Patent Publication Number: US-2016228210-A1

Title: Bearing unit for dental air turbine and dental air turbine

Description:
TECHNICAL FIELD 
     The present invention relates to a bearing unit for a dental air turbine and a dental air turbine, and more particularly, to a bearing unit for a dental air turbine and a dental air turbine which is a form of dental handpiece and uses air as a driving source. 
     BACKGROUND ART 
     A dental air turbine handpiece  120  includes a grip part  121  and a head part  122  provided at a tip end portion of the grip part  121 , as shown in  FIG. 37 . An operator holds the grip pail  121  and performs, for example, drilling on a tooth. 
     A related-art dental air turbine  100  which configures the head part  122  includes a rotating shaft  101  to which a dental treatment instrument (not shown) can be attached, a turbine blade  102  configured to rotate the rotating shaft  101  by compressed air, and a pair of ball bearings  103  and  104  which rotatably support the rotating shaft  101 , as shown in  FIG. 38 , and these components are contained in a head housing  105 . The outer rings  106  and  107  of the pair of ball bearings  103  and  104  are supported through rubber rings  108  fit in annular recesses  109  and  110  of the head housing  105 . The outer ring  107  of the lower ball bearing  104  is biased upward by a spring washer  111 , whereby a preload is applied to the pair of ball bearings  103  and  104 . That is, the pair of bail bearings  103  and  104  which support the rotating shaft  101  are arranged face-to-thee, and a constant preload is applied thereto by the elastic force of the spring washer  111  (see, for example, Patent Document 1). 
     As a bearing used in a dental air turbine, there is known an angular ball bearing having a counter-bore in its inner ring (see, (for example, Patent Document 2). In order to improve sealing performance, the angular ball bearing has a shield plate attached to the inner ring counter side. 
     BACKGROUND ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP-A-H11-37162 
     Patent Document 2: JP-A-H08-66411 
     SUMMARY OF THE INVENTION 
     Problems to be Solved 
     According to the dental air turbine  100  disclosed in Patent Document 1, the pair of ball bearings  103  and  104  are arranged face-to-face, and an end surface of the outer ring  107  fit in the head housing  105  by loose fitting is pressed by the spring washer  111 , whereby a preload is applied. Since the distance between the acting points of the pair of ball bearings  103  and  104  arranged face-to-face is short, and a load capability against a moment load is not necessarily high, if a radial load is applied to a tip end of the rotating shaft  101  during dental treatment or the like, the rotating shaft  101  is likely to be tilted. For this reason, in order to secure a load capability against a moment load by use of the pair of ball bearings  103  and  104  arranged face-to-face, the sizes of the bearings would become larger. Even in the dental air turbine disclosed in Patent Document 2. since the pair of angular ball bearings are arranged face-to-face, similar situation would occur. 
     The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a bearing unit for a dental air turbine and a dental air turbine capable of reducing a size of a rolling bearing and having high moment rigidity in a rotating shaft. 
     Means for Solving the Problems 
     (1) A dental air turbine comprising: 
     a turbine blade: 
     a rotating shaft which is integrally fixed with the turbine blade and includes one end to which a dental treatment instrument can be attached; and 
     a pair of rolling bearings which support the rotating shaft rotatably with respect to a housing, 
     wherein the pair of rolling bearings are arranged back-to-back while fitting inner rings of the pair of rolling bearings on the rotating shaft such that a preload is applied thereto. 
     (2) The dental air turbine according to (1), 
     wherein facing side surfaces of outer rings of the pair of rolling bearings come into contact with a step portion provided on the housing, and 
     wherein the inner rings of the pair of rolling bearings are fit on the rotating shaft to be fixed thereto such that a fixed position preload is applied. 
     (3) The dental air turbine according to (1), 
     wherein a spring member abuts on at least one of facing side surfaces of outer rings of the pair of rolling bearings and outer side surfaces of the inner rings in an axis direction such that a constant preload is applied to the pair of rolling bearings. 
     (4) The dental air turbine according to (3), 
     wherein the spring member is disposed between each of the facing side surfaces of the outer rings of the pair of the rolling bearings and a step portion provided on the housing, and 
     wherein the inner rings of the pair of rotting bearings are fit on the rotating shaft to be fixed thereto such that a constant preload is applied. 
     (5) The dental air turbine according to (3), 
     wherein each of the facing side surfaces of the outer rings of the pair of rolling bearings comes into contact with a step portion provided on the housing, and 
     wherein the inner rings of the pair of rolling beatings are fit on the rotating shaft in a state where the spring member is interposed between each of the outer side surfaces of the inner rings in the axis direction and a fixing ring fixed to the rotating shaft, such that a constant preload is applied. 
     (6) The dental air turbine according to (1), 
     wherein each of the pair of rolling bearings is an angular ball bearing which includes an outer ring having an outer ring raceway surface formed at an inner periphery thereof, an inner ring having an inner ring raceway surface formed at an outer periphery thereof, and a plurality of balls assembled with a contact angle with the outer ring raceway surface and the inner ring raceway surface. 
     (7) The dental air turbine according to (6), 
     wherein one of the inner periphery of the outer ring and the outer periphery of the inner ring of each of the pair of rolling bearings has a shoulder portion formed on one side with respect to the raceway surface in an axis direction, and a counter-bore formed on the other side in the axis direction, 
     wherein the other of the inner periphery of the outer ring and the outer periphery of the inner ring of each of the pair of rolling bearings has a pair of shoulder portions formed on both sides with respect to the raceway surface in the axis direction, and 
     wherein each of the pair of rotting bearings has a labyrinth structure between sealing means provided on the outer ring so as to face the outer periphery of the inner ring and the outer periphery of the inner ring at least on an outer side in the axis direction. 
     (8) The dental air turbine according to (7), 
     wherein in each of the pair of rolling bearings, the sealing means is configured by a sealing portion which is formed integrally with the outer ring and extends from the shoulder portion of the outer ring toward the inner rings in a radial direction. 
     (9) A bearing unit for a dental air turbine, the bearing unit comprising: 
     a turbine blade: 
     a rotating shaft which is integrally fixed with the turbine blade and includes one end to which a dental treatment instrument can be attached; and 
     a pair of rolling bearings which support the rotating shaft rotatably with respect to a housing, 
     wherein the pair of rolling bearings are arranged back-to-back while disposing a space member between outer rings of the pair of rolling bearings and fitting inner rings on the rotating shaft such that a preload is applied thereto. 
     (10) The bearing unit for a dental air turbine according to (9), 
     wherein facing side surfaces of the outer rings of the pair of rolling bearings come into contact with side surfaces of the space member in an axis direction, respectively, and 
     wherein the inner rings of the pair of rolling bearings are lit on the rotating shaft to be fixed thereto such that a fixed position preload is applied. 
     (11) The bearing unit for a dental air turbine according to (10), 
     wherein a spring member abuts on at least one of facing side surfaces of the outer rings of the pair of rolling bearings and outer side surfaces of the inner rings in an axis direction such that a constant preload is applied. 
     (12) The bearing unit for a dental air turbine according to (11), 
     wherein the spring member is disposed between each of the facing side surfaces of the outer rings of the pair of the rolling bearings and a corresponding one of the side surfaces of the space member in the axis direction, and 
     wherein the inner rings of the pair of rolling bearings are fit on the rotating shaft to be fixed thereto such that a constant preload is applied. 
     (13) The bearing unit for a dental air turbine according to (11), 
     wherein each of the facing side surfaces of the outer rings of the pair of rolling bearings comes into contact with a corresponding one of the side surfaces of the space member in the axis direction, and 
     wherein the inner rings of the pair of rolling bearings are fit on the rotating shaft in a state where the spring member is interposed between each of the outer side surfaces of the inner rings in the axis direction and a fixing ring fixed to the rotating shaft, such that a constant preload is applied. 
     (14) The bearing unit for a dental air turbine according to any one of (9) to (13), 
     wherein an external diameter of at least one of the outer rings is larger than an external diameter of the turbine blade, and 
     wherein the space member is a sleeve which is fixed to the housing and includes an air supply hole portion for supplying air to the turbine blade, an exhaust hole portion for discharging air, and a step portion which is provided on an inner side thereof in a radial direction so as to face side surfaces of the outer rings. 
     (15) The bearing unit for a dental air turbine according to any one of (9) to (13), 
     wherein the turbine blade is disposed on an outer side of the pair of rolling bearings in an axis direction, 
     wherein external diameters of the outer rings of the pair of rolling bearings are smaller than an external diameter of the turbine blade, and 
     wherein the space member is a sleeve which is fixed to the housing and includes an air supply hole portion for supplying air to the turbine blade, an exhaust hole portion for discharging air, and a step portion which is provided on an inner side thereof in a radial direction so as to face side surfaces of the outer rings. 
     (16) The bearing unit for a dental air turbine according to any one of (9) to (13), 
     wherein an external diameter of at least one of the outer rings is larger than an external diameter of the turbine blade, and 
     wherein the space member is an outer ring spacer which includes an air supply hole portion for supplying air to the turbine blade and an exhaust hole portion for discharging air and which is disposed between the pair of rolling bearings. 
     (17) The bearing unit for a dental air turbine according to (9), 
     wherein each of the pair of rolling bearings is an angular ball bearing which includes an outer ring having an outer ring raceway surface formed at an inner periphery thereof, an inner ring having an inner ring raceway surface formed at an outer periphery thereof, and a plurality of balls assembled with a contact angle with the outer ring raceway surface and the inner ring raceway surface. 
     (18) The bearing unit for a dental air turbine according to (17), 
     wherein one of the inner periphery of the outer ring and the outer periphery of the inner ring of each of the pair of rolling bearings has a shoulder portion formed on one side with respect to the raceway surface in an axis direction, and a counter-bore formed on the other side in the axis direction, 
     wherein the other of the inner periphery of the outer ring and the outer periphery of the inner ring of each of the pair of rolling bearings has a pair of shoulder portions formed on both sides with respect to the raceway surface in the axis direction, and 
     wherein each of the pair of rolling bearings has a labyrinth structure between sealing means provided on the outer ring so as to face the outer periphery of the inner ring and the outer periphery of the inner ring at least on an outer side in the axis direction. 
     (19) The bearing unit for a dental air turbine according to (18), 
     wherein in each of the pair of rolling bearings, the sealing means is configured by a sealing portion which is formed integrally with the outer ring and extends from the shoulder portion of the outer ring toward the inner rings in a radial direction. 
