Abstract:
A screw mounting hole and a female screw part are formed in a bearing part of a gear cover, and the screw insertion hole is formed in the idle shaft. The idle shaft is fixed to the gear cover by inserting a fixing screw through the screw mounting hole and screwing it into the female screw part through the screw insertion hole. A positioning groove in a predetermined position in relation to the screw insertion hole is formed in the end face of the idle shaft. When the positioning groove is disposed perpendicularly to an engine mounting surface and the idle shaft is fitted to the gear cover, the screw mounting hole is opposed to the screw insertion hole and, when the fixing screw is fitted, the confirmation of hole positions and the aligning of holes can be eliminated.

Description:
TECHNICAL FIELD 
     The present invention relates to a starter motor mounted on an engine of an automobile or the like, and more particularly, to a positioning structure of an idle shaft that supports an idle gear meshing with a ring gear of an engine. 
     BACKGROUND ART 
     In engines used in automobiles, two-wheeled motor vehicles, and large generators, a starting operation is generally performed by a starter motor mounted on an engine.  FIG. 7  is a perspective view showing an entire configuration of a starter motor  151  described above. The starter motor  151  is mounted on an engine (not shown) at an engine mounting surface  166  on a front side in the figure. The starter motor  151  is provided with an idle gear (middle gear)  152  that is movable in an axial direction and provided in a manner capable of meshing with and being separated from a ring gear (not shown) of the engine. The idle gear  152  is supported by an idle shaft  153  in a manner freely rotatable and movable in the axial direction, and meshes with a pinion (not shown) that is rotationally driven by a motor  154 . The pinion is connected to a rotational shaft of the motor  154  via an overrunning clutch. 
     The idle gear  152  meshes with the ring gear when the engine is started. Along with turning ON of an ignition key switch, the idle gear  152  moves in the axial direction (an upward direction in  FIG. 7 ) from a rest position to mesh with the ring gear, and rotates a crankshaft of the engine. If the engine is started and the number of revolutions of the pinion becomes higher than the number of revolutions of the motor  154 , the overrunning clutch achieves an overrunning state, and the idle gear  152  and the pinion are in an idle running state. After the engine is started, the idle gear  152  moves in the axial direction, and is separated from the ring gear. In this manner, meshing between both of the gears is released, and the idle gear  152  returns to the rest position. 
     On the other hand, in the starter motor  151  described above, the idle shaft  153  is supported by bearing sections  156  and  157  formed on a gear cover  155 , and is fixed on the gear cover  155  by an idle shaft fixing screw  158  (hereinafter abbreviated to the fixing screw  158 ) as shown in  FIG. 7 . In addition, to a top end section of the idle shaft  153 , a dustproof cap  159  made of rubber is attached.  FIG. 8  is an explanatory view showing a configuration of a screw fixing section of the idle shaft  153 . As shown in  FIG. 8 , a female screw section  161  and a screw mounting hole  162  are formed on the same axis line on the bearing section  157  of the gear cover  155 . A screw insertion hole  163  is formed on the idle shaft  153  corresponding to the female screw section  161  and the screw mounting hole  162 . By screwing the fixing screw  158  in the female screw section  161  via the screw insertion hole  163  from the screw mounting hole  162 , the idle shaft  153  is fixed on the gear cover  155 . 
       FIGS. 9 and 10  are explanatory views showing processes of assembling and fixing the idle shaft  153  to the gear cover  155 .  FIG. 11  is a process chart showing a main assembling process in the above case. In the starter motor  151 , an idle gear assembly  165  including the idle gear  152  is arranged on an idle gear mounting section  164  of the gear cover  155 . In this state, the idle shaft  153  is inserted through an axis hole formed on the bearing section  156  from an upper end side of the gear cover  155  in the figures. At this time, the idle shaft  153  is inserted in the gear cover  155  in a manner such that the screw mounting hole  162  of the gear cover  155  and the screw insertion hole  163  of the idle shaft  153  match with each other to a certain extent (step ( 1 )). 
     Next, the idle shaft  153  is inserted through the idle gear assembly  165 . Thereafter, the idle shaft  153  is inserted in an axis hole formed on the bearing section  157 . Then, positions of the screw mounting hole  162  and the screw insertion hole  163  are checked. In case these positions do not match with each other, the idle shaft  153  is rotated so that the holes are matched with each other (step ( 2 )). Thereafter, as shown in  FIG. 9 , the fixing screw  158  is inserted from the screw mounting hole  162 , and then the fixing screw  158  is screwed in the female screw section  161  via the screw insertion hole  163  (step ( 3 )). In this manner, after the idle shaft  153  is fixed on the gear cover  155 , the dustproof cap  159  is mounted (step ( 4 )).
