Patent Publication Number: US-11384767-B2

Title: Blower apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention claims priority under 35 U.S.C. § 119 to Japanese Application No. 2018-178990 filed on Sep. 25, 2018, the entire contents of which are hereby incorporated herein by reference. 
     1. FIELD OF THE INVENTION 
     The present disclosure relates to a blower apparatus including two impellers arranged in the axial direction. 
     2. BACKGROUND 
     Conventionally, a known blower apparatus is constructed by connecting a first casing and a second casing. The first casing houses a first impeller and a first motor. The second casing houses a second impeller and a second motor. When the first casing and the second casing are connected, the first impeller and the second impeller are axially aligned such that central axes serving as rotation centers are coaxial. 
     An intake port is provided on one side (for example, the upper side) in the axial direction of the first casing. A plurality of first ribs arranged in the circumferential direction are provided on the other side (for example, the lower side) of the first casing in the axial direction. First openings are formed between the circumferentially adjacent first ribs. A first support frame is provided radially inward of the plurality of first ribs. The first support frame supports the first motor. 
     A plurality of second ribs arranged in the circumferential direction are provided on one side (for example, the upper side) of the second casing in the axial direction. A discharge port is provided on the other side (for example, the lower side) of the second casing in the axial direction. Second openings are formed between the circumferentially adjacent second ribs. A second support frame is provided radially inward of the plurality of second ribs. The second support frame supports the second motor. 
     When the first impeller and the second impeller are respectively rotated by the first motor and the second motor, air is sucked into the interior of the first casing through the intake port. The sucked air flows toward the discharge port sequentially through the first openings and the second openings, and is discharged to the outside through the discharge port. 
     The rotation direction of the second impeller is opposite to the rotation direction of the first impeller. The orientations of blades of the first impeller and the second impeller are set such that, when the first impeller and the second impeller are rotated, the air flows from the intake port to the discharge port in the first casing and the second casing. 
     In the configuration described above, it is necessary to ensure a minimum necessary length (thickness) of the first ribs of the first casing and the second ribs of the second casing in the axial direction, from the viewpoint of preventing damage due to impact and ensuring reliability. For this reason, when the first casing and the second casing are connected in the axial direction, the axial length of the overall casing is increased, which may make it difficult to reduce the thickness of the blower apparatus. 
     In addition, if distortion occurs due to a manufacturing error on the bonding surfaces of the first casing and the second casing, such distortion may cause “rattling” or the like when the first casing and the second casing are combined. The “rattling” causes vibration and noise when the blower apparatus is used. Therefore, it is desirable to provide a blower apparatus that can avoid the occurrence of such vibration and noise. 
     Furthermore, development of blower apparatuses that are easy to assemble is also required nowadays. It is considered that, in the configuration described above, assembling is facilitated by connecting the first casing and the second casing. However, in the configuration in which the first casing and the second casing are connected, it is difficult to reduce the thickness of the blower apparatus, and vibration and noise are also generated due to distortion of the bonding surface, as described above. Therefore, it is desirable to achieve a blower apparatus that can be reduced in thickness with improved ease of assembly, and that can prevent occurrence of vibration and noise due to distortion of bonding surfaces. 
     SUMMARY 
     A blower apparatus according to an example embodiment of the present disclosure includes a casing including an intake port located on an upper side of a central axis that extends vertically, and a discharge port located on a lower side of the central axis, a plurality of ribs located radially inward of the casing and provided integrally with the casing, a motor housing located radially inward of the plurality of ribs and provided integrally with the plurality of ribs, a first housing located radially inward of the motor housing, a first motor supported on one side in an axial direction by the first housing, a first impeller rotatable around the central axis on the one side in the axial direction by the first motor, a second housing located on another side of the first housing in the axial direction, a second motor supported on the another side in the axial direction by the second housing, and a second impeller rotatable around the central axis on the another side in the axial direction by the second motor. The second housing is fixed to the first housing. 
     The above and other elements, features, steps, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of the example embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a blower apparatus according to an example embodiment of the present disclosure as viewed from an intake port side. 
         FIG. 2  is a perspective view of a configuration of a part of the inside of a casing of the blower apparatus as viewed from the intake port side. 
         FIG. 3  is a perspective view of the blower apparatus as viewed from a discharge port side. 
         FIG. 4  is a perspective view of a configuration of a portion of the inside of the casing of the blower apparatus as viewed from the discharge port side. 
         FIG. 5  is a longitudinal sectional view of the blower apparatus. 
         FIG. 6  is an exploded sectional view of a first housing and a second housing of the blower apparatus. 
         FIG. 7  is a longitudinal sectional view showing a configuration of a blower apparatus according to another example embodiment of the present disclosure. 
         FIG. 8  is an exploded sectional view of a motor housing, a first housing, and a second housing of the blower apparatus. 
         FIG. 9  is an exploded sectional view showing another configuration of the motor housing, the first housing, and the second housing of the blower apparatus. 
         FIG. 10  is an exploded sectional view showing still another configuration of the motor housing, the first housing, and the second housing of the blower apparatus. 
         FIG. 11  is a plan view of a second receiving portion of the motor housing as viewed from above in the axial direction. 
         FIG. 12  is a plan view of the second housing as viewed from above in the axial direction. 
         FIG. 13  is a plan view of a state in which the second housing is inserted into the second receiving portion as viewed from above in the axial direction. 
         FIG. 14  is a plan view of a state in which the second housing is rotated with respect to the second receiving portion as viewed from above in the axial direction. 
         FIG. 15  is a sectional view showing still another configuration of the motor housing, the first housing, and the second housing of the blower apparatus. 
         FIG. 16  is a longitudinal sectional view showing another configuration of the blower apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, example embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. It is assumed herein that: an axis serving as a rotation center of a first impeller and a second impeller is referred to as a “central axis”; and the direction in which the central axis extends is referred to by the term “axial direction”, “axial”, or “axially”. In addition, directions perpendicular to the central axis with respect to the central axis are each referred to simply by the term “radial direction”, “radial”, or “radially”. In this regard, in the radial direction, the side closer to the central axis is referred to by the term “radially inner side” or “radially inward”, and the side farther from the central axis is referred to by the term “radially outer side” or “radially outward”. Further, a direction along a circular arc around the central axis is referred to by the term “circumferential direction”, “circumferential”, or “circumferentially”. 
     It is also assumed herein that, for the sake of convenience of description, an axial direction is defined as a vertical direction, and the shape of each member or part and relative positions of different members or parts will be described on the assumption that a vertical direction of the blower apparatus corresponds to the vertical direction of the axial direction. In this regard, one of the directions of the axis is referred to by the term “upper” or “top”, and the other direction of the axis is referred to by the term “lower” or “bottom”. Further, one side in the axial direction is referred to by the term “axially above”, “above in the axial direction”, or “upper side in the axial direction”, and the other side in the axial direction is referred to by the term “axially below”, “below in the axial direction” or “lower side in the axial direction”. It should be noted, however, that the above definition of the vertical direction is not intended to restrict the orientation of, or relative positions of different members or parts of, the blower apparatus when in use. 
