modification Example of the annular member. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Thereinafter, a mode for carrying out this invention will be described and exemplified in detail on the basis of Examples, with reference to the drawings. However, Examples are not intended to exclusively limit the scope of the present invention to the dimensions, materials, shapes, relative arrangements, etc. of the components described therein, except when specified otherwise. 
     Example 1 
     A description will be given of a magnetic fluid seal according to Example 1 of the present invention, with reference to  FIGS. 1 to 3G . Note that a magnetic fluid seal  1  according to this Example is applicable to agitators, gas seals for VOC measures, various industrial apparatus such as vacuum devices for manufacturing semiconductors, fishing reels, shaft parts for various devices such as bicycles, as a leak prevention seal or a dust seal. 
     &lt;Overall Configuration of Magnetic Fluid Seal&gt; 
     A description will be given of an overall configuration of a magnetic fluid seal according to Example 1 of the present invention, in particular, with reference to  FIG. 1 .  FIG. 1  shows a stationary state (when a shaft  500  and a housing  600 , or two members, are stationary). 
     The magnetic fluid seal  1  is provided to seal an annular gap between the shaft  500  and the housing  600  that rotate relative to each other (including not only a case where one rotates and the other is stationary, but also a case where both rotate). In addition, the magnetic fluid seal  1  includes a magnetic circuit forming member  100  attached to the inner circumferential surface of a shaft hole in the housing  600 , an annular member  200  attached to the shaft  500 , and a magnetic fluid  300 . 
     Moreover, the magnetic circuit forming member  100  according to this Example includes an annular permanent magnet  110  which is fitted into and fixed to the inner circumferential surface of the shaft hole in the housing  600 , and a pair of disc-shaped magnetic pole members (pole pieces)  120  with a hole which are provided on the respective sides (N pole side portion and pole side portion) of the permanent magnet  110 . In addition, magnetic pole tip members  130  are disposed along the periphery of each of the holes and on the respective opposing sides of the pair of the magnetic pole members  120 . Note that both of the magnetic pole members  120  and both of the magnetic pole tip members  130  are composed of magnetic materials. 
     With the above configuration, a magnetic circuit M is formed, which passes through the permanent magnet  110 , the pair of magnetic pole members  120 , the pair of magnetic pole tip members  130 , and a gap between the pair of magnetic pole tip members  130 . 
     The annular member  200  is a disc-shaped member with a hole, and the inner circumferential surface thereof is fixed to the outer circumferential surface of the shaft  500 . This annular member  200  is composed of a flexible material, so that the outer circumferential surface side thereof can sway in the axial direction. As an example of this material, porous silicon, rubber, resin, fabric such as felt, paper or the like, can be given. 
     At least a portion of the annular member  200  which is in contact with the magnetic fluid  300  and its vicinity are structured to be able to absorb and retain the magnetic fluid  300 . Specifically, when porous silicon, fabric, paper, or the like as described above is employed for the material of the annular member  200 , the annular member  200  can absorb and retain the magnetic fluid  300  due to the property of the material itself. In addition, even when the material that cannot absorb and retain the magnetic fluid  300  due to the property itself, such as rubber, resin or the like, is used, the annular member  200  can absorb and retain the magnetic fluid  300  due to the capillary action by employing a foamable structure for the portion of the annular member  200  which is in contact with the magnetic fluid  300  and its vicinity. 
     The annular member  200  is configured such that its radially outward side opposes the magnetic pole tip member  130  of the magnetic circuit forming member  100  in the axial direction. By supplying the magnetic fluid  300  to a space between the respective opposing surfaces of the annular member  200  and the magnetic pole tip member  130 , the magnetic fluid  300  can be held in place between the opposing surfaces due to the magnetic attraction force while a part of it is absorbed and retained in the annular member  200 . Meanwhile, even when the material of the annular member  200  is not a magnetic material, at least a portion of the annular member  200  which is in contact with the magnetic fluid  300  and its vicinity retain the magnetic fluid  300  as described above, performing a function similar to that of a magnetic material. Consequently, a stable magnetic circuit is formed, and the magnetic fluid  300  is stably held in place. 
     With the above configuration, a combination of the magnetic circuit forming member  100 , the annular member  200 , and the magnetic fluid  300  seals the annular gap between the shaft  500  and the housing  600 . 
     &lt;Usage State&gt; 
     A description will be given of a usage state of the magnetic fluid seal  1  according to Example 1 of the present invention, in particular, with reference to  FIG. 2 . 
     In this Example, the shaft  500  and the housing  600  may rotate eccentrically upon relative rotation.  FIG. 2  illustrates a case where the housing  600  moves relative to the shaft  500  in a direction of an arrow X due to the eccentricity. In other words,  FIG. 2  illustrates a case where the shaft  500  and the housing  600  move relative to each other in the axial direction, and a gap between the shaft  500  and the housing  600  is widened within a cross-sectional area of this figure. 
