Abstract:
A bearing device has a sleeve that encircles a shaft, and two side-by-side roller bearings rotatably support the sleeve coaxially on the shaft. A spacer portion protrudes radially inwardly from the sleeve and extends between the two roller bearings. The sleeve axially overlaps a minor portion of one roller bearing but does not overlap a race surface thereof, and the sleeve axially overlaps a major portion of the other roller bearing including a race surface thereof.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bearing device and an information recording and reproducing apparatus. 
     2. Description of the Related Art 
     Hitherto, an information recording and reproducing apparatus such as a hard disk has been known, which allows a disk (magnetic recording medium) to store various types of information therein and to reproduce the information therefrom. In general, the information recording and reproducing apparatus includes: a head gimbal assembly having a slider that records a signal in the disk and reproduces the signal therefrom; and an arm (rotational member) in which the head gimbal assembly is mounted on a tip end. The arm is made rotatable by a bearing device provided on a base end thereof. The arm is rotationally moved, whereby the slider is moved to a predetermined position of the disk, thus making it possible to record the signal in the disk and to reproduce the signal therefrom. 
       FIG. 9  is a cross-sectional view of a bearing device  10  according to a conventional technology. The bearing device  10  includes a shaft  20  and a sleeve  40 , which are arranged coaxially with each other. The shaft  20  is connected to a cabinet of the information recording and reproducing apparatus, and an arm  8  of the information recording and reproducing apparatus is connected to an outer periphery of the sleeve  40 . Between the shaft  20  and the sleeve  40 , a pair of roller bearings  30   a  and  30   b  are arranged side by side in an axial direction. Outer peripheral surfaces of outer rings  34  of the respective roller bearings  30   a  and  30   b  and an inner peripheral surface of the sleeve  40  are fixedly adhered to each other by adhesive. 
     Mass of the sleeve  40  is large because the sleeve  40  is made of stainless steel. Therefore, a resonant frequency (resonance) between the sleeve  40  and the arm  8 , which are rotational portions, is decreased. Accordingly, there is a problem that the resonance is prone to occur at the time when the sleeve  40  and the arm  8  move rotationally, resulting in a decrease of positioning accuracy of the slider mounted on a tip end of the arm  8 . 
     In this connection, JP 2001-155448 A proposes a technology for increasing the resonant frequency in such a manner that a material of the sleeve is changed to aluminum to achieve weight reduction thereof. 
     However, the aluminum has a larger coefficient of linear expansion than the stainless steel, and accordingly, in the sleeve  40  made of the aluminum, an amount of thermal expansion thereof when a temperature thereof is high is increased. There is a problem that, if the amount of thermal expansion varies among the respective portions of the sleeve  40 , distortion occurs in the respective roller bearings  30   a  and  30   b.  Such a problem includes deformation of the respective roller bearings  30   a  and  30   b  from a perfect circular shape to an oval shape. Following such an occurrence of the distortion, an amount of rotation of the arm  8  with respect to rotation torque is changed, leading to the decrease of the positioning accuracy of the slider. 
     Further, the adhesive that fixedly adheres the roller bearings  30   a  and  30   b  and the sleeve  40  to each other is contracted at the time of being cured. There is a problem that, if an amount of contraction of the adhesive varies among the respective portions thereof, the distortion occurs in the roller bearings  30   a  and  30   b  in a similar way to the above. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the problems as described above. It is an object of the present invention to provide a bearing device capable of achieving the weight reduction of the rotational portions and capable of suppressing the distortion of the roller bearings. 
     In order to solve the above-mentioned problems, a bearing device according to the present invention includes: a shaft; a sleeve arranged on an outside of the shaft while a central axis thereof coincides with a central axis of the shaft; a first roller bearing and a second roller bearing, which are arranged side by side in a direction of the central axes, for supporting the shaft and the sleeve so that the shaft and the sleeve are capable of freely moving rotationally relative to each other; and a spacer portion formed between the first roller bearing and the second roller bearing so as to protrude from an inner peripheral surface of the sleeve, in which the sleeve is formed so as not to overlap a race surface of the first roller bearing in the direction of the central axes. 