     It is also assumed herein that a section parallel to the axial direction is referred to as a “longitudinal section”. Note that the wording “parallel” as used herein includes not only “exactly parallel” but also “substantially parallel”. 
       FIG. 1  is a perspective view of a blower apparatus  1  according to an example embodiment of the present disclosure as viewed from an intake port  81  side.  FIG. 2  is a perspective view of the configuration of a part of the inside of a casing  8  of the blower apparatus  1  as viewed from the intake port  81  side.  FIG. 3  is a perspective view of the blower apparatus  1  as viewed from a discharge port  82  side.  FIG. 4  is a perspective view of the configuration of a part of the inside of the casing  8  of the blower apparatus  1  as viewed from the discharge port  82  side.  FIG. 5  is a longitudinal sectional view of the blower apparatus  1 . For convenience,  FIG. 5  shows the cross section of only one side of the blower apparatus  1  in the radial direction with respect to the central axis C. 
     The blower apparatus  1  is a counter-rotating blower apparatus. Specifically, the blower apparatus  1  includes a first impeller  2 , a first motor  3 , a first circuit board  4 , a second impeller  5 , a second motor  6 , a second circuit board  7 , the casing  8 , a plurality of ribs  9 , a motor housing  10 , a first housing  11 , and a second housing  12 . The casing  8 , the plurality of ribs  9 , the motor housing  10 , the first housing  11 , and the second housing  12  are formed of, for example, resin. 
     The first impeller  2  is disposed axially above and radially outward of the first motor  3  in the casing  8 . The first impeller  2  is rotated about the central axis C by the first motor  3 . That is, the blower apparatus  1  includes the first impeller  2  which rotates around the central axis C on one side in the axial direction by the first motor  3 . 
     The first impeller  2  has a first impeller cup  22 , a plurality of first blades  21 , and a first fixing unit  23 . The first impeller cup  22  is fixed to the first motor  3  via the first fixing unit  23 . The first impeller cup  22  is a substantially cylindrical member having a lid on the upper side in the axial direction. A rotor yoke  341  of the first motor  3  is fixed to the inside of the first impeller cup  22 . 
     The plurality of first blades  21  are circumferentially arranged on the outer surface of the first impeller cup  22 . In the present example embodiment, the first impeller  2  has seven first blades  21  as shown in  FIG. 1 , but the number of first blades  21  is not limited to seven. The first fixing unit  23  is a member for fixing the first impeller cup  22  to a first shaft  31  of the first motor  3 . 
     The first motor  3  is supported by the first housing  11  on the upper side in the axial direction in the casing  8 . Specifically, the blower apparatus  1  includes the first motor  3  supported on one side in the axial direction by the first housing  11 . The first motor  3  has the first shaft  31 , a first bearing  32 , a first stator  33 , a first rotor  34 , and a first bearing holding unit  35 . 
     The first shaft  31  is arranged to extend along the central axis C. The first shaft  31  is, for example, a columnar member which is made of metal such as stainless steel, and extends in the axial direction. The first shaft  31  is rotatably supported about the central axis C by the first bearing  32 . The first shaft  31  is urged upward in the axial direction by a first spring  36  via the first fixing unit  23  with respect to the uppermost first bearing  32  in the axial direction. Thus, the downward movement of the first shaft  31  in the axial direction is suppressed. In addition, a first C retaining ring  37  is attached near the axially lower end of the first shaft  31 . This prevents dislodgement of the first shaft  31  toward the upper side in the axial direction. 
     The first bearing  32  is held radially inward of the first bearing holding unit  35  which is cylindrical around the central axis C, and rotatably supports the first shaft  31  around the central axis C. The first bearing  32  is, for example, a ball bearing, but may be a sleeve bearing or the like. 
     The first bearing holding unit  35  is made of metal such as stainless steel or resin. When the first bearing holding unit  35  is made of metal, the first bearing holding unit  35  may be integrally formed with the first housing  11  by, for example, insert molding. On the other hand, when the first bearing holding unit  35  is made of resin, the first bearing holding unit  35  may be integrally formed with the first housing  11  by injection molding. Alternatively, the first bearing holding unit  35  and the first housing  11  may be connected by another method such as press-fitting or adhesion using an adhesive. 
     The first stator  33  is fixed to the outer circumferential surface of the first bearing holding unit  35 . The first stator  33  includes a stator core  331 , an insulator  332 , and a coil  333 . 
     The stator core  331  is formed by laminating electromagnetic steel plates such as silicon steel plates in the vertical direction. The insulator  332  is made of an insulating resin. The insulator  332  is provided to surround the outer surface of the stator core  331 . The coil  333  is composed of a conducting wire wound around the stator core  331  via the insulator  332 . 
     The first rotor  34  is disposed axially above and radially outward of the first stator  33 . The first rotor  34  rotates around the central axis C with respect to the first stator  33 . The first rotor  34  has the rotor yoke  341  and a magnet  342 . 
     The rotor yoke  341  is a substantially cylindrical member that is made of a magnetic material and has a lid on the upper side in the axial direction. The rotor yoke  341  is fixed to the first shaft  31  via the first fixing unit  23 . The magnet  342  has a cylindrical shape and is fixed to the inner circumferential surface of the rotor yoke  341 . The magnet  342  is disposed radially outward of the first stator  33 . 
     The first circuit board  4  is disposed on the lower side of the first motor  3  in the axial direction, that is, on the second impeller  5  side, in the casing  8 . The first circuit board  4  has a disk shape extending in the radial direction about the central axis C, and is provided to drive the first motor  3 . The first circuit board  4  is held by the first motor  3  via the insulator  332 . 
     An electronic circuit for supplying a drive current to the coil  333  is mounted on the first circuit board  4  so as to be electrically connected to a lead wire  333   a  of the coil  333 . The electronic circuit includes electronic components such as a capacitor and a resistor. The first circuit board  4  is also mounted with a Hall element for detecting the rotational position of the first rotor  34  and a component such as a binding pin around which the lead wire  333   a  is wound and held as necessary. Hereinafter, various components mounted on the first circuit board  4  will be referred to as mounted components  41 . 
     The second impeller  5  is positioned in the casing  8  so as to be aligned with the first impeller  2  in the axial direction. The second impeller  5  is disposed axially below and radially outward of the second motor  6 . The second impeller  5  is rotated about the central axis C by the second motor  6 . That is, the blower apparatus  1  includes the second impeller  5  that rotates about the central axis C by the second motor  6  on the other side in the axial direction. 
     The second impeller  5  has a second impeller cup  52 , a plurality of second blades  51 , and a second fixing unit  53 . The second impeller cup  52  is fixed to the second motor  6  via the second fixing unit  53 . The second impeller cup  52  is a substantially cylindrical member having a lid on the lower side in the axial direction. A rotor yoke  641  of the second motor  6  is fixed to the inside of the second impeller cup  52 . 
