Patent Publication Number: US-2017356459-A1

Title: Blower apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a blower apparatus. 
     2. Description of the Related Art 
     A centrifugal blower apparatus which generates an air flow traveling radially outward by rotating an impeller including a plurality of blades is known. A known blower apparatus including an impeller is described in, for example, JP-A 2008-88985. 
     In the blower apparatus described in JP-A 2008-88985, a plurality of blades referred to as fan blades push surrounding gas to generate air flows traveling radially outward. 
     SUMMARY OF THE INVENTION 
     In recent years, there has still been a demand for reductions in the size and thickness of electronic devices. Accordingly, there has also been a demand for a reduction in the thickness of blower apparatuses used to cool the interiors of the electronic devices. 
     Here, in the case where an impeller is used to generate air flows, as in the blower apparatus described in JP-A 2008-88985, air flows pushed by a blade leak from axially upper and lower ends of the blade while the impeller is rotating. As a result, air pressure is lower at the axially upper and lower ends of the blade than in the vicinity of an axial middle of the blade. Accordingly, a reduction in the thickness of the blower apparatus, which involves a reduction in the axial dimension of the impeller, will result in a failure to secure sufficient air blowing efficiency. 
     An object of the present invention is to provide a technique for realizing a centrifugal blower apparatus which is excellent in air blowing efficiency. 
     A blower apparatus according to a preferred embodiment of the present invention includes an air blowing portion arranged to rotate about a central axis extending in a vertical direction; a motor portion arranged to rotate the air blowing portion; and a housing arranged to house the air blowing portion and the motor portion. The housing includes an air inlet arranged above the air blowing portion, and arranged to pass through a portion of the housing in an axial direction; and an air outlet arranged to face in a radial direction at at least one circumferential position radially outside of the air blowing portion. The air blowing portion includes a plurality of flat plates arranged in the axial direction with an axial gap defined between adjacent ones of the flat plates. The motor portion includes a stationary portion including an armature; and a rotating portion including a magnet arranged radially outside of the armature, and a hub arranged to hold the magnet. The hub includes a top plate portion arranged to cover an upper side of the armature; a magnet holding portion arranged to extend downward from the top plate portion to assume a cylindrical shape, and arranged to hold the magnet, with an inner circumferential surface thereof; and a flat plate holding portion arranged to extend radially on a radially outer side of the magnet holding portion, and hold at least one of the flat plates. 
     According to the above preferred embodiment of the present invention, once the air blowing portion starts rotating, an air flow traveling radially outward is generated in the axial gap between the adjacent ones of the flat plates by viscous drag of surfaces of the flat plates and a centrifugal force. Since the air flow is generated between the flat plates, the air flow does not easily leak upwardly or downwardly, and thus, an improvement in air blowing efficiency is achieved. Further, since the flat plates are held by the flat plate holding portion, the air blowing portion is able to stably rotate. Accordingly, a further improvement in the air blowing efficiency can be achieved. Thus, a reduced thickness of the blower apparatus according to the above preferred embodiment of the present invention does not result in a significant reduction in the air blowing efficiency. In addition, the blower apparatus according to the above preferred embodiment of the present invention is superior to a comparable centrifugal fan including an impeller in terms of being silent. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a blower apparatus according to a first preferred embodiment of the present invention. 
         FIG. 2  is a top view of the blower apparatus according to the first preferred embodiment. 
         FIG. 3  is a sectional view of the blower apparatus according to the first, preferred embodiment. 
         FIG. 4  is an exploded perspective view of the blower apparatus according to the first, preferred embodiment. 
         FIG. 5  is a partial sectional view of the blower apparatus according to the first preferred embodiment. 
         FIG. 6  is a partial sectional view of a blower apparatus according to a modification of the first preferred embodiment. 
         FIG. 7  is a partial sectional view of a blower apparatus according to a modification of the first preferred embodiment. 
         FIG. 8  is a partial sectional view of a blower apparatus according to a modification of the first preferred embodiment. 
         FIG. 9  is a partial sectional view of a blower apparatus according to a modification of the first preferred embodiment. 
         FIG. 10  is a partial sectional view of a blower apparatus according to a modification of the first preferred embodiment. 
         FIG. 11  is a partial sectional view of a blower apparatus according to a modification of the first preferred embodiment. 
         FIG. 12  is a partial sectional view of a blower apparatus according to a modification of the first preferred embodiment. 
         FIG. 13  is a partial sectional view of a blower apparatus according to a modification of the first preferred embodiment. 
         FIG. 14  is a top view of a blower apparatus according to a modification of the first preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, blower apparatuses according to preferred embodiments of the present, invention will be described. It is assumed herein that a side on which an upper plate portion is arranged with respect to a lower plate portion is an upper side, and the shape of each member or portion and relative positions of different members or portions will be described based on the above assumption. It should be noted, however, that the above definition of the upper and lower sides is not meant, to restrict in any way the orientation of a blower apparatus according to any preferred embodiment of the present invention at the time of manufacture or when in use. 
     1. First Preferred Embodiment 
       FIG. 1  is a perspective view of a blower apparatus  1  according to a first preferred embodiment of the present invention.  FIG. 2  is a top view of the blower apparatus  1 .  FIG. 3  is a sectional view of the blower apparatus  1  taken along line A-A in  FIG. 2 .  FIG. 4  is an exploded perspective view of the blower apparatus  1 .  FIG. 5  is a partial sectional view of the blower apparatus  1 . The blower apparatus  1  is a centrifugal blower apparatus designed to generate an air flow traveling radially outward by rotating an air blowing portion  40 . The blower apparatus  1  is, for example, installed in an electronic device, such as, for example, a personal computer, to cool an interior thereof. Note that the blower apparatus  1  according to a preferred embodiment of the present invention may alternatively be used for other purposes. 
