Patent Publication Number: US-9404502-B2

Title: Centrifugal fan device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a centrifugal fan device. 
     2. Description of the Related Art 
     Conventionally, there is known a centrifugal fan device for rotating an impeller with the drive power of a motor and discharging an axially drawn gas in a circumferential direction. The centrifugal fan device is used as, e.g., a suction means for a cleaner or an air cooling means for an electronic device. 
     A structure of a conventional centrifugal fan device is disclosed in, e.g., U.S. Patent Application Publication No. 2008/0069689 A1 (hereinafter US 2008/0069689). The centrifugal fan device of US 2008/0069689 includes a housing, an impeller arranged within the housing, and a drive device for rotating the impeller (see, for example, Claim 1 and FIGS. 1A and 3B of US 2008/0069689). In the centrifugal fan device of US 2008/0069689, a drawing hole is formed in a top shell of the housing (see, for example, Claim 5 and FIGS. 1A and 3B of US 2008/0069689). 
     In the centrifugal fan device disclosed in US 2008/0069689, a groove is formed on the lower surface of the top shell around the drawing hole. The upper end portion of a top panel of the impeller is inserted into the groove (see, for example, FIG. 3B of US 2008/0069689). However, with the structure of US 2008/0069689, the vibrations and noises of the top shell grow larger around the drawing hole. In particular, the problem of vibrations and noises becomes severe when driving the centrifugal fan device at a high speed. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide a centrifugal fan device capable of reducing vibrations and noises during an operation. 
     In accordance with a preferred embodiment of the present invention, a centrifugal fan device includes: an impeller supported to rotate about a center axis extending in an up-down direction; a motor arranged to rotate the impeller; and a housing arranged to surround the impeller. The housing includes an intake portion positioned above the impeller and an exhaust portion positioned radially outward of the impeller. The impeller includes a base extending in a direction orthogonal to the center axis, a ring-shaped shroud positioned above the base and having a diameter which decreases as it goes upward, and a plurality of blades arranged between the base and the shroud. The intake portion includes a cylinder portion positioned radially inward of an upper end portion of the shroud to extend in an axial direction, a top plate portion extending radially outward from an upper end portion of the cylinder portion, and an outer shell portion extending downward and radially outward from a radial outer edge of the top plate portion, and the housing further includes a plurality of ribs protruding from an inner surface of the intake portion toward the shroud. 
     With the illustrative first preferred embodiment of the present invention, the cylinder portion of the housing is preferably arranged radially inward of the upper end portion of the shroud. Therefore, a gas can be efficiently drawn into a space between the base and the shroud in the intake portion. The rigidity of the intake portion of the housing is increased by the ribs. Accordingly, it is possible to reduce vibrations and noises during an operation. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional view showing a centrifugal fan device according to a first preferred embodiment of the present invention. 
         FIG. 2  is a perspective view showing a centrifugal fan device according to a second preferred embodiment of the present invention. 
         FIG. 3  is a vertical sectional view of the centrifugal fan device according to the second preferred embodiment of the present invention. 
         FIG. 4  is a partial vertical sectional view showing an upper housing member and an impeller according to the second preferred embodiment of the present invention. 
         FIG. 5  is a bottom view of the upper housing member according to the second preferred embodiment of the present invention. 
         FIG. 6  is a vertical sectional view showing a lower impeller member according to the second preferred embodiment of the present invention. 
         FIG. 7  is a plan view of the lower impeller member according to the second preferred embodiment of the present invention. 
         FIG. 8  is a vertical sectional view showing an upper impeller member according to the second preferred embodiment of the present invention. 
         FIG. 9  is a bottom view of the upper impeller member according to the second preferred embodiment of the present invention. 
         FIG. 10  is a partial vertical sectional view showing an upper housing member and an impeller according to one modified example of a preferred embodiment of the present invention. 
         FIG. 11  is a partial vertical section view showing an upper housing member and an impeller according to another modified example of a preferred embodiment of the present invention. 
