JP5270393B2 - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve steering feeling by optimizing the operation of a ball screw mechanism in an electric power steering device. <P>SOLUTION: In this electric power steering device, an output from a hollow rotary shaft 11 of a motor 10 is transmitted to a steering shaft 20 via the ball screw mechanism 30. The ball screw mechanism 30 includes a screw shaft 31 integrated to the steering shaft 20, a nut 32 integrally fastened and fixed to the hollow rotary shaft 11 using an annular fixture 12, a plurality of deflectors (34, 36) attached axially both ends and an axially intermediate part of the nut 32 to form ball endless circulation passages, and a plurality of balls B1-B4 housed in the ball endless circulation passages to rotate freely and enabling the nut 32 and the screw shaft 31 to relatively rotate. A rigidity lowering part (elongated groove 32e) for lowering rigidity in the axial direction is provided to the nut 32. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an electric power steering apparatus provided in a vehicle.

  As one of the electric power steering devices, a hollow rotating shaft of an electric motor assembled to be rotatable with respect to the housing and not movable in the axial direction, and through the hollow rotating shaft and not rotatable with respect to the housing A steering shaft assembled to be movable in the axial direction, and coaxially interposed between the hollow rotating shaft and the steering shaft, the rotation of the hollow rotating shaft is converted into axial movement of the steering shaft and transmitted. There is a ball screw mechanism that transmits the output from the hollow rotary shaft to the steering shaft via the ball screw mechanism.

  In the electric power steering apparatus described in Patent Document 1 below, the ball screw mechanism includes a screw shaft that is integral with the steering shaft and provided with a male screw groove on an outer peripheral portion, and an annular shape with respect to the hollow rotary shaft. A cylindrical nut having an internal thread groove on an inner periphery thereof, which is integrally tightened and fixed using a fixture and forms a spiral ball rolling path with the male thread groove of the screw shaft, and the spiral ball rolling A plurality of balls which are accommodated in a motion path so as to be able to roll and allow the nut and the screw shaft to rotate relative to each other are provided.

JP 2000-168589 A

  In the electric power steering apparatus described in Patent Document 1 described above, the cylindrical nut in the ball screw mechanism is fastened and fixed to the hollow rotary shaft of the motor using an annular fixture, and the annular fixed The ball screw mechanism may not be able to obtain a smooth operation due to the axial deformation of the nut due to the tightening load of the tool, and a good feeling may not be obtained during steering. Such a problem is that a deflector (sometimes referred to as a top) is assembled in each mounting hole formed at a predetermined circumferential interval at both axial end portions and axial intermediate portions of the nut, When each ball infinite circulation path is formed by a ball rolling path, it occurs remarkably when the diameter (diameter) of a plurality of balls accommodated in each ball infinite circulation path is the same. The amount of axial deformation of the nut at the circumferential location where the mounting hole for assembling the deflector is formed is large in the vicinity of the mounting hole, so that the distance from the mounting hole is small, and the axial direction of the nut The maximum value of the axial deformation amount in the ball endless circulation path formed in the axial intermediate portion of the nut is larger than the maximum value of the axial deformation amount in the pair of ball infinite circulation paths formed at both ends. large Inventors bets is has become known by analyzing the various experiments.