     As shown, when the shaft  500  and the housing  600  move relative to each other in this manner, the vicinity of the radially outward side of the annular member  200  sways due to the magnetic attraction force so as to follow the movement of the magnetic pole tip member  130  of the circuit forming member  100 . This maintains the state where the magnetic fluid  300  is held in place. 
     &lt;Advantage of Magnetic Fluid Seal According to this Embodiment&gt; 
     In the magnetic fluid seal  1  according to this Example, even when the shaft  500  and the housing  600  move relative to each other due to the eccentricity or the like, the magnetic fluid  300  is stably held in place. This feature will be described in more detail. 
     The magnetic fluid seal  1  according to this Example is configured such that the magnetic fluid  300  is magnetically held between axially opposing surfaces of the annular members  200  and the magnetic pole tip member  130  of the magnetic circuit forming member  100 . Therefore, even when the interval of the gap between the shaft  500  and the housing  600  is varied, only a location within the annular member  200  where the magnetic fluid  300  makes contact is varied while a distance between the respective opposing surfaces of the annular member  200  and the magnetic pole tip members  130  isn&#39;t. 
     Moreover, the magnetic fluid seal  1  according to this Example is configured such that a portion of the annular member  200  which faces the magnetic pole tip member  130  sways so as to follow the magnetic pole tip member  130 . Accordingly, even when the shaft  500  and the housing  600  move relative to each other in the axial direction, the distance between the respective opposing surfaces of the annular member  200  and the magnetic pole tip member  130  is hardly varied. 
     As described, even when the interval of the gap between the shaft  500  and the housing  600  is varied, or even when the shaft  500  and the housing  600  move relative to each other in the axial direction, the distance between the respective opposing surfaces of the annular member  200  and the magnetic pole tip member  130  is hardly varied. Thus, the magnetic fluid  300  is stably held in place (magnetically held) within a region defined between the respective opposing surfaces of the annular member  200  and the magnetic pole tip member  130 . 
     In this Example, at least a portion of the annular member  200  which is in contact with the magnetic fluid  300  is configured to be able to absorb and retain the magnetic fluid  300 . This ensures that the magnetic fluid  300  is held in place. In addition, even when the amount of the magnetic fluid  300  is decreased due to the dispersion or the like, the magnetic fluid  300  retained in the annular member  200  can be supplied to the region between the respective opposing surfaces of the annular member  200  and the magnetic pole tip member  130 . This enables the magnetic fluid  300  to be supplied over an extended period by retaining a large amount of magnetic fluid  300  in the annular member  200 , which prolongs the lifetime. Furthermore, even when the material itself of the annular member  200  is a non-magnetic material, the portion of the annular member  200  which retains the magnetic fluid  300  performs a function equivalent to that of a magnetic material, thereby making it possible to form the magnetic circuit M stably. 
     In this Example, the magnetic circuit is formed through the vicinity of the end of the annular member  200  and a location of the magnetic circuit forming member  100 , enabling the magnetic fluid  300  to be held by this magnetic circuit. Therefore, the material of the shaft  500  can be either of a magnetic or non-magnetic material. In addition, even when the interval of the annular gap between the shaft  500  and the housing  600  is wide, a sleeve composed of a magnetic material or the like which has been conventionally provided for forming a magnetic circuit is no longer necessary. Meanwhile, in this Example, an amount of magnetic fluid  300  doesn&#39;t need to be increased since it is only necessary for the magnetic fluid  300  to be magnetically held in a small gap between one of the pair of magnetic pole members  120  (magnetic pole tip members  130 ) and the annular member  200 . Furthermore, the distance between the pair of magnetic pole members  120  (magnetic pole tip members  130 ) may also be long or short as long as the distance therebetween is adequate to form the magnetic circuit M, thereby providing a high degree of flexibility in the design. 
     Moreover, because the magnetic fluid  300  is present between the annular member  200  and the magnetic pole tip member  130 , the friction resistance therebetween can be extremely decreased due to the self-lubricating effect thereof. In addition, by applying a surface processing to the surface of the annular member  200  for reducing the friction resistance thereof, the friction resistance can be further reduced. 
