JPH06144246A - Motor-driven power steering - Google Patents

Abstract

PURPOSE:To provide a motor-driven power steering desirable in durability and fit for high-accuracy control without unreasonable force acting upon a steering shaft and a rack gear and without the power direction being reversed by noise or the like. CONSTITUTION:A motor-driven power steering 1 is provided with ball nuts 11, 13 to be engaged with a right screw 5 and a left screw 7 formed at a rack shaft 3, and an electric motor 15 disposed between them. Friction-engaged type clutches 25, 27 are formed at the facing parts between the respective ball nuts 11, 13 and a rotor 17. The respective ball nuts 11, 13 move to the left at the time of an input shaft 41 being rotated clockwise and move to the right at the time of the input shaft 41 being rotated counterclockwise by the action of a cam ring 43, a direct-acting converter rod 61 and a spring 63. With the engagement of the clutches 25, 27, the counterclockwise rotation of the electric motor 15 is transmitted to either of the ball nuts 11, 13 so as to execute power assist coinciding with the steering direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering device.

[0002]

2. Description of the Related Art Conventionally, as a power steering apparatus for a vehicle, a hydraulic type in which an oil passage is switched using a mechanical valve has been put into practical use, but the hydraulic type is inconvenient for precise control. Therefore, it has been desired to put the electric power steering device into practical use.

Against this background, for example, Japanese Patent Laid-Open No. 59-59
-50864, JP-A-60-25854, JP-A-6
No. 0-154955, Japanese Patent Laid-Open No. 62-261573, Japanese Patent Publication No. 3-15591, Japanese Patent Publication No. 4-28583, Japanese Patent Publication No. 4-57543, Japanese Utility Model No. 3-52606, Japanese Utility Model No. 4-27743 Has been proposed.

In these electric power steering devices, a torque sensor attached to a steering shaft converts a steering force applied by a driver into an electric signal, and a servo-type electric motor is controlled to rotate forward and backward to rotate a rack shaft to the left and right. I was moving to and was performing power assist.

[0005]

However, in these servo-type motors, when the electric signal obtained by the torque sensor becomes an erroneous signal in the opposite direction due to the influence of noise, etc. On the contrary, the power assist was executed, and sufficient fail-safe was needed. For this reason, in reality, in the electric type, a system that executes power assist only during low-speed traveling has been put into practical use for light vehicles.

Further, since the servo motor is prone to failure due to the problem of abrasion of the brush and it is necessary to stop the servo motor until the direction of power assist is instructed, the start-up is delayed. There was also. Therefore, there is also proposed a device that stops using a servomotor in the first place, uses a motor that rotates only in one direction, and two types of gear mechanisms, and engages and disengages the gear mechanism and the motor with an electromagnetic clutch. It is (actual public Sho 43-
19859). With this configuration, the motor can be always activated, so that there is no delay in activation and the problem of failure peculiar to the servomotor is solved. However, the control signal of the electromagnetic clutch may be an erroneous signal due to noise or the like, and again, it is not an essential fail-safe.

Under these circumstances, JP-A-59-128047
Japanese Patent Laid-Open No. 59-128048 discloses a system similar to the above-mentioned Japanese Utility Model Publication No. 43-19859, which mechanically engages and disengages the clutch by using a spring provided on the steering shaft. ing. In this device, the power assist direction does not reverse due to an abnormality in the electric signal.

However, in this system, since a large force is directly applied to the rack gear engaged with the pinion gear at the lower end of the steering shaft by the power assist, there is a problem that the rack gear is overloaded and is easily damaged. Further, there is a problem that there is a play between the pinion gear and the rack gear and that the steering shaft is strongly twisted by the electric motor, so that backlash occurs and delicate control cannot be performed.

Therefore, in the present invention, such an unreasonable force is not applied to the steering shaft and the rack gear, and the problem that the power assist direction is reversed due to noise or the like does not occur, the durability is good and the delicate control is performed. It is an object of the present invention to provide an electric power steering device that is also suitable for.

[0010]

SUMMARY OF THE INVENTION An electric power steering apparatus according to the present invention, which has been made to solve the above problems, is provided with an electric motor and a rack shaft side, and a clutch mechanism is provided for the electric motor. In an electric power steering apparatus including an assist mechanism that applies an assist force corresponding to the rotation of a steering shaft to a rack shaft itself when engaged via a motor, a motor that rotates only in one direction is used as the electric motor. A first engagement state in which the clutch mechanism is caused to function by using the rotational force of the electric motor as an assist force for left steering,
The rotation force of the electric motor is used as a right steering assist force to form a second engagement state that causes the assist mechanism to function, and the rotational movement of the steering shaft is converted into a linear movement. A mechanical switching mechanism for transmitting linear motion to the clutch mechanism and switching the clutch mechanism between the first engagement state, the second engagement state and the disengagement state is provided.

According to the electric power steering apparatus of the present invention, since the assist mechanism is provided on the rack shaft side and moves the rack shaft itself to the left and right, the steering shaft can be twisted strongly and the rack gear cannot be forced. No force is applied. Further, since the electric motor is not of the servo type, it has excellent durability in the first place. Moreover, since the clutch mechanism is switched by the mechanical switching mechanism, the power assist direction never reverses the direction intended by the driver.

