Abstract:
A shaft seal device is provided which is capable of sealing a rotary shaft in a positive and reliable manner without requiring a periodical lubricant-supplying operation, and is free from contamination of its environment, products, etc. by splashing of excess lubricant leaking from the oil seal lip. An annular sealing channel is formed in at least one of the opposite ends of a housing of a bearing unit, in which is accommodated an annular lubricant-bearing polymer member fitted on the rotary shaft. The polymer member has an inside diameter substantially equal to an outer diameter of the rotary shaft at an initial stage of rotation of the rotary shaft. The polymer member has at least part of opposite end surfaces or outer peripheral portion thereof disposed substantially in contact with opposite lateral wall surfaces of the sealing channel.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a shaft seal device, and more particularly to a shaft seal device for use in a bearing unit such as a plummer block used in iron and steel equipment and the like. 
     2. Prior Art 
     A conventional shaft seal device of a bearing unit used in iron and steel equipment and the like, such as an oil seal, requires applying lubricant to sliding parts (lips) thereof. To this end, in steel works, usually an operation of supplying lubricant to lips of oil seals is periodically carried out, which costs time and labor. Further, the conventional shaft seal device has a problem that excess lubricant leaking from lips of oil seals splashes with rotation of the rotary shaft to contaminate or stain its environment, rolled plates produced, etc. To solve this problem, a shaft seal device as shown in FIG. 13 has been proposed by Japanese Laid-Open Utility Model Publication (Kokai) No. 6-32837. 
     The proposed shaft seal device  101  is comprised of a pair of core metal members  102  and  103  with fitting collars  109  and  110  integrally fitted one upon another, and seal members  104  and  105  mounted on inner peripheral edges of main bodies  107  and  108  of the core metal members  102 ,  103 . The seal members  104 ,  105  have flexible lips which are suitably deflected and urged against an outer peripheral surface of a rotary shaft  111  to be sealed, in operation. A clip  115  is fitted on an outer peripheral surface of the lip of the seal member  105  so as to obtain a sufficient urging force of the lip. A lubricant-bearing polymer which is so-called “plastic grease”  106 ″ is filled in a gap between the opposed main bodies  107 ,  108  of the core metal members  102 ,  103 . 
     Lubricant continuously exudes or oozes from the lubricant-bearing polymer contained in the oil seal is led to the lips of the seal members  104 ,  105  to lubricate the same. This can dispense with the periodical operation of supplying lubricant to the lips. Besides, the amount of lubricant exuding from the polymer is so small as to solve the above-mentioned problem of environmental contamination. 
     According to the conventional arrangement, however, since the exudation amount of lubricant is very small, a sufficient amount of lubricant cannot reach the lips of the seal members  104 ,  105 . Further, even if the exudation amount of lubricant is sufficient, the lubricant flows along the inner peripheral portion of the lubricant-bearing polymer and stays at the lowermost inner peripheral portion of the polymer but does not reach the lips. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a shaft seal device which is capable of sealing a rotary shaft in a positive and reliable manner without requiring a periodical lubricant-supplying operation, and is free from contamination of its environment, products, etc. by splashing of excess lubricant leaking from the oil seal lip. 
     To attain the above object, the present invention provides a shaft seal device for a bearing unit having a bearing fitted on a rotary shaft, and a housing accommodating the rotary shaft extending therethrough, the housing having opposite ends, comprising: 
     an annular sealing channel formed in at least one of the opposite ends of the housing, the annular sealing channel having opposite lateral wall surfaces; and 
     an annular lubricant-bearing polymer member accommodated in the annular sealing channel and fitted on the rotary shaft, the polymer member having opposite end surfaces, and an outer peripheral portion; 
     wherein the polymer member has an inside diameter substantially equal to an outer diameter of the rotary shaft at an initial stage of rotation of the rotary shaft; and 
     wherein at least part of the opposite end surfaces or the outer peripheral portion of the polymer member is disposed substantially in contact with the opposite lateral wall surfaces of the annular sealing channel. 
     With the above arrangement, at an initial stage of rotation of the rotary shaft, the inside diameter of the annular lubricant-bearing polymer member is substantially equal to the outer diameter of the rotary shaft so that the polymer member closely contacts the rotary shaft at its inner peripheral surface or is rotated by the rotating rotary shaft while sliding thereon, while at the same time at least part of the opposite end surfaces or the outer peripheral portion of the polymer member substantially contacts the opposite lateral wall surfaces of the sealing channel. As a result, the surfaces of the polymer member in contact with its counterpart function as sealing surfaces to prevent dust and splashed water droplets from entering the bearing unit from the outside of the housing. 
     The lubricant-bearing polymer member has a function of exuding lubricant from its surfaces and has a nature that its inside diameter and outside diameter become reduced depending upon the exudation amount of lubricant. Therefore, when the inside diameter of the polymer member which initially closely contacts the rotary shaft or slides thereon becomes smaller than the outside diameter of the rotary shaft with exudation of the lubricant from the polymer member as the rotation of the rotary shaft continues, the polymer member becomes tightly fitted on the rotary shaft, whereby the former rotates together with the latter. Thereafter, the rotation of the polymer member is maintained with at least part of the opposite end surfaces or the outer peripheral portion of the polymer member in sliding contact with the opposite lateral wall surfaces of the sealing channel, or the polymer member rotates with delay relative to the rotation of the rotary shaft due to the sliding resistance of the sliding surfaces between the polymer member and the sealing channel. As a result, the surfaces of the polymer member in contact with its counterpart function as sealing surfaces to prevent dust and splashed water droplets from entering the bearing unit from the outside of the housing, as is the case with the initial stage of rotation of the rotary shaft. 
