Patent Publication Number: US-8123461-B2

Title: Axial flow fan unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a fan frame and an axial flow fan unit. 
     2. Description of the Related Art 
     Conventionally, a cooling fan is installed inside of a housing of various kinds of electronic devices to cool electronic parts thereof. Because the electronic parts suffer from increased heat generation due to high performance and have an increased arrangement density due to the reduction in size of the housing, there is a need to increase the static pressure and flow rate of the cooling fan. To meet this need, a serially connected axial flow fan unit has recently been used as a cooling fan that can secure a great enough static pressure and an increased flow rate. The serially connected axial flow fan unit includes a plurality of axial flow fans serially connected to one another by many different methods. 
     In a case where the axial flow fans are coupled together by screws, rivets or the like, however, there is a need to form through-holes in the housings of the axial flow fans, in addition to the through-holes used in attaching the axial flow fan unit to a device. With this structure, it is difficult to re-attach the axial flow fans even though the combination of axial flow fans is changed during the course of designing or installing the serially connected axial flow fan unit. 
     Once the axial flow fans are connected to one another, it is difficult to detach them without causing damage to the through-holes or the housings. Therefore, even if the combination of axial flow fans is changed during the course of designing or installing the serially connected axial flow fan unit, it is impossible to re-attach the axial flow fans without reducing the connection strength thereof. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention have been developed in order to overcome the above problems with conventional axial flow fan units as described above. 
     In one preferred embodiment, a fan frame includes, e.g., a first housing having a first end portion in which a first protrusion is arranged and a second housing having a second end portion in which a second protrusion opposing the first protrusion along a specified axis is arranged, the second end portion being arranged in an axially opposing relationship with the first end portion. A fixing member is attached to the first protrusion and the second protrusion to fix the first housing and the second housing together. 
     In another preferred embodiment, a fan unit includes a first impeller, a first housing, a second impeller and a second housing. 
     The first housing is arranged to accommodate the first impeller that is rotatable about a specified axis and has a first end portion in which a first protrusion is provided. 
     The second housing is arranged to accommodate the second impeller rotatable about the specified axis and has a second end portion in which a second protrusion axially opposing the first protrusion is provided. The second end portion is in an axially opposing relationship with the first end portion. A fixing member is attached to the first protrusion and the second protrusion to fix the first housing and the second housing together. 
     Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a serially connected axial flow fan unit in accordance with a first preferred embodiment of the present invention. 
         FIG. 2  is a vertical section view of the serially connected axial flow fan unit in accordance with the first preferred embodiment of the present invention. 
         FIG. 3  is a perspective view showing a first housing employed in the axial flow fan unit of the first preferred embodiment of the present invention. 
         FIG. 4  is a plan view showing a corner portion of the first housing employed in the axial flow fan unit of the first preferred embodiment of the present invention. 
         FIG. 5  is a perspective view showing a second housing employed in the axial flow fan unit of the first preferred embodiment of the present invention. 
         FIG. 6  is a perspective view showing a fixing member employed in the axial flow fan unit of the first preferred embodiment of the present invention. 
         FIG. 7  is another perspective view showing the fixing member employed in the axial flow fan unit of the first preferred embodiment of the present invention. 
         FIG. 8  is a plan view showing a corner portion employed in the axial flow fan unit of the first preferred embodiment of the present invention. 
         FIG. 9  is a perspective view showing a corner portion of the first housing and a corner portion of the second housing employed in the axial flow fan unit of the first preferred embodiment of the present invention. 
         FIG. 10  is another perspective view showing the corner portion of the first housing and the corner portion of the second housing employed in the axial flow fan unit of the first preferred embodiment of the present invention. 
         FIG. 11  is a view showing the manner in which the fixing members are attached to the first and second axial flow fans employed in the axial flow fan unit of the first preferred embodiment of the present invention. 
         FIG. 12  is a perspective view showing a serially connected axial flow fan unit in accordance with a second preferred embodiment of the present invention. 
         FIG. 13  is a perspective view showing a first housing employed in the axial flow fan unit of the second preferred embodiment of the present invention. 
         FIG. 14  is a perspective view showing a second housing employed in the axial flow fan unit of the second preferred embodiment of the present invention. 
         FIG. 15  is a plan view showing a corner portion employed in the axial flow fan unit of the second preferred embodiment of the present invention. 
         FIG. 16  is a perspective view showing a serially connected axial flow fan unit in accordance with a third preferred embodiment of the present invention. 
         FIG. 17  is a perspective view showing a first housing employed in the axial flow fan unit of the third preferred embodiment of the present invention. 
         FIG. 18  is a perspective view showing a second housing employed in the axial flow fan unit of the third preferred embodiment of the present invention. 
         FIG. 19  is a perspective view showing a first axial flow fan and a second axial flow fan employed in the axial flow fan unit of the third preferred embodiment of the present invention. 
         FIG. 20  is a perspective view showing a serially connected axial flow fan unit in accordance with a fourth preferred embodiment of the present invention. 
         FIG. 21  is a perspective view showing a serially connected axial flow fan unit in accordance with a fifth preferred embodiment of the present invention. 
         FIG. 22  is a section view showing a first housing and a second housing employed in the axial flow fan unit of the fifth preferred embodiment of the present invention. 
         FIG. 23  is a perspective view showing a fixing member employed in the axial flow fan unit of the fifth preferred embodiment of the present invention. 
         FIG. 24  is a perspective view showing an end portion of the first housing and an end portion of the second housing employed in the axial flow fan unit of the fifth preferred embodiment of the present invention. 
         FIG. 25  is a section view showing the first housing, the second housing and the fixing member employed in the axial flow fan unit of the fifth preferred embodiment of the present invention. 
         FIG. 26  is a perspective view showing a serially connected axial flow fan unit in accordance with a sixth preferred embodiment of the present invention. 
         FIG. 27  is a perspective view showing a serially connected axial flow fan unit in accordance with a seventh preferred embodiment of the present invention. 
         FIG. 28  is a perspective view showing a first housing employed in the axial flow fan unit of the seventh preferred embodiment of the present invention. 
         FIG. 29  is a plan view showing a third protrusion employed in the axial flow fan unit of the seventh preferred embodiment of the present invention. 
         FIG. 30  is a perspective view showing a second housing employed in the axial flow fan unit of the seventh preferred embodiment of the present invention. 
         FIG. 31  is a perspective view showing a fixing member employed in the axial flow fan unit of the seventh preferred embodiment of the present invention. 
         FIG. 32  is another perspective view showing the fixing member employed in the axial flow fan unit of the seventh preferred embodiment of the present invention. 
         FIG. 33A  is a plan view showing an arc portion to which the fixing member is attached. 
         FIG. 33B  is another plan view showing the arc portion to which the fixing member is attached. 
         FIG. 34  is a perspective view showing a lower end portion of the first housing and an upper end portion of the second housing employed in the axial flow fan unit of the seventh preferred embodiment of the present invention. 
         FIG. 35  is another perspective view showing a lower end portion of the first housing and an upper end portion of the second housing employed in the axial flow fan unit of the seventh preferred embodiment. 
         FIG. 36  is a view showing the manner in which the fixing members are attached to the first and second axial flow fans employed in the axial flow fan unit of the seventh preferred embodiment of the present invention. 
         FIG. 37  is a perspective view showing a serially connected axial flow fan unit in accordance with an eighth preferred embodiment of the present invention. 
         FIG. 38  is a plan view showing one preferred embodiment of the axial flow fan unit. 
         FIG. 39  is a plan view showing another preferred embodiment of the axial flow fan unit. 
         FIG. 40  is a section view of the upper portion of the fixing member and the first protrusion taken along a plane perpendicular to the circumferential direction. 
         FIG. 41  is a perspective view showing a further preferred embodiment of the axial flow fan unit. 
         FIG. 42  is a section view of the protrusion positioned at the left upper side in  FIG. 41  and the first housing. 
         FIG. 43  is a perspective view of a still further preferred modified example of the axial flow fan unit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 through 43 , preferred embodiments of the present invention will be described in detail. It should be noted that in the explanation of preferred embodiments of the present invention, when positional relationships among and orientations of the different components are described as being up/down or left/right, ultimately positional relationships and orientations that are in the drawings are indicated; positional relationships among and orientations of the components once having been assembled into an actual device are not indicated. Meanwhile, in the following description, an axial direction indicates a direction parallel or substantially parallel to an axis, and a radial direction indicates a direction perpendicular or substantially perpendicular to the axis. Further, in the drawings, like or similar elements are represented by the same or similar reference numerals to avoid confusion and repetitive description. 