  The present invention has been made on the basis of the above-mentioned known matters to solve the above-described problem (malfunction of the ball screw mechanism), and is assembled so as to be rotatable with respect to the housing and immovable in the axial direction. A hollow rotating shaft of the motor, a steering shaft that passes through the hollow rotating shaft and is assembled so as to be non-rotatable with respect to the housing and movable in the axial direction, and coaxially between the hollow rotating shaft and the steering shaft. And a ball screw mechanism that converts the rotation of the hollow rotary shaft into an axial movement of the steering shaft and transmits it, and the ball screw mechanism is formed integrally with the steering shaft and has a male screw groove on the outer periphery. A female screw groove that forms a spiral ball rolling path with the provided screw shaft and the male screw groove of the screw shaft that is integrally tightened and fixed to the hollow rotary shaft using an annular fixture. Cylindrical nut A plurality of deflectors which are assembled in mounting holes formed in both axial end portions and an axial intermediate portion of the nut to form a plurality of ball infinite circulation paths by the ball rolling paths, and the balls An electric power steering apparatus that includes a plurality of balls that are rotatably accommodated in an infinite circulation path and that allow the nut and the screw shaft to rotate relative to each other, wherein the deflectors are arranged at predetermined circumferential intervals. In the circumferential location where the mounting hole for assembling the deflector is formed at one end of the nut in the axial direction, axial rigidity is provided from the other end in the axial direction of the nut to the intermediate portion in the axial direction. In a circumferential place where a rigidity reduction portion for lowering is provided and a mounting hole for assembling the deflector is formed at the other axial end portion of the nut, the axial direction extends from one axial end portion of the nut. In the circumferential part where a rigidity reduction part for reducing the axial rigidity is provided in the intermediate part and a mounting hole for assembling the deflector is formed in the axial intermediate part of the nut, the nut shaft One feature is that a rigidity reduction portion for reducing the rigidity in the axial direction is provided at one end in the direction and the other end in the axial direction. In this case, it is preferable that four deflectors are arranged at intervals of 90 degrees in the circumferential direction, and the rigidity reduction portion is formed by partially scraping the outer periphery of the nut to make it thin. It is desirable.

  In the electric power steering apparatus according to the present invention, the cylindrical nut in the ball screw mechanism coaxially interposed between the hollow rotating shaft of the electric motor and the steering shaft penetrating the hollow rotating shaft uses the annular fixture. The nut is deformed in the axial direction by the tightening load of the annular fixture.

  By the way, in this invention, each deflector each assembled | attached to each attachment hole formed in the axial direction both ends and axial direction intermediate part of a nut is arrange | positioned by predetermined circumferential direction space | interval. Also, at the circumferential location where the mounting hole for assembling the deflector is formed at one end of the nut in the axial direction, the rigidity to reduce the axial rigidity from the other axial end of the nut to the axial middle A drop portion is provided. On the other hand, at a circumferential location where a mounting hole for assembling the deflector is formed at the other axial end of the nut, rigidity for reducing the axial stiffness from one axial end of the nut to the middle axial portion A drop portion is provided. Furthermore, at the circumferential location where a mounting hole for assembling the deflector is formed at the axially intermediate portion of the nut, rigidity for reducing axial rigidity at one axial end and the other axial end of the nut Each drop part is provided.

  For this reason, the rigidity in the axial direction of the nut is the same at a predetermined circumferential interval where each deflector is arranged, and the axial deformation of the nut due to the tightening load of the annular fixture is arranged in each deflector. It occurs equally at predetermined circumferential intervals. Therefore, it is possible to suppress local axial deformation at a specific location in the circumferential direction of the nut, and the amount of axial deformation in the pair of ball infinite circulation paths formed at both axial end portions of the nut. Can be made equal to the maximum value of the amount of axial deformation in the ball infinite circulation path formed in the axially intermediate portion of the nut. As a result, in the ball screw mechanism, even if the diameter of each ball accommodated in each ball infinite circulation path is the same in consideration of the maximum value of the axial deformation amount in each ball infinite circulation path, Each ball can move smoothly in an infinite circulation path, and a smooth motion can be obtained, and a good feeling can be obtained during steering.