     (20) A dental air turbine comprising: 
     the bearing unit according to any one of (9) to (19), 
     wherein the bearing unit is fixed to the housing with an elastic member interposed therebetween, which is disposed on an outer periphery of the space member or an outer periphery of the outer rings. 
     Advantage of the Invention 
     According to the dental air turbine of the present invention, since the pair of rolling bearings are arranged back-to-back while fitting the inner rings of the pair of rolling bearings on the rotating shaft such that a preload is applied thereto, if the bearings have the same sizes as those in the related art, it is possible to improve a load capability of the rotating shaft against a moment load by applying a preload to the pair of rolling bearings. Also, since it becomes possible to achieve the same moment rigidity as that achieved by the related-art bearing sizes by small-sized rolling bearings, it becomes possible to reduce the size of the turbine head while maintaining the operability and functions of the air turbine. Therefore, it is possible to accurately transmit the operating force of a dentist handling the dental air turbine to the rotating shaft, whereby the operability of the dental air turbine improves. 
     Also, according to the dental air turbine and the bearing unit for a dental air turbine of the present invention, since the pair of rolling bearings are arranged back-to-back while disposing the space member between the outer rings of the pair of rolling bearings and fitting the inner rings on the rotating shaft such that a preload is applied thereto, it becomes possible to achieve the same moment rigidity as that achieved by the related-art bearing sizes by small-sized rolling bearings. Therefore, it becomes possible to reduce the size of the turbine head while maintaining the operability and functions of the air turbine. Even if the bearings have the same sizes as those in the related art, it is possible to improve the load capability of the rotating shaft relative to a moment load by applying a preload to the pair of rolling bearings. Further, if the operability and functions of the air turbine are maintained with the same moment rigidity as that in the related art by bearings having the same sizes as those in the related art, since drilling performance improves, it becomes possible to reduce the rotating speed of the air turbine, and it becomes possible to reduce the amount of air required for rotation. Therefore, it is possible to reduce noise to be generated from the air turbine. Furthermore, since the rotating shaft having the turbine blade and the pair of rolling bearings are unitized while a preload is applied to the pair of rolling bearings, it is possible to easily assemble the dental air turbine in the housing, and since the bearing unit may be exchanged for exchanging the bearings, cumbersome preload adjustment work becomes not necessary. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a half cross-sectional view of a main portion of a dental air turbine according to a first embodiment of the present invention. 
         FIG. 2A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a first modification of the first embodiment of the present invention, and  FIG. 2B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a second modification of the first embodiment of the present invention. 
         FIG. 3A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a third modification of the first embodiment of the present invention, and  FIG. 3B  is a half cross-sectional view of a main portion of a beating unit for a dental air turbine according to a fourth modification of the first embodiment of the present invention. 
         FIG. 4  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fifth modification of the first embodiment of the present invention. 
         FIG. 5  is a half cross-sectional view of a main portion of a dental air turbine according to a second embodiment of the present invention. 
         FIG. 6A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a first modification of the second embodiment of the present invention, and  FIG. 6B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according, to a second modification of the second embodiment of the present invention. 
         FIG. 7  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a third embodiment of the present invention. 
         FIG. 8  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fourth embodiment of the present invention. 
         FIG. 9  is a half cross-sectional view of a main portion of bearing unit for a dental air turbine according to a modification of the fourth embodiment of the present invention. 
         FIG. 10A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fifth embodiment of the present invention, and  FIG. 10B  is a half cross-sectional view of a main portion showing a state where the bearing unit for a dental air turbine is fit into a housing. 
         FIG. 11  is a schematic cross-sectional view at an axis direction position where a turbine blade of  FIG. 10  is positioned. 
         FIG. 12  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a first modification of the fifth embodiment of the present invention. 
         FIG. 13A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a second modification of the fifth embodiment of the present invention, and  FIG. 13B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a third modification of the fifth embodiment of the present invention. 
         FIG. 14A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fourth modification of the fifth embodiment of the present invention, and  FIG. 14B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fifth modification of the fifth embodiment of the present invention. 
         FIG. 15A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a sixth modification of the fifth embodiment of the present invention, and FIG,  15 B is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a seventh modification of the fifth embodiment of the present invention. 
         FIG. 16  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to an eighth modification of the fifth embodiment of the present invention. 
         FIG. 17A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a ninth modification of the fifth embodiment of the present invention, and  FIG. 17B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a tenth modification of the fifth embodiment of the present invention. 
         FIG. 18A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to an eleventh modification of the fifth embodiment of the present invention, and  FIG. 18B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a twelfth modification of the fifth embodiment of the present invention. 
         FIG. 19  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a thirteenth modification of the fifth embodiment of the present invention. 
         FIG. 20  is a half cross-sectional view a main portion of a bearing unit for a dental air turbine according to a thirteenth embodiment of the present invention. 
         FIG. 21  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a first modification of the sixth embodiment of the present invention. 
         FIG. 22A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a second modification of the sixth embodiment of the present invention, and  FIG. 22B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a third modification of the sixth embodiment of the present invention. 
         FIG. 23  is a half cross-sectional view a main portion of a bearing unit for a dental air turbine according to a fourth modification of the sixth embodiment of the present invention. 
         FIG. 24A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fifth modification of the sixth embodiment of the present invention, and  FIG. 24B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a sixth modification of the sixth embodiment of the present invention. 
         FIG. 25  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a seventh modification of the sixth embodiment of the present invention. 
         FIG. 26A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to an eighth modification of the sixth embodiment of the present invention, and  FIG. 26B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a ninth modification of the sixth embodiment of the present invention. 
         FIG. 27  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a tenth modification of the sixth embodiment of the present invention. 
         FIG. 28A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to an eleventh modification of the sixth embodiment of the present invention, and  FIG. 28B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a twelfth modification of the sixth embodiment of the present invention. 
         FIG. 29A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a seventh embodiment of the present invention, and  FIG. 29B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according, to a first modification of the seventh embodiment of the present invention. 
         FIG. 30A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a second modification of the seventh embodiment of the present invention, and  FIG. 30B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a third modification of the seventh embodiment of the present invention. 
         FIG. 31  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fourth modification of the seventh embodiment of the present invention. 
         FIG. 32A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to an eighth embodiment of the present invention, and  FIG. 32B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a first modification of the eighth embodiment of the present invention. 
         FIG. 33A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a second modification of the eighth embodiment of the present invention, and  FIG. 33B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a third modification of the eighth embodiment of the present invention. 
         FIG. 34  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fourth modification of the eighth embodiment of the present invention. 
         FIG. 35A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fifth modification of the eighth embodiment of the present invention, and  FIG. 35B  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a sixth modification of the eighth embodiment of the present invention. 
         FIG. 36A  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a seventh modification of the eighth embodiment of the present invention, and  FIG. 36B  is a half cross-sectional view of a main portion of a beating unit for a dental air turbine according to an eighth modification of the eighth embodiment of the present invention. 
         FIG. 37  is a schematic side view illustrating a general dental air turbine handpiece. 
         FIG. 38  is a cross-sectional view of a main portion of a related-art dental air turbine. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a bearing unit for a dental air turbine and a dental air turbine according to each embodiment of the present invention will be described in detail while referring to the drawings. In  FIGS. 1 to 36 , a lower part ran instrument side) and an upper part (a counter-instrunmet side) of each dental air turbine are shown on a left side and a right side, respectively. 
     First Embodiment 
       FIG. 1  is a half cross-sectional view of a main portion of a dental air turbine of the first embodiment. A dental air turbine  10  includes a rotating shaft  11  having one end to which a dental treatment instrument (not shown) can be attached, a turbine blade  12  which is fixed to a substantially center portion of the rotating shaft  11  and is configured to rotate the rotating shaft  11  by compressed air, and a lower rolling bearing (a deep groove ball bearing)  13  and an upper rolling bearing (a deep groove ball healing)  14  which rotatably support the rotating shaft  11 , and these components are contained in a housing  15 . The turbine blade  12  is disposed between the lower rolling bearing  13  and the upper rolling bearing  14  and is fixed to the rotating shaft  11 . The housing  15  is configured by combining a lower first housing  16  and a middle second housing  17 , and a cap  18  is mounted on the second housing  17  from above. Incidentally, since  FIG. 1  is a half cross-sectional view of the main portion, it does not show a connection portion side of the housing  16  extending toward a grip part of a handpiece. 
     The lower rotting bearing  13  includes an outer ring  21 , an inner ring  23 , a plurality of balls (rolling elements)  25  rollably arranged between the outer ring  21  and the inner ring  23 , a crown type cage  27  which is disposed such that its annular portion is positioned on the inner side in the axis direction and holds the plurality of balls  25 , and a seal member  29  disposed on the outer side of the outer ring  21  in the axis direction, and the upper rolling bearing  14  includes an outer ring  22 , an inner ring  24 , a plurality of balls (rolling elements)  26  rollably arranged between the outer ring  22  and the inner ring  24 , a crown type cage  28  which is disposed such that its annular portion is positioned on the inner side in the axis direction and holds the plurality of balls  26 , and a seal member  30  disposed on the outer side of the outer ring  22  in the axis direction. 
     The outer ring  21  of the lower rolling bearing  13  is fit into a bearing hole  31  of the first housing  16  by light press-fitting, bonding, or the like, and the inner side surface  21   a  of the outer ring  21  comes into contact with a rib (a step portion)  32  formed so as to protrude from the bearing hole  31  toward the inner side in the radial direction. In a state where a preload is applied from the axis direction (from the left direction in  FIG. 1 ), the inner ring  23  is fixed to the rotating shaft  11  by light press-fitting, bonding, or the like. 
     The outer ring  22  of the upper rolling bearing  14  is fit into the second housing  17  by light press-fitting, bonding, or the like, and the facing inner side surface  22   a  of the outer ring  22  comes into contact with a rib (a step portion)  34  formed so as to protrude from a bearing hole  33  toward the inner side in the radial direction. In a state where a preload is applied from the axis direction (from the right direction in  FIG. 1 ), the inner ring  24  is fixed to the rotating shaft  11  by light press-fitting, bonding, or the like. 
     That is, the lower rolling bearing  13  and the upper rotting bearing  14  are preloaded in a back-to-back arrangement direction by a fixed position preload manner. Since the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, even if the sizes of the bearings are the same as the related-art bearings, it is possible to lengthen the distance L between the acting points, whereby the moment rigidity of the rotating shaft  11  improves. The outer rings  21  and  22  may be fixed in the bearing holes  31  and  33  by loose fitting or through O-rings, similarly to the related art. 