     Referring to  FIGS. 1 to 4 , the blower apparatus  1  includes a housing  20 , a motor portion  30 , and the air blowing portion  40 . 
     The housing  20  is a case arranged to house the motor portion  30  and the air blowing portion  40 . The housing  20  includes a lower plate portion  21 , a side wall portion  22 , and an upper plate portion  23 . 
     The lower plate portion  21  is arranged to define a bottom portion of the housing  20 . The lower plate portion  21  is arranged to extend radially below the air blowing portion  40  to cover at least a portion of a lower side of the air blowing portion  40 . In addition, the lower plate portion  21  is arranged to support the motor portion  30 . 
     The side wall portion  22  is arranged to extend upward from the lower plate portion  21 . The side wall portion  22  is arranged to cover a lateral side of the air blowing portion  40  between the lower plate portion  21  and the upper plate portion  23 . In addition, the side wall portion  22  includes an air outlet  201  arranged to face in a radial direction at one circumferential position. In the present preferred embodiment, the lower plate portion  21  and the side wall portion  22  are defined integrally with each other. Note that, the lower plate portion  21  and the side wall portion  22  may alternatively be defined by separate members. 
     The upper plate portion  23  is arranged to define a cover portion of the housing  20 . The upper plate portion  23  is arranged to extend radially above the lower plate portion  21 . In addition, the upper plate portion  23  includes an air inlet  202  arranged to pass therethrough in an axial direction. In other words, the upper plate portion  2   3  includes an inner edge portion  231  arranged to define the air inlet  202 . The air inlet  202  is, for example, circular and is centered on a central axis  9  in a plan view. 
     The motor portion  30  is a driving portion arranged to rotate the air blowing portion  40 . Referring to  FIG. 5 , the motor portion  30  includes a stationary portion  31  and a rotating portion  32 . The stationary portion  31  is fixed to the lower plate portion  21 . The stationary portion  31  is thus arranged to be stationary relative to the housing  20 . The rotating portion  32  is supported to be rotatable about the central axis  9  with respect to the stationary portion  31 . 
     The stationary portion  31  includes a stator fixing portion  311 , a stator  312 , and a bearing housing  313 . 
     The stator fixing portion  311  is fitted in a fixing hole  211  defined in the lower plate portion  21 . As a result, the stator fixing portion  311  is fixed to the lower plate portion  21 . The stator fixing portion  311  is arranged to extend upward from the fixing hole  211  to assume a cylindrical shape with the central axis  9  as a center thereof. The stator  312  is fixed to an outer circumferential portion of an upper portion of the stator fixing portion  311 . 
     The stator  312  is an armature arranged to generate magnetic flux in accordance with electric drive currents supplied from an external source. The stator  312  is arranged to annularly surround the central axis  9 , which extends in a vertical direction. The stator  312  includes, for example, an annular stator core defined by laminated steel sheets, and conducting wires wound around the stator core. 
     The bearing housing  313  is a member being cylindrical and having a closed bottom. Specifically, the bearing housing  313  includes a disk-shaped bottom portion, and a cylindrical portion arranged to extend upward from the bottom portion. The bearing housing  313  is fixed to an inner circumferential surface of the stator fixing portion  311 . 
     The rotating portion  32  includes a shaft  321 , a hub  322 , a bearing member  323 , and a magnet  324 . 
     The shaft  321  is a member arranged to extend along the central axis  9 . The shaft  321  according to the present preferred embodiment includes a columnar portion arranged inside of a first cylindrical portion  52 , which will be described below, and arranged to extend with the central axis  9  as a center thereof, and a disk-shaped portion arranged to extend radially from a lower end portion of the columnar portion. 
     The hub  322  is fixed to the shaft  321 . The hub  322  is made up of a hub body member  501  and a flange member  502 . The hub body member  501  includes a top plate portion  51 , the first cylindrical portion  52 , a second cylindrical portion  53 , and a magnet holding portion  54 . The flange member  502  includes an outer wall portion  55 , a top plate fixing portion  56 , and a flat plate holding portion  57 . 
     The top plate portion  51  is a disk-shaped portion arranged to extend radially with the central axis  9  as a center thereof. The top plate portion  51  is arranged above the stator  312 . The top plate portion  51  has a recessed portion  511  recessed from an upper surface thereof at an outer edge portion thereof. 
     The first cylindrical portion  52  is arranged to extend downward from the top plate portion  51  to assume a cylindrical shape with the central axis  9  as a center thereof. The columnar portion of the shaft  321  is housed in the first cylindrical portion  52 . In addition, the shaft  321  is fixed to the first cylindrical portion  52 . 
     The second cylindrical portion  53  is arranged to extend downward from the top plate portion  51  to assume a cylindrical shape with the central axis  9  as a center thereof. The second cylindrical portion  53  is arranged to have an inside diameter greater than an outside diameter of the first cylindrical portion  52 . In other words, the second cylindrical portion  53  is arranged radially outside of the first cylindrical portion  52 . 
     The magnet holding portion  54  is arranged to extend downward from a radially outer end of the top plate portion  51  to assume a cylindrical shape with the central axis  9  as a center thereof. The magnet holding portion  54  is arranged radially outside of the stator  312 . The magnet  324  is fixed to an inner circumferential surface of the magnet holding portion  54 . 
