Patent Publication Number: US-2007114863-A1

Title: Brushless motor

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to a brushless motor equipped with a roller bearing.  
      2. Description of the Related Art  
      A roller bearing is used for a brushless motor to obtain a highly reliable and durable brushless motor. In such a brushless motor, the roller bearing is axially positioned by abutting against an annular step provided on a shaft of the brushless motor. In another example, the roller bearing is abutted against an annular member arranged on the shaft for axially positioning the roller bearing. It is also known that a plurality of swells each having a doughnut shape are provided on the shaft for axially positioning the roller bearing by abutting the roller bearing against the swells.  
      Recently, a brushless motor having a competitive price and reliable performance is in demand. However, the conventional method of providing the annular step on the shaft does not meet the demand in terms of a cost for processing the shaft. Similarly, upon using the annular member for axially positioning the ball bearing, it is necessary to provide an annular groove on the shaft to arrange the annular member in a predetermined position. Such an additional component (i.e., the annular member) and additional processing procedures drive up the price of the brushless motor.  
      The plurality of the doughnut shape swells are conventionally formed by pressurizing a portion of the shaft surface to be deformed around the pressurized portion. As a result of the pressurization, the holes and the swells arranged around the holes are formed on the shaft. When force directed in a radial direction is applied to the shaft, stress is concentrated at portions in which holes are provided. Especially when a gear which engages with a gear wheel of another device is arranged on a bottom end of the shaft, an excessive force directed in the radial direction is applied to the shaft. Therefore, as a result of the long duration of driving the brushless motor, the shaft may be damaged.  
     SUMMARY OF THE INVENTION  
      In order to overcome the problems described above, preferred embodiments of the present invention provide a reliable and durable brushless motor. In addition, manufacturing of the brushless motor is effectively facilitated.  
      According to a preferred embodiment of the present invention, the protruding portion is formed on the shaft by a rolling process. According to another preferred embodiment of the present invention, a plurality of protruding portions may be circumferentially arranged on the shaft in an equally circumferentially spaced manner. The protruding portion protrudes radially outwardly from about 3% to about 5% of an outer diameter of a portion of the shaft where the ball bearing is attached. By virtue of this configuration, the manufacturing process of the motor is facilitated, and the durability of the shaft is preferably maintained. Additionally, axial positioning of the ball bearing is facilitated.  
      According to another preferred embodiment of the present invention, the gear is provided on an axial end portion of the shaft. The gear is arranged axially outside of the motor, and the protruding portion is provided axially between the gear and the ball bearing. Upon providing the gear on the end portion of the shaft, a force in the radial direction is applied to the shaft and stress is concentrated on a portion near where the protruding portion is arranged. In the present preferred embodiment of the present invention, the protruding portion is formed by rolling and the durability of the portion of the shaft is preferably maintained.  
      According to another preferred embodiment of the present invention, a plurality of ball bearings are arranged in an axially spaced manner, and at least one ball bearing arranged near the gear is secured to the shaft by press-fitting. By virtue of this configuration, a reliable and durable motor is provided.  
      Other features, elements, steps, processes, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic sectional view illustrating a brushless motor according to a preferred embodiment of the present invention.  
       FIG. 2A  is a magnified view illustrating a tip end portion of the shaft and a ball bearing provided on the brushless motor according to the present preferred embodiment of the present invention.  
       FIG. 2B  is a magnified view illustrating a tip end portion of a shaft and the ball bearing provided on a brushless motor according to the conventional art.  
       FIG. 3  illustrates a cross section of the shaft along a line X-X line shown in  FIG. 2A .  
       FIG. 4A  is a magnified view illustrating a tip end portion of the shaft and a ball bearing provided on the brushless motor according to another preferred embodiment of the present invention.  
       FIG. 4B  illustrates a cross section of the shaft along a line Y-Y shown in  FIG. 4A .  
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Referring to FIGS.  1  to  4 B, a brushless motor according to preferred embodiments of the present invention will be described in detail. It should be understood that in the explanation of the present invention, when positional relationships among and orientations of the different components are described as being such as top/bottom, up/down or left/right, positional relationships and orientations that are in the drawings are indicated, and positional relationships among and orientations of the components once having been assembled into an actual device are not indicated. Additionally, in the following description, an axial direction indicates a direction parallel to a rotation axis, and a radial direction indicates a direction perpendicular to the rotation axis.  
      Configuration of the Brushless Motor  
      Referring to  FIG. 1 , the configuration of the brushless motor according to a preferred embodiment of the present invention will be described in detail.  
