JP2004338537A - 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 in a ball screw even when a moment load is applied to a nut of the ball screw, maintaining a high transmission efficiency, and enhancing the steering feeling. <P>SOLUTION: The electric power steering apparatus comprises a case 4, a rack shaft 1, a ball screw 7 comprising a conversion mechanism to convert the rotational force from a steering wheel into the axial moving force of the rack shaft 1 and a nut 5 externally fitted to a screw shaft forming a part of the rack shaft 1 via a plurality of balls 6, an electric motor 10 to assist the steering force, and a pair of rolling bearings 8 and 9 which are externally fitted to the nut 5 and support the nut 5 in a rotatable and axially non-movable manner with respect to the case 4. The output of the electric motor 10 is transmitted to the nut 5 via a timing belt 12. The pair of rolling bearings 8 and 9 are disposed across the timing belt 12, and the nut 5 is double-supported. <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 electric power steering device in which an electric motor and a ball screw are separately provided.
[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 an electric motor and a ball screw are used. There is another type in which the output of an electric motor is transmitted to a ball screw via a belt or a gear and applied as an axial moving force of the ball screw.
[0003]
FIG. 5 shows an example of this type of EPS. In the EPS, a rack shaft 51 supported by a case 54 so as to be movable in the axial direction, and a pinion gear (not shown) that meshes with the rack shaft 51 are provided integrally, and connected to a steering wheel (not shown). And a pinion shaft 52. Therefore, the rotation of the steering wheel is converted into the axial movement force of the rack shaft 51, and the steered wheels (not shown) are swung through tie rods and knuckle arms (not shown) connected to both ends of the rack shaft 51, thereby turning the vehicle. Steering is possible.
[0004]
A spiral groove 51 a is formed in the rack shaft 51, and a nut 55 having a spiral groove 55 a on the inner peripheral surface is fitted around the outer periphery of the spiral groove 51 a via a plurality of balls 56. The rack shaft 51, the nut 55 and the ball 56 form a ball screw 57.
[0005]
A rotating member 58 is fitted around the outer periphery of the rack shaft 51, and the rotating member 58 is rotatably supported on the case 54 via an angular contact bearing 59. The rotary member 58 is integrally provided with a large-diameter toothed pulley 60. On the other hand, an electric motor 61 is disposed in the case 54 substantially in parallel with the rack shaft 51, and a small-diameter toothed pulley 62 is mounted on a rotating shaft of the electric motor 61. A timing belt 63 is wound between the large-diameter toothed pulley 60 and the small-diameter toothed pulley 62.
[0006]
Further, a nut 55 of a ball screw 57 is fastened to the rotating member 58 by a bolt 65 via elastic members 64a and 64b, and the nut 55 is supported on the rotating member 58 so as to be integrally rotated and radially displaceable. The structure has been.
[0007]
In the EPS configured as described above, the rotation of the electric motor 61 is transmitted to the rotating member 58 via the timing belt 63, and further transmitted from the rotating member 58 to the nut 55 of the ball screw 57 via the bolt 65. 57 converts the force into the axial movement force of the rack shaft 51 and transmits the force. The nut 55 of the ball screw 57 is electrically operated even when the rack shaft 51 bends due to a bending moment acting on the rack shaft 51 from the steered wheels or when vibration occurs due to poor accuracy between the pinion and the rack. Since the rotating member 58 that transmits the rotation of the motor 61 is connected via the elastic members 64a and 64b so as to be radially displaceable, only the thrust load from the rack shaft 51 can be applied to the ball screw 57, It is possible to reduce adverse effects such as bending due to other shear loads and bending moments, and maintain high transmission efficiency of the ball screw 57 (for example, see Patent Document 1).
[0008]
[Patent Document 1]
Japanese Utility Model Publication No. 6-49489 (pages 2, 3; FIG. 1)
[0009]
[Problems to be solved by the invention]
However, in such a conventional EPS, the rotating member 58 integrally provided with the large-diameter toothed pulley 60 is substantially supported by an angular contact bearing 59, so-called cantilever support. Therefore, the tension of the timing belt 63 is applied to the nut 55 as a moment load, and the nut 55 elastically supported by the rotating member 58 via the elastic members 64a and 64b can absorb the radial displacement. Is also repeatedly rotated in a twisted state.