         FIG. 12  is a bottom view of the upper housing member shown in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described with reference to the drawings which form a part hereof. In the subject application, the direction extending along the center axis of a motor will be referred to as “axial direction”. The direction orthogonal to the center axis of a motor will be referred to as “radial direction”. The direction extending along a circular arc about the center axis of a motor will be referred to as “circumferential direction”. In the subject application, the shape and positional relationship of individual components will be described under the assumption that the axial direction is an up-down direction and the side of an intake portion with respect to an impeller is an “upper side”. However, these definitions are made merely for the sake of convenience in description and are not intended to limit the direction when the present centrifugal fan device is used. 
     First Preferred Embodiment 
       FIG. 1  is a vertical section view showing a centrifugal fan device  1 A in accordance with a first preferred embodiment of the present invention. Referring to  FIG. 1 , the centrifugal fan device  1 A preferably includes a motor  10 A, an impeller  20 A, and a housing  30 A. The impeller  20 A is supported to rotate about a center axis  9 A. The impeller  20 A is rotated by the motor  10 A. 
     As shown in  FIG. 1 , the impeller  20 A preferably includes a base  61 A, a shroud  62 A, and a plurality of blades  63 A. The base  61 A extends in the direction orthogonal to the center axis  9 A. The shroud  62 A is positioned above the base  61 A. The shroud  62 A preferably has an annular shape and the diameter of the shroud  62 A decreases as it goes upward. In other words, the shroud  62 A is inclined in such a direction that the angle of the shroud  62 A with respect to the center axis  9 A grows smaller as the shroud  62 A extends radially inward. In this example, the innermost edge of the shroud  62 A is parallel or substantially parallel to the center axis  9 A. The blades  63 A are preferably arranged between the base  61 A and the shroud  62 A. 
     The housing  30 A surrounds the impeller  20 A. The housing  30 A preferably includes an intake portion  71 A and an exhaust portion  72 A. The intake portion  71 A is positioned above the impeller  20 A. The exhaust portion  72 A is positioned radially outward of the impeller  20 A. 
     As shown in  FIG. 1 , the intake portion  71 A preferably includes a cylinder portion  711 A, a top plate portion  712 A, and an outer shell portion  713 A. The cylinder portion  711 A is positioned radially inward of the upper end portion of the shroud  62 A and extends in the axial direction. The top plate portion  712 A extends radially outward from the upper end portion of the cylinder portion  711 A. The outer shell portion  713 A extends downward and radially outward from the radial outer edge of the top plate portion  712 A. 
     In the centrifugal fan device  1 A, as set forth above, the cylinder portion  711 A of the housing  30 A is preferably arranged radially inward of the upper end portion of the shroud  62 A. For that reason, a gas can be efficiently drawn from the intake portion  71 A into a space between the base  61 A and the shroud  62 A. 
     The housing  30 A preferably further includes ribs  73 A protruding from the inner surface of the intake portion  71 A toward the shroud  62 A. The rigidity of the intake portion  71 A of the housing  30 A is preferably increased by the ribs  73 A. This reduces vibrations and noises generated during the operation of the centrifugal fan device  1 A. 
     Second Preferred Embodiment 
     Next, description will be made of a second preferred embodiment of the present invention.  FIG. 2  is a perspective view showing a centrifugal fan device  1  in accordance with the second preferred embodiment.  FIG. 3  is a vertical section view of the centrifugal fan device  1 . The centrifugal fan device  1  is preferably mounted to a suction-type cleaning device such as, for example, a self-propelled cleaning robot, a hand-held cleaner, a vacuum cleaner, etc., and is used as suction device of the cleaning device. As shown in  FIGS. 2 and 3 , the centrifugal fan device  1  of the present preferred embodiment preferably includes a motor  10 , an impeller  20 , and a housing  30 . 
     The motor  10  preferably includes a stationary unit  40  and a rotary unit  50 . The stationary unit  40  is kept stationary with respect to the housing  30 . The rotary unit  50  is supported to rotate with respect to the stationary unit  40 . The stationary unit  40  preferably includes an attachment plate  41 , a bearing holder  42 , a sleeve  43 , a stator core  44 , a coil  45 , and a circuit board  46 . The rotary unit  50  preferably includes a shaft  51 , a rotor holder  52 , and a plurality of magnets  53 . 