     The plurality of second blades  51  are circumferentially arranged on the outer surface of the second impeller cup  52 . In the present example embodiment, the second impeller  5  has five second blades  51  as shown in  FIG. 3 , but the number of second blades  51  is not limited to five. The second fixing unit  53  is a member for fixing the second impeller cup  52  to a second shaft  61  of the second motor  6 . 
     The second motor  6  is supported by the second housing  12  on the lower side in the axial direction in the casing  8 . Specifically, the blower apparatus  1  includes the second motor  6  supported on the other side in the axial direction by the second housing  12 . The second motor  6  has the second shaft  61 , a second bearing  62 , a second stator  63 , a second rotor  64 , and a second bearing holding unit  65 . 
     The second shaft  61  is arranged to extend along the central axis C. The second shaft  61  is, for example, a columnar member which is made of metal such as stainless steel, and extends in the axial direction. The second shaft  61  is rotatably supported about the central axis C by the second bearing  62 . The second shaft  61  is urged downward in the axial direction by a second spring  66  via the second fixing unit  53  with respect to the lowermost second bearing  62 . Thus, the upward movement of the second shaft  61  in the axial direction is suppressed. In addition, a second C retaining ring  67  is attached near the axially upper end of the second shaft  61 . This prevents dislodgement of the second shaft  61  toward the lower side in the axial direction. 
     The second bearing  62  is held radially inward of the second bearing holding unit  65  which is cylindrical around the central axis C, and rotatably supports the second shaft  61  around the central axis C. The second bearing  62  is, for example, a ball bearing, but may be a sleeve bearing or the like. 
     The second bearing holding unit  65  is made of metal such as stainless steel or resin. When the second bearing holding unit  65  is made of metal, the second bearing holding unit  65  may be integrally formed with the second housing  12  by insert molding. On the other hand, when the second bearing holding unit  65  is made of resin, the second bearing holding unit  65  may be integrally formed with the second housing  12  by injection molding. Alternatively, the second bearing holding unit  65  and the second housing  12  may be connected by another method such as press-fitting or adhesion using an adhesive. 
     The second stator  63  is fixed to the outer circumferential surface of the second bearing holding unit  65 . The second stator  63  includes a stator core  631 , an insulator  632 , and a coil  633 . 
     The stator core  631  is formed by laminating electromagnetic steel plates such as silicon steel plates in the vertical direction. The insulator  632  is made of an insulating resin. The insulator  632  is provided to surround the outer surface of the stator core  631 . The coil  633  is composed of a conducting wire wound around the stator core  631  via the insulator  632 . 
     The second rotor  64  is disposed axially below and radially outward of the second stator  63 . The second rotor  64  rotates around the central axis C with respect to the second stator  63 . The second rotor  64  has a rotor yoke  641  and a magnet  642 . 
     The rotor yoke  641  is a substantially cylindrical member made of a magnetic material and having a lid on the lower side in the axial direction. The rotor yoke  641  is fixed to the second shaft  61  via the second fixing unit  53 . The magnet  642  is cylindrical and fixed to the inner circumferential surface of the rotor yoke  641 . The magnet  642  is disposed radially outward of the second stator  63 . 
     The second circuit board  7  is disposed on the upper side of the second motor  6  in the axial direction, that is, on the first impeller  2  side, in the casing  8 . The second circuit board  7  has a disk shape extending in the radial direction about the central axis C, and is provided to drive the second motor  6 . The second circuit board  7  is held by the second motor  6  via the insulator  632 . 
     An electronic circuit for supplying a drive current to the coil  633  is mounted on the second circuit board  7  so as to be electrically connected to a lead wire  633   a  of the coil  633 . The electronic circuit includes electronic components such as a capacitor and a resistor. The second circuit board  7  is also mounted with a Hall element for detecting the rotational position of the second rotor  64  and a component such as a binding pin around which the lead wire  633   a  is wound and held as necessary. Hereinafter, various components mounted on the second circuit board  7  will be referred to as mounted components  71 . 
     The casing  8  has an intake port  81  and a discharge port  82 . The intake port  81  is an opening for taking in external air into the casing  8 . The intake port  81  is positioned on the upper side of the casing  8  in the axial direction. The discharge port  82  is an opening for discharging the air in the casing  8  to the outside. The discharge port  82  is positioned on the lower side of the casing  8  in the axial direction. That is, the blower apparatus  1  has the casing  8  having the intake port  81  positioned on the upper side of the vertically extending central axis C and the discharge port  82  positioned on the lower side of the central axis C. In the present example embodiment, the casing  8  has a unitary structure, and is not constructed by bonding separate casings. 
     The plurality of ribs  9  are positioned radially inward of the casing  8 . The plurality of ribs  9  are located substantially at the center of the casing  8  in the axial direction. The ribs  9  are arranged in the circumferential direction with opening  9   a  therebetween. The opening  9   a  is a hole through which air flowing from the intake port  81  to the discharge port  82  in the casing  8  passes when the first impeller  2  and the second impeller  5  rotate. Each rib  9  is integrally formed with the casing  8 . That is, the blower apparatus  1  has a plurality of ribs  9  which are positioned radially inward of the casing  8  and integrally formed with the casing  8 . 
     The motor housing  10  is located radially inward of the plurality of ribs  9  in the casing  8  and is formed so as to surround the central axis C. The motor housing  10  is supported to the casing  8  by the plurality of ribs  9 . In the present example embodiment, the motor housing  10  is integrally formed with the plurality of ribs  9 . That is, the blower apparatus  1  has the motor housing  10  located radially inward of the plurality of ribs  9  and integrally formed with the plurality of ribs  9 . 
     The first housing  11  supports the first motor  3  on the upper side in the axial direction in the casing  8 . The first housing  11  is located radially inward of the motor housing  10 . That is, the blower apparatus  1  has the first housing  11  located radially inward of the motor housing  10 . In the present example embodiment, the first housing  11  is integrally formed with the motor housing  10 . 
     The second housing  12  supports the second motor  6  on the lower side in the axial direction in the casing  8 . The second housing  12  is located below the first housing  11 . That is, the blower apparatus  1  has the second housing located on the other side in the axial direction with respect to the first housing  11 . The details of the first housing  11  and the second housing  12  will be described later. 
     In the above configuration, when a drive current is supplied from the first circuit board  4  to the coil  333  of the first motor  3 , a magnetic flux in the radial direction is generated in the stator core  331 . A magnetic field generated by the magnetic flux of the stator core  331  and a magnetic field generated by the magnet  342  act to generate torque in the circumferential direction of the first rotor  34 . The generated torque causes the first rotor  34  and the first impeller  2  to rotate about the central axis C together with the first shaft  31 . 
     In addition, when the drive current is supplied from the second circuit board  7  to the coil  633  of the second motor  6 , a magnetic flux in the radial direction is generated in the stator core  631 . A magnetic field generated by the magnetic flux of the stator core  631  and a magnetic field generated by the magnet  642  act to generate torque in the circumferential direction of the second rotor  64 . The generated torque causes the second rotor  64  and the second impeller  5  to rotate about the central axis C together with the second shaft  61 . 
