Patent Publication Number: US-9890798-B2

Title: Axial fan

Description:
TECHNICAL FIELD 
     The present invention relates to a axial fan. More specifically, the present invention relates to a axial fan used for cooling inside of a device such as an electronic device, and the like. 
     BACKGROUND ART 
     In general, a axial fan comprises a casing having a cavity portion in the center, an impeller having a plurality of vanes which rotates together with a rotation axis, a motor for rotating the rotation axis, a motor base for holding the motor. The impeller, the motor and the motor base are housed in the cavity portion of the casing. 
     A boss is formed integrally with the motor base. A bearing housing of a hollow cylindrical shape provided with a bearing for supporting the rotation axis in the center is fitted and mounted to the boss. A stator of the motor is mounted on the outside of the bearing part, and the rotation axis is mounted inside thereof, rotatably through a bearing. In addition, the motor base is connected to the casing by a plurality of spokes. 
     Then, when the motor is driven, the rotation axis rotates together with the impeller. By the rotation of the impeller, fluid (air) is sucked into the impeller through one end side of the cavity portion of the casing, or a suction port, and passes through the inside of the casing, and is blown out to the outside of the casing through the other end side of the cavity portion of the casing, or a discharge port. At this time, when it is required to increase the pressure of the fluid blown out to the outside of the casing, a fixed vane is provided in the vicinity of the discharge port of the casing in some case, and the spoke serves as the fixed vane in another case. In addition, the casing, the motor base, and the spoke are formed by molding integrally using a resin or a metal, and the like. 
     Meanwhile, the axial fan as described above is used to cool an electronic device, by mounting the axial fan to the electronic device and efficiently discharging the heat generated from an electronic component inside the electronic device to the outside of the electronic device. Therefore, high air volume of the axial fan (high blowing out air volume per unit time of the axial fan) is required. In addition, with regard to electronic devices such as a server and the like, the interior space in which the air flows grows smaller and smaller, due to the high density packaging inside the housing. Incidentally, high static pressure (high power of the axial fan for blowing out the air) is required in addition to the above mentioned high air volume to the axial fan for cooling the inside of the housing. 
     In order to obtain the high air volume and the high static pressure of the axial fan, it is necessary to rotate the motor for rotating the impeller at a high speed. However, when the motor is rotated at a high speed, vibration caused by the high speed rotation of the motor is transmitted to the casing through a bearing, and the vibration is transmitted to the electronic device equipped with the axial fan, there generates a problem that the vibration may occur to the electronic device. In particular, when the vibration caused by the rotation of the motor resonates with the natural frequency of the casing, the vibration grows larger, and, as a result, there may generate a big problem that an abnormal vibration occurs in the electronic device. 
     With regard to such problems, a axial fan so as to suppress the generation of the vibration, by strengthening the structure of the housing through forming a plurality of reinforcement ribs to the motor base is proposed, as described in Japanese Laid-Open Patent Application Publication No. 2006-57631, for example. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Laid-Open Patent Application Publication No. 2006-57631 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, although the technology described in Japanese Laid-Open Patent Application Publication No. 2006-57631 proposes a axial fan having a housing structure strengthened by forming a plurality of reinforcement ribs to the motor base, the shape and the structure of the plurality of reinforcement ribs formed on the motor base are not fully disclosed. In addition, in an electronic device such as a server and the like, the axial fan is rotated at 20000 rpm at the maximum rotation speed, by performing a high density packaging inside the housing. If the axial fan is rotated at such a high speed, there generates a problem that the vibration transmitted to the casing through the motor base cannot be necessarily reduced sufficiently. 
