JP2004338536A - Electric power steering apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering apparatus capable of suppressing vibrations and noise attributable to free movement of balls with a simple configuration. <P>SOLUTION: In the electric power steering apparatus comprising a housing 1, a steering shaft 2 connected to a steering mechanism to steer wheels, a conversion mechanism 6 to convert the rotational force from a steering wheel into the force to move the steering shaft 2 in the longitudinal direction, a ball screw 20 comprising a screw shaft 21 forming a part of the steering shaft 2 and a nut 22 which is externally fitted to the screw shaft 21 via a plurality of balls 23, a rolling bearing 13 which is fitted to the nut 22 to rotatably support the nut, and a motor 8 fitted to the nut 22, a rolling bearing 13 is a four-point contact ball bearing, and a rolling surface 14a of an inner ring 14 is super-finished. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a ball screw type rack assist type electric power steering device, and more particularly to an improvement of an electric power steering device which suppresses generation of vibration and noise.
[0002]
[Prior art]
2. Description of the Related Art There are various types of electric power steering devices (hereinafter referred to as EPS) for automobiles, which assist the steering force of a steering wheel with an electric motor. As one of them, an axial moving force is applied from a steering wheel to a retractable steering shaft connected to a wheel steering mechanism via a conversion mechanism such as a rack and pinion mechanism, and the output of an electric motor is output. There is one that is applied as an axial moving force via a ball screw.
[0003]
In a ball screw mechanism that is used to convert the rotation of the motor for steering force assistance by the steering wheel into the axial movement of the steering shaft, vibration and noise generated due to the play of the ball can be reduced with a simple configuration. EPS is known to suppress.
[0004]
In this EPS, as shown in FIG. 9, a steering force assist motor 55 is formed by expanding a middle part of a rack housing H1 surrounding a steering shaft (rack shaft) 51 over a suitable length. It is configured as a three-phase brushless motor having a stator 61 provided inside the motor housing H3 and a rotor 62 provided coaxially inside the stator 61 inside the motor housing H3.
[0005]
The rotor 62 is configured by holding a magnetic pole 63 facing the inner periphery of the stator 61 through a slight radial clearance on the outer periphery of a cylindrical body having an inner diameter larger than the outer diameter of the steering shaft 51. Are rotatably supported around the axis of the motor housing H3. The rotor 62 can rotate in both forward and reverse directions in accordance with energization of the stator 61.
[0006]
The motor 55 for assisting the steering force moves in the direction of the steering torque applied to the steering wheel based on a detection signal of a torque sensor (not shown) disposed inside the pinion housing H2 that houses the pinion shaft 60. Drive control is performed to generate a rotational force corresponding to the magnitude of the steering torque. By this drive control, a motion converting ball screw mechanism provided on one side of the motor 55 is converted into an axial moving force and applied to the steering shaft 51, and the steering generated by the movement of the steering shaft 51 in the rack housing H 1. Is assisted. Reference numeral 60a denotes a pinion, and 51a denotes rack teeth formed on the steering shaft 51, which mesh with the pinion 60a.
[0007]
The ball screw mechanism for motion conversion is coaxially press-fitted and fixed to one end of the rotor 62, and the cylindrical nut member 52 is rotatable into the rack housing H1 by a four-point contact ball bearing 66. I support it. Then, the female screw groove 56 formed on the inner circumference of the nut member 52 and the male screw groove 54 formed on the outer circumference of the steering shaft 51 are rotated around the axis by the transmission from the motor 55 for assisting the steering force. Are screwed together through a plurality of balls 53 rolling along.
[0008]
In the EPS that assists the steering by converting the rotation of the motor 55 driven in accordance with the steering torque into the axial movement of the steering shaft 51 by the ball screw mechanism, the vibration from the engine or the running Vibration from the road surface is transmitted to the EPS via the wheels. Since the rolling bearings 65 and 66 that rotatably support the nut member 52 with respect to the housing H3 have a clearance between the rolling surfaces of the inner ring and the outer ring and the rolling elements (balls), these vibrations are reduced. When this occurs, a rolling noise is generated, and a high-frequency vibration is sometimes generated in addition to the rolling noise.