     The housing  15 , that is, the first housing  16  has an air supply passage and an exhaust passage for supplying compressed air to the turbine blade  12  or discharging the air, and the turbine blade  12  is rotated at 300,000 rpm to 500,000 rpm by the compressed air supplied from the air supply passage. 
     According to the dental air turbine  10  (If the first embodiment configured as described above, since the inner rings  23  and  24  of the lower rolling bearing  13  and the upper rolling bearing  14  are fit on the rotating shaft  11  such that the lower rolling bearing  13  and the upper rolling bearing  14  are preloaded and arranged back-to-back, it is possible to lengthen the distance L between the acting points, and it is possible to improve the moment rigidity of the rotating shaft  11  having one end to which a dental treatment instrument can be attached. 
     Accordingly, even if the external diameters of the outer rings  21  and  22  are set to be smaller by about 30% as compared to related-art bearings arranged face-to-face, it is possible to secure moment rigidity having the same level as that in the related art, and it becomes possible to reduce the size of the dental air turbine  10 . Further, it is possible to accurately transmit the operating force of a dentist to the rotating shaft  11 , whereby the operability of the dental air turbine  10  improves. 
     Since the facing inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  are brought into contact with the ribs  32  and  34  formed on the first and second housings  16  and  17 , and the inner rings  23  and  24  are fit on the rotating shaft  11  to be fixed thereto, it is possible to apply a fixed position preload to the pair of the lower rolling bearing  13  and the upper rolling bearing  14  arranged back-to-back by a simple configuration. 
     In the above-described embodiment, a case where the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a fixed position preload is applied thereto has been described. However, as long as the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, they may be configured as shown in  FIGS. 2 to 4  such that a constant preload is applied thereto. In this case, a spring member  35  abuts on at least one of the facing inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction. 
     That is, like a dental air turbine  10  of a first modification shown in  FIG. 2A , the spring members  35  (leaf springs or spring washers) are disposed between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the ribs  32  and  34  formed on the first and second housings  16  and  17 , respectively, and the respective inner rings  23  and  24  are fit on the rotating shall  11  to be fixed thereto. Accordingly, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Alternatively, like a dental air turbine  10  of a second modification shown in  FIG. 2B , the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  come into contact with the ribs  32  and  34  formed on the housings, and the respective inner rings  23  and  24  are fit on the rotating shaft  11  while fixing the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to fixing rings  36  with the spring members  35  (leaf springs or spring washers) interposed therebetween, respectively. Even in this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Also, like a dental air turbine  10  of a third modification shown in  FIG. 3A , a spring member  35  is disposed only in any one of the gaps between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the ribs  32  and  34  formed on the first and second housings  16  and  17 , and the inner rings  23  and  24  are fit on the rotating shall  11  to be fixed thereto. Even in this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Also, like a dental air turbine  10  of a fourth modification shown in  FIG. 3B , the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  come into contact with the ribs  32  and  34  formed on the first and second housings  16  and  17 , and the respective inner rings  23  and  24  are fit on the rotating shaft  11  while fixing only one of the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to a fixing ring  36  with a spring member  35  interposed therebetween and fitting the other one of the inner rings  23  and  24  on the rotating shaft  11  to be fixed thereto. Even in this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Further, like a dental air turbine  10  of a fifth modification shown in  FIG. 4 , a spring member  35  is disposed in any one of the gaps between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the ribs  32  and  34  formed on the first and second housings  16  and  17 , and the respective inner rings  23  and  24  are fit on the rotating shaft  11  while fixing only one of the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to a fixing ring  36  with a spring member  35  interposed therebetween and fitting the other one of the inner rings  23  and  24  on the rotating shaft  11  to be fixed thereto. Even in this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Second Embodiment 
       FIG. 5  is a cross-sectional view of a main portion of a dental air turbine of a second embodiment. The second embodiment is different from the first embodiment in the configurations of the lower rolling bearing  13  and the upper rolling bearing  14 . Therefore, parts identical to or equivalent to those of the first embodiment are denoted by the same reference symbols, and a description thereof will not be made or will be shortly made. 
     In the second embodiment, the lower rolling bearing  13  is configured by an angular bail bearing including an outer ring  21  having an outer ring raceway surface  21   c  at an inner periphery thereof, an inner ring  23  having an inner ring raceway surface  23   c  at an outer periphery thereof, a plurality of balls (rolling elements)  25  assembled with a contact angle with the outer ring raceway surface  21   c  and the inner ring raceway surface  23   c,  and a machined cage  27  which holds the plurality of balls  25 . The upper rolling bearing  14  is configured by an angular ball bearing including an outer ring  22  having an outer ring raceway surface  22   c  at an inner periphery thereof, an inner ring  24  having an inner ring raceway surface  24   c  at an outer periphery thereof, a plurality of bails (rolling elements)  26  assembled with a contact angle with the outer ring raceway surface  22   c  and the inner ring raceway surface  24   c,  and a machined cage  28  which holds the plurality of balls  26 . In the rolling bearings  13  and  14 , the diameters of the inner circumferential surfaces of the inner rings  23  and  24  are 1.5 mm to 5 mm, and the diameters of the outer circumferential surfaces of the outer rings  21  and  22  are 4 mm to 10 mm. 
     The outer periphery of the inner ring  23  of the lower rolling bearing  13  has a shoulder portion  23   d  formed on one side with respect to the inner ring raceway surface  23   c  in the axis direction (the outer side in the axis direction), and a counter-bore  23   f  formed on the other side in the axis direction (the inner side in the axis direction), and the outer periphery of the inner ring  24  of the upper rolling bearing  14  has a shoulder portion  24   d  formed on one side with respect to the inner ring raceway surface  24   c  in the axis direction (the outer side in the axis direction), and a counter-bore  24   f  formed on the other side in the axis direction (the inner side in the axis direction). 
     Meanwhile, the inner periphery of the outer  21  of the lower rolling bearing  13  has a pair of shoulder portions  21   d  and  21   e  formed on both sides with respect to the outer ring raceway surface  21   c  in the axis direction, and the inner periphery of the outer ring  22  of the upper rolling bearing  14  has a pair of shoulder portions  22   d  and  22   e  formed on both sides with respect to the outer ring raceway surface  22   c  in the axis direction. 
     The lower rolling bearing  13  and the upper rolling bearing  14  of the second embodiment have open type configurations without any sealing means to be described below, and thus can be suitably used in a dental air turbine having restrictions in space in the axis direction. 
     According to the dental air turbine  10  of the second embodiment configured as described above, since the lower rolling bearing  13  and the upper rolling bearing  14  are configured by angular ball bearings and are arranged back-to-back while fitting the inner rings  23  and  24  of the lower rolling bearing  13  and the upper rolling bearing  14  on the rotating shaft  11  such that a preload is applied thereto, it is possible to lengthen the distance L between the acting points, and it is possible to improve the moment rigidity of the rotating shaft  11  having one end to which a dental treatment instrument can be attached. 
     Accordingly, even if the external diameters of the outer rings  21  and  22  are set to be smaller by about 30% as compared to related-art bearings arranged face-to-face, it is possible to secure moment rigidity having the same level as that in the related art, and it becomes possible to reduce the size of the dental air turbine  10 . Also, it is possible to accurately transmit the operating force of a dentist to the rotating shaft  11 , whereby the operability of the dental air turbine  10  improves. 
     The other configurations and effects are the same as those of the first embodiment. 
     In the above-described embodiment, a case where the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back and a fixed position preload is applied thereto has been described. However, as long as the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, they may be configured as shown in  FIG. 6  such that a constant preload is applied thereto. In this case, a spring member  35  abuts on at least one of the facing inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction. 
     That is, like a dental air turbine  10  of first modification shown in  FIG. 6A , the spring members  35  (leaf springs or spring washers) are disposed between facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the ribs  32  and  34  formed on the first and second housings  16  and  17 , respectively, and the respective inner rings  23  and  24  are fit on the rotating shaft  11  to be fixed thereto. Accordingly, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Alternatively, like a dental air turbine  10  of a second modification shown in  FIG. 6B , the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  come into contact with the ribs  32  and  34  formed on the housings, and the respective inner rings  23  and  24  are fit on the rotating shaft  11  while fixing the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to fixing rings  36  with the spring members  35  (leaf springs or spring washers) interposed therebetween, respectively. Even in this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Also, the present invention is not limited to the dental air turbines  10  of the first and second modifications, and like the first embodiment, a spring member  35  may be disposed only in any one of the gaps between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the ribs  32  and  34  formed on the first and second housings  16  and  17 , and the inner rings  23  and  24  may be fit on the rotating shaft  11  to be fixed thereto. 
     Alternatively, the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  may come into contact with the ribs : 32  and  34  formed on the first and second housings  16  and  17 , and the respective inner rings  23  and  24  may be fit on the rotating shaft  11  while fixing only one of the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to a fixing ring  36  with a spring member  36  interposed therebetween and fitting the other one of the inner rings  23  and  24  on the rotating shaft  11  to be fixed thereto. 
     Further, a spring member  35  may be disposed in any one of the gaps between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the ribs  32  and  34  formed on the first and second housings  16  and  17 , and the respective inner rings  23  and  24  may be fit on the rotating shaft  11  while fixing only one of the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to a fixing ring  36  with a spring member  35  interposed therebetween and fitting the other one of the inner rings  23  and  24  on the rotating shaft  11  to be fixed thereto. 
     Third Embodiment 
       FIG. 7  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a third embodiment. In a dental air turbine  10  of the third embodiment, the lower rolling bearing  13  includes sealing means  81  and  83  provided on both sides of the outer ring  21  in the axis direction so as to face the outer periphery of the inner ring  23  and has a labyrinth structure between the sealing means  81  and  83  and the outer periphery of the inner ring  23 , and the upper rolling bearing  14  includes sealing means  82  and  84  provided on both sides of the outer ring  22  in the axis direction so as to face the outer periphery of the inner ring  24  and has a labyrinth structure between the sealing means  82  and  84  and the outer periphery of the inner ring  24 . 