     The outer wall portion  55  is a cylindrical portion arranged to extend in the vertical direction with the central axis  9  as a center thereof. The outer wall portion  55  is arranged to extend along an outer circumferential surface of the magnet holding portion  54  of the hub body member  501 . 
     The top plate fixing portion  56  is arranged to extend radially inward from an upper end portion of the outer wall portion  55  to assume the shape of a circular ring. The top plate fixing portion  56  is arranged in the recessed portion  511 , which is defined in the upper surface of the top plate portion  51  of the hub body member  501 . In addition, the upper surface of the top plate portion  51  and an upper surface of the top plate fixing portion  56  are thus arranged at the same axial position. 
     The flat plate holding portion  57  is arranged to extend radially outward from a lower end portion of the outer wall portion  55 . The flat plate holding portion  57  is arranged to hold the air blowing portion  40  on a radially outer side of the magnet holding portion  54  of the hub body member  501 . In the present preferred embodiment, the air blowing portion  40  is mounted on an upper surface of the flat plate holding portion  57 . The flat plate holding portion  57  is thus arranged to stably hold a plurality of flat plates  410  included in the air blowing portion  40 . 
     The bearing member  323  is a cylindrical member arranged to extend in the vertical direction with the central axis  9  as a center thereof. The bearing member  323  is arranged to extend along an outer circumferential surface of the first cylindrical portion  52  of the hub body member  501 . In addition, the bearing member  323  is fixed to the outer circumferential surface of the first cylindrical portion  52 . The cylindrical portion of the bearing housing  313  is arranged radially outside of the bearing member  323  and radially inside of the second cylindrical portion  53  of the hub body member  501 . 
     The magnet  324  is fixed to the inner circumferential surface of the magnet holding portion  54  of the hub body member  501 . In addition, the magnet  324  is arranged radially outside of the stator  312 . The magnet  324  according to the present preferred embodiment is in the shape of a circular ring. A radially inner surface of the magnet  324  is arranged radially opposite to the stator  312  with a slight gap therebetween. In addition, an inner circumferential surface of the magnet  324  includes north and south poles arranged to alternate with each other in a circumferential direction. Note that a plurality of magnets may be used in place of the magnet  324  in the shape of a circular ring. In the case where the plurality of magnets are used, the magnets are arranged in the circumferential direction such that north and south poles of the magnets alternate with each other. 
     As illustrated in an enlarged view in  FIG. 5 , a lubricating fluid  300  is arranged between the bearing housing  313  and a combination of the shaft  321 , the bearing member  323 , and the hub body member  501 . A polyolester oil or a diester oil, for example, is used as the lubricating fluid  300 . The shaft  321 , the hub  322 , and the bearing member  323  are supported to be rotatable with respect to the bearing housing  313  through the lubricating fluid  300 . Thus, in the present preferred embodiment, the bearing housing  313 , which is a component of the stationary portion  31 , the combination of the shaft  321 , the bearing member  323 , and the hub body member  501 , each of which is a component of the rotating portion  32 , and the lubricating fluid  300  together define a fluid dynamic bearing. 
     A surface of the lubricating fluid  300  is defined in a seal portion  301 , which is a gap between an outer circumferential surface of the bearing housing  313  and an inner circumferential surface of the second cylindrical portion  53  of the hub body member  501 . In the seal portion  301 , the distance between the outer circumferential surface of the bearing housing  313  and the inner circumferential surface of the second cylindrical portion  53  is arranged to increase with decreasing height. In other words, in the seal portion  301 , the distance between the outer circumferential surface of the bearing housing  313  and the inner circumferential surface of the second cylindrical portion  53  is arranged to increase with increasing distance from the surface of the lubricating fluid  300 . Since the radial width of the seal portion  301  thus increases with decreasing height, the lubricating fluid  300  is attracted upward in the vicinity of the surface of the lubricating fluid  300 . This reduces the likelihood that the lubricating fluid  300  will leak out of the seal portion  301 . 
     Use of the fluid dynamic bearing as a bearing mechanism that connects the stationary portion  31  and the rotating portion  32  allows the rotating portion  32  to rotate stably. Thus, the likelihood of an occurrence of an unusual sound from the motor portion  30  can be reduced. 
     Once electric drive currents are supplied to the stator  312  in the motor portion  30  as described above, magnetic flux is generated around the stator  312 . Then, interaction between the magnetic flux of the stator  312  and magnetic flux of the magnet  324  produces a circumferential torque between the stationary portion  31  and the rotating portion  32 , so that the rotating portion  32  is caused to rotate about the central axis  9  with respect to the stationary portion  31 . The air blowing portion  40 , which is held by the flat plate holding portion  57  of the rotating portion  32 , is caused to rotate about the central axis  9  together with the rotating portion  32 . 
     Referring to  FIGS. 4 and 5 , the air blowing portion  40  includes the plurality of flat plates  410  and a plurality of spacers  420 . The flat plates  410  and the spacers  420  are arranged to alternate with each other in the axial direction. In addition, adjacent ones of the flat plates  410  and the spacers  420  are fixed to each other through, for example, adhesion. 
     Referring to  FIGS. 4 and 5 , in the present preferred embodiment, the flat plates  410  include a top flat plate  411 , which is arranged at the highest position, a bottom flat plate  412 , which is arranged at the lowest position, and four intermediate flat plates  413 , which are arranged below the top flat plate  411  and above the bottom flat plate  412 . That is, the number of flat plates  410  included in the air blowing portion  40  according to the present preferred embodiment is six. The flat plates  410  are arranged in the axial direction with an axial gap  400  defined between adjacent ones of the flat plates  410 . 
