Abstract:
An inside space of a starter motor having a motor, a magnetic switch and a reduction gear train is ventilated through a ventilation passage formed in the starter motor. The ventilation passage includes a labyrinth passage formed at a portion connecting a front housing containing the reduction gear and a center housing containing the motor and the magnetic switch. The labyrinth passage connecting a motor chamber and a switch chamber prevents brush dusts generated in the motor chamber from being scattered in the other spaces. Grease splash from the gear train does not enter into the ventilation passage separated the gear train. Thus, the ventilation passage well functions for a long time without being clogged with the grease, brush dusts or the mixture thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of priority of Japanese Patent Application No. 2000-319266 filed on Oct. 19, 2000, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a starter motor for cranking an internal combustion engine, the starter motor including a labyrinth-shaped ventilation passage. 
     2. Description of Related Art 
     An example of a starter motor having a ventilation passage is disclosed in JP-A-7-103116. A relevant portion of the starter motor is illustrated in FIG.  6 . Cavities  110  are formed in a clutch case  100 , an inner space of the starter motor (a motor chamber) communicates with outside through the cavities  110  and a clutch chamber  120 . In this manner, a ventilation passage connecting the inner space of the starter motor to the outside can be made long and complex, and accordingly it is possible to prevent outside water from entering into the inner space of the starter motor through the ventilation passage. 
     In the conventional motor, however, a communicating hole  130  connecting the cavities  110  to the clutch chamber  120  is positioned at an outer peripheral portion of a reduction gear  140  that is housed in an upper portion of the clutch chamber  120 . Therefore, there is a problem that the communicating hole  130  is closed by lubricating grease scattered from the rotating gear  140 . Moreover, brush dusts generated in the motor chamber enter into the clutch chamber  120  through the ventilation passage and mixes with the grease. The reduction gear  140  and associated parts are abnormally abraded by the brush dusts mixed with the grease. Further, there is a possibility that the brush dusts contained in ventilation air adhere to and close the ventilating hole  130 . 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved ventilation passage in the starter motor, so that the ventilation passage is not clogged with the scattered grease, and the brush dusts generated in the motor chamber is prevented from being scattered in the ventilation passage. 
     The starter motor is composed of a motor for generating a rotational torque for cranking an internal combustion engine, a magnetic switch for supplying electric power from a battery to the motor, and a reduction gear train for transferring a rotational torque of the motor to the engine. An inside space of the starter motor is ventilated by air flowing through a ventilation passage that is open to outside of the starter motor. The ventilation passage includes a communicating passage connecting a switch chamber containing the magnetic switch therein and a motor chamber containing the motor therein. 
     The communicating passage is formed along a boundary surface between a front housing containing the reduction gear train and a center housing having the motor space and the switch space. Cavities formed in the front housing and cavities formed in the center housing are positioned to partially overlap one another when both housings are connected. The cavities overlapped in this manner form a labyrinth passage. The communicating passage that includes the labyrinth passage is formed around the reduction gear train, and the gear train is separated from the communicating passage by a separating wall. 
     When the motor rotates upon actuation of the magnet switch, pressure inside the motor chamber and switch chamber vibrates, and the inside air is heated. The heated air is ventilated through the ventilation passage. The labyrinth passage formed in the communicating passage prevents brush dusts generated in the motor chamber from being scattered in the ventilation passage and being mixed with grease lubricating the reduction gear train. Further, since one of the cavities constituting the labyrinth passage is positioned at the bottom of the front housing, the brush dusts are kept therein. Since the ventilation passage is separated from the reduction gear train, the grease splash from the gear train does not enter into the ventilation passage. 
     The ventilation passage according to the present invention is kept free from the grease and the brush dusts without being clogged therewith. The inside space of the starter motor is well ventilated by the air flowing through the ventilation passage. 
    
    
     Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing a front housing and a center housing of a starter motor according to the present invention, taken along line I-I shown in both FIGS. 2 and 3; 
     FIG. 2 is a plan view showing the front housing in which a gear chamber is formed, viewed in direction F shown in FIG. 1; 
     FIG. 3 is a plan view showing the center housing, viewed in direction C shown in FIG. 1; 
     FIG. 4 is a perspective view showing the front housing and a gear train housed therein; 
     FIG. 5 is a side view, partly cross-sectioned, showing an entire structure of the starter motor according to the present invention; and 
     FIG. 6 is a plan view showing a clutch case of a conventional starter motor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described with reference to FIGS. 1-5. FIG. 5 shows an entire structure of a starter motor  1  according to the present invention. FIG. 1 shows a cross-sectional view of a front housing  4  and a center housing  5 , in which a communicating passage  27  as a part of a ventilation passage is formed. FIG. 2 shows a plan view of the front housing  4 , viewed in direction F shown in FIG. 1, in which a gear train having three gears  8 ,  9  and  10  is housed. FIG. 3 shows a plan view of the center housing  5 , viewed from direction C shown in FIG.  1 . FIG. 4 is a perspective view showing the front housing  4  and the gear train. 