     Specifically, the sealing means  81 ,  82 ,  83 , and  84  include sealing portions  81  and  82  formed integrally with the outer rings  21  and  22  on the outer sides in the axis direction so as to extend from the shoulder portions  21   d  and  22   d  of the outer rings  21  and  22  toward the inner rings  23  and  24  on the inner sides in the radial directions, respectively, and sealing members  83  and  84  disposed in seal grooves formed in the shoulder portions  21   e  and  22   e  of the outer rings  21  and  22  on the inner sides in the axis direction by use of retaining rings  85 , respectively. 
     It is preferable to set the thicknesses of the sealing portions  81  and  82  and the sealing members  83  and  84  in the axis direction to 0.1 mm or greater. If they are set to 0.1 mm or greater, since passages through foreign substances needs to pass in order to enter the bearings lengthen, it is difficult for foreign substances to enter the bearings. Also, the upper limit values of the thicknesses of the sealing portions  81  and  82  and the sealing members  83  and  84  in the axis direction are set in view of the axial gap of the bearings, so as not to interfere with the balls and the cages. 
     It is preferable to set the radial gaps of the labyrinth structures, that is, the radial gaps between the tip ends of the sealing portions  81  and  82  and the sealing members  83  and  84  and the outer peripheries of the inner rings  23  and  24  to 0.1 mm or less. If the radial gaps become 0.1 mm or less, it becomes difficult for foreign substances to enter the bearings from the outside, and it is more preferable to set the radial gaps to 0.05 mm to 0.1 mm. Meanwhile, in the related-art structure, if the radial gaps become 0.05 mm or less, there is a possibility that the sealing portions  81  and  82  and the sealing members  83  and  84  aright come into contact with the outer peripheries of the inner rings  23  and  24 . 
     However, in the structure according to the third embodiment, moment rigidity becomes high, tilting of the bearings is suppressed, and it becomes possible to set the radial gaps between the outer peripheries of the inner rings  23  and  24  and the sealing portions  81  and  82  and the sealing members  83  and  84  to 0.05 mm or less. 
     The inner circumferential aces of the tip ends of the sealing portions  81  and  82  and the sealing members  83  and  84  and the outer circumferential surfaces of the shoulder portions  23   d  and  24   d  of the inner rings  23  and  24  are polished, whereby it is possible to keep a non-contact state with high accuracy. 
     In the third embodiment, labyrinth structures may be provided only on the outer sides in the axis direction by the sealing portions  81  and  82  provided on the outer sides in the axis direction and the outer peripheries of the inner rings  23  and  24 , without providing the sealing members  83  and  84  provided on the inner sides in the axis direction. 
     The sealing members  83  and  84  provided on the inner sides in the axis direction and the sealing portions  81  and  82  provided on the outer sides in the axis direction may be inversely configured. 
     The other configurations and effects are the same as those of the second embodiment. 
     Fourth Embodiment 
       FIG. 8  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a third embodiment. In a dental air turbine  10  of the fourth embodiment, the inner periphery of the outer ring  21  of the lower rolling bearing  13  has the shoulder portion  21   e  formed on one side with respect to the outer ring raceway surface  21   c  in the axis direction (the inner side in the axis direction) and the counter-bore  21   f  formed on the other side in the axis direction (the outer side in the axis direction), and the inner periphery of the outer ring  22  of the upper rolling bearing  14  has the shoulder portion  22   e  formed on one side with respect to the outer ring raceway surface  22   c  in the axis direction (the inner side in the axis direction) and the counter-bore  22   f  formed on the other side in the axis direction (the outer side in the axis direction). 
     Meanwhile, the outer peripheries of the inner rings  23  and  24  of the lower rolling bearing  13  and the upper rolling bearing  14  have the pair of shoulder portions  23   d  and  23   e  formed on both sides with respect to the inner ring raceway surface  23   c  in the axis direction, and the pair of shoulder portions  24   d  and  24   e  formed on both sides with respect to the inner ring raceway surface  24   c  in the axis direction, respectively. 
     In the fourth embodiment, in the same manner as those of the first and second embodiments, the lower rolling bearing  13  and the upper rolling bearing  14  are preloaded in a back-to-back arrangement direction by a fixed position preload manner. Since the lower rolling bearing  13  and the upper rolling beating  14  are arranged back-to-back, even if the sizes of the bearings are the same as those in the related art, it is possible to lengthen the distance L between the acting points, whereby the moment rigidity of the rotating shaft  11  improves. 
     The other configurations and effects are the same as those of the second embodiment. 
     Also, in a modification of the fourth embodiment shown in  FIG. 9 , like the third embodiment, the lower rolling bearing  13  includes sealing means  81   a  and  83   a  provided on both sides of the outer ring  21  in the axis direction so as to face the outer periphery of the inner ring  23 , and has a labyrinth structure between the sealing means  81  and  83  and the inner ring  23 , and the upper rolling bearing  14  includes sealing means  82   a  and  84   a  provided on both sides of the outer ring  22  in the axis direction so as to face the outer periphery of the inner ring  24 , and has a labyrinth structure between the sealing means  82  and  84  and the inner ring  24 . 
     Specifically, the sealing means  81   a,    82   a,    83   a,  and  84   a  include sealing members  81   a  and  82   a  disposed in seal grooves formed in the counter-bores  21   f  and  22   f  of the outer rings  21  and  22  on the outer sides in the axis direction by use of retaining rings  85 , and sealing portions  83   a  and  84   a  formed integrally with the outer rings  21  and  22  on the inner sides in the axis direction so as to extend from the shoulder portions  21   e  and  22   e  of the outer rings  21  and  22  toward the inner rings  23  and  24  on the inner sides in the radial directions, respectively. 
     Even in this modification, labyrinth structures may be provided only on the outer sides in the axis direction by the sealing members  81   a  and  82   a  provided on the outer sides in the axis direction and the outer peripheries of the inner rings  23  and  24 , without providing the sealing members  83   a  and  84   a  on the inner sides in the axis direction. 
     Fifth Embodiment 
       FIG. 10  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a fifth embodiment, and  FIG. 11  is a schematic cross-sectional view at an axis direction position where a turbine blade of  FIG. 10  is positioned. In a dental air turbine  10  of the fifth embodiment, the rotating shaft  11 , the turbine blade  12 , the pair of rolling beatings  13  and  14 , and a sleeve (a space member)  41  are unitized as a bearing unit  20  for a dental air turbine, which is disposed in the housing  15 . 
     The external diameters D 1  of the outer rings  21  and  22  are set to be larger than the external diameter D 2  of the turbine blade  12 , and the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back on both sides of the turbine blade  12  fixed to the center of the rotating shaft  11  and rotatably support the rotating shaft  11 . 
     The sleeve  41  which is fit on the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  has a step portion  42  provided at the center in the axis direction, and protruding toward the internal diameter side, and abutting on the inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22 . As shown in  FIGS. 10 and 11 , at positions of the sleeve  41  which face the turbine blade  12 , an air supply hole portion  43  for supplying compressed air to the turbine blade  12 , and an exhaust hole portion  44  for discharging air are provided. 
     The outer rings  21  and  22  are fit in the sleeve  41  such that the inner side surfaces  21   a  and  22   a  of the outer rings abut on the step portion  42  of the sleeve  41 , and the inner rings  23  and  24  are fit on the rotating shaft  11  to be fixed thereto such that a preload is applied. In this way, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, whereby a bearing unit  20  for a dental air turbine is configured. The hearing unit  20  for a dental air turbine is assembled in the housing  15  as shown in  FIG. 10B . Similarly to the related art, the outer rings  21  and  22  may be fixed to the sleeve  41  by loose fitting or through O-rings  72  (see a first modification shown in  FIG. 12 ). 
     As described above, according to the bearing unit  20  for a dental air turbine of the fifth embodiment, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back by fitting the outer rings  21  and  22  into the sleeve  41  and fitting the inner rings  23  and  24  on the rotating shaft  11  such that a preload is applied thereto. Therefore, a preload is applied to the lower rolling bearing  13  and the upper rolling bearing  14 , whereby it is possible to improve the load capability of the rotating shaft  11  against a moment load. Also, it is possible to unitize the rotating shaft  11  having the turbine blade  12  and the lower and upper rolling bearings  13  and  14 , and it becomes easy to assemble them in the housing  15  of the dental air turbine  10 . 
     Since the facing inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  are brought into contact with the side surfaces of the sleeve  41  in the axis direction, and the inner rings  23  and  24  are fit on the rotating shaft  11  to be fixed thereto, it is possible to apply a fixed position preload to the pair of the lower rolling beating  13  and the upper rolling bearing  14  arranged back-to-back, by a simple configuration. 
     Also, the external diameters D 1  of the outer rings  21  and  22  are larger than the external diameter D 2  of the turbine blade  12 , and the sleeve  41  is fixed to the housing  15 , and has the air supply hole portion  43  for supplying air to the turbine blade  12 , the exhaust hole portion  44  for discharging air, and the step portion  42  provided on the inner side in the radial direction so as to come into contact with the inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22 . Therefore, it is possible to bring the inner side surfaces  21   a  and  22   a  of the pair of outer rings  21  and  22  into contact with the sleeve  41 , thereby regulating their positions, and apply a preload to the pair of rolling bearings  13  and  14  arranged back-to-back, by a simple structure, and it is possible to configure the high-accuracy bearing unit  20 , whereby the cost of the unit is suppressed. 
     Also, in  FIG. 10 , the turbine blade  12  is disposed between the lower rolling bearing  13  and the upper rolling bearing  14 . However, the turbine blade may be fixed to a portion of the rotating shaft  11  lower than the lower rolling bearing  13 , like a second modification shown in  FIG. 13A , or may be fixed to a portion of the rotating shaft  11  higher than the upper rolling bearing  14 . Alternatively, like a third modification shown in  FIG. 13B , a pair of turbine blades  12   a  and  12   b  may be fixed to portions of the rotating shaft  11  positioned on both outer sides of the lower rolling bearing  13  and the upper rolling bearing  14  in the axis direction. 
     That is, in a bearing unit  20  for a dental air turbine according to a second modification shown in  FIG. 13A , the inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  are brought into contact with a step portion  52  protruding toward the inner side of a sleeve  51  in the radial direction, whereby the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back and close to each other. At positions of the sleeve  51  which face the turbine blade  12 , an air supply hole portion  53  for supplying compressed air to the turbine blade  12  and an exhaust hole portion (not shown) for discharging air are provided. 