     The attachment plate  41  is preferably a substantially flat member extending in the direction orthogonal to the center axis  9 . The attachment plate  41  is fixed to a lower housing member  31 . The bearing holder  42  is a cup-shaped member fixed to the attachment plate  41 . The sleeve  43  is held within the bearing holder  42 . The sleeve  43  preferably has a substantially cylindrical inner circumferential surface corresponding to an outer circumferential surface of the shaft  51 . 
     The stator core  44  is preferably fixed to the outer circumferential surface of the bearing holder  42 . The stator core  44  preferably includes a plurality of radially extending teeth  441 . The coil  45  is preferably defined by conductive wires wound around the respective teeth  441 , but any other desirable type of coil  45  could be used. The circuit board  46  is fixed to the upper surface of the attachment plate  41 . An electronic circuit arranged to apply a drive current to the coil  45  is preferably mounted to the circuit board  46 . 
     The shaft  51  is preferably a columnar member extending in the axial direction. The shaft  51  is supported by the sleeve  43  and the bearing holder  42  to rotate about the center axis  9 . The upper end portion of the shaft  51  preferably protrudes upward beyond the upper surface of the sleeve  43 . The rotor holder  52  is fixed to the shaft  51  and is rotated together with the shaft  51 . The magnets  53  are positioned radially outward of the stator core  44  and are fixed to the rotor holder  52 . The magnets  53  are arranged along the circumferential direction such that N-poles and S-poles alternately stand side by side. 
     If a drive current is applied to the coil  45  through the circuit board  46 , magnetic fluxes are generated in the teeth  441  of the stator core  44 . Circumferential torque is generated by the action of the magnetic fluxes between the teeth  441  and the magnets  53 . As a result, the rotary unit  50  is rotated about the center axis  9  with respect to the stationary unit  40 . 
     The impeller  20  is supported to rotate together with the rotary unit  50  of the motor  10 . In the present preferred embodiment, the impeller  20  preferably includes a lower impeller member  21  and an upper impeller member  22  arranged above the lower impeller member  21 . As shown in  FIG. 3 , the lower impeller member  21  preferably includes a substantially disc-shaped base  61  extending in the direction orthogonal to the center axis  9 . The upper end portion of the shaft  51  is preferably, for example, press-fitted to an attachment hole  611  defined on the lower surface of the base  61 . Consequently, the shaft  51  and the base  61  are fixed together. 
     The upper impeller member  22  preferably includes a shroud  62  and a plurality of blades  63 . The shroud  62  is preferably positioned above the base  61  and extends to have an annular shape. The shroud  62  is inclined such that the shroud  62  goes upward as it comes close to the center axis  9 . Therefore, the diameter of the shroud  62  decreases as it goes upward. In other words, the shroud  62  is preferably inclined in such a direction that the angle of the shroud  62  with respect to the center axis  9  decreases as the shroud  62  extends radially inward. A circular opening  622  arranged to draw a gas therethrough is preferably defined inside the upper end portion of the shroud  62 . 
     The blades  63  extend downward from the lower surface of the shroud  62 . Therefore, the blades  63  are preferably arranged between the base  61  and the shroud  62 . Each of the blades  63  obliquely extends with respect to the radial direction and the circumferential direction. The lower end portions of the blades  63  are welded to the upper surface of the base  61 . Consequently, the lower impeller member  21  and the upper impeller member  22  are preferably fixed together. Details of the fixing structure of the lower impeller member  21  and the upper impeller member  22  will be described later. 
     The housing  30  preferably includes a lower housing member  31  and an upper housing member  32  arranged above the lower housing member  31 . The lower housing member  31  preferably includes a bottom plate portion  311  positioned below the impeller  20  and a groove portion  312  positioned radially outward of the bottom plate portion  311 . The bottom plate portion  311  extends radially outward from a position near the outer peripheral portion of the motor  10 . The groove portion  312  is preferably positioned radially outward of the impeller  20  and extends in the circumferential direction so as to surround the bottom plate portion  311 . As shown in  FIG. 3 , the groove portion  312  has a substantially U-shaped upper surface when seen in a cross-sectional view. 
