Patent Publication Number: US-2019178288-A1

Title: Rolling bearing unit

Description:
TECHNICAL FIELD 
     The present invention relates to a rolling bearing including, inside thereof, a circulation path for oil so that the rolling bearing is lubricated by oil passing through the circulation path, and particularly a rolling bearing unit capable of preventing foreign objects generated in the bearing due to breakage of e.g., rolling surfaces, i.e., peeled-off metal pieces, from flowing out of the bearing. 
     BACKGROUND ART 
     Rolling bearings are used for moving parts of transportation machinery industrial machines, and other machines and apparatus. Some of such machinery and apparatus include, besides the rolling bearings, which need oil lubrication, operating mechanism portions which also need lubrication, and which are lubricated by the same oil as used to lubricate the rolling bearings. Such operating mechanism portions include meshing portions of gears, and slide contact portions of sliding parts. 
     Some of such machinery include, inside thereof, rolling bearings and operating mechanisms. For example, an oil pump includes rolling bearings and an operating mechanism therein, and is configured to feed lubricating oil in the oil pump to a separate, external operating mechanism. 
     In a lubrication system including such an oil pump, the external operating mechanism is disposed at an intermediate portion of the oil circulation path such that lubricating oil returned from the external operating mechanism through the circulation path is passed through the interior of the rolling bearings in the pump, and fed again to the external operating mechanism. 
     In such a lubrication system, foreign objects generated in the rolling bearings as well as in the internal and external operating mechanisms, such as peeled-off metal pieces and wear dust, mix into the circulating lubricating oil, and flow into operating mechanism portions in the rolling bearings themselves and the external operating mechanism. This results in reduced endurance of the machine due to wedging of foreign objects, and also could results in malfunction, failure or breakage of the machine. 
     Thus, the below-identified Patent Documents 1-3 propose to close one side opening of the bearing space defined between the inner and outer races of the bearing with a seal member (such as a seal ring) with a filter to prevent, with this seal member, entry of foreign objects, such as iron dust, that have mixed into the lubricating oil flowing through the oil circulation path, into the bearing. 
     The below-identified Patent Document 4 proposes a seal member (seal ring) closing an end of the rolling bearing space (space between the inner and outer races), and including a filter for catching foreign objects. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: JP2628526B 
         Patent Document 2: JP2002-250354A 
         Patent Document 3: JP2011-256895A 
         Patent Document 4: JP5600555B 
       
    
     SUMMARY OF THE INVENTION 
     Object of the Invention 
     It is not preferable that foreign objects generated in the rolling bearing enter an operating mechanism disposed at an intermediate portion of the oil circulation path. 
     Among such foreign objects, large peeled-off pieces generated in a rolling bearing unit for an oil pump tend to cause especially significant damage to the operating mechanism portions of the oil pump itself as well as parts of external operating mechanisms, which could result in malfunction, failure or breakage of these machines. 
     Patent Documents 1-4 offers a partial solution to this problem because the seal member with the filter is disposed at the outlet of a portion of the lubricating oil circulation path in the bearing (side opening of the bearing space through which lubricating oil leaves the bearing), so that the filter of the seal member can filter out foreign objects generated in the bearing. 
     However, in Patent Documents 1-4, since the seal member includes a support frame formed with window holes used as lubricating oil passages, and the window holes are closed with the filter, the area of the filter where foreign objects are caught is small, so that the filter easily becomes clogged. 
       FIG. 2  of Patent Document 2 shows a structure in which substantially the entire area of a side opening between the inner and outer races is covered with an outer filter and an inner filter. With this structure, however, it is difficult to stably hold the filters in position because the filters receive flow pressure of the lubricating oil. 
     It would be possible to reduce clogging of the filter(s) by providing, between the seal member and one of the inner and outer rings of the bearing, a gap through which lubricating oil can leave the bearing without passing through the filter(s). 
     However, irrespective of whether such a gap is provided between the bearing outer race and the (annular) support frame of the seal member or between the bearing outer race and the annular support frame, such a gap would inevitably expand to some extent due e.g., to a manufacturing error of the seal member and/or a difference in thermal expansion between the seal member and the bearing. 
     This could result in foreign objects leaking out through such a gap, i.e., the gap between seal member and one of the inner and outer bearing races. 