     According to the present invention, a length of the sleeve in the direction of the central axes is shortened, and accordingly, weight of the sleeve can be reduced. Further, it becomes possible to reduce a constituent material and machining process of the sleeve, and hence manufacturing cost thereof can be reduced. 
     Further, on an inner peripheral surface of an outer ring of each of the roller bearings, the race surface having a groove on the entire circumference thereof is formed on a center portion in the direction of the central axes. In this center portion, a thickness thereof in a radial direction is reduced, and accordingly, rigidity thereof is low, and the distortion is prone to occur therein. The sleeve of the present invention is formed so as not to overlap a center portion of the first roller bearing in the direction of the central axes. Accordingly, even if the sleeve is thermally expanded, the distortion of the first roller bearing can be suppressed. 
     Further, it is preferred that the sleeve be formed so as not to overlap the first roller bearing in the direction of the central axes. 
     According to the present invention, the length of the sleeve in the direction of the central axes is further shortened, and accordingly, the weight of the sleeve can be reduced. Further, the first roller bearing is hardly affected by the thermal expansion of the sleeve, and accordingly, the distortion of the first roller bearing can be prevented. 
     Further, it is preferred that the bearing device include a pedestal portion formed so as to protrude from an outer peripheral surface of the sleeve, in which a rotational member made freely rotatable by the bearing device is fixedly attached to a first end surface of the pedestal portion. 
     According to the present invention, the pedestal portion is formed so as to protrude from the sleeve, and accordingly, rigidity of the sleeve is enhanced. Therefore, even if the rotational member is thermally expanded, it becomes possible to suppress deformation of the sleeve. Following such suppression of the deformation, the distortion of each of the roller bearings can be suppressed. 
     Further, it is preferred that a second end surface of the pedestal portion be formed so as to be flush with an end surface of the sleeve. 
     According to the present invention, it becomes possible to simultaneously machine the second end surface of the pedestal portion and the end surface of the sleeve, and hence the manufacturing cost can be reduced. 
     Further, it is preferred that the second end surface of the pedestal portion be formed between both end surfaces of the sleeve. 
     According to the present invention, a height (distance between the first end surface and the second end surface) of the pedestal portion is decreased, and accordingly, weight of the pedestal portion can be reduced. 
     Further, it is preferred that the first end surface of the pedestal portion be formed between both end surfaces of the spacer portion. 
     The rigidity of the sleeve is increased in a portion on which the spacer portion is formed so as to protrude therefrom. According to the present invention, the first end surface is formed between both the end surfaces of the spacer portion, and the rotational member is fixedly attached to the first end surface. Accordingly, the rotational member can be fixedly attached to the portion of the sleeve, in which the rigidity is high. Therefore, even if the rotational member is thermally expanded, it becomes possible to suppress the distortion of the sleeve. Following the suppression of the deformation, the distortion of each of the roller bearings can be suppressed. 
     Further, it is preferred that the sleeve be formed so as not to overlap a race surface of the second roller bearing in the direction of the central axes. 
     According to the present invention, the length of the sleeve in the direction of the central axes is further shortened, and accordingly, the weight of the sleeve can be reduced. Further, the sleeve of the present invention is formed so as not to overlap the center portions of both of the roller bearings in the direction of the central axes. Accordingly, even if the sleeve is thermally expanded, the distortion of both of the roller bearings can be suppressed. 
     On the other hand, an information recording and reproducing apparatus according to the present invention includes: the above-mentioned bearing device; a cabinet connected to the shaft; a rotational member connected to the sleeve; and a slider mounted on the rotational member, for recording information in a magnetic recording medium and reproducing information from the magnetic recording medium. 
     According to the present invention, the weight of the rotational portions (sleeve, pedestal portion, and the like) of the bearing device is reduced, and accordingly, the resonant frequency among the rotational portions can be increased. In such a way, it becomes possible to suppress the resonance at the time when the rotational portions move rotationally, and positioning accuracy of the slider with respect to the magnetic recording medium can be enhanced. 