     First Preferred Embodiment 
       FIG. 1  is a perspective view showing a serially connected axial flow fan unit  1  in accordance with a first preferred embodiment of the present invention. Preferably, the axial flow fan unit  1  includes a first axial flow fan  2 , a second axial flow fan  3  and a plurality of, e.g., two, fixing members  4 . As will be described below, the first and second axial flow fans  2  and  3  are respectively provided with a first housing  23  and a second housing  33 . The first and second housings  23  and  33  are preferably fixed to each other through the fixing members  4  to define a hollow frame. In the axial flow fan unit  1 , an air is admitted into the frame through the first axial flow fan  2  and then discharged to the outside. 
       FIG. 2  is a vertical section view of the axial flow fan unit  1  taken along a plane containing an axis J 1  and extending parallel or substantially parallel to the upper end sides of the first axial flow fan  2 . The axial flow fan unit  1  is preferably a so-called double contra-rotating axial flow fan unit. The rotating direction of a first impeller  21  is opposite to the rotating direction of a second impeller  31 . 
     Preferably, the first axial flow fan  2  includes a first impeller  21 , a first motor unit  22 , a first housing  23  and a plurality of first support ribs  24 . The first impeller  21  is arranged to be rotated about the axis J 1  by the first motor unit  22 . The first housing  23  has a wind tunnel portion defined by its inner circumferential surface surrounding the outer circumferences of the first impeller  21  and the first motor unit  22 . The first support ribs  24  are arranged to support the first motor unit  22  and interconnect the first housing  23  and the first motor unit  22 . In this preferred embodiment, the first housing  23  and the first support ribs  24  are preferably defined by a single unitary member that is preferably formed by, e.g., injection-molding a resin. 
     The first impeller  21  is preferably provided with a first cup  212  and a plurality of first blades  211 . The cup  212  has a substantially cylindrical closed-top shape and preferably covers the outer circumference of the first motor unit  22 . The first blades  211  extend radially outwards from the outer surface of the first cup  212  and are arranged at a uniform pitch in the circumferential direction. The first motor unit  22  is preferably provided with a first rotor portion  221  and a first stator portion  222 . 
     The first rotor portion  221  is preferably provided with a first yoke  2211 , a first field magnet  2212  and a first shaft  2213 . The first yoke  2211  is preferably made of metal and has a substantially cylindrical closed-top shape so that it can cover the first cup  212 . The first field magnet  2212  preferably has a substantially cylindrical shape and is fixed to the inner surface of the first yoke  2211 . The first shaft  2213  is fixed to a cover portion of the first yoke  2211 . The first rotor portion  221  is preferably defined by a single unitary member including the first impeller  21 . 
     The first stator portion  222  is preferably provided with a first base portion  2221 , a first bearing holder portion  2222 , a first armature  2223  and a first circuit board  2224 . The first base portion  2221  preferably has a substantially disk shape with an opening. The first bearing holder portion  2222  preferably has a substantially cylindrical shape and is arranged at the center of the first base portion  2221 . The first armature  2223  is attached to the outer circumference of the first bearing holder portion  2222  and is in an opposing relationship with the first field magnet  2212 . The first circuit board  2224  is arranged below the first armature  2223  and is electrically connected to the first armature  2223 . 
     The first base portion  2221  is fixed to the substantially cylindrical inner surface of the first housing  23  through the first support ribs  24  to thereby hold the respective portions of the first stator portion  222  in place. If an electric current flows from an external power source (not shown) to the first armature  2223 , a torque acting about the axis J 1  is generated between the first armature  2223  and the first field magnet  2212 . Ball bearings  2225  and  2226  are preferably arranged inside the first bearing holder portion  2222  at upper and lower positions thereof to rotatably support the first shaft  2213  inserted into the first bearing holder portion  2222 . However, it is noted that any other desirable type of bearings could be used, for example, hydrodynamic bearings could be used. 
     The second axial flow fan  3  preferably has substantially the same structure as that of the first axial flow fan  2  and, preferably, includes a second impeller  31 , a second motor unit  32 , a second housing  33  and a plurality of second support ribs  34 . The second impeller  31  has a plurality of second blades  311  arranged preferably at uniform intervals and in a reverse pitch with respect the first impeller  21 . 
     The second motor unit  32  preferably has substantially the same structure as that of the first motor unit  22  and, preferably, includes a second rotor portion  321  and a second stator portion  322 . The second rotor portion  321  preferably includes a second yoke  3211 , a second field magnet  3212  and a second shaft  3213 . The structures of the second yoke  3211 , the second field magnet  3212  and the second shaft  3213  are substantially the same as those of the first motor unit  22 . 
     Preferably, the second stator portion  322  includes a second base portion  3221 , a second bearing holder portion  3222 , a second armature  3223 , a second circuit board  3224  and ball bearings  3225  and  3226 , the structures of which are substantially the same as those of the first stator portion  222 . Again, it is noted that any other desirable type of bearings could be used, for example, hydrodynamic bearings could be used. 
     The first motor unit  22  rotates the first impeller  21  to generate an air stream flowing along the axis J 1 . The second motor unit  32  rotates the second impeller  31  in a direction opposite to the rotating direction of the first motor unit  22 , thereby generating an air stream flowing in the same direction as the flowing direction of the air stream caused by the first impeller  21 . Accordingly, this makes it possible for the axial flow fan unit  1  to secure a great enough air flow rate. 
       FIGS. 3 and 5  are perspective views showing the first housing  23  and the second housing  33 , respectively.  FIG. 4  is a plan view showing a first protrusion  236  of a corner portion  2351 . In  FIGS. 3 and 5 , the circumferential direction about the axis J 1  is indicated by an arrow  91 . When assembling the axial flow fan unit  1 , a lower end portion  232  of the first housing  23  comes into an opposing relationship with an upper end portion  331  of the second housing  33 . 
     As shown in  FIG. 3 , an upper end portion  231  and the lower end portion  232  preferably have a generally square shape when seen in a plan view. Each of the upper end portion  231  and the lower end portion  232  has a plurality of (e.g., four) flange-shaped corner portions extending outwards generally perpendicularly to the axis J 1 . The contour  233  of the first housing  23 , indicated by double-dotted chain lines, is in the shape of a generally square imaginary column defined by axially joining the upper end portion  231  and the lower end portion  232 . 
     Through-holes  234  are defined in the four corner portions of the upper end portion  231  and in the four corner portions  2351  to  2354  of the lower end portion  232 , respectively. Screws or other fastening elements or material will be preferably be inserted into the through-holes  234 , e.g., when the axial flow fan unit  1  is to be mounted to a specified device. 
     Referring to  FIG. 5 , the second housing  33  preferably includes an upper end portion  331  and a lower end portion  332 . In the second housing  33 , the upper end portion  331  and the lower end portion  332  preferably have a generally square contour shape when seen in a plan view. As in the first housing  23 , each of the upper end portion  331  and the lower end portion  332  of the second housing  33  has four generally flange-shaped corner portions with through-holes  334 . The contour  333  of the second housing  33  is similar to the contour  233  of the first housing  23 , and is also indicated by double-dotted chain lines is in the shape of a generally square imaginary column. 
     In the lower end portion  232 , the corner portions  2351  and  2353  oppose each other with respect to the axis J 1 . Both of the corner portions  2351  and  2353  have a first protrusion  236  protruding radially outwards. Likewise, the corner portions  2352  and  2354  oppose each other with respect to the axis J 1 . Each of the corner portions  2352  and  2354  preferably has a first locking portion  237  cut along the circumferential direction. 
     As shown in  FIGS. 3 and 4 , the first protrusion  236  of the corner portion  2351  is preferably provided with an increased width portion  2362  whose circumferential width gradually increases radially outwards at its tip end  2361 . Similarly, the first protrusion  236  of the corner portion  2353  is provided at its tip end  2361  with an increased width portion  2362 . 
     As shown in  FIG. 3 , the first locking portion  237  is preferably defined by a first side surface  2371  and a second side surface  2372 . The first side surface  2371  is perpendicular or substantially perpendicular to the circumferential direction. The second side surface  2372  extends along the circumferential direction and defines a portion of the substantially cylindrical outer surface of the first housing  23 . The second side surface  2372  preferably has an upper region and a lower region protruding radially outwards relative to the upper region. 
     As can be seen in  FIG. 5 , the corner portions  3351  and  3353  are opposed to each other with respect to the axis J 1 . Each of the corner portions  3351  and  3353  has a second protrusion  336  protruding radially outwards. The second protrusion  336  preferably has the same shape as the first protrusion  236  and makes axial contact with the first protrusion  236  when assembling the axial flow fan unit  1 . The second protrusion  336  is preferably provided with an increased width portion  3362  whose circumferential width gradually increases radially outwards at its tip end  3361 . 