     In other words, the shaft seal device assumes an initial state in which the inside diameter of the lubricant-bearing polymer member is substantially equal to the outside diameter of the rotary shaft, and a state in which the inside diameter of the polymer member is smaller than the outside diameter of the rotary shaft such that the polymer member is tightly fitted on the rotary shaft. Between the two states, however, no change occurs in the position of the sealing surfaces, though the state of the sealing surfaces changes, and in the both states, it is possible to dust and splashed water droplets from entering the bearing unit. 
     Further, since the lubricant continuously exudes from the sliding surfaces of the lubricant-bearing polymer member to well lubricate the sliding surfaces, which prevents wear of the polymer member due to the friction between the polymer member and its counterpart as well as formation of a gap between the two members, whereby dust and splashed water droplets can be prevented from entering the bearing unit. The lubricant can exude from the polymer member over a long time, making it unnecessary to periodically supply lubricant. Besides, the exudation amount of lubricant is very small but equal to the minimum amount required for lubrication such that the exuded lubricant will not be splashed with rotation of the rotary shaft, and therefore the environment and the products cannot be contaminated by the lubricant. 
     Preferably, the shaft seal device according to the present invention includes an annular covering member formed of an elastic material with an inner peripheral channel formed therein. The annular covering member is fitted in the annular sealing channel in an elastically compressed fashion, and the lubricant-bearing polymer member is accommodated in the inner peripheral channel of the covering member. With this arrangement, the lubricant-bearing polymer member will not directly slide on the associated end portion of the housing but slides on the covering member. As a result, the possibility of wearing of the polymer member is reduced. 
     Preferred forms of the present invention are as follows: 
     1. At least one of the opposite lateral wall surfaces of the annular sealing channel is tapered, and at least one of opposite outer end surfaces of the covering member is tapered according to the tapered at least one of the opposite lateral walls of the sealing channel. 
     2. The covering member has opposite inner end surfaces, at least one of which is roughened. 
     3. The covering member has opposite end walls, at least one of which has at least one through hole formed therein. 
     4. The opposite end surfaces of the lubricant-bearing polymer member are spaced from respective associated ones of the opposite inner end surfaces of the covering member with a gap formed therebetween, the gap extending along one of the opposite end surfaces of the polymer member, an outer peripheral surface thereof, and the other end surface thereof so as to present a form of a labyrinth, and lubricant exuding from the polymer member is filled in the gap. 
     5. A plurality of circumferentially extending protuberances are formed on at least one of the opposite end surfaces of the lubricant-bearing polymer member and the opposite inner end surfaces of the covering member. 
     6. The outer peripheral portion of the lubricant-bearing polymer member is formed therein with a plurality of circumferentially extending grooves, while an inner peripheral surface of the covering member has formed thereon a plurality of circumferentially extending protuberances at locations corresponding respectively to the circumferentially extending grooves of the lubricant-bearing polymer member, the protuberances being inserted in respective associates ones of the circumferentially extending grooves. In this case, more preferably, the opposite end surfaces of the lubricant-bearing polymer member are spaced from the opposite inner end surfaces of the covering member so as to present a gap in the form of a labyrinth between the two members. 
     7. An inner peripheral portion of the lubricant-bearing polymer member has formed thereon at least one lip disposed for sliding contact with the rotary shaft, a part of the inner peripheral portion other than a part thereof formed with the at least one lip has an inside diameter larger than the outside diameter of the rotary shaft. 
     8. At least one of the outer peripheral portion of the lubricant-bearing polymer member and an inner peripheral portion of the covering member has at least one lip formed thereon, the at least one lip being disposed in elastic contact with an inner peripheral surface of the covering member or the outer peripheral surface of the polymer member. In this case, more preferably, the at least one lip is formed on the outer peripheral portion of the lubricant-bearing polymer member and projects from an associated one of the opposite end surfaces of the polymer member in a longitudinal direction of the rotary shaft. Alternatively, the at least one lip entirely may lie in the range of the thickness of the polymer member such that the end surfaces of the polymer member and the inner end surfaces of the covering member are in contact with each other. 
     9. At least one of at least one of the opposite end surfaces of the lubricant-bearing polymer member and at least one of opposite inner end surfaces of the covering member has at least one lip formed thereon, the at least one lip being each disposed in elastic contact with an associated one of the opposite end surfaces of the polymer member or an associated one of the opposite inner end surfaces of the covering member. 
     10. The outer peripheral portion of the lubricant-bearing polymer member has a plurality of circumferentially extending grooves formed therein, while an inner peripheral surface of the covering member has formed thereon a plurality of circumferentially extending protuberances at locations corresponding respectively to the circumferentially extending grooves of the lubricant-bearing polymer member, the protuberances being inserted in respective associates ones of the circumferentially extending grooves, and further, at least one of at least one of the opposite end surfaces of the lubricant-bearing polymer member and at least one of opposite inner end surfaces of the covering member has at least one lip formed thereon, the at least one lip being each disposed in elastic contact with an associated one of the opposite end surfaces of the polymer member or an associated one of the opposite inner end surfaces of the covering member. 
     11. The outer peripheral portion of the covering member has a circumferentially extending outer peripheral groove formed therein, the covering member having formed therein a plurality of through holes for supply of lubricant circumferentially arranged and extending from the outer peripheral groove to the inner peripheral surface of the covering member, and the housing of the bearing unit has formed therein a port for supply of lubricant upwardly extending from an uppermost position of the outer peripheral groove. 
     A description will be made of the chemical composition of a lubricant-bearing polymer that forms the lubricant-bearing polymer member used in the bearing unit employing the shaft seal device according to the present invention hereinbelow. 