     An object of the present invention is to keep relatively large objects, such as peeled-off pieces, that have been generated in the rolling bearing, in the bearing space between the inner and outer races, and thus to prevent them from flowing out of the bearing. 
     Means for Achieving the Object 
     In order to achieve this object, according to the present invention, a conventional rolling bearing unit, i.e., a rolling bearing unit comprising an inner race supporting a rotary shaft; an outer race fixed to a housing, the inner race and the outer race defining a bearing space therebetween, the bearing space having an opening at one axial end thereof; rolling elements disposed in the bearing space; and a seal member attached to one axial end of the outer race at the one axial end of the bearing space so as to cover the opening of the bearing space, wherein the bearing space defines a circulation path for lubricating oil, the circulation path having an outlet at a position where there is the seal member, is configured as follows: 
     That is, the seal member includes a circular annular support frame having a plurality of window holes, and a filter having a predetermined mesh size, the filter being fixedly joined to the support frame, or the support frame and the filter are formed by integral molding such that the window holes are closed by the filter, wherein the support frame has an inner diameter determined such that a passage through which lubricating oil can pass is defined between the support frame and a radially outer surface of the inner race, and wherein the filter includes a protruding portion protruding radially inwardly beyond a radially inner surface of the support frame such that a radially inner edge of the protruding portion is in contact with the inner race, or such that the protruding portion surrounds the radially outer surface of the inner race through a gap defined therebetween and smaller than the mesh size of the filter. 
     The mesh size of the filter is preferably 0.2 mm or more and 0.5 mm or less, and the filter is preferably made of a resin such as a polyamide resin. 
     Preferably, the rolling bearing unit according to the present invention further comprises an additional foreign object catching arrangement other than the filter. 
     The additional foreign object catching arrangement may comprise a permanent magnet attached to the seal member at the outlet of the bearing space as the circulation path for lubricating oil, or may comprise a labyrinth disposed at the outlet of the circulation path, and having a bent portion. 
     If the permanent magnet is used, the permanent magnet may be fixedly embedded in the support frame, and the support frame may have dust-collecting recesses surrounding the permanent magnet, and configured to receive foreign objects therein. 
     Each dust-collecting recess may be shaped so as to gradually narrow from its opening at the surface of the support frame toward its bottom. If the permanent magnet is cylindrical, each dust-collecting recess may have an inner surface including a circular arc portion extending along the cylindrical outer surface of the permanent magnet. 
     Further preferably, the support frame of the seal member has foreign object guiding grooves each extending from the radially inner portion to an area inward of the window holes and configured to receive foreign objects generated in the bearing, or the rolling bearing unit may further comprise dust-collecting pockets disposed inward of the respective window holes, and configured to receive foreign objects that have moved through the respective foreign object guiding grooves. 
     The labyrinth disposed at the outlet of the circulation path for lubricating oil preferably narrows gradually from its inlet toward its outlet. 
     If the labyrinth narrows gradually from its inlet toward its outlet, and the permanent magnet is used, the permanent magnet is disposed at a position where its magnetic field reaches large portions of the labyrinth including its inlet. 
     Advantages of the Invention 
     According to the present invention, by using the seal member having the above-described structure to close the opening of the bearing space between the inner and outer races at one end thereof, it is possible to define, between the inner race and the radially inner surface of the support frame of the seal member, a passage through which lubricating oil can smoothly flow out of the bearing unit. 
     Since this passage is closed by the protruding portion of the filter which protrudes radially inwardly beyond the radially inner surface of the support frame of the seal member such that lubricating oil can pass through this passage, compared with filters of conventional seal members, this filter has a large area where foreign objects are caught, so that this filter is less likely to become clogged with foreign objects. 
     Since the outlet of the circulation path for lubricating oil is closed by the seal member with no gap defined that is larger than the mesh size of the filter, it is possible to reliably prevent foreign objects generated in the bearing, such as peeled-off pieces, from flowing out of the bearing unit through a gap between the seal member and the inner race. 
     Since the support frame of the seal member is capable of retaining shape, the seal member can be stably supported by the bearing outer race or by a housing supporting the outer race. 