     &lt;Others&gt; 
     In the above description, the case has been exemplified where the magnetic circuit forming member  100  includes the annular permanent magnet  110 , the pair of magnetic pole members  120 , and the pair of magnetic pole tip members  130  having a rectangular cross-section, as illustrated in  FIGS. 1 and 2 . However, a magnetic circuit forming member that is applicable to the present invention is not limited to such a configuration. A description will be given of another example of a magnetic circuit forming member that is applicable to the present invention, with reference to  FIGS. 3A to 3G .  FIGS. 3A to 3G  are schematic cross-sectional views illustrating various Modification Examples of a magnetic circuit forming member, and illustrates the cross-section of only the main parts. Note that in  FIGS. 3C to 3G , a permanent magnet is omitted from the magnetic circuit forming member, and only one of a pair of magnetic pole members is illustrated. 
       FIG. 3A  illustrates one exemplified case where a magnetic circuit forming member is configured with magnets alone. Specifically, a pair of disc-shaped permanent magnets  111  each with a hole is provided on the inner circumferential surface of the shaft hole in the housing  600 . In between the pair of permanent magnets  111 , one has an N pole on the inner circumferential side and an S pole on the outer circumferential side, while the other has an S pole on the inner circumferential side and an N pole on the outer circumferential side. This forms a magnetic circuit M as illustrated in the figure, making it possible to magnetically hold the magnetic fluid  300  to one end of the permanent magnet  111 . 
       FIG. 3B  illustrates a case based on the configuration illustrated in  FIG. 3A  where annular magnetic pole tip members  130  are provided on the respective opposing surfaces of the pair of permanent magnets  111  and along the respective circumferences of the holes. This enables the magnetic fluid  300  to be concentrated only on the surface side that opposes the annular member  200  (not illustrated in  FIGS. 3A to 3G ). 
       FIG. 3C  illustrates a case where the magnetic pole tip members  130  are not provided in the configuration illustrated in  FIGS. 1 and 2 . In this case, the magnetic fluid  300  that is magnetically held on the end of the magnetic pole member  120  comes around not only to the side opposing the annular member  200  (not illustrated in  FIGS. 3A to 3G ) but also to the opposite side, but the holding function for the magnetic fluid  300  is not affected so much. 
       FIG. 3D  illustrates a case where a cross-sectional shape of a magnetic pole tip member  131  is triangular. Moreover,  FIG. 3E  illustrates a case where a cross-sectional shape of a magnetic pole tip member  132  is semielliptical. By employing these configurations, a location where the magnetic fluid  300  is held can be concentrated within a smaller region. 
       FIG. 3F  illustrates a case where magnetic pole tip members  133  each of which has a triangular cross-section are provided at two locations while being adjacent to each other, and  FIG. 3G  illustrates a case where a plurality of grooves  121   a  each with a triangular cross-section are formed on the side of the magnetic pole member  121  that opposes the annular member  200  (not illustrated in  FIGS. 3A to 3G ) and in the vicinity of an end of the magnetic pole member  121 . When these configurations are employed, it is possible to suppress the movement of a location where the magnetic fluid  300  is held, with respect to magnetic pole tip members  133  or the magnetic pole member  121 , thereby holding the magnetic fluid  300  in place more stably. 
     Although the above description has exemplified the case where the cross-sectional shape of the annular member  200  is rectangular as illustrated in  FIG. 1 or 2 , a shape of an annular member that is applicable to the present invention is not limited thereto. For example, any appropriate cross-sectional shape, such as a triangular shape, or one having an arc-shaped end or an ellipse-shaped end may be employed. Furthermore, although the above various examples have exemplified the case where the magnetic pole member and the magnetic pole tip member are separate members, a single member that integrates them may be employed. 
     Example 2 
       FIGS. 4A to 4D  illustrate Example 2 according to the present invention. This Example will describe a configuration where an annular dispersion preventing member is provided to prevent a magnetic fluid from being dispersed, in addition to the above configuration exemplified in Example 1. The same reference numerals are assigned to the same components as those of Example 1, and descriptions therefor will be omitted as appropriate. Note that  FIGS. 4A to 4D  illustrate schematic cross-sectional views of a magnetic fluid seal, and only a cut surface obtained by cutting the main part is illustrated. 
     This Example exemplifies a case where an annular dispersion preventing member that prevents a magnetic fluid  300  from being dispersed is disposed radially outward of a portion on which the magnetic fluid  300  is magnetically held. 
     In an example illustrated in  FIG. 4A , an annular dispersion preventing member  410  is disposed on the magnetic pole member  120 . This dispersion preventing member  410  is configured such that one end thereof is fixed to the magnetic pole member  120  and the other end (free end) thereof extends toward the end of an annular member  200 . This achieves the configuration where the dispersion preventing member  410  covers the radially outward side of a portion where the magnetic fluid  300  is magnetically held. 