[0009]
Various measures have been taken to solve these problems, one of which is shown in FIG. The rolling bearing 70 includes an inner ring 71 and an outer ring 72 having rolling surfaces 71a, 72a, and a plurality of balls 73, 74 which are rotatably interposed between the rolling surfaces 71a, 72a by a retainer 75. In this embodiment, a part of the plurality of balls is reduced in size, and a clearance is formed between the reduced size small ball 74 and the inner rings 71 and 72. Further, the outer peripheral surface of the steel portion 74b of the small ball 74 is covered with a cushioning member 74a such as rubber.
[0010]
As a result, the small ball 74 reduced in size acts as a weight and collides with the inner surfaces of the inner ring 71 and the outer ring 72 on the vibration direction side, so that energy is lost due to sliding friction and the collision, and the vibration is attenuated. At the same time, the rolling noise caused by the vibration can be reduced (for example, see Patent Document 1).
[0011]
[Patent Document 1]
JP-A-2002-161921 (page 3, FIG. 1)
[0012]
[Problems to be solved by the invention]
In the rolling bearing 70 in such an EPS, a load is not applied to the ball 74 having an apparently large clearance, and the rolling bearing 70 can freely move in an annular space formed by the rolling surfaces 71a and 72a. As a result, the random movement of the small ball 14 generates a vibration having a random waveform like a white noise, so that it acts as a damper for attenuating noise having a resonance peak, thereby reducing the overall sound level.
[0013]
However, when the noise generated in the actual EPS is measured, a noise peak due to a resonance phenomenon having a peak at a specific frequency is not so much recognized, and a noise in a low frequency range of 2 to 4 KHz or less. But this is often a problem. This is because the rolling of the ball of the rolling bearing or ball screw generates an alternating force that occurs when the ball and the raceway roll while receiving an external load, and the alternating force causes the components to vibrate. Conceivable. Therefore, as in the case of the conventional rolling bearings described above, it may be effective to reduce noise by secondary measures, but it is hard to say that this measure is effective for acoustic measures in all EPSs. It is desired to cut off the fundamental noise sources and take measures against acoustics.
[0014]
The present invention has been made in view of such circumstances, and in an electric power steering apparatus having a ball screw mechanism that is used to convert a rotation of a motor for steering force into an axial movement of a steering shaft, It is an object of the present invention to provide an electric power steering device that suppresses vibration and noise generated due to play of a ball with a simple configuration.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, an invention according to claim 1 of the present invention provides a substantially cylindrical housing, a steering shaft penetrating the housing and connected to a steering mechanism for steering wheels, and a steering wheel. A conversion mechanism for converting the rotational force from the steering shaft into a force for moving the steering shaft in the longitudinal direction, and a part of the steering shaft becomes a screw shaft, which is externally fitted to the screw shaft via a plurality of balls. A ball screw comprising a nut, a rolling bearing fitted to the nut of the ball screw and rotatably supporting the nut with respect to the housing; a stator fixed to an inner peripheral surface of the housing; A motor comprising a rotor opposed to an inner peripheral surface of the nut via a predetermined radial clearance and fitted to the nut. A three-point or ball bearings 4-point contact, and employs a configuration subjected to superfinishing to at least the inner ring of the rolling run surface.
[0016]
As described above, the rolling bearing is a three-point or four-point contact ball bearing, and at least the rolling surface of the inner ring is super-finished to reduce noise over a range from a low speed region to a high speed region of the EPS. Can be.
[0017]
Preferably, when the maximum amplitude width of the undulation in the rolling direction on the rolling surface is restricted to 1 μm or less, the noise reduction effect is remarkably exhibited.
[0018]
More preferably, if the surface roughness in the rolling direction on the rolling surface is regulated to Ra 0.1 or less as in the invention according to claim 3, the noise reduction effect becomes remarkable.
[0019]
In the invention described in claim 4, since the rolling bearing is fitted to the housing via an elastic member, vibration and noise generated due to the play of the ball in the ball screw are reduced by the elastic member. It can be buffered, and can be efficiently controlled at a low cost with a simple configuration. Furthermore, a ball screw type rack assist type electric power steering device which can tolerate misalignment due to assembly with this elastic member and has no torque unevenness can be provided.
[0020]
Further, the same effect can be obtained even if the rolling bearing is fitted to the nut via an elastic member as in the invention described in claim 5.