     In a bearing unit  20  for a dental air turbine according to a third modification shown in  FIG. 13B , the inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  are brought into contact with a step portion  56  protruding from the center of a sleeve  55  toward the inner side in the radial direction, whereby the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back and close to each other. Also, at positions of the sleeve  55  which face the turbine blades  12   a  and  12   b,  air supply hole portions  57   a  and  57   b  for supplying compressed air to the turbine blades  12   a  and  12   b  and exhaust hole portions (not shown) for discharging air are provided. 
     In  FIG. 10 , the external diameters D 1  of the outer rings  21  and  22  are set to be larger than the external diameter D 2  of the turbine blade  12 . However, it is only needed to set the external diameter D 1  of at least one of the outer rings  21  and  22  larger than the external diameter D 2  of the turbine blade  12 . For example, in a bearing unit  20  for a dental air turbine according to a fourth modification shown in  FIG. 14A , the turbine blade  12  is disposed between the lower rolling bearing  13  and the upper rolling bearing  14 , and the external diameter D 3  of the outer ring  21  of the lower rolling bearing  13  is set to be smaller than the external diameter D 2  of the turbine blade  12 , and the external diameter D 1  of the outer ring  22  of the upper rolling bearing  14  is set to be larger than the external diameter D 2  of the turbine blade  12 . 
     A sleeve  61  which is fit on the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  has a small-diameter hole portion  62 , a mid-diameter hole portion  63 , and a large-diameter hole portion  64  formed corresponding to the lower rolling bearing  13 , the turbine blade  12 , and the upper rolling bearing  14 , respectively. The outer ring  21  of the lower rolling bearing  13  is fit into the small-diameter hole portion  62 , whereby the inner side surface  21   a  of the outer ring  21  comes into contact with a step portion  65  protruding from the small-diameter hole portion  62  toward the inner side in the radial direction. Also, the outer ring  22  of the upper rolling bearing  14  is fit into the large-diameter hole portion  64 , whereby the inner side surface  22   a  of the outer ring  22  comes into contact with a step portion  66  between the mid-diameter hole portion  63  and the large-diameter hole portion  64 , and the inner rings  23  and  24  are fit on the rotating shaft  11  to be fixed thereto, whereby a preload is applied to the lower rolling bearing  13  and the upper rolling bearing  14 . At positions of the sleeve  61  which face the turbine blade  12 , an air supply hole portion  67  for supplying compressed air to the turbine blade  12  and an exhaust hole portion (not shown) for discharging air are provided. 
     The configuration of the fourth modification can be applied to the bearing units  20  for dental air turbines according to the fifth embodiment of  FIG. 10  and the second and third modifications of  FIGS. 13A and 13B . 
     Also, like a bearing unit  20  for a dental air turbine according to a fifth modification shown in FIG.  14 B, the outer circumferential surface of the small-diameter hole portion  62  of the sleeve  61  may be formed in a stepped shape, and have a diameter smaller than those of the outer circumferential surfaces of the mid-diameter hole portion  63  and the large-diameter hole portion  64 . 
     In bearing units  20  for dental air turbines like the second and third modifications of  FIGS. 13A and 13B , the external diameters D 1  of the outer rings  21  and  22  may be set to be smaller than the external diameter D 2  of the turbine blade  12 , and for example, rolling bearings smaller than bearings which are generally used may be applied. Specifically, in a bearing unit  20  for a dental air turbine according to a sixth modification shown in  FIG. 15A , a turbine blade  12  having the same size as that of the turbine blade  12  shown in  FIG. 13A  is used, and small-sized rolling bearings  13  and  14  are used. Therefore, although the external diameters of the rotating shaft  11 , the turbine blade  12 , and the sleeve  51  are the same as those of the bearing unit  20  for a dental air turbine according to the second modification shown in  FIG. 13A , it is possible to reduce the lengths of the sleeve  51  and the rotating shaft  11  in the axis direction, and it is possible to improve rotation performance. 
     Also, in a bearing unit  20  for a dental air turbine according to a seventh modification shown in  FIG. 15B , although the external diameter of the rotating shaft  11  is the same as that of the rotating shaft  11  shown in  FIG. 13A , since small-sized rolling bearings  13  and  14  are used, the external diameter and length of the turbine blade  12  decreases, and the external diameter and length of the sleeve  51  decreases, whereby the size of the whole unit decreases. Therefore, although the size of the turbine blade  12  is small, since the sizes of the bearings are small, it is possible to maintain rotation performance and functions. 
     Further, the bearing unit  20  for a dental air turbine having the sleeve according to the fifth embodiment may be assembled in the housing  15  with elastic members interposed therebetween. For example, a bearing unit  20  for a dental air turbine according to an eighth modification shown in  FIG. 16  is an example of application to the bearing unit  20  for a dental air turbine shown in  FIG. 10 , and an O-ring groove  41   b  is formed at a position of the outer circumferential surface  41   a  of the sleeve  41  corresponding to the outer ring  22 , and an O-ring groove  15   b  is formed at a position of the inner circumferential surface  15   a  of the housing  15  corresponding to the outer ring  21 . O-rings  70  are installed in the O-ring grooves  15   b  and  41   b,  and the sleeve  41  and the housing  15  are fit together. Also, even between the outer end surfaces of the sleeve  41  in the axis direction and the inner end surfaces of the housing  15  in the axis direction, O-rings  71  may be installed. Accordingly, it is possible to suppress vibration of the rotating shaft  11  rotating at high speed from being transmitted to the housing  15 . 
     Although a case where the O-ring grooves  15   b  and  41   b  are formed in the inner circumferential surface  15   a  of the housing  15  and the outer circumferential surface  41   a  of the sleeve  41 , respectively, has been described, the two O-ring grooves may be formed in any one of the inner circumferential surface  15   a  of the housing  15  and the outer circumferential surface  41   a  of the sleeve  41 . 
     Also, the configuration of the eighth modification can be applied to the bearing units  20  for dental air turbines according to the first to seventh modifications. 
     Even in the fifth embodiment, as long as the lower rolling bearing  13  and the upper rolling bearing  14  arranged back-to-back, they may be configured such that a constant preload is applied thereto. Even in this case, a spring member  35  abuts on at least one of the facing inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction. 
     That is, like a bearing unit  20  for a dental air turbine according to a ninth modification shown in  FIG. 17A , spring members  35  (leaf springs or spring washers) are disposed between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, respectively, and the respective inner rings  23  and  24  are fit on the rotating shaft  11  to be fixed thereto. Accordingly, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back and a constant preload is applied thereto. 
     Alternatively, like a bearing unit  20  for a dental air turbine according to a tenth modification shown in  FIG. 17B , the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  come into contact with the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, and the respective inner rings  23  and  24  are fit on the rotating shaft  11  while fixing the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to fixing rings  36  with the spring members  35  (leaf springs or spring washers) interposed therebetween, respectively. Even in this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Also, like a bearing unit  20  for a dental air turbine according to an eleventh modification shown in  FIG. 18A , a spring member  35  is disposed in any one of the gaps between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, and the respective inner rings  23  and  24  are fit on the rotating shaft  11  to be fixed thereto. Even in this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Alternatively, like a bearing unit  20  for a dental air turbine according to a twelfth modification shown in  FIG. 18B , the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  come into contact with the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, and the respective inner rings  23  and  24  are fixed to the rotating shaft  11  while fixing only one of the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to a fixing ring  36  with a spring member  35  interposed therebetween and fitting the other one of the inner rings  23  and  24  on the rotating shaft  11  to be fixed. Even in this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Further, like a bearing unit  20  for a dental air turbine according to a thirteenth modification shown in  FIG. 19 , a spring member  35  is disposed in any one of the gaps between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, and the respective inner rings  23  and  24  are fit on the rotating shaft  11  while fixing only one of the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to a fixing ring  36  with a spring member  35  interposed therebetween and fitting the other one of the inner rings  23  and  24  on the rotating shaft  11  to be fixed thereto. Even in this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     The configurations of the ninth to third modifications can be applied to the bearing units  20  for dental air turbines according to the first to eight modifications. 
     Sixth Embodiment 
       FIG. 20  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a sixth embodiment. The sixth embodiment is different from the fifth embodiment in the configurations of the lower rolling bearing  13  and the upper rolling bearing  14 . Therefore, parts identical to or equivalent to those of the fifth embodiment are denoted by the same reference symbols, and a description thereof will not be made or will be shortly made. 
     That is, in the sixth embodiment, the lower rolling beating  13  and the upper rolling bearing  14  are angular ball bearings, and the outer periphery of the inner ring  23  has a shoulder portion  23   d  formed on one side with respect to the inner ring raceway surface  23   c  in the axis direction (the outer side in the axis direction), and a counter-bore  23   f  formed on the other side in the axis direction (the inner side in the axis direction), and the outer periphery of the inner ring  24  has a shoulder portion  24   d  formed on one side with respect to the inner ring raceway surface  24   c  in the axis direction (the outer side in the axis direction), and a counter-bore  24   f  formed on the other side in the axis direction (the inner side in the axis direction). Also, the inner periphery of the outer ring  21  has a pair of shoulder portions  21   d  and  21   e  formed on both sides with respect to the outer ring raceway surface  21   c  in the axis direction, and the inner periphery of the outer ring  22  has a pair of shoulder portions  22   d  and  22   e  formed on both sides with respect to the outer ring raceway surface  22   c  in the axis direction. 
     That is, according to the bearing unit  20  for a dental air turbine according to the sixth embodiment, the lower rolling bearing  13  and the upper rolling bearing  14  are configured by angular ball bearings, and are arranged back-to-back while fitting the outer rings  21  and  22  into the sleeve  41  and fitting the inner rings  23  and  24  on the rotating shaft  11  such that a preload is applied thereto. Therefore, a preload is applied to the lower rolling bearing  13  and the upper rolling bearing  14 , whereby it is possible to improve the load capability of the rotating shaft  11  against a moment load. Also, it is possible to unitize the rotating shaft  11  having the turbine blade  12  and the lower and upper rolling bearings  13  and  14 , and it becomes easy to assemble them in the housing  15  of the dental air turbine  10 . 
     The other configurations and effects are the same as those of the fifth embodiment. 
     Even in the sixth embodiment, like a first modification shown in  FIG. 21 , similarly to the related art, the outer rings  21  and  22  may be fixed to the sleeve  41  by loose fitting or through O-rings  72 . 