     With the above configuration, the dispersion preventing member  410  can suppress the dispersion of the magnetic fluid to the exterior of a magnetic fluid seal  1 , even when a centrifugal force is applied to the magnetic fluid  300  along with the relative rotation of the shaft  500  and the housing  600 , and part of the magnetic fluid  300  is separated radially outwardly against the magnetic attraction force from the portion where the magnetic fluid  300  is held. Note that the magnetic fluid that has been separated from the portion is kept adhered around the dispersion preventing member  410  while undergoing the centrifugal force, but can be returned to the initial location due to the magnetic attraction force, once the centrifugal force is not applied. 
     An example illustrated in  FIG. 4B  illustrates a case where an annular dispersion preventing member  420  is disposed on a permanent magnet  110 , and an example illustrated in  FIG. 4C  illustrates a case where an annular dispersion preventing member  430  is disposed on the annular member  200 . Either case can produce a functional effect similar to that of the above case. 
     Note that each of the arrows in  FIGS. 4A to 4C  indicates a direction in which part of the magnetic fluid separates from the magnetic attraction portion due to the centrifugal force. 
     It is desirable for the above dispersion preventing members  410 ,  420  and  430  to be composed of a non-magnetic material, so that the magnetic fluid adhered to each dispersion preventing member is not magnetically attracted thereto. 
     When the configuration with the above dispersion preventing member  410 ,  420  or  430  being disposed is employed, such configuration that the magnetic fluid  300  is filled into a spatial region defined by the dispersion preventing member  410 ,  420  or  430 , the annular member, the magnetic pole members, and the like can also be employed. Employing such a configuration increases the amount of the magnetic fluid  300  to be reserved, thereby making it possible to prolong the lifetime of the magnetic fluid seal  1 . Note that  FIG. 4D  illustrates a case where a magnetic fluid is filled into the above spatial region in the example illustrated in  FIG. 4A . 
     Example 3 
       FIG. 5  illustrates Example 3 of the present invention. Example 1 described above has exemplified the configuration where the single annular member holds the magnetic fluid at a single location, whereas this Example has exemplified a configuration where two annular members hold a magnetic fluid at two locations. The same reference numerals are assigned to the same components as those of Example 1, and descriptions therefor will be omitted as appropriate.  FIG. 5  illustrates a schematic cross-sectional view of a magnetic fluid seal, and only a cut surface obtained by cutting the main part. 
     As illustrated in  FIG. 5 , a configuration of a magnetic circuit forming member  100  is the same as that of Example 1. In this Example, two annular members  200  are disposed on a shaft  500 . The configuration itself of each annular member  200  is the same as that of Example 1 described above. 
     Respective configurations of magnetically holding a magnetic fluid  300  are employed between one of the pair of annular members  200  and one of a pair of magnetic pole tip members  130  and between the other of the pair of annular members  200  and the other of the pair of magnetic pole tip members  130 . 
     According to this Example, the annular gap between the shaft  500  and a housing  600  can be sealed at two locations. 
     Example 4 
       FIG. 6  illustrates Example 4 of the present invention. Example 1 described above has exemplified the case where the annular member is fixed to the shaft directly, whereas this Example exemplifies a configuration where an annular member is disposed on a sleeve fitted onto the shaft. The same reference numerals are assigned to the same components as those of Example 1, and descriptions therefor will be omitted as appropriate.  FIG. 6  illustrates a schematic cross-sectional view of a magnetic fluid seal, and only a cut surface obtained by cutting the main part. 
     In this Example, a sleeve  210  fitted onto the shaft  500  is disposed. In addition, an annular member  200  is fixed to this sleeve  210 . Rest of the configuration thereof is the same as that of Example 1 described above. 
     It is effective to dispose the annular member  200  on the sleeve  210  as in this Example, in the case where the annular member  200  cannot be fixed to the shaft  500  for some technical reasons or in view of some circumstances, or the case where annular member  200  is expendable and needs to be exchanged as appropriate. In this Example, thus, the sleeve  210  with the annular member  200  disposed thereon can be handled as a single component. 
     Example 5 
       FIGS. 7A and 7B  illustrate Example 5 of the present invention. In this Example, a description will be given of a configuration where a labyrinth seal structure is formed, in addition to the above configuration exemplified in Example 1. The same reference numerals are assigned to the same components as those of Example 1, and descriptions therefor will be omitted as appropriate.  FIG. 7A  illustrates schematic cross-sectional views of a magnetic fluid seal, and only a cut surface obtained by cutting the main part. 