     Referring again to  FIG. 5 , the corner portions  3352  and  3354  oppose each other with respect to the axis J 1 . Each of the corner portions  3352  and  3354  has a second locking portion  337  extending in the axial direction. The second locking portion  337  is preferably defined by a first side surface  3371  and a second side surface  3372 . The first side surface  3371  is perpendicular or generally perpendicular to the circumferential direction. The second side surface  3372  extends from the first side surface  3371  along the circumferential direction. The second side surface  2372  has a lower region and an upper region protruding radially inwards relative to the lower region. 
     If the axial flow fan unit  1  is arranged as shown in  FIG. 1 , the first locking portions  237  and the second locking portions  337  are locked against one another in the corner portions  2352 ,  2354 ,  3352  and  3354 . At this time, the first side surface  2371  and the first side surface  3371  come into contact with each other in the circumferential direction, whereas the lower region of the second side surface  2372  and the upper region of the second side surface  3372  engage with each other in the axial direction. 
     Thus, the lower end portion  232  of the first housing  23  and the upper end portion  331  of the second housing  33  are prevented from relative movement in the axial direction and in the direction indicated by the arrow  92 . 
       FIG. 6  is a perspective view showing a fixing member  4 .  FIG. 7  is another perspective view of the fixing member  4 , in which the fixing member  4  is horizontally inverted from the state shown in  FIG. 6 . The fixing member  4  is preferably made of, e.g., a resin material, and is preferably provided with an upper portion  41 , a lower portion  42  and a plurality of (e.g., two) side portions  43  interconnecting the upper portion  41  and the lower portion  42 . The fixing member  4  has a hole portion  44  surrounded by the upper portion  41 , the lower portion  42  and the side portions  43 . A generally semi-cylindrical groove portion  45  is defined on the inner surface of each of the upper portion  41  and the lower portion  42 . 
     As shown in  FIG. 1 , the first protrusion  236  and the second protrusion  336  are inserted into the hole portion  44  such that they are overlapping with each other, thereby allowing the fixing member  4  to engage with the first protrusion  236  and the second protrusion  336 . More specifically, once inserted into the hole portion  44 , the overlapping first and second protrusions  236  and  336  are circumferentially interposed between the side portions  43  and axially interposed between the upper portion  41  and the lower portion  42 . 
     Consequently, the lower end portion  232  is locked against the upper end portion  331 , which prevents the lower end portion  232  and the upper end portion  331  relative movement in the axial direction and in the directions indicated by the arrows  91  and  93 . Thus the first housing  23  is prevented from moving relative to the second housing  33  in the axial direction and in the direction perpendicular or substantially perpendicular to the axis J 1 . 
     Referring again to  FIG. 1 , the fixing member  4  defines a portion of the flange portion  5  in cooperation with the corner portions  2351 ,  2353 ,  3351  and  3353  and is attached to each of the corner portions in such a way as to not protrude outwards beyond either contour  233  or  333 . 
     As a result, even when the fixing member  4  is attached to each of the corner portions, the flange portion  5  is kept inside a substantially cylindrical columnar imaginary contour defined by the contours  233  and  333 . This makes it possible to reduce the size of the axial flow fan unit  1 . 
       FIG. 8  is a plan view showing the flange portion  5  that corresponds to the corner portions  2351  and  3351  shown in  FIG. 1 . At the inner side of the flange portion  5 , a through-hole  51  is preferably defined by the groove portion  45 , the through-hole  234  and the through-hole  334 . 
     In the through-hole  51 , the radius of a semi-cylindrical surface of the groove portion  45  is greater than the radius of inner surfaces of the through-holes  234  and  334 . Therefore, the screw or other fastening element preferably used in fixing the axial flow fan unit  1  to a specified device can be reliably inserted through the through-hole  51  with little likelihood of the through-holes  234  and  334  being blocked by the groove portion  45 . 
     The through-hole  51  may be, e.g., a through-hole into which a screw is removably inserted to fix the first housing  23  to the second housing  33 . However, the usage of the through-hole  51  is not particularly limited. The flange portion  5  corresponding to the corner portions  2353  and  3353  preferably has the same structure as set forth above. 
     Next, a description will be provided of an example of the fabrication process of the axial flow fan unit  1 .  FIGS. 9 and 10  are perspective views showing the corner portions  2352  and  3352 . In order to assemble the axial flow fan unit  1 , the first housing  23  and the second housing  33  are first brought into an axially opposing relationship so that the lower end portion  232  and the upper end portion  331  can oppose each other. At this time, each of the corner portions  2351  to  2354  are out of alignment with the corresponding corner portions  3351  to  3354  in the direction opposite to the direction indicated by the arrow  91 . 
     Thereafter, the first housing  23  is axially moved toward the second housing  33  to make contact with the same. As can be seen in  FIG. 9 , the first locking portion  237  is positioned opposite to the second locking portion  337  with the through-hole  334  positioned therebetween. 
     If the lower end portion  232  is rotated relative to the upper end portion  331  in the direction indicated by the arrow  91 , the first side surface  2371  comes into contact with the first side surface  3371  as shown in  FIG. 10 . In addition, the lower region of the second side surface  2372  comes into contact with the upper region of the second side surface  3372  in the axial direction. 
     As a result, the first locking portion  237  and the second locking portion  337  engage with each other in the axial direction and in the direction indicated by the arrow  91 . This holds true with the corner portions  2354  and  3354  also. 
     Through the process noted above, the first housing  23  is locked against (i.e., tentatively fixed to) the second housing  33  and is prevented from movement relative to the second housing  33  in the direction indicated by the arrow  92  or making rotation relative thereto. This makes it possible to easily align the first housing  23  with the second housing  33  when attaching the fixing member  4  after this tentative fixing operation, thereby reducing the number of steps and the time required in the assembling process. 
     The engagement between the first housing  23  and the second housing  33  is released by rotating the first housing  23  relative to the second housing  33  in the direction opposite to the direction indicated by the arrow  91 . This makes it possible to separate the first housing  23  and the second housing  33  from each other. 
       FIG. 11  is a perspective view of the axial flow fan unit  1 , illustrating the manner in which the fixing members  4  are attached to the first axial flow fan  2  and the second axial flow fan  3 . After the first housing  23  is tentatively fixed to the second housing  33 , the fixing members  4  are slid in the directions indicated by arrows  94  and  95  and are attached to the first and second protrusions  236  and  336 . 
     As can be seen in  FIG. 8 , the minimum circumferential width of the hole portion  44  (i.e., the minimum distance between the two side portions  43 ) is preferably smaller than the maximum width of the increased width portions  2362  and  3362 . Thus, the hole portion  44  is elastically deformed as the first and second protrusions  236  and  336  are inserted into the hole portion  44 . Under the action of the restoring forces imparted by the surrounding portions of the hole portion  44 , each of the fixing members  4  is firmly fixed to the first and second protrusions  236  and  336 . 
     The axial flow fan unit  1  is completely assembled through the operation set forth above. 
     Next, a description will be provided of the operation of separating the first axial flow fan  2  and the second axial flow fan  3  from each other. With the axial flow fan unit  1  kept in the state shown in  FIG. 1 , the fixing members  4  are first removed from the first and second housings  23  and  33 . 
     Then the first housing  23  is rotated relative to the second housing  33  in the direction opposite to the direction indicated by the arrow  91 , thereby releasing the engagement between the first locking portion  237  and the second locking portion  337 . Thereafter, the first housing  23  is axially moved away from the second housing  33  to separate the first and second housings  23  and  33  from each other. 
     This minimizes or prevents damage to the afore-mentioned engagement structures, which would otherwise be caused during the course of separating the first and second housings  23  and  33  from each other. This also makes it possible to easily dismantle the axial flow fan unit  1  assembled before. And as a result, it is possible to reuse the first axial flow fan  2  and the second axial flow fan  3 . 
     In this preferred embodiment, the fixing members  4  are preferably made of resin, for example. This prevents damage of the first and second housings  23  and  33  which may be caused in the process of attaching the fixing members  4 . Alternatively, the fixing members  4  may be made of any other desirable material other than resin. 
     Since the fixing members  4  and the first and second locking portions  237  and  337  are provided in the flange-shaped portions of the first and second housings  23  and  33 , they are prevented from affecting the size of wind tunnel portions of the first and second housings  23  and  33 . This prevents reduction of the static pressure and flow rate characteristics in the axial flow fan unit  1 . 
