Title of Invention

A MOTOR FOR AN ELECTRIC POWER STEERING DEVICE

Abstract

This invention relates to a motor for an electric power steering device, which helps to achieve a reduction in requisite man-hours for running-in of the motor and a reduction in production cost and which gives no discomfort to a driver, for example, in a normal "continuous rest steering" operating state. In the motor for an electric power steering device of the present invention, which includes: a shaft, an armature fixed to the shaft, a commutator fixed to an end portion of the shaft, and a carbon brush whose distal end portion is held in contact with an outer peripheral surface of the commutator, when a rated current flowing through the carbon brush is of a value that ranges from 25A to 8OA, and the carbon brush contains copper in an amount that ranges from 35% to 55%, the ratio of an area of a portion of the carbon brush which is held in contact with the commutator to a radial projection area of the carbon brush ranges from 4 to 8%.

Full Text

MOTOR FOR AN ELECTRIC POWER STEERING DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a motor for an electric power steering device for assisting a driver with an operating force in operating a steering wheel of a vehicle. 2. Description of the Related Art Conventionally, a motor for an electric power steering device has been known which is equipped with a shaft, an armature fixed to the shaft, a commutator fixed to an end of the shaft, and a carbon brush whose distal end is in contact with an outer peripheral surface of the commutator (see, for example, JP 2001-8402 A). In this conventional motor for an electric power steering device, when a driver rotates a steering wheel, an electric current is supplied from outside to the armature through the carbon brush held in contact with the commutator, and the armature is rotated together with the shaft by an electromagnetic action. When mounted, for example, to a steering column, this motor for the electric power steering device, constructed as described above, is situated near the driver, so sound generated when the carbon brush slides on the outer peripheral surface of the commutator may be experienced by the driver as unpleasant noise. This noise is mitigated by causing the carbon brush to slide on the outer peripheral surface of the commutator in advance for a certain period of time. Thus, there are taken countermeasures for the noise by executing running-in of the motor beforehand, providing a soundproof material between the motor and the driver, etc., which means a corresponding amount of man-hours is required. Further, a high production cost is involved. SUMMARY OF THE INVENTION The present invention has been made with a view toward solving the above problems. It is an object of the present invention to provide a motor for an electric power steering device which helps to achieve a reduction in requisite man-hours for running-in of the motor and a reduction in production cost and which gives no discomfort to a driver in a normal "continuous rest steering" operating state. According to the present invention, a motor for an electric power steering device includes: a shaft, an armature fixed to the shaft, a commutator fixed to an end portion of the shaft, and a carbon brush whose distal end portion is held in contact with an outer peripheral surface of the commutator. In the motor, when a rated current flowing through the carbon brush is of a value that ranges from 25A to 80A, and the carbon brush contains copper in an amount that ranges from 35% to 55%, the ratio of an area of a portion of the carbon brush which is held in contact with the commutator to a radial projection area of the carbon brush ranges from 4 to 8%. According to the present invention, the motor for an electric power steering device helps to achieve a reduction in the requisite man-hours for the running-in of the motor and a reduction in production cost and which gives no discomfort to the driver in the normal "continuous rest steering" operating state. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a side sectional view of a motor for an electric power steering device according to Embodiment 1 of the present invention; Fig. 2 is a front view of a carbon brush of Fig. 1; Fig. 3 is a bottom view of the carbon brush of Fig. 2; Fig. 4 is a characteristic chart showing a relationship between ratio a, noise (dB), and permissible rated current supply time; Fig. 5 is a front view of a carbon brush of a motor for an electric power steering device according to Embodiment 2 of the present invention; and Fig. 6 is a bottom view of the carbon brush of Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Fig. 1 is a side sectional view of a motor for an electric power steering device according to Embodiment 1 of the present invention (hereinafter simply referred to as motor). In this motor, quadrupole field permanent magnets 2 are fixed to an inner wall surface of a yoke 1 in the form of a bottomed cylinder at equal circumferential intervals. A bearing accommodating portion 3 on a bottom side of the yoke 1 accommodates a first bearing 4. One end portion of a shaft 5 is rotatably supported by the first bearing 4. The other end portion of the shaft 5 is rotatably supported by a second bearing 13. An armature 6 is fixed to an intermediate portion of the shaft 5. A housing 12 closes an opening of the yoke 1, and is integrated with the yoke 1 by a screw 11. A commutator 7 is fixed to a portion of the shaft 5 between the second bearing 13 attached to the housing 12 and the armature 6. A distal end portion of a carbon brush 9 is held in contact with a surface of the commutator 7 by an elastic force of a spring 8. The carbon brush 9, which is formed as a parallelepiped, is accommodated in a brush holder 10 so as to be capable of reciprocating radially. A connecting member 14 is press-fitted for fixation onto a forward end portion of the shaft 5 on the second bearing 13 side. Agrommet 16 is provided between the housing 12 and the yoke 1, and a lead wire 15 extends through the grommet 16. The armature 6 is equipped with a core 17 extending in an axial direction and having a plurality of slots formed at circumferential intervals, and a winding 18 formed by winding a conductor in a lap winding fashion around the slots. Fig. 2 is a front