     The lubricant-bearing polymer consists essentially of a polyolefine resin, and a lubricant such as a paraffin mineral oil and grease. The polymer may be produced by heating a raw material prepared from a mixture of the polyolefine resin and the lubricant to a temperature above its melting point into a plastic state, and then cooling the same into a solid state. Alternatively, the lubricant-bearing polymer may be polyurethane rubber which has been hardened with grease contained therein, or may be produced from a material consisting essentially of a polyester elastomer resin, and a lubricant such as ester oil and ester oil-base grease by heating a raw material prepared from a mixture of the polyester elastomer resin and the lubricant to a temperature above its melting point into a plastic state, and then cooling the same into a solid state. The possible chemical composition of the polymer is 10-50 weight % the resin or the rubber, and 90-50 weight % the lubricant. Since the strength required for the polymer member to serve as a structural member is lower as the content of the resin or the rubber is lower, the preferable chemical composition is 20-50 weight % the resin or the rubber, and 80-50 weight % the lubricant. As the lubricant, other materials such as paraffin hydrocarbon oil, naphthene hydrocarbon oil, mineral oil, ether oil, and ester oil may be used. 
     The above and other objects, feature, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary axial sectional view showing the construction of a bearing unit employing a shaft seal device according to a first embodiment of the present invention; 
     FIG. 2A is a fragmentary axial sectional view showing the construction of a shaft seal device according to a second embodiment of the invention; 
     FIG. 2B is a similar view to FIG. 2A showing the construction of a variation of the second embodiment; 
     FIG. 3A is a fragmentary axial sectional view showing the construction of a shaft seal device according to a third embodiment of the invention; 
     FIG. 3B is a fragmentary enlarged sectional view showing a portion of FIG. 3A on an enlarged scale; 
     FIG. 4 is a fragmentary axial sectional view showing the construction of a shaft seal device according to a fourth embodiment of the invention; 
     FIG. 5 is a fragmentary axial sectional view showing the construction of a shaft seal device according to a fifth embodiment of the invention; 
     FIG. 6A is a fragmentary axial sectional view showing the construction of a shaft seal device according to a sixth embodiment of the invention; 
     FIG. 6B is a similar view to FIG. 6A showing the construction of a variation of the sixth embodiment; 
     FIG. 6C is a fragmentary axial sectional view showing the construction of another variation of the sixth embodiment; 
     FIG. 7A is a fragmentary axial sectional view showing the construction of a shaft seal device according to a seventh embodiment of the invention; 
     FIG. 7B is a similar view to FIG. 7A showing the construction of a variation of the seventh embodiment; 
     FIG. 8A is a fragmentary axial sectional view showing the construction of a shaft seal device according to an eighth embodiment of the invention; 
     FIG. 8B is a similar view to FIG. 8A, showing the shaft seal device with a lubricant-bearing polymer member  40  reduced in size; 
     FIG. 8C is a similar view to FIG. 8A showing the construction of a variation of the eighth embodiment; 
     FIG. 9A is a fragmentary axial sectional view showing the construction of a shaft seal device according to a ninth embodiment of the invention; 
     FIG. 9B is a similar view to FIG. 9A showing the construction of a variation of the ninth embodiment; 
     FIG. 9C is a similar view to FIG. 9A showing the construction of another variation of the ninth embodiment; 
     FIG. 10A is a fragmentary axial sectional view showing the construction of a shaft seal device according to a tenth embodiment of the invention; 
     FIG. 10B is a similar view to FIG. 9A showing the construction of a variation of the tenth embodiment; 
     FIG. 10C is a similar view to FIG. 9A showing the construction of another variation of the tenth embodiment; 
     FIG. 10D is a similar view to FIG. 9A showing the construction of a further variation of the tenth embodiment; 
     FIG. 11 is a fragmentary axial sectional view showing the construction of a shaft seal device according to an eleventh embodiment of the invention; 
     FIG. 12A is a fragmentary axial sectional view showing the construction of a shaft seal device according to a twelfth embodiment of the invention; 
     FIG. 12B is a sectional view taken along line A—A in FIG. 12A; and 
     FIG. 13 is a fragmentary axial sectional view showing the construction of a conventional shaft seal device. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to the accompanying drawings showing preferred embodiments thereof. In drawings showing all embodiments of the present invention and variations thereof, corresponding elements and parts are designated by identical reference numerals. 
     Referring first to FIG. 1, there is shown the construction of a bearing unit employing a shaft seal device according to a first embodiment of the present invention. The illustrated bearing unit is a plummer block. 
     In FIG. 1, reference numeral  10  designates a rotary shaft which has thinned portions  11  and  11   a  at an end portion thereof. A self-aligning rolling bearing  12  is mounted on the thinned portion  11 . The bearing  12  is accommodated in a housing  13 , and is mainly comprised of an inner ring  14  fitted on the thinned portion  11 , an outer ring  15  fitted in an inner peripheral portion of the housing  13 , double-train rollers  16  disposed to roll along track grooves formed in the outer peripheral surface of the inner ring  14  and the inner peripheral surface of the outer ring  15 , and a solid cage that retains the rollers  16 . 
     A lubricant-bearing polymer member  20  in the form of a ring or annulus is fitted in a space defined between the track grooves in a fashion urgingly contacting the inner peripheral surface of the outer ring  15 . The lubricant-bearing polymer member  20  has opposite end surfaces thereof disposed in contact with the rollers  16  so as to guide the rollers  16  along a track surface of the outer ring  15 . 
     An annular sealing channel  30  is formed in one axial end of the housing  13 , in which is accommodated a lubricant-bearing polymer member  40  in the form of a ring or annulus. The polymer member  40  is fitted on the rotary shaft  10 , to seal between the axial end of the housing  13  and the rotary shaft  10 . At an initial stage of rotation of the rotary shaft  10 , the lubricant-bearing polymer member  40  has an inside diameter almost equal to the outside diameter of the rotary shaft  10 . On this occasion, at least one end surface of the polymer member  40  is in contact with at least one associated lateral wall surface of the sealing channel  30 . The polymer member  40  also functions to prevent dust and water droplets from entering the bearing unit from the outside of the housing  13 . 
