Abstract:
A starter for an internal combustion engine is provided which includes a pair of angular contact ball bearings used to support an armature shaft. The bearings are disposed close to opposed ends of a core of an armature. The core has disposed thereon coils which define a commutator on one of the ends of the core. One of the bearings is disposed within an inner periphery of the commutator. This structure results in a decreased interval between the bearings, thus ensuring stability in supporting the armature shaft during rotation, which results in decreased mechanical vibrations, magnetic noises, and wear of the bearings.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates generally to a starter working to start internal combustion engines, and more particularly to a support structure for an armature of a motor of such a starter. 
   2. Background Art 
   In recent years, demands for light weight, compact, and low cost structures of automotive starters have been getting severe all the more. In order to reduce the size, starters have become prevalent which are equipped with a high-speed motor and a reduction gear working to reduce the speed of the motor to produce an increased torque output, however, there has been an increasing need for a higher reduction gear ratio. This has resulted in an increase in speed of an armature of the motor up to 20000 rpm or more, thus requiring further improvement in balancing the armature mechanically. 
   The above, however, conflicts with reduction in manufacturing cost of the starters. The omission of the balancing operation will result in deflection of the armature shaft and vibrations of bearings during rotation of the motor, which leads to a change in air gap in the motor, thus resulting in mechanical and magnetic noises of the motor and wear of the bearings. 
   In order to avoid the above problems, Japanese Patent First Publication No. 11-117839 (U.S. Pat. No. 6,114,772 assigned to the same assignees as that of this application) teaches installation of two bearings close to an armature shaft of a starter motor to minimize mechanical deflection of the armature shaft. The starter motor, however, has a commutator which results in an increased interval between the bearings, thus encouraging ease of bending of the armature shaft. This requires the need for balancing the armature in a conventional manner, thus resulting in a difficulty in reducing the manufacturing costs. 
   SUMMARY OF THE INVENTION 
   It is therefore a principal object of the invention to avoid the disadvantages of the prior art. 
   It is another object of the invention to provide a light weight and low cost structure of a starter for internal combustion engines designed to ensure a desired operation of the starter without undesirable mechanical vibrations and/or magnetic noises. 
   According to one aspect of the invention, there is provided a starter for an internal combustion engine which comprises: (a) an armature made up of an armature shaft, a core, and coils, the core being made of a laminate of plates and installed on the armature shaft, the core having a first and a second end opposed to each other, the coils being disposed therearound and defining a surface commutator on the first end thereof, the surface commutator having an inner periphery formed therein; and (b) a first and a second ball bearing which work to support the armature shaft rotatably. The first and second ball bearings are disposed close to the first and second ends of the core. Specifically, the first and second ball bearings are arranged adjacent the core directly without any undesirable parts such as the commutator disposed between each of the bearings and the core, thus resulting in a decreased interval between the ball bearings, thereby minimizing deformation of the armature shaft during rotation thereof. The first ball bearing is disposed within the inner periphery of the surface commutator, which results in a further decrease in interval between the bearings, thereby increasing mechanical balance of the armature shaft during rotation. 
   In the preferred mode of the invention, the first and second ball bearings have inner peripheries fitted on the armature shaft and outer peripheries fitted on a frame surrounding the armature. 
   The commutator includes a conductor coil that is a portion of the coils of the armature. This facilitates ease of installation of the first ball bearing within the inner periphery of the commutator without need for increasing an inner diameter of the commutator. This is also advantageous to withstand the centrifugal force acting on the commutator. 
   The starter further comprises a thrust applying mechanism working to apply thrust to the first and second ball bearings to minimize clearances or play in the first and second ball bearings. This minimizes mechanical vibrations, magnetic noises, and/or wear of the bearings without need for balancing the armature. 
   The thrust applying mechanism may include an elastic member working to apply a spring pressure to brushes used to supply power to the surface commutator. For example, a brush spring may be used as the elastic member, thereby avoiding an increase in component parts of the starter. 
   The thrust applying mechanism may include a spring working to apply a spring pressure to brushes used to supply power to the surface commutator and a disc spring working to apply a spring pressure to the first and second bearings. The disc spring is simple in structure and inexpensive, thus minimizing an increase in production cost of the starter. 
   The first and second ball bearings are implemented by angular contact ball bearings which are suitable for minimizing the clearances in the bearings using the thrust acting on the bearings. Use of the angular contact ball bearings minimizes a shift in center of the armature shaft. 
   Ech of the angular contact ball bearings has an outer and an inner ring at least one of which is made by pressing a plate. This results in a decrease in manufacturing cost of the starter. 
   The starter further comprises a planetary reduction mechanism disposed within a starter housing, an output shaft, and a pinion stalled on the output shaft. The planetary reduction mechanism such as a planetary reduction gear works to reduce speed of the armature and transmit it to the output shaft. The pinion meshes with a ring gear of the engine when the engine is started. Use of the planetary reduction mechanism eliminates addition of bending moment of the ring gear to the armature shaft, thereby further decreasing the mechanical vibrations of the starter. 
   The starter further includes a cylindrical yoke, frames, and a brush holder. The yoke includes magnetic poles made of permanent magnets or electromagnets. The frames are so disposed on sides of the yoke as to surround the armature. The planetary reduction mechanism and the brushes are installed outside the frames and the yoke. Specifically, only the yoke and the frames are used to retain the bearings firmly, thus ensuring mechanical stability of the armature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
       FIG. 1  is a longitudinal sectional view which shows an internal structure of a starter according the invention; 
       FIG. 2  is a partial sectional view which shows an armature installed in the stator of  FIG. 1 ; 
       FIG. 3  is a side view of the armature as illustrated in  FIG. 2 ; 
       FIG. 4  is a perspective view which shows the armature as illustrated in  FIG. 2 ; and 
       FIG. 5  is a sectional view which shows an angular contact ball bearing used to support an armature shaft of the stator of FIG.  1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to  FIG. 1 , there is shown a starter according to the invention which may be used in starting automotive internal combustion engines. 