     As illustrated in  FIG. 7A , in this Example, an annular labyrinth seal forming member  450  that forms a labyrinth seal structure is disposed on the shaft  500  axially outside of a portion where a magnetic fluid  300  is magnetically held. This labyrinth seal forming member  450  is a disc-shaped member with a hole (a hole having a same diameter as the outer diameter of the shaft  500 ), and the inner circumferential surface thereof is fixed to the outer circumferential surface of the shaft  500 . A small gap is formed between the labyrinth seal forming member  450  and the magnetic pole member  120 , and this small gap forms a labyrinth structure. This can suppress the partial leak of the magnetic fluid to the exterior, or the entry of foreign debris (dust, etc.) into the interior. In addition, it is more effective to provide projections or grooves on the inner wall side of the labyrinth seal forming member  450  for producing a pump effect, in case the shaft  500  rotates.  FIG. 7B  illustrates one example thereof. In this figure, reference numerals  450   a ,  450   b  and  450   c  are projections for producing a pump effect. 
     Note that it is desirable that the rotational direction of the labyrinth seal forming member  450  and the shape of projections or grooves be combined to produce a pumping effect in the radially outward direction. This makes it possible to enhance the prevention of the entry of foreign debris from the exterior. Even in this case, it is possible to sufficiently suppress the partial leak of the magnetic fluid to the exterior, due to the effect of the above labyrinth seal and the disposition of the projections or grooves. 
     Example 6 
       FIG. 8  illustrates Example 6 of the present invention. Examples described above have exemplified the configurations where the magnetic circuit forming member is disposed on the housing, and the annular member is disposed on the shaft, whereas this Example exemplifies a configuration where the magnetic circuit forming member is disposed on the shaft, and the annular member is disposed on the housing. The same reference numerals are assigned to the same components as those of Example 1, and descriptions therefor will be omitted as appropriate.  FIG. 8  illustrates a schematic cross-sectional view of a magnetic fluid seal, and only a cut surface obtained by cutting the main part. 
     In this Example, a magnetic circuit forming member  100  is disposed on a shaft  500 , and an annular member  200  is disposed on a housing  600 . The magnetic circuit forming member  100  and the annular member  200  basically have the same configurations as those having been described in Example 1 above, except that each employs a radially-symmetrical shape to that of Example 1, as the inner circumferential surface side being in a radially symmetric-fashion to the outer circumferential surface side. Needless to say, even this Example produces a functional effect similar to that of Example 1 described above. 
     &lt;Others&gt; 
     Among the various configurations of the magnetic circuit forming member illustrated in  FIGS. 3A to 3G  which have been exemplified in Example 1 described above, the various configurations of the dispersion preventing member which have been exemplified in Example 2, the configuration provided with the two annular members which has been exemplified in Example 3, the configuration provided with the sleeve which has been exemplified in Example 4, and the configuration provided with the labyrinth seal forming member which has been exemplified in Example 5, any given combination is possible. 
     As for the configuration exemplified in Example 6, the each configuration having been exemplified in Examples 1 to 5 may be applied thereto arbitrarily. However, when the each configuration exemplified in Examples 1 to 5 is applied to the configuration exemplified in Example 6, each employs a radially-symmetrical shape to that of the original, as the inner circumferential surface side being in a radially symmetric-fashion to the outer circumferential surface side. Note that when the dispersion preventing member exemplified in Example 2 is applied to the configuration exemplified in Example 6, the dispersion preventing member is disposed on the inner circumferential surface of the shaft hole in the housing  600  or on the annular member  200 . 
     In the case where a target to be sealed is a liquid, surface processing may be performed as appropriate, in order to prevent the liquid from being absorbed in the interfaces of the various members. 
     Example 7 
       FIGS. 9A to 9C  illustrate Example 7 of the present invention. Examples described above have exemplified the configurations where the magnetic pole members or the magnets are arranged on the respective sides of the annular member in the axial direction, whereas this Example exemplifies a configuration where a magnet or the like is disposed on only one side of the annular member in the axial direction. The same reference numerals are assigned to the same components as those of Example 1, and descriptions therefor will be omitted as appropriate.  FIGS. 9A to 9C  illustrate schematic cross-sectional views of a magnetic fluid seal, and only a cut surface obtained by cutting the main part. In addition,  FIGS. 9A to 9C  illustrate various Modification Examples. 
       FIG. 9A  illustrates a configuration where a magnetic circuit forming member  100  is composed of a single permanent magnet  112  alone. In this Example, specifically, the outer circumferential surface of the annular permanent magnet  112  is fixed to the inner circumferential surface of the shaft hole in a housing  600 . Note that one surface of the permanent magnet  112  in the axial direction has an N pole, and the other surface thereof has an S pole. The configuration of the annular member  200  is the same as that of Example 1 described above. Thus, a magnetic fluid seal  1  is configured by holding the magnetic fluid  300  in place between the respective opposing surfaces of the permanent magnet  112  and the annular member  200 . 
     As described above, the structure of this Example is simpler than that of Example 1, and enables the number of the components to be reduced and an assembly operation to be carried out easily. 
