Patent Publication Number: US-7909568-B2

Title: Counter-rotating axial-flow fan

Description:
This application is a divisional of U.S. Ser. No. 11/531,510, entitled COUNTER-ROTATING AXIAL-FLOW FAN, filed Sep. 13, 2006 now U.S. Pat. No. 7,445,423. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a counter-rotating axial-flow fan used to cool an interior of an electric appliance. 
     As an electric appliance becomes smaller in size, so does a space inside a case of the electric appliance in which air flows. To cool an interior of the small case, a fan with features of a large amount of air and a high static pressure is called for. As a fan with such features, a counter-rotating axial-flow fan has come to be used in recent years. 
     For example, Japanese Patent Publication No. 2004-278370 (US2005/0106026) (FIG. 1) shows a conventional counter-rotating axial-flow fan of this kind. 
     In recent years some applications call for higher performance than that of the existing counter-rotating axial-flow fan. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a counter-rotating axial-flow fan which is capable of producing a larger amount of air and a higher static pressure than conventional fans do. 
     The counter-rotating axial-flow fan or axial-flow fan with double impellers rotating in mutually opposite directions of this invention comprises a housing, a first impeller, a first motor, a second impeller, a second motor, and a plurality of stationary blades. The housing includes an air channel which has a suction opening portion at one side in an axial direction thereof and a discharge opening portion at the other side in the axial direction. The first impeller includes a plurality of front blades that rotate in the suction opening portion. The first motor rotates the first impeller about an axial line of the fan in one of two rotating directions. The second impeller has a plurality of rear blades that rotate in the discharge opening portion. The second motor rotates the second impeller about the axial line in the other rotating direction opposite to the one rotating direction. The stationary blades are arranged stationary in the housing between the first impeller and the second impeller and extend radially. Here, the word “radially” applies to not only a case where the blades extend radially in straight lines but also a case where they extend radially in curved lines. In the counter-rotating axial-flow fan of the present invention, the number of the plurality of front blades is defined to be N, the number of the plurality of stationary blades is defined to be M, and the number of the plurality of rear blades is defined to be P. Each of N, M and P is a positive integer, and their relationship is defined as N&gt;P&gt;M. 
     In the counter-rotating axial-flow fan of this invention, a length L 1  of each of the N front blades, measured in the axial direction is defined to be longer than a length L 2  of each of the P rear blades, measured in the axial direction. A relationship between the length L 1  and the length L 2  has been studied. The finding is that a larger amount of air and a higher static pressure can be generated when the length L 1  is set longer than the length L 2 . In the counter-rotating axial-flow fan of this invention, the air amount and the static pressure can be increased, compared with conventional fans. 
     The first impeller includes an annular member having a peripheral wall on which N blades are mounted and disposed at a predetermined interval in a circumferential direction. End portions of the N blades, located at the other side in the axial direction, extend toward the other side beyond an end portion of the peripheral wall of the annular member, located at the other side in the axial direction. The second impeller includes an annular member having a peripheral wall on which the P blades are mounted and disposed at a predetermined interval in a circumferential direction. End portions of the P blades, located at the one side in the axial direction, do not substantially extend beyond an end portion of the peripheral wall of the annular member located at the one side in the axial direction. End portions of the P blades, located at the other side in the axial direction, do not substantially extend beyond the end portion of the peripheral wall of the annular member located at the other side in the axial direction. 
     The housing may be formed as one integral structure but it may also be formed of two or more constitutional parts. For example, when the counter-rotating axial-flow fan of this invention is made by coupling two axial-flow fan units, the housing is constructed by coupling the cases of the two axial-flow fan units. 
     When a first axial-flow fan unit and a second axial-flow fan unit are coupled together to form the counter-rotating axial-flow fan, the first axial-flow fan unit includes a first case, a first impeller, a first motor and a plurality of webs. The first case includes an air channel having a suction opening portion at one side in an axial direction thereof and a discharge opening portion at the other side in the axial direction. The first impeller includes a plurality of front blades that rotate in the suction opening portion. The first motor rotates the first impeller about the axial line in one of two rotating directions. The plurality of webs are located in the discharge opening portion and disposed at a predetermined interval in a circumferential direction to fix the first motor to the first case. Similarly, the second axial-flow fan unit includes a second case, a second impeller, a second motor and a plurality of webs. The second case includes an air channel having a suction opening