     As shown in FIG. 5, the starter motor  1  is composed of a motor  6  having an armature  2 , a magnetic switch  7  and a reduction gear train. The motor  6  and the magnetic switch  7  are combined together, so that both axes extend in parallel to each other. The armature  2  is housed in a space formed by the front housing  4 , a yoke  15  and a end cover  16 . The magnetic switch  7  is housed in the center housing  5  forming a switch case  20 , the rear end of the center housing  5  being closed with an end plate  23 . The gear train is contained in the front housing  4 . 
     The reduction gear train is composed of, as shown in FIG. 2, a drive gear  8  connected to an armature shaft  2   a , an idle gear  9  engaging with the drive gear  8  and a clutch gear  10  engaging with the idle gear  9 . A rotational speed of the armature  2  is reduced and its rotational torque is increased by the gear train. The rotational torque of the clutch gear  10  is transferred to a pinion shaft  11  via a one-way clutch (not shown) installed inside the clutch gear  10 . A pinion gear  12  spline-connected to the pinion shaft  11  is rotated by the pinion shaft  11 . 
     As shown in FIG. 5, the motor  6  is composed of the armature  2 , brushes  13  and other components housed in a motor housing. The motor housing is composed of a cylindrical yoke  15  carrying stationary poles  14  positioned around the armature  2 , the center housing  5  closing a front opening of the yoke  15 , and the end cover  16  closing a rear opening of the yoke  15  and having a ventilating pipe  17 . The armature  2  is rotatably supported by bearings  18  and  19  held in the center housing  5  and the end cover  16 , respectively. Upon closing a motor switch (not shown) installed in the magnetic switch  7 , electrical power is supplied to the armature  2  through the brushes  13 , and thereby the armature  2  is rotated. 
     The magnetic switch  7  composed of a coil  21 , a plunger  22  having a plunger rod  24 , the motor switch and other components is installed in the switch case  20  integrally formed with the center housing  5  (refer to FIG. 1) The rear opening of the switch case  20  is hermetically closed with the end plate  23 . Upon energization of the coil  21 , the plunger  22  is driven by the magnetic force of the coil  21 , thereby closing the motor switch and pushing the pinion shaft  11  frontward (to the leftward in FIG.  1 ). Upon de-energization of the coil  21 , the plunger  22  returns to its original position, and thereby the motor switch is opened and the pinion shaft  11  returns to its original position. 
     As shown in FIG. 1, a communicating passage  27  forming a part of a ventilation passage in the starter motor  1  is formed along a surface connecting the front housing  4  and the center housing  5 . The communicating passage  27  (shown with an arrowed line in FIG. 1) is composed of: cavities  28  formed in the front housing  4 ; cavities  29  formed in the center housing  5 ; a motor side ventilating hole  30  formed through a vertical wall of the center housing  5  as a passage between the lower cavities  28 ,  29  and a motor chamber  25 ; a communicating chamber  31  formed at a side of the gear chamber  3  and communicating with one of the cavities  28  positioned at a upper portion; and a switch side ventilation hole  32  as a passage between the communicating chamber  31  and a switch chamber  26 . The motor chamber  25  formed in the motor housing and the switch chamber  26  confined by the switch case  20  and the end plate  23  communicate with each other through the communicating passage  27 . 
     As shown in FIG. 2, the cavities  28  in the front housing  4  are formed along a bottom portion of the gear chamber  3  that contains the gear train therein. A rib  33  formed between the gear chamber  3  and the cavities  28  serves as a connecting surface  4   a  that abuts a connecting surface  5   a  of the center housing  5  (refer to FIG.  3 ). The cavities  28  are thus separated from the gear chamber  3  by the rib  33 . As shown in FIG. 3, the cavities  29  in the center housing  5  are formed, being aligned along an arc line, on the connecting surface  5   a  that closes a front opening of the yoke  15 . When both the front housing  4  and the center housing  5  are connected by abutting both connecting surfaces  4   a  and  5   a , the cavities  28  and  29  are positioned not to completely overlap but to partly overlap each other. In this manner, the communicating passage  27  is formed as a labyrinth passage. 
