Abstract:
A brush-less motor has a center piece, which has a flange portion fixed to a motor holder and a cylindrical portion fixed to a stator core at the outer periphery of the cylindrical portion. The cylindrical portion has a pair of bearings disposed inside the cylindrical portion to rotatably support said shaft. The center piece is formed by a pressing machine to omit a finishing machine work.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority from Japanese Patent Applications: Hei 10-191743, filed on Jul. 7, 1998 and Hei 10-214977, filed on Jul. 30, 1998, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a center piece structure of a brush-less motor and a method of manufacturing the brush-less motor. 
     2. Description of the Related Art 
     A common outer rotor type brush-less motor has a stator core fixed to an upper surface of a motor holder and a stator winding supported by a center piece which has a flange portion and a cylindrical portion. The cylindrical portion has an inner periphery which supports a pair of bearings, which rotatably supports a rotor shaft. 
     Usually, such a center piece has been made of aluminum alloy die-cast. However, such a center piece may need another machining step for finishing to provide necessary precision and bearing&#39;s retaining surfaces. This increases production cost and time. 
     SUMMARY OF THE INVENTION 
     A main object of the invention is to provide a brush-less motor having an improved center piece structure. 
     According to a preferred embodiment of the invention, a brush-less motor includes a press-formed center piece which does not require additional finishing step. 
     The cylindrical portion may have a cut-down projection extending radially inward to position bearing members in the axial direction thereof. 
     According to another preferred embodiment of the invention, a method of manufacturing a brush-less motor including a step of providing a stator winding on the stator core, and a step of fixing an outer periphery of the cylindrical portion to the stator core thereafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
     FIG. 1 is a cross-sectional view illustrating a brush-less motor according to a preferred embodiment of the invention; 
     FIG. 2 is an exploded view illustrating a main portion of the brush-less motor illustrated in FIG. 1; and 
     FIG. 3 is a center piece of the brush-less motor illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, stator  2  is fixed to upper surface  1   a  of motor holder  1 . Stator  2  has center piece  3 , stator core  4  and stator winding  5 . Stator core  4  has a central opening  4   a,  a plurality of teeth extending radially outward, and slots formed therebetween. Stator winding  5  is wound around the teeth in a well-known manner, however, before stator core  4  is fixed to center piece  3 , because sufficient space for a winding tool can be provided. This can provide stator  2  with any one of various types of stator windings easily. 
     As shown in FIG. 3, center piece  3  is press-formed from a metal plate such as iron alloy or steel to have semi-annular flange  3   a  and cylindrical portion  3   b  extending from a central portion of flange  3   a.  Three projections  3   c  project downward from the lower peripheral surface of flange  3   a.  Each projection  3   c  has a through screw hole  3   d  therein. Three openings  3   e  are also formed in flange  3   a  near cylindrical portion  3   b,  through which lead wires of stator winding  5  pass to be connected to printed circuit board  11 . Flange  3   a  has straight edge  3   f  and a circular edge having concavity  3   g.  Cylindrical portion  3   b  has end portion  3   h  of a reduced outside diameter and inner periphery  3   i.  Four upper cut-down projections  3   j  and four lower cut-down projections  3   k  are respectively formed at the axially middle of cylindrical portion  3   b  at equal intervals to project radially inward therefrom. Cut-down projections  3   j,    3   k  provide retaining surfaces  3   m,    3   n.    
     Because center piece  3  is press-formed, cylindrical portion  3   b  can be inserted into center hole  4   a  of stator core  4 , as shown in FIG. 2, without additional machine-finishing step. 
     Upper bearing  6   a  has a bearing portion which supports rotor shaft  10  and an axially extending portion  6   c.  Axially extending portion  6   c  has an outer periphery press-fitted into inner periphery  3   i  of cylindrical portion  3   b  from the upper end thereof and an inner periphery whose diameter is much larger than the diameter of the rotor shaft  10 . Lower bearing  6   b  has a bearing portion which supports rotor shaft  10  and an axially extending portion  6   d.  Axially extending portion  6   d  has an outer periphery press-fitted into inner periphery  3   i  from the lower end thereof and an inner periphery whose diameter is much larger than the diameter of the rotor shaft  10 . Each of the bearing portions of bearings  6   a,    6   b  has outer periphery whose diameter is smaller than the diameter of axially extending portions  6   c,    6   d.    
     Although the outer periphery of extending portions  6   c,    6   d  may shrink when bearings  6   a,    6   b  are press-fitted into the inner periphery  3   i  of cylindrical portion  3   b,  the bearing portions do not shrink. As a result, both bearings  6   a  and  6   b  are respectively retained and positioned in the axial direction by retaining surfaces  3   m,    3   n  so that inner bearing surfaces  6   e,    6   f  of bearings  6   a,    6   b  correctly support rotor shaft  10 . 
     Rotor shaft  10  is then press-fitted into bearing surfaces  6   e,    6   f,  so that rotor  7  can be rotatably supported by stator  2  via center piece  3 . Bell-shaped yoke  8 , which has a center boss and a plurality of permanent magnets  9  fixed to the inner periphery thereof, is also press-fitted to rotor shaft  10  at the center boss to rotate therewith. Yoke  8  surrounds stator  2  so that permanent magnets  9  can face stator core  4 . 
     Printed circuit board  11 , which has a field exciting circuit therein, is fixed to the lower surface of motor holder  1 . When stator winding  5  is energized by the exciting circuit, stator  2  provides a rotating magnetic field to rotate rotor  7 . 
     In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the broader spirit and scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention in this document is to be regarded in an illustrative, rather than restrictive, sense.