     Patent Document 1: Jap. Pat. Appln. Laid-Open Publication No. 2003-239834   

     However, in the starter motor  151  described above, work is carried out after positions of the screw mounting hole  162  and the screw insertion hole  163  are made to match with each other to a certain extent when the idle shaft  153  is inserted. Nevertheless, when the idle shaft  153  is assembled, both of the holes need to be matched with each other for fixing the shaft with a screw. Then, if the positions of the holes do not match with each other to some extent at the time of screwing, the positions of the holes need to be matched again in a state where the idle shaft  153  is inserted. That is, checking of the positions of the holes and alignment of the holes is necessary at the time of fixing the shaft. Therefore, there has been a problem in which workability is not excellent and also additional steps are necessary to thereby increase the manufacturing cost. In particular, in the starter motor  151  in  FIG. 7 , the screw insertion hole  163  is positioned at 30° from the engine mounting surface  166  in consideration of interference with other sections. Therefore, alignment is difficult in comparison with a case in which the holes are at positions, such as positions in the right angle, where a reference can be easily set, and improvement in this respect has been required. 
     In addition, the alignment of holes described above is carried out in a manner such that a P part in  FIG. 10  is held to rotate the idle shaft  153  while looking through the screw mounting hole  162  in order to match the screw insertion hole  163  with the screw mounting hole  162 . With respect to this point, after the idle shaft  153  is inserted, a width of the P part is narrow and the idle shaft  153  is difficult to rotate, and also a part between the idle gear  152  and the idle shaft  153  is a grease application section which is not preferably touched by a worker for quality reasons. Further, a hand of the worker is stained at the time of alignment, which is not preferable in view of a working environment. In addition, in case the insertion and the screwing of the idle shaft  153  are carried out in an automated line, a step of detecting whether the holes match with each other or not is difficult to be set, and has been an obstacle to automation of assembly of the starter motor  151 . 
     An object of the present invention is to provide a starter motor enabling screwing of an idle shaft to be fixed on a gear cover in a manner such that holes can be aligned easily and accurately when an idle shaft is mounted and such alignment is not carried out again. 
     SUMMARY OF THE INVENTION 
     A starter motor of the present invention includes an idle gear that meshes with a ring gear of an engine in a manner rotationally driven by a motor and moving in an axial direction, an idle shaft that supports the idle gear in a manner such that the idle gear can be rotated freely and moved in the axial direction, a covering member that is mounted to the engine and supports both end parts of the idle shaft, and a shaft fixing member that is inserted from a mounting hole formed on the covering member to an insertion hole provided on the idle shaft. The idle shaft is provided with a positioning part that is arranged at a predetermined position to make the mounting hole and the insertion hole face each other in a state in which the shaft fixing member can be inserted. 
     In the present invention, the positioning part is provided on the idle shaft, and is arranged at the predetermined position. In this manner, the idle shaft can be mounted in a state in which the mounting hole on a covering member side and the insertion hole on an idle shaft side face each other. Accordingly, positioning of the mounting hole and the insertion hole can be carried out easily and accurately at the time of mounting the idle shaft. For this reason, the shaft fixing member can be mounted without checking whether the mounting hole and the insertion hole match with each other or not, and checking of positions of the holes and alignment of the holes are not necessary at the time of mounting the shaft fixing member. In addition, work for adjusting the positions of the holes by holding the idle shaft can also be eliminated. 
     In the above starter motor, the positioning part may be provided on an end surface of the idle shaft under a predetermined positional relationship with the insertion hole. In addition, the mounting hole and the insertion hole may be made facing each other by arranging the positioning part under a predetermined positional relationship with respect to an engine mounting surface of the gear cover. In this case, the mounting hole and the insertion hole may be made facing each other by arranging the positioning part at a position in the right angle with respect to the engine mounting surface of the gear cover. Alternatively, the mounting hole and the insertion hole may be made facing each other by allocating the positioning part at a position in parallel with the engine mounting surface of the gear cover. Further, a groove or a protrusion extending in a radial direction may be formed on the end surface of the idle shaft as the positioning part, and the groove or the protrusion may be provided at a position rotated from a position of the insertion hole for a predetermined angle in a circumferential direction with a center axis line of the idle shaft being a center. 
     In the above starter motor, a covering member mounted on one end side of the idle shaft may also be provided, and a junction section that is connected to the positioning part in a predetermined positional relationship may be provided on the covering member. In addition, when the covering member is mounted on the positioning part through the junction section, a positioning surface that is flush with the engine mounting surface of the gear cover may be provided on the covering member, and in this manner, the covering member can be used to align the mounting hole and the insertion hole. In this case, when the positioning surface is provided in a manner flush with the engine mounting surface, the mounting hole and the insertion hole may be made facing each other. 