     Second Preferred Embodiment 
       FIG. 12  is a perspective view showing a serially connected axial flow fan unit  1   a  in accordance with a second preferred embodiment of the present invention.  FIGS. 13 and 14  are perspective views showing a first housing  23   a  and a second housing  33   a .  FIG. 15  is an enlarged plan view showing the corner portions  2352  and  3352  of the axial flow fan unit  1   a  shown in  FIG. 12 , in which view the corner portions  2352  and  3352  are illustrated in an axially overlapping state. 
     As shown in  FIGS. 12 ,  13  and  15 , a first protrusion  236  is defined in each of the corner portions  2351  and  2353 , whereas a first locking portion  237   a  is defined in each of the corner portions  2352  and  2354 . 
     The first locking portion  237   a  preferably has a general U-shape when seen in a plan view and includes a plurality of (e.g., two) step portions  2373  and an inner piercing portion  2374 . The step portions  2373  are indicated by broken lines in  FIG. 15  and are arranged radially outwards of the first locking portion  237   a  in a mutually opposing relationship. The inner piercing portion  2374  is arranged radially inwards of the step portions  2373  and is axially pierced through the corresponding one of the corner portions. 
     As shown in  FIGS. 12 and 14 , a second protrusion  336  is defined in each of the corner portions  3351  and  3353 , whereas a second locking portion  337   a  is defined in each of the corner portions  3352  and  3354 . 
     The second locking portion  337   a  has a tip end  3373  whose upper region protrudes outwards in the circumferential direction more than the lower region thereof. The tip end  3373  is preferably shaped such that the circumferential width thereof gradually increases as it moves downwards. 
     As can be seen in  FIG. 15 , the radial width D 1  of the second locking portion  337   a  preferably is generally equal to the radial width D 2  of the step portions  2373  and is preferably equal to or smaller than the radial width D 3  of the inner piercing portion  2374 . 
     The circumferential width W 1  of the tip end  3373  is greater than the circumferential width W 2  between the lower regions of the step portions  2373  and is equal to or smaller than the circumferential width between the upper regions of the step portions  2373  and the circumferential width of the inner piercing portion  2374 . 
     When the first housing  23   a  and the second housing  33   a  are combined together as shown in  FIG. 12 , the second locking portion  337   a  is fit into the step portions  2373  in the corner portions  2352 ,  2354 ,  3352  and  3354  so that the first locking portion  237   a  and the second locking portion  337   a  can engage with each other. 
     This prevents the lower end portion  232  from movement relative to the upper end portion  331  in the axial direction and in the direction indicated by the arrow  92 . As shown in  FIG. 15 , a through-hole is defined by the radially inwardly facing surface of the second locking portion  337   a  and the inner surface of the inner piercing portion  2374 . 
     Referring again to  FIG. 12 , the fixing members  4  are attached to the first and second protrusions  236  and  336  in the corner portions  2351 ,  2353 ,  3351  and  3353 . As in the first preferred embodiment, the lower end portion  232  is axially fixed relative to the upper end portion  331  and is prevented from relative movement in the axial direction and in the directions indicated by the arrows  91  and  93 . 
     The use of the aforementioned structures in the second preferred embodiment prevents the first housing  23   a  from movement relative to the second housing  33   a  in the axial direction and in the direction perpendicular to the axis J 1 . 
     As in the first preferred embodiment, when coupling the first housing  23   a  and the second housing  33   a  together, the first housing  23   a  is moved toward the second housing  33   a  such that the contour  233  of the first housing  23   a  can be aligned with the contour  333  of the second housing  33   a.    
     In the corner portions  2352 ,  2354 ,  3352  and  3354 , the second locking portion  337   a  is inserted between the step portions  2373  from below while undergoing elastic deformation in the circumferential direction such that it comes into engagement with the step portions  2373  in the axial direction and in the direction indicated by the arrow  93 . 
     Through the process set forth above, the first housing  23   a  is tentatively fixed to the second housing  33   a . This makes it easy to perform the task of aligning the first housing  23   a  with the second housing  33   a  and to attach the fixing members  4  to the first and second housings  23   a  and  33   a.    
     As in the first preferred embodiment, the fixing members  4  are attached to the first and second protrusion portions  236  and  336  after the first and second housings  23   a  and  33   a  have been tentatively fixed to each other. 
     The axial flow fan unit  1   a  is completely assembled through the operation set forth above. 
     When one wishes to bring the first locking portion  237   a  into engagement with the second locking portion  337   a  by sliding the first locking portion  237   a  within a plane perpendicular or substantially perpendicular to the axis J 1 , there would be a need to insert one of the two second locking portion  337   a  into the inner piercing portion  2374  of the corresponding first locking portion  237   a  and to cause the other to radially oppose the corresponding first locking portion  237   a  with increased accuracy. This may possibly make the operation complicated. 
     Next, description will be provided of the operation of separating the first housing  23   a  and the second housing  33   a  from each other. As in the first preferred embodiment, the fixing members  4  are first removed from the axial flow fan unit  1   a . Then the first housing  23   a  is moved relative to the second housing  33   a  in the direction opposite to the direction indicated by the arrow  92 . This releases the engagement between the first locking portion  237   a  and the second locking portion  337   a , thereby making it possible to separate the first and second housings  23   a  and  33   a  from each other. 
     The radial width and circumferential width of the tip end  3373  is smaller than the radial width and circumferential width of the inner piercing portion  2374 . This makes it possible to easily separate the first and second housings  23   a  and  33   a  from each other while minimizing damage thereto. 
     Third Preferred Embodiment 
       FIG. 16  is a perspective view showing a serially connected axial flow fan unit  1   b  in accordance with a third preferred embodiment of the present invention.  FIG. 17  is a perspective view showing a first housing  23   b , in which a view of the lower end portion  232  is fully depicted.  FIG. 18  is a perspective view showing a second housing  33   b , in which view the upper end portion  331  is fully depicted. 
     As shown in  FIGS. 16 and 17 , a first protrusion  236  is defined in each of the corner portions  2351  and  2353 . 
     A third locking portion  2381  having a circumferentially extending protrusion  2381   a  is arranged in the corner portion  2352 . 
     A fourth locking portion  2382  including two raised portions  2382   a  is arranged in the corner portion  2354 . The raised portions  2382   a  are arranged side by side in the circumferential direction and are preferably provided with generally L-shaped lower ends protruding toward each other. 
     As shown in  FIGS. 16 and 18 , a second protrusion  336  is defined in each of the corner portions  3351  and  3353 . 
     A fifth locking portion  3381  having a tip end  3381   a  protruding in the direction opposite to the direction indicated by the arrow  91  is arranged in the corner portion  3352 . 
     A sixth locking portion  3382  including two recessed portions  3382   a  is arranged in the corner portion  3354 . When seen from the radial direction, the recessed portions  3382   a  have a generally L-like shape complementary to the shape of the raised portions  2382   a.    
     Referring to  FIG. 16 , the fixing members  4  are attached to the first and second protrusions  236  and  336  as is the case in the preceding preferred embodiments. 
     In the corner portions  2352  and  3352 , the third locking portion  2381  engages with the fifth locking portion  3381 . More specifically, the protrusion  2381   a  makes contact with the tip end  3381   a  in the vertical direction and the fifth locking portion  3381  makes contact with the circumferential opposite sides of the third locking portion  2381 . In the corner portions  2354  and  3354 , the raised portions  2382   a  make contact with the recessed portions  3382   a  in the axial and circumferential directions. 
     This prevents the first housing  23   b  from making movement relative to the second housing  33   b  in the axial direction and in the directions perpendicular or substantially perpendicular to the axis J 1  including the directions indicated by the arrows  91 ,  92  and  93 . 
     Next, a description will be provided of the task of coupling the first housing  23   b  and the second housing  33   b  together. As in the preceding preferred embodiments, the upper end portion  331  is first arranged in an opposing relationship with the lower end portion  232  but out of alignment with the lower end portion  232  in the direction indicated by an arrow  92 A. Then the lower end portion  232  is moved in the direction indicated by the arrow  92 A to oppose the upper end portion  331 . 
     Subsequently, the third locking portion  2381  and the fifth locking portion  3381  engage with each other in the axial direction and in the direction indicated by the arrow  92 . In the corner portions  2354  and  3354 , the raised portions  2382   a  and the recessed portions  3382   a  engage with each other in the axial direction and in the direction indicated by the arrow  91 . As in the preceding preferred embodiments, the fixing members  4  are then attached to the first and second protrusions  236  and  336  overlapping with each other. 