     Accordingly, the present invention has been made in view of the above described problems. It is an object of the present invention to provide a axial fan which enables to reduce the effect of the vibration transmitted to the casing through the motor base, by optimizing the shape and the arrangement of the reinforcement ribs formed on the motor base, even when the maximum rotation speed of the axial fan is increased to the vicinity of 20000 rpm. 
     Solution to Problem 
     The present invention has been proposed to achieve the above described object. In accordance with an aspect of the present invention, a axial fan of the present invention comprises: an impeller having a plurality of vanes; a motor rotating the impeller; a casing housing the impeller and the motor; a motor base mounting the motor; a plurality of spokes connecting the motor base and the casing, wherein the motor base has a plurality of reinforcement ribs thereon, the plurality of reinforcement ribs being equal to or more than the plurality of spokes in number, and wherein the casing has a natural frequency equal to or higher than a frequency being transmitted to the casing from the motor when the motor is rotated at a rotation speed of 20000 rpm or more. 
     Preferably, the plurality of spokes comprises at least seven spokes. 
     Preferably, each of the plurality of spokes is inclined at a predetermined angle with regard to a plane perpendicular to a rotation axis rotating integrally with the impeller. 
     Preferably, the axial fan further comprises a hollow boss protrusively disposed around a center of the motor base, wherein each of the plurality of reinforcement ribs is formed in an equal width with extending distance from an outer periphery side of the boss to an outer periphery side of the motor base. 
     Preferably, the axial fan further comprises a hollow boss protrusively disposed around a center of the motor base, wherein each of the plurality of reinforcement ribs is formed in a gradually decreasing width with extending distance from an outer periphery side of the boss to an outer periphery side of the motor base. 
     Preferably, each of the plurality of reinforcement ribs is formed extending toward a connecting portion of each spoke and the motor base when the reinforcement ribs are equal to the spokes in number. 
     Advantageous Effects of Invention 
     According to the present invention, a axial fan being free from abnormal vibration can be provided, even in a case the axial fan rotates at a maximum rotation speed up to 20000 rpm. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a axial fan shown as one embodiment of the present invention. 
         FIG. 2  is a perspective view showing a casing of the axial fan shown in  FIG. 1 . 
         FIG. 3A  is a plan view showing the casting of  FIG. 2 . 
         FIG. 3B  is an enlarged cross-sectional view taken along line A-A of  FIG. 3A . 
         FIG. 4  is a graph showing a relationship between a number of the reinforcement ribs formed on the motor base and natural frequency of the housing. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a preferred embodiment for carrying out the present invention (hereinafter referred to as “embodiment”) will be described in detail, with reference to the accompanying drawings. Hereinafter, in the following description, an expression indicating a vertical direction is no absolute. It is appropriate if the expression represents a posture in which respective part of the axial fan of the present invention is depicted, but the posture is changed, the expression should be construed in response to the changes of the posture. 
     As shown in  FIG. 1 , a axial fan  10  according to one example of the embodiment of the present invention is an axial flow fan and comprises, a casing  11 , a rotation axis  12 , an impeller  14  having a plurality of vanes  13  and rotating integrally with the rotation axis  12 , a motor  15  for rotating the rotation axis  12 , a hollow cylindrical shape bearing housing  17  being mounted with a vertical pair of bearings  16   a ,  16   b  for supporting the rotation axis  12 , a motor base  18  having a boss  18   a  for supporting the bearing housing  17 , and the like. The rotation axis  12 , the impeller  14 , the motor  15 , the bearing housing  17  and the motor base  18  are housed in the casing  11 . 