     ADVANTAGES OF THE INVENTION 
     The starter motor of the present invention includes an idle gear that meshes with a ring gear of the engine and is supported by an idle shaft in a manner such that the idle gear is freely rotatable and movable in the axial direction, a covering member that supports both end parts of the idle shaft, and a shaft fixing member that is inserted from a mounting hole formed on the covering member to an insertion hole provided on the idle shaft. According to the starter motor of the present invention, the positioning part that is allocated in a predetermined position to make the mounting hole and the insertion hole face each other in a state in which the shaft fixing member can be inserted is provided on the idle shaft, therefore the mounting hole and the insertion hole can be aligned when the idle shaft is mounted. For this reason, in steps thereafter, checking of the positions of the holes and alignment of the holes become unnecessary, and workability in an assembly step is improved and also reduction in the number of steps can be attempted. In addition, work of adjusting the positions of the holes by holding the idle shaft becomes unnecessary, and a worker never touches the grease application section. In this manner, quality of a product is improved, and hands of the worker are not stained as well as the working environment can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a configuration of a starter motor adopting an idle shaft positioning structure of the present invention; 
         FIG. 2  is a bottom view (viewed from an angle X in  FIG. 1 ) of the starter motor of  FIG. 1 ; 
         FIG. 3  is an explanatory view showing shapes of an idle shaft and a dustproof cap of the starter motor of  FIG. 1  in comparison with conventional shapes, where (a) and (b) indicate configurations of the idle shaft and the dustproof cap, respectively; 
         FIG. 4  is an explanatory view showing a state in which the idle shaft is mounted on a gear cover; 
         FIG. 5  is an explanatory view showing a step of assembling and fixing the idle shaft in the gear cover; 
         FIG. 6  is a process chart showing a main assembly step if the idle shaft is assembled and fixed in the gear cover; 
         FIG. 7  is a perspective view showing an entire configuration of a conventional starter motor; 
         FIG. 8  is an explanatory view showing a configuration of a screw fixing section of an idle shaft in the starter motor in  FIG. 7 ; 
         FIG. 9  is an explanatory view showing a conventional step of assembling and fixing the idle shaft in the gear cover; 
         FIG. 10  is an explanatory view showing a conventional step of assembling and fixing the idle shaft in the gear cover; and 
         FIG. 11  is a process chart showing a main assembly step in case the idle shaft is assembled and fixed in the gear cover in the conventional starter motor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, the present invention will be described in greater detail by referring to the accompanying drawings.  FIG. 1  is a cross-sectional view showing a configuration of a starter motor adopting an idle shaft positioning structure of the present invention,  FIG. 2  is a bottom view (viewed from an angle X in  FIG. 1 ) of the starter motor of  FIG. 1 . The electric starter motor  1  of  FIG. 1  is used for starting an automotive engine, giving rotations to a resting engine required for intake, atomization, compression and ignition of fuel. 
     Roughly speaking, the electric starter motor  1  comprises a motor section  2 , a gear section  3 , a magnet switch section  4 , a case section  5  and an idle section  6 . In the motor section  2 , there is provided a motor (electric motor)  11  as a driving source, and in the gear section  3 , there are provided a planetary gear mechanism  12 , an overrunning clutch  13  and a pinion  14  as reduction gears. In the idle section  6 , there is provided an idle gear  15  engaging with the pinion  14 . The idle gear  15  is mounted so as to be movable axially (in the left and right directions in the figure), and when moving in the left direction in the figure, the idle gear  15  engages with a ring gear  16  of the engine. The torque of the motor  11  is transmitted to the pinion  14  via the planetary gear mechanism  12  and the overrunning clutch  14 , and then, from the idle gear  15  to the ring gear  16 , starting the engine. 
     The motor  11  is configured to arrange an armature  22  rotatably within a cylindrical motor housing  21 . The motor housing  21  acts also as the yoke of the motor  11  and is made of a magnetic metal such as iron. A metallic end cover  23  is mounted to the right end section of the motor housing  21 . On the other hand, a gear cover (cover member)  24  of the case section  5  is mounted to the left end section of the motor housing  21 . The end cover  23  is secured to the gear cover  24  by a set bolt  25 , and the motor housing  21  is secured between the end cover  23  and the gear cover  24 . 
     A plurality of permanent magnets  26  are secured to the inner circumferential surface of the motor housing  21  in a circumferential direction, and an armature  22  is provided inside each of the permanent magnets  26 . The armature  22  is composed of an armature core  28  secured to a motor shaft  27  and an armature coil  29  wound on the armature core  28 . The right end section of the motor shaft  27  is supported rotatably by a metal bearing  31  mounted on the end cover  23 . On the other hand, the left end section of the motor shaft  27  is supported rotatably by an end section of a drive shaft (output shaft)  32  to which the pinion  14  is mounted. In the right end section of the drive shaft  32  a bearing section  33  is provided concavely, and the motor shaft  27  is supported rotatably by a metal bearing  34  mounted to the bearing section  33 . 
     On one end side of the armature core  28 , there is arranged adjacently a commutator  35  secured to the motor shaft  27  with being fitted thereon. A plurality of commutator pieces  36  made of a conductive material are fitted to the outer circumferential surface of the commutator  35 , and the end section of the armature coil  29  is secured to each of the commutator pieces  36 . A brush holder  37  is mounted to the left end section of the motor housing  21 . Four brush holding sections  38  are arranged in the brush holder  37  with being spaced in a circumferential direction. A brush  39  is contained in each brush holding section  38  so as to be able to appear freely. The projecting distal end (inner diameter side distal end) of the brush  39  is in sliding contact with the outer circumferential surface of the commutator  35 . 