     Each flat plate  410  is made of, for example, a metal material, such as stainless steel, or a resin material. Each flat plate  410  may alternatively be made of, for example, paper. In this case, paper including a glass fiber, a metal wire, or the like in addition to plant fibers may be used. The flat plate  410  is able to achieve higher dimensional accuracy when the flat plate  410  is made of a metal material than when the flat plate  410  is made of a resin material. 
     In the present preferred embodiment, each of the top flat plate  411  and the four intermediate flat plates  413  is arranged to have the same shape and size. Referring to  FIGS. 1, 2, and 5 , each of the top flat plate  411  and the intermediate flat plates  413  includes an inner annular portion  61 , an outer annular portion  62 , a plurality of ribs  63 , and a plurality of air holes  60 . In the present preferred embodiment, the number of ribs  63  and the number of air holes  60  included in each of the top flat plate  411  and the intermediate flat plates  413  are both five. 
     The inner annular portion  61  is an annular portion centered on the central axis  9 . The inner annular portion  61  has a central hole  65  (see  FIG. 4 ) arranged to pass therethrough in the vertical direction in a center thereof. The outer annular portion  62  is an annular portion arranged radially outside of the inner annular portion  61  with the central axis  9  as a center thereof. Each rib  63  is arranged to join the inner annular portion  61  and the outer annular portion  62  to each other. Each air hole  60  is arranged to be in communication with a space radially outside of the air blowing portion  40  through the axial gap(s)  400  adjacent to the flat plate  410  including the air hole  60  on the upper and/or lower sides of the flat plate  410 . Each air hole  60  is arranged at a position overlapping with the air inlet  202  of the housing  20  when viewed in the axial direction. 
     The bottom flat plate  412  is an annular and plate-shaped member centered on the central axis  9 . The bottom flat plate  412  has a central hole  65  arranged to pass therethrough in the vertical direction in a center thereof. 
     Referring to  FIG. 4 , each spacer  420  is a member in the shape of a circular ring. The spacers  420  are arranged between the flat plates  410  to secure the axial gaps  400  between the flat plates  410 . Each spacer  420  has a central hole  429  arranged to pass therethrough in the vertical direction in a center thereof. The motor portion  30  is arranged in the central holes  65  of the flat plates  410  and the central holes  429  of the spacers  420 . 
     Each spacer  420  is arranged at a position axially coinciding with the inner annular portion  61  of each of the top flat plate  411  and the intermediate flat plates  413 . Thus, the spacer  420  is arranged in a region in the corresponding axial gap  400 , the region covering only a portion of the radial extent of the corresponding axial gap  400 . 
     Once the motor portion  30  is driven, the air blowing portion  40  is caused to rotate together with the rotating portion  32 . As a result, viscous drag of a surface of each flat plate  410  and a centrifugal force together generate an air flow traveling radially outward in the vicinity of the surface of the flat plate  410 . Thus, an air flow traveling radially outward is generated in each of the axial gaps  400  between the flat, plates  410 . Thus, gas above the housing  20  is supplied to each axial gap  400  through the air inlet  202  of the housing  20  and the air holes  60  of the top flat plate  411  and the intermediate flat plates  413 , and is discharged out of the blower apparatus  1  through the air outlet  201 , which is defined in a side portion of the housing  20 . 
     Here, each flat plate  410  is arranged to have an axial thickness of about 0.1 mm. Meanwhile, each axial gap  400  is arranged to have an axial dimension of about 0.3 mm. The axial dimension of the axial gap  400  is preferably in the range of 0.2 mm to 0.5 mm. An excessively large axial dimension of the axial gap  400  would lead to a separation between an air flow generated by a lower surface of the flat plate  410  on the upper side and an air flow generated by an upper surface of the flat plate  410  on the lower side during rotation of the air blowing portion  40 . This separation could result in a failure to generate sufficient static pressure in the axial gap  400  to discharge a sufficient volume of air. Moreover, an excessively large axial dimension of the axial gap  400  would make it difficult to reduce the axial dimension of the blower apparatus  1 . Accordingly, in this blower apparatus  1 , the axial dimension of the axial gap  400  is arranged to be in the range of 0.2 mm to 0.5 mm. This arrangement allows the blower apparatus  1  to achieve a reduced thickness while allowing an increase in the static pressure in the axial gap  400  to discharge a sufficient volume of air. 
     Each of the fop flat plate  411  and the intermediate flat plates  413  includes the air holes  60 . Accordingly, in each of the top flat plate  411  and the intermediate flat plates  413 , the outer annular portion  62 , which is arranged radially outside of the air holes  60 , defines an air blowing region which generates an air flow in the vicinity of a surface thereof. Meanwhile, the bottom flat-plate  412  includes no air hole  60 . Therefore, in an upper surface of the bottom flat plate  412 , an entire region radially outside of a portion of the bottom flat plate  412  which makes contact with the spacer  420  defines an air blowing region. In other words, in the upper surface of the bottom flat plate  412 , a region which axially coincides with the air holes  60  and the ribs  63  of the top flat plate  411  and the intermediate flat plates  413 , and a region which axially coincides with the outer annular portions  62  thereof, together define the air blowing region. In addition, in a lower surface of the bottom flat plate  412 , an entire region radially outside of a portion of the bottom flat plate  412  which makes contact with the flat plate holding portion  57  defines an air blowing region. Notice that an air flow is generated by a lower surface of the flat plate holding portion  57  as well. 