     When the first impeller  2  and the second impeller  5  rotate, a stream of air flowing from the intake port  81  toward the discharge port  82  is generated by the plurality of first blades  21  and the plurality of second blades  51 . That is, air is taken into the casing  8  through the intake port  81 . The air taken into the casing  8  passes through the openings  9   a  between the circumferentially adjacent ribs  9  and flows toward the discharge port  82 . The air reaching the discharge port  82  is discharged to the outside through the discharge port  82 . Therefore, in the configuration of the present example embodiment, air can be blown in one direction from the intake port  81  to the discharge port  82 . 
     Next, the details of the first housing  11  and the second housing  12  will be described.  FIG. 6  is an exploded sectional view of the first housing  11  and the second housing  12 . 
     The first housing  11  has a cylindrical part  111  and a connection part  112 . The cylindrical part  111  is formed to surround the central axis C. The inner diameter of the cylindrical part  111  is smaller than the inner diameter of the motor housing  10 . 
     The connection part  112  connects the motor housing  10  and the cylindrical part  111  in the radial direction. More specifically, the connection part  112  radially connects a central part  101  located substantially at the center of the motor housing  10  in the axial direction and an axially lower end  111   a  which is on the lower side of the cylindrical part  111 . Thus, the motor housing  10 , the cylindrical part  111 , and the connection part  112  are integrally formed. That is, the motor housing  10  and the first housing  11  are integrally formed. 
     The connection part  112  has a holding part  113  on the radially inner side. Here, the holding part  113  is formed as a recess upwardly recessed in the axial direction and having an annular shape as viewed from below in the axial direction, as shown in  FIG. 4 . The recess of the holding part  113  is formed into a shape conforming to the shape of a flange  121  described later of the second housing  12 . 
     The second housing  12  has the flange  121  and a cylindrical part  122 . The flange  121  is a thin plate having an annular shape when viewed from above in the axial direction. The cylindrical part  122  is connected to the flange  121  so as to extend downward in the axial direction. 
     The second housing  12  is held and fixed to the first housing  11  by inserting the flange  121  of the second housing  12  into the holding part  113  of the first housing  11  from below in the axial direction. That is, in the blower apparatus  1 , the second housing  12  is fixed to the first housing  11 . The second housing  12  is fixed by a snap-fit  13  so as not to be disengaged from the first housing  11 , the detail of which will be described later. 
     In the present example embodiment, the plurality of ribs  9  are located radially inward of the casing  8  as described above. The motor housing  10  is located radially inward of the plurality of ribs  9 . The first housing  11  is located radially inward of the motor housing  10 . The first motor  3  is supported on the upper side in the axial direction by the first housing  11 . The second motor  6  is supported on the lower side in the axial direction by the second housing  12 . The second housing  12  is fixed to the first housing  11 . 
     Due to the casing  8 , the plurality of ribs  9 , the motor housing  10 , the first housing  11 , and the second housing  12  being arranged to have the above-described positional relationship, the blower apparatus  1  can be achieved which uses the casing  8  having a unitary structure and which is provided with the first motor  3  and the second motor  6  arranged in the axial direction in the casing  8 . Since the casing  8  has a unitary structure, the ribs  9  integrally formed with the casing  8  can also be configured to be unitary in the axial direction. In this case, the thickness of each of the ribs  9  in the axial direction for ensuring strength can be reduced as compared with the conventional configuration in which two casings are bonded to each other. 
     Thus, the thickness in the axial direction of the motor housing  10  located radially inward of the ribs  9  can also be reduced as compared with the configuration in which two casings are bonded to each other. As a result, the casing  8  can be reduced in thickness in the axial direction, as compared to a configuration in which two casings are bonded to each other, whereby the blower apparatus  1  can be reduced in thickness. That is, the casing  8  and the blower apparatus  1  can be reduced in thickness with reliability being ensured by ensuring the required strength of the ribs  9 . 
     Conversely, when the thickness of the casing  8  in the axial direction is constant, for example, the strength of each rib  9  can be quadrupled by simply doubling the thickness of each rib  9  in the axial direction, as compared with the case where two parts are bonded to each other in the axial direction to construct the casing. Therefore, in this case, it is possible to achieve the casing  8  that is resistant to impact and not easily broken, and the reliability of the blower apparatus  1  can be further improved. 
     In addition, in the configuration where two parts are bonded to each other to construct the casing, if distortion occurs in the bonding surface of at least one of the two parts, vibration occurs, and noise is generated when the blower apparatus is driven. However, when the casing  8  has a unitary structure as in the present example embodiment, there is no problem of vibration and noise unique to the above-described configuration where two parts are bonded to each other. 
     Furthermore, since the second housing  12  is fixed to the first housing  11 , the blower apparatus  1  can be assembled as follows. Specifically, the first bearing holding unit  35  is inserted into the first housing  11  from below in the axial direction, for example. Next, the first stator  33  with the first circuit board  4  is inserted into the casing  8  from above in the axial direction until the stator core  331  contacts the cylindrical part  111  of the first housing  11 . Then, the first bearing holding unit  35  is press fitted into the radially inner side of the first stator  33 , and the first bearing holding unit  35  is fixed to the first stator  33 . Note that the first bearing holding unit  35  and the first stator  33  may be fixed using an adhesive agent. 
     Next, the first bearing  32 , the first spring  36 , and the first impeller  2  with the first shaft  31  are sequentially inserted into the casing  8  from above in the axial direction. Then, the first C retaining ring  37  is inserted into the casing  8  from below in the axial direction and attached to the first shaft  31 . In this way, the attachment of the first impeller  2  and the first motor  3  to the inside of the casing  8  is completed. 
     On the other hand, the second stator  63 , the second bearing holding unit  65 , the second bearing  62 , the second spring  66 , and the second impeller  5  with the second shaft  61  are mounted to the second housing  12 , and the second C retaining ring  67  is attached near the upper end of the second shaft  61  in the axial direction. 
     Next, the second housing  12  is attached to the first housing  11  from below in the axial direction. Thus, the attachment of the second impeller  5  and the second motor  6  to the inside of the casing  8  is completed, and the assembly of the blower apparatus  1  is completed. 
     As described above, when the blower apparatus  1  is assembled, a method for assembling some of the components of the blower apparatus  1  at the outside of the casing  8 , and then, inserting the assembled components to the casing  8  can be used. As a result, the blower apparatus  1  can be easily assembled, and ease of assembly can be improved. That is, even if the casing  8  of the blower apparatus  1  accommodating inside the first impeller  2 , the second impeller  5 , the first motor  3 , and the second motor  6  has a unitary structure, the assembly of the blower apparatus  1  can be facilitated. 
     Further, as described above, the motor housing  10 , and the cylindrical part  111  and the connection part  112  which constitute the first housing  11  are integrally formed. Further, the second housing  12  is a component fixed to the first housing  11 , and therefore, the second housing  12  is a separate component from the motor housing  10  and the first housing  11 . That is, the motor housing  10  and the first housing  11  are an integral member, and the motor housing  10  and the second housing  12  are separate members. 