     Similarly, an annular sealing channel  31  is formed in the other axial end of the housing  13 , in which is accommodated a lubricant-bearing polymer member  41  having the same construction and function as the polymer member  40  and fitted on the rotary shaft  10 , to seal between the other axial end of the housing  13  and the rotary shaft  10 . At least one end surface of the polymer member  41  is in contact with at least one associated lateral wall surface of the sealing groove  31 . 
     The chemical composition of a lubricant-bearing polymer forming the lubricant-bearing polymer member  20  is 5 weight % ultrahigh molecular weight (UHMW) polyethylene (that belongs to UHMW), 20 weight % high density polyethylene (that belongs to relatively low molecular weight), 5 weight % polyethylene wax (that belongs to wax), and 70 weight % paraffin mineral oil. 
     The chemical composition of a lubricant-bearing polymer forming the lubricant-bearing polymer members  40 ,  41  is 10 weight % ultrahigh molecular weight (UHMW) polyethylene (that belongs to UHMW), 20 weight % high density polyethylene (that belongs to relatively low molecular weight), and 70 weight % paraffin mineral oil. 
     The shaft seal device according to the first embodiment is constituted by the sealing channel  30  and the lubricant-bearing polymer member  40  and the sealing channel  31  and the lubricant-bearing polymer member  41 . 
     The operation of the shaft seal device according to the present embodiment constructed as above will now be described with respect to the sealing channel  30  and the lubricant-bearing polymer member  40  provided at the one axial end of the housing  13 . 
     At an initial stage of rotation of the rotary shaft  10 , the inside diameter of the lubricant-bearing polymer member  40  is almost equal to the outside diameter of the rotary shaft  10 , and then the polymer member  40  and the rotary shaft  10  slide against each other such that the polymer member  40  is stationary or rotated by the rotation of the rotary shaft  10 . If the polymer member  40  is stationary, at least one end surface thereof is in stationary close contact with at least one corresponding lateral wall surface of the sealing channel  13  which is stationary, while if the polymer member  40  is rotated by the rotation of the rotary shaft  10 , the at least one end surface of the polymer member  40  and the at least one corresponding lateral wall surface of the sealing channel  13  slide against each other. 
     Irrespective of whether the polymer member  40  is stationary or it is rotated by the rotating rotary shaft  10 , the at least one surface of the polymer member  40  that is in contact with its counterpart functions as a sealing surface to prevent dust and splashed water droplets from entering the bearing unit from the outside of the housing  13 . Thereafter, as the lubricant exudes from the lubricant-bearing polymer member  40  with further rotation of the rotary shaft  10 , the inside diameter and outside diameter of the polymer member  40  become reduced depending upon the exudation amount of lubricant, so that the inside diameter of the polymer member  40  becomes smaller than the outside diameter of the rotary shaft  10 , and accordingly the polymer member  40  becomes tightly fitted on the rotary shaft  10 . 
     In this state, the polymer member  40  rotates together with the rotary shaft  10 , or it rotates with delay relative to the rotation of the rotary shaft  10  due to the sliding resistance of the sliding surfaces between the polymer member  40  and the sealing channel  30 . When the polymer member  40  rotates together with the rotary shaft  10 , they are in stationary close contact with each other, and on the other hand, when the polymer member  40  rotates with delay relative to the rotation of the rotary shaft  10 , they slide against each other. In either case, the at least one end surface of the polymer member  40  and the at least one corresponding lateral wall surface of the sealing channel  30  always slide against each other. Thus, when the polymer member  40  is reduced in size after continued rotation of the rotary shaft  10 , the at least one surface of the polymer member  40  that is in contact with its counterpart functions as a sealing surface to prevent dust and splashed water droplets from entering the bearing unit from the outside of the housing  13 , as is the case with the initial stage of rotation of the rotary shaft  10 . 
     The lubricant continuously exudes from the sliding surface of the lubricant-bearing polymer member  40  to well lubricate the sliding surface of the member  50  with its counterpart. Therefore, the sliding friction between the the polymer member  40  and its counterpart is so small that the polymer member  40  is not worn by its sliding against the counterpart, and hence no gap cannot be formed between the sliding surfaces of the two members, whereby entry of alien substances such as dust and water droplets into the bearing unit can be prevented. 
     The operation of the shaft seal device at the other axial end of the housing  13  is identical or similar to the operation described as above. 
     Second to twelfth embodiments of the present invention will now be described. In FIGS. 2A to  12 B showing these embodiments, only shaft seal devices at the one axial end of the housing  13  are shown, and shaft seal devices at the other axial end of the housing  13  are similarly constructed, illustration and description of which are therefore omitted. 
     A second embodiment of the present invention will now be described with reference to FIG. 2A showing an axial section of a shaft seal device according to the second embodiment. 
     The shaft seal device according to the present embodiment is mounted in the bearing unit as shown in FIG. 1, and description of the construction, operation and effects thereof which are common or identical to those of the first embodiment described above is omitted. 
     Fitted in the annular sealing channel  30  formed in the axial end of the housing  13  is a covering member  50  in the form of an annulus, which is formed of an elastic material such as rubber and has a U-shaped axial section. The covering member  50  has an inner peripheral channel  50   a  formed therein, in which is accommodated the lubricant-bearing polymer member  40  in the form of a ring or annulus, which is fitted on the rotary shaft  10 , to seal between the axial end of the housing  13  and the rotary shaft  10 . At an initial stage of rotation of the rotary shaft  10 , the lubricant-bearing polymer member  40  has an inside diameter almost equal to the outside diameter of the rotary shaft  10 . On this occasion, at least one end surface of the polymer member  40  is in contact with at least one associated lateral wall surface of the sealing groove  30 . 
     Next, the operation of the present embodiment will be described. With the above arrangement, the polymer member  40  and the rotary shaft  10  slide against each other such that at least one end surface of the polymer member  40  is in stationary close contact with at least one corresponding inner end surface of the covering member  50  or slides against the latter. With further rotation of the rotary shaft  10 , the lubricant exudes from the lubricant-bearing polymer member  40  so that the inside diameter of the polymer member  40  becomes smaller than the outside diameter of the rotary shaft  10 , and accordingly the polymer member  40  becomes tightly fitted on the rotary shaft  10 . On this occasion, the polymer member  40  and the rotary shaft  10  rotate together with each other, or they slide against each other such that at least one end surface of the polymer member  40  slides against at least one corresponding inner end surface of the covering member  50 . 