     Further, the distortion of the roller bearings is suppressed, and accordingly, each of the rotational members can be moved rotationally by a predetermined amount according to rotation torque. Hence, the positioning accuracy of the slider with respect to the magnetic recording medium can be enhanced. 
     According to the bearing device of the present invention, the length of the sleeve in the direction of the central axes is shortened, and accordingly, the weight of the sleeve can be reduced. 
     Further, in the end portions of each of the roller bearings, the distortion is less likely to occur than in the center portion thereof. Accordingly, even if the sleeve is thermally expanded, the distortion of the first roller bearing can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a perspective view of an information recording and reproducing apparatus of the present invention; 
         FIG. 2  is a side cross-sectional view of a bearing device according to a first embodiment of the present invention; 
         FIG. 3  is a side cross-sectional view of a bearing device according to a second embodiment of the present invention; 
         FIG. 4  is a side cross-sectional view of a bearing device according to a third embodiment of the present invention; 
         FIG. 5  is a side cross-sectional view of a bearing device according to a fourth embodiment of the present invention; 
         FIG. 6  is a side cross-sectional view of a bearing device according to a fifth embodiment of the present invention; 
         FIG. 7  is a side cross-sectional view of a bearing device according to a sixth embodiment of the present invention; 
         FIG. 8  is a side cross-sectional view of a bearing device according to a modification example of the sixth embodiment of the present invention; and 
         FIG. 9  is a side cross-sectional view of a bearing device according to a conventional technology. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description is made below of embodiments of the present invention with reference to the accompanying drawings. 
     (Information Recording and Reproducing Apparatus) 
       FIG. 1  is a perspective view of an information recording and reproducing apparatus  1 . This information recording and reproducing apparatus  1  is an apparatus that performs writing to and reading from disks (magnetic recording media) D having recording layers. The information recording and reproducing apparatus  1  includes: arms (rotational members)  8 ; head gimbal assemblies  4  supported on tip ends of the arms  8 ; sliders  2  mounted on tip ends of the head gimbal assemblies  4 ; an actuator (voice coil motor: VCM)  6  that moves the head gimbal assemblies  4  in a scanning manner; a spindle motor  7  that rotates the disks D; a control unit  5  that supplies the sliders  2  with currents modulated in response to information; and a housing (cabinet)  9  that houses these respective components therein. 
     The housing  9  is made of a metal material, such as aluminum, and has a box shape with an opening in an upper portion thereof. The housing  9  is composed of a bottom  9   a  rectangular when viewed from above, and a peripheral wall (not shown) erected on a peripheral edge of the bottom  9   a  in a vertical direction with respect to the bottom  9   a.  In an inside of the housing  9 , which is surrounded by the peripheral wall, a recessed portion that houses the above-mentioned respective components therein is formed. Note that, in  FIG. 1 , the peripheral wall that surrounds the housing  9  is omitted in order to make the description easy to understand. The above-described spindle motor  7  is mounted on a substantial center of the bottom  9   a , and center holes of the disks D are fitted onto the spindle motor  7 , whereby the disks D are freely detachably fixed thereto. 
     A bearing device  10  according to this embodiment is arranged on the side of the disks D. The arms  8  are fixedly attached to an outer peripheral surface of the bearing device  10 . One-side ends of the arms  8 , which are closer to the bearing device  10 , are connected to the above-mentioned actuator  6 . Further, the other-side ends of the arms  8  are extended in parallel to surfaces of the disks D, and the head gimbal assemblies  4  are connected to tip ends of the other-side ends concerned. Each of the head gimbal assemblies  4  includes: a suspension  3 ; and a slider  2  that is mounted onto a tip end of the suspension  3 , and is arranged so as to be opposed to the surface of the disk D. The slider  2  includes: a recording element that performs the writing (recording) of information to the disk D; and a reproducing element that performs the reading (reproduction) of information from the disk D. 