portion at one side in an axial direction thereof and a discharge opening portion at the other side in the axial direction. The second impeller includes a plurality of rear blades that rotate in the discharge opening portion. The second motor rotates the second impeller about the axial line in the other rotating direction opposite to the one rotating direction. The plurality of webs are located in the suction opening portion and disposed at a predetermined interval in a circumferential direction to fix the second motor to the second case. The first case of the first axial-flow fan unit and the second case of the second axial-flow fan unit are coupled together to form the housing. In that case, the plurality of webs of the first axial-flow fan unit and the plurality of webs of the second axial-flow fan unit are preferably coupled to form a plurality of radially extending stationary blades arranged stationary in the housing between the first impeller and the second impeller. With this arrangement, there is no need to construct a case having a plurality of stationary blades separately from the axial-flow fan units, reducing the number of parts used in the counter-rotating axial-flow fan. Further, compared with a case where a separate unit having a plurality of stationary blades is used, the counter-rotating axial-flow fan of this invention can be reduced in an axial overall size. 
     Specifically, in the present invention a length L 3  of the first case, measured in the axial direction is defined to be longer than a length  4  of the second case, measured in the axial direction. The lengths L 1  and L 2  are defined so that a ratio of the two lengths L 1 /L 2  is 1.3 to 2.5. The lengths L 3  and L 4  are defined so that a ratio of the two lengths L 3 /L 4  is 1.2 to 1.8. 
     More specifically, the front blades are curved in a transverse cross section of the front blades as taken along a direction parallel to the axial line (or along the axial line) so that their concave portions are open toward the rotating direction of the first impeller, i.e. in the one rotating direction as described above. The rear blades are curved in a transverse cross section of the rear blades as taken along a direction parallel to the axial line so that their concave portions are open toward the rotating direction of the second impeller, i.e. in the other rotating direction as described above. In this construction, the stationary blades are preferably curved in a transverse cross section of the stationary blades as taken along a direction parallel to the axial line so that their concave portions are open toward the other rotating direction (the rotating direction of the second impeller) and toward a direction in which the rear blades are located. With this arrangement, it is possible to increase the maximum amount of air and the maximum static pressure while reducing the suction noise. 
     More specifically, the first impeller may include an annular member having a peripheral wall surrounding the axial line on which base portions of five front blades are integrally mounted. The second impeller may include an annular member having a peripheral wall surrounding the axial line on which base portions of four rear blades are integrally mounted. This arrangement allows the first and second impellers to be formed easily by resin injection molding. 
     The rotating speed of the second impeller is preferably set slower than that of the first impeller for reducing noise. 
     In the counter-rotating axial-flow fan of the present invention, a length L 1  of each of the N front blades, measured in the axial direction is set longer than a length L 2  of each of the P rear blades, measured in the axial direction. Then the air amount and the static pressure can be increased, compared with conventional fans. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A ,  1 B,  1 C,  1 D and  1 E are a perspective view as viewed from a suction opening portion, a perspective view as viewed from a discharge opening portion, a front side elevation view as viewed from the suction opening portion, a rear side elevation view as viewed from the discharge opening portion and the right side elevation view of the front side elevation view respectively of a counter-rotating axial-flow fan of one embodiment of the present invention. 
         FIG. 2  is a vertical cross-sectional view of the counter-rotating axial-flow fan in this embodiment. 
         FIG. 3  is a perspective view showing a first axial-flow fan unit in this embodiment. 
         FIG. 4  is a perspective view showing a second axial-flow fan unit in this embodiment. 
         FIG. 5  is an enlarged vertical cross-sectional view for illustrating a coupling structure of the counter-rotating axial-flow fan in this embodiment. 
         FIG. 6  is a transverse cross-sectional view of a front blade, a rear blade and a stationary blade when the counter-rotating axial-flow fan is cut in a direction parallel to an axial direction in this embodiment. 
         FIG. 7  is a characteristic chart showing the relationship between an amount of air and a static pressure generated by the counter-rotating axial-flow fan having a structure of the present invention, the counter-rotating axial-flow fan of an comparative example and a conventional counter-rotating axial-flow fan. 