It is the principal part expanded sectional view which showed roughly one Embodiment of the electrically driven power steering device by this invention. FIG. 2 is a left side view of the nut shown in FIG. 1 (a nut in which four deflectors are assembled at equal intervals in the circumferential direction). It is the figure seen from the arrow 3 direction of FIG. FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. FIG. 2 is a developed view of the inner periphery of the nut and the deflector in the state shown in FIG. 1 (a state where the nut is tightened by an annular fixture). FIG. 6 is a view corresponding to FIG. 3 schematically showing a first modified embodiment of the nut shown in FIGS. 1 to 5 (a nut in which four deflectors are assembled at equal intervals in the circumferential direction). FIG. 7 is a cross-sectional view corresponding to FIG. 4 of the nut according to the first modified embodiment shown in FIG. 6 (a nut in which four deflectors are assembled at equal intervals in the circumferential direction). FIG. 8 is a developed inner peripheral view corresponding to FIG. 5 of the nut of the first modified embodiment shown in FIGS. 6 and 7 (a nut in which four deflectors are assembled at equal intervals in the circumferential direction). FIG. 6 is a view corresponding to FIG. 3 schematically showing a second modified embodiment of the nut shown in FIGS. 1 to 5 (a nut in which four deflectors are assembled at equal intervals in the circumferential direction). FIG. 10 is a cross-sectional view corresponding to FIG. 4 of the nut of the second modified embodiment shown in FIG. 9 (a nut in which four deflectors are assembled at equal intervals in the circumferential direction). FIG. 11 is an inner peripheral development corresponding to FIG. 5 of the nut of the second modified embodiment shown in FIGS. 9 and 10 (a nut in which four deflectors are assembled at equal intervals in the circumferential direction). (A) shows the relationship between a virtual nut that is not tightened by an annular fixture (a nut that does not have a deflector mounting hole and is not assembled with a deflector) and the screw shaft shown in FIG. FIG. 2B is a cross-sectional view showing a relationship between a virtual nut fastened by an annular fixture and the screw shaft shown in FIG. 1. A state in which a nut (with four deflectors assembled at equal intervals in the circumferential direction) that is not provided with a rigidity reduction portion for reducing the axial rigidity of the nut is tightened by an annular fixture FIG.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 schematically shows an essential part of an electric power steering apparatus according to the present invention. The electric power steering apparatus includes a hollow rotating shaft 11 of an electric motor 10 and a housing that passes through the hollow rotating shaft 11. A steering shaft 20 that is assembled so as to be non-rotatable with respect to 40 and movable in the axial direction, and is coaxially interposed between the hollow rotating shaft 11 and the steering shaft 20 to rotate the hollow rotating shaft 11. A ball screw mechanism 30 is provided that converts it into axial movement and transmits it.

  The rotating shaft 11 of the electric motor 10 is assembled to the housing 40 via a bearing (not shown) at its right end (not shown) so as to be rotatable and immovable in the axial direction, and at its left end via a bearing 50. The housing 40 is assembled so as to be rotatable and immovable in the axial direction. On the other hand, the steering shaft 20 is provided so as to extend in the left-right direction of the vehicle, and is assembled with the housing 40 so as not to rotate and to move in the axial direction. (Not shown) is steered.

  As shown in FIGS. 1 to 5, the ball screw mechanism 30 is coaxially and integrally rotatable with respect to the hollow rotary shaft 11 and a screw shaft 31 that is integral with the steering shaft 20 and has a male screw groove on the outer periphery. A cylindrical nut 32 disposed in the four, four deflectors 33, 34, 35, 36 assembled to the nut 32, the deflectors 33, 34, 35, 36, the screw shaft 31 and the nut 32. A plurality of balls (steel balls) B1, B2, B3, and B4 are provided so as to be rollable in the formed four ball infinite circulation paths S1, S2, S3, and S4.

  The nut 32 has a female screw groove that forms a spiral ball rolling path Ho with the male screw groove of the screw shaft 31, and is integrated with the hollow rotary shaft 11 using the annular fixture 12. It is tightened and fixed. The annular fixture 12 is screwed to a female screw provided on the inner periphery of the left end of the hollow rotary shaft 11 with a male screw provided on the outer periphery thereof, and the nut 32 is attached to the hollow rotary shaft 11 with a predetermined tightening load. On the other hand, it is fixed so that it can rotate integrally.

  The deflector 33 assembled to the right end portion of the nut 32 in the axial direction is assembled to the mounting hole 32a formed at the right end portion of the nut 32 in the axial direction, and has itself for circulating the ball B1. The first ball infinite circulation path S1 is formed by the communication path (see FIG. 3) and a part of the spiral ball rolling path Ho (substantially one round). The deflector 36 assembled at the left end of the nut 32 in the axial direction is assembled into a mounting hole 32d formed at the left end of the nut 32 in the axial direction. The deflector 36 has itself for circulating the ball B4. A fourth ball infinite circulation path S4 is formed by the communication path and a part (substantially one round) of the spiral ball rolling path Ho.