     In this configuration, it is only sufficient to use two members, which are the motor housing  10  integral with the first housing  11  and the second housing  12 , for the housings necessary for supporting the first motor  3  and the second motor  6  in the casing  8 . Therefore, the cost for the components of the blower apparatus  1  can be reduced. Further, since the motor housing  10  and the first housing  11  are integrated, a structure for supporting the first housing  11  on the motor housing  10  is unnecessary. Therefore, the structure for supporting the first motor  3  and the second motor  6  in the casing  8  is simplified. 
     As shown in  FIG. 6 , the first housing  11  has a dent part  132  in the holding part  113  described above. The dent part  132  is formed to be recessed radially outward from the inner surface  113   a  of the holding part  113 . The dent part  132  is formed into a shape conforming to the shape of a protrusion  131  of the second housing  12 . 
     The second housing  12  has the protrusion  131 . The protrusion  131  is formed to protrude further outward in the radial direction from the radially outer end  121   a  of the flange  121  of the second housing  12 . 
     When the second housing  12  is brought close to the first housing  11  from below in the axial direction, the protrusion  131  of the second housing  12  contacts a corner  113   b  of the holding part  113  of the first housing  11 . When the second housing  12  is further pushed upward in the axial direction, a force for pushing the protrusion  131  radially inward is exerted due to a reaction force to the force applied to the corner  113   b  by the protrusion  131 . As a result, the second housing  12  is slightly deformed such that the protrusion  131  moves to the inside of the holding part  113 . 
     When the protrusion  131  reaches the dent part  132  due to the second housing  12  being further pushed upward in the axial direction, the force for pushing the protrusion  131  radially inward is released. Thus, the deformation of the second housing  12  is released, and the protrusion  131  fits into the dent part  132 . As a result, the second housing  12  is prevented from being dislodged axially downwardly from the first housing  11 , and is fixed to the first housing  11 . 
     The structure in which the second housing  12  is fixed to the first housing  11  by the protrusion  131  being fitted into the dent part  132  in this manner is referred to as the snap-fit  13 . That is, the second housing  12  is fixed to the first housing  11  by the snap-fit  13 . In this case, since the first housing  11  and the second housing  12  can be easily fixed by the snap-fit  13 , the assembly of the blower apparatus  1  is further facilitated. 
     The method for fixing the second housing  12  and the first housing  11  is not limited to the above method using the snap-fit  13 . For example, the second housing  12  and the first housing  11  may be fixed by screwing, may be fixed using a rivet, or may be fixed using an adhesive. However, from the viewpoint of further improving the ease of assembly, a fixing method using the snap-fit  13  as in the present example embodiment is desirable. 
     As shown in  FIGS. 5 and 6 , the first housing  11  has a first recessed part  100 P and a second recessed part  100 Q. The first recessed part  100 P is formed to be open at the upper side in the axial direction and to be closed at the lower side in the axial direction. Any of the mounted components  41  on the first circuit board  4  is inserted into the first recessed part  100 P from above in the axial direction. 
     Here, from among the mounted components  41 , a component protruding to the side opposite to the stator core  331  from the first circuit board  4  (that is, protruding downward in the axial direction) can be considered to be inserted into the first recessed part  100 P. In particular, the binding pin which is tall in the axial direction can be considered to be one of the mounted components  41  which is to be inserted into the first recessed part  100 P. However, it is obvious that any other components such as a capacitor may be inserted into the first recessed part  100 P. 
     The second recessed part  100 Q is formed to be open at the lower side in the axial direction and to be closed at the upper side in the axial direction. Any of the mounted components  71  on the second circuit board  7  is inserted into the second recessed part  100 Q from below in the axial direction. Here, from among the mounted components  71 , a component protruding to the side opposite to the stator core  631  from the second circuit board  7  (that is, protruding upward in the axial direction) can be considered to be inserted into the second recessed part  100 Q. In particular, the binding pin which is tall in the axial direction can be considered to be one of the mounted components  71  which is to be inserted into the second recessed part  100 Q. However, it is obvious that any other components such as a capacitor may be inserted into the second recessed part  100 Q. 
     In this configuration, the mounted component  41  mounted on the first circuit board  4  and protruding downward in the axial direction is inserted into the first recessed part  100 P in the casing  8 . Further, the mounted component  71  mounted on the second circuit board  7  and protruding upward in the axial direction is inserted into the second recessed part  100 Q in the casing  8 . As a result, even if the distance between the first circuit board  4  and the second circuit board  7  in the axial direction is shortened, electrical insulation can be ensured. Therefore, the casing  8  can be entirely reduced in thickness in the axial direction by bringing the first impeller  2  and the second impeller  5  close to each other in the axial direction, whereby the blower apparatus  1  can be further reduced in thickness. 
     In the first housing  11 , it is desirable that the first recessed part  100 P and the second recessed part  100 Q are offset from each other in at least one of the circumferential direction and the radial direction. In this configuration, the first recessed part  100 P and the second recessed part  100 Q do not make a through hole by being connected to each other in the axial direction. Therefore, it is not necessary to provide a wall between the first recessed part  100 P and the second recessed part  100 Q for separating the opening of the first recessed part  100 P and the opening of the second recessed part  100 Q in the axial direction. Accordingly, the thickness of the first housing  11  (particularly, the connection part  112 ) in the axial direction can be reduced because it is not necessary to provide the wall. As a result, the axial distance between the first circuit board  4  and the second circuit board  7  can be shortened, whereby the axial distance between the first impeller  2  and the second impeller  5  can be shortened. Consequently, it is possible to further reduce the thickness of the casing  8  and the blower apparatus  1 . 
     Another exemplary example embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. In a blower apparatus  1  according to the present example embodiment, the configuration other than the motor housing  10 , the first housing  11 , and the second housing  12  is the same as that of the first example embodiment, and thus the description thereof will be omitted below. 
       FIG. 7  is a longitudinal sectional view showing the blower apparatus  1  according to the present example embodiment. For convenience,  FIG. 7  shows the cross section of only one side of the blower apparatus  1  in the radial direction with respect to the central axis C.  FIG. 8  is an exploded sectional view of the motor housing  10 , the first housing  11 , and the second housing  12  of the blower apparatus  1  shown in  FIG. 7 . In the present example embodiment, the motor housing  10 , the first housing  11 , and the second housing  12  are different members. 
     The first housing  11  and the second housing  12  are arranged in the axial direction. In particular, the first housing  11  is located above the second housing  12  in the axial direction. When a distance from the central axis C to an outer surface  11   a  which is a radially outer surface of the first housing  11  is defined as L 1  (mm), and a distance from the central axis C to an outer surface  12   a  which is a radially outer surface of the second housing  12  is defined as L 2  (mm), L 2 &lt;L 1  is established. That is, the outer surface  11   a  of the first housing  11  is located radially outward of the outer surface  12   a  of the second housing  12 . 