     In more detail, as shown in  FIG. 1  to  FIG. 3 , the casing  11  is formed in a rectangular shaped frame in a plan view in which a circular shaped cavity portion  19  for ventilation penetrating back and forth is provided in the center portion. In addition, a motor base  18  is disposed at the center of the cavity portion  19 , and the inner periphery of the cavity portion  19  and the outer periphery of the motor base  18  are connected by a plurality of spokes  20  (seven in Example 1), and the motor base  18  and the plurality of spokes  20  are integrally molded using a resin. Further, as shown in  FIG. 1 , at the upper and the lower ends of the cavity portion  19 , a suction port  19   a  and a discharge port  19   b  are formed in turn. 
     As shown in  FIG. 1 , the plurality of spokes  20  are disposed on the side of the discharge port  19   b  of the casing  11 . Further, as shown in  FIG. 3A , each of the plurality of spokes  20  is formed radially toward the inner periphery surface of the cavity portion  19  from the outer periphery surface of the motor part base  18 , at an equal angle in the circumferential direction. Each of the plurality of spokes  20  increases the pressure of the air blown to the outside of the casing  11 , and also acts as a fixed vane because each of the plurality of spokes  20  is inclined at a predetermined angle with regard to a plane perpendicular to the rotation axis  12  so as to rectify the exhaled air. 
     The motor base  18 , which is supported to the casing  11  by the plurality of spokes  20 , is provided on the side of the discharge port  19   b  of the casing  11  together with the plurality of spokes  20 . In addition, as shown in  FIG. 2  and  FIG. 3 , the motor base  18  is provided integrally with a hollow cylindrical shape boss  18   a  which protrudes toward the side of the suction port  19   a  at the center, and is formed in a disk shape in a plan view. One end of the bearing housing  17  is inserted into the boss  18   a  and is fixedly attached concentrically. An adhesive is used to secure the bearing housing  17 . 
     Further, on the surface of the motor base  18  where the boss  18   a  protrudes, that is, on the surface facing the suction port  19   a , corresponding to each of the plurality of spokes  20 , a plurality of reinforcement ribs  21  (seven in Example 1) extending radially in the outer circumferential direction of the motor base  18 , and toward a connecting portion C of the motor base  18  and the plurality of spokes  20 , from the outer periphery surface of the boss  18   a . As shown in  FIG. 3B  as a cross-section along line A-A of  FIG. 3A , each of the plurality of reinforcement ribs  21  protrudes in a mountain like cross-sectional shape, outward from the rear surface of the motor base  18 , and is formed integrally with the motor base  18 . In addition, without limited to the shape shown in  FIG. 3B , the cross-sectional shape of each of the plurality of reinforcement ribs  21  may be formed triangle, square, trapezoidal and the like, otherwise. 
     As shown in  FIG. 3A  and  FIG. 3B , each of the plurality of reinforcement ribs  21  is formed at an angle equal in the circumferential direction, and also in a same shape, and in a same width (in the circumferential direction) W and a same height (in the axial direction) H over the portion between the outer periphery surface of the boss  18   a  and the connecting portion C. Then, by providing the plurality of reinforcement rib  21  in this way, the strength of the connecting portion C is increased, and at the same time, the overall casing  11  can be strengthened. As described later in detail, when the motor  15  is rotated at a rotation speed of 20000 rpm, the natural frequency of the casing  11  is set so as to be equal to or higher than a frequency being transmitted to the casing from the rotation of the motor. 
     In addition, the casing  11 , the motor base  18 , the plurality of spokes  20  and the plurality of reinforcement ribs  21  are integrally molded using a resin, but they can be integrally molded using a metal and the like. Further, the bearing housing  17  can be resin molded integrally with the boss  18   a  of the motor base  18   
     As shown in  FIG. 1 , the rotation axis  12  penetrates vertically a pair of bearings  16   a ,  16   b  mounted to the inner periphery of the bearing housing  17 , and is secured rotatably. The impeller  14  is integrally attached to the upper end of the rotation axis  12 . 
     Meanwhile, although the bearing housing  17  is described as a separate part in the present embodiment, it can be integral with the boss  18   a.    
     The impeller  14  has a hub  22  which rotates integrally with the rotation axis  12 , and a plurality of vanes  13  are provided on the outer periphery surface of the hub  22 . 