   The starter includes an armature  20  which consists of a core  21 , an armature shaft  22 , outer coils  23 , and inner coils  24 . The core  21  is made of a laminate of plates and affixed to the armature shaft  22  in a known manner. The outer and inner coils  23  and  24  are, as clearly shown in  FIGS. 3 and 4 , disposed in grooves formed in the periphery of the core  21 , respectively. The outer and inner coils  23  and  24  are, as clearly shown in  FIG. 2 , of a substantially C-shape and joined together at ends  23   a  and  24   a  to form a winding as a whole. 
   The outer coils  23  define an end surface  25  which works as a surface commutator to perform commutation together with brushes  51 . 
   The stator also includes angular contact ball bearings  30  and  40  which are, as clearly shown in  FIG. 2 , installed on the armature shaft  22  close to opposed end surfaces of the core  21  and surrounded by the ends  24   a  of the inner coils  24   a . Each of the bearings  30  and  40  is preferably located at an interval away from one of the end surfaces of the core  21  which is shorter than the width of the bearings  30  and  40 . 
   The angular contact ball bearings  30  and  40 , as clearly shown in  FIG. 1 , have outer peripheries secured to a cylindrical yoke  10  through frames  11  and  12 , respectively. The bearing  40  is, as clearly shown in  FIG. 5 , made up of an outer ring (also called an outer race)  41 , an inner ring (also called an inner race)  42 , balls  43 , and a cage (not shown). The outer and inner rings  41  and  42  each may be made by pressing a plate. Application of thrust loads to the bearing  40  will causes the outer and inner rings  41  and  42  to push the balls  43  at round portions  41   a  and  42   a  tightly to carry the thrust load and eliminate clearances between the outer and inner rings  41  and  42  and the balls  43 . The bearing  30  has the same structure as that of the bearing  40 , and explanation thereof in detail will be omitted here. 
   The yoke  10  has magnetic poles  14  formed by permanent magnets or electromagnets. The yoke  10  and the armature  20  constitute a stator motor. The bearings  30  and  40  have the inner rings  42  affixed to the armature shaft  22  and the outer rings  41  fitted within inner peripheral walls of the frames  11  and  12  to be slidable in lengthwise directions of the armature shaft  22 . The frame  11  has a shoulder  11   a  which holds the bearing  30  from moving in one of the lengthwise directions of the armature shaft  22  (the left direction as viewed in FIG.  1 ). 
   Two types of thrust loads are applied to the bearings  30  and  40 . One is produced by a spring pressure of a coil spring  52  (typically called a brush spring) which works to push the brushes  51  disposed within a brush holder  53  and act on the bearing  30 . The second is produced by a spring pressure of a disc spring  71  acting on the outer ring  41  of the ball bearing  40  directly. Specifically, the second thrust load is transmitted from the outer ring  41  of the bearing  40 , to the balls of the bearing  40 , to the inner ring  42  of the bearing  40 , to the armature shaft  22 , to the inner ring  42  of the bearing  30 , the balls of the bearing  30 , to the outer ring  41  of the bearing  30 , and to the frame  11 . The torque produced by the armature  20  is transmitted to a pinion  74  through a planetary reduction gear  61 , a clutch  72 , and an output shaft  73 . A magnet switch  80  is installed adjacent the stator motor (i.e., the armature  20 ). The magnet switch  80  works to open or close contacts leading to a battery (not shown) and the armature  22 , respectively, to turn on or off the stator motor. The magnet switch  80  also works to move a connecting rod  81  and a stopper  82  to hold the pinion  74  from rotating and to move the output shaft  73  in a lengthwise direction thereof to establish engagement with a ring gear (not shown) connected to the engine. 
   In operation, when a key switch (not shown) such as an ignition switch of the engine is turned on, the magnet switch  80  attracts the connecting rod  81 , so that the stopper  82  engages a gear installed on the pinion  74  to hold the pinion  74  from rotating. The magnet switch  80  also closes the contacts to supply the power to the armature  20 , so that the armature  20  starts to rotate, thereby rotating the output shaft  73 . The output shaft  73  meshes with the pinion  74  through spiral splines, so that the pinion  74 , which is prohibited by the stopper  82  from rotating, moves in the left direction, as viewed in  FIG. 1 , to engage the ring gear for stating the engine. This operation is discussed in detail in Japanese Patent First Publication No. 10-115274 and U.S. Pat. No. 6,114,772, issued Sep. 5, 2000, assigned to the same assignees as that of this application, disclosures of which are incorporated herein by reference. 
   The angular contact ball bearings  30  and  40  are, as described above, disposed inside the coils  23  and  24  (i.e., the surface communtator). In other words, the bearings  30  and  40  are located close to the core  21 , thereby resulting in a greatly decreased deflection of the armature shaft  22  during rotation thereof. The springs  52  and the  71  apply the thrust loads to the bearings  30  and  40  to decrease a bearing clearance to zero (0), thereby eliminating any play of the bearings  30  and  40  during rotation of the armature shaft  22 . This eliminates the need for physically balancing the armature  20 . The starter is usually not used frequently, that is, employed only in starting the engine. A certain decrease in load capacity of the bearings  30  and  40  is, therefore, not objectionable in use of the stator, thus permitting at least one of the outer and inner rings  41  and  42  of each of the bearings  30  and  40  to be made by pressing a plate, which results in a decrease in manufacturing cost of the starter. 
   While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.