Miniature motor with spark quenching connection between commutator and brushes
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a miniature motor, and more particularly to a miniature motor being adapted to suppress spark generation between a plurality of commutator segments and brushes by connecting a resistors to each of the commutator segments. 
     2. Description of the Prior Art 
     Miniature motors having a spark-quenching means to suppress spark generation between a plurality of commutator segments and brushes by connecting resistors to each of the commutator segments are generally known. 
     A well-known arrangement for quenching sparks between the commutator segments and the brushes is to utilize a ring resistor in which thick-film resistors as many as the rotor poles are disposed on a ring-shaped printed circuit board with the adjoining thick-film resistors being connected with copper foil, for example; the copper foil being soldered to the commutator segments. With this arrangement, however, soldering is required to connect the ring resistor to the commutator segments, resulting in increased manufacturing cost in addition to the expensive ring resistor. 
     Another conventional spark-quenching arrangement is to connect rotor windings to commutator terminals provided integrally with the commutator segments, and then adhere a short cylindrical electrically conductive rubber by an electrically conductive adhesive so as to cover the commutator terminals. This arrangement also involves an unwanted problem of an electrically conductive adhesive required to adhere the ring-shaped electrically conductive rubber to the commutator. 
     SUMMARY OF THE INVENTION 
     This invention is intended to overcome the aforementioned problems. 
     It is an object of this invention to provide a miniature motor having a spark-quenching resistor connecting each of the commutator segments, and constructed so that a disc-shaped electrically conductive rubber ring is fixedly fitted to a insulating cylinder in such a manner that the end face of the disc-shaped rubber ring comes in contact with commutator terminals provided on commutator segments; the electrically conductive rubber ring being brought to intimate contact with the commutator terminals by heat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of a commutator embodying this invention. 
     FIG. 2 is a diagram of assistance in explaining the assembly process of the embodiment shown in FIG. 1; FIG. 2(A) being a diagram of assistance in explaining the assembly procedures; FIG. 2(B) being a perspective view of the electrically conductive rubber used in this invention; FIG. 2(C) being a perspective view of the commutator segment used in this invention. 
     FIG. 3 is a diagram illustrating the curves of changes with time in resistance values between commutator segments. 
     FIG. 4 shows another embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     In FIGS. 1 through 3, reference numeral 1 refers to a motor rotating shaft; 2 to an insulating cylinder which is fixedly fitted to the motor rotating shaft 1; 2&#39; to a flange formed in a projecting fashion on the insulating cylinder 2; 3 to an electrically conductive rubber ring having such an inside diameter as to fit the insulating cylinder 2; 4 to a commutator segment; 4&#39; to a commutator terminal integrally formed with the commutator segment 4, to which a rotor winding is connected; and 5 to an insulating washer for fixing the commutator segment 4 disposed on the insulating cylinder 2, respectively. 
     In FIG. 1 illustrating an example of commutator used in a miniature motor embodying this invention, the spark-quenching electrically conductive rubber ring 3 is assembled by procedures which will be described later, and installed so as to be in contact with the commutator terminals 4&#39;. Consequently, when rotor windings (not shown) are welded to the commutator terminals 4&#39; by spot welding or soldering, the heat generated by spot welding or soldering causes the contact surface of the electrically conductive rubber ring 3 with the commutator terminals 4&#39; to be melted or softened, resulting in an intimate contact of the electrically conductive rubber ring 3 with the commutator terminals 4&#39;. In other words, an electrical connection between the spark-quenching electrically conductive rubber ring 3 and the commutator terminals 4&#39; is automatically effected by welding the rotor windings to the commutator terminals, eliminating the need for joining the electrically conductive rubber ring 3 with the commutator terminals 4&#39;. Whereas FIG. 1 shows the state where the commutator according to this invention has been assembled, the assembly process of the embodiment shown in FIG. 1 will be described to facilitate the understanding of this invention, referring to FIGS. 2(A) through (C). 