Here, as described in claim 2, the clutch mechanism is formed integrally with the assist mechanism, and the mechanical switching mechanism directly moves the assist mechanism to the left and right, whereby the clutch mechanism operates. The assist mechanism and the mechanical switching mechanism switch between the first and second engaged states and the disengaged state, and the reaction force received by the assist mechanism causes the clutch mechanism to self-touch the clutch mechanism in each of the first and second engaged states. With such a configuration, the greater the request for power assist, the stronger the engagement of the clutch mechanism, and the power assist can be executed accurately.

As a more specific configuration of the electric power steering apparatus of the present invention, as described in claim 3, the assist mechanism is engaged with the screw groove formed on the rack shaft and the screw groove. It can be configured by a ball screw mechanism including a ball nut. The baud screw clew mechanism is suitable for suppressing rattling and performing delicate control without bending the rack shaft.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of an electric power steering apparatus to which the present invention is applied will be described below with reference to the drawings. 1 to 3 show the first embodiment. The embodiment is for a right-hand drive vehicle, and FIG. 1 is a cross-sectional view of a main part when the vehicle is viewed from the rear side to the front side. The electric power steering apparatus 1 according to the first embodiment includes a ball nut 1 that engages with each of a right screw 5 and a left screw 7 formed on a rack shaft 3.
1, 13 and an electric motor 15 arranged between the ball nuts 11, 13. In the electric motor 15, the rack shaft 3 is inserted through the center of the rotor 17, and the stator 19 is arranged and fixed around the rotor 17. The rotor 17 is supported by a ball bearing 21 so that it cannot freely move in the left-right direction within the gear housing 23, but is rotatable.

In addition, the ball nuts 11 and 13 and the rotor 17 are provided integrally with the facing portions of the rotor 17.
A pair of friction-engagement type clutches 25 and 27 are provided. Each ball nut 11 and 13 has a clutch 2
Rotational force from the rotor 17 is transmitted via 5, 27.

The ball nuts 11 and 13 are respectively attached to the ball bearings 35 and 37 with respect to the cylindrical frame bodies 31 and 33 which are movable in the left and right directions along the rack shaft 3 in the gear housing 23. And is rotatably supported. The frame bodies 31 and 33 are connected by a connecting rod 39 and are configured to move to the left and right at the same time.

On the other hand, the input shaft 41 connected to the handle (not shown) has a cam ring 43 as shown in detail in FIG.
Is attached. This cam ring 43 has a structure shown in FIG.
As shown in (A), an oblique elongated hole 45 extending obliquely downward to the right
And a vertical elongated hole 4 extending vertically as shown in FIG.
7 are provided. Then, a pin 51 protruding from the handle side portion 41a of the input shaft 41 is engaged with the diagonal elongated hole 45, and a pin 53 protruding from the pinion side portion 41b is engaged with the vertical elongated hole 47. There is. Also, the cam ring 43
Springs 55 and 57 are provided above and below the.
When the input shaft 41 rotates, the springs 55 and 57 are provided for the pinion side portion 41 with respect to the handle side portion 41a.
This is for allowing b to rotate with a predetermined phase angle.

Therefore, when the steering wheel is steered to the right, the steering wheel side portion 41a first starts to rotate to the right, but the pinion side portion 41b does not immediately rotate to the right. Therefore, the diagonal elongated hole 45 is pushed by the pin 51 engaged therewith on the contact surface on the left side in the drawing. As a result, the cam ring 43 begins to descend. Then, when the input torque due to the right steering reaches a certain value, the cam ring 4 is passed through the upper spring 55.
3 and the pinion side part 41b start to rotate integrally to the right. The cam ring 43 continues to descend until the pinion side portion 41b starts rotating rightward integrally with the handle side portion 41a, and thereafter, rotates rightward while maintaining its height. On the contrary, when the steering wheel is steered to the left, first, the cam ring 43 starts to rise and does not rise at a certain position, and thereafter, the cam ring 43 and the pinion side portion 41b integrally rotate left.

Here, when the cam ring 43 descends, the linear conversion rod 61, which is in contact with this cam surface, moves to the left in the figure, and the frame members 31, 33 and the ball nuts 11, 13 are moved.
To the left. Further, since the spring 63 disposed behind the left ball nut 13 applies a pressing force to the linear motion conversion rod 61 in the right direction in the drawing, when the cam ring 43 rises, the frame bodies 31, 33 are And the ball nuts 11 and 13 are moved to the right. In this way, the rotation of the input shaft 41 due to the operation of the handle is converted into a linear motion, and the ball nuts 11 and 13 are moved left and right. In addition, a spring 65 is also placed behind the ball nut 11 on the right side.
Is provided, but the spring force is smaller than that of the spring 63 on the left side.