     As shown in FIG. 1, the motor side ventilating hole  30  is formed through a vertical wall of the center housing  5  that closes the front opening of the motor chamber  25 . The motor side ventilating hole  30  communicates with a cavity  29  positioned at the lowest. 
     The center housing  5  and the front housing  4  are connected by engaging a circular wall  37  formed at the rear opening of the front housing  4  with a circular groove  34  formed at the front opening of the center housing  5 , as shown in FIG.  1 . The circular groove  34  is formed outside of a circular bearing holder  36  that holds a bearing  35  therein. A portion of the circular groove  34  is enlarged toward the cavity  29  of the center housing  5 , thereby forming the communication chamber  31 , as shown in FIGS. 1 and 3. The communicating chamber  31  communicates with the cavity  28  of the front housing  4  and the switch side ventilating hole  32  formed through a front wall of the center housing  5 . 
     The motor chamber  25  and the switch chamber  26  communicate with each other through the communicating passage  27  that includes the switch side ventilating hole  32 , the communicating chamber  31 , the cavities  28 ,  29 , and the motor side ventilating hole  30 . Thus, the ventilation passage in the starter motor  1  is formed. The ventilation passage further communicates with the ventilating pipe  17  connected to the end cover  16  (shown in FIG.  5 ). 
     As shown in FIGS. 2 and 4, one end of the circular wall  37  is extended to a position where the upper cavity  28  of the front housing  4  is separated from the gear chamber  3  by the end of the circular wall  37 . The end of the circular wall  37  extends in the communicating chamber  31  when the front housing  4  and the center housing  5  are connected, as shown in FIG.  1 . 
     The operation of the starter motor  1  including the magnetic switch  7  is not explained here because it is well known. The motor chamber  25  and the switch chamber  26  are ventilated through the ventilation passage described above in the following manner. The pressure in the switch chamber  26  varies when the plunger  22  is activated upon energization of the coil  21 . A ventilating airflow is generated through the ventilation passage that includes the switch chamber  26 , the communicating passage  27 , the motor chamber  25  and the ventilating pipe  17 . The communicating passage  27  includes the labyrinth structure formed by the cavities  28  of the front housing  4  and the cavities  29  of the center housing. 
     Since the communicating passage  27  connecting the switch chamber  26  and the motor chamber  25  is formed around the gear chamber  3  bypassing the gear chamber  3 , the communicating passage  27  is not clogged with the lubricating grease of the reduction gear train if the grease is scattered due to its rotation. Therefore, the ventilation passage is always maintained in good conditions. Further, since the circular wall  37  extends into the communicating chamber  31 , as shown in FIG. 1, the grease scattered by the rotation of gears  8 - 10  is prevented from entering into the cavities  28 ,  29 . 
     The communicating passage  27  includes the labyrinth structure that is formed by positioning the cavities  28 ,  29  at offset positions not to completely overlap each other. Therefore, there is almost no chance for the brush dusts generated in the motor chamber  25  to enter into the gear chamber  3  or the switch chamber  26  through the labyrinth structure. As a result, the brush dusts do not mix with the grease, and accordingly abnormal attrition of the gears by the brush dusts is prevented. Further, since the brush dusts do not enter into the switch chamber  26 , the sliding operation of the plunger  22  is not hindered by the brush dusts. 
     One of the cavities  28  formed in the front housing  4  and communicating with the motor side ventilating hole  30  is positioned at the lowest portion of the front housing  4 . Therefore, the brush dusts entering into the lowest cavity  28  through the motor side ventilating hole  30  can be accumulated and kept therein. In this manner, the brush dusts are prevented from being scattered in the communicating passage  27 , and the motor side ventilating hole  30  is not clogged by the brush dusts. 
     Though the magnetic switch  7  is positioned at an upper side of the motor  6  in the embodiment described above, the magnetic switch  7  may be positioned at an lower side of the motor  6 . Though the ventilating pipe  17  is connected to the end cover  16  in the embodiment described above, it may be connected to the magnetic switch  7 , for example, to the switch case  20 . 
     While the present invention has been shown and described with reference to the foregoing preferred embodiment, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.