[0010]
When the nut is rotated in such a state, the nut 55 and the rack shaft 51 are driven in a relatively inclined state, and vibration and noise are generated. In addition, an unnecessary load is applied to the ball 56 interposed between the spiral grooves 51a and 55a, so that the ball 56 cannot roll smoothly. As a result, the transmission efficiency of the rotational torque of the electric motor 61 decreases. This was a factor that hindered the improvement of steering feeling.
[0011]
The present invention has been made in view of such circumstances, and suppresses generation of vibration and noise in a ball screw even when a moment load is applied to a nut of the ball screw, while maintaining high transmission efficiency. It is an object of the present invention to provide an EPS that improves steering feeling.
[0012]
[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 case, a steering shaft penetrating the case 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 composed of a nut, an electric motor that is arranged side by side substantially in parallel with the steering shaft and assists a steering force of the steering wheel, and is externally fitted to a nut of the ball screw, and the nut is attached to the case. A pair of rolling bearings rotatably and axially immovable, transmitting the output of the electric motor to the nut via a transmission mechanism, and sandwiching the transmission mechanism with the pair of rolling bearings. Employing the configuration so as to support said nut has both be disposed a gully bearing.
[0013]
As described above, in the EPS in which the electric motor and the ball screw are separately provided, the output of the electric motor is transmitted to the ball screw via a transmission mechanism such as a belt or a gear, and is applied as an axial moving force of the ball screw. , A pair of rolling bearings is arranged with the transmission mechanism in between, and the nut is supported by both ends, so that the nut and the steering shaft are not relatively inclined and driven, and Only the thrust load can be applied to the ball screw, and high transmission efficiency of the ball screw can be maintained. Therefore, vibration and noise can be prevented from being generated in the ball screw, and the ball interposed between the spiral grooves of the ball screw can roll smoothly, so that the rotational torque of the electric motor is smoothly transmitted and the steering is performed. Feeling can be improved.
[0014]
Further, the invention according to claim 2 is characterized in that the pair of rolling bearings is a rolling bearing configured to apply a radial load and a thrust load on one hand and a radial load on the other, so that the thrust load from the steering shaft is provided. Can be effectively loaded by one of the rolling bearings, eliminating the cause of slippage of the ball serving as a rolling element, and improving the durability of the rolling bearing.
[0015]
Further, as in the invention according to claim 3, one of the pair of rolling bearings may be a four-point contact ball bearing and the other may be a deep groove ball bearing. One of the pair of rolling bearings may be a four-point contact ball bearing, and the other may be a needle roller bearing.
[0016]
Further, in the invention according to claim 5, since at least the rolling surface of the inner ring of the four-point contact ball bearing is subjected to superfinishing, the noise level of EPS is reduced.
[0017]
Preferably, as in the invention according to claim 6, if the maximum amplitude width of the undulation in the rolling direction on the rolling surface is regulated to 1 μm or less and the surface roughness is regulated to Ra 0.1 or less, Compared with the conventional rolling bearing, the noise level can be reduced from the low speed range to the high speed range.
[0018]
In the invention described in claim 7, the axial clearance of one of the pair of rolling bearings is set smaller than that of the other rolling bearing, so that the thrust load can be reliably reduced by one of the rolling bearings. In the event that the nut can heat up during operation and thermally expands in the axial direction, the other rolling bearing can function as a free-side bearing to allow thermal expansion of the nut, and this rolling bearing Can be prevented from being subjected to an alternating load or the like.
[0019]
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 longitudinal sectional view of a main part showing an embodiment of an EPS according to the present invention.
[0020]
In the EPS, a rack shaft 1 supported by a case 4 so as to be movable in the axial direction, and a pinion gear (not shown) that meshes with the rack shaft 1 are integrally provided, and connected to a steering wheel (not shown). And a pinion shaft 2. Therefore, the rotation of the steering wheel is converted into the axial movement force of the rack shaft 1, and the steered wheels (not shown) are swung via tie rods and knuckle arms (not shown) connected to both ends of the rack shaft 1, thereby causing the vehicle to swing. Steering is possible.
[0021]
Further, a spiral groove 1 a is formed in the rack shaft 1, and a nut 5 having a spiral groove 5 a on an inner peripheral surface is fitted around the outer periphery of the spiral groove 1 a via a plurality of balls 6. The rack shaft 1, the nut 5, and the ball 6 constitute a ball screw 7.