     To the rear end side of the brush  39 , there is mounted a pig tail (not shown), which is connected electrically to a conductive plate  41  of the brush holder  37 . A switch section  42  is provided on the conductive plate  41 , and when a switch plate  43  comes into contact with the conductive plate  41 , an electric connection is made between a power source terminal  44  and the brushes  39 , supplying electric power to the commutator  35 . The switch plate  43  is mounted to a switch shaft  45 , and when the magnet switch section  4  turns on electricity, the switch shaft  45  moves to the left to bring the switch plate  43  into contact with the conductive plate  41 . 
     In the planetary gear mechanism  12  of the gear section  3 , there are provided an internal gear unit  46  and a drive plate unit  47 . The internal gear unit  46  is secured to the right end side of the gear cover  24 , and on the inner circumferential side thereof, an internal gear  48  is formed. A metal bearing  49  is contained in the center of the internal gear unit  46 , supporting the right end side of the drive shaft  32  rotatably. The drive plate unit  47  is secured to the right end side of the drive shaft  32 , and three planetary gears  51  are mounted with being equally spaced. The planetary gears  51  are supported rotatably by a support pin  53  secured to a base plate  52  via a metal bearing  54 . The planetary gears  51  engage with the internal gear  48 . 
     In the left end side of the motor shaft  27 , a sun gear  55  is formed. The sun gear  55  engages with the planetary gears  51 , and the planetary gears  51  rotate and revolve between the sun gear  55  and the internal gear  48 . When the motor  11  is operated, the sun gear  55  rotates together with the motor shaft  27 , and the rotations of the sun gear  55  are accompanied by the revolutions of the planetary gears  51  around the sun gear  55  with the planetary gears  51  engaging with the internal gear  48 . Thereby, the base plate  52  secured to the drive shaft  32  is rotated, transmitting the decelerated rotations of the motor shaft  27  to the drive shaft  32 . 
     The overrunning clutch  13  transmits the rotations decelerated by the planetary gear mechanism  12  to the pinion  14  in one rotation direction. The overrunning clutch  13  is configured to arrange a roller  58  and a clutch spring  59  between a clutch outer  56  and a clutch inner  57 . The clutch outer  56  comprises a boss section  56   a  and a clutch section  56   b , and the boss section  56   a  is mounted to a helical spline section  61  of the drive shaft  32 . On the inner circumferential side of the boss section  56   a , there is formed a spline section  62  engaging with the helical spline section  61 . Thereby, the clutch outer  56  is made movable axially on the drive shaft  32  along the helical spline section  61 . 
     A stopper  63  is mounted to the drive shaft  32 . The stopper  63  is hindered from moving axially by a circlip  64  fitted to the drive shaft  32 . One end side of a gear return spring  65  is attached to the stopper  63 . The other end side of the gear return spring  65  is in contact with the inner end wall  66  of the boss section  56   a . The clutch outer  56  is pushed to the right by this gear return spring  65 , and at normal times (at the time of no power distribution), the clutch outer  56  is held while being in contact with a clutch stopper  67  secured to the gear cover  24 . 
     On the inner circumference of the clutch section  56   b  of the clutch outer  56 , there is provided a clutch inner  57  formed integrally with the pinion  14 . A plurality of pairs of rollers  58  and clutch springs  59  are arranged between the clutch outer  56  and clutch inner  57 . In addition, on the outer circumference of the clutch section  56   b , a clutch cover  68  is provided, and a clutch washer  69  is fitted between the left end surface of the clutch section  56   b  and the clutch cover  68 . By this clutch washer  69 , the roller  58  and the clutch spring  59  are contained on the inner circumferential side of the clutch section  56   b  with being hindered to move axially. 
     The inner circumferential wall of the clutch section  56   b  is formed as a cam surface including a cuneiform slope section and a curved section. The roller  58  is usually pushed by the clutch spring  59  toward the curved section side. When the clutch outer  56  rotates and the roller  58  is interposed between the cuneiform slope section and the outer circumferential surface of the clutch inner  57  against the pushing force of the clutch spring  59 , the clutch inner  57  rotates together with the clutch outer  56  via the roller  58 . Thereby, when the motor  11  is operated and the drive shaft  32  rotates, the rotations thereof are transmitted from the clutch outer  56  via the roller  58  to the clutch inner  57 , rotating the pinion  14 . 
     On the contrary, when the engine is started and the clutch inner  57  rotates faster than the clutch outer  56 , the roller  58  moves to the curved section side, bringing the clutch inner  57  into an idle running state to the clutch outer  56 . That is, when the clutch inner  57  comes into an overrunning state, the roller  58  is not interposed between the cuneiform slope section and the outer circumferential surface of the clutch inner  57  and the rotations of the clutch inner  57  are not transmitted to the clutch outer  56 . Accordingly, even if the clutch inner  57  is rotated faster from the engine side after the engine start, the rotations thereof are interrupted by the overrunning clutch  13  and are not transmitted to the motor  11  side. 