     In order to perform the recording or reproduction of the information in the information recording and reproducing apparatus  1  configured as described above, the spindle motor  7  is first driven, and the disks D are rotated about a center axis L 2 . Further, the actuator  6  is driven, and the arms  8  are moved rotationally about a center axis L 1  of the bearing device  10 . In such a way, the sliders  2  arranged on the tip ends of the head gimbal assemblies  4  can be moved in a scanning manner to the respective portions of the surfaces of the disks D. Then, the recording elements or reproducing elements of the sliders  2  are driven, whereby the recording of the information to the disks D or the reproduction of the information from the disks D can be performed. 
     (Bearing Device) 
       FIG. 2  is a side cross-sectional view of the bearing device  10  according to the first embodiment. Note that the respective drawings of  FIG. 2  to  FIG. 9  are cross-sectional views of portions corresponding the line A-A of  FIG. 1 . Though, in  FIG. 1 , a plurality of the arms are fixedly attached to the outer peripheral surface of the bearing device  10 , in each of  FIG. 2  to  FIG. 9 , the case where only one arm is fixedly attached thereto is illustrated. Though, in  FIG. 2 , a fixing structure of a shaft  20  to the housing  9  is illustrated, illustration of the fixing structure is omitted in each of the drawings of  FIG. 3  to  FIG. 9 . In each of the drawings of  FIG. 2  to  FIG. 9 , a direction going along a central axis of the bearing device  10  and leaving the bottom  9   a  of the housing is defined as a +Z-direction, and a radial direction of the bearing device  10  is defined as an R-direction. 
     As illustrated in  FIG. 2 , the bearing device  10  includes: the inner shaft  20  and an outer sleeve  40 , which are arranged coaxially with each other; and a first roller bearing  30   a  and a second roller bearing  30   b,  which are arranged between the shaft  20  and the sleeve  40 . 
     The shaft  20  is made of a metal material such as stainless steel, and includes: a cylinder portion  22 ; and a flange portion  24  formed on an end of the cylinder portion  22  in a −Z-direction. The cylinder portion  22  is inserted onto an outside of a protruding portion  12  of the housing  9 , and the flange portion  24  abuts against the bottom  9   a  of the housing  9 . An annular fixing member  14  is mounted onto an end surface of the cylinder portion  22  in the +Z-direction. A bolt  15  is inserted into an inside of the cylinder portion  22  through a center hole of the fixing member  14  from an outside of the fixing member  14 . The bolt  15  is screwed to a female thread  13  formed on a tip end surface of the protruding portion  12 . By the fixing structure described above, the shaft  20  is fixed to the housing  9 . 
     The sleeve  40  is arranged on an outside of the shaft  20  and encircles the shaft while allowing a central axis thereof to coincide with that of the shaft  20 . The sleeve  40  includes a cylinder portion  42  made of a metal material such as stainless steel. To an outer peripheral surface of the sleeve  40 , the arm  8  of the information recording and reproducing apparatus is fixedly attached by adhesive, press fitting or the like. 
     The first roller bearing  30   a  and the second roller bearing  30   b  support the shaft  20  and the sleeve  40  so that the shaft  20  and the sleeve  40  are capable of freely moving rotationally relative to each other. Each of the roller bearings  30   a  and  30   b  includes an inner ring  32  and an outer ring  34 , which are made of a metal material such as stainless steel and arranged coaxially with each other. The inner ring  32  is fixed to an outer peripheral surface of the cylinder portion  22  of the shaft  20  by the adhesive, and the outer ring  34  is fixed to an inner peripheral surface of the sleeve  40  by the adhesive. Anaerobic adhesive is used as this adhesive. The anaerobic adhesive is cured by being shielded from the air and contacting a metal. Note that it is desirable to combine heating treatment with such curing treatment of the anaerobic adhesive. 