         FIG. 8  is a vertical cross-sectional view of the conventional counter-rotating axial-flow fan. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Now, an embodiment of the present invention will be described in detail by referring to  FIGS. 1A to 1E  through  FIG. 6 .  FIGS. 1A ,  1 B,  1 C,  1 D and  1 E are a perspective view as viewed from a suction opening portion, a perspective view as viewed from a discharge opening portion, a front side elevation view as viewed from the suction opening portion, a rear side elevation view as viewed from the discharge opening portion and the right side elevation view of the front side elevation view respectively of a counter-rotating axial-flow fan of one embodiment of the present invention.  FIG. 2  is a vertical cross-sectional view of the counter-rotating axial-flow fan in this embodiment.  FIG. 3  is a perspective view showing a first axial-flow fan unit in this embodiment.  FIG. 4  is a perspective view showing a second axial-flow fan unit in this embodiment.  FIG. 5  is an enlarged vertical cross-sectional view for illustrating a coupling structure of the counter-rotating axial-flow fan in this embodiment.  FIG. 6  is a transverse cross-sectional view of a front blade, a rear blade and a stationary blade when the counter-rotating axial-flow fan is cut in a direction parallel to an axial direction in this embodiment. 
     A counter-rotating axial-flow fan of this embodiment is constructed via a coupling structure of the first axial-flow fan unit  1  and the second axial-flow fan unit  2 . 
     The first axial-flow fan unit  1  has a first case  5 , a first impeller (front impeller)  7 , a first motor  25 , and three webs  19 ,  21 ,  23  spaced apart 120 degrees circumferentially, all of which are arranged in the first case  5 . The first case  5  has an annular suction-side flange  9  at one side in the axial direction in which the axial line A extends and an annular discharge-side flange  11  at the other side. The first case  5  also has a cylindrical portion  13  between the two flanges  9 ,  11 . The flanges  9 ,  11  and an inner space in the cylindrical portion  13  all together form an air channel. 
       FIG. 3  is a perspective view of the first case  5  of the first axial-flow fan unit  1  as seen from the coupled portion between the first case  5  and the second axial-flow fan unit  3  by separating the second axial-flow fan unit  3  from the first axial-flow fan unit  1  of the counter-rotating axial-flow fan of  FIGS. 1A to 1E . The suction flange  9  has an almost rectangular outline, with a circular suction opening portion  15  formed therein. The suction flange  9  has, at its four corner portions, flat faces  9   a  facing toward the cylindrical portion  13  and through-holes  9   b  for mounting screws. 
     The discharge flange  11  also has an almost rectangular outline with a circular discharge opening portion  17  formed therein. In the discharge opening portion  17 , three radially extending webs  19 ,  21 ,  23  are arranged at circumferentially equal intervals. Through the three webs  19 ,  21 ,  23 , a motor case in which a stator of the first motor  25  is fixed is secured to the first case  5 . Of the three webs  19 ,  21 ,  23 , the web  19  has a groove-shaped recessed portion  19   a  opening toward the second axial-flow fan unit  3 . In this recessed portion  19   a  is installed a feeder wire not shown which is connected to an excitation winding of the first motor  25 . The three webs  19 ,  21 ,  23  are respectively combined with three webs  43 ,  45 ,  47 , described later, of the second axial-flow fan unit  3  to form M stationary blades  61 , three in the embodiment, ( FIG. 6 ) described later. 
     The first motor  25  comprises a rotor not shown, to which the first impeller  7  of  FIG. 2  is mounted, and a stator for rotating the rotor. The first motor  25  rotates the first impeller  7  in the suction opening portion  15  of the first case  5  counterclockwise in  FIG. 1  (i.e., in a direction of arrow R 1 , or in one rotating direction). The first motor  25  rotates the first impeller  7  at a speed faster than a second impeller  35  described later. The first impeller  7  has an annular member  27  fitted with a cup-shaped member, not shown, of the rotor which is fixed onto a shaft, not shown, of the first motor  25 , and N front blades  28 , five in the embodiment, integrally provided on an outer peripheral surface of an annular wall  27   a  of the annular member  27 . 
     The discharge-side flange  11  has flat faces  11   a  formed at each of four corner portions  12 A to  12 D facing the cylindrical portion  13 . At the four corner portions  12 A to  12 D are formed four first fitting grooves  29  that constitute engaged portions of a first kind, as shown in  FIG. 3 . These first fitting grooves  29  are formed by through-holes passing through the discharge-side flange  11 . Here a construction of the first fitting groove  29  formed in the corner portion  12 A will be explained. The first fitting groove  29  has a hook passing hole  29   a  and a hook moving hole  29   b  contiguous with the hook passing hole  29   a . The hook passing hole  29   a  has a semi-arc portion  29   al which also serves as a through-hole through which a mounting screw passes. The hook moving hole  29   b  is shaped like an arc. At end portion  29   c  when seen in the rotating direction R 1  of the first impeller  7 , the hook moving hole  29   b , as shown in  FIG. 5 , is formed with a first engaged surface  29   d  and a second engaged surface  29   e  to be engaged by a hook  53  described later.  FIG. 5  is a partial cross-sectional view of the corner portion  12 A as taken along the first fitting groove  29  and a second fitting groove  31  described later. The first engaged surface  29   d  is situated at the corner portion  12 A and is formed by a part of the flat face  11   a  situated close to the end portion  29   c  of the hook moving hole  29   b . The second engaged surface  29   e  is formed of an end face, at the rotating direction side, of the hook moving hole  29   b.    