     The pinion  14  is a steel member formed by cold forging and meshes with the idle gear  15 . The pinion  14  is formed integrally with the clutch inner  57 , and a gear section  71  and a boss section  94  are formed on the left side of the clutch inner  57 . An outer diameter of the boss section  94  is smaller than a root outer diameter of the gear section  71 , and the pinion  14  can be easily cold-forged. By cold-forging the pinion  14 , accuracy of a dimension in an axial direction of the gear section  71  is improved. Thereby, a gap between parts, such as between the pinion  14  and the idle gear  15 , can be made smaller, and wearing away and breakage of the parts can be prevented. In addition, by forming the gear section  71  by cold forging, work hardening is generated and strength of the gear section  71  is increased, and thereby strength of a gear connecting section can be increased. 
     A pinion washer  72  made of steel is mounted on the boss section  94 . The pinion washer  72  is fixed by the C-ring  73  mounted on the boss section  94  in a manner such that disconnection is prevented in the axial direction. An outer circumferential section of the pinion washer  72  contacts with a left side surface of the idle gear  15 . In this manner, when the pinion  14  moves to a rightward direction, the idle gear  15  also moves to the rightward direction, and the idle gear  15  is separated from the ring gear  16  after the engine is started. 
     On the inner circumferential side of the pinion  14 , there are formed a shaft hole  74  and a spring holding section  75 . In the shaft hole  74 , a pinion gear metal  76  is fitted, and the pinion  14  is supported rotatably by the drive shaft  32  via the pinion gear metal  76 . The spring holding section  75  is formed on the inner circumferential side of the clutch inner  57 , and the stopper  63  and the gear return spring  65  are held therein. 
     The magnet switch section  4  is arranged concentrically with the motor  11  and the planetary gear mechanism  12  on the left side of the planetary gear mechanism  12 . The magnet switch section  4  comprises a steel secured section  77  secured to the gear cover  24  and a movable section  78  arranged movably in the left and right directions along the drive shaft  32 . In the secured section  77 , there are provided a case  79  secured to the gear cover  24 , a coil  81  held in a case  79  and a stationary iron core  82  mounted to the inner circumferential side of the case  79 . In the movable section  78 , there is provided a movable iron core  83  to which the switch shaft  45  is mounted, and on the inner circumferential side of the movable iron core  83 , a gear plunger  84  is mounted. On the outer circumferential side (lower end side in the figure) of the movable iron core  83 , a switch return spring  90  is fitted. The other end side of the switch return spring  90  is in contact with the gear cover  24 , and the movable iron core  83  is pushed to the right. 
     To the inner circumference of the movable iron core  83 , a bracket plate  85  is secured further. One end of a plunger spring  86  is secured to the bracket plate  85  by caulking. When the ignition key switch is turned OFF (in the state of  FIG. 1 ), the other end of the plunger spring  86  contacts with a gear plunger  84 , and the gear plunger  84  is pushed by the plunger spring  86  to the left. The gear plunger  84  is mounted axially movably to the drive shaft  32 , and a slide bearing  87  is provided between the gear plunger  84  and the inner circumferential surface of the movable iron core  83 . 
     The case section  5  is provided with the aluminum die-cast gear cover  24 , and the left end side of the drive shaft  32  is supported rotatably by the gear cover  24  via a metal bearing  88 . To the gear cover  24 , there is further mounted an idle shaft  89  supporting the idle gear  15 . The left end side of the idle shaft  89  is retained by an idle shaft stopper (not shown). Within the gear cover  24 , as described above, the synthetic resin (for example, glass-fiber-reinforced polyamide) clutch stopper  67  and the case  79  are secured, and to the right end side thereof, the motor housing  21  and the end cover  23  are secured by the set bolt  25 . 
     The idle section  6  is provided with the idle gear  15 . The gear cover  24  is provided with an idle gear mounting section  100 , and the idle gear  15  is arranged in the idle gear mounting section  100  in a state where the idle gear  15  is supported by the idle shaft  89 . The idle gear  15  is provided with a gear section  92  and a boss section  93 , and the gear section  92  meshes with the gear section  71  of the pinion  14 . A collar  95  made of synthetic resin (for example, glass fiber reinforced polyamide) is fitted on the boss section  93 . The collar  95  includes a flange section  95   a  and a boss section  95   b , and an outer circumference section of the flange section  95   a  is interposed between an end surface of the idle gear  15  and the clutch cover  68 . Grease is applied as lubricant between the collar  95  and the clutch cover  68  and between the collar  95  and the idle gear  15  for improving a sliding property. An end part of the boss section  95   b  contacts with a C-ring  96  attached to the boss section  93 . The collar  95  is fixed by the C-ring  96  in a manner such that disconnection is prevented in the axial direction, and forms an idle gear assembly  97  together with the idle gear  15 . 
     The idle gear  15  is supported by the idle shaft  89  in a manner freely rotatable through a metal bearing  91 . The idle shaft  89  is supported by the bearing sections  98  and  99  provided on the gear cover  24 . An axis opening  101  and an axis hole  102  are formed on the bearing sections  98  and  99 . The idle shaft  89  is inserted through the axis opening  101  in a state in which the idle gear  15  is supported and further inserted into the axis hole  102 . An end part of the idle shaft  89  on a bearing section  98  side is attached with a dustproof cap (covering member)  103  made of rubber. An end part of the idle shaft  89  on a bearing section  99  side is formed with a screw insertion hole  104 . The bearing section  99  is formed with a female screw section  105  and a screw mounting hole  106  on the same axis line similar to  FIG. 8 . By screwing an idle shaft fixing screw  107  (an idle shaft fixing member, hereinafter abbreviated to the fixing screw  107 ) from the screw mounting hole  106  to the female screw section  105  through the screw insertion hole  104 , the idle shaft  89  is fixed on the gear cover  24 . 