     The sleeve  40  is composed of stainless steel with a small coefficient of linear expansion, and accordingly, an amount of thermal expansion of the sleeve  40  is decreased at the time when the adhesive is cured and at the time when the information recording and reproducing apparatus is used at a high temperature. Thus, distortion can be suppressed from occurring in each of the roller bearings  30   a  and  30   b . Further, the outer rings  34  of the pair of roller bearings  30   a  and  30   b  and the sleeve  40  are both composed of stainless steel, and accordingly, the distortion can be suppressed from occurring in each of the roller bearings  30   a  and  30   b  owing to a difference in amount of thermal expansion between both of the outer rings  34  and the sleeve  40 . 
     Between the inner ring  32  and outer ring  34  of each of the roller bearings  30   a  and  30   b , a plurality of balls (rolling elements)  33  are arranged. On an inner peripheral surface of the outer ring  34  and on an outer peripheral surface of the inner ring  32 , a race surface  34   a  on which the balls  33  roll is formed. The race surfaces  34   a  are formed into a groove shape over the entire circumference of the inner ring  32  and the outer ring  34 , and a cross-sectional shape of the race surface  34   a  substantially coincides with an outline of the balls  33 . In such a way, the balls  33  are capable of smoothly rolling along the race surface  34   a  while being held on the race surface  34   a . Note that a similar race surface is also formed on an outer peripheral surface of the inner ring  32 . 
     The pair of roller bearings  30   a  and  30   b  are arranged side by side in the Z-direction. A −Z-side end surface of the inner ring  32  of the first roller bearing  30   a  arranged on a −Z-side abuts against the flange portion  24  of the shaft  20 . 
     Between the first roller bearing  30   a  and the second roller bearing  30   b,  a spacer portion  44  is formed so as to protrude from the inner peripheral surface of the sleeve  40 . The spacer portion  44  is formed into an annular shape over the entire circumference of the sleeve  40 . A +Z-side end surface of the outer ring  34  of the first roller bearing  30   a  arranged on the −Z-side abuts against a −Z-side end surface of the spacer portion  44 . Further, a −Z-side end surface of the outer ring  34  of the second bearing  30   b  arranged on a +Z-side abuts on a +Z-side end surface of the spacer portion  44 . 
     (First Embodiment) 
     As illustrated in  FIG. 2 , the sleeve  40  of the first embodiment is formed so as not to axially overlap the race surface  34   a  of the first roller bearing  30   a  in the Z-direction. In this application, the inner ring  32  and the outer ring  34  of the first and second roller bearings  30   a ,  30   b  each have a center portion  35  and end portions  36  in the Z-direction. The center portion  35  is a portion on which the race surface  34   a  is formed, and the end portions  36  are portions on which the race surface  34   a  is not formed. The sleeve  40  of this embodiment is adhered only to one of the end portions  36  of the outer ring  34  of the first bearing  30   a , and is not adhered to the center portion  35  thereof. The sleeve  40 , therefore, axially overlaps a minor portion of the first roller bearing  30   a  and does not overlap the race surfaces  34   a  of the roller bearing  30   a , whereas the sleeve  40  axially overlaps a major portion of the second roller bearing  30   b  including the race surfaces of the roller bearing  30   b . Even in this case, the pair of roller bearings  30   a  and  30   b  and the sleeve  40  are adhered to each other, and hence the sleeve  40  can be supported so as to be freely rotatable with respect to the shaft  20 . 
     In this embodiment, a length of the sleeve  40  in the Z-direction is shortened more than in the conventional technology illustrated in  FIG. 9 . Therefore, in this embodiment illustrated in  FIG. 2 , weight of the sleeve  40  can be reduced even if the sleeve  40  is not composed of the aluminum. In such a way, a resonant frequency between the sleeve  40  and the arm  8 , which are the rotational portions, is increased, and resonance can be suppressed from occurring when the sleeve  40  and the arm  8  move rotationally. Hence, positioning accuracy of the slider with respect to the disk can be enhanced. 
     Further, it becomes possible to reduce a constituent material and machining process of the sleeve  40  because the length of the sleeve  40  in the Z-direction is shortened. Thus, manufacturing cost thereof can be reduced. 