     Also, in  FIG. 20 , the turbine blade  12  is disposed between the lower rolling bearing  13  and the upper rolling beating  14 . However, the turbine blade may be fixed to a portion of the rotating shaft  11  lower than the lower rolling bearing  13 , like a second modification shown in  FIG. 22A , or may be fixed to a portion of the rotating shaft  11  higher than the upper rolling bearing  14 . Alternatively, like a third modification shown in  FIG. 22B , a pair of turbine blades  12   a  and  12   b  may be fixed to portions of the rotating shaft  11  positioned on both outer sides of the lower rolling bearing  13  and the upper rolling bearing  14  in the axis direction. 
     That is, in a bearing unit  20  for a dental air turbine of a according to a second modification shown in  FIG. 22A , the inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  are brought into contact with a step portion  52  protruding toward the inner side of a sleeve  51  in the radial direction, whereby the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back and close to each other. At positions of the sleeve  51  which face the turbine blade  12 , an air supply hole portion  53  for supplying compressed air to the turbine blade  12  and an exhaust hole portion (not shown) for discharging air are provided. 
     In a bearing unit  20  for a dental air turbine according to a third modification shown in  FIG. 22B , the inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  are brought into contact with a step portion  56  protruding from the center of a sleeve  55  toward the inner side in the radial direction, whereby the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back and close to each other. At positions of the sleeve  55  which face the turbine blades  12   a  and  12   b,  air supply hole portions  57   a  and  57   b  for supplying compressed air to the turbine blades  12   a  and  12   b  and exhaust hole portions (not shown) for discharging air are provided. 
     In  FIG. 20 , the external diameters D 1  of the outer rings  21  and  22  are set to be larger than the external diameter D 2  of the turbine blade  12 . However, it is only needed to set the external diameter D 1  of at least one of the outer rings  21  and  22  larger than the external diameter D 2  of the turbine blade  12 . For example, in a bearing unit  20  for a dental air turbine according to a fourth modification shown in  FIG. 23 , the turbine blade  12  is disposed between the lower rolling bearing  13  and the upper rolling bearing  14 , and the external diameter D 3  of the outer ring  21  of the lower rolling bearing  13  is set to be smaller than the external diameter D 2  of the turbine blade  12 , and the external diameter D 1  of the outer ring  22  of the upper rolling bearing  14  is set to be larger than the external diameter D 2  of the turbine blade  12 . 
     A sleeve  61  which is fit on the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  has a small-diameter hole portion  62 , a mid-diameter hole portion  63 , and a large-diameter hole portion  64  formed corresponding to the lower rolling bearing  13 , the turbine blade  12 , and the upper rolling bearing  14 , respectively. The outer ring  21  of the lower rolling bearing  13  is fit into the small-diameter hole portion  62 , whereby the inner side surface  21   a  of the outer ring  21  comes into contact with a step portion  65  protruding from the small-diameter hole portion  62  toward the inner side in the radial direction. The outer ring  22  of the upper rolling bearing  14  is fit into the large-diameter hole portion  64 , whereby the inner side surface  22   a  of the outer ring  22  comes into contact with a step portion  66  between the mid-diameter hole portion  63  and the large-diameter hole portion  64 , and the inner rings  23  and  24  are fit on the rotating shaft  11  to be fixed thereto, whereby a preload is applied to the lower rolling bearing  13  and the upper rolling bearing  14 . At positions of the sleeve  61  which face the turbine blade  12 , an air supply hole portion  67  for supplying compressed air to the turbine blade  12  and an exhaust hole portion (not shown) for discharging air are provided. 
     The configuration of the fourth modification can be applied to the bearing units  20  for dental air turbines according to the fourth embodiment of  FIG. 20  and the second and third modifications of  FIGS. 21A and 21B . 
     Also, in bearing units  20  for dental air turbines like the second and third modifications of  FIGS. 22A and 22B , the external diameters D 1  of the outer rings  21  and  22  may be set to be smaller than the external diameter D 2  of the turbine blade  12 , and for example, tolling bearings smaller than beatings which are generally used may be applied. Specifically, in a bearing unit  20  for a dental air turbine according to a fifth modification shown in  FIG. 24A , a turbine blade  12  having the same size as that of the turbine blade  12  shown in  FIG. 22A  is used, and small-sized rolling bearings  13  and  14  are used. Therefore, although the external diameters of the rotating shaft  11 , the turbine blade  12 , and the sleeve  51  are the same as those of the bearing unit  20  for a dental air turbine according to the second modification shown in  FIG. 22A , it is possible to reduce the lengths of the sleeve  51  and the rotating shaft  11  in the axis direction, and it is possible to improve rotation performance. 
     In a bearing unit  20  for a dental air turbine according to a sixth modification shown in  FIG. 24B , although the external diameter of the rotating shaft  11  is the same as that of the rotating shaft  11  shown in  FIG. 22A , since small-sized rolling bearings  13  and  14  are used, the external diameter and length of the turbine blade  12  decreases, and the external diameter and length of the sleeve  51  decreases, whereby the size of the whole unit decreases. Therefore, although the size of the turbine blade  12  is small, since the sizes of the bearings are small, it is possible to maintain rotation performance and functions. 
     Further, the bearing unit  20  for a dental air turbine having the sleeve according to the sixth embodiment may be assembled in the housing  15  with elastic members interposed therebetween. For example, a bearing unit  20  for a dental air turbine according to a seventh modification shown in  FIG. 25  is an example of application to the bearing unit  20  for a dental air turbine shown in  FIG. 20 , and an O-ring groove  41   b  is formed at a position of the outer circumferential surface  41   a  of the sleeve  41  corresponding to the outer ring  22 , and an O-ring groove  15   b  is formed at a position of the inner circumferential surface  15   a  of the housing  15  corresponding to the outer ring  21 . O-rings  70  are installed in the O-ring grooves  15   b  and  41   b,  and the sleeve  41  and the housing  15  are fit together. Also, even between the outer end surfaces of the sleeve  41  in the axis direction and the inner end surfaces of the housing  15  in the axis direction, O-rings  71  may be installed. Accordingly, it is possible to suppress vibration of the rotating shaft  11  rotating at high speed from being transmitted to the housing  15 . 
     Although a case where the O-ring grooves  15   b  and  41   b  are formed in the inner circumferential surface  15   a  of the housing  15  and the outer circumferential surface  41   a  of the sleeve  41 , respectively, has been described, the two O-ring grooves may be formed in any one of the inner circumferential surface  15   a  of the housing  15  and the outer circumferential surface  41   a  of the sleeve  41 . 
     The configuration of the seventh modification can be applied to the bearing units  20  for dental air turbines according to the first to sixth modifications. 
     Even in the sixth embodiment, as long as the lower rolling bearing  13  and the upper rolling bearing  14  arranged back-to-back, they may be configured such that a constant preload is applied thereto. In this case, a spring member  35  abuts on at least one of the facing inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction. 
     That is like a bearing unit  20  for a dental air turbine according to an eighth modification shown in  FIG. 26A , spring members  35  (leaf springs or spring washers) are disposed between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, respectively, and the respective inner rings  23  and  24  are lit on the rotating shaft  11  to be fixed thereto. Accordingly, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Alternatively, like a bearing unit  20  for a dental air turbine according to a ninth modification shown in  FIG. 26B , the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  come into contact with the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, and the respective inner rings  23  and  24  are fit on the rotating shaft  11  while fixing the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to fixing rings  36  with the spring members  35  (leaf springs or spring washers) interposed therebetween, respectively. In this case, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, and a constant preload is applied thereto. 
     Also, the present invention is not limited to the bearing units  20  for dental air turbines according to the eighth and ninth modifications, and similarly to the fifth embodiment, a spring member  35  may be disposed in any one of the gaps between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, and the respective inner rings  23  and  24  may be fit on the rotating shaft  11  to be fixed thereto. 
     Alternatively, the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  may come into contact with the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, and the respective inner rings  23  and  24  may be fit on the rotating shaft  11  while fixing only one of the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to a fixing ring  36  with a spring member  35  interposed therebetween and fitting the other one of the inner rings  23  and  24  on the rotating shaft  11  to be fixed therebetween, 
     Further, a spring member  35  may be disposed in any one of the gaps between the facing side surfaces of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rotting bearing  14  and the side surfaces of the step portion  42  of the sleeve  41  in the axis direction, and the respective inner rings  23  and  24  may be fit on the rotating shaft  11  while fixing only one of the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction to a fixing ring  36  with a spring member  36  interposed therebetween and fitting the other one of the inner rings  23  and  24  on the rotating shaft  11  to be fixed thereto. 
     Also, in a bearing unit  20  for a dental air turbine according to a tenth modification shown in  FIG. 27 , similarly to the fourth embodiment, the lower rolling bearing  13  and the upper rolling bearing  14  may be configured by the angular ball bearings having counterbores  21   f  and  22   f  formed in the outer rings  21  and  22 . 
     That is, the inner periphery of the outer ring  21  of the lower rolling bearing  13  has the shoulder portion  21   e  formed on one side with respect to the outer ring raceway surface  21   c  in the axis direction (the inner side in the axis direction) and the counter-bore  21   f  formed on the other side in the axis direction (the outer side in the axis direction), and the inner periphery of the outer ring  22  of the upper rolling bearing  14  has the shoulder portion  22   e  formed on one side with respect to the outer ring raceway surface  22   c  in the axis direction (the inner side in the axis direction) and the counter-bore  22   f  formed on the other side in the axis direction (the outer side in the axis direction). The outer peripheries of the inner rings  23  and  24  have the pair of shoulder portions  23   d  and  23   e  formed on both sides with respect to the inner ring raceway surface  23   c  in the axis direction, and the pair of shoulder portions  24   d  and  24   e  formed on both sides with respect to the inner ring raceway surface  24   c  in the axis direction, respectively. 
     Moreover, in bearing units  20  for dental air turbines according to eleventh and twelfth modifications shown in  FIGS. 28A and 28B , similarly to the third embodiment, the lower rolling bearing  13  includes sealing means  81  and  83  ( 81   a  and  83   a ) provided on both sides of the outer ring  21  in the axis direction so as to face the outer periphery of the inner ring  23  and has a labyrinth structure between the sealing means  81  and  83  ( 81   a  and  83   a ) and the outer periphery of the inner ring  23 , and the upper rolling bearing  14  includes sealing means  82  and  84  ( 82   a  and  84   a ) provided on both sides of the outer ring  22  in the axis direction so as to face the outer periphery of the inner ring  24  and has a labyrinth structure between the sealing means  82  and  84  ( 82   a  and  84   a ) and the outer periphery of the inner ring  24 . 