     As described above, the bottom flat plate  412  has air blowing regions wider than the air blowing regions of the top flat plate  411  and the intermediate flat plates  413 . Therefore, the axial gap  400  between the lowest one of the intermediate flat plates  413  and the bottom flat plate  412  is able to have higher static pressure than any other axial gap  400 . 
     Air flows passing downward through the air inlet  202  and the air holes  60  are drawn radially outward in each axial gap  400 . Therefore, the air flows passing through the air holes  60  become weaker as they travel downward. In the present, preferred embodiment, the bottom flat plate  412  is arranged to have an air blowing region wider than the air blowing regions of the top flat plate  411  and the intermediate flat plates  413  to cause a stronger air flow to be generated in the lowest one of the axial gaps  400  than in any other axial gap  4   00  to cause the air flows passing downward through the air holes  60  to be drawn toward the lowest axial gap  400 . Thus, a sufficient volume of gas is supplied to the lowest axial gap  400  as well. As a result, the air blowing portion  40  achieves improved air blowing efficiency. 
     In a related-art blower apparatus that generates air flows by rotating an impeller including a plurality of blades, air flows generated by the impeller leak at upper and lower end portions of the impeller. This leakage of the air flows occurs regardless of the axial dimension of the blower apparatus. Therefore, as the blower apparatus is designed to be thinner, an effect, of this leakage on the blower apparatus as a whole becomes greater, resulting in lower air blowing efficiency. Meanwhile, in the blower apparatus  1  according to the present preferred embodiment, the air flows are generated in the vicinity of the surfaces of the flat plates  410 , and therefore, the air flows do not easily leak upward or downward. Therefore, even when the axial dimension of the air blowing portion  40 , which generates the air flows, is reduced, a reduction in air blowing efficiency due to leakages of the air flows does not easily occur. That is, even when the blower apparatus  1  has a reduced thickness, a reduction in air blowing efficiency thereof does not easily occur. Further, since the air blowing portion  40  is held by the flat plate holding portion  57 , which extends radially, the air blowing portion  40  is able to stably rotate. 
     In addition, in a blower apparatus including an impeller, periodic noise occurs owing to the shape, number, arrangement, and so on of blades. However, this blower apparatus  1  is superior to a comparable blower apparatus including an impeller in terms of being silent, because the air flows are generated by the viscous drag of the surface of each flat plate  410  and the centrifugal force in the blower apparatus  1 . In particular, since the air blowing portion  40  is able to stably rotate as described above, a further reduction in noise can be achieved. 
     In addition, from the viewpoint of P-Q characteristics (i.e., flow rate-static pressure characteristics), the blower apparatus including the flat plates  410  is able to produce a higher static pressure in a low flow rate region than the blower apparatus including the impeller. Therefore, when compared to the blower apparatus including the impeller, the blower apparatus  1  is suitable for use in a densely packed case, from which only a relatively small volume of air can be discharged. Examples of such cases include cases of electronic devices, such as, for example, personal computers. 
     In the present preferred embodiment, the top flat plate  411  and all the intermediate flat plates  413  include the air holes  60 . Accordingly, all the axial gaps  400  are in axial communication with a space above the housing  20  through the air inlet  202  and the air holes  60 . 
     In a process of manufacturing the blower apparatus  1 , the six flat plates  410  are placed one above another from one axial side (i.e., an upper surface side) of the flat plate holding portion  57 . This results in higher manufacturing efficiency than that of another preferred embodiment of the present invention in which the flat plates are placed from both axial sides of the flat plate holding portion  57 . 
     Referring to  FIG. 2 , the air inlet  202  is centered on the central axis  9 . That is, a center of the air inlet  202  coincides with the central axis  9 . Meanwhile, the air blowing portion  40  is also centered on the central axis  9 . Accordingly, differences in pressure do not easily occur at different circumferential positions in the air blowing portion  40 . This contributes to reducing noise. It is assumed that, the term “coincide” as used here includes not only “completely coincide” but also “substantially coincide”. 
     2. Example Modifications 
     While a preferred embodiment of the present invention has been described above, it is to be understood that the present invention is not limited to the above-described preferred embodiment. 
       FIG. 6  is a partial sectional view of a blower apparatus  1 A according to a modification of the above-described preferred embodiment. In the blower apparatus  1 A according to the modification illustrated in  FIG. 6 , an air blowing portion  40 A includes a plurality of flat plates  410 A arranged in the axial direction with an axial gap  400 A defined between adjacent ones of the flat plates  410 A. The flat plates  410 A include a top flat plate  411 A, which is arranged at the highest position, a bottom flat plate  412 A, which is arranged at the lowest position, and four intermediate flat plates  413 A, which are arranged below the top flat plate  411 A and above the bottom flat plate  412 A. 
     In addition, a hub  322 A includes a hub body member  501 A, which includes a top plate portion  51 A and a magnet holding portion  54 A, and a flange member  502 A, which includes a flat plate holding portion  57 A arranged to extend radially. 
     In this blower apparatus  1 A, the flat plate holding portion  57 A is arranged to perform a function as the bottom flat plate  412 A of the air blowing portion  40 A. In other words, the bottom flat plate  412 A defines a portion of the flat plate holding portion  57 A. In addition, the flat plate holding portion  57 A is arranged to hold the other flat plates  410 A, i.e., the top flat plate  411 A and the four intermediate flat plates  413 A, above an upper surface thereof. 