     By using the additional foreign object catching arrangement other than the filter, foreign objects generated in the bearing are partially caught by the additional foreign object catching arrangement, so that the filter is further less likely to be clogged with foreign objects. 
     By using the additional foreign object catching arrangement comprising a permanent magnet, peeled-off pieces of magnetic material are attracted toward and gathered around the permanent magnet so as not to be escapable therefrom. 
     By using the additional foreign object catching arrangement comprising the labyrinth, foreign objects such as peeled-off pieces get caught or stuck in the labyrinth, so that they are less likely to flow out of the bearing space. By using both the labyrinth and the permanent magnet, it is possible to more reliably prevent the escape of foreign objects caught. 
     The filter is preferably made of a resin because a resin filter never damages the bearing inner race when it touches the inner race, so that it is possible to completely eliminate a gap between the seal member and the inner race, which in turn makes it possible to reduce the distance between the outer periphery of the inner race and the inner edge of the filter (i.e., the dimension of the gap therebetween), to a value smaller than the mesh size of the filter. 
     The reason why the mesh size of the filter is preferably 0.2 mm or more and 0.5 mm or less is described later. 
     The following reasons are also described later: the reason why the support frame of the seal member preferably has the guiding grooves and/or the dust-collecting pockets, the reason why the labyrinth preferably narrows gradually from its inlet toward its outlet; and the reason why, if the labyrinth narrowing gradually from its inlet toward its outlet is used in combination with the permanent magnet, the permanent magnet is preferably disposed at a position where its magnetic field reaches large portions of the labyrinth including its inlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an oil pump using a bearing unit according to the present invention. 
         FIG. 2  is a sectional view taken along line X-X of  FIG. 1 , and showing a portion of the oil pump of  FIG. 1 . 
         FIG. 3  is a front view of an exemplary seal member of the rolling bearing unit according to the present invention. 
         FIG. 4  is a sectional view of a portion of the rolling bearing unit with the seal member of  FIG. 2  attached thereto. 
         FIG. 5  is a front view of a different seal member to be attached to rolling bearing unit according to the present invention. 
         FIG. 6  is a sectional view of a portion of the rolling bearing unit with the seal member of  FIG. 6  attached thereto. 
         FIG. 7  is a sectional view of a portion of the rolling bearing unit with a seal member attached thereto, the seal member being different from the seal member of  FIG. 6  in that permanent magnets are disposed at different locations. 
         FIG. 8  is a front view of still another seal member to be attached to the rolling bearing unit according to the present invention. 
         FIG. 9  is a front view of yet another seal member to be attached to the rolling bearing unit according to the present invention. 
         FIG. 10  is a sectional view of a portion of the rolling bearing unit with the seal member of  FIG. 9  attached thereto. 
         FIG. 11  is a front view of a still different seal member to be attached to the rolling bearing unit according to the present invention. 
         FIG. 12( a )  is an enlarged front view of a portion of  FIG. 11 ; and  FIG. 12( b )  is a sectional view of  FIG. 12( a ) . 
         FIG. 13  is a sectional view of a modification of  FIG. 11 . 
         FIG. 14  is a sectional view of another rolling bearing unit according to the present invention, which includes a labyrinth. 
         FIG. 15  is an enlarged sectional view taken along line Y-Y of  FIG. 14 . 
         FIG. 16  is a sectional view of a portion of a rolling bearing unit to which the seal member of  FIG. 2  is attached such that the seal member defines a labyrinth for preventing foreign objects from moving in a straight line. 
         FIG. 17  is a sectional view of a portion of a rolling bearing unit to which the seal member of  FIG. 7  is attached such that the seal member defines a labyrinth for preventing foreign objects from moving in a straight line. 
         FIG. 18  is an enlarged sectional view of a portion of an inner ring of a support frame of a seal member, the inner ring being formed with grooves on its radially outer surface, instead of forming grooves on the radially inner surface of an outer annular portion of a non-linear-path-defining ring as shown in  FIG. 14 . 
         FIG. 19  is a sectional view of a portion of another rolling bearing unit according to the present invention to which the seal member of  FIG. 7  is attached such that the seal member defines a labyrinth for preventing foreign objects from moving in a straight line. 
