Abstract:
A direct current machine, such as a motor or a dynamo, has a commutator. The commutator has a contact surface which includes a generally cylindrical contact surface. The commutator is rotatable around an axis. A brush assembly of the direct current machine includes at least two brushes. Each brush has a distal surfaces which is engagable to a contact surface of the commutator. A brush holder movably holds the brushes toward the commutator. A clearance is defined between the brush and the brush holder. Each spring urges the corresponding brush against the contact surface of the commutator. Further, the spring urges the brush to be off-set toward the front side of a rotational direction of the commutator, within the brush holder.

Description:
BACKGROUND OF THE INVENTION 
     This application claims the priority of Japanese Patent application No. 4-132885 filled May 25, 1992. 
     FIELD OF THE INVENTION 
     The present invention relates to a direct current machine, such as a motor or a dynamo, having a brush assembly and a commutator. More specifically, it pertains to a direct current machine having a brush assembly which slidably contacts the commutator under the brush. 
     DESCRIPTION OF THE RELATED ART 
     In a conventional direct current motor of this type, a brush holder 23 is secured to a case (not shown) of a motor, as shown in FIG. 6. Each brush holder 23 accommodates a brush 22, which is arranged generally perpendicular to the axial line of a commutator 21. A sliding contact surface 22a, which is formed at the distal end portion of each brush 22, is urged against a peripheral surface 21a of the commutator 21, by the urging force of a spring (not shown). 
     As shown in FIG. 7, the sliding contact surface 22a is inclined with respect to the axial line of the commutator 21, i.e., to the peripheral surface 21a. Therefore, when the brushes 22 are first installed, a distal edge of the sliding contact surface 22a tangentially contacts the surface 21a of the commutator 21. Each surface 22a of the brush 22 has a generally equal curvature with respect to that of the surface 21a of the commutator 21, in order to increase the contacting area between the brush 22 and the commutator 21. The portion indicated by the arrow A of the brush 22 starts wearing gradually, with the rotational movement of the commutator 21, as shown in FIG. 7. 
     However, there is a clearance between the brush 22 and the brush holder 23, as shown in FIGS. 8 and 9. Thus, each brush 22 is caused to shift transversely, or to incline within the brush holder 23, due to sliding friction against the commutator 21, while the commutator 21 is revolving. As a result, the central line L1 of the width of the brush 22, does not coincide with the central line L of the brush holder 23. The central portion of the surface 22a of the brush 22, corresponding to the central line L1 of the width of the brush 22, no longer makes a slidable contact with the surface 21a of the commutator 21. For example, as shown in FIG. 9, a corner 22b of the brush 22 makes a slidable contact with the commutator 21. In this case, noise is generated. 
     FIG. 10 shows the wear progressing process in which a contact position of surface 22a of the brush 22 against the peripheral surface 21a changes with time, in response to the wear of the brush 22. The central line L1 of the width of the brush 22 does not coincide with a central line L2 of the sliding contact surface 22a. The reason being that a contact area of the sliding contact surface 22b against the peripheral surface 21a gradually increases, due to the wear of the brush 22. At the same time, the central portion of the surface 22a of the brush 22 tends to make a slidable contact with the surface 21a of the commutator 21. Therefore, the central portion of the surface 22a mainly wears out. As a result, in a conventional motor, a slidable contact position between the surface 22a of the brush 22 and the commutator 21 is difficult to maintain in the fixed position. Therefore, the resulting shortcome is that the output characteristics of the conventional motor is easily caused to fluctuate. 
     In an attempt to overcome this disadvantage, Japanese Unexamined Patent Publication No. 2-164244 discloses a direct current motor. In the motor of the above-described publication, as shown in FIG. 11, a curvature of the surface 22a of the brush 22 is set larger than a radius r of the commutator 21. The sliding contact surface 22a is formed as an arch with a curvature. 
     Therefore, as shown in FIG. 11, even if the brush 22 were shifted transversely within the brush holder 23, the corner 22b of the brush 22 does not make slidable contact with the surface 21a of the commutator 21. As shown in FIG. 12, even if the brush 22 were inclined within the brush holder 23, the corner 22b of the brush 22 does not make slidable contact with the surface 21a of the commutator 21. 
     However, in the direct current motor described in this publication (2-164244), when the brush 22 is shifted transversely within the brush holder 23, the surface 22a of the brush 22 does not make slidable contact with the surface 21a of the commutator 21, at the central portion thereof, with respect to the central line L1 of the width of the brush 22. Therefore, a slidable contacting position of the surface 22a of the brush 22 with respect to the surface 22a of the commutator 21 is difficult to maintain in the fixed position. As a result, the fluctuation of the motor characteristic is still unresolved. 