     Except for the corner portion  12 B adjacent to the web  19  in which a wire not shown is installed, the plurality of corner portions  12 A,  12 C,  12 D are each formed with a second fitting groove  31  that constitutes an engaged portion of a second kind. As shown in  FIG. 5 , the second fitting groove  31  has a protrusion moving groove  31   a  and an engaging groove  31   b  contiguous with the protrusion moving groove  31   a . The protrusion moving groove  31   a  has an opening  31   c  opening toward a side surface of the discharge-side flange  11 . The protrusion moving groove  31   a  has a bottom surface  31   d  which is sloping in such a manner that the bottom surface becomes closer to the second axial-flow fan unit  3  as it extends from the opening  31   c  toward the engaging groove  31   b . As a result, a step is formed between the engaging groove  31   b  and the protrusion moving groove  31   a . An inner surface of the engaging groove  31   b  situated at the protrusion moving groove  31   a  side constitutes a third engaged surface  31   e.    
     The second axial-flow fan unit  3  has a second case  33 , a second impeller (rear impeller)  35  in  FIG. 2 , a second motor  49  in  FIG. 2  and  FIG. 4 , and three webs  43 ,  45 ,  47  in  FIG. 4 , all of which are arranged in the second case  33 . The second case  33 , as shown in  FIG. 1  and  FIG. 4 , has a suction-side flange  37  at one side in the axial direction in which the axial line A extends and a discharge-side flange  39  at the other side. The second case  33  also has a cylindrical portion  41  between the two flanges  37 ,  39 . The flanges  37 ,  39  and an inner space in the cylinder portion  41  all together form an air channel.  FIG. 4  is a perspective view of the second case  33  of the second axial-flow fan unit  3  as seen from the coupled portion between the second case  33  and the first axial-flow fan unit  1 , which is separated from the second axial-flow fan unit  3  of the counter-rotating axial-flow fan in  FIG. 1  and  FIG. 2 . 
     The suction-side flange  37  has an almost rectangular outline, with a circular suction opening portion  42  formed therein. In the suction opening portion  42 , three radially extending webs  43 ,  45 ,  47  are arranged at circumferentially equal intervals. The second motor  49  is secured to the second case  33  through the plurality of webs  43 ,  45 ,  47 . Of the plurality of webs  43 ,  45 ,  47 , the web  43  has a groove-shaped recessed portion  43   a  opening toward the first axial-flow fan unit  1 . In this recessed portion  43   a  is installed a feeder wire not shown which is connected to an excitation winding of the second motor  49 . The three webs  43 ,  45 ,  47  combine respectively with three webs  19 ,  21 ,  23  of the first axial-flow fan unit  1  to form M stationary blades  61  (three in the embodiment) described later. 
     The second motor  49  comprises a rotor not shown to which the second impeller  35  of  FIG. 2  is mounted and a stator that rotates this rotor. The second motor  49  rotates the second impeller  35  in a discharge opening portion  57  clockwise in  FIG. 2  [in the direction of arrow R 2  in the figure, i.e., in a direction opposite to the rotating direction (an arrow R 1 ) of the first impeller  7 . As described above, the second impeller  35  is rotated at a speed slower than that of the first impeller  7 . 
     The second impeller  35  has an annular member  50  fitted with a cup-shaped member, not shown, of the rotor which is secured to a shaft, not shown, of the second motor  49 , and P rear blades  51  (four in the embodiment) integrally provided on an outer peripheral surface of an annular wall  50   a  of the annular member  50 . 
     Four corner portions  36 A to  36 D of the suction-side flange  37  are formed with a through-hole  38  through which a mounting screw passes, as shown in  FIG. 4 . Each of the four corner portions  36 A to  36 D also has a hook  53  formed integrally therewith which constitutes an engaging portion of a first kind. The hooks  53  protrude toward the first case  5 . The construction of the hook  53  at the corner portion  36 A will be explained. The hook  53  has a body portion  53   a  rising along the axial line A from the corner portion and a head portion  53   b  attached at an end of the body portion  53   a . The head portion  53   b  at the end of the body portion  53   a  protrudes outwardly in a radial direction, gradually away from the axial line A, thus forming a step between the head portion  53   b  and the body portion  53   a . A surface of this step forms a first engaging surface  53   d  that engages with the first engaged surface  29   d  shown in  FIG. 5 . Except for the corner portion  36 B adjacent to the web  43 , the plurality of corner portions  36 A,  36 C,  36 D are each formed integrally with a protrusion  55  to constitute an engaging portion of a first kind in such a manner that the through-hole  38  is located between the hook  53  and the protrusion  55 . The protrusion  55  protrudes toward the first case  5  along the axial line A, as with the hooks  53 . The protrusion  55  has an inclined surface  55   a  which inclines in such a manner that the inclined surface becomes closer to the first case  5  as it departs away from the hook  53  situated in the same corner portion. This inclined surface  55   a  slides on a sloped surface forming the bottom surface  31   d  of the protrusion moving groove  31   a  shown in  FIG. 5 . The protrusion  55  has an end face  55   b  extending along the axial line from an end of the inclined surface  55   a  toward the second case  33 . This end face  55   b  forms a third engaging surface that engages with the third engaged surface  31   e  formed in the engaging groove  31   b.    