     Note that a thickness of the end portions  36  in the R direction on which the race surface  34   a  is not formed is larger than that of the center portion  35  on which the race surface  34   a  is formed. Therefore, rigidity of the end portions  36  is higher than in the center portion  35 , and the distortion is less likely to occur in the end portions  36 . 
     The sleeve  40  of this embodiment is adhered only to one of the end portions  36  of the first roller bearing  30   a,  and is not adhered to the center portion  35  thereof. Hence, even if the sleeve  40  is thermally expanded at the time when the information recording and reproducing apparatus is used at the high temperature, the distortion of the first rollerbearing  30   a  can be suppressed. Further, even if the adhesive is contracted and the sleeve  40  is thermally expanded at the time when the adhesive itself is cured, the distortion of the first roller bearing  30   a  can be suppressed. 
     (Second Embodiment) 
       FIG. 3  is a side cross-sectional view of a bearing device  10  according to a second embodiment. A sleeve  40  of the second embodiment is formed so as not to overlap the first bearing  30   a  in the Z-direction. The sleeve  40  of the first embodiment, which is illustrated in  FIG. 2 , overlaps the end portion  36  of the first roller bearing  30   a.  The sleeve  40  of the second embodiment, which is illustrated in  FIG. 3 , is different from the sleeve  40  of the first embodiment in not overlapping the first roller bearing  30   a  at all. Note that the +Z-side end surface of the outer ring  34  of the first roller bearing  30   a  and the −Z-side end surface of the spacer portion  44  do not have to be joined to each other though are desirably joined by the adhesive, laser welding or the like. In the second embodiment, the length of the sleeve  40  in the Z-direction is shortened more than in the first embodiment. Therefore, the weight of the sleeve  40  is reduced, and the resonant frequency between the rotational portions is increased. Hence, the positioning accuracy of the slider with respect to the disk can be enhanced. 
     Further, the first roller bearing  30   a  is hardly affected by the thermal expansion of the sleeve  40 . Therefore, the distortion of the first roller bearing  30   a  can be prevented. 
     Further, a −Z-side end surface of the sleeve  40  is formed so as to be flush with the −Z-side end surface of the spacer portion  44 . In such a way, it becomes possible to simultaneously machine the −Z-side end surfaces of the sleeve  40  and the spacer portion  44 , whereby manufacturing cost of the sleeve  40  can be reduced. 
     (Third Embodiment) 
       FIG. 4  is a side cross-sectional view of a bearing device  10  according to a third embodiment. The bearing device  10  according to the third embodiment includes a pedestal portion  46  formed so as to protrude from the outer peripheral surface of the sleeve  40 . The pedestal portion  46  is formed into a cylindrical shape over the entire circumference of the sleeve  40 . A +Z-side end surface (first end surface)  46   a  of the pedestal portion  46  is arranged between both end surfaces  40   a  and  40   b  of the sleeve  40 . The arm  8  of the information recording and reproducing apparatus is mounted on the first end surface  46   a,  and is fixedly adhered thereto by the adhesive or the like. In the third embodiment in which the arm  8  is fixedly attached to the pedestal portion  46 , the arm  8  can be supported more stably than in the first embodiment in which the arm  8  is fixedly attached to the outer peripheral surface of the sleeve  40 . 
     In general, the arm  8  is composed of the aluminum, and hence an amount of thermal expansion thereof at the time when a temperature thereof is high is increased. Therefore, it is apprehended that the sleeve  40  may be deformed, and further, the distortion may occur in each of the roller bearings  30   a  and  30   b.  Further, in the case of fixedly attaching the arm  8  to the sleeve  40 , it is apprehended that the distortion may occur in each of the roller bearings  30   a  and  30   b  owing to variations in amount of contraction of the adhesive at the time when the adhesive is cured. 
     In contrast, in this embodiment, the pedestal portion  46  is formed so as to protrude from the sleeve  40 , and accordingly, rigidity of the sleeve  40  is enhanced. Therefore, it becomes possible to suppress deformation of the sleeve  40 , and hence the distortion of each of the roller bearings  30   a  and  30   b  can be prevented. 