     Depending upon whether the rotation of the rotary shaft  10  is in an initial state or in a continued state, the shaft seal device assumes either of the above-mentioned two states. In either case, the at least one surface of the polymer member  40  that is in contact with its counterpart functions as a sealing surface to prevent dust and splashed water droplets from entering the bearing unit from the outside of the housing  13 . 
     While in the first embodiment, if the housing  13  is formed of a metal material, there is a possibility that the lubricant-bearing polymer member  40  is worn down at its sliding surface with the axial end of the housing  13 , in the second embodiment, however, the intervention of the covering member  50  formed of an elastic material such as rubber between the axial end of the housing  13  and the polymer member  40  makes it possible to prevent the polymer member  40  from directly sliding against the axial end of the housing  13 . Since the polymer member  40  slides against the covering member  50  which is formed of an elastic material, the amount of wear of the polymer member  40  is greatly reduced. Also in this embodiment, the lubricant continuously exudes from the sliding surfaces of the polymer member  40  that are in sliding contact with the covering member  50  to lubricate the sliding surfaces of the two members, to thereby prevent formation of gaps between the two members due to wear of them, and hence prevent degradation of the sealing performance. The covering member  50  is fitted in the sealing channel  30  at the axial end of the housing  13  in an elastically compressed fashion, and therefore it will not be rotated in the housing  13  even if it receives torque from the polymer member  40  in sliding contact with the covering member  50 . 
     FIG. 2B shows the construction of a variation of the second embodiment. This variation is distinguished from the second embodiment described above in that the sealing channel  30  formed in the housing  13  has a trapezoidal section with its opposite lateral wall surfaces tapered, and the covering member  50  has its opposite outer end surfaces tapered correspondingly to the taped opposite lateral wall surfaces of the sealing channel  30 . This construction can prevent gaps from being formed between the lubricant-bearing polymer member  40  and the opposite lateral wall surfaces of the sealing channel  30  due to reduction of the outside and inside diameters of the polymer member  40 , to thereby exhibit a further reliable sealing performance. 
     The material of the covering member  50  may include elastomers, in addition to rubber. 
     Shaft seal devices according to the third to twelfth embodiments which will be described hereinbelow are all mounted in the bearing unit as shown in FIG.  1  and have substantially the same construction with that of the second embodiment described above. Description of the construction, operation, and effects thereof which are common or identical to those of the second embodiment described above is omitted. 
     A third embodiment of the present invention will now be described with reference to FIGS. 3A and 3B. FIG. 3A shows an axial section of a shaft seal device according to the third embodiment, and FIG. 3B is an enlarged sectional view of a portion encircled by a dashed line in FIG.  3 A. 
     The third embodiment is distinguished from the second embodiment in that the opposite inner end surfaces of the covering member  50  are roughened into satin finished surfaces, for example. The surface roughness of the satin finished surfaces is in a range of approximately 10 S to approximately 500 S. 
     Next, the operation of the present embodiment will be described. In the second embodiment described above, the lubricant-bearing polymer member  40  has at least one end surface thereof disposed in sliding contact with at least one associated inner end surface of the covering member  50 . However, if the inner end surfaces of the covering member  50  are very smooth and at the same time a thin film of lubricant is formed between the sliding surfaces of the polymer member  40  and the covering member  50 , there can occur a phenomenon that the inner end surface(s) of the covering member  50  and the associated end surface(s) of the polymer member  40  become stuck together, whereby the covering member  50  receives torque from the rotating polymer member  40  and accordingly can easily rotate in the sealing channel  30  at the axial end of the housing  13 , which results in that the covering member  50  and the inner walls of the sealing channel  30  rub each other and hence the covering member  50  wears down. To prevent the above phenomenon, in the third embodiment, the inner end surfaces of the covering member  50  are roughened into satin finished surfaces, for example. The roughened surfaces of the covering member  50  can prevent the inner end surface(s) of the covering member  50  and the associated end surface(s) of the polymer member  40  from becoming stuck together and hence prevent the covering member  50  from rotating. 
     A fourth embodiment of the present invention will be described with reference to FIG. 4 showing the construction of a shaft seal device according to the fourth embodiment. 
     The fourth embodiment is distinguished from the second embodiment in that each of the end walls of the covering member  50  is formed with at least one through hole  51  which extends through the end wall in the longitudinal direction of the rotary shaft  10 . The suitable diameter of the through hole  51  may range from 0.1 mm to 10 mm. The number of the through hole  51  may preferably be 8 or more for each of the end walls, and a plurality of the through holes  51  may also preferably be circumferentially arranged at equal intervals. 
     Next, the operation of the fourth embodiment will be described. In the second embodiment described above, the end surface(s) of the polymer member  40  slides (slide) against the inner end surface(s) of the covering member  50 . As mentioned above, if the inner end surfaces of the covering member  50  are very smooth and at the same time a thin film of lubricant is formed between the sliding surfaces of the polymer member  40  and the covering member  50 , there can occur a phenomenon that the inner end surface(s) of the covering member  50  and the associated end surface(s) of the polymer member  40  become stuck together, whereby the covering member  50  receives torque from the rotating polymer member  40  and accordingly can easily rotate in the sealing channel  30  at the axial end of the housing  13 , which results in that the covering member  50  and the inner walls of the sealing channel  30  rub each other and hence the covering member  50  wears down. To prevent the above phenomenon, in the fourth embodiment, one or more longitudinally extending through holes  51  are formed in each of the end walls of the covering member  50 . Therefore, air passes through the holes  51 , which cuts off the thin film of lubricant formed on the polymer member  40  at portions thereof corresponding to the locations of the through holes  51 , so that the end surfaces of the two members  40 ,  50  are not stuck together at the cut-off portions of the thin film, thus preventing the covering,member  50  from rotating. 