     The first housing  11  has a large diameter part  114 , a cylindrical part  115 , and a connection part  116 . The large diameter part  114  is the outermost part of the first housing  11  in the radial direction, and is formed to surround the connection part  116 . The connection part  116  is formed to surround the cylindrical part  115 . The cylindrical part  115  is formed to surround the central axis C. 
     The connection part  116  connects the large diameter part  114  and the cylindrical part  115  in the radial direction. More specifically, the connection part  116  radially connects a lower end  114   a  of the large diameter part  114  in the axial direction and a lower end  115   a  of the cylindrical part  115  in the axial direction. Thus, the first housing  11  in which the large diameter part  114 , the cylindrical part  115 , and the connection part  116  are integrated is configured. 
     The second housing  12  has a plate-shaped part  123  and a cylindrical part  124 . The plate-shaped part  123  is a plate member extending radially outward from an upper end  124   a  of the cylindrical part  124  in the axial direction. The cylindrical part  124  is formed to surround the central axis C. 
     The first housing  11  is fixed to the first bearing holding unit  35  of the first motor  3 , thereby supporting the first motor  3 . The second housing  12  is fixed to the second bearing holding unit  65  of the second motor  6 , thereby supporting the second motor  6 . 
     The blower apparatus  1  according to the present example embodiment also has a first recessed part  100 P and a second recessed part  100 Q in the first housing  11  and the second housing  12 . The first recessed part  100 P is formed to be open at the upper side in the axial direction and to be closed at the lower side in the axial direction. The second recessed part  100 Q is formed to be open at the lower side in the axial direction and to be closed at the upper side in the axial direction. 
     The first recessed part  100 P has a through hole  116   a  and a lid part  123   a . The through hole  116   a  is a hole which passes through the connection part  116  of the first housing  11  in the axial direction. The lid part  123   a  is located at the second housing  12  and closes the through hole  116   a . The lid part  123   a  is constituted by a portion of the plate-shaped part  123  of the second housing  12 . 
     The second recessed part  100 Q has a through hole  123   b  and a lid part  116   b . The through hole  123   b  is a hole that passes through the plate-shaped part  123  of the second housing  12  in the axial direction. The lid part  116   b  is located at the first housing and closes the through hole  123   b . The lid part  116   b  is constituted by a portion of the connection part  116  of the first housing  11 . 
     A mounted component  41  mounted on the first circuit board  4  and protruding downward in the axial direction is inserted into the first recessed part  100 P from above in the axial direction. Further, a mounted component  71  mounted on the second circuit board  7  and protruding upward in the axial direction is inserted into the second recessed part  100 Q from below in the axial direction in the casing  8 . This can provide the effect of reducing the blower apparatus  1  in thickness by shortening the axial distance between the first circuit board  4  and the second circuit board  7 , as in the first example embodiment. 
     The first housing  11  and the second housing  12  having the above-described configurations are fixed to the motor housing  10 , and therefore, the motor housing  10  is slightly different from that shown in  FIGS. 5 and 6 . 
     The motor housing  10  has a first receiving portion  10   a  and a second receiving portion  10   b . The first receiving portion  10   a  is located radially inward of the plurality of ribs  9 . The first receiving portion  10   a  receives the first housing  11 , which is inserted from above in the axial direction, at a position axially above and radially outward of the second receiving portion  10   b . The second receiving portion  10   b  receives the second housing  12  which is inserted from above in the axial direction. 
     That is, the motor housing  10  has the first receiving portion  10   a  located radially inward of the plurality of ribs  9  and receiving the first housing  11  in the axial direction, and the second receiving portion  10   b  located radially inward of the first receiving portion  10   a  and receiving the second housing  12  in the axial direction. In the present example embodiment, the blower apparatus  1  can be assembled as follows. 
     First, the second stator  63  with the second circuit board  7 , the second bearing holding unit  65 , and the second bearing  62  are attached to the second housing  12  outside the casing  8 . Then, the second housing  12  is inserted into the casing  8  from above in the axial direction. Thereafter, the second spring  66  and the second impeller  5  with the second shaft  61  are sequentially inserted from below in the axial direction, and the second C retaining ring  67  is attached in the vicinity of the axially upper end of the second shaft  61 . 
     Next, the first stator  33  with the first circuit board  4 , the first bearing holding unit  35 , the first bearing  32 , the first spring  36 , the first impeller  2  with the first shaft  31 , and the first C retaining ring  37  are sequentially attached to the first housing  11  outside the casing  8 . Thereafter, the first housing  11  is inserted into the casing  8  from above in the axial direction and fixed. Thus, the blower apparatus  1  is completed. 
     As described above, in the present example embodiment, the motor housing  10 , the first housing  11 , and the second housing  12  are separate members. Therefore, outside the casing  8 , a part of the second motor  6  can be attached to the second housing  12 , and the first motor  3  can be attached to the first housing  11 , as described above. Finally, the first housing  11  and the second housing  12  can be attached to the motor housing  10  of the casing  8  to complete the blower apparatus  1 . As described above, since a part of the blower apparatus  1  can be assembled outside the casing  8 , the workability at the time of assembly is improved. 
     In particular, the motor housing  10  has the first receiving portion  10   a  and the second receiving portion  10   b  described above. Therefore, the blower apparatus  1  can be assembled by inserting the second housing  12  into the casing  8  from above in the axial direction, and then inserting the first housing  11  similarly from above in the axial direction. As described above, since the insertion directions of the first housing  11  and the second housing  12  into the casing  8  are the same, the assembly of the blower apparatus  1  can be facilitated. 
     Further, when the second housing  12  is inserted into the casing  8  from above in the axial direction during the assembly of the blower apparatus  1  described above, the second housing  12  comes in contact with the second receiving portion  10   b  of the motor housing  10  at its radially outer end  12   b , and stops. When the first housing  11  is then inserted into the casing  8  similarly from above in the axial direction, the first housing  11  comes in contact with the first receiving portion  10   a  of the motor housing  10  at its radially outer end  114   a , and stops. In this state, the second housing  12  is axially held between the second receiving portion  10   b  and the first housing  11 , and is fixed to the motor housing  10 . 
     That is, the second housing  12  is fixed to the motor housing  10  by being axially held between the second receiving portion  10   b  of the motor housing  10  and the first housing  11  received by the first receiving portion  10   a . Since the second housing  12  is fixed in this manner, a separate fixing member (for example, an adhesive) for fixing the second housing  12  to the motor housing  10  can be eliminated, whereby the second housing  12  can be fixed to the motor housing  10  by a simple configuration. 
     As shown in  FIG. 8 , the first housing  11  has a protrusion  141 . The protrusion  141  is formed to protrude radially outward from the outer surface  11   a  of the first housing  11 . The motor housing  10  also has a dent part  142 . The dent part  142  is formed to be recessed radially outward from the inner surface  10   c  of the motor housing  10 . The inner surface  10   c  of the motor housing  10  is located above the first receiving portion  10   a  in the axial direction. The inner surface  10   c  contacts the outer surface  11   a  of the first housing  11 , by which the first housing  11  slides in the axial direction. The dent part  142  is formed into a shape conforming to the shape of the protrusion  141  of the first housing  11 . 