     With the structures set forth above, the first housing  23   b  is easily and firmly fixed relative to the second housing  33   b  in the axial direction and in the direction perpendicular or substantially perpendicular to the axis J 1 . 
     Erroneous coupling of the first and second housings  23   b  and  33   b  is prevented by the difference in shape between the third locking portion  2381  and the fourth locking portion  2382  and the difference in shape between the fifth locking portion  3381  and the sixth locking portion  3382 . 
     When separating the first housing  23   b  and the second housing  33   b  from each other, the fixing members  4  are first removed as in the preceding preferred embodiments. Then the first housing  23   b  is moved relative to the second housing  33   b  in the direction opposite to the direction indicated by the arrow  92 . Consequently, the respective locking portions are disengaged so that the first and second housings  23   b  and  33   b  can be separated from each other. 
     Fourth Preferred Embodiment 
       FIG. 20  is a perspective view showing a serially connected axial flow fan unit  1   c  in accordance with a fourth preferred embodiment of the present invention. First and second protrusions  236  and  336  are located in the respective corner portions  2351  to  2354  and  3351  to  3354  of the first and second housings  23   c  and  33   c . Fixing members  4  are attached to the first and second protrusions  236  and  336  of the respective corner portions  2351  to  2354  and  3351  to  3354 . 
     Since the fixing members  4  are arranged in the respective corner portions mutually opposed with respect to the axis J 1 , it is possible, as in the preceding preferred embodiments, to prevent the first housing  23   c  from moving relative to the second housing  33   c  in the axial direction and in the direction perpendicular or substantially perpendicular to the axis J 1 . 
     When coupling the first and second housings  23   c  and  33   c  together, the lower end portion  232  and the upper end portion  331  are brought into contact with each other in a state that the contour  233  and the contour  333  are aligned in the axial direction as in the preceding preferred embodiments. Then the fixing members  4  are attached to the respective corner portions, thereby coupling the first and second housings  23   c  and  33   c  together. 
     The first and second housings  23   c  and  33   c  can be separated from each other by removing the fixing members  4  and moving the first housing  23   c  relative to the second housing  33   c  as in the preceding preferred embodiments. 
     Fifth Preferred Embodiment 
       FIG. 21  is a perspective view showing the first and second housings  23   d  and  33   d  of a serially connected axial flow fan unit  1   d  in accordance with a fifth preferred embodiment of the present invention. In  FIG. 21 , the impeller  21  is depicted by double-dotted chain lines.  FIG. 22  is a section view of the first and second housings  23   d  and  33   d  taken along a plane containing the axis J 1  and extending from the front side to the rear side in  FIG. 21 . 
     The lower end portion of the first housing  23   d  makes contact with the upper end portion of the second housing  33   d . A plurality of (e.g., two) fixing members  4  is attached to the first and second housings  23   d  and  33   d  in an opposing relationship with respect to the axis J 1 . 
     Preferably, the first and second housings  23   d  and  33   d  include cylinder portions  61  and  71 , upper increased diameter portions  621  and  721 , lower increased diameter portions  622  and  722 , and thin wall portions  63  and  73 . 
     The upper increased diameter portions  621  and  721  extend upwards from the upper ends of the cylinder portions  61  and  71  in an inclined relationship with the axis J 1 . Similarly, the lower increased diameter portions  622  and  722  extend downwards from the lower ends of the cylinder portions  61  and  71  in an inclined relationship with the axis J 1 . 
     The thin wall portions  63  have a generally rectilinear shape when seen in a plan view and interconnect the upper increased diameter portions  621  and  621  while interconnecting the lower increased diameter portions  622  and  622 . This holds true in case of the thin wall portions  73 . The thin wall portions  73  interconnect the upper increased diameter portions  721  and  721  while interconnecting the lower increased diameter portions  722  and  722 . 
     As shown in  FIGS. 21 and 22 , end portions  6211 ,  6221 ,  7211  and  7221  extending in the direction generally perpendicular to the axis J 1  are provided in the upper increased diameter portions  621  and  721  and the lower increased diameter portions  622  and  722 . The end portions  6211 ,  6221 ,  7211  and  7221  include a generally arc-like region and a generally rectilinear region when seen in a plan view and have a plurality of through-holes  64  and  74 . The through-holes  64  are aligned in position with the through-holes  74  when seen in a plan view. Screws or other fastening elements may be inserted through the through-holes  64  and  74  when fixing the axial flow fan unit  1   d  to a specified device. 
       FIG. 23  is a perspective view showing a fixing member  4   a . Preferably, the fixing member  4   a  includes side portions  43   a , an upper portion  41   a , a lower portion  42   a , a groove portion  45   a  and raised portions  411  and  421 . The upper portion  41   a  is joined to the lower portion  42   a  by way of the side portions  43   a . The groove portion  45   a  preferably has a semi-cylindrical shape that is preferably formed by axially cutting out the upper portion  41   a  and the lower portion  42   a . The raised portions  411  and  421  are respectively located at the circumferential opposite sides of tip ends of the upper portion  41   a  and the lower portion  42   a.    
       FIG. 24  is an enlarged view showing the end portions  6221  and  7211 . 
     An axially extending cutout  623  is formed in each of the end portions  6221  mutually opposing with respect to the axis J 1 . In the lower region of the cutout  623 , there is formed a protrusion  6231  that protrudes radially outwards. The protrusion  6231  has an axially extending through-hole  6231   a  and a recessed portion  6231   b  formed on the upper surface thereof. 
     As shown in  FIG. 24 , a cutout  723  overlapping with the cutout  623  is formed in each of the end portions  7211  mutually opposing with respect to the axis J 1 . In the upper region of the cutout  723 , there is formed a protrusion  7231  that protrudes radially outwards. At the center of the protrusion  7231 , there is formed a through-hole overlapping with the through-hole  6231   a . Recessed portions are provided at the circumferential opposite sides of the lower surface of the protrusion  7231 . 
       FIG. 25  is a section view of the axial flow fan unit  1   d  taken along a plane containing the axis J 1 . The positions of the raised portions  411  and  421  of the fixing member  4   a  shown in  FIG. 21  are clearly illustrated in  FIG. 25 . 
     The fixing members  4   a  are slid as indicated by arrows  94   a  and  95   a  in  FIG. 21  and then attached to the protrusion  6231  and  7231 , at which time the raised portions  411  and  421  are fitted to the recessed portions  6231   b  and  7231   b , respectively. 
     Thus, the first and second housings  23   d  and  33   d  are coupled together with ease. This prevents the first housing  23   d  from making movement relative to the second housing  33   d  in the axial direction and in the direction perpendicular or substantially perpendicular to the axis J 1 . At the same time, the downwardly extending through-holes  64  and  74  are defined by the groove portion  45   a , the through-hole  6231   a  and the through-hole of the cutout  723 . 
     The axial height of the side portions  43   a  is equal to the sum total of the height of the end portion  6221  and the height of the end portion  7211 . The radial width of the upper portion  41   a  and the lower portion  42   a  is equal to the radial width of the cutouts  623  and  723 . 
     This prevents the fixing member  4  from protruding beyond the end portions  6221  and  7211 . 
     Sixth Preferred Embodiment 
       FIG. 26  is a perspective view showing the frame of an axial flow fan unit  1   e  in accordance with a sixth preferred embodiment of the present invention. Preferably, the axial flow fan unit  1   e  includes an axial flow fan  2   a  with a third housing  25 , a fourth housing  8  and a plurality of (e.g., two) fixing members  4 . 
     The fourth housing  8  has a wind tunnel portion through which the air sent from the axial flow fan  2   a  flows and makes contact with the lower end portion  251  of the third housing  25  in the axial direction. The fixing members  4  are preferably made of resin and are capable of fixing the third housing  25  and the fourth housing  8  together. 
     The axial flow fan  2   a  preferably has substantially the same structure as that of the first axial flow fan  2  described above. A seventh locking portion  252  is formed in each of the corner portions of the lower end portion  251  mutually opposing with respect to the axis J 1 . 
     The fourth housing  8  is preferably provided with an upper end portion  81  and a cylinder portion  82 . The upper end portion  81  has a generally square flange-like shape when seen in a plan view and is joined to the cylinder portion  82 . An eighth locking portion  811  having the same shape as the second locking portion  337  described above is formed in each of the corner portions of the upper end portion  81  mutually opposing with respect to the axis J 1 . 
     Referring again to  FIG. 26 , protrusions having the same shape as the first and second protrusions  236  and  336  described above are formed in the left and right corner portions of the lower end portion  251  and the upper end portion  81 . The fixing members  4  are attached to these protrusions. The inner surface of the cylinder portion  82  has a shape obtainable by extending the inner surface of the lower end portion  251  parallel or substantially parallel to the axis J 1 . 