     The upper housing member  32  is preferably a ring-shaped member arranged to cover the upper surface and side portion of the impeller  20 . The lower end of the outer peripheral portion of the upper housing member  32  is preferably fixed to the upper end of the outer peripheral portion of the lower housing member  31 . The impeller  20  is preferably surrounded by the lower housing member  31  and the upper housing member  32 . The housing  30  preferably includes an intake portion  71  positioned above the impeller  20  and an exhaust portion  72  positioned radially outward of the impeller  20 . An intake port  710  arranged to draw a gas from the outside therethrough is preferably defined in the intake portion  71 . An exhaust port  720  arranged to discharge a gas toward the outside therethrough is preferably provided in the exhaust portion  72 . 
     When the motor  10  is driven, the impeller  20  is rotated together with the rotary unit  50  of the motor  10 . Then, a gas is admitted into the housing  30  through the intake port  710 . The gas admitted into the housing  30  is accelerated by the impeller  20 . As a result, the gas flows in the circumferential direction along the upper surface of the groove portion  312  of the lower housing member  31 . Thereafter, the gas is discharged to the outside of the housing  30  through the exhaust port  720 . 
     Subsequently, the detailed structure of the upper housing member  32  will be discussed.  FIG. 4  is a partial vertical section view of the upper housing member  32  and the impeller  20 .  FIG. 5  is a bottom view of the upper housing member  32 . 
     As set forth above, the upper housing member  32  is provided with the intake portion  71 . The intake portion  71  preferably includes a cylinder portion  711 , a top plate portion  712 , and an outer shell portion  713 . The cylinder portion  711  is preferably arranged around the intake port  710  and extends in the axial direction to have a cylindrical shape. The top plate portion  712  extends radially outward from the upper end portion of the cylinder portion  711 . The outer shell portion  713  extends downward and radially outward from the radial outer edge of the top plate portion  712 . The upper housing member  32  preferably includes an annular groove  714  provided on the lower surface of the intake portion  71 . The upper end portion of the shroud  62  is arranged within the groove  714 . 
     The cylinder portion  711  is positioned radially inward of the upper end portion of the shroud  62 . This prevents a gas from flowing from the intake port  710  to a space between the shroud  62  and the upper housing member  32 . The gas drawn from the outside is efficiently admitted into a space between the base  61  and the shroud  62  through the intake port  710 . In particular, in the present preferred embodiment, the lower end portion of the cylinder portion  711  is positioned lower than the upper end portion of the shroud  62 . For that reason, a gas is efficiently admitted from the intake port  710  into the space between the base and the shroud  62 . As a result, it becomes possible to increase the static pressure of the centrifugal fan device  1 . 
     As shown in  FIGS. 4 and 5 , the upper housing member  32  preferably further includes a plurality of flat ribs  73 . Each of the flat ribs  73  protrudes from the inner surface of the intake portion  71  toward the shroud  62 . In the present preferred embodiment, the rigidity of the intake portion  71  is preferably increased by the flat ribs  73 . As a result, it is possible to reduce vibrations and noises of the upper housing member  32  during the operation of the centrifugal fan device  1 . 
     In a hypothetical case where flat ribs are provided on the outer surface of an intake portion, structural constraints on, for example, a cleaning device, grow larger when a centrifugal fan device is installed in the cleaning device. Accordingly, in the present preferred embodiment, the flat ribs  73  are preferably provided on the inner surface of the intake portion  71 . Also, on the outer surface of the intake portion  71  there are preferably no irregularities which may otherwise be generated by ribs. Accordingly, it is possible to reduce constraints on the installation of the centrifugal fan device  1 . 