     As shown in  FIG. 4 , the discharge-side flange  39  has an almost rectangular outline, with a circular discharge opening portion  57  formed therein. The discharge-side flange  39  has flat faces  39   a  formed at each of the four corner portions at the side of the cylinder portion  41 . The four corner portions are each formed with a through-hole  39   b  through which a mounting screw passes. 
       FIG. 6  shows a front blade  28 , a rear blade  51  and a stationary blade  61  in a transverse cross-sectional view as taken along a direction parallel to the axial line, with the first case  5  and the second case  33  coupled together. In the example shown in  FIG. 6 , the stationary blade  61  is formed by coupling the web  23  of the first axial-flow fan unit  1  and the web  47  of the second axial-flow fan unit  3 . As shown in the figure, the front blade  28  is curved in the transverse cross section so that its concave portion opens toward the direction R 1  while the rear blade  51  is curved in the transverse cross section so that its concave portion opens toward the other direction R 2 . The stationary blade  61  is curved in the transverse cross section so that its concave portion opens toward the other direction R 2  and also toward a direction in which the rear blade  51  is located. 
     When the number of the front blades  28  is N, that of the stationary blades  61  is M, and that of the rear blades  51  is P, each of N, M and P is a positive integer, and a relationship among N, M and P is defined as N&gt;P&gt;M in the counter-rotating axial-flow fan of the present invention. Since N=5, P=4 and M=3 in this embodiment, the relationship among N, M and P is 5&gt;4&gt;3. 
     Specifically in the counter-rotating axial-flow fan of the present invention, a length L 1 , of each the N front blades  28  of the first axial-fan unit  1 , measured in an axial direction is set longer than the length L 2 , of each the P rear blades  51  of the second axial-fan unit  3 , measured in the axial direction as shown in  FIG. 2 . 
     More specifically, end portions  28   a  of the N front blades  28  of the first axial-fan unit  1 , located at the other side in the axial direction (at the discharge opening portion  17 ), extend toward a direction of the other side (at the discharge opening portion  17 ) beyond an end portion  27   aa  of the peripheral wall  27   a  of the annular member  27 , located at the other side in the axial direction (at the discharge opening portion  17 ). End portions  51   b  of the rear P blades  51  of the second axial-flow fan unit  3 , located at the one side in the axial direction (at the suction opening portion  42 ), do not substantially extend beyond an end portion  50   ab  of the peripheral wall  50   a  of the annular member  50  located at the one side in the axial direction (at the suction opening portion  42 ). End portions  51   a  of the rear P blades, located at the other side in the axial direction (at the discharge opening portion  57 ), do not substantially extend beyond the end portion  50   aa  of the peripheral wall  50   a  of the annular member  50  located at the other side in the axial direction (at the discharge opening portion  57 ). 
     Each of the end portions  28   a , of the N front blades  28 , located at the other side (at the discharge opening portion  17 ) in the axial direction extends beyond the end portion  27   aa , of the peripheral wall  27   a  of the annular member  27 , located at the other side (at the discharge opening portion  17 ) in the axial direction. A length La of an extended part for each of the end portions  28   a  of the N front blades  28 , which extends toward the other side in the axial direction beyond the end portion  27   aa  of the peripheral wall  27   a  of the annular member  27  is within a range from 10 percent to 15 percent of the length L 1 . 
     A length L 3  of the first case  5  measured in the axial direction A is set longer than a length L 4  of the second case  3  measured in the axial direction. The length L 3  is set longer than the length L 4 . In this embodiment, the length L 3  is set to 30 millimeter and the length L 4  is set to 26 millimeter. Preferably the length L 3  and the length L 4  are determined so that a ratio of the two lengths L 3 /L 4  is a value from 1.2 to 1.8. 
     In this embodiment of the fan, the first case  5  of the first axial-flow fan unit  1  and the second case  33  of the second axial-flow fan unit  3  are coupled as follows. First, the end portion of the first case  5  and the end portion of the second case  33  are brought close together, and the head portions  53   b  of the four hooks  53  of the second case  33  are inserted into the corresponding hook passing holes  29   a  of the four first fitting grooves  29  in the first case  5 . At this time, the plurality of protrusions  55  of the second case  33  fit into the openings  31   c  of the plurality of second fitting grooves  31  in the first case  5 . Next, as shown in  FIG. 3  and  FIG. 4 , these cases  5 ,  33  are rotated clockwise in one rotating direction (indicated by arrow D 1 ) relative to each other. This rotation may be achieved either by rotating both of the cases or only one case relative to the other. This rotation causes the body portions  53   a  of the hooks  53  to move in the hook moving holes  29   b  of the first fitting grooves  29  until the first engaging surfaces  53   d  of the head portions  53   b  of the hooks  53  abut onto the first engaged surfaces  29   d  at the flat faces  11   a  of the discharge-side flange  11  and the second engaging surfaces  53   e  of the body portions  53   a  abut onto the second engaged surfaces  29   e  of the discharge-side flange  11 , thus preventing the hooks  53  from coming off the first fitting grooves  29 . Further, the protrusions  55  move in the protrusion moving grooves  31   a  of the second fitting grooves  31  until they fit into the engaging grooves  31   b . The end faces  55   b  of the protrusions  55  engage with the third engaged surfaces  31   e  formed in the engaging grooves  31   b.    