     As in the preceding preferred embodiments, the axial flow fan unit  1   e  is assembled by moving the third housing  25  and the fourth housing  8  toward each other and causing the seventh locking portion  252  and the eighth locking portion  811  to engage with each other. Then the fixing members  4  are attached to the lower end portion  251  and the upper end portion  81 . 
     This makes it possible to easily and firmly fix the fourth housing  8  to the third housing  25 , which prevents the lower end portion  251  from making movement relative to the upper end portion  81  in the axial direction and in the direction perpendicular or substantially perpendicular to the axis J 1 . 
     The method of fixing the third housing  25  and the fourth housing  8  together is not limited to the one shown in  FIG. 26 . As an alternative example, it may be possible to use the fixing members  4   a  shown in  FIG. 21  or to use the different kinds of fixing methods set forth earlier with respect to the preceding preferred embodiments. 
     Seventh Preferred Embodiment 
       FIG. 27  is a perspective view showing a serially connected axial flow fan unit  1   f  in accordance with a seventh preferred embodiment of the present invention. A first housing  23   e  and a second housing  33   e  are kept in contact with each other and are fixed together by fixing members  4   b.    
       FIG. 28  is a perspective view showing the first housing  23   e . As depicted in  FIGS. 27 and 28 , arc portions  2321  to  2324  are located in the lower end portion  232   b  of the first housing  23   e . The contour of the lower end portion  232   b  preferably has a generally octagonal shape when seen in a plan view and lies inside the contour  233  of the first housing  23   e.    
       FIG. 29  is a plan view showing a third protrusion  236   b  formed in the arc portion  2321 . As shown in  FIGS. 28 and 29 , the arc portions  2321  and  2323  are diametrically opposed to each other with respect to the axis J 1 , each of which has a third protrusion  236   b  protruding radially outwards. The third protrusion  236   b  is provided with a radially outwardly extending tip end  2361   b  that has an increased width portion  2362   b  whose width increases in the circumferential direction. 
     As shown in  FIG. 28 , the arc portions  2322  and  2324  are diametrically opposed to each other with respect to the axis J 1 , each of which has a ninth axially extending locking portion  237   b.    
     The ninth locking portion  237   b  is preferably provided with a flange portion  2371   b , a raised portion  2372   b  and a first pressing portion  2373   b.    
     The flange portion  2371   b  protrudes radially outwards and is positioned axially above the lower end surface of the arc portion  2322  by a distance corresponding to the axial height of the first pressing portion  2373   b . The radially outer surface of the flange portion  2371   b  has a substantially cylindrical shape. The raised portion  2372   b  protrudes axially downwards from the flange portion  2371   b . The first pressing portion  2373   b  is arranged at the lower end of the raised portion  2372   b  to extend in the circumferential direction. 
     At the trailing side of the arc portion  2322  from the raised portion  2372   b  along the direction indicated by an arrow  91  in  FIG. 28 , the flange portion  2371   b  protrudes axially downwards. The outer surface of the flange portion  2371   b  is positioned radially outwards of the corresponding through-hole  234 . The through-hole  234  and the flange portion  2371   b  do not overlap with each other when seen in the axial direction. 
     As shown in  FIG. 28 , the radially outer surfaces of the raised portions  2372   b  and the first pressing portion  2373   b  are configured to define a portion of the cylindrical outer surface of the flange portion  2371   b . In the arc portion  2322 , the lower end surface of the flange portion  2371   b  positioned at the right side of the raised portion  2372   b  is substantially flush with the lower end surface of the arc portion  2322 . 
       FIG. 30  is a perspective view showing the second housing  33   e . The second housing  33   e  includes an upper end portion  331   b  whose plan-view contour has a generally octagonal shape and lies inside the contour  333  of the second housing  33   e.    
     A fourth protrusion  336   b  that makes contact with the third protrusion  236   b  is preferably provided in each of the arc portions  3311  and  3313 . A tenth locking portion  337   b  that engages with the ninth locking portion  237   b  is preferably provided in each of the arc portions  3312  and  3314 . 
     The fourth protrusion  336   b  is generally identical in shape with the third protrusion  236   b . The fourth protrusion  336   b  is provided with a radially outwardly protruding tip end  3361   b  having an increased width portion  3362   b  whose width increases in the circumferential direction. 
     As shown in  FIG. 30 , the tenth locking portion  337   b  preferably includes a flange portion  3371   b , a raised portion  3372   b  and a second pressing portion  3373   b.    
     The flange portion  3371   b  is positioned axially below the upper end surface of the arc portion  3312  by a distance corresponding to the axial height of the second pressing portion  3373   b . The radially outer surface of the flange portion  3371   b  forms a portion of the cylindrical surface of the arc portion  3312  extending about the axis J 1 . 
     The raised portion  3372   b  protrudes axially upwards from the flange portion  3371   b . The second pressing portion  3373   b  is arranged at the upper end of the raised portion  3372   b  to extend in the circumferential direction. 
     As shown in  FIG. 30 , the flange portion  3371   b  protrudes axially upwards at the leading side of the arc portion  3312  from the raised portion  3372   b  along the direction indicated by the arrow  91 . The outer surface of the flange portion  3371  is positioned radially outwards of the corresponding through-hole  334  so as not to overlap with the through-hole  334  when seen in the axial direction. 
     The radially outer surfaces of the raised portion  3372   b  and the second pressing portion  3373   b  are configured to form a portion of the cylindrical outer surface of the flange portion  3371   b . At the leading side of the flange portion  3371   b  from the raised portion  3372   b  along the direction indicated by the arrow  91 , the upper end surface of the flange portion  3371   b  is flush with the upper end surface of the arc portion  3312 . 
     Referring to  FIG. 27 , the first pressing portion  2373   b  is fitted to the gap between the second pressing portion  3373   b  and the flange portion  3371   b  in the arc portions  2322 ,  2324 ,  3312  and  3314 . The surface of the first pressing portion  2373   b  facing toward the first housing  23   e  and the surface of the second pressing portion  3373   b  facing toward the second housing  33   e  are pressed against each other, thereby preventing the first housing  23   e  from moving relative to the second housing  33   e  in the axial direction. 
       FIG. 31  is a perspective view depicting the fixing member  4   b  shown in  FIG. 27 .  FIG. 32  is another perspective view of the fixing member  4   b , in which view the fixing member  4   b  is horizontally inverted from the state depicted in  FIG. 31 . 
     The fixing member  4   b  is preferably made of resin and has a generally arc-like shape when seen in a plan view. The fixing member  4   b  has a contact surface  48  that makes contact with the outer surfaces the arc portions  2321  and  3311  in the radial direction when assembling the axial flow fan unit  1   f.    
     As shown in  FIGS. 27 and 31 , the radius of curvature of the contact surface  48  is substantially the same as that of the outer surfaces of the flange portions  2371   b  and  3371   b . It can be seen in  FIG. 27  that the fixing member  4   b  is positioned radially inwards of the corresponding through-hole  234  so as not to overlap with the through-hole  234  when seen in the axial direction. 
     Referring to  FIGS. 31 and 32 , a hole portion  44   b  extending through the convex surface  49  and the contact surface  48  is formed in the central region of the fixing member  4   b . The hole portion  44   b  has an increased width portion  492  formed at the side of the convex surface  49  and a reduced width portion  491  formed at the side of the contact surface  48 , the reduced width portion  491  having a circumferential width smaller than that of the increased width portion  492 . The circumferential width of the reduced width portion  491  is preferably smaller than the maximum width of the increased width portions  2362   b  and  3362   b  mentioned earlier. 
     Therefore, when attached to the first and second housings  23   e  and  33   e , the fixing member  4   b  is capable of restraining the first housing  23   e  from making movement relative to the second housing  33   e  in the directions indicated by the arrows  91  and  93 . 
     The radius of curvature of the contact surface  48  is preferably smaller than that of the outer surfaces of the arc portions  2321  and  3311 . During the course of attaching the fixing member  4   b  to the first and second housings  23   e  and  33   e , the fixing member  4   b  is deformed is such a direction that the radius of curvature of the contact surface  48  becomes greater. This creates internal stresses in the fixing member  4   b . In other words, when attaching the fixing member  4   b , an elastic force acting radially outwards relative to the third and fourth protrusions  236   b  and  336   b  is generated in the fixing member  4   b . At this time, the longitudinal opposite end portions of the fixing member  4   b  serves as fulcrums. Under the action of this elastic force, the fixing member  4   b  is attached to the third and fourth protrusions  236   b  and  336   b.    