      A bearing holder  10  has a substantially cylindrical shape with a through hole at a middle portion thereof. The bearing holder  10  is preferably formed by aluminum or zinc die casting. The bearing holder  10  includes an upper step portion  11  which is an upper portion of the bearing holder having an axially upwardly facing recess and a bottom step portion  12  which is a bottom portion of the bearing holder having an axially downwardly facing recess. An upper ball bearing  20  is arranged in the upper step portion  11 , and a bottom ball bearing  21  is arranged in the bottom step portion  12 . A substantially cylindrical shaft  30  is inserted into a hollow portion  13  of the bearing holder  10  and center openings of the ball bearings  20  and  21 . By virtue of the configuration described above, the shaft  30  is rotatably supported by the ball bearings  20  and  21 . The bottom ball bearing  21  has a first axial side (i.e., an axially upper side) and a second axial side (i.e., an axially lower side), and a resilient member  22 , such as an O-shape spring washer, is arranged axially between the first axial side of the bottom ball bearing  21  and the bearing holder  10 . The resilient member  22  supports a radially outward portion of the bottom ball bearing  21  and applies a pre-load to the ball bearing  21 .  
      A gear  31  defined by any suitable gear wheel such as a helical gear is arranged on a bottom portion of the shaft  30 . The gear  31  is formed on a bottom portion of the shaft  30 . The gear  31  may be provided integrally with, or separately from, the bottom portion of the shaft  30 . The gear  31  is engaged with another gear wheel provided on a device to which the brushless motor is installed, and therefore, torque generated by the brushless motor is transferred to the device.  
      In this preferred embodiment of the present invention, three first convex portions  14  and three second convex portions  15  are arranged radially outside of a bottom portion of the bearing holder  10 . The first convex portions  14  are axially upwardly arranged from the second convex portions  15 . The first convex portions  14  are arranged in an equally circumferentially spaced manner (i.e., at about 120°), and the second convex portions  15  are arranged in the same manner.  
      An attaching board  40  is arranged at a bottom end portion  16  of the bearing holder  10 . The attaching board  40  is processed by a method such as press working of a steel plate and is secured to the bottom end portion  16  by caulking, for example, or other suitable method. The attaching board  40  includes a mounting hole  41  used for mounting the brushless motor on an electronic device. The bearing holder  10  includes an annular convex portion  18  protruding axially downwardly from the bottom end portion  16  of the bearing holder  10 . The annular convex portion  18  is used for axial positioning of the brushless motor against the device to which the brushless motor is installed.  
      A stator  50  having an annular shape is abutted against and secured to an upper surface of a first convex portion  14  and an outer circumferential surface of a cylindrical body of the bearing holder  10 . In this preferred embodiment of the present invention, the first convex portion  14  includes a threaded hole, and the stator  50  includes a through hole which is axially aligned with the threaded hole. A screw  60  is inserted into and passes through the through hole, and is tightened to the threaded hole to secure the stator  50 . A circuit board  70 , such as a paper-phenol board, is secured to an upper surface of the second convex portion  15  by a screw  61 . The circuit board  70  includes a hall element  71  which detects a rotation speed of the brushless motor, and an integrated circuit (IC)  72  which processes a signal generated by the hall element  70 .  
      A rotor holder  80  having a substantially hollowed cylindrical shape is formed by such method as press working of the steel plate. The rotor holder  80  includes a hollow portion  81 , and an upper end portion of the shaft  30  is secured to the rotor holder  80  by press-fitting (i.e., the shaft  30  is pressed into the hollow portion  81  and secured by friction after the parts are pushed together) The rotor holder  80  and the shaft  30  are arranged to be coaxial with the stator  50  and radially covering the stator  50 . Furthermore, a substantially annular rotor magnet  90  is arranged at an inner surface of a cylindrical body  82  of the rotor holder  80  in a manner coaxial with the shaft  30  and radially facing the stator  50 . The rotor magnet  90  is secured to the cylindrical body  82  with an adhesive or the like.  
      When electricity is provided to the stator  50 , the stator  50  generates a magnetic field which interacts with the rotor magnet  90 . The interaction between the rotor magnet  90  and the magnetic field generates torque which rotates the brushless motor. In this preferred embodiment of the present invention, the bearing holder  10  and the stator  50  may be collectively referred to as a stationary member, and the rotor holder  80  and the rotor magnet  90  may be referred to as a rotor member. However, it should be noted that the components of the rotor member and/or the stationary member may be changed in accordance with a motor configuration.  