     
    
    
     EMBODIMENTS 
     Now referring to  FIGS. 1-19 , a bearing unit embodying the present invention, as used in an oil pump, is described. 
     The oil pump is designated by numeral  10  in  FIG. 1 , and includes, inside thereof, the bearing unit  20 , and an operating mechanism  30  including a pump rotor (not shown) that sucks, compresses, and discharge oil. 
     The bearing unit  20  includes three rolling bearings  21 ,  22  and  23  that are juxtaposed to each other in a housing  11 , and lubricated by oil. 
     The rolling bearings  21 ,  22  and  23  support a rotary shaft  12  of the oil pump, and the rotary shaft  12  drive the pump rotor of the operating mechanism  30  so that the pump rotor sucks, compresses, and discharges oil. 
     The rolling bearing  21 ,  22  and  23  are known bearings each including an inner (bearing) race  1  having a raceway  1   a,  an outer (bearing) race  2  having a raceway  2   a,  and rolling elements (tapered rollers in the example shown)  3  disposed between the raceways  1   a  and  2   a  of the inner and outer races. The rolling elements  3  are retained by a retainer  4  so as to be circumferentially equidistantly spaced apart from each other. 
     The outer races  2  of the respective rolling bearings  21 ,  22  and  23  are press-fitted in the radially inner surface of the housing  11  so as to be non-rotatable. 
     The inner races  1  of the respective rolling bearings  21 ,  22  and  23  are fixed to the outer periphery of the rotary shaft  12  so as to be non-rotatable relative to the rotary shaft  12 . 
     The rolling elements  3  of the rolling bearings  21 ,  22  and  23  may be spherical or cylindrical rolling elements. The number of rolling bearings of the bearing unit  20  is not limited. Spacers  5 ,  6  and  7  shown in  FIG. 1  maintain the positional relationships between the rolling bearings  21 ,  22  and  23 . 
     Lubricating oil compressed in and discharged from the pump rotor passes through a circulation path  13  in the oil pump  10 . 
     A hole  13   a  in the rotary shaft  12  along its center axis forms part of the circulation path  13 . Lubricating oil that has passed through the hole  13   a  passes through the bearing space between the inner and outer races  1  and  2  of the rolling bearing  22 , through the bearing space between the inner and outer races  1  and  2  of the rolling bearing  21 , and through a discharge passage  13   b  in the housing  11 , and flows into an operating mechanism  50  disposed outside the pump. 
     From the operating mechanism  50 , lubricating oil flows through a return passage  13   c  in the housing  11  into the operating mechanism  30  inside the oil pump, where the lubricating oil is sucked by the pump rotor and discharged back into the circulation path  13 . 
     In the case of the oil pump  10  shown, if peeling occurs on the raceways  1   a  or  2   a  of the rolling bearing  21  or  22 , or on the rolling surface of any rolling element  3 , peeled-off pieces could mix into the oil flowing through the circulation path  13 , and flow toward the operating mechanism  50 . 
     The bearing unit  20  includes a seal member  40  attached to the rolling bearing  21 , which is located at the downstream end, in the oil flow direction, of the portion of the circulation path  13  in the bearing unit  20 , at one of the two open ends of the rolling bearing  21 , i.e., at the side opening D of the bearing space of the rolling bearing  21  through which lubricating oil leaves the rolling bearing  21 . 
     Referring to  FIGS. 2-4 , the seal member  40  includes a circular annular support frame  41  having window holes  42 , and a filter  43  having a predetermined mesh size and fixedly joined to the support frame  41  to close the window holes  42 . The filter  43  and support frame  41  may be formed by integral molding, too. 
     In the example shown, the support frame  41  includes a cylindrical portion  41   a;  an end wall  41   b  integrally connected to one end of the inner periphery of the cylindrical portion  41   a  and having the window holes  42 ; and an inner ring  41   c  integrally connected to the inner edge of the end wall  41   b  to extend toward the other end of the cylindrical portion  41   a.  The support frame  41  is fixed in position by e.g., press-fitting the cylindrical portion  41   a  into a hole of the housing  11 , or by coupling the cylindrical portion  41   a  to the outer race  2  of the rolling bearing  21  with a coupling member (now shown). 