  The deflector 34 assembled on the right side in the axial direction intermediate portion of the nut 32 is assembled in the mounting hole 32b formed on the right side in the axial direction intermediate portion of the nut 32 and has itself to circulate the ball B2. The second ball infinite circulation path S2 is formed by the communication path for this purpose and a part of the spiral ball rolling path Ho (substantially one round). The deflector 35 assembled to the left side of the intermediate portion in the axial direction of the nut 32 is assembled to a mounting hole 32c formed on the left side of the intermediate portion of the nut 32 in order to circulate the ball B3. The third ball infinite circulation path S3 is formed by the communication path and a part of the spiral ball rolling path Ho (approximately one round). Each deflector 31-34 is formed in the same shape, and as shown in FIG. 2, it is arrange | positioned at equal intervals (90 degree intervals) in the circumferential direction.

  The ball B1 is filled (accommodated in a large number) in the first ball infinite circulation path S1, and the nut 32 and the screw shaft 31 can be rotated relative to each other. The balls B2 are filled (accommodated in a large number) in the second ball infinite circulation path S2, and the nut 32 and the screw shaft 31 can be rotated relative to each other. The ball B3 is filled (accommodated in a large number) in the third ball infinite circulation path S3, and the nut 32 and the screw shaft 31 can be rotated relative to each other. The ball B4 is filled (accommodated in a large number) in the fourth ball infinite circulation path S4, and the nut 32 and the screw shaft 31 can be rotated relative to each other.

  By the way, in this embodiment, in the circumferential location where the mounting hole 32a for assembling the deflector 33 is formed at the right end of the nut 32 in the axial direction, the nut 32 is moved from the left end in the axial direction to the intermediate portion in the axial direction. A plurality of long grooves 32e are formed. Further, at the circumferential position where the mounting hole 32d for assembling the deflector 36 is formed at the left end in the axial direction of the nut 32, three long grooves 32e are formed from the right end in the axial direction of the nut 32 to the intermediate portion in the axial direction. Has been.

  Further, at the circumferential location where the mounting hole 32 b for assembling the deflector 34 is formed on the right side of the axially intermediate portion of the nut 32, one long groove 32 e is formed at the axially right end portion of the nut 32. Two long grooves 32e are formed at the left end in the axial direction. Further, at a circumferential location where a mounting hole 32 c for assembling the deflector 35 is formed on the left side in the axially intermediate portion of the nut 32, two long grooves 32 e are formed at the axially right end portion of the nut 32. One long groove 32e is formed at the left end in the axial direction.

  Each long groove 32e described above is a rigidity reduction portion for reducing the axial rigidity of the nut 32, and is formed so as to be parallel to the attachment holes 32a, 32b, 32c, and 32d. Each long groove 32e is formed by partially scraping the outer peripheral portion of the nut 32 with a predetermined width to make it thin, and the center portion in the circumferential direction is deepest and the both end portions in the circumferential direction are shallowest. Has been.

  In addition, in the nut 32 of this embodiment, similarly to the imaginary nut 132 shown in FIG. 12 (the nut in which the mounting hole for the deflector is not provided in the nut and the deflector is not assembled), the annular fixture 12 is used. The pitch P1 of the female thread groove in the untightened free state (see FIG. 12 (a)) is the pitch P2 of the female thread groove in the assembled state (see FIG. 12 (b)) tightened by the annular fixture 12. It is larger by a predetermined amount. For this reason, in the hypothetical nut 132 tightened by the annular fixture (12), the pitch P2 of the female thread groove coincides with the pitch P2 of the male thread groove on the screw shaft 31, as shown in FIG. It is set to be.

  In this embodiment configured as described above, the deflectors 33, 34, and 35 respectively assembled to the mounting holes 32a, 32b, 32c, and 32d formed in the both axial end portions and the axial intermediate portion of the nut 32. , 36 are arranged at predetermined circumferential intervals (intervals of 90 degrees). Further, at a circumferential position where a mounting hole 32a for assembling the deflector 33 is formed at the right end of the nut 32 in the axial direction, three long grooves 32e (nuts from the left end of the nut 32 in the axial direction to the intermediate portion in the axial direction are provided. 32, a rigidity reduction portion for reducing the rigidity in the axial direction at 32 is formed. Further, at the circumferential position where the mounting hole 32d for assembling the deflector 36 is formed at the left end in the axial direction of the nut 32, three long grooves 32e are formed from the right end in the axial direction of the nut 32 to the intermediate portion in the axial direction. Has been.

