Patent Publication Number: US-2011070081-A1

Title: Cooling fan with internally tapered housing

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates generally to cooling fans, and more particularly to an axial fan with a fan housing which can be easily fabricated. 
     2. Description of Related Art 
     Cooling fans are commonly used in combination with heat sinks for cooling electronic components, such as CPUs. Normally, the heat sink is arranged on the electronic component to absorb heat therefrom, while the cooling fan is arranged on the heat sink to produce forced airflow flowing through the heat sink to take away the heat. 
     Generally, the cooling fan includes a stator, a rotor rotatably supported by the stator, and a fan housing surrounding the rotor for guiding the forced airflow through the heat sink. The fan housing usually has a rectangular or annular profile. An inner surface of the fan housing facing the rotor is cylindrical, to avoid interference of the rotor and the fan housing during rotation of the rotor. The fan housing is usually made of plastic, by a process of injection molding. During the molding process, after the fan housing is molded in a mold, it is difficult to separate the fan housing from the mold because the inner surface of the fan housing is cylindrical. As a result, the fan housing is liable to sustain abrasion of the inner surface thereof and/or deformation during the removal process. 
     What is need, therefore, is a cooling fan which can overcome the above limitations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing is a cross-sectional view of a cooling fan according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The drawing shows a cooling fan according to an exemplary embodiment, the cooling fan including a fan housing  12 , a stator  20 , a rotor  14 , and a pair of bearings  129 . 
     The fan housing  12  is annular (or cylindrical) and hollow. An air inlet  121  is formed at a top of the fan housing  12 , and an air outlet  123  is formed at a bottom of the fan housing  12  opposite to the air inlet  121 . The fan housing  12  has an inner surface  124  surrounding the rotor  14 . The inner surface  124  converges (tapers) slightly in a downward direction, i.e., the inner surface  124  of the fan housing  12  slants towards a central axis O of the fan housing  12  from top to bottom. Put another way, the inner surface  124  of the fan housing  12  expands slightly in an upward direction away from the central axis O of the fan housing  12 , with an inner space  126  surrounded by the inner surface  124  correspondingly expanding slightly in the upward direction. Accordingly, the air inlet  121  at the top of the fan housing  12  is a little larger than the air outlet  123  at the bottom of the fan housing  12 . 
     Since the inner surface  124  of the fan housing  12  expands upwardly along the central axis O, after the fan housing  12  is molded in a mold, it is relatively easy to separate the fan housing  12  from the mold by moving the mold in an upward direction along the central axis O of the fan housing  12 . Furthermore, during the separation process, friction between the inner surface  124  of the fan housing  12  and the mold is avoided. Thus the quality of the fan housing  12  obtained should be good. In order to maintain the cooling capability of the cooling fan and still facilitating separation of the fan housing  12  from the mold, a first angle θ 1  between the inner surface  124  and the central axis O of the fan housing  12  is in the range of 0.3˜3 degrees. 
     A base  125  is received in the fan housing  12 , and is arranged at the air outlet  123 . A central tube  128  extends upwardly from a center of the base  125 . A central hole  30  extends through the central tube  128 , such that top and bottom ends of the central tube  128  are open. That is, the central hole  30  is a through hole. In addition, an annular recess  32  communicating with the central hole  30  is formed at an inner periphery of each of the top and bottom ends of the central tube  128 . Each recess  32  has a diameter exceeding that of the central hole  30 . Thus the top and bottom ends of the central tube  128  have an inner diameter exceeding that of a middle portion of the central tube  128 . 
     The stator  20  is mounted around the central tube  128  of the base  125 . The stator  20  includes a stator core  22  with coils  24  wound thereon to establish an alternating magnetic field, and a PCB (printed circuit board)  26  electrically connected with the coils  24  to control electrical current flowing through the coils  24 . 
     The rotor  14  includes a hub  146  forming a shaft seat  147  at a central portion thereof, a plurality of rotary blades  142  extending radially and outwardly from an outer periphery of the hub  146 , a magnet  148  adhered to an inner surface  124  of the hub  146  and facing the coils  24  of the stator  20 , and a shaft  144  extending downwardly from the shaft seat  147  of the rotor  14 . The shaft  144  defines an annular notch  140  at a distal end thereof. 
     Each of the rotary blades  142  has an outer edge  145  confronting the inner surface  124  of the fan housing  12 . The outer edge  145  of each rotary blade  142  is approximately parallel to the inner surface  124  of the fan housing  12 . That is, the outer edge  145  of the rotary blade  142  slants towards the central axis O of the fan housing  12  from top to bottom. A second angle θ 2  between the outer edge  145  of the rotary blade  142  and the central axis O of the fan housing  12  is equal to the first angle θ 1  between the central axis O and the inner surface  124  of the fan housing  12 . Thus a distance between the outer edge  145  of the rotary blade  142  and the inner surface  124  of the fan housing  12  is constant from top to bottom. 
     As shown in the drawing, a height of the outer edge  145  of the rotary blade  142  as measured parallel to the central axis O of the fan housing  12  is designated as H, a first distance between a bottom end of the outer edge  145  of the rotary blade  142  and the central axis O of the fan housing  12  is designated as R 1 , and a second distance between a top end of the outer edge  145  of the rotary blade  142  and the central axis O of the fan housing  12  is designated as R 2 . The second distance R 2  is slightly larger than the first distance R 1 , and the second angle θ 2  between the outer edge  145  of the rotary blade  142  and the central axis O should satisfy the equation:  θ   2 =tan −1 ((R 2 −R 1 )/H). 
     The bearings  129  are received in the top and bottom recesses  32  of the central tube  128 , respectively, and surround the shaft  144 . When assembled, the rotor  14  is received in the inner space  26  and surrounded by the inner surface  124 , with the shaft  144  extending through the bearings  129 . Thus the shaft  144  of the rotor  14  is rotatably supported by the pair of bearings  128 . A locking ring  40  is arranged in the bottom recess  32  of the central tube  128  and engages in the notch  140  of the shaft  144  to limit movement of the shaft  144  along an axial direction thereof. A coil spring  50  is arranged between the bottom bearing  129  and the central tube  128  for applying a preset engaging pressure between the rotor  14  and the top bearing  129 , thereby ensuring that the top bearing  129  remains stationary relative to the hub  146  in the axial direction of the shaft  144 . 
     During operation, the rotor  14  is rotated by the interaction of the alternating magnetic field established by the stator  20  and the magnetic field of the magnet  148  of the rotor  14 . The rotary blades  142  thus produce forced airflow to take away heat generated in an application environment that employs the cooling fan. Since the distance between the outer edge  145  of each rotary blade  142  and the inner surface  124  and the fan housing  12  is uniform from top to bottom, during rotation of the rotor  14 , interference of the rotor  14  and the fan housing  12  is avoided, and the cooling fan can operate smoothly and quietly. 
     It is to be understood, however, that even though numerous characteristics and advantages of embodiments have been set forth in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.