[0021]
In the invention described in claim 6, since the elastic member is made of synthetic resin or rubber, the productivity is good, the weight can be reduced, and the cost can be reduced.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a partially cutaway side view showing an embodiment of an electric power steering device according to the present invention.
[0023]
The motor housing 1 is fixed to the vehicle body via a bracket (not shown). A steering shaft 2 extends through the motor housing 1. Tie rods 3 and 4 are connected to both ends of the steering shaft 2 and are connected to a steering mechanism (not shown) for steering wheels. A pinion shaft 5 is provided so as to extend obliquely upward from one side of the motor housing 1, and the pinion shaft 5 is connected at an upper end to a steering wheel (not shown). The pinion shaft 5 is rotatably supported, and its rotation is transmitted to the steering shaft 2 as a moving force in the axial direction via the conversion mechanism 6 at the lower end. The conversion mechanism 6 includes a rack tooth 7 formed at a part of the steering shaft 2 in the longitudinal direction, and a pinion (not shown) provided at a lower end of the pinion shaft 5 and meshing with the rack tooth 7. A torque detector (not shown) for detecting the steering torque and transmitting the detection signal to a motor control device (not shown) is provided at a part of the pinion shaft 5.
[0024]
The motor housing 1 is formed in a substantially cylindrical shape, and is configured such that support members 1b and 1c are respectively coupled to both ends of a central cylindrical body 1a. A stator 9 constituting the motor 8 is press-fitted and fixed at the center of the inner periphery of the motor housing 1. The stator 9 includes a core and a stator coil, and a rotor 10 of the motor 8 is disposed on the inner periphery of the stator 9 with a predetermined radial clearance therebetween. The rotor 10 is formed of a magnetic material into a cylindrical shape, and is integrally fixed to the outer periphery of the sleeve 11. The steering shaft 2 is inserted through the inner periphery of the sleeve 11 so as to be movable in the axial direction. The motor 8 is controlled by a motor control device (not shown) according to the detection signal of the above-described torque detector.
[0025]
One end (left side in the figure) of the sleeve 11 is rotatably supported in the motor housing 1 by a rolling bearing (ball bearing) 12. The rolling bearing 12 may be a single-row bearing or a double-row bearing, as long as it can apply a radial load and a thrust load.
[0026]
The rotation of the motor 8 is transmitted via the ball screw 20 to the steering shaft 2 as an axial moving force. In the ball screw 20, a part of the steering shaft 2 in the axial direction is configured as a ball screw shaft 21. As shown in FIG. 2, a nut 22 constituting the ball screw 20 is rotatably supported in the motor housing 1 via a rolling bearing 13 fitted to an outer diameter portion thereof. Further, one end of the rotor 10 is fitted to one end of the nut 22.
[0027]
The rolling bearing 13 is a four-point contact ball bearing as shown in FIG. 3, and includes an inner ring 14, an outer ring 15, and a plurality of balls 16 interposed between the inner and outer rings 14, 15. The rolling surface 14a formed on the outer peripheral surface of the inner ring 14 and the rolling surface 15a formed on the inner peripheral surface of the outer ring 15 have a pair of centers of curvature which are offset equidistantly in the axial direction with respect to the axial center of the bearing. It is formed in an arc surface, a so-called Gothic arch shape. Therefore, this four-point contact ball bearing can apply a radial load and a thrust load in both directions while being a single-row bearing. In addition, the range of the axial clearance can be suppressed with respect to the radial clearance, and the play of the ball screw 20 after assembly into the EPS can be suppressed.
[0028]
Conventionally, rolling bearings consisting of four-point contact ball bearings are used in the low rotation range, and because the shape of the rolling surface is difficult to machine with a gothic arch shape, the rolling surface is super-finished by grinding only. (Hereinafter referred to as SF) was not applied. Here, the rolling surfaces 14a and 15a of the rolling bearing 13 are subjected to SF after the grinding process, and in particular, the shape error components such as roughness and undulation in the rolling direction of the ball 16 on the rolling surfaces 14a and 15a are improved. Have been.
[0029]
FIGS. 4A and 4B show the surface roughness in the axial direction depending on the presence or absence of SF on the rolling surface 14a of the inner ring 14 of the rolling bearing 13. FIG. The sample is a four-point contact ball bearing having an inner diameter of φ40, an outer diameter of φ75, and a width of 16 mm. As can be seen from this, the surface roughness is improved to approximately 1/2 in (b) of this embodiment as compared with the conventional rolling surface shown in FIG. Preferably, by regulating the surface roughness in the rolling direction to Ra 0.1 or less, this effect is remarkably exhibited.