     In this embodiment, the hooks  53  (engaging portions of first kind) and the first fitting grooves  29  (engaged portions of first kind) are coupled to form an engaging structure of first kind. The protrusions  55  (engaging portions of second kind) and the second fitting grooves  31  (engaged portions of second kind) are coupled to form a second kind of an engaging structure. With this construction, when a separating action to move in the axial direction the first case  5  and the second case  33  out of engagement with each other, the first engaging surfaces  53   d  of the head portions  53   b  of the hooks  53  engage with the first engaged surfaces  29   d  at the flat faces  11   a  of the discharge-side flange  11 , activating the first kind of engaging structure to resist the separating action. Further, when a first rotating action is performed to rotate the first case  5  and the second case  33 , in a coupled state, about the axial line A in one rotating direction indicated by arrow D 1 , the second engaging surfaces  53   e  of the body portions  53   a  engage with the second engaged surface  29   e  of the discharge-side flange  11 , activating the first kind of engaging structure to resist the first rotating action. When a second rotating action is performed to rotate the first case  5  and the second case  33 , in a coupled state, about the axial line A in a direction indicated by arrow D 2 , opposite to the one rotating direction (arrow D 1 ), the end faces  55   b  of the protrusions  55  forming the third engaging surfaces engage with the third engaged surfaces  31   e  of the engaging grooves  31   b  of the second fitting grooves  31 , activating the second kind of engaging structure to resist the second rotating action. Thus, in the fan of this embodiment, even if the first case  5  and the second case  33  are subjected to a force acting in the direction of arrow D 1  or a force acting in the direction of arrow D 2 , they are prevented from being disconnected. 
     As shown in  FIG. 1  and  FIG. 2 , in the fan of this embodiment, the first case  5  and the second case  33  are coupled to form a housing  59 ; and the webs  19 ,  21 ,  23  of the first axial-flow fan unit  1  and the webs  43 ,  45 ,  47  of the second axial-flow fan unit  3  are coupled to form a plurality of radially extending stationary blades  61  ( FIG. 6 ) disposed stationarily in the housing  59  between the first impeller  7  and the second impeller  35 . When the first impeller  7  rotates in one rotating direction R 1  and the second impeller  35  in the other rotating direction R 2 , air is moved in a direction F from the suction opening portion  15  toward the discharge opening portion  57 . 
       FIG. 7  shows a relationship between an amount of air and a static pressure generated by each of three types of the counter-rotating axial-flow fan. The first one of the three types is the counter-rotating axial-flow fan having a structure of the present invention as shown in  FIG. 1  to  FIG. 6 . The second one of the three types is a counter-rotating axial-flow fan in a comparative example in which a portion, extending beyond the end portion  27   aa  of the peripheral wall  27   a  of the annular member  27  located at the other side (the discharge opening portion  17 ) in the axial direction, is cut away. The last one is a conventional counter-rotating axial-flow fan as shown in  FIG. 8 .  FIG. 8  shows the counter-rotating axial-flow fan having the conventional structure while  FIG. 1  to  FIG. 6  show the counter-rotating axial-flow fan having a structure of the present invention. The parts in the  FIG. 8  corresponding to those in  FIG. 1  to  FIG. 6  are indicated with reference numerals each of which is made by adding 100 to each of the reference numerals in  FIG. 1  to  FIG. 6 . Also the reference numerals in  FIG. 8 , indicating lengths and corresponding to reference numerals in  FIG. 2 , are indicated by adding dashes to the reference numerals in  FIG. 2 . 
       FIG. 7  is a characteristic chart showing the amount of air and the static pressure, when a ratio of a length L 1  and a length L 2 , L 1 /L 2  is varied from 1.3 (Embodiment 1 of the present invention, indicated by a line connecting symbols of ◯), 2.0 (Embodiment 2 of the present invention, indicated by a line connecting symbols of Δ), to 2.5 (Embodiment 3 of the present invention, indicated by a line connecting symbols of □). 
       FIG. 7  also shows the characteristics of the amount of air and the static pressure in the conventional counter-rotating axial-flow fan shown in  FIG. 8  using dashed lines. 
     Table 1 shows an actual length L 3  of the first case, an actual length L 4  of the second case, and a ratio of L 3 /L 4 , as well as an actual length L 1  of the front blade an actual length L 2  of the rear blade, and a ratio of L 1 /L 2 , in connection with the characteristics of the amount of wind and a static pressure shown in  FIG. 7 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Ratio of case 
                 Ratio of blade 
               