     The hub  22  is made by injection molding using a typical resin material (a synthetic resin such as PBT, ABS and the like). At the molding, a magnetic yoke having a circular cross-section and being formed in a cup shaped outline which is closed at one end (upper end) side and opened at the other end (lower end) side, and is equipped with a rotor magnet  23  of the motor  15  on the inner periphery surface, and the rotation axis are placed in a molding die (not shown). Then, by injecting the resin material into the molding die, as shown in  FIG. 1 , a cylindrical portion  22   b  which is connected to and supported by the rotation axis  12  and is provided to extend in the axial direction of a disk shape ceiling portion  22   a  is provided, and a molded product of a circular cross-section and of a cup shaped outline which is closed at the upper end side and opened at the lower end side is formed. Further, at the same time, a plurality of vanes  13  are formed integrally on the outer periphery surface of the cylindrical portion  22   b  of the hub  22 . 
     As shown in  FIG. 1 , the motor  15  is composed of a rotor  15   a  comprising a magnetic yoke  24  in the side of the impeller  14  and a rotor magnet  23  being mounted to the inner periphery side of the magnetic yoke  24 , and a stator  15   b  being mounted and fixed to the outer periphery of the bearing housing  17  in the side of the casing  11 . By rotating the rotor  15   a  to the stator  15   b , the impeller  14  and the rotation axis  12  are rotated integrally. 
     As shown in  FIG. 1 , the stator  15   b  is mounted to the outer periphery surface of the bearing housing  17  from the side of the suction port  19   a . The stator  15   b  is provided with an iron core  25  being fitted and mounted to the outer periphery of the bearing housing  17 , and a driving coil  27  being wound to the iron core  25  through an insulator  26 . In the lower portion of the stator  15   b , a circuit board  28  for motor drive being packaged with electronic components for controlling a driving current supplied to the coil  27  is fixed and mounted to the insulator  26 . The circuit board  28  is electrically connected to an external power source (not shown) through a lead wire (not shown too). 
     In the axial fan  10  having such a configuration, when a drive current is supplied to the coil  27  of the motor  15  from the circuit board  28 , the rotor  15   a  operates and the rotation axis  12  and the impeller  14  rotates integrally. In addition, when the impeller  14  rotates, air or fluid is sucked into the inside of the plurality of vanes  13  of the impeller  14  from the suction port  19   a  of the casing  11 , passes through the inside of casing  11 , and is blown out to the outside of the casing  11  from the discharge port  19   b  of the casing  11 . At this time, the plurality of spokes  20  being provided on the side of the discharge port  19   b  of the casing  11  rectify the air being blown out to the outside of the casing  11 , and also increases the pressure of the air. As a result, in an electronic device such as a server and the like to which the axial fan is mounted, ventilation (or air circulation) is performed and internal cooling is performed. 
     Meanwhile, in an electronic device such as a server and the like, the inner space in which air flows is getting smaller, due to the high density packaging of the inside of the housing. Therefore, with regard to the axial fan  10  for cooling the inside of the above mentioned housing, the axial fan  10  being provided with high air volume and high static pressure is required. For this reason, in some cases, the motor  15  which rotates the impeller  14  is rotated at a maximum rotation speed up to 20000 rpm, in order to the axial fan  10  being provided with high air volume and high static pressure. When the motor  15  is rotated at a high speed, the accompanying vibration is transmitted to the casing  11  through the bearings  16   a ,  16   b , and is transmitted to the electronic device being equipped with the axial fan  10 , and as a result, vibration also generates in the electronic device. In this case, when the vibration caused by the rotation of the motor  15  resonates with the natural frequency of the casing  11 , the vibration becomes even larger, as a result, there is a possibility that abnormal vibration occurs in the electronic device. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Natural 
                   