     On an end surface (left end surface in the figure) on a bearing section  98  side of the idle shaft  89 , a positioning groove (positioning part)  108  is recessed for alignment between the screw mounting hole  106  and the screw insertion hole  104 . In addition, in the dustproof cap  103 , a fitting protrusion  109  (junction part) is provided in a projecting manner in accordance with the positioning groove  108 .  FIG. 3  is an explanatory view showing shapes of the idle shaft  89  and the dustproof cap  103  in comparison with conventional shapes, and (a) indicates a configuration of the idle shaft  89  and (b) indicates a configuration of the dustproof cap  103 . A left side in  FIG. 3  is the configuration of conventional configurations and a right side in  FIG. 3  is that of the present invention. In addition,  FIG. 4  is an explanatory view showing a state in which the idle shaft  89  is mounted on the gear cover  24  (the idle gear assembly  97  is omitted to clearly show a mounting structure of the idle shaft  89 ). 
     As clearly seen from a plan view shown on a top part of the right figure of  FIG. 3(   a ), the positioning groove  108  (center line O 2 ) and the screw insertion hole  104  (center line O 1 ) are formed at positions shifted for only an angle θ 1  (60° here) with a center axis line O S  of the idle shaft  89 . Here, the screw mounting hole  106  is located at a position of 30° (O 2 ) with respect to an engine mounting surface  111  (Y) of the gear cover  24 . Therefore, when the idle shaft  89  is mounted on the gear cover  24 , and the positioning groove  108  is arranged in the right angle (θ 3 ) with respect to the engine mounting surface  111  as shown in  FIG. 4 , the screw mounting hole  106  and the screw insertion hole  104  just face each other (θ 3 =θ 1 +θ 2 ; 90°=60°+30°). 
     In addition, as understood from a plan view shown in a bottom part of the right figure of  FIG. 3(   b ), a notched surface (positioning surface)  112  of the dustproof cap  103  is formed at a position of 90° (θ 4 ) with respect to the fitting protrusion  109 . Accordingly, when the dustproof cap  103  is mounted on the idle shaft  89  in a manner that the fitting protrusion  109  is fitted with the positioning groove  108 , the notched surface  112  of the dustproof cap  103  is flush with the engine mounting surface  111  (θ 4 =θ 3 ). 
     Now, the starting operation of an engine using such an electric starter motor  1  will be described. First, as shown in  FIG. 1 , when the ignition key switch of a car is turned OFF, the clutch outer  56  contacts the clutch stopper  67  by the pushing force of the gear return spring  65 . At this time, the switch plate  43  is spaced from the conductive plate  41 , supplying no current to the motor  11 . Further, the idle gear  15  is in the disengagement position on the right and is disengaged from the ring gear  16 . On the other hand, as shown in  FIG. 4 , when the ignition key switch is turned ON, the idle gear  15  moves to the left, engaging with the ring gear  16 . 
     That is, when the ignition key switch is turned ON, current flows first to the coil  81 , creating suction at the magnet switch section  4 . When the coil  81  is excited, a magnetic path extending through the case  79  and the stationary iron core  82  is formed, sucking the movable iron core  83  to the left. When the movable iron core  83  moves to the left against the pushing force of the switch return spring  90 , the switch shaft  45  moves also to the left, bringing the switch plate  43  into contact with the conductive plate  41  to close a contact. Thereby, an electric connection is made between the power source terminal  44  and the brush  39 , supplying power to the commutator  35  to start the motor  11  and rotate the armature  22 . In addition, the bracket plate  85  moves also to the left, thereby compressing the plunger spring  86 . 
     When the armature  22  is rotated, the drive shaft  32  is rotated via the planetary gear mechanism  12 . The rotations of the drive shaft  32  are accompanied by the rotations of the clutch outer  56  mounted to the helical spline section  61 . The twisting direction of the helical spline section  61  is set in consideration of the rotation direction of the drive shaft  32 . As the clutch outer  56  rotates faster, the clutch outer  56  moves to the left along the helical spline section  61  (rest position→operation position) due to the inertial mass thereof. When the clutch outer  56  protrudes to the left, the pinion  14  also moves to the left together with the clutch outer  56 . At this time, the gear return spring  65  is also compressed by being pushed by the clutch outer  56 . 
     When the clutch outer  56  moves to the left, the idle gear  15  also moves to the left by being pushed by the clutch outer  56 , engaging with the ring gear  16 . When the idle gear  15  engages with the ring gear  16 , the rotations of the motor  11  are transmitted to the ring gear  16 , rotating the ring gear  16 . The ring gear  16  is connected to a crankshaft of the engine. The rotations of the ring gear  16  are accompanied by the rotations of the crankshaft, starting the engine. When the engine is started, the pinion  14  is rotated with a high rotation speed by the ring gear  16  via the idle gear  15 . However, the rotations thereof are not transmitted to the motor  11  side by the action of the overrunning clutch  13 . 