     In particular, in the present preferred embodiment, the upper surface of the top plate portion  712  is preferably a smooth surface substantially orthogonal to the center axis  9 . This makes it easy to bring the upper surface of the top plate portion  712  into close contact with a member of, e.g., a cleaning device. If the upper surface of the top plate portion  712  is brought into close contact with the member of the cleaning device, it becomes possible to prevent leakage of a gas from the vicinity of the top plate portion  712 . As a result, it is possible to increase the static pressure of the centrifugal fan device  1  when mounted to the cleaning device. 
     The flat ribs  73  preferably extend radially and axially from the inner surface of the intake portion  71 . For that reason, it is possible to reduce the radial and axial vibration components of the intake portion  71 . The flat ribs  73  included in the present preferred embodiment are preferably connected to both the lower surface of the top plate portion  712  and the radial inner surface of the outer shell portion  713 . Since the flat ribs  73  extend along two surfaces in this manner, it is possible to increase the rigidity of the intake portion  71  and to reduce vibrations thereof. 
     In the present preferred embodiment, the radial inner end portions of the flat ribs  73  are preferably arranged radially outward of the upper end portion of the shroud  62 . This makes it possible to have the upper end portion of the shroud  62  and the lower surface of the top plate portion  712  be close to each other while preventing the flat ribs  73  from making contact with the upper end portion of the shroud  62 . As a result, it is possible to reduce the total axial dimension of the centrifugal fan device  1 . 
     In the present preferred embodiment, the radial dimension of the flat ribs  73  is decreased as it extends radially outward. When seen in a vertical cross section including the center axis  9  and the flat ribs  73 , the lower edges of the flat ribs  73  and the lower surface of the outer shell portion  713  are smoothly joined to each other. When seen in the vertical cross section, the lower surfaces of the flat ribs  73  and the outer shell portion  713  are inclined to gradually descend radially outward. The lower surface of the section of the outer shell portion  713  lying radially outward of the flat ribs  73  is preferably positioned lower than the lower end portions of the flat ribs  73 . 
     For that reason, it is easy to have the flat ribs  73  and the upper surface of the shroud  62  be close to each other while preventing the flat ribs  73  from making contact with the shroud  62 . If the flat ribs  73  and the upper surface of the shroud  62  are close to each other, it is possible to prevent a gas from flowing from the intake portion  71  to the space between the shroud  62  and the upper housing member  32 . 
     In this regard, as shown in  FIG. 4 , it is assumed that the radial distance between the radial inner edge of each of the flat ribs  73  and the outer circumferential surface of the upper end portion of the shroud  62  is d 1 . It is also assumed that the radial distance between the radial inner edge of the outer shell portion  713  and the outer circumferential surface of the upper end portion of the shroud  62  is d 2 . It is further assumed that the axial distance between the lower surface of the section of the outer shell portion  713  lying radially outward of the flat ribs  73  and the upper surface of the shroud  62  is d 3 . 
     In order to prevent a gas from flowing into the space between the shroud  62  and the upper housing member  32 , it is desirable to shorten the distance between the flat ribs  73  and the upper end portion of the shroud  62 . Therefore, d 3  is preferably set larger than d 1 . In order to increase the rigidity of the intake portion  71 , it is desirable to increase the radial dimension of the flat ribs  73 . Therefore, d 2  is preferably set to be larger than d 3 , which in turn is set to be larger than d 1 . 
     In the present preferred embodiment, as shown in  FIG. 5 , preferably seven, for example, flat ribs  73  are provided in the upper housing member  32 . The seven flat ribs  73  are preferably arranged substantially at an equal interval in the circumferential direction. The number of the flat ribs  73  of the upper housing member  32  may alternatively be any number other than seven if so desired. It is however preferred that the number of the flat ribs  73  do not match up with any one of the number of the blades  63  of the impeller  20 , the pole number of the motor  10 , and the slot number of the motor  10 . This makes it possible to restrain the flat ribs  73  from resonating with other members. The pole number is preferably equal to the magnetic pole number of the magnets of the motor  10 . The slot number is preferably equal to the number of the teeth  441  of the motor  10 . 