     In addition to the above, as shown in FIG. 13, it is known that the brush 22 is accommodated within the brush holder 23, which is inclined by a range of 10 to 20 degrees, with respect to a line passing through the center of the commutator 21. In this conventional motor, the brush 22 is forced to contact the inner wall surface of the holder 23, by means of a frictional force generated with respect to the commutator 21, while the commutator 21 is revolving. However, even in this case, the fluctuation of characteristic of the motor still occurs for similar reasons as indicated. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a direct current machine, which has the capability to prevent noise generation and fluctuation of characteristic of the motor, generated by the brush when it slidably contacts the commutator. 
     To achieve the foregoing and other objects and in accordance with the purpose of the present invention, the new and improved direct current machine has a generally cylindrical contact surface made around a peripheral surface thereof, and a commutator which is rotatable around a single axial line. A brush assembly of the machine includes a brush, which has a distal end surface that contacts a contact surface of the commutator. A brush holder slidably holds the brush toward the commutator. A clearance is defined between the brush and the brush holder. An urging mechanism urges the brush against the contact surface of the commutator. This urging mechanism further urges the brush to be off-set toward the front side of a rotational direction of the commutator, within the brush holder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments, together with the accompanying drawings in which: 
     FIG. 1 is a cross sectional view of a direct current motor according to a first embodiment of the present invention; 
     FIG. 2 is a cross sectional view illustrating a brush urged against an inner wall surface within a brush holder of the motor of FIG. 1; 
     FIG. 3 illustrates a progressive wear process of the brush of the motor of FIG. 1; 
     FIG. 4 is a cross sectional view illustrating the brush inclined within the brush holder of a direct current motor employed in a second embodiment according to the present invention; 
     FIG. 5 is a cross sectional view of the brush and the brush holder of a direct current motor employed in a third embodiment according to the present invention; 
     FIG. 6 is a cross sectional view of the brush and the others of a conventional direct current motor; 
     FIG. 7 is a perspective view of the brush employed in the motor of FIG. 6; 
     FIG. 8 is a cross sectional view illustrating a conventional brush urged against an inner wall surface of the brush holder; 
     FIG. 9 is a cross sectional view illustrating the conventional brush being inclined within the brush holder; 
     FIG. 10 illustrates a progressive wear process of the conventional brush; 
     FIG. 11 is a cross sectional view illustrating another conventional brush being urged against the right surface of the brush holder; 
     FIG. 12 is a cross sectional view illustrating another conventional brush being inclined within the brush holder; and 
     FIG. 13 is a cross sectional view illustrating an inclined brush holder in a still another conventional direct current motor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first preferred embodiment of a direct current motor according to the present invention will now be described referring to FIGS. 1 through 3. 
     As shown in FIG. 1, a rotary shaft 13 is rotatably supported by a pair of bearings 11 and 12 within a case 10 of the direct current motor. An armature 14 and a commutator 1 are integrally rotatably secured to the rotary shaft 13. A plurality of magnets 15 are equiangularly secured to an inner peripheral surface of the case 10, around the peripheral surface of the armature 14. A support plate 16 is provided within the case 10 in the vicinity of the commutator 1. 
     A portion of the commutator 1 is inserted into a through hole 17 formed in the support plate 16. A pair of brush holders 2, each of which has a substantially tubular form with a generally rectangular cross section, are attached to the support plate 16, around the peripheral surface of the commutator 1, and are separated from each other by a predetermined distance. As shown in FIG. 2, a central line L of the each brush holder 2 is set in a plane which includes a central line 0 of the commutator 1. 
     Each brush holder 2 accommodates a reciprocatably movable brush 3, which includes a generally square pole, along the central line L of the brush holder 2. A predetermined clearance is defined between the brush holder 2 and the brush 3. A spring 18 is attached to each brush holder 2 via attachment means 2a. This spring 18 is generally a flat spring, and is secured to the brush holder 2, via a proximal wound section 18a. A distal portion 18b of the spring 18 engages the outer distal surface of the brush 3. 
     The spring 18 urges the brush 3 against the commutator 1, under the force indicated by the arrow f, as shown in FIG. 2. More specifically, the brush 3 is urged against the commutator 1 under the force component fz of the urging force f, along the z direction. The brush 3 is also urged against one of inner side surfaces (an inner surface located in the right of FIG. 2) of the brush holder 2, under the force component fx of the urging force f, along the x direction. A sliding contact surface 3a, which contacts a peripheral surface 1a of the commutator 1, is formed at an inner distal portion of each brush 3. The contact surface 3a is inclined with respect to the surface 1a of the commutator 1. That is with respect to the central line 0 of the commutator 1. 
     Therefore, while the commutator 1 is rotating in the direction indicated by the arrow B, the brush 3 is urged against the inner wall surface located at the right side of the brush holder 2, under the friction caused by the rotation of the commutator 1 and the force component fx of the spring 18, along x direction. Therefore, the brush 3 is eccentrically retained at the right side with respect to the center line L of the brush holder 2, as shown in FIG. 2. 