               
               
                   
                 No. of 
                 No. of 
                 frequency of 
                 Resonance rotation 
               
               
                 Example 
                 spokes 
                 ribs 
                 casing, Hz 
                 number of casing, rpm 
               
               
                   
               
             
            
               
                 Comparative 
                 4 
                 4 
                 approx. 252 
                 approx. 15095 
               
               
                 Example 1 
               
               
                 Comparative 
                 6 
                 6 
                 approx. 275 
                 approx. 16508 
               
               
                 Example 2 
               
               
                 Example 1 
                 7 
                 7 
                 approx. 365 
                 approx. 21910 
               
               
                   
               
            
           
         
       
     
     In a case the number of the plurality of reinforcement ribs  21  and the number of the plurality of spokes are taken as a parameter, a value of the natural frequency (Hz) of the casing  11  estimated by an analysis and a resonance rotation speed of the motor  15  based on it are indicated in TAB. 1. Here, an estimated value D of natural frequency on a developed fan may be obtained by the following calculation formulas.
 
 X =( B−A )/ A  
 
 D=C /(1+ X )
 
     wherein A is a measured value of natural frequency on a referential fan, B is a simulated value of natural frequency on the referential fan, X is a parameter as shown in the above, and C is a simulated value of natural frequency on the developed fan. 
     In TAB. 1, the Comparative Example 1 represents the casing  11  where the number of the plurality of spokes  20  is formed to four and the number of the plurality of reinforcement ribs is formed to four too. The Comparative Example 2 represents the casing  11  where the number of the plurality of spokes  20  is formed to six and the number of the plurality of reinforcement ribs is formed to six too. Example 1 represents the casing  11  according to the embodiment of the present invention, as shown in  FIG. 1  to  FIG. 3 , where the number of the plurality of spokes  20  is formed to seven and the number of the plurality of reinforcement ribs is formed to seven too. Meanwhile, the shape of the casing  11  is the same for all the Comparative Example 1, the Comparative Example 2, and the Example 1, each of the plurality of reinforcement ribs  21  is formed so as to mate at the connecting portion C where each of the plurality of spokes  20  is connected to the motor base  18 . 
     As shown in TAB. 1, the natural frequency of the casing estimated by the analysis is approx. 252 Hz in Comparative Example 1, approx. 275 Hz in Comparative Example 2 and approx. 365 Hz in Example 1. Therefore, when the number of the plurality of reinforcement ribs  21  and the number of the plurality of spokes  20  are set to be equal, it can be seen that the natural frequency of the casing  11  increases with increasing number of the plurality of reinforcement ribs  21  and of the plurality of spokes  20 . Here, in Example 1, a value of X (referred to as vibration frequency) in a case the maximum rotation speed of the motor  15  for rotating the impeller  14  of the axial fan  10  is assumed to 20000 rpm is calculated from the rotation speed ratio (365:X=21910:20000) as 334. That is, it can be seen that the vibration frequency for the rotation of the motor  15  is approx. 334 Hz. 
     Therefore, if the natural frequency of the casing  11  estimated by the analysis in advance is lower than the vibration frequency of 334 Hz, there occurs a possibility that the frequency of the casing  11  associated with the rotation of the motor  15  and the natural frequency of the casing  11  resonate. As a result, there is a possibility that abnormal vibration is generated in the electronic device and the like. 
     Therefore, as shown in TAB. 1, with regard to the casings  11  in Comparative Example 1 and Comparative Example 2, the natural frequency of the casing  11  is 252 Hz and 275 Hz in turn, which are both lower than 334 Hz. Therefore, in the case the maximum rotation speed of the motor  15  for rotating the impeller  14  of the axial fan  10  is assumed to 20000 rpm, there is a possibility that the frequency of the casing  11  that occurs with this is to resonate with the natural frequency of the casing  11 . 
     In contrast, the casing  11  of Example 1 shown in  FIG. 1  to  FIG. 3  shows the natural frequency (365 Hz) which is higher than 334 Hz. As a result, even if the maximum rotation speed of the motor  15  for rotating the impeller  14  of the axial fan  10  is assumed to 20000 rpm, it is possible to prevent generation of abnormal vibration in the electronic device and the like equipped with the axial fan  10 , without generation of the resonance. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Natural 
                   
               
               
                   
                 No. of 
                 No. of 
                 frequency of 
                 Resonance rotation 
               
               
                 Casing 
                 spokes 
                 ribs 
                 casing, Hz 
                 number of casing, rpm 
               
               
                   
               
             
            
               