     The window holes  42  of the support frame  41  are circumferentially spaced apart from each other, and closed by the filter  43 , through which oil can pass. 
     In the example shown, the support frame  41  of the seal member  40  is made of a fiber-reinforced polyamide resin, while the filter  43  is a polyamide resin mesh filter. While the materials of the support frame  41  and the filter  43  are not particularly limited, for lower cost and lightness in weight, resins that are resistant to oil and ensure necessary strength are preferable. 
     Resins that meet these requirements include super-engineering plastics such as polysulfone (PSF), polyethersulfone (PES), polyphenylene sulfide (PPS), polyarylate (PAR), polyamide imide (PAI), polyether imide (PEI), polyetheretherketone (PEEK), liquid crystal polymer (LCP), thermoplastic polyimide (TPI), polybenzimidazole (PBI), polymethyl-pentene (TPX), poly 1,4-cyclohexane dimethylene terephthalate (PCT), polyamide 46 (PA46), polyamide 6T (PA6T), polyamide 9T (PA9T), polyamide 11, 12 (PA11, 12), polytetrafluoroethylene (PTFE), and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-ethylene copolymer (ETFE). 
     Among them, self-lubricating synthetic resins, such as polyetheretherketone (PEEK) and polyphenylene sulfide (PPS), are high in oil resistance and can be used in hostile environments such as where there is a refrigerant (by adding, if necessary, a filler such as carbon fiber (CF) or glass fiber (GF)). 
     Among such self-lubricating synthetic resins, polyamide resins (PA: PA66 and PA46) are used widely in industrial machines (by adding, if necessary, a filler such as carbon fiber (CF) or glass fiber (GF)), because they are easily injection moldable and are inexpensive. 
     The inner ring  41   c  of the support frame  41  of the seal member  40  has an inner diameter determined such that a passage  9  (see  FIG. 4 ) through which oil can flow is defined between the inner ring  41   b  and the inner race  1  of the rolling bearing  21 . 
     The filter  43  has a protruding portion  43   a  protruding radially inwardly beyond the radially inner surface  41   d  of the inner ring  41   c.  In the example shown, the radially inner edge of the protruding portion  43   a  is in contact with the inner race  1  of the rolling bearing  21 , but the protruding portion  43   a  may be sized such that it does not contact the inner race  1  of the rolling bearing  21 , and instead, there is a gap therebetween that is smaller than the mesh size of the filter  43 . However, the protruding portion  43   a  preferably has a sufficiently large radial dimension such that, even when the filter  43  is thermally expanded, no gap will form between the protruding portion  43   a  and the inner race  1 , or even if such a gap forms, it will not increase excessively. 
     The protruding portion  43   a  of the filter  43  may be a separate part from the portion of the filter  43  closing the window holes  42 . However, the seal member  40  can be more easily manufactured by attaching, as the filter  43 , a one-piece annular filter material having an outer diameter larger than the diameter of the circle passing through the radially outer peripheries of the window holes  42 , to the support frame  41  by molding such that the radially inner portion of the one-piece annular filter material protrudes radially inwardly beyond the radially inner surface of the support frame  41  to provide the protruding portion  43   a.  Such a protruding portion  43   a  can be more stably held in position too. 
     The mesh size of the filter  43  is preferably 0.2 mm or more and 0.5 mm or less. If the mesh size is less than 0.2 mm, the filter  43  is more likely to become clogged. 
     If the mesh size of the filter  43  is larger than 0.5 mm, foreign objects produced in the bearing unit and larger than 0.5 mm can flow out of the bearing unit. 
     It has been confirmed by experiments that foreign objects larger than 0.5 mm leave impressions larger than 1 mm on rolling surfaces and slide contact surfaces if such foreign objects become wedged into these surfaces, and such large impressions quickly shorten the lives of the affected devices. 
     By choosing a mesh size of 0.5 mm or less, foreign objects larger than 0.5 mm can be filtered out, so that it is possible to prevent such large foreign objects from shortening the lives of devices. 
     In order to effectively prevent foreign objects generated in the bearing unit, such as peeled-off pieces, from flowing out of the bearing unit, it is also important to prevent the foreign objects caught by the filter from leaving the filter. For that purpose, as shown in  FIGS. 5-9 , permanent magnets  44  may be attached to the seal member  40  as an additional foreign object catching arrangement other than the filter  43 . 