     Specifically, in the eleventh modification shown in  FIG. 28A , the sealing means  81 ,  82 ,  83 , and  84  include sealing portions  81  and  82  formed integrally with the outer rings  21  and  22  on the outer sides in the axis direction so as to extend from the shoulder portions  21   d  and  22   d  of the outer rings  21  and  22  toward the inner rings  23  and  24  in the radial directions, and the sealing members  83  and  84  disposed in seal grooves formed in the shoulder portions  21   e  and  22   e  of the outer rings  21  and  22  on the inner sides in the axis direction by use of retaining rings  85 . 
     The sealing members  83  and  84  provided on the inner sides in the axis direction and the sealing portions  81  and  82  provided on the outer sides in the axis direction may be inversely configured. 
     In the twelfth modification shown in FIG,  28 B, the sealing means  81   a,    82   a,    83   a,  and  84   a  include sealing members  81   a  and  82   a  disposed in seal grooves formed in the counter-bores  21   f  and  22   f  of the outer rings  21  and  22  on the outer sides in the axis direction by use of retaining rings  85 , and sealing portions  83   a  and  84   a  formed integrally with the outer rings  21  and  22  on the inner sides in the axis direction so as to extend from the shoulder portions  21   e  and  22   e  of the outer rings  21  and  22  toward the inner rings  23  and  24  in the radial directions. 
     In the eleventh and twelfth modifications, labyrinth structures may be provided only on the outer sides in the axis direction by the sealing means  81  and  82  ( 81   a  and  82   a ) provided on the outer sides in the axis direction and the outer peripheries of the inner rings  23  and  24 , without providing the sealing means  83  and  84  ( 83   a  and  84   a ) on the inner sides in the axis direction. 
     Seventh Embodiment 
       FIG. 29  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to a seventh embodiment, and in a dental air turbine  10  of the seventh embodiment, the rotating shaft  11 , the turbine blade  12 , the pair of rolling bearings  13  and  14 , and an outer ring spacer (a space member)  45  are unitized as a bearing unit  20  for a dental air turbine, which is disposed in the housing  15 . 
     The external diameters D 1  of the outer rings  21  and  22  are set to be larger than the external diameter D 2  of the turbine blade  12 , and the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back on both sides of the turbine blade  12  fixed to the center of the rotating shaft  11  and rotatably support the rotating shaft  11 . 
     The outer ring spacer  45  is disposed between the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14 , and the inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  come into contact with side surfaces of the outer ring spacer  45  in the axis direction, respectively. At positions of the outer ring spacer  45  which face the turbine blade  12 , an air supply hole portion  46  for supplying compressed air to the turbine blade  12  and an exhaust hole portion (not shown) for discharging air are provided. 
     In a state where the outer ring spacer  45  is interposed between the outer rings  21  and  22 , the inner rings  23  and  24  are fit into the rotating shaft  11  to be fixed thereto such that a preload is applied. In this way, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back, whereby a bearing unit  20  for a dental air turbine is configured. 
     As described above, according to the bearing unit  20  for a dental air turbine according to the seventh embodiment, the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back while interposing the outer ring spacer  45  between the outer rings  21  and  22  and fitting the inner rings  23  and  24  on the rotating shaft  11  such that a preload is applied thereto. Therefore, a preload is applied to the lower rolling bearing  13  and the upper rolling bearing  14 , whereby it is possible to improve the load capability of the rotating shaft  11  relative against a moment load. Also, it is possible to unitize the rotating shaft  11  having the turbine blade  12  and the lower and upper rolling bearings  13  and  14 , and it becomes easy to assemble them in the housing  15  of the dental air turbine  10 . 
     Since the external diameters D 1  of the outer rings  21  and  22  are larger than the external diameter D 2  of the turbine blade  12 , and the outer ring spacer  47  is disposed between the pair of outer rings  21  and  22 , it is possible to simplify the configuration of the bearing unit  20  for a dental air turbine. 
     The other configurations and effects are the same as those of the bearing unit  20  for a dental air turbine according to the fifth embodiment, and thus will not be described. 
     In  FIG. 29A , the external diameters D 1  of the outer rings  21  and  22  of the lower and upper rolling bearings  13  and  14  are set to be equal to each other. However, like a first modification shown in  FIG. 29B , the outer ring spacer  45  may be formed so as to have substantially L-shaped cross sections, whereby it is possible to apply, as the lower rolling bearing  13 , a bearing having an outer ring  21  having an external diameter D 3  smaller than the external diameter D 1  of the outer ring  22  of the upper rolling bearing  14 . That is, at an end portion of the outer ring spacer  45  positioned on the lower rolling bearing  13  side, an inward flange portion  48  is provided. Therefore, the outer ring spacer  45  is disposed between the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14 , and comes into contact with the inner side surfaces  21   a  and  22   a  of the inner rings  23  and  24 . 
     At the inner end portion of the inward flange portion  48  of the outer ring spacer  45 , a cylindrical portion  49  is formed and is fit into a fitting groove  21   g  of the outer ring  21  of the lower rolling bearing  13 . 
     In  FIG. 29A , the turbine blade  12  is disposed between the lower rolling bearing  13  and the upper rolling bearing  14 . However, the turbine blade may be fixed to a portion of the rotating shaft  11  lower than the lower rolling bearing  13 , like a second modification shown in  FIG. 30A , or may be fixed to a portion of the rotating shaft  11  higher than the upper rolling bearing  14 . Alternatively, like a third modification shown in  FIG. 30B , a pair of turbine blades  12   a  and  12   b  may be fixed to portions of the rotating shaft  11  positioned on both outer sides of the lower rolling bearing  13  and the upper rolling bearing  14  in the axis direction. 
     That is, in the bearing unit  20  for a dental air turbine according to any one of the second modification shown in  FIG. 30A  and the third modification shown in  FIG. 30B , an outer ring spacer  47  is interposed between the outer rings  21  and  22 , and the inner rings  23  and  24  are fixed to the rotating shaft  11 , whereby a preload is applied, whereby the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back and close to each other. 
     The bearing unit  20  for a dental air turbine having the outer ring spacer according to the seventh embodiment may be assembled in the housing with elastic members interposed therebetween. For example, a bearing unit  20  for a dental air turbine according to a fourth modification shown in  FIG. 31  is an example of application to the bearing unit  20  for a dental air turbine shown in  FIG. 30A , and an O-ring groove  22   c  is formed at a position of the outer circumferential surface  22   b  of the outer ring  22 , and an O-ring groove  15   b  is formed at a position of the inner circumferential surface  15   a  of the housing  15  corresponding to the outer ring  21 . O-rings  70  are installed in the O-ring grooves  22   c  and  15   b,  respectively, and the outer rings  21  and  22  are fit in the housing  15 . 
     Although a case where the O-ring grooves  15   b  and  22   c  are formed in the inner circumferential surface  15   a  of the housing  15  and the outer circumferential surface  22   b  of the outer ring  22 , respectively, has been described, the two O-ring grooves may be formed in the inner circumferential surface  15   a  of the housing  15 , or may be provided in the outer circumferential surfaces of the outer rings  21  and  22 , respectively. 
     The configuration of the fourth modification can be applied to the bearing units  20  for dental air turbines according to the first to third modifications. 
     In the bearing unit  20  for a dental air turbine having the outer ring spacer according to the seventh embodiment, like the fifth embodiment, as long as the lower rolling bearing  13  and the upper rolling bearing  14  arranged back-to-back, they may be configured such that a constant preload is applied thereto. In this case, a spring member abuts on at least one of the facing inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction. 
     Eighth Embodiment 
       FIG. 32  is a half cross-sectional view of a main portion of a bearing unit for a dental air turbine according to an eighth embodiment. The eighth embodiment is different from the seventh embodiment in the configurations of the lower rolling bearing  13  and the upper rolling bearing  14 . Therefore, parts identical to or equivalent to those of the seventh embodiment are denoted by the same reference symbols, and a description thereof will not be made or will be shortly made. 
     That is, in the eighth embodiment, the lower rolling bearing  13  and the upper rolling bearing  14  are angular ball bearings, and the outer periphery of the inner ring  23  has a shoulder portion  23   d  formed on one side with respect to the inner ring raceway surface  23   c  in the axis direction (the outer side in the axis direction), and a counter-bore  23   f  formed on the other side in the axis direction (the inner side in the axis direction), and the outer periphery of the inner ring  24  has a shoulder portion  24   d  formed on one side with respect to the inner ring raceway surface  24   c  in the axis direction (the outer side in the axis direction), and a counter-bore  24   f  formed on the other side in the axis direction (the inner side in the axis direction). The inner periphery of the outer ring  21  has a pair of shoulder portions  21   d  and  21   e  formed on both sides with respect to the outer ring raceway surface  21   c  in the axis direction, and the inner periphery of the outer ring  22  has a pair of shoulder portions  22   d  and  22   e  formed on both sides with respect to the outer ring raceway surface  22   c  in the axis direction. 
     That is, according to the bearing unit  20  for a dental air turbine according to the eighth embodiment, the lower rolling bearing  13  and the upper rolling bearing  14  are configured by angular ball bearings, and are arranged back-to-back while interposing the outer ring spacer  45  between the outer rings  21  and  22  and fitting the inner rings  23  and  24  on the rotating shaft  11  such that a preload is applied thereto. Therefore, a preload is applied to the lower rolling bearing  13  and the upper rolling bearing  14 , whereby it is possible to improve the load capability of the rotating shaft  11  against a moment load. Also, it is possible to unitize the rotating shaft  11  having the turbine blade  12  and the lower and upper rolling bearings  13  and  14 , and it becomes easy to assemble them in the housing  15  of the dental air turbine  10 . 
     Since the external diameters D 1  of the outer rings  21  and  22  are larger than the external diameter D 2  of the turbine blade  12 , and the outer ring spacer  47  is disposed between the pair of outer rings  21  and  22 , it is possible to simplify the configuration of the beating unit  20  for a dental air turbine. 
     The other configurations and effects are the same as those of the bearing unit  20  for a dental air turbine according to the seventh embodiment, and thus will not be described. 