                 A 
                 7 
                 — 
                 approx. 225 
                 approx. 13500 
               
               
                 B 
                 7 
                 4 
                 approx. 308 
                 approx. 18480 
               
               
                 C 
                 7 
                 5 
                 approx. 321 
                 approx. 19260 
               
               
                 D 
                 7 
                 6 
                 approx. 337 
                 approx. 20220 
               
               
                 E 
                 7 
                 7 
                 approx. 365 
                 approx. 21910 
               
               
                 F 
                 7 
                 9 
                 approx. 369 
                 approx. 22140 
               
               
                   
               
            
           
         
       
     
     TAB. 2 shows a natural frequency of the casing  11  estimated by the analysis and a resonance rotation speed of the motor  15  based thereon, when, in the casing  11  of the axial fan  10  (Example 1) of the embodiment shown in  FIG. 1  to  FIG. 3 , the number of the plurality of spokes  20  is set to seven, and the number of the plurality of reinforcement ribs  21  formed on the motor base  18  is taken as a parameter.  FIG. 4  is a graph showing the estimated natural frequency of the casing  11  when the number of the plurality of reinforcement ribs  21  shown in TAB. 2 is changed. 
     In TAB. 2, the number of the spokes  20  is seven and the reinforcement rib  21  is not formed in Casing A. The number of the spokes  20  is seven and the number of the reinforcement ribs  21  is formed to four in Casing B. The number of the spokes  20  is seven and the number of the reinforcement ribs  21  is formed to five in Casing C. The number of the spokes  20  is seven and the number of the reinforcement ribs  21  is formed to six in Casing D. The number of the spokes  20  is seven and the number of the reinforcement ribs  21  is formed to seven in Casing E (Example 1) which is the embodiment of the present invention shown in  FIG. 1  to  FIG. 3 . The number of the spokes  20  is seven and the number of the reinforcement ribs  21  is formed to nine in Casing F. Meanwhile, all the casings have the same shape, and only in Casing E of Example 1 which is the embodiment of the present invention, each of the plurality of reinforcement ribs  21  is formed so as to mate at the connecting portion C where each of the plurality of spokes  20  is connected to the motor base  18 . 
     As shown in TAB. 2 and  FIG. 4 , when the number of the spokes of the casing  11  is formed to seven, by forming the number of the reinforcement ribs  21  formed on the motor base  18  to a number being equal to or more than six, the natural frequency of the casting  11  can be set to a higher value (337 Hz) than the value of 334 Hz which is estimated by the analysis, even if the maximum rotation speed of the motor  15  for rotating the impeller of the axial fan  10  is 20000 rpm. 
     However, when the number of the reinforcement ribs  21  formed on the motor basement part  18  is six, it cannot be said that the margin is enough because the natural frequency of 337 Hz of the casing  11  is slightly higher than the vibration frequency of 334 Hz of the axial fan  10 . For this reason, by forming the number of the reinforcement ribs  21  formed on the motor basement part  18  to a number being equal to or more than seven which is equal to the number of the spokes  20 , the casing  11  having the natural frequency being equal to or higher than 365 Hz can be obtained with higher margin with regard to the vibration frequency of 334 Hz of the axial fan  10 . As a result, when the number of the reinforcement ribs  21  formed on the motor basement part  18  is set to at least six, or more preferably to a number being equal to or more than seven, even if the maximum rotation speed of the motor  15  for rotating the impeller  14  of the axial fan  10  is 20000 rpm, the effect of the vibration transmitted to the casing  11  through the motor base  18  is lessened and so the abnormal vibration generating in the electronic devices equipped with the axial fan  10  can be prevented. 
     In addition, when the number of the reinforcement ribs  21  formed on the motor base  18  is the same with the number of the spokes  20 , it is preferred that the reinforcement ribs  21  are formed so as to mate with the portion where the spokes  20  are connected to the motor base  18 . 