  Further, at the circumferential location where the mounting hole 32 b for assembling the deflector 34 is formed on the right side of the axially intermediate portion of the nut 32, one long groove 32 e is formed at the axially right end portion of the nut 32. Two long grooves 32e are formed at the left end in the axial direction. Further, at a circumferential location where a mounting hole 32 c for assembling the deflector 35 is formed on the left side in the axially intermediate portion of the nut 32, two long grooves 32 e are formed at the axially right end portion of the nut 32. One long groove 32e is formed at the left end in the axial direction.

  For this reason, the rigidity of the nut 32 in the axial direction is equal at a predetermined circumferential interval at which the deflectors 33, 34, 35, and 36 are disposed, and the nut 32 is caused by the tightening load of the annular fixture 12. These axial deformations occur equally at predetermined circumferential intervals at which the deflectors 33, 34, 35, and 36 are disposed. Accordingly, local deformation in the axial direction at a specific location in the circumferential direction of the nut 32 (each ball infinite circulation occurring at both circumferential ends of each deflector 33, 34, 35, 36 in the development view of the nut 232 shown in FIG. 13). It is possible to suppress the occurrence of the deformation (see the deformation of the paths S1 to S4).

  For this reason, in this embodiment, the axial deformation of the nut 32 can be brought close to an ideal state (the axial deformation of the virtual nut 132 shown in FIG. 12), The maximum amount of axial deformation in the pair of ball infinite circulation paths S1 and S4 formed and the amount of axial deformation in the pair of ball infinite circulation paths S2 and S3 formed in the intermediate portion of the nut 32 in the axial direction. It is possible to make the maximum values equivalent. Thereby, in the said ball screw mechanism 30, in consideration of the maximum value of the axial direction deformation amount in each said ball infinite circuit S1-S4, each ball B1- accommodated in each ball infinite circuit S1-S4. Even if the diameter of B4 is the same, each ball B1 to B4 can move smoothly in each ball infinite circulation path S1 to S4, it is possible to obtain a smooth operation, and a good feeling during steering is obtained. It is possible.

  Note that the nut 232 shown in FIG. 13 is not provided with the long groove 32e of the above-described embodiment on the outer periphery thereof. In this nut 232, each of the deflectors 33, 34, 35, and 36 for assembling the deflectors is assembled. The amount of axial deformation of the nut 232 at the circumferential location where the mounting holes 232a, 232b, 232c, 232d are formed is large in the vicinity of the mounting holes 232a, 232b, 232c, 232d, and the mounting holes 232a, 232b. 232c and 232d are small in size, and local axial deformation occurs in each of the ball infinite circulation paths S1 to S4 at the circumferential ends of the deflectors 33, 34, 35, and 36. In addition, a pair of balls formed at the intermediate portion in the axial direction of the nut 232 as compared with the maximum amount of axial deformation in the pair of ball infinite circulation paths S1 and S4 formed at both ends in the axial direction of the nut 232 The maximum value of the axial deformation amount in the infinite circulation paths S2 and S3 is large.

  Therefore, in the ball screw mechanism employing the nut 232, when the diameters of the balls B1 to B4 accommodated in the ball infinite circulation paths S1 to S4 are the same, for example, the axial direction intermediate of the nut 232 is used. Even if the balls B2 and B3 move smoothly in the pair of ball endless circulation paths S2 and S3 formed in the portion, the pair of ball endless circulation paths S1 and S4 formed at both ends in the axial direction of the nuts 232 In this case, the balls B1 and B4 do not move smoothly, a smooth operation cannot be obtained, and a good feeling may not be obtained during steering.

  In the above-described embodiment, each of the long grooves 32e that is thinned by partially scraping the outer peripheral portion of the nut 32 with a predetermined width is employed as a rigidity reducing portion for reducing the axial rigidity of the nut 32. However, instead of the long grooves 32e, the large, medium, and small round holes 32f1, 32f2, and 32f3 shown in FIGS. 6 to 8, or the large, medium, and small notches 32g1 shown in FIGS. , 32g2, and 32g3 can be employed. The large / medium / small round holes 32f1, 32f2, 32f3 shown in FIGS. 6 to 8 and the large / medium / small notches 32g1, 32g2, and 32g3 shown in FIGS. Similar to each long groove 32e, the central portion in the circumferential direction is deepest and both end portions in the circumferential direction are formed shallowest.