     In a process of manufacturing the blower apparatus  1 A, the other flat plates  410 A, i.e., the top flat plate  411 A and the four intermediate flat plates  413 A, are placed one above another from one axial side (i.e., an upper surface side) of the flat plate holding portion  57 A. This results in higher manufacturing efficiency than that of another preferred embodiment of the present invention in which the flat plates are placed from both axial sides of the flat plate holding portion  57 A. 
       FIG. 7  is a partial sectional view of a blower apparatus  1 B according to another modification of the above-described preferred embodiment. In the blower apparatus  1 B according to the modification illustrated in  FIG. 7 , an air blowing portion  40 B includes a plurality of flat plates  410 B arranged in the axial direction with an axial gap  400 B defined between adjacent ones of the flat plates  410 B. The flat plates  410 B include a top flat plate  411 B, which is arranged at the highest position, a bottom flat plate  412 B, which is arranged at the lowest position, and four intermediate flat, plates  413 B, which are arranged below the top flat plate  411 B and above the bottom flat plate  412 B. 
     In addition, a hub  322 B includes a hub body member  501 B, which includes a top plate portion  51 B and a magnet holding portion  54 B, and a flange member  502 B, which includes a flat plate holding portion  57 B arranged to extend radially. 
     In this blower apparatus  1 B, the flat plate holding portion  57 B is arranged to perform a function as the top flat plate  411 B of the air blowing portion  40 B. In other words, the top flat plate  411 B defines a portion of the flat plate holding portion  57 B. In addition, the flat plate holding portion  57 B is arranged to hold the other flat plates  410 B, i.e., the four intermediate flat plates  413 B and the bottom flat, plate  412 B, below a lower surface thereof. Thus, the flat plate holding portion  57 B may be arranged to hold the flat plates  410 B on a lower surface side thereof. 
     In a process of manufacturing the blower apparatus  1 B, the other flat plates  410 B, i.e., the four intermediate flat plates  413 B and the bottom flat plate  412 B, are placed one below another from one axial side (i.e., the lower surface side) of the flat, plate holding portion  57 B. This results in higher manufacturing efficiency than that of another preferred embodiment of the present invention in which the flat plates are placed from both axial sides of the flat plate holding portion  57 B. Further, the top plate portion  51 B of the hub  322 B, which has a flat surface, is placed on a work (i.e., a portion of assembling equipment) at the time of assembling. Accordingly, a mounting surface of the work on which the top plate portion  51 B of the hub  322 B is placed does not need to have a complicated shape, resulting in an improved productivity. 
       FIG. 8  is a partial sectional view of a blower apparatus  1 C according to yet another modification of the above-described preferred embodiment. In the blower apparatus  1 C according to the modification illustrated in  FIG. 8 , an air blowing portion  40 C includes a plurality of flat plates  410 C arranged in the axial direction with an axial gap  400 C defined between adjacent ones of the flat plates  410 C. The flat plates  410 C include a top flat plate  411 C, which is arranged at the highest position, a bottom flat plate  412 C, which is arranged at the lowest position, and four intermediate flat plates  413 C, which are arranged below the top flat plate  411 C and above the bottom flat plate  412 C. The four intermediate flat plates  413 C will be hereinafter referred to as, from highest to lowest, a first intermediate flat plate  414 C, a second intermediate flat plate  415 C, a third intermediate flat plate  416 C, and a fourth intermediate flat plate  417 C. 
     In addition, a hub  3220  includes a hub body member  501 C, which includes a top plate portion  510  and a magnet holding portion  54 C, and a flange member  502 C, which includes a flat plate holding portion  57 C arranged to extend radially. 
     In this blower apparatus  10 , the flat plate holding portion  57 C is arranged to perform a function as the third intermediate flat plate  416 C of the air blowing portion  40 C. In other words, the third intermediate flat plate  4160 , which is one of the intermediate flat plates  413 C, defines a portion of the flat plate holding portion  57 C. The flat plate holding portion  57 C is arranged to hold the top flat plate  411 C, the first intermediate flat plate  414 C, and the second intermediate flat plate  415 C above an upper surface thereof. In addition, the flat plate holding portion  57 C is arranged to hold the fourth intermediate flat plate  417 C and the bottom flat plate  412 C below a lower surface thereof. Thus, the flat plate holding portion  57 C may be arranged to hold the flat plates  410 C on both an upper surface side and a lower surface side thereof. 
     As in each of the blower apparatuses  1 A,  1 B, and  1 C according to the modifications illustrated in  FIGS. 6, 7 , and  8 , respectively, at least one of the plurality of flat plates of the air blowing portion may define a portion of the flat plate holding portion. A reduction in the number of parts can be achieved by defining one of the flat plates and the flat plate holding portion integrally with each other. The number of parts to be assembled can thus be reduced, resulting in an improved productivity. 
       FIG. 9  is a partial sectional view of a blower apparatus  1 D according to yet another modification of the above-described preferred embodiment. In the blower apparatus  1 D according to the modification illustrated in  FIG. 9 , a hub  322 D is defined by a single monolithic member including a top plate portion  51 D, a magnet holding portion  54 D, and a flat plate holding portion  57 D. The flat plate holding portion  57 D is arranged to extend radially outward from a lower end portion of the magnet holding portion  54 D. In addition, an air blowing portion  40 D includes a plurality of flat plates  410 D arranged in the axial direction with an axial gap  400 D defined between adjacent ones of the flat plates  410 D. The flat plate holding portion  57 D of the hub  322 D is arranged to hold the air blowing portion  40 D. That is, the flat plate holding portion  57 D is arranged to hold the flat plates  410 D. 