[0030]
FIGS. 5A and 5B show the roundness depending on the presence or absence of SF on the rolling surface 14 a of the inner ring 14 of the rolling bearing 13. Compared to the conventional rolling surface shown in FIG. 5 (a), in (b) of this embodiment, the tendency of the roundness itself has not changed, but it can be seen that the swell component in a narrow range is improved. . Preferably, by regulating the maximum amplitude width of the rolling in the rolling direction to 1 μm or less, this effect becomes remarkable.
[0031]
Further, the present applicant incorporated the rolling bearing 13 having such an inner ring 14 into an EPS, performed acoustic measurement at a position 300 mm apart, and made a comparison with a conventional one. FIG. 6 shows the result. As for the noise level, a reduction effect of 2.3 to 3.5 dB-A was confirmed from the low speed range to the high speed range as compared with the conventional rolling bearing. In the present embodiment, an example in which SF is applied only to the rolling surface 14a of the inner ring 14 in the rolling bearing 13 has been described. However, by applying SF to the rolling surface 15a of the outer ring 15 as well, there is no effect equivalent to that of the inner ring 14. However, an effect of about 50% was further recognized.
[0032]
Normally, rolling of a ball of a rolling bearing or a ball screw generates an alternating force generated when the ball and the raceway roll while receiving an external load, and the alternating force causes the components to vibrate. In the embodiment, SF is applied to the rolling surface 14a of the inner ring 14 on the rotating side, and the surface roughness and roundness in the rolling direction are regulated to predetermined values. Noise could be reduced.
[0033]
Further, in the present embodiment, a pair of annular grooves 15 b are provided in parallel on the outer peripheral surface of the outer ring 15 of the rolling bearing 13. An elastic member 17 made of a ring-shaped PA (polyamide) 11 or the like is mounted in the annular groove 15b. Here, as the material of the elastic member 17, PA11 having a relatively high heat-resistant temperature and a high electrical insulation property is used by adding an appropriate amount of graphite which is advantageous for settling or the like to PA11. The material of the elastic member 17 is not limited to this, and may be any material having a sufficiently smaller Young's modulus than steel. Therefore, other synthetic resins and rubbers, for example, urethane rubber, natural rubber, chloropyrene rubber, nitrile rubber and the like can be exemplified. Further, the number of the annular grooves 15b formed on the outer peripheral surface of the outer ring 15 is not limited to one pair, and may be one or three or more. Reference numeral 18 denotes a seal which is mounted on the outer ring 15 and seals an annular space between the outer ring 15 and the inner ring 14. It is preventing.
[0034]
As described above, since the rolling bearing 13 that supports the nut 22 is fitted to the supporting member 1b that constitutes the motor housing 1 via the elastic member 17, vibration generated due to the free movement of the ball in the ball screw 20 can be reduced. Noise can be buffered by the elastic member 17 and can be efficiently suppressed at low cost with a simple configuration. Further, misalignment due to assembly can be tolerated by the elastic member 17, and electric corrosion of the rolling bearing 13 due to leakage current of the motor 8 can be prevented. Therefore, it is suitable for mass production because it does not increase the accuracy of each part and does not lower the workability of assembly.
[0035]
According to the results of the noise level verification performed by the present applicant, in the four-point contact ball bearing 13 illustrated in FIG. 3, when urethane rubber is used as the elastic member 17, if the rotation speed of the nut increases to some extent, the elastic member is used. The noise suppression effect was remarkable, and the rolling surface 14a of the inner ring 14 was reduced by 2 dB-A at the maximum without applying SF. Therefore, it is considered that noise can be reduced by 4 dB-A or more by further applying the above-described SF to the rolling surface 14a.
[0036]
Alternatively, an elastic member may be integrally fixed to the outer peripheral surface of the outer ring 15 by injection molding or bonding so as to cover the outer peripheral surface, or a pair of annular grooves 15b may be arranged on the outer peripheral surface of the outer ring 15 in parallel. An elastic member may be integrally formed by injection molding so as to cover the outer peripheral surface including the annular groove 15b and the end surface.