[0022]
A large-diameter toothed pulley 3 is fitted on the outer diameter of the nut 5, and rolling bearings 8 and 9 are fitted on both sides of the large-diameter toothed pulley 3, respectively. Therefore, the nut 5 is supported by these rolling bearings 8 and 9 in a double-supported state so as to be rotatable with respect to the case 4 and not movable in the axial direction. On the other hand, an electric motor 10 is arranged in the case 4 substantially in parallel with the rack shaft 1, and a small-diameter toothed pulley 11 is mounted on a rotating shaft 10 a of the electric motor 10. A timing belt 12 is wound between the large-diameter toothed pulley 3 and the small-diameter toothed pulley 11.
[0023]
A stator 10 b constituting the electric motor 10 is press-fitted and fixed to the inner periphery of the motor housing 13. The stator 10b is composed of a core and a stator coil, and a rotor 10c of the electric motor 10 is disposed on the inner periphery of the stator 10b with a predetermined radial clearance therebetween. The rotor 10c is formed in a cylindrical shape by a magnetic material, and is integrally fixed to an outer periphery of a motor shaft 10a rotatably supported by a pair of rolling bearings 14 and 15 with respect to a motor housing 13. The electric motor 10 is controlled by a motor control device (not shown) according to a detection signal of a torque detector (not shown).
[0024]
In the EPS configured as described above, the rotation of the electric motor 10 is transmitted to the nut 5 via the timing belt 12, and is converted into the axial moving force of the rack shaft 1 by the ball screw 7 and transmitted. Even if the tension of the timing belt 12 is applied to the nut 5 as a radial load or a moment load, or if the rack shaft 1 bends due to the bending moment acting on the rack shaft 1 from the steered wheels, the ball screw 7, the nut 5 is supported by both ends with the timing belt 12 interposed therebetween, so that the nut 5 and the rack shaft 1 are not driven to be inclined relatively to each other, and only the thrust load from the rack shaft 1 is provided. Can be applied to the ball screw 7, and the high transmission efficiency of the ball screw 7 can be maintained. Therefore, vibration and noise can be prevented from being generated in the ball screw 7, and the ball 6 interposed between the spiral grooves 1a, 5a can roll smoothly, and the rotational torque of the electric motor 10 can be smoothly reduced. The steering feeling transmitted to the rack shaft 1 can be improved.
[0025]
In the present embodiment, the rolling bearings 9 and 8 that support the nut 5 include a deep groove ball bearing and a four-point contact ball bearing as shown in FIG. The rolling bearing 8 composed of the four-point contact ball bearing includes an inner ring 16, an outer ring 17, and a plurality of balls 18 interposed between the inner and outer rings 16 and 17. A rolling surface 16a formed on the outer peripheral surface of the inner ring 16 and a rolling surface 17a formed on the inner peripheral surface of the outer ring 17 are formed by a pair of centers of curvature which are axially offset by the same distance from the axial center of the bearing. , Which is a so-called Gothic arch shape. Therefore, the rolling bearing 8 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 7 after assembly into the EPS can be suppressed.
[0026]
Of the two rolling bearings 8, 9, the rolling bearing 8 formed of a four-point contact ball bearing applies a radial load and a thrust load, and the rolling bearing 9 formed of a deep groove ball bearing is configured to apply a radial load. (See FIG. 1). As a result, the thrust load from the rack shaft 1 can be effectively applied to one of the rolling bearings 8, eliminating the cause of ball slippage of the rolling bearing 9 composed of a deep groove ball bearing, and improving the durability of the rolling bearing 9. Can be improved.
[0027]
Further, among the two rolling bearings 8 and 9, the rolling bearing 8 composed of a four-point contact ball bearing is set to have a smaller axial clearance than the rolling bearing 9 composed of the other deep groove ball bearing. Therefore, when the temperature of the nut 5 rises during operation and thermally expands in the axial direction, the rolling bearing 9 having a large axial clearance can allow thermal expansion of the nut 5 as a free-side bearing. It is possible to prevent an unreasonable alternating load from being applied to the bearing 9.
[0028]
Conventional four-point contact ball bearings are used in the low rotation range, and the rolling surface is a gothic arch shape, which is difficult to machine. Therefore, the rolling surface is only ground and super-finished (hereinafter referred to as SF). ) Had not been applied. However, the rolling surface 16a of the rolling bearing 8 is subjected to SF after the grinding process, and particularly, the shape error components such as the roughness in the rolling direction of the ball 18 and the undulation on the rolling surface 16a are improved.