       FIG. 9B  is Modification Example of the magnetic fluid seal  1  illustrated in  FIG. 9A . In this example, a magnetic circuit forming member  100  includes a permanent magnet  112  and a retention member  122 , made of a magnetic material, which retains the permanent magnet  112 . In more detail, an annular groove  122   g  is provided on the disc-shaped retention member  122  with a hole, and the annular permanent magnet  112  is fitted into and retained by this annular groove  122   g . Note that one surface of the permanent magnet  112  in the axial direction has an N pole, whereas the other surface thereof has an S pole. This example has a slightly more complicated structure than the magnetic fluid seal  1  illustrated in  FIG. 9A , but can narrow a region where the magnetic fluid  300  is held, thereby decreasing the amount of the magnetic fluid  300 . 
       FIG. 9C  is Modification Example of the magnetic fluid seal  1  illustrated in  FIG. 9A . In this example, a magnetic circuit forming member  100  includes a permanent magnet  112 , and a pair of magnetic pole members  122   a  and  122   b  provided on the respective ends of this permanent magnet  112  in the axial direction. Note that one surface of the permanent magnet  112  in the axial direction has an N pole, whereas the other surface thereof has an S pole. In addition, a small gap is formed between the respective inner circumferential end sides of the pair of magnetic pole members  122   a  and  122   b , so that the magnetic fluid  300  can be retained in this small gap. This example has a slightly more complicated structure than the magnetic fluid seal  1  illustrated in  FIG. 9A , but can narrow a region where the magnetic fluid  300  is held, thereby decreasing the amount of the magnetic fluid  300 . 
     In this Example, it is desirable that the housing  600  to which the permanent magnet  112 , the retention member  122 , and the magnetic pole members  122   a  and  122   b  are fixed be made of a non-magnetic material, so as to cause a magnetic force to concentrate, as much as possible, on a portion where the magnetic fluid  300  is held. In the case where the housing  600  itself is made of a magnetic material, an annular sleeve made of a non-magnetic material may be disposed across the inner circumference of the shaft hole in the housing  600 , and the permanent magnet  112  and the like may be fixed to the inner circumference of the sleeve. 
     The various configurations of the dispersion preventing member which have been exemplified in Example 2 or the configuration provided with the labyrinth seal forming member which has been exemplified in Example 5 can also be employed for this Example as appropriate. Furthermore, the configuration where the magnetic circuit forming member  100  is disposed on the shaft  500  and the annular member  200  is disposed on the housing  600  can also be employed for this Example, as described in Example 6. In this case, it is desirable for the shaft  500  to be made of a non-magnetic material. 
     &lt;Various examples of Magnet&gt; 
     Although a single annular permanent magnet that has one surface of an N pole and the other surface of an S pole in the axial direction can be employed as the permanent magnet used in Examples described above, a permanent magnet that can be employed for each Example is not limited thereto. Here, a description will be given of one example of a magnet that can be employed for each Example, with reference to  FIGS. 10A to 10F . 
     A magnet  113  illustrated in  FIG. 10A  includes a plurality of disc-shaped permanent magnets  113   a , and a retention member  113   b , made of a non-magnetic material, which retains the plurality of permanent magnets  113   a . Note that the retention member  113   b  is a disc-shaped member with a hole, and is provided with a plurality of circular holes  113   b   1  for retaining the permanent magnets  113   a . The permanent magnets  113   a  are fitted into and retained by the plurality of holes  113   b   1 . Note that the disc-shaped permanent magnets  113   a  each have one surface of an N pole and the other surface of an S pole, and are retained by the retention member  113   b  in such a way the same poles are oriented to the same surface side. However, a configuration where the N and S poles are combined to be arranged alternately on one surface side may be employed. 
     A magnet  114  illustrated in  FIG. 10B  constitutes an annular magnet as a whole by combining sector-shaped permanent magnets  114   a . The sector-shaped permanent magnets  114   a  each have one surface of an N pole and the other surface of an S pole, and are combined in such a way the same poles are oriented to the same surface side. However, a configuration where the N and S poles are combined to be arranged alternately on one surface side may be employed. 
     A magnet  115  illustrated in  FIG. 10C  is a combination of a plurality of annular permanent magnets  115   a , each of which has one surface of an N pole and the other surface of an S pole and which are combined in such a way the N and S poles thereof are arranged alternately on one surface side in a concentric fashion. 
     A magnet  116  illustrated in  FIG. 100D  has an overall constitution of an annular magnet which is formed by combining a plurality of substantially rod-shaped permanent magnets  116   a , each of which has one surface of an N pole and the other surface of an S pole, in such a way the N and S poles are arranged alternately on one surface side. 