     Meanwhile, although the description is made for a case where the number of the spokes  20  is seven in Example, it is enough that the number of the spokes  20  is equal to or more than seven. If the reinforcement ribs  21  is formed to seven or a number more than seven on the motor base  18  in a case the number of the spokes is formed to at least seven, when the maximum rotation speed of the motor  15  for rotating the impeller  14  is 20000 rpm, the natural frequency of the easing  11  grows higher than the frequency due to the vibration caused by the rotation of the motor  15 , and so the occurrence of the abnormal vibration can be prevented. 
     Further, although the configuration is disclosed that the plurality of the reinforcement ribs  21  extend from the outer periphery side of the boss  18   a  to the outer periphery side of the motor base  18 , in the same width W (in the circumferential direction) and the same height H (in the axial direction) (see  FIGS. 3A and 3B ), a configuration is possible that the plurality of the reinforcement ribs  21  extend such that the width H decreased gradually and the height H decreases gradually with increasing extension distance of the plurality of the reinforcement ribs  21  from the outer periphery side of the boss  18   a  to the outer periphery side of the motor base  18 . In this way, by decreasing gradually the width of the reinforcement ribs  21  which is wide at the boss  18   a , with extension distance toward the outer periphery side of motor base  18 , the boss  18   a  can be strengthened effectively, excess reinforcement can be suppressed, and the weight of the casing  11  can be reduced. 
     In conclusion, according to the above configuration, the plurality of reinforcement ribs are provided radially on the motor base being formed in a disk shape in a plan view, while the plurality of the spokes connecting the motor base and the casing are provided, and as a result, the housing structure is strengthened. Moreover, a number of the plurality of reinforcement ribs is formed to be equal to or more than a number of the plurality of spokes, and, when the motor is rotated at a rotation speed of 20000 rpm or more, a natural frequency of the casing is set so as to be equal to or higher than a frequency being transmitted to the casing from the rotation of the motor. Thus, even if the motor is rotated at a rotation speed of 20000 rpm or more, the motor and the housing does not resonate, and as a result, occurrence of abnormal vibration in the housing can be suppressed. 
     Further, in a case the number of the plurality of the spokes is formed to be at least seven, if the number of the plurality of the reinforcement ribs is formed to be equal to or more than seven, the natural frequency of the casing grows higher than the frequency due to rotation of the motor at the maximum rotation speed of 20000 rpm or more, and so occurrence of abnormal vibration of the casing due to resonance of the motor and the casing can be prevented. 
     Furthermore, each of the plurality of spokes increases the air pressure blown to the outside of the casing, and acts as a fixed vane for rectifying the air exhaled. 
     Furthermore, the motor base is reinforced uniformly, by forming each of the plurality of the reinforcement ribs in an equal width to the outer periphery of the boss to the outer periphery side of the motor base, and occurrence of abnormal vibration can be prevented. 
     Furthermore, by gradually decreasing the width of each of the plurality of reinforcement ribs having a wider width at the boss, with extending distance from the outer periphery side of the boss to the outer periphery side of the motor base, it is possible to reinforce the boss efficiently, to suppress excess reinforcement, and to reduce the weight of the casing. 
     Furthermore, by providing each of the plurality of reinforcement ribs so as to extend toward the connecting portion of the plurality of spokes and the motor base, the strength of the connecting portion can be increased, and further, the overall structure of the casting can be strengthened. 
     It is noted that the present invention is not limited to the above described embodiment. A modification, an improvement and the like within the scope where the object of the present invention can be achieved are included in the present invention. 
     REFERENCE SIGNS LIST 
       10  . . . axial fan,  11  . . . casing,  12  . . . rotation axis,  13  . . . vane,  14  . . . impeller,  15  . . . motor,  15   a  . . . rotor,  15   b  . . . stator,  16   a ,  16   b  . . . bearing,  17  . . . bearing housing,  18  . . . motor base,  18   a  . . . boss,  19  . . . cavity portion,  19   a  . . . suction port,  19   b  . . . discharge port,  20  . . . spoke,  21  . . . reinforcement rib