     In a process of manufacturing the blower apparatus  1 D, the flat plates  410 D, which are six in number, are placed one above another from one axial side (i.e., an upper surface side) of the flat plate holding portion  57 D. This results in higher manufacturing efficiency than that of another preferred embodiment of the present invention in which the flat plates are placed from both axial sides of the flat plate holding portion  57 D. 
       FIG. 10  is a partial sectional view of a blower apparatus  1 E according to yet another modification of the above-described preferred embodiment. In the blower apparatus  1 E according to the modification illustrated in  FIG. 10 , a hub  322 E is defined by a single monolithic member including a top plate portion  51 E, a magnet holding portion  54 E, and a flat plate holding portion  57 E. The flat plate holding portion  57 E is arranged to extend radially outward from a lower end portion of the magnet holding portion  54 E. 
     In addition, an air blowing portion  40 E includes a plurality of flat plates  410 E arranged in the axial direction with an axial gap  400 E defined between adjacent ones of the flat plates  410 E. The flat plates  410 E include a top flat plate  411 E, which is arranged at the highest position, a bottom flat plate  412 E, which is arranged at the lowest position, and four intermediate flat plates  413 E, which are arranged below the top flat plate  411 E and above the bottom flat plate  412 E. 
     In this blower apparatus  1 E, the flat plate holding portion  57 E is arranged to perform a function as the bottom flat plate  412 E of the air blowing portion  40 E. In other words, the bottom flat plate  412 E defines a portion of the flat plate holding portion  57 E. In addition, the flat plate holding portion  57 E is arranged to hold the other flat plates  410 E, i.e., the top flat plate  411 E and the four intermediate flat, plates  413 E, above an upper surface thereof. 
     In a process of manufacturing the blower apparatus  1 E, the other flat plates  410 E, i.e., the top flat plate  411 E and the four intermediate flat plates  413 E, are placed one above another from one axial side (i.e., an upper surface side) of the flat plate holding portion  57 E. This results in higher manufacturing efficiency than that of another preferred embodiment of the present invention in which the flat plates are placed from both axial sides of the flat plate holding portion  57 E. 
       FIG. 11  is a partial sectional view of a blower apparatus  1 F according to yet another modification of the above-described preferred embodiment. In the blower apparatus  1 F according to the modification illustrated in  FIG. 11 , a hub  322 F is defined by a single monolithic member including a top plate portion  51 F, a magnet holding portion  54 F, and a flat plate holding portion  57 F. The flat plate holding portion  57 F is arranged to extend radially outward from a side surface of the magnet, holding portion  54 F. 
     In addition, an air blowing portion  40 F includes a plurality of flat plates  410 F arranged in the axial direction with an axial gap  400 F defined between adjacent ones of the flat plates  410 F. The flat plates  410 F include a top flat plate  411 F, which is arranged at the highest position, a bottom flat plate  412 F, which is arranged at the lowest position, and four intermediate flat plates  413 F, which are arranged below the top flat plate  411 F and above the bottom flat plate  412 F. 
     In this blower apparatus  1 F, the flat plate holding portion  57 F is arranged to perform a function as the top flat plate  411 F of the air blowing portion  40 F. In other words, the top flat plate  411 F defines a portion of the flat plate holding portion  57 F. In addition, the flat plate holding portion  57 F is arranged to hold the other flat plates  410 F, i.e., the four intermediate flat plates  413 F and the bottom flat plate  412 F, below a lower surface thereof. 
     In a process of manufacturing the blower apparatus  1 F, the other flat plates  410 F, i.e., the four intermediate flat plates  413 F and the bottom flat plate  412 F, are placed one below another from one axial side (i.e., a lower surface side) of the flat plate holding portion  57 F. This results in higher manufacturing efficiency than that of another preferred embodiment of the present invention in which the flat plates are placed from both axial sides of the flat plate holding portion  57 F. Further, the top plate portion  51 F of the hub  322 F, which has a flat surface, is placed on a work (i.e., a portion of assembling equipment) at the time of assembling. Accordingly, a mounting surface of the work on which the top plate portion  51 F of the hub  322 F is placed does not need to have a complicated shape, resulting in an improved productivity. 
       FIG. 12  is a partial sectional view of a blower apparatus  1 G according to yet another modification of the above-described preferred embodiment. In the blower apparatus  1 G according to the modification illustrated in  FIG. 12 , a hub  322 G is defined by a single monolithic member including a top plate portion  51 G, a magnet holding portion  54 G, and a flat plate holding portion  57 G. The flat plate holding portion  57 G is arranged to extend radially outward from a side surface of the magnet, holding portion  54 G. 
     In addition, an air blowing portion  40 G includes a plurality of flat plates  410 G arranged in the axial direction with an axial gap  400 G defined between adjacent ones of the flat plates  410 G. The flat plates  410 G include a top flat plate  411 G, which is arranged at the highest position, a bottom flat plate  412 G, which is arranged at the lowest position, and four intermediate flat plates  413 G, which are arranged below the top flat plate  411 G and above the bottom flat plate  412 G. The four intermediate flat plates  413 G will be hereinafter referred to as, from highest to lowest, a first intermediate flat plate  414 G, a second intermediate flat plate  415 G, a third intermediate flat plate  416 G, and a fourth intermediate flat plate  417 G. 