     This makes it possible for the inner circumferential surface of the hole portion  44   b  of the fixing member  4   b  to fix the third and fourth protrusions  236   b  and  336   b  in the radial direction as well as in the axial and circumferential directions. 
     This also prevents the first and second housings  23   e  and  33   e  from making relative movement in the axial direction and in the direction perpendicular to the axis J 1 . 
     As shown in  FIGS. 31 and 32 , the fixing member  4   b  has a symmetrical shape in both the longitudinal direction and the vertical direction. This means that either the upper portion  41   b  or the lower portion  42   b  may be positioned at the side of the first housing  233  when attaching the fixing member  4   b  to the third and fourth protrusions  236   b  and  336   b . Furthermore, it does not matter which one of the longitudinal end portions of the fixing member  4   b  is positioned at the left side or the right side in  FIG. 27 . In other words, no particular restriction is imposed on the direction in which the fixing member  4   b  is attached to the first and second housings  23   e  and  33   e . This makes it possible to reduce the number of steps and the time required in attaching the fixing member  4   b  to the first and second housings  23   e  and  33   e.    
     The third protrusion  236   b  and the ninth locking portion  237   b  are preferably defined by a single unitary member including the first housing  23   e , which is preferably formed, e.g., by resin injection molding. The third protrusion  236   b  and the ninth locking portion  237   b  preferably are located substantially in the same radial position from the axis J 1 . This reduces warpage of the first housing  23   e  which may be generated in the axial upward or downward direction when the first housing  23   e  is molded with resin. This holds true in case of the second housing  33   e.    
     With the seventh preferred embodiment, it is possible to mount the axial flow fan unit  1   f  to a specified device, e.g., by penetrating a single screw through the through-holes  234  and  334 . No portion present between the through-holes  234  and  334  impedes penetration of the screw. This makes it easy to penetrate a screw or other fastening element through the through-holes  234  and  334 . 
     Since the through-holes  234  and  334  are not used in coupling the first and second housings  23   e  and  33   e  together, there is no need to exactly align the positions of the through-holes  234  and  334 . This eliminates the need to design the molds for resin injection molding with high dimensional accuracy, which leads to reduction in the manufacturing cost and the number of fabrication steps of the axial flow fan unit  1   f.    
       FIGS. 33A and 33B  are plan views showing the arc portions  2321  and  3311  to which the fixing member  4   b  is attached. As shown in  FIG. 33A , the increased width portion  2362   b  is locked against the step difference between the increased width portion  492  and the reduced width portion  491 . As shown in  FIG. 33B , the tip end  2361   b  and the convex surface  49  do not overlap with the through-hole  234  in the axial direction. 
     This eliminates the possibility that the fixing member  4   b  and the third protrusion  236   b  may impede penetration of a screw, e.g., when the screw is penetrated through the through-hole  234 . Therefore, it becomes easy to attach the axial flow fan unit  1   f  to an electronic device or the like. 
     Next, description will be provided of an example of the process flow for fabrication of the axial flow fan unit  1   f . First, the first housing  23   e  is brought into an axially opposing relationship with the second housing  33   e . At this time, the arc portions  2321  to  2324  are arranged out of alignment with the corresponding arc portions  3311  to  3314  in the circumferential direction. 
     Thereafter, the first housing  23   e  is moved toward the second housing  33   e  so that the lower end portion  232   b  can axially oppose the upper end portion  331   b . At this time, the first pressing portion  2373   b  is in a circumferentially opposing relationship with the second pressing portion  3373   b  as shown in  FIG. 34 . 
     Then, if the lower end portion  232   b  is rotated relative to the upper end portion  331   b  in the direction indicated by the arrow  91 , the ninth locking portion  237   b  and the tenth locking portion  337   b  come into engagement with each other in the axial direction and in the rotational direction as shown in FIG.  35 . More specifically, in the arc portions  2322  and  3312 , the circumferential tip end of the first pressing portion  2373   b  makes contact with the raised portion  3372   b  while the tip end of the second pressing portion  3373   b  comes into contact with the raised portion  2372   b . The same engagement structure is available in the arc portions  2324  and  3314 . 
     In the manner as set forth above, the lower end portion  232   b  is tentatively fixed relative to the upper end portion  331   b . Thus the lower end portion  232   b  is prevented from moving relative to the upper end portion  331   b  in the axial direction and in the direction indicated by the arrow  91 . This makes it easy to align the first housing  23   e  with the second housing  33   e  when attaching the fixing member  4   b.    
     With the engagement structures mentioned above, the first housing  23   e  and the second housing  33   e  can be separated from each other by rotating the first housing  23   e  relative to the second housing  33   e  in the direction opposite to the direction indicated by the arrow  91 . 
       FIG. 36  is a view showing the manner in which the fixing members  4   b  are attached to the first and second axial flow fans  2  and  3 . In the arc portions  2321 ,  2323 ,  3311  and  3313 , the fixing members  4   b  are slid toward the third and fourth protrusions  236   b  and  336   b  overlapping with each other and are attached thereto while being elastically deformed. 
     More specifically, the increased width portion  492  of the hole portion  44   b  engages with the tip ends  2361   b  and  3361   b , thereby holding the third and fourth protrusions  236   b  and  336   b  at the circumferential opposite sides thereof. In addition, the third and fourth protrusions  236   b  and  336   b  are axially interposed between the upper portion  41   b  and the lower portion  42   b.    
     This prevents the lower end portion  232   b  from moving relative to the upper end portion  331   b  in the axial direction and in the direction indicated by the arrow  93 . The fixing members  4   b  are elastically deformed and attached to the third and fourth protrusions  236   b  and  336   b  under the action of the restoration force thereof. This assures firm attachment of the fixing members  4   b . The convex surface  49  is arranged substantially flush with the tip ends  2361   b  and  3361   b.    
     The axial flow fan unit  1   f  is completely assembled through the operation described above. 
     When dismantling the axial flow fan unit  1   f  shown in  FIG. 27 , the fixing members  4   b  are first detached from the first and second housings  23   e  and  33   e . Then the ninth locking portion  237   b  and the tenth locking portion  337   b  are disengaged from each other by rotating the ninth locking portion  237   b  relative to the tenth locking portion  337   b  in the direction opposite to the engaging direction. 
     This makes it possible to easily separate the first and second housings  23   e  and  33   e  from each other and to reuse the first and second housings  23   e  and  33   e . This also prevents (or restrains) the engagement structures of the first and second housings  23   e  and  33   e  from being damaged in the separating process. 
     In this preferred embodiment, the fixing members  4   b , the third and fourth protrusions  236   b  and  336   b  and the ninth and tenth locking portions  237   b  and  337   b  are positioned radially inwards of the through-holes  234  and  334 . This makes it possible to secure a space radially inwards of the contours  233  and  333 . The space may accommodate, e.g., lead wires or the like. 
     The outer diameter of the respective arc portions of the first and second housings  23   e  and  33   e  is smaller than the outer diameter of the contours  233  and  333 . This makes it easy for the operator to hold the axial flow fan unit  1   f  during the operation of assembling the axial flow fan unit  1   f  and mounting the same to an electronic device or the like, thereby enabling the operator to efficiently perform the operation. 
     Eighth Preferred Embodiment 
       FIG. 37  is a perspective view showing a serially connected axial flow fan unit  1   g  in accordance with an eighth preferred embodiment of the present invention. The first housing  23   f  has third and fourth protrusions  236   b  and  336   b  formed in the respective arc portions  2321  to  2324  and  3311  to  3314 . The third and fourth protrusions  236   b  and  336   b  make contact with each other. A fixing member  4   b  is attached to the third and fourth protrusions  236   b  and  336   b.    
     Therefore, the first housing  23   f  is firmly and stably fixed relative to the second housing  33   f  in the axial direction and in the direction perpendicular to the axis J 1 . 
     Preferred Modified Examples 
       FIG. 38  is a plan view showing one preferred modified example of the axial flow fan unit  1  in accordance with another preferred embodiment of the present invention. A flange portion  5  having a fixing member  4   c  is shown in  FIG. 38 . The fixing member  4   c  and the first housing  23  are depicted in a separated state in  FIG. 38 . The tip end  2361  of the first protrusion  236  is provided with increased width portions  2363  at the lateral opposite ends thereof. The increased width portions  2363  have two surfaces  2363   a  extending in the axial direction and facing radially inwards. Although not shown in  FIG. 38 , the tip end of the second protrusion  336  is provided with increased width portions having the same shape as that of the increased width portions  2363 . The side portions  43   c  of the fixing member  4   c  have surfaces  431  extending in the axial direction and facing radially outwards. 