     A fifth embodiment of the present invention will be described with reference to FIG. 5 showing an axial section of a shaft seal device according to the fifth embodiment. 
     The fifth embodiment is distinguished from the second embodiment in that the lubricant-bearing polymer member  40  has a thickness set to a value smaller than the distance between the opposite inner end surfaces of the covering member  50  such that the end surfaces of the polymer member  40  and the respective associated inner end surfaces of the covering member  50  are spaced from each other, providing a gap  60  therebetween. The gap  60  extends along one end surface of the polymer member  40 , along the outer peripheral surface thereof, and then along the other end surface thereof, thus presenting a form of a labyrinth. The labyrinth gap  60  is filled with the lubricant exuding from the polymer member  40 . The gap  60  has a size from approximately 0.1 mm to approximately 1 mm. 
     Next, the operation of the fifth embodiment will be described. In the initial stage of rotation of the rotary shaft  10 , the lubricant-bearing polymer member  40  is stationary or rotating relative to the covering member  50 . On this occasion, however, the labyrinth gap  60  is maintained, so that alien substances such as dust and water droplets will not enter the bearing unit via the outer peripheral portion of the polymer member  40 . As the rotation of the rotary shaft  10  continues and accordingly the polymer member  40  becomes reduced in size with exudation of the lubrication therefrom, the polymer member  40  rotates together with the rotary shaft  10 . On this occasion, as is the case with the initial stage of rotation of the rotary shaft, the labyrinth gap  60  is maintained and filled with the lubricant, providing a higher sealing performance than in the initial stage of rotation of the rotary shaft  10 . When the polymer member  40  rotates together with the rotary shaft  10 , the polymer member  40  does not slide against the other part or the covering member  50 , and therefore the torque of the rotary shaft  10  can be reduced as compared with the shaft seal device according to the second embodiment. 
     A sixth embodiment of the present invention will be described with reference to FIG. 6A showing an axial section of a shaft seal device according to the sixth embodiment. 
     The sixth embodiment is distinguished from the second embodiment in that a plurality of protuberances  41  are formed on the opposite end surfaces of the lubricant-bearing polymer member  40  in a fashion circumferentially extending in concentricity with the inner peripheral surface of the polymer member  40 . The tip of each protuberance  41  is flat and disposed in contact with the associated inner end surface of the covering member  50 . In the illustrated embodiment of FIG. 6A, three protuberances  41  are formed on each end surface of the polymer member  40 , and totally sixth protuberances  41  are formed on the both end surfaces of the polymer member  40 . When the polymer member  40  rotates, these protuberances  41  and the respective associated inner end surfaces of the covering member  50  slide against each other. 
     Next, the operation of the sixth embodiment will be described. In the initial stage of rotation of the rotary shaft  10 , the flat tip surface of each protuberance  41  on the polymer member  40  and the associated inner end surface of the covering member  50  are in stationary close contact with each other or slide against each other. As the rotation of the rotary shaft  10  continues and accordingly the polymer member  40  becomes reduced in size with exudation of the lubricant therefrom, the polymer member  40  becomes tightly fitted on the rotary shaft  10 , so that the protuberances  41  and their respective associated inner end surfaces of the covering member  50  slide against each other. Considering the path of entry of dust, water droplets, or the like into the bearing unit, a plurality of sets of the tip surfaces of the protuberances  41  in sliding contact with the inner end surfaces of the covering member and spaces between the protuberances  41  and the covering member  50  are present, that is, the spaces between the protuberances  41  and the covering member  50  present gaps in the form of a labyrinth. Therefore, in addition to the effects of the second embodiment, the labyrinth effect is additionally provided by the protuberances  41 , to thereby further enhance the sealing performance. 
     FIG. 6B shows a variation of the sixth embodiment, in which, in place of the protuberances  41  provided on the opposite end surfaces of the lubricant-bearing polymer member  40 , similar protuberances  51  are formed on the opposite inner end surfaces of the covering member  50 . 
     FIG. 6C shows another variation of the sixth embodiment, which employs a combination of the protuberances  41  in FIG.  6 A and the protuberances  51  in FIG.  6 B. 
     Also these variations can provide substantially the same effects as mentioned above. 
     A seventh embodiment of the present invention will be described with reference to FIG. 7A showing an axial section of a shaft seal device according to the seventh embodiment. 
     The seventh embodiment is distinguished from the second embodiment in that a plurality of peripheral grooves  42  are formed in the outer peripheral surface of the lubricant-bearing polymer member  40 , and a plurality of peripheral protuberances  52  are formed on the inner peripheral surface of the covering member  50  at locations corresponding respectively to the peripheral grooves  42 . In the illustrated embodiment of FIG. 7A, three peripheral grooves  42  and three peripheral protuberances  52  are provided. Each peripheral protuberance  52  is inserted in an associated peripheral groove  42  in a fashion being spaced from the latter with a constant clearance therebetween. The clearance ranges from approximately 0.1 mm to approximately 1 mm. 
     Next, the operation of the seventh embodiment will be described. Irrespective of whether the lubricant-bearing polymer member  40  is rotating or stationary, the path of entry of alien substances such as dust and water droplets into the bearing unit includes gaps in the form of a labyrinth consisting of alternate recesses and projections, provided at the outer periphery of the polymer member  40 . Therefore, compared with the construction of the second embodiment, the gaps in the form of a labyrinth at the outer periphery of the polymer member  40  are added, to thereby achieve a stronger sealing effect. 