     In the present preferred embodiment, the upper housing member  32  is preferably, for example, a resin molded article that can be obtained by injection molding, however, any other desirable material and forming method could be used. Use of the injection molding makes it possible to easily mold the flat ribs  73 . In a hypothetical case that an attempt is made to obtain high rigidity by increasing the overall thickness of an intake portion instead of forming flat ribs, a partial depression is easily generated on the surface of the intake portion when cooling and solidifying a molten resin. In the present preferred embodiment, however, the rigidity of the intake portion  71  is preferably increased by the flat ribs  73  while reducing the thickness of the intake portion  71 . This prevents generation of a partial depression. It is also possible to reduce the use amount of a resin as compared with a case where the thickness of the intake portion is increased as a whole. 
     Next, description will be made on a fixing structure of the lower impeller member  21  and the upper impeller member  22 .  FIG. 6  is a vertical section view of the lower impeller member  21 .  FIG. 7  is a plan view of the lower impeller member  21 .  FIG. 8  is a vertical section view of the upper impeller member  22 .  FIG. 9  is a bottom view of the upper impeller member  22 . 
     As shown in  FIGS. 6 and 7 , a plurality of first recesses  211  is preferably provided on the upper surface of the base  61  of the lower impeller member  21 . Each of the first recesses  211  obliquely extends with respect to the radial direction and the circumferential direction. The first recesses  211  are arranged substantially at an equal interval along the circumferential direction in a corresponding relationship with the blades  63 . Second recesses  212  depressed further downward from the first recesses  211  are defined in the base  61 . The second recesses  212  are substantially circular depressions smaller in size than the first recesses  211  when seen in a top view. 
     On the other hand, as shown in  FIGS. 8 and 9 , first protrusions  221  and second protrusions  222  are preferably provided on the lower surfaces of the blades  63  of the upper impeller member  22 . The first protrusions  221  extend along the blades  63 . In other words, the first protrusions  221  obliquely extend with respect to the radial direction and the circumferential direction. The lower end portions of the first protrusions  221  are preferably thinned in a downward direction. The second protrusions  222  are preferably substantially cylindrical columnar lugs smaller in size than the blades  63  when seen in a bottom view. The second protrusions  222  are arranged in a corresponding relationship with the second recesses  212 . 
     When manufacturing the impeller  20 , the lower end portions of the blades  63  are preferably fitted to the first recesses  211  of the base  61 . The second protrusions  222  of the blades  63  are preferably fitted to the second recesses  212  of the base  61 . At this time, the lower end portions of the first protrusions  221  of the blades  63  make contact with the bottom portions of the first recesses  211  of the base  61 . 
     Thereafter, ultrasonic vibrations, for example, are applied to at least one of the lower impeller member  21  and the upper impeller member  22 . Then, because of the ultrasonic vibrations, frictional heat is generated between the upper surface of the base  61  and the first protrusions  221  of the blades  63 . Thus, the first protrusions  221  are melted by the frictional heat. As a result, the lower impeller member  21  and the upper impeller member  22  are welded together. 
     From the standpoint of manufacture, it is difficult to bring the gravity center positions of the lower impeller member  21  and the upper impeller member  22  into perfect alignment with the center axis  9 . In a hypothetical case where the gravity center deviation of the lower impeller member  21  and the gravity center deviation of the upper impeller member  22  overlap in the same direction, the gravity center deviation of the impeller  20  as a whole becomes larger. For that reason, when manufacturing the impeller  20 , the relative rotation positions of the lower and upper impeller members  21  and  22  with respect to the center axis  9  is preferably set such that the gravity center deviation of the lower impeller member  21  and the gravity center deviation of the upper impeller member  22  are cancelled each other. 
     In the present preferred embodiment, the diameter of the second recess  212   s , one of the second recesses  212 , is preferably set larger than the diameter of the remaining second recesses  212 . Likewise, the diameter of the second protrusion  222   s , one of the second protrusions  222 , is preferably set larger than the diameter of the remaining second protrusions  222 . In the following description, the second recess  212   s  having a large diameter will be referred to as large-diameter second recess  212   s . Moreover, the second protrusion  222   s  having a large diameter will be referred to as large-diameter second protrusion  222   s.    