view of the carbon brush 9 of Fig. 1, and Fig. 3 is a bottom view of the carbon brush 9 of Fig. 2. The carbon brush 9 contains 40% of copper, and a ratio a of an area of a portion of the carbon brush 9 held in contact with the commutator 7 (shaded portions (2S) of Fig. 3) to a radial projection area (a X b) of the carbon brush 9 (i.e., a = 2S/(a x b)) is set to be in a range of 4 to 8%. In the motor constructed as described above, an electric current is supplied from the lead wire 15 to the winding 18 through the brush 9 held in contact with the commutator 7, whereby the armature 6 is rotated together with the shaft 5 by an electromagnetic action. The forward end portion of the shaft 5 is press-fitted into a press-fitting hole 19 of the connecting member 14, and an input shaft (not shown) is spline-connected to an engagement portion 20 of the connecting member 14, so the rotational torque of the shaft 5 is reliably transmitted to the input shaft to thereby assist the driver with a steering force in operating the steering wheel (not shown). Fig. 4 is a characteristic chart showing a relationship between the ratio a, the noise (dB), and the permissible rated current supply time (sec) as obtained through experiment by the present inventor This characteristic chart shows experiment results when a copper content of the carbon brush 9 is 40%, and a rated current is 45A. It can be seen from this chart that the noise increases as the area of the portion of the carbon brush 9 in contact with the surface of the commutator 7 increases, that is, as the above-mentioned ratio a increases, and that, to suppress the noise, it is desirable to make a value of the ratio a as small as possible. On the other hand, voltage drop occurs between the carbon brush 9 and the commutator 7, and the carbon brush 9 generates heat. When, in the "continuous rest steering" state, rated current is continued to be supplied to the motor, the carbon brush 9 continues to generates heat due to this voltage drop, and there is a fear of the brush holder 10, which is formed of resin, being melted. In reality, to avoid this, in the "continuous rest steering" state, a current value and time of the electric current flowing through the armature 6 are controlled by an electronic control unit (ECU). The voltage drop between the carbon brush 9 and the commutator 7 increases as the value of the ratio a decreases, so the current value of the electric current flowing through the armature 6 cannot help but be reduced as the ratio a decreases, that is, the assisting force for the steering force for the steering wheel cannot help but decrease. The present inventor has found that as long as the ratio a does not exceed an upper limit value of 8%, no such sound as experienced by the driver as noise is generated, and that as long as the ratio a does not become less than a lower limit value of 3%, no discomfort is given to the driver during the "continuous rest steering" operation. While Fig. 4 is a characteristic chart obtained when the copper content of the carbon brush 9 is 40%, and the rated current is 45A, it has been found out that when the copper content of the carbon brush 9 ranges from 35% to 55%, and the rated current ranges from 25Ato 80A, there is generated no such sound as experienced by the driver as noise, and normal "continuous rest steering" operation is possible without giving any discomfort to the driver as in the case in which the carbon content of the carbon brush 9 is 40% and in which the rated current is 45A, as long as the ratio a ranges from 3 to 8%. As described above, in this motor, when the rated current flowing through the carbon brush 9 ranges from 25A to 80A, and the copper content of the carbon brush 9 ranges from 35% to 55%, the ratio of the area of the portion of the carbon brush 9 in contact with the commutator 7 to the radial projection area of the carbon brush 9 ranges from 4 to 8%, so there is no need to perform running-in of the motor beforehand or to provide a soundproof material between the motor and the driver, thus making it possible to reduce the operating man-hours for motor running-in and to achieve a reduction in production cost. Further, it is possible to conduct normal "continuous rest steering" operation without giving any discomfort to the driver. Embodiment 2 Fig. 5 is a front view of a carbon brush 21 of a motor according to Embodiment 2, and Fig. 6 is a bottom view of the carbon brush 21 of Fig. 5. In this embodiment, chamfered portions 22 are formed at the commutator 7 side end portion of the carbon brush 21 and on the outer sides of both edge portions thereof arranged in the axial direction of the shaft 5. Otherwise, this embodiment is of the same construction as Embodiment 1. In the motor of this embodiment, it is possible to obtain the same effect as that of Embodiment 1. Further, due to the formation of the chamfered portions 22, which are slopes, at both edge portions of the carbon brush 21, it is possible to mitigate chipping damage at the edge portions of the carbon brush 21. The chamfered portions 22 of the carbon brush 21 are of a configuration allowing releasing in one direction when molding the carbon brush 21 in a mold, and there is no need to take the trouble to perform machining for forming chamfered portions. Thus, the carbon brush of this embodiment can be produced by the same process as that for the carbon brush 9 of Embodiment 1. While in the above-described embodiments the carbon brush 9 is held in contact with the outer peripheral surface of the commutator 7 of a motor for an electric power steering device at two positions with the same contact area, it is also possible for the carbon brush to be held in contact with the outer peripheral surface of the commutator at two positions with different contact areas. Further, the present invention is also applicable to a motor for an electric power steering device in which the carbon brush is held in contact with the outer peripheral surface of the commutator solely on one side, that is, at one position.

Documents:

724-che-2006 abstract.jpg

724-che-2006-abstract.pdf

724-che-2006-claims.pdf

724-che-2006-correspondnece-others.pdf

724-che-2006-description(complete).pdf

724-che-2006-drawings.pdf

724-che-2006-form 1.pdf

724-che-2006-form 3.pdf

724-che-2006-form 5.pdf

724-che-2006-prioritydocument.pdf