     FIG. 7B shows a variation of the seventh embodiment. While according to the seventh embodiment at least one end surface of the polymer member  40  may be in contact with the associated inner end surface of the covering member  50 , the opposite end surfaces of the polymer member  40  may be slightly spaced from the opposite inner end surfaces of the covering member  50  with a certain clearance, as shown in in FIG. 7B, in addition to the provision of the gaps in the form of a labyrinth. The clearance ranges from approximately 0.1 mm to approximately 1 mm. 
     An eighth embodiment of the present invention will be described with reference to FIG. 8A showing an axial section of a shaft seal device according to the eighth embodiment, and FIG. 8B showing a state in which the lubricant-bearing polymer member  40  is reduced in size. 
     The eighth embodiment is distinguished from the second embodiment in that a lip  43  is provided at a portion of the inner periphery of the lubricant-bearing polymer member  40 , for sliding contact with the rotary shaft  10 . The rest portion of the inner periphery of the polymer member  40  other than the portion provided with the lip  43  defines a hollow cylindrical shape concentrical with the lip  43  and has a diameter larger than the outside diameter of the rotary shaft  10 . 
     Next, the operation of the eighth embodiment will be described. In the initial stage of rotation of the rotary shaft  10 , the lip  43  and the rotary shaft  10  slide against each other, exhibiting a sealing function by the lip  43 . Since the lubricant exudes from the lip  43  as well, the lip  43  will not be worn down by its sliding against the rotary shaft  10 . As the rotation of the rotary shaft  10  continues and the lubricant exudes from the polymer member  40 , the inside diameter of the polymer member  40  becomes reduced and accordingly the diameter of the hollow cylindrical portion at the inner periphery of the polymer member  40  becomes smaller than the outside diameter of the rotary shaft  10  so that the polymer member  40  becomes tightly fitted on the rotary shaft  10 , as shown in FIG. 8B, whereby the end surfaces of the polymer member  40  and the inner end surfaces of the covering member  50  slide against each other, thus serving as sealing surfaces. With the arrangement of the eighth embodiment, in the initial stage of rotation of the rotary shaft  10 , the lip  43  exhibits a sealing function, and therefore, compared with the second embodiment, the sealing performance is enhanced. Since the hollow cylindrical portion at the inner periphery of the polymer member  40 , which becomes tightly fitted on the rotary shaft  10 , has a hollow cylindrical shape, the polymer member  40  can be prevented from being fitted aslant on the rotary shaft  10  and hence stuck thereto when the former become tightly fitted on the latter. 
     FIG. 8C shows a variation of the eighth embodiment, in which a plurality of lips  43  are provided, to thereby further enhance the sealing performance. 
     A ninth embodiment of the present invention will be described with reference to FIG. 9A showing an axial section of a shaft seal device according to the ninth embodiment. 
     The ninth embodiment is distinguished from the second embodiment in that a pair of lips  44  are formed in a circumferentially extending fashion at the outer periphery of the lubricant-bearing polymer member  40  and disposed in elastic contact with the inner peripheral surface of the covering member  50 . The lips  44  each project from an associated end surface of the polymer member in the longitudinal direction of the rotary shaft  10 . Therefore, the opposite end surfaces of the polymer member  40  are out of contact with the opposite inner end surfaces of the covering member  50 . Only one lip  44  or more than two lips  44  may be provided, as is distinct from the example of FIG.  9 A. 
     Next, the operation of the ninth embodiment will be described. In the initial stage of rotation of the rotary shaft  10 , the rotary shaft  10  rotates in sliding contact with the inner peripheral surface of the lubricant-bearing polymer member  40  to thereby form a sealing surface there. As the rotation of the rotary shaft  10  continues and the lubricant exudes from the polymer member  40  so that the inside diameter of the polymer member  40  becomes smaller than the outside diameter of the rotary shaft  10 , the polymer member  40  rotates together with the rotary shaft  10 . Then, the lips  44 , which is elastically deflectable, at the outer periphery of the polymer member  40  and the inner peripheral surface of the covering member  50  slide against each other to thereby form a sealing surface there. The outside diameter of the polymer member  40  is also reduced as the rotation of the rotary shaft  10  continues. However, the amount of deflection of the lips  44  is set to a sufficient amount for the amount of reduction in the outside diameter. Therefore, there is no possibility that the lips  44  become separated from the inner peripheral surface of the covering member  50  to spoil the sealing performance. 
     FIG. 9B shows a variation of the ninth embodiment. The tips of the lips  44  need not project from the respective associated end surfaces of the polymer member  40  in the longitudinal direction of the rotary shaft  10 . As shown in FIG. 9B, the lips  44  are formed on the outer periphery of the polymer member  40  in such a fashion that they entirely lie in the range of the thickness of the polymer member  40 . Therefore, the end surfaces of the polymer member  40  and the inner end surfaces of the covering member  50  can be brought into contact with each other, providing a sealing surface between the end surfaces of the two members. That is, compared with the construction of the second embodiment, additional sealing by the lips  44  is provided at the outer periphery of the polymer member  40 . 
     FIG. 9C shows another variation of the ninth embodiment, in which, in place of the lips  44  formed at the outer periphery of the lubricant-bearing polymer member  40 , lips  53  are formed on the inner periphery of the covering member  50 . In this case, the outer peripheral surface of the polymer member  40  is cylindrically shaped. Even with this arrangement in which lips are formed on the covering member  50  side, substantially the same sealing performance can be obtained as that obtained with the arrangement of FIG.  9 A. 
     A tenth embodiment of the present invention will be described with reference to FIG. 10A showing an axial section of a shaft seal device according to the tenth embodiment. 
     The tenth embodiment is distinguished from the second embodiment in that lips  45  are formed on the opposite end surfaces of the lubricant-bearing polymer member  40 , which circumferentially extend in concentricity with the inner peripheral surface of the polymer member  40  and disposed in elastic contact with the respective associated inner end surfaces of the covering member  50 . In the illustrated embodiment, a pair of lips  45  are formed on the opposite end surfaces of the polymer member, one lip  45  being oriented radially outward, and the other being oriented radially inward. 