[0029]
In the present embodiment, the surface roughness is improved to approximately 比 べ compared with the conventional rolling surface, and particularly, the surface roughness in the rolling direction is regulated to Ra 0.1 or less. Further, the undulation component in a narrow range is improved, and the maximum amplitude width of the undulation in the rolling direction is regulated to 1 μm or less. As a result of installing the rolling bearing 8 having the inner ring 16 in the EPS and performing acoustic measurement, the present applicant has found that the noise level is reduced, and from the low speed range to the high speed range as compared with the conventional rolling bearing, 2. A reduction effect of 3 to 3.5 dB-A was confirmed. In the present embodiment, an example in which SF is applied only to the rolling surface 16a of the inner ring 16 in the rolling bearing 8 is exemplified. However, by applying SF to the rolling surface 17a of the outer ring 17, there is no effect equivalent to that of the inner ring 16. However, an effect of about 50% was further recognized.
[0030]
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 16a of the inner ring 16 on the rotating side, and the surface roughness and roundness in the rolling direction are regulated to predetermined values, so that the EPS extends from a low speed region to a high speed region of the EPS. Noise could be reduced.
[0031]
As described above, a four-point contact ball bearing has been exemplified as the rolling bearing 8, but such a rolling bearing 8 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 one. A so-called three-point contact ball bearing that contacts the ball 6 at two points may be used. Further, it is effective to apply SF to at least the rolling surface 16a of the inner ring 16, 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.
[0032]
FIG. 3 is a longitudinal sectional view of a main part 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.
[0033]
In the present embodiment, the rolling bearings 8 and 19 that support the nut 5 include a four-point contact ball bearing and a needle roller bearing. A rolling bearing 19 composed of a needle roller bearing includes a plurality of needle rollers 20 and an outer ring 21 having a rolling surface 21a on an inner peripheral surface. The inner ring has no inner ring, and the outer peripheral surface of the nut 5 has an inner rolling surface. Has become.
[0034]
Of the two rolling bearings 8, 19, the rolling bearing 8 is configured to apply a radial load and a thrust load, and the rolling bearing 19 is configured to apply a radial load. Thus, the thrust load from the rack shaft 1 can be effectively applied to the one rolling bearing 8. Here, when the nut 5 rises in temperature during operation and thermally expands in the axial direction, the rolling bearing 19 formed of a needle roller bearing can allow the thermal expansion of the nut 5 as a free-side bearing.
[0035]
In the EPS according to the present invention, the pair of rolling bearings rotatably support the nut 5 in a double-supported state with the timing belt 12 interposed therebetween, and can bear a radial load and a moment load generated by belt tension, and Any rolling bearing having a configuration capable of applying a thrust load from the rack shaft 1 may be used. The present invention is not limited to the above-described type of rolling bearing. For example, one rolling bearing is a double-row angular ball bearing, and the other is a deep groove ball bearing. Alternatively, both may be deep groove ball bearings. Here, in consideration of the case where the nut 5 heats up during operation and thermally expands in the axial direction, the axial clearance of one of the rolling bearings is set smaller than the other, and the other rolling bearing having a larger axial clearance is used. It is desirable that the bearing can allow thermal expansion of the nut 5 as a free-side bearing.
[0036]
The present applicant incorporated the rolling bearing having the above-described configuration into the EPS, performed acoustic measurement at a position 300 mm apart, and made a comparison with a conventional one. The result is shown in FIG. The vertical axis indicates the noise level, and the horizontal axis indicates the belt tension. As can be seen from the measurement results, compared to the conventional rolling bearing in which a single rolling bearing is externally fitted to the nut 5 and the belt tension is cantilevered, one is a four-point contact ball bearing and the other is a deep groove ball bearing. The product of the present invention (see FIG. 1) held and supported has a reduced noise level in the range of 2.5 to 4.0 dB-A. Also, in the present invention product (see FIG. 3) in which one is supported by a four-point contact ball bearing and the other is supported by a needle roller bearing, the noise level is reduced in the range of 1.5 to 3.0 dB-A. I have. Therefore, in the EPS according to the present invention in which a pair of rolling bearings are externally fitted to the nut 5 and supported with both ends of the belt tension, even if a moment load is applied to the nut 5 of the ball screw 7, the SF treatment of the rolling surface is performed. In combination with the effect described above, the generation of vibration and noise in the ball screw 7 can be suppressed.