     In each of the embodiments of  FIGS. 5 and 6  and  FIGS. 8 and 9 , the permanent magnets  44  are embedded in the radially inner surface of the inner ring  41   c  of the support frame  41  of the seal member  40  at locations upstream of the protruding portion  43   a  of the filter  43  (i.e., within the bearing space) so as to be opposed to the passage  9  defined between the support frame  41  and the inner race  1  of the rolling bearing  21 . 
     In the embodiment of  FIG. 7 , the permanent magnets  44  are attached to the inner surfaces of ribs  41   e  defining the window holes  42  in the end wall  41   b  of the support frame  41  of the seal member  40 . 
     The permanent magnets  44  attract foreign objects of magnetic material, thereby stopping the flow of foreign objects in the oil flow in the circulation path  13 . This makes it more difficult for foreign objects to flow out of the bearing unit. Also, by attracting foreign objects, the permanent magnets  44  more effectively reduce the possibility of clogging of the filter  43 . 
     In order to more effectively prevent foreign objects from flowing out of the bearing unit, the seal member  40  may include foreign object guiding grooves  46  shown in  FIGS. 8 and 9 , and/or dust-collecting pockets  47  shown in  FIGS. 9 and 10 . 
     The foreign object guiding grooves  46  shown in  FIGS. 8 and 9  are formed in the inner surface of the inner ring  41   c  of the support frame of the seal member to extend (preferably obliquely to the oil flow direction) from the passage  9  to respective ones of the window holes  42 . 
     By the provision of the foreign object guiding grooves  46 , foreign objects Fm that have flowed into the passage  9  without being attracted by the permanent magnets  44  due to the permanent magnets  44  being unable to attract all of foreign objects can be moved back into the window holes  42  together with the oil flow (in the direction of arrows in  FIG. 8 ) by centrifugal force, so that foreign objects are less likely to flow back into the passage  9 . 
     The dust-collecting pockets  47  shown in  FIGS. 9 and 10  are disposed inward of the window holes  42 , and collect foreign objects Fm that have flowed into the window holes  42 . This more effectively reduces the possibility of foreign objects Fm flowing back into the passage  9  and out of the bearing unit through the radially inner edge of the filter  43 . 
     By collecting foreign objects, the dust-collecting pockets  47  prevent scattering of foreign objects, thereby reducing the possibility of the surfaces of the portions of the filter  43  covering the window holes  42  becoming clogged with scattered foreign objects. 
     In the embodiment of  FIG. 11 , the permanent magnets  44  are fixedly embedded in the support frame  41  of the seal member  40 , at locations close to the radially inner edge of the support frame  41 . In the specific example shown, the permanent magnets  44  are fixed to the inner ring  41   c  of the support frame  41 . While, in the example shown, the permanent magnets  44  are exposed to the axial end surface of the inner ring  41   c,  the permanent magnets  44  may be completely embedded in the inner ring  41   c  so as not to be exposed. Also, the permanent magnets  44  may be fixed to portions of the seal member  40  other than the inner ring  41   c.    
     The support frame  41  includes, around, i.e., on both sides of, each permanent magnet  44 , pocket-shaped dust-collecting recesses  51  for collecting foreign objects Fm. The permanent magnets  44  and the dust-collecting recesses  51  constitute an additional foreign object catching arrangement of the seal member  40  other than the filter. 
     By the provision of the pocket-shaped dust-collecting recesses  51  on both sides of the respective permanent magnets  44 , foreign objects Fm magnetically attracted to the permanent magnets  44  are retained in the dust-collecting recesses  51  so as not to flow out of the bearing unit. 
     In this embodiment, the permanent magnets  44  are cylindrical member having cylindrical surfaces  44   a  on the outer peripheries thereof. The inner surface of each dust-collecting portion  51  has, as shown in.  FIG. 12( a ) , a circular arc portion  51   a  extending along the cylindrical outer surface  44   a  of the corresponding permanent magnet  44 . This causes substantially equal magnetic forces to be generated in the space of the dust-collecting recess  51  along the circular arc portion  51   a,  so that foreign objects Fm can be caught over the entire dust-collecting recess  51 . 