     In addition, when the engine is started and the idle gear  15  rotates at a high speed, a difference in the numbers of revolutions due to an overrunning condition is generated between the clutch cover  68  and the idle gear  15  and the pinion  14 . By the difference in the numbers of revolutions, the collar  95  interposed between the clutch cover  68  and the idle gear  15  slides and contacts with the clutch cover  68  and the idle gear  15 . At that time, since the collar  95  is formed with synthetic resin and grease is applied thereon, ablation of the clutch cover  68 , and seizing of each member hardly occurs. 
     Further, when the clutch outer  56  moves to the left, the gear plunger  84  moves to the left by the pushing force of the compressed plunger spring  86 , and then contacts with the right end surface of the clutch outer  56 . At this time, the plunger spring  86  goes into a natural length state, creating a small gap between the gear plunger  84  contacting with the clutch outer  56  and the plunger spring  86 . 
     When the engine is started, the pinion  14  is rotated with a high rotation speed, and the overrunning clutch  13  is rotated in an idle running direction. When the overrunning clutch  13  is rotated in the idle running direction, idle running torque is created in the clutch, applying torque called cutting torque to the clutch outer  56 . This torque creates rightward thrust force in the clutch outer  56  via the helical spline section  61 , moving the clutch outer  56  to the right. As a result, the idle gear  15  may be disengaged from the ring gear  16 . Thus, in the electric starter motor  1 , the clutch outer  56  is held by the gear plunger  84  in the operated position, regulating the rightward movement of the idle gear  15  to prevent the idle gear  15  from being disengaged. 
     On the other hand, when the ignition key switch is turned OFF after the engine has been started, the power distribution to the magnet switch section  4  is stopped, and the suction thereof disappears. Then, the bracket plate  85  is pushed by the pushing force of a switch return spring  90  to the right, moving the movable iron core  83  held on the left by the suction of the stationary iron core  82  to the right. When the movable iron core  83  moves to the right, the switch shaft  45  also moves to the right, separating the switch plate  43  from the conductive plate  41  to open the contact. Thereby, the power supply to the motor  11  is shut off, stopping the rotations of the drive shaft  32  to also stop the rotations of the clutch outer  56 . 
     When the rotations of the clutch outer  56  are stopped, the axial moving force due to the inertial mass thereof also disappears. Thus, by the pushing force of the compressed gear return spring  65 , the clutch outer  56  moves to the right from the operated position to the rest position along the helical spline section  61 . At this time, the gear plunger  84  is also pushed by the clutch outer  56  and returns to the state of  FIG. 1 . In addition, the pushing force of the gear return spring  65  is set to be greater than that of the plunger spring  86  at that time. 
     When the clutch outer  56  moves to the right, the pinion  14  also moves to the right. When the pinion  14  moves to the right, the pinion washer  72  contacts with a left end surface of the idle gear  15 . In this manner, the idle gear  15  moves to the right by the pinion washer  72 , and the idle gear  15  is separated from the ring gear  16 . 
     On the other hand, in the starter motor  1  including the above configuration, the idle shaft  89  is assembled in the following manner.  FIG. 5  is an explanatory view showing a step of assembling and fixing the idle shaft  89  on the gear cover  24 , and  FIG. 6  is a process chart showing a main assembling step in such a case. As shown in  FIG. 5 , in the starter motor  1 , the idle gear assembly  97  including the idle gear  15  is firstly allocated in an idle gear mounting section  100  of the gear cover  24 . In this state, from a top side of the gear cover  24  in  FIG. 5 , the idle shaft  89  is inserted through the axis opening  101  formed on the bearing section  98 . At this time, the idle shaft  89  is inserted in the axis opening  101  in a manner that the positioning groove (positioning part)  108  of the idle shaft  89  is oriented at a right angle (90°) with respect to the engine mounting surface  111  (step ( 1 ) of  FIG. 6(   a )). In this manner, the idle shaft  89  is mounted on the gear cover  24  so that the screw mounting hole  106  and the screw insertion hole  104  just facing-face each other. 
     Next, the idle shaft  89  is inserted though the idle gear assembly  97 , and thereafter, inserted into the axis hole  102  formed on the bearing section  99 . As described above, at that time, the screw mounting hole  106  and the screw insertion hole  104  are aligned with each other by disposing the positioning groove  108  and the engine mounting surface  111  in right angle relation. For this reason, the fixing screw  107  can be inserted from the screw mounting hole  106  without checking positions of the holes, and the fixing screw  107  can be screwed into the female screw section  105  through the screw insertion hole  104  (step ( 2 ) of  FIG. 6(   a )). In the above manner, after the idle shaft  89  is fixed on the gear cover  24 , the dustproof cap  103  is mounted (step ( 3 ) of  FIG. 6(   a )). The dustproof cap  103  is mounted on the idle shaft  89  in a manner such that the fitting protrusion  109  of the dustproof cap  103  is fitted with the positioning groove  108 . At this time, the notched surface  112  of the dustproof cap  103  is flush with the engine mounting surface  111 . 