[0037]
FIG. 7 is an enlarged view of a main part with a part broken away showing another embodiment of the EPS according to the present invention. The same parts as those in the above-described embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0038]
A nut 27 constituting the ball screw 26 is rotatably supported in the motor housing 1 via a rolling bearing 28 fitted to an outer diameter portion thereof. Further, one end of the rotor 10 is fitted to one end of the nut 27. The rolling bearing 28 is positioned and fixed in the axial direction by a flange 27 a formed at an end of the nut 27 and a retaining ring 31 mounted on the outer peripheral surface of the nut 27. A pair of annular grooves 29 a are provided in parallel on the inner peripheral surface of the inner ring 29 of the rolling bearing 28. An elastic member 30 made of a synthetic resin such as a ring-shaped PA (polyamide) 11 is mounted in the annular groove 29a. The number of the annular grooves 29a formed on the outer peripheral surface of the inner ring 29 is not limited to one pair, and may be single or three or more. Then, the elastic member 30 may be attached.
[0039]
FIG. 8 is an enlarged view of a main part with a part broken away showing another embodiment of the EPS according to the present invention. The same parts as those in the above-described embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0040]
The nut 32 constituting the ball screw 26 'is rotatably supported in the motor housing 1 via a rolling bearing 33 fitted to an outer diameter portion thereof. A pair of annular grooves 32a are arranged in parallel on the outer peripheral surface of the nut 32, and an elastic member 34 made of a synthetic resin such as a ring-shaped PA (polyamide) 11 is mounted in the annular grooves 32a. The number of the annular grooves 32a formed on the outer peripheral surface of the nut 32 is not limited to one pair, and may be a single or three or more.
[0041]
In this manner, the rolling bearings 28, 33 supporting the nuts 27, 32 are fitted to the nuts 27, 32 via the elastic members 30, 34, and SF is applied to at least the rolling surfaces of the inner rings 29, 14. Therefore, similarly to the above-described embodiment, it is possible to reduce the rolling noise generated due to the play of the balls in the ball screws 26, 26 ′, and it is possible to efficiently suppress the rolling noise with a simple configuration. Further, misalignment due to assembly can be tolerated by the elastic members 30 and 34, and electrolytic corrosion of the rolling bearings 28 and 33 due to the leakage current of the motor 8 can be prevented.
[0042]
As described above, the four-point contact ball bearing is exemplified as the rolling bearing. However, such a rolling bearing is not limited to this. For example, the outer ring has two points, the inner ring has one point, or the opposite outer ring has one point, and the inner ring has two points. In this case, a so-called three-point contact ball bearing that contacts the ball may be used. Further, it is effective to apply SF to at least the rolling surface of the inner wheel which is a rotating wheel. Further, the SF may be one that swings (oscillates) the grindstone for SF, or may be a traverse method without swinging. Furthermore, the SF is not limited to the SF using a grindstone, and may be the SF based on wrapping of paper or the like.
[0043]
As described above, the embodiments of the present invention have been described. However, the present invention is not limited to these embodiments at all, but is merely an example, and may be variously modified without departing from the gist of the present invention. The scope of the present invention is, of course, indicated by the appended claims, and further includes the equivalent meanings described in the appended claims and all modifications within the scope. Including.
[0044]
【The invention's effect】
As described in detail above, the electric power steering apparatus according to the present invention includes a substantially cylindrical housing, a steering shaft that penetrates the housing and is connected to a steering mechanism that steers wheels, and rotation from the steering wheel. A conversion mechanism for converting the force into a force for moving the steering shaft in the longitudinal direction, and a nut in which a part of the steering shaft is a screw shaft and which is externally fitted to the screw shaft via a plurality of balls. A ball screw, a rolling bearing fitted to a nut of the ball screw and rotatably supporting the nut with respect to the housing; a stator fixed to an inner peripheral surface of the housing; and an inner periphery of the stator. A motor consisting of a rotor opposed to the surface via a predetermined radial clearance and fitted to the nut. Is a three-point or four-point contact ball bearing, and at least the rolling surface of the inner ring is super-finished, so that noise can be effectively reduced over a low to high speed range of the EPS with a simple configuration. be able to.
[Brief description of the drawings]
FIG. 1 is a partially cutaway side view showing an embodiment of an electric power steering device according to the present invention.
FIG. 2 is an enlarged view of a main part of the same, partially cut away.