     The seal member  40  is formed by injecting molten resin into a mold with the permanent magnets  44  disposed in the mold so that the permanent magnets  44  are integral with the seal member  40 . It is usually difficult to prevent the permanent magnets  44  from being moved in the mold by the injected molten resin from the predetermined positions, by the time the molten resin hardens. However, since the mold used to form the seal member  40  of the embodiment of  FIGS. 11, 12 ( a ) and  12 ( b ) includes protrusions for forming the dust-collecting recesses  51 , such protrusions serve to keep the permanent magnets  44  at the predetermined positions. The effect of preventing movements of the permanent magnets  44  is further strengthened by the fact that the permanent magnets  44  are cylindrical, and the inner surface of each dust-collecting recess  51  has a circular arc portion  51   a  extending along the cylindrical outer surface  44   a.    
       FIG. 12( b )  shows how foreign objects Fm are magnetically attracted to one of the permanent magnets  44 . If the flow of lubricating oil, shown by arrow in  FIG. 12( b ) , is fast, without the dust-collecting recesses  51 , the foreign objected Fm attracted to the permanent magnet  44  could flow out of the bearing unit. However, by the provision of the dust-collecting recesses  51  adjacent to the permanent magnets  44 , even if foreign objects Fm separate from the permanent magnets  44 , such foreign objects Fm enter the dust-collecting recesses  51 . Once in the dust-collecting recesses  51 , foreign objects are not affected by the flow of lubricating oil, so that they can be retained in the dust-collecting recesses  51 . Thus, it is possible to prevent foreign objects Fm caught by the permanent magnets  44  from flowing out of the bearing unit. The dust-collecting recesses  51  are preferably positioned where foreign objects Fm can be magnetically attracted to the respective permanent magnets  44 , but even if they are positioned outside the influence of any permanent magnet  44  (i.e., where foreign objects Fm cannot be magnetically attracted to any permanent magnet  44 ), foreign objects Fm can still be caught in such dust-collecting recesses  51 . 
     As an alternative, as shown in  FIG. 13 , each dust-collecting recess  51  may be shaped so as to gradually narrow from its opening at the surface of the support frame  41  toward its bottom. In the particular example shown in  FIG. 13 , the portion of the inner surface of each dust-collecting recess  51  opposite from the circular arc portion  51   a  comprises an inclined portion  51   a  inclined so as to gradually approaches the circular arc portion  51   a  from its opening toward its bottom. 
     By shaping each dust-collecting recess  51  such that its opening is wider than its bottom, a larger amount of foreign objects Fm can be caught in the dust-collecting recess  51 . The larger the amount of foreign objects Fm caught in the dust-collecting recesses  51 , the less likely foreign objects Fm are to close the openings of the dust-collecting recesses  51 . 
     In order that each dust-collecting recess  51  narrows gradually from its opening toward its bottom, the portion of its inner surface remote from the permanent magnet  44  may comprise the inclined portion  51   b  as shown in  FIG. 13 , or the portion of its inner surface close to the permanent magnet  44  may be inclined. Also, a portion of its inner surface other than the above two portions may be inclined. 
     The seal member  40  of  FIGS. 16 and 17  includes an additional foreign object catching arrangement other than the filter that comprises a portion of the circulation path  13 . 
     In particular, such an additional foreign object catching arrangement other than the filter comprises a labyrinth  45  at the outlet of the portion of the circulation path  13  defined by the bearing space of the rolling bearing  21 . 
     The labyrinth  45  is defined by a non-linear-path-defining ring  48  having a “ ” -shaped cross-section, and an anti-separation ring  49  engaging the non-linear-path-defining ring  49 . 
     The non-linear-path-defining ring  48  includes an inner annular portion  48   a,  an outer annular portion  48   b,  and an end wall  48   c  integrally connected to one end of each annular portion. The non-linear-path-defining ring  48  is disposed inwardly of the seal member  40  (i.e., within the bearing space) while being properly spaced from the seal member  40  to define the labyrinth  45  between the non-linear-path-defining ring  48  and the support frame  41  of the seal member  40 . 