     In this blower apparatus  1 G, the flat plate holding portion  57 G is arranged to perform a function as the third intermediate flat plate  416 G of the air blowing portion  40 G. In other words, the third intermediate flat plate  416 G, which is one of the intermediate flat plates  413 G, defines a portion of the flat plate holding portion  57 G. The flat plate holding portion  57 G is arranged to hold the top flat plate  411 G, the first intermediate flat plate  414 G, and the second intermediate flat plate  415 G above an upper surface thereof. In addition, the flat plate holding portion  57 G is arranged to hold the fourth intermediate flat plate  417 G and the bottom flat plate  412 G below a lower surface thereof. 
     As in each of the blower apparatuses  1 D,  1 E,  1 F, and  1 G according to the modifications illustrated in  FIGS. 9, 10, 11, and 12 , respectively, the hub may be defined by a single monolithic member. In the case where the hub is defined by a single monolithic member, a reduction in the number of parts can be achieved when compared to the case where the hub is defined by a plurality of members. The number of parts to be assembled can thus be reduced, resulting in an improved productivity. 
     In addition, as in each of the blower apparatuses  1 E,  1 F, and  1 G according to the modifications illustrated in  FIGS. 10, 11, and 12 , respectively, at least one of the plurality of flat plates of the air blowing portion may define a portion of the flat plate holding portion. A further reduction in the number of parts can be achieved by defining one of the flat plates and the flat plate holding portion integrally with each other. The number of parts to be assembled can thus be further reduced, resulting in a further improved productivity. 
       FIG. 13  is a partial sectional view of a blower apparatus  1 H according to yet another modification of the above-described preferred embodiment. In the blower apparatus  1 H according to the modification illustrated in  FIG. 13 , a motor portion  30 H includes a stationary portion  31 H, a rotating portion  32 H, and two ball bearings  33 H. 
     The stationary portion  31 H includes a stator fixing portion  311 H and a stator  312 H. The stator fixing portion  311 H is a member being cylindrical and having a closed bottom and fixed to a housing  20 H. The stator  312 H is an armature fixed to an outer circumferential surface of the stator fixing portion  311 H. 
     The rotating portion  32 H includes a shaft  321 H, a hub  322 H, and a magnet  324 H. At least a lower end portion of the shaft  321 H is arranged inside of the stator fixing portion  311 H. In addition, an upper end portion of the shaft  321 H is fixed to the hub  322 H. The magnet  324 H is fixed to the hub  322 H. The magnet  324 H is arranged radially opposite to the stator  312 H. 
     Each ball bearing  33 H is arranged to connect the rotating portion  32 H to the stationary portion  31 H such that the rotating portion  32 H is rotatable with respect to the stationary portion  31 H. Specifically, an outer race of each ball bearing  33 H is fixed to an inner circumferential surface of the stator fixing portion  311 H of the stationary portion  31 H. In addition, an inner race of each ball bearing  33 H is fixed to an outer circumferential surface of the shaft  321 H of the rotating portion  32 H. Further, a plurality of balls, each of which is a spherical rolling element, are arranged between the outer race and the inner race. As described above, instead of a fluid dynamic bearing, rolling-element bearings, such as, for example, bail bearings, may be used as a bearing structure of the motor portion  30 H. 
     In the modification illustrated in  FIG. 13 , the motor portion  30 H includes the two ball bearings  33 H. The ball bearings  33 H are arranged near an upper end and a lower end of an axial range over which the inner circumferential surface of the stator fixing portion  311 H and the shaft  321 H are opposed to each other. This contributes to preventing the shaft  321 H from being inclined with respect to a central axis  9 H. 
       FIG. 14  is a top view of a blower apparatus U according to yet another modification of the above-described preferred embodiment. In the blower apparatus  1 J according to the modification illustrated in  FIG. 14 , a housing  20 J includes a plurality of air outlets  201 J. Specifically, a side wall portion  22 J includes the air outlets  201 J, each of which is arranged to face in a radial direction, at a plurality of circumferential positions. The housing  20 J includes tongue portions  203 J, each of which is arranged near a separate one of the air outlets  201 J. In addition, an air blowing portion  40 J includes a plurality of flat plates  410 J arranged in the axial direction with an axial gap defined between adjacent ones of the flat plates  410 J. 
     In a centrifugal fan including an impeller, periodic noise occurs owing to the shape, number, arrangement, and so on of blades. In addition, such noise tends to easily occur around a tongue portion. Accordingly, when air is to be discharged in a plurality of directions, a deterioration in noise characteristics occurs because of an increased number of tongue portions. However, in this blower apparatus  1 J, air flows traveling radially outward are generated by rotation of the flat plates  410 J, and therefore, the blower apparatus  1 J is able to achieve reduced periodic noise when compared to the centrifugal fan including the impeller. Therefore, the blower apparatus  1 J, which is designed to discharge air in a plurality of directions, does not significantly deteriorate in noise characteristics due to the tongue portions  203 J. 
     Note that, although the number of flat plates included in the air blowing portion is six in each of the above-described preferred embodiment and the modifications thereof, this is not essential to the present invention. The number of flat plates may alternatively be two, three, four, five, or more than six. 
     Also note that, although the hub is defined by one or two members in each of the above-described preferred embodiment and the modifications thereof, this is not essential to the present invention. The hub may alternatively be defined by three or more members. 
     Also note that the detailed shape of any member may be different from the shape thereof as illustrated in the accompanying drawings of the present application. For example, the shape of any of the housing, the air blowing portion, and the motor portion may be different from that according to each of the above-described preferred embodiment and the modifications thereof. Also note that features of the above-described preferred embodiment and the modifications thereof may be combined appropriately as long as no conflict arises. 
     Preferred embodiments of the present invention are applicable to, for example, blower apparatuses. 
     While preferred embodiments of the present invention have been described above, it is to be understood that, variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.