       FIG. 10E  is a cross-sectional view of the magnet  115  illustrated in  FIG. 10C , and illustrates a state where the magnetic fluid  300  is held. 
     The magnet  118  illustrated in  FIG. 10F  is Modification Example of the magnet  118  illustrated in  FIGS. 10C and 10E , and is an example of partially employing a bilayer structure. Thus, the bilayer structure is partially employed only for the portion which holds the magnetic fluid  300  in place, thereby being able to narrow a region where the magnetic fluid  300  is held. 
     &lt;Various Examples of Ring-Shaped Member&gt; 
     The annular member for use in each Example described above may be merely a disc-shaped member with a hole which has a flexibility so as to sway in the axial direction. Accordingly, for example, a member that has a one-layer structure made of a single non-magnetic material can be employed, but an annular member which can be employed for each Example is not limited to such an annular member. Here, a description will be given of one example of an annular member that can be employed for each Example, with reference to  FIGS. 11A to 13D . 
     It is desirable for the annular member to be composed of a magnetic material, since the annular member constitutes a part of the magnetic circuit. However, the annular member requires flexibility as described above, and no single material having both magnetization and flexibility is found. Accordingly, a configuration can be employed, where a flexible material, such as porous silicon, rubber, resin, fabric such as felt, paper or the like, contains magnetic units, such as fillers, wires or the like, in order to provide both magnetization and flexibility.  FIGS. 11A to 11G  illustrate such one example. 
       FIGS. 11A-11G  illustrate various examples of an annular member in which a disc-shaped member with a hole is used as a base material, and magnetic units are arranged in the inside of the base material. Note that this base material is composed of a flexible non-magnetic material.  FIG. 11A  illustrates an annular member  211  in which a plurality of rod-shaped magnetic units N are arranged in the inside of the base material so as to be along the circumferential direction.  FIG. 11B  illustrates an annular member  212  in which a plurality of disc-shaped magnetic units N are arranged in the inside of the base material so as to be along the circumferential direction.  FIG. 11C  illustrates an annular member  213  in which a plurality of rod-shaped magnetic units N are arranged radially in the inside of the base material.  FIG. 11D  illustrates an annular member  214  in which a plurality of annular magnetic units N are arranged concentrically in the inside of the base material.  FIG. 11E  illustrates an annular member  215  in which regions where a magnetic unit N is disposed and regions where a magnetic unit N is not disposed are arrayed in the inside of the base material in a lattice form.  FIG. 11F  is a view illustrating an XX cross-section in  FIG. 11C . In the annular members illustrated in  FIGS. 11A to 11F , a method of arranging the magnetic units N in the inside of the base material is not limited to a specific one. For example, in the case where the base material is a fabric, the magnetic units N can be arranged in the inside of the base material by inweaving the magnetic units N.  FIG. 11G  illustrates an annular member  216  in which powdery magnetic units N are arranged in the inside of the base material while being distributed therein by mixing the powdery magnetic units N into the base material. 
     As described above, these annular members have both magnetization and flexibility. An annular member having magnetization improves the magnetic property, which can increase the holding force for a magnetic fluid, thereby enhancing the sealing property. Note that it is possible to adjust the balance between the magnetization and the flexibility by adjusting the amount or arrangement of the magnetic units. The size of each magnetic unit N is not limited to a specific one, but it is desirable that it be larger than a magnetic particle in the magnetic fluid  300 . 
     Next, a description will be given of an annular member that can increase the amount of the magnetic fluid to be held, with reference to  FIGS. 12A to 12C . An annular member  217  illustrated in  FIG. 12A  includes capillary portions  217   a  that cause the capillary action, and a hollow portion  217   b  that is connected to the capillary portions  217   a . Note that the annular member  217  is composed of a material (rubber or resin) that cannot absorb and retain the magnetic fluid  300 . In the annular member  217  configured above, the magnetic fluid  300  can be supplied between the annular member  217  and the magnetic pole member, etc. over an extended period by reserving the magnetic fluid  300  in the hollow portion  217   b . This enables the lifetime to be prolonged. 
     A annular member  218  illustrated in  FIG. 12B  has a multilayered structure including a first layer  218   a  that is provided with capillary portions for causing the capillary action, a second layer  218   b  that can absorb and retain a magnetic fluid due to a property of its own material, and a third layer  218   c . Note that the first layer  218   a  and the third layer  218   c  are composed of a material that cannot absorb and retain the magnetic fluid  300 . In the annular member  218  configured above, the magnetic fluid  300  can be supplied between the annular member  218  and the magnetic pole member, etc. over an extended period by reserving the magnetic fluid  300  in the second layer  218   b . This enables the lifetime to be prolonged. 