      Shaft and Bearing  
      Referring to  FIGS. 2A  to  4 B, the shaft  30  and the ball bearing  21  according to the present preferred embodiment of the present invention will be described in detail.  FIGS. 2A and 2B  are magnified views illustrating a tip end portion of the shaft  30  and the ball bearing  21 .  FIG. 2A  illustrates a preferred embodiment of the present invention, and  FIG. 2B  illustrates the conventional art.  FIG. 3  illustrates a cross section of the shaft  30  along a line X-X shown in  FIG. 2A .  FIGS. 4A and 4B  illustrate another preferred embodiment of the present invention.  FIG. 4B  illustrates a cross section of the shaft  30  along a line Y-Y shown in  FIG. 4A .  
      As illustrated in  FIG. 2A , the shaft  30  includes a plurality of protruding portions  32  circumferentially arranged at a position axially upward from the gear  31  of the shaft  30 . The axially second side of the bottom ball bearing  21  is abutted against the protruding portions  32  such that the ball bearing  21  and the shaft  30  are axially positioned. The protruding portions  32  are formed by rolling with a rolling mill such as knurling. Through the rolling process, the protruding portion is formed so as to extend in substantially circumferential and axial directions and not to include a recess extending radially inwardly into the shaft. By virtue of the configuration, the durability of the shaft  30  against the force directed in the radial direction is increased. Generally the durability of the shaft  30  is decreased through the cutting work. In the preferred embodiment of the present invention, however, the durability of the shaft  30  is increased through the rolling process.  
      When the gear  31  provided on the bottom portion of the shaft  30  engages with the gear wheel of the device, the force F directed in a radial direction illustrated in  FIGS. 2A and 2B  by arrows is applied to a bottom end portion of the shaft  30 . Therefore, stress is concentrated around the portion below ball bearing  21 . Without the protruding portions  32 , the shaft  30  may be damaged or broken by the force F at the bottom end portion. In the present preferred embodiment of the present invention, however, the protruding portions  32  formed by rolling are provided on the portion below the ball bearing  21 . Consequently, the durability of the shaft  30  is increased. Moreover, since the protruding portions  32  are formed by rolling, no burrs are produced during a forming process of the protruding potions  32 . By virtue of this configuration, it is possible to provide a reliable brushless motor.  
      As illustrated in  FIG. 2B , conventional protruding portions  33  are provided on the shaft by forming a hole  33   a  so as to deform a portion of the shaft surface. When the force F is applied, stress is concentrated at the portions where the holes are provided. As a result, the shaft may be damaged.  
      As illustrated in  FIG. 3 , the protruding portions  32  may protrude in the radial direction by about 3% to about 5% of an outer diameter of a bearing support portion  21   a  of the shaft  30 , arranged radially inward from the ball bearing  21 . By virtue of this configuration, the ball bearing  21  is axially positioned while a bottom surface of the ball bearing  21  is maintained substantially perpendicular to the shaft  30 . If the protruding portions  32  protrude less than about 3% of the outer diameter of the bearing support portion  21   a , the protruding portions  32  may extend into a curved, or chamfered, portion  21   b  of the ball bearing(shown in  FIG. 2A ). As a result, the ball bearing  21  may not be positioned axially and/or the bottom surface of the ball bearing  21  may be slanted. If the protruding portions  32  are formed to protrude more than about 5% of the outer diameter of the bearing support portion  21   a , an excessive force, which degrades deflection accuracy and/or roundness of the shaft  30 , is applied to the shaft  30 . Additionally, in order to form such protruding portions, a strong force is applied to a chucking tool which holds the shaft  30  during the rolling process. As a result of such a force, the chucking tool may be broken or damaged. With the protruding portions radially protruding in the range of about 3% to about 5% of the bearing supporting portion  21   a , it is possible to axially position the ball bearing  21  while preferably keeping the deflection accuracy and/or roundness of the shaft  30 . p In this preferred embodiment of the present invention, the outer dimension of the bearing support portion  21   a  is about 6 mm, i.e., the protruding portions protrude from about 0.2 mm to about 0.3 mm in the radial direction. Generally, the radial width of the curved portion of the ball bearing is less than about 0.2 mm.  
      In the above preferred embodiments of the present invention, a cross section in the radial direction of the protruding portion preferably has a substantially trapezoidal shape. However, the shape of the protruding portion may be semicircular, triangular, or the like.  
      In the preferred embodiments of the present invention, three protruding portions  32  are provided as shown in  FIG. 3 . However, the number of the protruding portions  32  provided on the shaft is not limited thereto. Moreover, the protruding portions may be formed by any suitable rolling process. For example, as shown in  FIG. 4B , a plurality of protruding portions  32   a  may be formed on the shaft by knurling or thread rolling.  
      While the present invention has been described above with respect to the preferred embodiments, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.