     When manufacturing the impeller  20 , the large-diameter second protrusion  222   s  is fitted to the large-diameter second recess  212   s . It is not possible to fit the large-diameter second protrusion  222   s  to the second recesses  212  other than the large-diameter second recess  212   s . Accordingly, the relative rotation positions of the lower impeller member  21  and the upper impeller member  22  are uniquely specified. 
     In the present preferred embodiment, the impeller  20  is a resin molded article that can be obtained by injection molding. When manufacturing the impeller  20 , the tendency of the gravity center deviations of the lower impeller member  21  and the upper impeller member  22  is inspected in advance. Then, the positions of the large-diameter second recess  212   s  and the large-diameter second protrusion  222   s  are set such that, when the lower and upper impeller members  21  and  22  are combined together, the gravity center deviations of the lower and upper impeller members  21  and  22  are in the substantially opposite directions. 
     This makes it possible to reduce the gravity center deviation of the impeller  20  as a whole even when each of the lower and upper impeller members  21  and  22  has a gravity center deviation. It is also preferably possible to simplify or omit a step of correcting the gravity center of the impeller  20  after the lower impeller member  21  and the upper impeller member  22  are combined together. As a result, it becomes possible to shorten the manufacturing process of the impeller  20 . 
     MODIFIED EXAMPLES OF PREFERRED EMBODIMENTS 
     While one illustrative preferred embodiment of the present invention has been described above, the present invention is not limited thereto. 
       FIG. 10  is a partial vertical section view showing an upper housing member  32 B and an impeller  20 B in accordance with one modified example of a preferred embodiment of the present invention. In the example shown in  FIG. 10 , a plurality of first flat ribs  731 B and a plurality of second flat ribs  732 B are provided on the inner surface of an intake portion  71 B. The first flat ribs  731 B are positioned radially outward of the upper end portion of a shroud  62 B. The second flat ribs  732 B are positioned radially inward of the upper end portion of the shroud  62 B. By installing the flat ribs in different radial positions in this manner, it is preferably possible to further increase the rigidity of the intake portion  71 B. 
     In the example shown in  FIG. 10 , a cutout  733 B is arranged between the first flat ribs  731 B and the second flat ribs  732 B. The upper end portion of the shroud  62 B is arranged within the cutout  733 B. This makes it possible to prevent the upper end portion of the shroud  62 B from making contact with the first flat ribs  731 B and second flat ribs  732 B. Since the upper end portion of the shroud  62 B can be caused to come close to the lower surface of the top plate portion  712 B, it is possible to reduce the backflow of a gas and to increase the static pressure of the centrifugal fan device. With this configuration, it is possible to have the upper end portion of the shroud  62 B and the lower surface of the top plate portion  712 B of the upper impeller member  22 B come close to each other in the axial direction. Accordingly, it is possible to reduce the axial dimension of the centrifugal fan device. 
     In the example shown in  FIG. 10 , the first flat ribs  731 B are connected to both the lower surface of the top plate portion  712 B and the radial inner surface of the outer shell portion  713 B. The second flat ribs  732 B are connected to both the lower surface of the top plate portion  712 B and the outer circumferential surface of the cylinder portion  711 B. Since each of the flat ribs extends along two surfaces of the intake portion  71 B in this manner, it is preferably possible to further reduce the vibrations of the intake portion  71 B. The top plate portion  712 B need not necessarily be planar. The axial cross section of the top plate portion  712 B may have an arch shape. In that case, the top plate portion refers to the portion having an arch-shaped cross section, which is positioned above the upper end portion of the shroud  62 B. 