     In this embodiment, the surface 3a of the brush 3 is formed along an arch Q1 (indicated by a double dotted line as shown in FIG. 2), which passes through a point P located on the inner distal end surface of the brush 3, lying on the central line L1 of the width of the brush 3. Further, a center 01 of the arch Q1 is located on an extended line of a line X1, which lies on a plane including the point P and the central line 0 of the commutator 1. The radius R of the arch Q1 is larger than the radius r of the commutator 1. Therefore, the surface 3a always slidably contacts the peripheral surface 1a of the commutator 1 on the central line L1 of the width of the brush 3. 
     According to this embodiment, as shown in FIG. 2, the brush 3 is kept at the position displaced transversely by the urging force of the spring 18, within the brush holder 2. As shown in FIG. 3, the central line L2 of the sliding contact surface 3a which passes through the point P and the central line L1 of the width of the brush 3, coincide on the surface 3a of the brush 3. As the brush 3 becomes electrically charged, the rotary shaft 13, armature 14 and commutator 1 are integrally rotated in a direction shown by an arrow B. While the commutator 1 is rotating, the surface 3a of the brush 3 is always slidably contacted to the surface 1a of the commutator 1 on the central line L1 of the width of the brush 3. The sliding contact position therebetween is never out of the central line L2 of the sliding contact surface 3a. 
     As a result, the direct current motor can smoothly rotate. Therefore, the fluctuation of the motor characteristic is minimized. As the surface 3a of the brush 3 is formed along the arch which has a larger curvature of the surface 1a, the sliding contact between a corner 3b of the brush 3 and the surface 1a of the commutator 1 is averted, so that noise generation is minimized. 
     A direct current motor employed in a second embodiment according to the present invention will now be described referring to FIG. 4, mainly emphasizing the difference relative to the first embodiment. In this second embodiment, a brush 33 is accommodated within a corresponding holder 32, which is kept in the tilted position and which is reciprocally movable. An inner distal portion of the brush 33 which is located in the front side of the rotational direction of the commutator 1 engages an inner peripheral surface of the holder 32, and an outer distal portion of the brush 33, which is located in the rear side of the rotational direction of the commutator 1, and which engages the other peripheral surface of the holder 32. A coil spring 34 is attached to the holder 32, which is disposed in parallel to the central line L of the holder 32. The spring 34 engages the outer end surface of the brush 33. The brush 33 is urged toward the center of the commutator 1, under the urging force f of the spring 34, and at the same time, the brush 33 is maintained in the inclined position. 
     Further, in this embodiment, the point P located on the central line L2 of the sliding contact surface 33a of the brush 33, is off-set toward the front side, in the rotational direction of the commutator 1, with respect to the central line L of the brush holder 32. The surface 33a is formed along an arch Q2 which has a central point O2 located on a line X2. The line X2 lies in a plane including the point P and the central line 0 of the commutator 1, and has a larger radius R than the radius r of the commutator 1. 
     Therefore, similar to the first embodiment, in the present embodiment, while the commutator 1 is rotating, the slidably contacting position between the brush 33 and the commutator 1 always coincides with the central line L1 of the sliding contact surface 33a. As a result, the direct current motor can smoothly rotate. Therefore, the fluctuation of the motor characteristic is averted. The sliding contact between a corner 33b of the brush 33 and the surface 1a of the commutator 1 is also averted, so that noise generation is controlled. 
     A direct current motor employed in a third embodiment according to the present invention will now be described referring to FIG. 5, mainly emphasizing the difference relative to the first embodiment. In the present embodiment, a brush holder 42 is inclined by a predetermined angle θ (for example, 10 to 20 degrees) with respect to the vertical plane which includes a central line 0 of the commutator 1. The spring 44 is similar to that used in the first embodiment. 
     Further, in the present embodiment, the point P lying on the central line L2 of the sliding contact surface 43a of the brush 43, is off-set toward the front side of the rotational direction of the commutator 1, with respect to the central line L of the brush holder 42. The surface 43a is formed along an arch Q3, which has a central point 03 that is located on a line X3. The line X3 lies in a plane including the point P and the central line 0 of the commutator 1. The arc Q3 has a larger radius R than the radius r of the commutator 1. 
     Therefore, while the commutator 1 is rotating, the slidably contacting position between the brush 43 and the commutator 1 always coincides with the central line L2 of the sliding contact surface 43a, similar to the above-described embodiments. Therefore, the fluctuation of the motor characteristics is averted. The sliding contact between a corner 43b of the brush 43 and the surface 1a of the commutator 1 is averted, so that noise generation is minimized. 
     Although only three embodiments of the present invention have been described herein, it should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the following embodiments are contemplated by the present invention: 
     For example, a sliding contact surface of a brush could be formed as a substantially cylindrical tube around the axial line of the commutator. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details giving herein, but may be modified within the scope of the appended claims.