     As described above, in the starter motor  1  according to the present invention, the positioning groove  108  indicating a position of the screw insertion hole  104  is provided on an upper end surface of the idle shaft  89 . Therefore, by adjusting a rotational position of the idle shaft  89  so as to mount the positioning groove  108  in a predetermined direction (right angle with respect to the engine mounting surface  111  in the present embodiment), the idle shaft  89  can be mounted on the gear cover  24  in a manner such that the screw mounting hole  106  and the screw insertion hole  104  just face each other. That is, before the idle shaft  89  is inserted, a screw hole for fixing the idle shaft can be positioned. For this reason, without checking whether the screw mounting hole  106  and the screw insertion hole  104  match with each other or not, the fixing screw  107  can be inserted from the screw mounting hole  106 , and checking of positions of the holes and alignment of the holes become unnecessary in subsequent steps. Accordingly, workability in an assembly step is improved and reduction in the number of steps is attempted, and thereby a manufacturing cost can be reduced. 
     In addition, since alignment of the holes is not necessary, the adjustment of poor workability carried out by holding the P part in  FIG. 9  is not necessary as well. For this reason, the worker does not touch the part to which the grease is applied. In this manner, quality of product is improved, and hands of the worker are not stained as well as the working environment is improved. Further, whether the holes are matched or not need not be checked, and insertion and screwing of the idle shaft  89  are easily carried out in the automated line. In this manner, assembly of the starter motor  1  can be automated. 
     On the other hand, the screw mounting hole  106  and the screw insertion hole  104  can be aligned by using the dustproof cap  103 . That is, when the dustproof cap  103  is mounted on the idle shaft  89  and the notched surface  112  of the idle shaft  89  is made flush with the engine mounting surface  111 , the idle shaft  89  can be inserted in a manner such that both of the holes face each other. Therefore, an assembling step of the idle shaft  89  can be carried out in a manner shown in  FIG. 6(   b ). Here, the idle shaft  89  is inserted in the axis opening  101  and the axis hole  102  in a state where the dustproof cap  103  is mounted on the idle shaft  89 , and at that time, the notched surface  112  is made flush with the engine mounting surface  111  ( FIG. 6(   b ), step ( 1 )). In this manner, the idle shaft  89  is mounted on the gear cover  24  so that the screw mounting hole  106  and the screw insertion hole  104  just face each other. 
     Then, the idle shaft  89  is inserted through the idle gear assembly  97 , and thereafter the idle shaft  89  is inserted to the axis hole  102  formed in the bearing section  99 . At this time, the screw mounting hole  106  and the screw insertion hole  104  match with each other. Therefore, without checking the positions of the holes, the fixing screw  107  can be inserted into the screw mounting hole  106 , and then the fixing screw  107  can be screwed into the female screw section  105  through the screw insertion hole  104  ( FIG. 6(   b ), step ( 2 )). As described above, when the dustproof cap  103  is mounted on the idle shaft  89  in advance and the idle shaft  89  is assembled by using the dustproof cap  103  as an indicator for positioning, the assembly step can be further simplified as shown in  FIG. 6(   b ). The idle shaft  89  may be inserted into the axis opening  101  and the axis hole  102  without specifically carrying out alignment, and then the dustproof cap  103  may be mounted, and at this time, the dustproof cap  103  may be appropriately rotated to align the positions of the holes. 
     Needless to say, the present invention is not limited to the above embodiment, and a variety of modifications can be made in a scope not deviating from the gist of the present invention. 
     In the embodiment described above, the screw mounting hole  106  and the screw insertion hole  104  are configured to just face each other by mounting the positioning groove  108  at a position in the right angle with respect to the engine mounting surface  111 . However, the position of the positioning groove  108  is not limited to the right angle as long as the configuration is such that positioning can be easily carried out, and setting can be made appropriately, such as in a direction matching with the engine mounting surface  111  (in parallel position). For example, the positioning groove  108  may be provided in the same direction as the screw insertion hole  104 , and an indication (arrow or the like) indicating a mounting position may be provided on the gear cover  24  or an upper end surface of the bearing section  98  in  FIG. 5 . In addition, the indication of the positions of the holes is not limited to a groove, and may be a protrusion or a notch, or may be indicated by an arrow which is labeled or painted, as long as the indication is easily visually recognized by the worker. Further, in order to effect the positioning of the idle shaft  89 , convexo-concave fitting parts may be provided on the axis opening  101  and the idle shaft  89 . 
     In addition, in the embodiment described above, the configuration in which the fixing screw  107  is used as the idle shaft fixing member is shown. However, a fixing material other than a screw, such as a pin, can be used as well. Further, in the embodiment described above, the starter motor in which the overrunning clutch  13  is mounted on the drive shaft  32  rotated by the motor  11  via the planetary gear mechanism  12  is shown. However, there is no specific limitation with respect to the form of the starter motor. The present invention may be applied to starter motors of a variety of forms, including a starter motor in which an overrunning clutch is mounted on a front end part of the motor shaft  27 .