     FIG. 10B shows a variation of the ninth embodiment, in which a lip  44  is formed only on one end surface of the polymer member  40 . In this case, the other end surface of the polymer member  40  is in direct contact with the associated inner end surface of the covering member  50 . It suffices to provide at least one lip  45 . Also, a plurality of lips  44  may be formed on one or each end surface of the polymer member  40 . 
     Next, the operation of the tenth embodiment and the variation thereof will be described. In the initial stage of rotation of the rotary shaft  10 , the rotary shaft  10  rotates in sliding contact with the inner peripheral surface of the polymer member  40  to form a sealing surface there. As the rotation of the rotary shaft  10  proceeds and the lubricant exudes from the polymer member  40  so that the inside diameter of the polymer member  40  becomes smaller than the outside diameter of the rotary shaft  10  and hence the former becomes tightly fitted on the latter, the polymer member  40  rotates together with the rotary shaft  10 . Then, the lips  45  at the end surfaces of the polymer member  40  and the inner end surfaces of the covering member  50  slide against each other to form sealing surfaces therebetween. The lubricant exudes from the lips  45 , to thereby prevent the lips  45  from being worn down due to sliding friction to form gaps between the lips  45  and the inner end surfaces of the covering member  50 . According to the tenth embodiment and the variation, compared with the second embodiment, sealing is effected by the lips  45 , to thereby further enhance the sealing performance. 
     FIG. 10C shows another variation of the tenth embodiment, in which, in place of the lips  45  formed at the end surfaces of the polymer member  40 , lips  54  may be formed on the opposite inner end surfaces of the covering member  50 . In this case, each end surface of the polymer member  40  is a flat surface. Even if the lips are thus formed on the covering member  50  side, substantially the same sealing effects can be achieved. 
     FIG. 10D shows a further variation of the tenth embodiment, in which both lips  45  and  54  are provided at the end surfaces of the polymer member  40  and the inner end surfaces of the covering member  50 , to thereby further enhance the sealing performance. 
     An eleventh embodiment of the present invention will be described with reference to FIG. 11 showing an axial section of a shaft seal device according to the eleventh embodiment. 
     The eleventh embodiment is a combination of the seventh embodiment and the tenth embodiment described above. 
     According to the present embodiment, similarly to the seventh embodiment, peripheral grooves  42  are formed in the outer peripheral surface of the lubricant-bearing polymer member  40 , and peripheral protuberances  52  are formed on the inner peripheral surface of the covering member  50  at locations corresponding to the respective peripheral grooves  42  and inserted in the latter with a constant clearance of approximately 0.1 mm to approximately 1 mm therebetween. Further, similarly to the tenth embodiment, lips  54  are formed on the inner end surfaces of the covering member in concentricity with the inner peripheral surface of the covering member  50  and disposed in elastic contact with the respective end surfaces of the polymer member  40 . Alternatively, a lip similar to the lips  54  may be formed only on one inner end surface of the covering member  50 , or one or more similar lips may be formed on one or both end surfaces of the polymer member  40 , or on both of the polymer member  40  and the covering member  50 . 
     The operation of the eleventh embodiment is a combination of the operations of the seventh and tenth embodiments, providing both of the effects of the two embodiments as described above. That is, the path of entry of alien substances such as dust and water droplets into the bearing unit includes a gap in the form of a labyrinth consisting of alternate recesses and projections provided at the outer periphery of the polymer member  40  irrespective of whether the polymer member  40  is rotating or stationary. Therefore, alien substances cannot easily enter the interior of the bearing unit. Besides, the lips formed on the inner end surfaces of the covering member  50  elastically contact the end surfaces of the polymer member  40 , which renders it more difficult for alien substances to enter the bearing unit. As a result, the sealing performance can be further enhanced. 
     A twelfth embodiment of the present invention will be described with reference to FIG. 12A showing an axial section of a shaft seal device according to the twelfth embodiment and FIG. 12B showing a section taken along line A—A in FIG.  12 A. 
     The twelfth embodiment is distinguished from the second embodiment in that a circumferentially extending outer peripheral groove  55  is formed in the outer peripheral surface of the covering member  50 . The covering member  50  is further formed therein with four lubricant supply holes  56  each extending in the peripheral wall of the covering member  50  from the outer peripheral groove  55  to the inner peripheral surface of the covering member  50 . The through holes  56  are circumferentially arranged at equal intervals. By so arranging the holes  56 , irrespective of the angular position in which the covering member  50  is placed about a horizontal line, at least one of the holes  56  is necessarily positioned above the horizontal line and opens upward. A lubricant supply port  70  is formed in the housing  13  in a fashion upwardly extending from an uppermost portion of the outer peripheral groove  56  formed in the covering member  50  which is fitted in the sealing channel  30  at the axial end of the housing  13 . Although the four lubricant supply holes  56  are circumferentially arranged at equal intervals, it is more desirable that more than four holes  56  are provided. Lubricant may be directly poured into the lubricant supply port  70 , or an oiler may be connected to the port  70 . 
     Next, the operation of the twelfth embodiment will be described. Lubricant poured into the lubricant supply port  70  is guided to the outer periphery of the covering member  50 . Then, the lubricant is guided along the outer peripheral groove  55  of the covering member  50  to reach the first upwardly directed lubricant supply hole  56 . The lubricant passes the same hole  56  to move from the outer peripheral surface of the covering member  50  to the inner peripheral surface of the same, and finally permeates into the polymer member  40 . Thus, lubricant is supplied to the polymer member  40  and stored therein to supplement lubricant exuding from the polymer member  40  for lubrication. With the arrangement of the twelfth embodiment, since lubricant is supplied to the lubricant-bearing polymer member for supplement of exuded lubricant, though the polymer member  40  inherently can operate for a long time without supply of lubricant, the effective life of the polymer member  40  can be further prolonged.