               
                   
                 lengths L3/L4 
                 lengths L1/L2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Conventional 
                 1 
                 0.77 
               
               
                   
                 Example 
               
               
                   
                 Embodiment 1 
                 1.2 
                 1.3 
               
               
                   
                 Embodiment 2 
                 1.5 
                 2.0 
               
               
                   
                 Embodiment 3 
                 1.8 
                 2.5 
               
               
                   
                   
               
            
           
         
       
     
     As shown most clearly in  FIG. 7 , it has been found that the characteristics of the amount of air and the static pressure can be improved in the counter-rotating axial-flow fan having a structure of the present invention, compared with those of the comparison examples and the conventional example, when two lengths of L 1  and L 2  are set so that a ratio of the two lengths L 1 /L 2  is a value from 1.3 to 2.5. A ratio of the length L 3  of the first case and the length L 4  of the second case, L 3 /L 4 , is a value from 1.2 to 1.8. 
     In other words, of the present invention, the characteristics of the amount of air and the static pressure can be improved when the length L 1  of the front blade is longer than the length L 2  of the rear blade. The characteristics of the amount of air and the static pressure will be lowered, when the length L 1  of the front blade is too long, while the length of the rear blade is too short. 
     While a preferred embodiment of the invention has been described with a certain degree of particularity with reference to the drawings, obvious modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.