     When assembling the axial flow fan unit  1 , the fixing member  4   c  is elastically deformed and attached to the first and second protrusions  236  and  336  in the same manner as mentioned above. At this time, the surfaces  431  of the side portions  43   c  make contact with the surfaces  2363   a  of the first protrusion  236  and the like surfaces of the second protrusion  336  in the radial direction. 
     With this structure, the lower end portion  232  is prevented from moving relative to the upper end portion  331  in the axial direction and in the direction perpendicular or substantially perpendicular to the axis J 1 . 
     The first and second housings  23  and  33  can be separated from each other by breaking the fixing member  4   c . Even in that case, the first and second housings  23  and  33  are protected from damage, which makes it possible to reuse the first and second housings  23  and  33  thus separated. 
     The strength with which the fixing member  4   c  is fixed to the first and second housings  23  and  33  is set substantially equal to the breaking strength of the fixing member  4   c . This prevents the fixing member  4   c  from being inadvertently removed from the first and second housings  23  and  33 . 
     In case the first and second housings  23  and  33  are fixed together by the fixing member  4   c , it may be possible to omit the first locking portion  237  of the corner portions  2352  and  2354  and the second locking portion  337  of the corner portions  3352  and  3354 . 
       FIG. 39  is a plan view showing another preferred modified example of the axial flow fan unit  1  in accordance with the preferred embodiment. A flange portion  5  having a fixing member  4   d  including side portions  43   d  is shown in  FIG. 39 . A recessed portion  2364  is formed on each of the upper surface of the tip end  2361  and the lower surface of the tip end  3361 . A raised portion  46  is formed on each of the lower surface of the upper portion  41   d  and the upper surface of the lower portion  42   d.    
       FIG. 40  is a section view of the upper portion  41   d  of the fixing member  4   d  and the first protrusion  236  taken along a plane perpendicular to the circumferential direction. In  FIG. 40 , the fixing member  4   d  is attached to the first and second housings  23  and  33 . 
     When assembling the axial flow fan unit  1 , the raised portion  46  engages with the recessed portion  2364  of each of the first and second protrusions  236  and  336 . 
     This makes it possible to fix the first and second housings  23  and  33  together with ease and to prevent the first housing  23  from making movement relative to the second housing  33  in the direction perpendicular to the axis J 1 . 
       FIG. 41  is a perspective view showing a further preferred modified example of the axial flow fan unit  1 . A flange portion  5  having a fixing member  4   e  is shown in  FIG. 41 . 
     As shown in  FIG. 41 , the outer surface of the flange in the corner portions  2351  and  2353  has a shape conforming to the contour  233 . Similarly, the outer surface of the flange in the corner portions  3351  and  3353  has a shape conforming to the contour  333 . 
     The fixing member  4   e  preferably is a generally rectangular metallic plate curved along the outer surfaces of the corner portions  2351  and  3351 . Similarly, a fixing member  4   e  preferably having the same shape is attached to the corner portions  2353  and  3353 . 
     The fixing member  4   e  has a plurality of (e.g., four) protrusions  47 . The protrusions  47  protrude along the upper surface of the flange of the first housing  23  and the lower surface of the flange of the second housing  33 . Each of the protrusions  47  has a downwardly protruding raised portion  471  or an upwardly protruding raised portion  472 . 
       FIG. 42  is a section view of the protrusion  47  positioned at the left upper side in  FIG. 41  and the first housing  23 , which view is taken along a plane perpendicular to the outer surface of the first housing  23  but parallel to the axis J 1  (see  FIG. 1 ). A downwardly recessed portion  239  and an upwardly recessed portion (not shown) are formed in the first and second housings  23  and  33  in such positions corresponding to the raised portions  471  and  472 . As the fixing member  4   e  is attached to the first and second housings  23  and  33 , the raised portions  471  and  472  are fitted to the respective recessed portions. 
     This makes it possible to easily and firmly fix the first housing  23  relative to the second housing  33  against movement in the axial direction and in the direction perpendicular or substantially perpendicular to the axis J 1 . 
     The present invention is not limited to the preferred embodiments and the preferred modified examples described above but may be changed in many different forms. For example, as shown in  FIG. 43 , the axial height of the fixing member  4  may be set equal to the axial height of the corner portions of the first and second housings  23  and  33 . The axial height of the fixing member is not particularly limited but may be arbitrarily changed in the preferred embodiments and the preferred modified examples. 
     The number of the fixing members used herein is not particularly limited and one or more fixing members may be used depending on the circumstances. For example, in case of the first preferred embodiment, the first and second housings  23  and  33  may be fixed at one side by the engagement structure of the first and second locking portions  237  and  337  instead of the fixing member  4 . 
     In other words, the corner portions  3351  and  3353  may be axially fixed relative to the corner portions  2351  and  2353  through the use of a single fixing member  4  in combination with the first and second locking portions  237  and  337 . 
     This holds true in case of the other preferred embodiments and the preferred modified examples. Provision of at least one fixing member realizes easy and secure fixing of the first and second housings. 
     The shape and size of the fixing member is not particularly limited to the ones described above. For example, the first and second housings  23  and  33  may be fixed together in the corner portions  2351 ,  2353 ,  3351  and  3353  using other clip-like axial fixing members instead of the fixing member  4 . 
     In this case, axially extending protrusions and substantially cylindrical hole portions for engagement with the protrusions are preferably formed in two or more corner portions of the first housing  23  and in the corresponding corner portions of the second housing  33 . Through the engagement of the protrusions and the hole portions, the first housing  23  is fixed relative to the second housing  33  in the direction perpendicular or substantially perpendicular to the axis J 1 . These alternative structures may be suitably used in the other preferred embodiments and the preferred modified examples. 
     As another alternative example of the fixing member, each of the first and second protrusions  236  and  336  may be provided with a radially extending slit in case of the first preferred embodiment. In this case, the increased width portions  2362  and  3362  are elastically deformed toward each other in the circumferential direction when the fixing member  4  is fitted to the first and second protrusions  236  and  336 . As a consequence, the fixing member  4  is fixed to the first and second housings  23  and  33 . These alternative structures may be suitably used in the other preferred embodiments and the preferred modified examples. 
     The shape and size of the respective locking portions may be arbitrarily changed. For example, in case of the second preferred embodiment, the first and second locking portions  237   a  and  337   a  may have other shapes insofar as at least one of the first and second locking portions  237   a  and  337   a  is elastically deformable during the engagement process thereof. 
     In addition, an axially extending slit may be formed in the tip end  3373  of the second locking portion  337   a . The first and second locking portions  237   a  and  337   a  may engage with each other through the elastic deformation of the second locking portion  337   a.    
     In case of the fifth preferred embodiment, no particular restriction is imposed on the position where the first and second housings  23   d  and  33   d  are fixed together by the fixing member  4   d . For example, the attachment position of the fixing member  4   d  may be far away from the through-holes  64  and  74 . The arrangement of the locking portions and the protrusions and the position of the fixing member may be suitably changed in the other preferred embodiments and the preferred modified examples. 
     The number of the fixing members is not particularly limited and may be, e.g., three or more in case of the fifth preferred embodiment. 
     When fixing the first and second housings together, the fixing member may be used in combination with the engagement structures included in the other preferred embodiments and the preferred modified examples. Even in that case, an error-proofing mechanism is provided so as to ensure correction positioning of the first housing with respect to the second housing. 
     The ribs  24  and  34  for supporting the motor units  22  and  32  of the first and second axial flow fans  2  and  3  may be provided at the intake side, too. In the preferred embodiments described above, the first axial flow fan  2  may be arranged at the exhaust side of the axial flow fan unit, with the second axial flow fan  3  arranged at the intake side thereof. The number of the axial flow fans constituting the axial flow fan unit may be three or more. Likewise, the fan frame may be constructed by combining three or more housings. 
     The shape and size of the respective protrusions to which the fixing member is attached may be suitably changed. For example, a plurality of protrusions formed in a single housing may differ in shape and size from one another. Even in that case, it is possible to fix the housings together and to realize an error-proofing mechanism. 
     In the preferred embodiments and the preferred modified examples described above, a duct rather than the second axial flow fan may be fixed to the first axial flow fan using the afore-mentioned engagement structures. 
     The number of the corner portions and the arc portions provided in the housings is not particularly limited. The number of the corner portions and the arc portions of the first housing may be different from that of the second housing. The contour of the housings and the shape and contour of the upper and lower end portions are not particular limited. 
     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 the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.