     In FIG. 2(A), the insulating cylinder 2 is first fixedly fitted to the motor rotating shaft 1. Then, the electrically conductive rubber ring 3 formed from electrically conductive rubber into a ring shape as shown in FIG. 2(B) is fitted onto the insulating cylinder 2 and slid in the direction shown by an arrow in the figure until the ring 3 comes in contact with the flange 2&#39; provided on the insulating cylinder 2. Next, the commutator segments 4 shown in FIG. 2(C) are disposed on the insulating cylinder 2 in such a manner that the commutator terminals 4&#39; formed integrally with the commutator segments 4 come in contact with the electrically conductive rubber ring 3. The insulating washer 5 is then press-fitted onto the commutator segments 4 until the commutator terminals 4&#39; come in contact with the electrically conductive rubber ring 3. 
     The embodiment shown in FIG. 1 is assembled by the assembly process described in the foregoing, referring to FIG. 2. Since the commutator terminals 4&#39; are kept pressed onto the electrically conductive rubber ring 3 by the insulating washer 5, an electrical connection between the commutator terminals 4&#39; and the electrically conductive rubber ring 3 is positively effected by the heat generated during the welding of the abovementioned rotor windings to the commutator terminals 4&#39;. 
     Now, changes with time in resistance value between commutator terminals according to this invention will be described in the following, referring to Table 1 below and FIG. 3. 
     Table 1 shows the measurement results of resistance value between the commutator terminals at atmospheric temperature of 50° C. and relative humidity of 95%. FIG. 3 is a graphic representation of the results shown in Table 1. In the table and figure, (A) indicates the results obtained with this invention, (B) the results obtained with a conventional method where an electrically conductive rubber ring as described at the beginning of the present Specification, and (C) the results obtained with the abovementioned conventional method where connection with an electrically conductive adhesive was omitted. Numerical values given in parentheses in Table 1 represent normalized resistance values based on a figure of 100 for the initial values. In FIG. 3, the coordinate represents the abovementioned normalized values and the abscissa elapsed time. 
     
                       TABLE 1                                                     
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TIME                                                                      
0           131    371        539   1035                                  
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(A)    385      374    375      377   379                                 
       (100)     (97)   (97)     (98)  (98)                               
(B)    195      205    203      200   204                                 
       (100)    (105)  (104)    (103) (105)                               
(C)    500      675    1000     2000  5000                                
       (100)    (135)  (200)    (400) (1000)                              
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     Table 1 and FIG. 3 clearly indicate the superior stability with time of the resistance value of the spark-quenching means according to this invention. 
     FIG. 4 shows another embodiment of this invention. In the figure, like numerals correspond with like parts in FIG. 1. In FIG. 4, the commutator terminals 4&#39; are sandwiched between the flange 2&#39; and the electrically conductive rubber ring 3. In this state, the end face of the electrically conductive rubber ring 3 is brought to intimate contact with the commutator terminal 4&#39; by the heat generated during the welding of the rotor windings to the commutator terminals. In other words, the commutator terminals 4&#39; are sandwiched between the flange 2&#39; and the electrically conductive rubber ring 3 in the embodiment shown in FIG. 4, whereas the commutator terminals 4&#39; are sandwiched between the electrically conductive rubber ring 3 and the insulating washer 5 in the embodiment shown in FIG. 1. That the commutator terminals 4&#39; are brought to intimate contact with the electrically conductive rubber ring 3 by heating is common to both the embodiments shown in FIGS. 1 and 4. Consequently, the embodiment shown in FIG. 4 has the same characteristics as the embodiment shown in FIG. 1. 
     As described above, this invention makes it possible to achieve an effective electrical connection between a spark-quenching electrically conductive rubber ring and commutator terminals, and to provide miniature motors having a stable spark-quenching means at low cost, coupled with the availability of low-cost electrically conductive rubber.