     In  FIG. 32A , the external diameters D 1  of the outer rings  21  and  22  of the lower and upper rolling bearings  13  and  14  are set to be equal to each other. However, like a first modification shown in  FIG. 32B , the outer ring spacer  45  may be formed so as to have substantially L-shaped cross sections, whereby it is possible to apply, as the lower rolling bearing  13 , a bearing having an outer ring  21  having an external diameter D 3  smaller than the external diameter D 1  of the outer ring  22  of the upper rolling bearing  14 . That is, at an end portion of the outer ring spacer  45  positioned on the lower rolling bearing  13  side, an inward flange portion  48  is provided. Therefore, the outer ring spacer  45  is disposed between the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14 , and comes into contact with the inner side surfaces  21   a  and  22   a  of the inner rings  23  and  24 . 
     At the inner end portion of the inward flange portion  48  of the outer ring spacer  45 , a cylindrical portion  49  is formed and is fit into a fitting groove  21   g  of the outer ring  21  of the lower rolling bearing  13 . 
     In  FIG. 32A , the turbine blade  12  is disposed between the lower rolling bearing  13  and the upper rolling bearing  14 . However, the turbine blade may be fixed to a portion of the rotating shaft  11  lower than the lower rolling bearing  13 , like a second modification shown in  FIG. 33A , or may be fixed to a portion of the rotating shaft  11  higher than the upper rolling bearing  14 . Alternatively, like a third modification shown in  FIG. 33B , a pair of turbine blades  12   a  and  12   b  may be fixed to portions of the rotating shaft  11  positioned on both outer sides of the lower rolling bearing  13  and the upper rolling beating  14  in the axis direction. 
     That is, even in the bearing unit  20  for a dental air turbine according to any one of the second modification shown in  FIG. 33A  and the third modification shown in  FIG. 33B , an outer ring spacer  47  is interposed between the outer rings  21  and  22 , and the inner rings  23  and  24  are fixed to the rotating shaft  11 , whereby a preload is applied, whereby the lower rolling bearing  13  and the upper rolling bearing  14  are arranged back-to-back and close to each other. 
     The bearing unit  20  for a dental air turbine having the outer ring spacer according to the eighth embodiment may be assembled in the housing with elastic members interposed therebetween. For example, a bearing unit  20  for a dental air turbine according to a fourth modification shown in  FIG. 34  is an example of application to the bearing unit  20  for a dental air turbine shown in  FIG. 33A , and an O-ring groove  22   c  is formed at a position of the outer circumferential surface  22   b  of the outer ring  22 , and an O-ring groove  15   b  is formed at a position of the inner circumferential surface  15   a  of the housing  15  corresponding to the outer ring  21 . O-rings  70  are installed in the O-ring grooves  22   c  and  15   b,  respectively, and the outer rings  21  and  22  are fit in the housing  15 . 
     Although a case where the O-ring grooves  15   b  and  22   c  are formed in the inner circumferential surface  15   a  of the housing  15  and the outer circumferential surface  22   b  of the outer ring  22 , respectively, has been described, the two O-ring grooves may be formed in the inner circumferential surface  15   a  of the housing  15 , or may be provided in the outer circumferential surfaces of the outer rings  21  and  22 , respectively. 
     The configuration of the fourth modification can be applied to the bearing units  20  for dental air turbines according to the first to third modifications. 
     In the bearing unit  20  for a dental air turbine having the outer ring spacer according to the eighth embodiment, like the seventh embodiment, as long as the lower rolling bearing  13  and the upper rolling bearing  14  arranged back-to-back, they may be configured such that a constant preload is applied thereto. In this case, a spring member abuts on at least one of the facing inner side surfaces  21   a  and  22   a  of the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and the outer side surfaces  23   a  and  24   a  of the inner rings  23  and  24  in the axis direction. 
     In a bearing unit  20  for a dental air turbine according to a fifth modification shown in  FIG. 35A , similarly to the fourth embodiment, the lower rolling bearing  13  and the upper rolling bearing  14  may be configured by the angular ball bearings having counter-bores  21   f  and  22   f  formed in the outer rings  21  and  22 . 
     That is, the inner periphery of the outer ring  21  of the lower rolling bearing  13  has the shoulder portion  21   e  formed on one side with respect to the outer ring raceway surface  21   c  in the axis direction (the inner side in the axis direction) and the counter-bore  21   f  formed on the other side in the axis direction (the outer side in the axis direction), and the inner periphery of the outer ring  22  of the upper rolling bearing  14  has the shoulder portion  22   e  formed on one side with respect to the outer ring raceway surface  22   c  in the axis direction (the inner side in the axis direction) and the counter-bore  22   f  formed on the other side in the axis direction (the outer side in the axis direction). Also, the outer peripheries of the inner rings  23  and  24  have the pair of shoulder portions  23   d  and  23   e  formed on both sides with respect to the inner ring raceway surface  23   c  in the axis direction, and the pair of shoulder portions  24   d  and  24   e  formed on both sides with respect to the inner ring raceway surface  24   c  in the axis direction, respectively. 
     Also, like a sixth modification shown in  FIG. 35B , the outer ring spacer  45  may be formed so as to have substantially L-shaped cross sections, whereby it is possible to apply, as the lower rolling bearing  13 , a bearing including an outer ring  21  having an external diameter D 3  smaller than the external diameter D 1  of the outer ring  22  of the upper rolling bearing  14 . That is, at an end portion of the outer ring spacer  45  positioned on the lower rolling bearing  13  side, an inward flange portion  48  is provided. Therefore, the outer ring spacer  45  is disposed between the outer rings  21  and  22  of the lower rolling bearing  13  and the upper rolling bearing  14  and comes into contact with the inner side surfaces  21   a  and  22   a  of the inner rings  23  and  24 . 
     At the inner end portion of the inward flange portion  48  of the outer ring spacer  45 , a cylindrical portion  49  is formed and is fit into a fitting groove  21   g  of the outer ring  21  of the lower rolling bearing  13 . 
     Moreover, in bearing units  20  for dental air turbines according to seventh and eighth modifications shown in  FIGS. 36A and 36B , similarly to the third embodiment, the lower rolling bearing  13  includes sealing means  81  and  83  ( 81   a  and  83   a ) provided on the outer ring  21  on both sides in the axis direction so as to face the outer periphery of the inner ring  23 , and has a labyrinth structure between the sealing means  81  and  83  ( 81   a  and  83   a ) and the inner ring  23 , and the upper rolling bearing  14  includes sealing means  82  and  84  ( 82   a  and  84   a ) provided on the outer ring  22  on both sides in the axis direction so as to face the outer periphery of the inner ring  24 , and has a labyrinth structure between the sealing means  82  and  84  ( 82   a  and  84   a ) and the inner ring  24 . 
     Specifically, in the seventh modification shown in  FIG. 36A , the sealing means  81 ,  82 ,  83 , and  84  include sealing portions  81  and  82  formed integrally with the outer rings  21  and  22  on the outer sides in the axis direction so as to extend from the shoulder portions  21   d  and  22   d  of the outer rings  21  and  22  toward the inner rings  23  and  24  in the radial directions, and sealing members  83  and  84  disposed in seal grooves formed in the shoulder portions  21   e  and  22   e  of the outer rings  21  and  22  on the inner sides in the axis direction by use of retaining rings  85 . 
     The sealing members  83  and  84  provided on the inner sides in the axis direction and the sealing portions  81  and  82  provided on the outer sides in the axis direction may be inversely configured. 
     In the eighth modification shown in  FIG. 36B , the sealing means  81   a,    82   a,    83   a,  and  84   a  include sealing members  81   a  and  82   a  disposed in seal grooves formed in the counter-bores  21   f  and  22   f  of the outer rings  21  and  22  on the outer sides in the axis direction by use of retaining rings  85 , and sealing portions  83   a  and  84   a  formed integrally with the outer rings  21  and  22  on the inner sides in the axis direction so as to extend from the shoulder portions  21   e  and  22   e  of the outer rings  21  and  22  toward the inner rings  23  and  24  in the radial directions. 
     In the seventh and eighth modifications, labyrinth structures may be provided only on the outer sides in the axis direction by the sealing means  81  and  82  ( 81   a  and  82   a ) provided on the outer sides in the axis direction and the outer peripheries of the inner rings  23  and  24 , without providing the sealing means  83  and  84  ( 83   a  and  84   a ) on the inner sides in the axis direction. 
     It is noted that the present invention is not limited to each embodiment described above, and modifications, improvements, and the like can be made properly. 
     For example, in the rolling bearings, the types and arrangements of the cages and the scaling members may be appropriately changed. 
     Further, the present invention is not limited to each embodiment described above, and modifications, improvements, and the like can be made properly. 
     More specifically, as the ball bearings which configure the rolling bearings, as described above, angular ball bearings having counter-bores in their outer rings or their inner rings may be used, or deep groove rolling bearings having no counter-bores may be used. Also, the types and arrangements of the cages and the sealing members may be appropriately changed. 
     This application is based on Japanese Patent Application No. 2013-196875, filed on Sep. 24, 2013, and Japanese Patent Application No. 2014-034542, filed on Feb. 25, 2014, and Japanese Patent Application No. 2014-182122, filed on Sep. 8, 2014; and the entire contents of which are incorporated herein by reference. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           10 : dental air turbine 
           11 : rotating shaft 
           12 ,  12   a,    12   b:  turbine blade 
           13 : lower rolling bearing (rolling bearing) 
           14 : upper rolling bearing (rolling bearing) 
           15 : housing 
           20 : bearing unit for dental air turbine 
           21 ,  22 : outer ring 
           21   a,    22   a  inner side surface 
           21   b,    22   b:  outer circumferential surface 
           23 ,  24 : inner ring 
           21   f,    22   f,    23   f,    24   f:  counter-bore 
           32 ,  34 : rib (step portion) 
           35 : spring member 
           41 ,  51 ,  55 ,  61 : sleeve (space member) 
           41   a:  outer circumferential surface 
           42 ,  52 ,  56 ,  65 ,  66 : step portion 
           43 ,  46 ,  53 ,  57   a,    57   b,    67 : air supply hole portion 
           44 : exhaust hole portion 
           45 ,  47 : outer ring spacer (space member) 
           70 : O-ring (elastic member) 
           81 ,  82 ,  81   a,    82   a:  sealing portion (sealing means) 
           83 ,  84 ,  83   a,    84   a:  sealing member (sealing means) 
         D 1 , D 3 : external diameter of outer ring 
         D 2 : external diameter of turbine blade