  In the above-described embodiment, the pair of ball infinite circulation paths S <b> 2 and S <b> 3 are set to be formed in the axially intermediate portion of the nut 32, but are formed in the axially intermediate portion of the nut 32. The ball infinite circuit may be single or three or more. In the above-described embodiment, the deflectors 33 to 36 are arranged at regular intervals in the circumferential direction. However, the circumferential intervals between the deflectors 33 to 36 can be changed as appropriate. is there.

  Further, in the above-described embodiment, the maximum value of the axial deformation amount in the pair of ball infinite circulation paths S1 and S4 formed at both axial ends of the nut 32 by each long groove 32e (rigidity reduction portion), Although the case where the maximum value of the axial deformation amount in the pair of ball infinite circulation paths S2 and S3 formed in the axially intermediate portion of the nut 32 can be made equal is described, each long groove 32e (stiffness reduction) Part), the maximum value of the axial deformation amount in the pair of ball infinite circulation paths S1 and S4 formed at both ends in the axial direction of the nut 32, and the pair of balls infinitely formed in the intermediate portion in the axial direction of the nut 32. Since the maximum value of the axial deformation amount in the circulation paths S2 and S3 cannot be made equal, the maximum value of the axial deformation amount in the pair of ball infinite circulation paths S1 and S4 is the middle of the nut 32 in the axial direction. A pair of parts formed in the part When the maximum amount of axial deformation in the infinite circulation paths S2 and S3 is increased, the balls B1 and B4 have the same diameter, the balls B2 and B3 have the same diameter, and the balls B1 , B4 can be carried out with a diameter smaller than that of each of the balls B2, B3.

DESCRIPTION OF SYMBOLS 10 ... Electric motor, 11 ... Hollow rotating shaft, 12 ... Annular fixture, 20 ... Steering shaft, 30 ... Ball screw mechanism, 31 ... Screw shaft, 32 ... Nut, 32a-32d ... Mounting hole, 32e ... Long groove (stiffness reduction part) ) 33-36 ... Deflector, Ho ... Spiral ball rolling path, S1-S4 ... Infinite ball circulation path, B1-B4 ... Ball, 40 ... Housing

Claims (3)

A hollow rotating shaft of an electric motor that is assembled to be rotatable with respect to the housing and is not movable in the axial direction, and a steering that is assembled so as to pass through the hollow rotating shaft and be unrotatable with respect to the housing and movable in the axial direction. A shaft and a ball screw mechanism that is coaxially interposed between the hollow rotary shaft and the steering shaft, converts the rotation of the hollow rotary shaft into an axial movement of the steering shaft, and transmits it.
The ball screw mechanism includes a screw shaft that is integral with the steering shaft and provided with a male screw groove on an outer peripheral portion thereof, and is integrally fastened and fixed to the hollow rotary shaft using an annular fixture. The male screw of the screw shaft A cylindrical nut having a female thread groove on the inner periphery that forms a spiral ball rolling path with the groove, and each mounting hole formed in both axial end portions and axial intermediate portions of the nut. A plurality of deflectors forming a plurality of ball infinite circulation paths by the ball rolling paths, and a plurality of deflectors accommodated in the balls infinite circulation paths so as to be capable of rolling relative to each other. An electric power steering apparatus equipped with a ball of
Each of the deflectors is disposed at a predetermined circumferential interval, and the other end in the axial direction of the nut is provided at a circumferential position where a mounting hole for assembling the deflector is formed at one axial end of the nut. In the circumferential place where the rigidity reduction part for reducing the rigidity in the axial direction is provided in the axially intermediate part from the axial direction other end part of the nut, the mounting hole for assembling the deflector is formed, A rigidity reduction portion for reducing axial rigidity is provided from one axial end portion of the nut to an axial intermediate portion, and a mounting hole for assembling the deflector is formed in the axial intermediate portion of the nut. The electric power steering apparatus according to claim 1, wherein a rigidity reduction portion for reducing the rigidity in the axial direction is provided at one end portion in the axial direction and the other end portion in the axial direction of the nut.   The electric power steering apparatus according to claim 1, wherein four deflectors are arranged at intervals of 90 degrees in the circumferential direction.   3. The electric power steering apparatus according to claim 1, wherein the rigidity reduction portion is formed by partially scraping an outer peripheral portion of the nut to make it thin. 3. Steering device.

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