     Further, a −Z-side end surface (second end surface)  46   b  of the pedestal portion  46  is formed so as to be flush with the −Z-side end surface  40   b  of the sleeve  40  and a −Z-side end surface  44   b  of the spacer portion  44 . In such a way, it becomes possible to simultaneously machine the −Z-side end surfaces of the pedestal portion  46 , the cylindrical portion  42 , and the spacer portion  44 , whereby manufacturing cost of the sleeve  40  can be reduced. 
     (Fourth Embodiment) 
       FIG. 5  is a side cross-sectional view of a bearing device  10  according to a fourth embodiment. In the fourth embodiment, the second end surface  46   b  of the pedestal portion  46  is arranged between both end surfaces  40   a  and  40   b  of the sleeve  40 . In such a way, a height (distance in the Z-direction between the first end surface  46   a  and the second end surface  46   b ) of the pedestal portion  46  is decreased. Accordingly, weight of the pedestal portion  46  is reduced, and the resonant frequency between the rotational portions is increased. Hence, the positioning accuracy of the slider with respect to the disk can be enhanced. 
     (Fifth Embodiment) 
       FIG. 6  is a side cross-sectional view of a bearing device  10  according to a fifth embodiment. In the fifth embodiment, the first end surface  46   a  is formed between both end surfaces  44   a  and  44   b  of the spacer portion  44 , and the arm  8  is fixedly attached to the first end surface  46   a  concerned. Rigidity of the sleeve  40  is increased in a portion on which the spacer portion  44  is formed so as to protrude therefrom, and accordingly, the arm  8  can be fixedly attached to such a highly rigid portion in the fifth embodiment. Therefore, even if the amount of thermal expansion of the arm  8  is increased at the time when the temperature thereof is high, the deformation of the sleeve  40  can be suppressed. Following such suppression of the distortion, the distortion of each of the roller bearings  30   a  and  30   b  can be suppressed. 
     (Sixth Embodiment) 
       FIG. 7  is a side cross-sectional view of a bearing device  10  according to a sixth embodiment. A sleeve  40  of the sixth embodiment is formed so as not to overlap the race surface  34   a  of the second roller bearing  30   b  in the Z-direction. In other words, the sleeve  40  is adhered only to the end portion  36  of the outer ring  34  of the second roller bearing  30   b,  and is not adhered to the center portion  35  thereof. Note that the sleeve  40  does not overlap the first roller bearing  30   a  in the Z-direction at all. 
     In the sixth embodiment, the length of the sleeve  40  in the Z-direction is shortened more than in the second embodiment. Therefore, the weight of the sleeve  40  is reduced, and the resonant frequency between the rotational portions is increased. Hence, the positioning accuracy of the slider with respect to the disk can be enhanced. 
     Further, in the end portion  36 , the distortion is less likely to occur than in the center portion  35 . Accordingly, the distortion of the second roller bearing  30   b  can be suppressed even if the sleeve  40  is thermally expanded. 
     (Modified Embodiment) 
       FIG. 8  is a side cross-sectional view of a bearing device  10  according to a modified embodiment of a sixth embodiment. A sleeve  40  of the modified embodiment is formed so as not to overlap the race surface  34   a  of each of the roller bearings  30   a  and  30   b  in the Z-direction. In other words, the sleeve  40  is adhered only to the end portion  36  of the outer ring  34  of each of the roller bearings  30   a  and  30   b,  and is not adhered to the center portion  35  thereof. 
     Even in this case, similar effects to those of the sixth embodiment can be exerted. 
     Note that the technical scope of the present invention is not limited to the above-mentioned embodiments, and incorporates those in which a variety of alterations are added to the above-mentioned embodiments within the scope without departing from the spirit of the present invention. In other words, specific materials, layer configurations, and the like, which are mentioned in the embodiments, are merely examples, and are changeable as appropriate. 
     For example, the configuration of the information recording and reproducing apparatus is not limited to those described in the embodiments.