     When the first housing  11  is moved downward in the axial direction, the protrusion  141  of the first housing  11  comes in contact with the corner  10   d  of the motor housing  10 . When the first housing  11  is further pushed downward in the axial direction, a force for pushing the protrusion  141  radially inward is exerted due to a reaction force to the force applied to the corner  10   d  by the protrusion  141 . As a result, the first housing  11  is slightly deformed such that the protrusion  141  moves toward the radially inner side of the motor housing  10 . 
     When the protrusion  141  reaches the dent part  142  due to the first housing  11  being further pushed downward in the axial direction, the force for pushing the protrusion  141  radially inward with respect to the dent part  142  is released. Thus, the deformation of the first housing  11  is released, and the protrusion  141  fits into the dent part  142 . As a result, the first housing is prevented from being dislodged upwardly from the motor housing  10  in the axial direction, and is fixed to the motor housing  10 . 
     The structure in which the first housing  11  is fixed to the motor housing  10  by the protrusion  141  being fitted into the dent part  142  in this manner is referred to as a snap-fit  14 . That is, the first housing  11  is fixed to the motor housing  10  by the snap-fit  14 . Since the first housing  11  and the motor housing  10  can be easily fixed by the snap-fit  14 , the assembly of the blower apparatus  1  is further facilitated. 
     The method for fixing the first housing  11  and the motor housing  10  is not limited to the above method using the snap-fit  14 . That is, the first housing  11  and the motor housing  10  may be fixed by screwing, may be fixed using a rivet, or may be fixed using an adhesive. However, from the viewpoint of further improving the ease of assembly, a fixing method using the snap-fit  14  is desirable. 
       FIG. 9  is an exploded sectional view showing another configuration of the motor housing  10  and the second housing  12 . In the configuration shown in  FIG. 9 , when the distance from the central axis C to the outer surface  11   a  of the first housing  11  is defined as L 1  (mm), and the distance, that is, the radial length, from the central axis C to the outer surface  12   a  of the second housing  12  is defined as L 2  (mm), L 2 =L 1  is established. That is, the outer surface  11   a  of the first housing  11  is located at the same position as the outer surface  12   a  of the second housing  12  in the radial direction. 
     Further, the motor housing  10  has a housing receiving portion  10   e . The housing receiving portion  10   e  is located radially inward of the plurality of ribs  9 . The housing receiving portion  10   e  simultaneously receives the first housing  11  and the second housing  12  which are inserted from above in the axial direction. That is, the motor housing  10  has the housing receiving portion  10   e  located radially inward of the plurality of ribs  9  and receiving the second housing  12  together with the first housing  11  in the axial direction. 
     When the radial lengths of the first housing  11  and the second housing  12  are the same, both the first housing  11  and the second housing  12  of the motor housing  10  can be received by the single housing receiving portion  10   e  in the axial direction. Therefore, it is not necessary to provide different receiving portions for individually receiving the first housing  11  and the second housing  12  to the motor housing  10 , as shown in  FIG. 8 . In other words, it is only sufficient that only one receiving portion which is the minimum necessary is provided. Therefore, the configuration of the motor housing  10  can be simplified as compared with the configuration in  FIG. 8 . 
     Further, when the second housing  12  is inserted into the casing  8  from above in the axial direction in the configuration shown in  FIG. 9 , the second housing  12  comes in contact with the housing receiving portion  10   e  of the motor housing  10  at the radially outer end  12   b , and stops. When the first housing  11  is then inserted into the casing  8  similarly from above in the axial direction, the first housing  11  comes in contact with the second housing  12  and stops. In this state, the second housing  12  is axially held between the housing receiving portion  10   e  and the first housing  11 , and is fixed to the motor housing  10 . 
     That is, the second housing  12  is fixed to the motor housing  10  by being axially held between the housing receiving portion  10   e  of the motor housing  10  and the first housing  11 . Since the second housing  12  is fixed in this manner, a separate fixing member (for example, an adhesive) for fixing the second housing  12  to the motor housing  10  can be eliminated, whereby the second housing  12  can be fixed to the motor housing  10  by a simple configuration. 
     In the configuration shown  FIG. 9 , the first housing  11  may also be fixed to the motor housing  10  by the snap-fit  14  as in the configuration shown in  FIG. 8 . 
       FIG. 10  is an exploded sectional view showing still another configuration of the motor housing  10 , the first housing  11 , and the second housing  12 . When the housing receiving portion  10   e  of the motor housing  10  receives, in the axial direction, the first housing  11  and the second housing  12  which have the same radial length, a member having the same shape as the second housing  12  can be used for the first housing  11 . That is, when the second housing  12  is vertically inverted and arranged, the inverted second housing  12  can be used as the first housing  11 . Therefore, it is only sufficient that only one type of member is used for the first housing  11  and the second housing  12 , whereby cost for the components can be reduced. 
     When the member having the same shape as the second housing  12  is used for the first housing  11 , the motor housing  10 , the first housing  11 , and the second housing  12  may be fixed to one another by screwing, by using an adhesive, or by snap-fit. Moreover, when the first housing  11  and the second housing  12  which have the same shape are used, it is desirable to fix them by rotating one of the first housing  11  and the second housing  12  with respect to the other in the circumferential direction. In this case, the thickness of the blower apparatus  1  can be reduced by providing the first recessed part  100   p  and the second recessed part  100 Q so as not to overlap each other in the axial direction. 
       FIG. 11  is a plan view of the second receiving portion  10   b  of the motor housing  10  as viewed from above in the axial direction. The second receiving portion  10   b  has an inner surface  102 . The inner surface  102  is on the radially inner side of the second receiving portion  10   b  so as to surround the central axis C. The inner surface  102  is a spline-shaped uneven surface, and is formed by alternately providing first grooves  102   a  and first protruding portions  102   b  in the circumferential direction. 
     The first grooves  102   a  extend along the axial direction. That is, the first grooves  102   a  are recessed outward in the radial direction. The first protruding portions  102   b  protrude inward in the radial direction. That is, the second receiving portion  10   b  has the inner surface  102  having first grooves  102   a  recessed outward in the radial direction and first protruding portions  102   b  protruding inward in the radial direction, the first grooves  102   a  and the first protruding portions  102   b  being alternately arranged in the circumferential direction. 
       FIG. 12  is a plan view of the second housing  12  as viewed from above in the axial direction. The second housing  12  has an outer surface  125 . The outer surface  125  is on the radially outer side of the second housing  12  so as to surround the central axis C. The outer surface  125  is a spline-shaped uneven surface, and is formed by alternately providing second grooves  125   a  and second protruding portions  125   b  in the circumferential direction. The outer surface  125  is formed into a shape conforming to the shape of the inner surface  102  of the second receiving portion  10   b.    