[0037]
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.
[0038]
【The invention's effect】
As described in detail above, the electric power steering device according to the present invention includes a substantially cylindrical case, a steering shaft that penetrates the case 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, an electric motor arranged side by side substantially in parallel with the steering shaft and assisting the steering force of the steering wheel, and a nut fitted to the ball screw, the nut being rotatable with respect to the case. And a pair of rolling bearings that support the motor in an axial direction so that the output of the electric motor is transmitted to the nut via a transmission mechanism. The rolling bearing is disposed so that the nut is supported by both ends.Therefore, the nut and the steering shaft are not relatively inclined to be driven, and only the thrust load from the steering shaft is transferred to the ball screw. , And high transmission efficiency of the ball screw can be maintained. Therefore, vibration and noise can be prevented from being generated in the ball screw, and the ball interposed between the spiral grooves of the ball screw can roll smoothly, so that the rotational torque of the electric motor is smoothly transmitted and the steering is performed. Feeling can be improved.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view of a main part showing an embodiment of an electric power steering device according to the present invention.
FIG. 2 is an enlarged sectional view of a main part of the above.
FIG. 3 is a longitudinal sectional view of a main part showing another embodiment of the electric power steering device according to the present invention.
FIG. 4 is a graph showing a result of comparing and measuring noise levels of the electric power steering apparatus according to the present invention and a conventional electric power steering apparatus.
FIG. 5 is a longitudinal sectional view of a main part showing a conventional electric power steering device.
[Explanation of symbols]
1 ······ Rack shaft 1a, 5a ····· Spiral groove 2・ ・ ・ ・ ・ ・ Pinion shaft 3 ・ ・ ・ ・ ・ ・ ・ ・ ・ Large diameter toothed pulley 4 ・ ・ ・ ・ ・ ・ ・ ・ ・Case 5 Nut 6, 18 Ball 7 ... Ball screws 8, 9, 14, 15, 19 ... Rolling bearing 10 ... Electric motor 10a ... ··········· Motor shaft 10b ························ Rotor 11 ..... Small diameter toothed pulley 12 Timing belt 13 Motor housing 16 Inner rings 16a, 17a, 21a ..Tolling surfaces 17, 21 Outer ring 20 Needle rollers 51 ... Rack shafts 51a, 55a Spiral groove 52Pinion shaft 54 ... Case 55 ... Nut 56 ... Ball 57 Ball screw 58 Rotating member 59 ..... Rolling bearing 60 Large diameter toothed pulley 61 ... Electric motor 62 ... Small diameter toothed pulley 63 ... ... Timing belts 64a, 64b Elastic member 65 Bolt

Claims (7)

A substantially cylindrical case, a steering shaft that penetrates the case, 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 conversion mechanism, and a ball screw composed of a nut that is partly fitted to the steering shaft via a screw shaft, and a nut externally fitted to the screw shaft via a plurality of balls. An electric motor that assists the steering force of the steering wheel, and a pair of rolling bearings that are fitted around the nut of the ball screw and rotatably support the nut with respect to the case, but cannot move in the axial direction. Transmitting the rotation of the electric motor to the nut via a transmission mechanism, and arranging the pair of rolling bearings across the transmission mechanism to support the nut with both ends. Electric power steering device that. The electric power steering device according to claim 1, wherein the pair of rolling bearings are rolling bearings configured to load a radial load and a thrust load on one hand and load a radial load on the other. The electric power steering device according to claim 2, wherein one of the pair of rolling bearings is a four-point contact ball bearing, and the other is a deep groove ball bearing. The electric power steering device according to claim 2, wherein one of the pair of rolling bearings is a four-point contact ball bearing, and the other is a needle roller bearing. 5. The electric power steering device according to claim 3, wherein at least a rolling surface of an inner ring of the four-point contact ball bearing is subjected to superfinishing. The electric power steering apparatus according to claim 5, wherein the maximum amplitude width of the undulation in the rolling direction on the rolling surface is regulated to 1 µm or less, and the surface roughness is regulated to Ra 0.1 or less. The electric power steering device according to any one of claims 1 to 6, wherein an axial clearance of one of the pair of rolling bearings is set to be smaller than that of the other rolling bearing.

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