     The non-linear-path-defining ring  48  is fitted on the inner race  1  of the bearing with other ends of the inner and outer annular portions  48   a  and  48   b  directed outwardly, and the free end of the inner ring  41   c  of the seal member  40  is inserted between the inner and outer annular portions  48   a  and  48   b  of the non-linear-path-defining ring  48 . 
     The labyrinth  45  therefore extends downward, inward, downward, and then outward, so that foreign objects Fm, such as peeled-off pieces, that have flowed into the labyrinth  45  get caught or stuck at bent corners of the labyrinth  45 , and cannot easily flow out of the bearing space. 
     In this embodiment, a large number of passage grooves  48   d  are defined between the inner annular portion  48   a  of the non-linear-path-defining ring  48 , which has the outer annular portion  48   b,  and the inner ring  41   c  of the support frame of the seal member so that even if, as shown in  FIG. 14 , the inner ring  41   c  is thermally expanded until the gap between the inner annular portion  48   a  and the inner ring  41   c  of the support frame of the seal member disappears, a flow path remains therebetween. Otherwise, the inner annular portion  48   a  may be fitted to the outer periphery of inner ring  41   c  such that they contact with each other from the beginning. 
     The passage grooves  48   d  may comprise, as shown in  FIG. 15 , axial grooves formed in the radially inner surface of the inner annular portion  48   a,  or, as shown in  FIG. 18 , axial grooves formed in the radially outer surface of the inner ring  41   c  of the support frame of the seal member. 
     While the shapes of the passage grooves  48   d  are not particularly limited, if they are sized to be equivalent to the mesh size of the filter  43 , foreign objects can be filtered at the inlets of the passage grooves. 
     The non-linear-path-defining ring  48  may be a ring having an L-shaped cross-section formed by an inner annular portion  48   a  and an end wall  48   c  shown in  FIG. 19 . 
     The shape of the labyrinth  45  may be determined su h that its flow path size gradually decreases from its inlet toward its outlet so that foreign objects that have entered the labyrinth can hardly pass therethrough, and thus to further effectively catching foreign objects. 
     The seal member  40  may include both such a labyrinth  45 , i.e., a labyrinth having a flow path size that gradually decreases from its inlet toward its outlet, and the above-described permanent magnets  44 . In that case, as shown in  FIG. 12 , the permanent magnets  44  may be arranged near the inlet of the labyrinth  45 , or at positions where their magnetic fields reach large portions of the labyrinth  45  so that foreign objects can be caught at a plurality of locations, and thus to more effectively prevent foreign objects from flowing out of the bearing unit. 
     The anti-separation ring  49  is attached to the outer periphery of the inner race  1  of the bearing by e.g., press-fitting, and prevents separation of the non-linear-path-defining ring  48  from the inner race  1 . 
     DESCRIPTION OF THE NUMERALS 
     
         
           1 . Inner race 
           1   a.  Raceway 
           2 . Outer race 
           2   a.  Raceway 
           3 . Rolling element 
           4 . Retainer 
           5 ,  6 ,  7 . Spacer 
           8 . Presser member 
           9 . Passage 
           10 . Oil pump 
           11 . Housing 
           12 . Rotary shaft 
           13 . Circulation path for lubricating oil 
           13   a.  Hole 
           13   b.  Discharge passage 
           13   c.  Return passage 
           20 . Bearing unit 
           21 ,  22 ,  23 . Rolling bearing 
           30 . Operating mechanism 
           40 . Seal member 
           41 . Support frame 
           41   a.  Cylindrical portion 
           41   b.  End wall 
           41   c.  Inner ring 
           41   d.  Radially inner surface 
           41   e.  Rib 
           42 . Window hole 
           43 . Filter 
           43   a.  Radially inwardly protruding portion 
           44 . Permanent magnet 
           44   a.  Cylindrical outer surface 
           45 . Labyrinth 
           46 . Foreign object guiding groove 
           47 . Dust-collecting pocket 
           48 . Non-linear-path-defining ring 
           48   a.  Inner annular portion 
           48   b.  Outer annular portion 
           48   c.  End wall 
           48   d.  Passage groove 
           49 . Anti-separation ring 
           50 . Operating mechanism 
           51 . Dust-collecting recess 
           51   a.  Circular arc portion 
           51   b.  Inclined portion 
         Fm. Foreign objects