       FIG. 11  is a partial vertical section view showing an upper housing member  32 C and an impeller  20 C in accordance with another modified example of a preferred embodiment of the present invention.  FIG. 12  is a bottom view of the upper housing member  32 C shown in  FIG. 11 . In the example shown in  FIGS. 11 and 12 , a plurality of flat ribs  73 C and a single ring-shaped rib  74 C are preferably provided on the inner surface of the intake portion  71 C. The ring-shaped rib  74 C is positioned radially outward of the upper end portion of the shroud  62 C. The ring-shaped rib  74 C surrounds the upper end portion of the shroud  62 C and extends in the axial direction. The positioning the ring-shaped rib  74 C preferably makes it possible to further increase the rigidity of the intake portion  71 C. 
     In this example, as shown in  FIG. 11 , the lower end portion of the ring-shaped rib  74 C is positioned lower than the upper end portion of the shroud  62 C. The ring-shaped rib  74 C, the upper end portion of the shroud  62 C and the cylinder portion  711 C overlap in the radial direction. This makes it possible to further prevent a gas from flowing from the intake port  710 C into a space between the shroud  62 C and the upper housing member  32 C. Accordingly, it is possible to efficiently draw a gas from the intake port  710 C into a space between the base  61 C and the shroud  62 C. As a result, it is possible to further increase the static pressure of the centrifugal fan device. 
     As a further modified example of a preferred embodiment of the present invention, the flat ribs may be omitted and only the ring-shaped rib may be provided on the inner surface of the intake portion of the upper housing member. 
     The material of the impeller  20  and the housing  30  may preferably be a resin as in the aforementioned preferred embodiments or may be other desirable materials. For example, one or both of the impeller  20  and the housing  30  may be made of metal if so desired. 
     The second protrusions may be provided in the lower impeller member and the second recesses may be provided in the upper impeller member. The lower impeller member may include the base and the blades and the upper impeller member may be defined by only the shroud. In that case, for example, the positions of the upper surfaces of the blades and the lower surface of the shroud may be decided by welding or fitting. The impeller may alternatively be defined by one member or three or more members. 
     The diameter of the second recess  212   s , one of the second recesses  212 , may be set smaller than the diameter of the remaining second recesses  212 . Likewise, the diameter of the second protrusion  222   s , one of the second protrusions  222 , may be set smaller than the diameter of the remaining second protrusions  222 . 
     The centrifugal fan device according to various preferred embodiments of the present invention may be mounted to a device other than a cleaning device. For example, the centrifugal fan device according to various preferred embodiments of the present invention may be mounted to an electronic device, such as, for example, a personal computer or the like, to cool the inside thereof. The centrifugal fan device according to various preferred embodiments of the present invention may be mounted to other different OA devices, medical instruments, home appliances, or transport machines. 
     Detailed configurations of the centrifugal fan device may differ from those of the preferred embodiments and modified examples described above. The individual components included in the preferred embodiments and modified examples described above may be arbitrarily combined unless contradictory to one another. 
     Preferred embodiments of the present invention in which the “ribs” are not essential elements but in which “recesses” and “protrusions” which determine the rotation positions are alternative essential elements is also possible. This preferred embodiment corresponds to a centrifugal fan device preferably including: an impeller supported to rotate about a center axis extending in an up-down direction; a motor arranged to rotate the impeller; and a housing arranged to surround the impeller. The housing preferably includes an intake portion positioned above the impeller and an exhaust portion positioned radially outward of the impeller. The impeller preferably further includes a lower impeller member and an upper impeller member positioned above the lower impeller member, one of the lower impeller member and the upper impeller member including a plurality of recesses and the other of the lower impeller member and the upper impeller member including a plurality of protrusions fitted to the recesses. The recesses preferably include a single large-diameter recess that is larger in diameter than the remaining recesses, the protrusions including a single large-diameter protrusion that is larger in diameter than the remaining protrusions and the large-diameter protrusion being fitted to the large-diameter recess. 
     With the preferred embodiment of the present invention described just above, the relative rotation positions of the lower impeller member and the upper impeller member with respect to the center axis can be decided by fitting the large-diameter recess and the large-diameter protrusion together. The individual components included in the preferred embodiments and modified examples described above may be combined with the preferred embodiment of the present invention described just above. 
     Preferred embodiments of the present invention can find their application in a centrifugal fan device. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.