     The second grooves  125   a  extend along the axial direction. That is, the second grooves  125   a  are recessed inward in the radial direction. The second protruding portions  125   b  protrude outward in the radial direction. That is, the second housing  12  has the outer surface  125  having second grooves  125   a  recessed inward in the radial direction and second protruding portions  125   b  protruding outward in the radial direction, the second grooves  125   a  and the second protruding portions  125   b  being alternately arranged in the circumferential direction. The outer surface  125  of the second housing  12  has a shape conforming to the shape of the inner surface  102  of the second receiving portion  10   b.    
     In the above configuration, the blower apparatus  1  can be assembled as follows. Specifically, the second stator  63  with the second circuit board  7 , the second bearing holding unit  65 , the second bearing  62 , the second spring  66 , and the second impeller  5  with the second shaft  61  are attached to the second housing  12  outside the casing  8 , and the second C retaining ring  67  is attached to the second shaft  61 . In this state, the second housing  12  is inserted into the second receiving portion  10   b  from below in the axial direction, and after the second housing  12  passes through the second receiving portion  10   b  toward the upper side in the axial direction, the second housing  12  is rotated in the circumferential direction. 
       FIG. 13  is a plan view of the state in which the second housing  12  is inserted into the second receiving portion  10   b  as viewed from above in the axial direction. The second housing  12  and the second receiving portion  10   b  are positioned such that the first protruding portions  102   b  of the second receiving portion  10   b  are engaged with the second grooves  125   a  of the second housing  12 , and the second protruding portions  125   b  of the second housing  12  are engaged with the first grooves  102   a  of the second receiving portion  10   b , whereby the second housing  12  can be inserted into the second receiving portion  10   b  in the axial direction. 
       FIG. 14  is a plan view showing a state in which the second housing  12  is rotated with respect to the second receiving portion  10   b  as viewed from above in the axial direction. When the second housing  12  is rotated, the second protruding portions  125   b  of the second housing  12  overlap with the first protruding portions  102   b  of the second receiving portion  10   b  as viewed in the axial direction. In this state, the second protruding portions  125   b  are caught by the first protruding portions  102   b , which prevents the second housing  12  from being dislodged downwardly in the axial direction. That is, the second housing  12  is fixed to the second receiving portion  10   b . Then, the first impeller  2  and the first motor  3  are placed in the casing  8  in the same manner as in  FIG. 8 . 
     As described above, since the inner surface  102  of the second receiving portion  10   b  and the outer surface  125  of the second housing  12  have a spline shape, the method for inserting the second housing  12  to the motor housing  10 , which is integral with the casing  8 , from below in the axial direction can be employed. Therefore, both the first motor  3  and the second motor  6  can be attached to the first housing  11  and the second housing  12 , respectively, outside the casing  8 , and then, can be placed in the casing  8 . Thus, the ease of assembly can be further improved. 
       FIG. 15  is a sectional view showing still another configuration of the first housing  11 . When the inner surface  102  of the second receiving portion  10   b  and the outer surface  125  of the second housing  12  have a spline shape, the first housing  11  may have a projection  114   b . The projection  114   b  has a shape that fits into a hole portion  130  shown in  FIG. 14  when the first housing  11  is inserted downward into the motor housing  10  in the axial direction. The hole portion  130  is a hole formed from the first groove  102   a  and the second groove  125   a  overlapping with each other in the axial direction when the second housing  12  is inserted into the second receiving portion  10   b  from below in the axial direction and rotated in the circumferential direction. That is, the first housing  11  has the projection  114   b  fitted in the hole portion  130  formed from the first groove  102   a  of the second receiving portion  10   b  and the second groove  125   a  of the second housing  12  overlapping with each other in the axial direction. 
     The engagement between the projection  114   b  of the first housing  11  and the hole portion  130  can prevent the second housing from rotating in the circumferential direction and being disengaged from the second receiving portion  10   b . In addition, when the projection  114   b  of the first housing  11  is engaged with the hole portion  130 , a snap-fit can be formed. That is, the projection  114   b  can be used as a part constituting a snap-fit. 
       FIG. 16  is a longitudinal sectional view showing another configuration of the blower apparatus  1  according to the present example embodiment. For convenience,  FIG. 16  shows the cross section of only one side of the blower apparatus  1  in the radial direction with respect to the central axis C. In the case where the motor housing  10  and the first housing  11  are fixed by the snap-fit  14  shown in  FIG. 8 , it is desirable that the air blowing direction is opposite to the air blowing direction in the configuration shown in  FIGS. 7 and 8 . 
     That is, in the configuration shown in  FIG. 7 , the intake port  81  is located on the upper side of the casing  8  in the axial direction, and the discharge port  82  is located on the lower side of the casing  8  in the axial direction, so that air is blown from top to bottom in the axial direction. In contrast, when the fixing method using the snap-fit  14  is employed, it is desirable that the intake port  82  is located on the lower side of the casing  8  in the axial direction, the discharge port  82  is located on the upper side of the casing  8  in the axial direction, and the first motor  3  and the second motor  6  are driven to blow air from bottom to top in the axial direction, as shown in  FIG. 16 . That is, it is desirable to rotate the first impeller  2  and the second impeller  5  in the direction opposite to the direction in the configuration shown in  FIG. 7 . 
     Specifically, in the blower apparatus  1  in  FIG. 16 , the intake port  81  of the casing  8  is located on the side opposite to the first housing  11  with respect to the second housing  12 , and the discharge port  82  of the casing  8  is located on the side opposite to the second housing  12  with respect to the first housing  11 , in relation to the axial direction. Further, the first motor  3  and the second motor  6  rotate the first impeller  2  and the second impeller  5 , respectively, so that air flows from the intake port  81  toward the discharge port  82 . 
     Generally, in a blower apparatus in which two impellers are arranged coaxially, the impellers receive a force (reaction force) in the direction opposite to the stream of air flowing from the intake port to the discharge port in the casing. When the intake port  81 , the discharge port  82 , the first housing  11 , and the second housing  12  have the positional relationship shown in  FIG. 16 , the first impeller  2  on the first housing  11  side receives a force (reaction force) toward the intake port  81  side with respect to the stream of air flowing from the intake port  81  toward the discharge port  82 . Due to this reaction force, a force of the first housing  11  pressing the second housing  12  to the intake port  81  side is generated. This force is in a direction in which the first housing  11  is engaged with the motor housing  10 , and is in a direction opposite to the direction in which the snap-fit  14  is released in the axial direction (a direction in which the first housing  11  is removed from the motor housing  10  in the axial direction). As a result, the snap-fit  14  is less likely to be disengaged. As a result, the fixing of the first housing  11  to the motor housing  10  can be maintained. 
     While example embodiments of the present disclosure have been described above, it will be understood that the scope of the present disclosure is not limited to the above-described example embodiments, and that various modifications are possible without departing from the spirit of the present disclosure. In addition, features of the above-described example embodiments and the modifications thereof may be combined appropriately as desired. 
     The blower apparatus according to the present disclosure is applicable to serial axial blowing apparatuses. 
     While example embodiments of the present disclosure 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 disclosure. The scope of the present disclosure, therefore, is to be determined solely by the following claims.