FIG. 3 is a sectional view showing an embodiment of the rolling bearing according to the present invention.
FIG. 4 shows the surface roughness in the axial direction depending on the presence or absence of SF on the rolling surface of the inner ring of the rolling bearing, where (a) shows the conventional surface roughness and (b) shows the inner surface rolling surface according to the present invention. Is shown.
5A and 5B show the roundness of the rolling surface of the inner ring of the rolling bearing depending on the presence or absence of SF. FIG. 5A shows the conventional rolling surface, and FIG. 5B shows the rolling surface of the inner ring according to the present invention. .
FIG. 6 is a graph for verifying a noise level depending on the presence or absence of SF on a rolling surface of an inner ring. FIG. is there.
FIG. 8 is an enlarged view of a main part with a part cut away showing still another embodiment of the electric power steering apparatus according to the present invention.
FIG. 9 is a partially cutaway side view showing a conventional electric power steering device.
FIG. 10 is a partially broken perspective view showing a conventional rolling bearing.
[Explanation of symbols]
1 Motor housing 1a Cylindrical body 1b 1c・ ・ ・ ・ ・ Support member 2 ・ ・ ・ ・ ・ ・ ・ ・ ・ Steering shafts 3、4 ・ ・ ・ ・ ・ ・ Tie rods 5 ・ ・ ・... Pinion shaft 6 ... Conversion mechanism 7 ... ··· Rack teeth 8 ··· Motor 9 ··· Stator 10 ... Rotator 11 ... Sleeve 12, 13, 28, 33 ... Rolling bearing 14, 29 ... Inner rings 14a, 15a Rolling surface 15 ... Outer rings 15b, 29a, 32a ... Annular grooves 16 ... Balls 17, 30, 34 Elastic member 18 Seal 20, 26, 26 'Ball screw 21 ... Screw shafts 22, 27, 32 Nuts 27a Flanges 31 Retaining ring 51 Steering shaft 51a ... Rack teeth 52Nut member 53Ball 54・ ・ ・ ・ ・ ・ ・ ・ ・ ・ Male thread groove 55 ・ ・ ・ ・ ・ ・ ・ Mo Taper 56 Female thread groove 60 Pinion shaft 60a ... Pinion 61 ... Stator 62 ... Rotor 63・ ・ ・ ・ ・ ・ ・ ・ ・ Magnetic poles 64, 65 ・ ・ ・ ・ ・ ・ Ball bearing 66 ・ ・ ・ ・ ・ ・ ・ ・ ・ 4 point contact ball bearing 70 Rolling bearings 71 Inner rings 71a, 72a・ Rolling surface 72 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ Ball 74 ・ ・ ・ ・ ・ ・ ・ ・ ・... Small ball 74a・ ・ ・ Steel part 75 ・ ・ ・ Holder H1 ・ ・ ・ Rack Housing H2: Pinion housing H3: Motor housing

Claims (6)

A substantially cylindrical housing, a steering shaft that penetrates the housing and is connected to a steering mechanism that steers wheels, and a rotational force from the steering wheel is converted into a force for moving the steering shaft in the longitudinal direction. A ball screw composed of a conversion mechanism, a part of the steering shaft serving as a screw shaft, a nut externally fitted to the screw shaft via a plurality of balls, and a housing fitted with the nut of the ball screw, A rolling bearing that rotatably supports the nut with respect to the stator, a stator fixed to the inner peripheral surface of the housing, and a predetermined radial clearance that faces the inner peripheral surface of the stator, and is fitted to the nut. And a motor comprising a combined rotor,
An electric power steering device, wherein the rolling bearing is a three-point or four-point contact ball bearing, and at least the rolling surface of the inner ring is subjected to superfinishing. The electric power steering device according to claim 1, wherein the maximum amplitude width of the undulation in the rolling direction on the rolling surface is restricted to 1 µm or less. The electric power steering device according to claim 1, wherein a surface roughness of the rolling surface in a rolling direction is regulated to Ra 0.1 or less. The electric power steering device according to claim 1, wherein the rolling bearing is fitted to the housing via an elastic member. The electric power steering device according to claim 1, wherein the rolling bearing is fitted to the nut via an elastic member. The electric power steering device according to claim 4 or 5, wherein the elastic member is made of synthetic resin or rubber.

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