     An annular member  219  illustrated in  FIG. 12C  has a multilayered structure including a first layer  219   a  that is provided with capillary portions for causing the capillary action, a second layer  219   b  that can absorb and retain a magnetic fluid due to a property of its own material, and a third layer  219   c  that is provided with a hollow portion. Note that the first layer  219   a  and the third layer  219   c  are composed of a material that cannot absorb and retain the magnetic fluid  300 . In the annular member  219  configured above, a magnetic fluid  300  can be supplied between the annular member  219  and the magnetic pole member, etc. over an extended period by reserving the magnetic fluid  300  in the second layer  218   b  as well as reserving the magnetic fluid  300  in the hollow portion of the third layer  219   c . This enables the lifetime to be prolonged. 
     Note that as for the location where the capillary portions are provided in the annular member  217 ,  218  or  219 , figures illustrate the case where they are provided entirely. However, it may be only at a location where the magnetic fluid  300  is held between the magnetic pole members, etc. In addition, it is possible to adjust the flexibility of the annular member by adjusting the arrangement or size of the hollow portion or by adjusting the thickness of each layer when the layered structure is employed. 
     Next, with reference to  FIG. 13A to 13D , a description will be given of an annular member that enables the magnetic fluid  300  to be stably held in place even under the situation where the shaft  500  and the housing  600  move relatively over a long distance in the axial direction. The magnetic fluid seal  1  illustrated in  FIG. 13A  gives an example in which only a configuration of an annular member  220  is changed in the configuration exemplified in Example 1 described above. Specifically, the annular member  200  of Example 1 described above is a disc-shaped (flat-shaped) member with a hole. Meanwhile, the annular member  220  of this Modification Example is different from the annular member  200  of Example 1 described above in that it is configured, not in a flat shape, but in an accordion shape. 
     In the case where the annular member  200  has a flat-shape, when a fixed end of the annular member  200  moves as far as the opposite side of a surface of the magnetic pole member  120  where the magnetic fluid  300  is held, due to the movement of the shaft  500  relative to the housing  600  in the axial direction, there is a possibility of the outer circumferential end of the annular member  200  moving in a direction away from the magnetic pole member  120  (see  FIG. 13B ). This makes it difficult to stably hold the magnetic fluid  300  in place. 
     Meanwhile, in the case where the annular member  220  has an accordion shape, the outer circumferential side thereof can be flexibly deformed about the inner circumferential side thereof in the axial direction. Therefore, even when the fixed end of the annular member  220  moves in the above manner, a positional relationship of the outer circumferential end of the annular member  220  relative to the magnetic pole member  120  (magnetic pole tip member  130 ) is hardly changed (see  FIG. 13C ) This enables the magnetic fluid to be stably held in place. In addition, because the annular member  220  can be expanded and contracted readily in the radial direction by configuring the annular member  220  in an accordion shape, the magnetic fluid can be stably held in place, even upon the great eccentricity. In this case, even when the outer circumferential end of the annular member  220  touches the inner circumferential surface of the permanent magnet  110 , the magnetic fluid  300  can be stably held in place since it can be deformed as to be contracted. Furthermore, when being configured in an accordion shape, the annular member  220  can also hold the magnetic fluid at multiple locations, thus further enhancing the sealing performance. 
     As for an annular member, even when an annular member  220   a  that is configured, not in an accordion shape, but in a multi-stepped shape when viewing a cross-section thereof, for example, as illustrated in  FIG. 13D  is employed, a similar functional effect can be produced. In this case, in an initial state, the inner circumferential side of the annular member  220   a  may be fixed to the outer side of the magnetic pole member  120 , etc. or the inner side thereof. 
     REFERENCE SIGNS LIST 
     
         
           1  magnetic fluid seal 
           100  magnetic circuit forming member 
           110 ,  111 ,  112 ,  113   a ,  114   a ,  115   a ,  116   a  and  117   a  permanent magnet 
           113 ,  114 ,  115 ,  116 ,  117  and  118  magnet 
           120 ,  121 ,  122   a  and  122   b  magnetic pole member 
           121   a  groove 
           122  retention member 
           122   g  annular groove 
           130 ,  131 ,  132  and  133  magnetic pole tip member 
           200  annular member 
           210  sleeve 
           211 ,  212 ,  213 ,  214 ,  215 ,  216 ,  217 ,  218  and  219  annular member 
           217   a  capillary portion 
           217   b  hollow portion 
           218   a  first layer 
           218   b  second layer 
           218   c  third layer 
           219   a  first layer 
           219   b  second layer 
           219   c  third layer 
           220  and  220   a  annular member 
           300  magnetic fluid 
           410 ,  420  and  430  dispersion preventing member 
           450  labyrinth seal forming member 
           500  shaft 
           600  housing 
         M magnetic circuit 
         N magnetic unit