With such a direct-acting conversion mechanism, when the steering wheel is steered to the right and the cam ring 43 descends, the frame 31,
Both 33 and the ball nuts 11 and 13 move to the left, and the right clutch 25 is engaged. Therefore, the ball nut 11 on the right side starts to rotate by receiving the rotational force of the electric motor 15. Here, the electric motor 15 is always rotating slowly to the left, and acceleration / deceleration control is performed by a controller (not shown) so that the electric motor 15 has a rotation speed corresponding to the steering amount. This acceleration / deceleration control is executed by the torque detector 67 provided around the cam ring 43, which detects the torque generated in the input shaft 41 due to the steering, and according to the detected value. In the present embodiment, the torque detector 67 is configured to detect the input torque by detecting the movement of the cam ring 43 with a coil arranged around the cam ring 43.

In this way, when the steering wheel is steered to the right and the left rotation of the electric motor 15 is transmitted to the ball nut 11 on the right side, the rack shaft 3 is power assisted in the right direction in the drawing.
At this time, the reaction force that the ball nut 11 receives from the rack shaft 3 is a force to the left in the drawing, so that the clutch 25 is in the self-touching state. Therefore, when strong power assist is required, the clutch 25 naturally enters a stronger engaged state and can smoothly transmit force without slipping. Since this strong force is a force that moves the rack shaft 3 itself straight to the right, the rack shaft 3 is not bent. Further, since the pinion gear 71 itself is not strongly twisted, the rack gear 73 is not subjected to an excessive force. afterwards,
When the right steering is completed, the cam ring 43 returns to the neutral position, so that the right clutch 25 is in the disengaged state like the left clutch 27.

When the steering wheel is steered to the left, conversely,
The cam ring 43 rises, the frame members 31, 33 and the ball nuts 11, 13 both move to the right, and the left clutch 27
Engage. Accordingly, the ball nut 13 on the left side starts rotating to the left this time, and the rack shaft 3 is power assisted to the left in the drawing. At this time as well, the clutch 27 is in the self-touching state.

As described above, when the input torque by steering is applied to the input shaft 41, the clutch OFF region and the clutch ON region are mechanically determined by the magnitude thereof. This state is schematically shown in FIG. Here, the reason why the clutch OFF range is set to a certain degree of input torque is that the driver often drives while slightly moving the steering wheel to the left or right even when the vehicle is traveling straight, and the wheels are slightly affected by the road surface. This is because the input shaft 41 may slightly rotate left and right due to such vibration, so that the power assist does not work in such a state.

As a result of such a configuration, according to the power steering apparatus of the first embodiment, the power assist can be executed without bending the rack shaft 3 and without exerting any force on the rack gear 73. it can. Moreover, since the electric motor 15 is used, it is possible to execute highly accurate and delicate power assist.

Further, since the electric motor 15 is a motor that rotates only in one direction, it does not easily break down like a servo motor. Then, the clutch 25, 27
Since the presence / absence and the direction of the power assist are controlled by engaging and disengaging with, the electric motor 15 can be kept in the activated state at all times, and there is no delay in rising at the time of activation. In addition, the motor inertia is not applied to the rack shaft 3 when the clutch is not engaged. Therefore, it is easy to turn back the handle.

Moreover, since the direct-acting conversion rod 61 is adopted and the mechanism for mechanically switching the clutch according to the input torque is used, an error in the electric signal does not occur when the clutch is engaged or disengaged, which is contrary to the driver's intention. There is no need for power assistance in the direction. In addition, in the torque detector 67, regardless of the direction of the input torque, only the magnitude needs to be detected, and an error in the signal is unlikely to occur. Further, even if the detected torque value is incorrect, the direction of the power assist will not be reversed, and the driver will only have the impression that the steering wheel is heavy or light.

Further, since the clutch mechanism of the present embodiment is constructed to be self-touching, the engagement force of the clutch increases in proportion to the power assist force. Therefore, a proper power assist can be naturally obtained, and slippage of the clutch portion is unlikely to occur even when a strong power assist is applied, so that the durability of the clutch itself is also improved.

Next, a second embodiment will be described with reference to FIG. It should be noted that components having the same role as those in the first embodiment are displayed by adding 100 to the reference numerals used in the first embodiment, and detailed description thereof will be omitted. In the power steering device 101 of the second embodiment, unlike the first embodiment, the left and right ball nuts 111 and 113 are arranged so as not to move in the axial direction, and a sleeve that is spline-fitted to each ball nut 111 and 113. 124, 126 and rotor 117 of electric motor 115
And friction-engagement type clutches 125 and 127. The linear motion conversion rod 161 is configured to switch engagement / disengagement of the clutch by moving the sleeves 124 and 126 to the left and right. The operation and effect are almost the same as those of the first embodiment, but the self-touch function is not exerted. On the other hand, since the reaction force against the direct conversion rod 161 is small, the durability of the direct conversion rod 161 itself is good.

Next, a third embodiment will be described with reference to FIG. In addition, about the same component as 1st Example etc., the hundreds place of the code | symbol used in 1st Example etc. is changed and displayed to "2", and detailed description is abbreviate | omitted. The power steering device 201 of the third embodiment is different from that of the first embodiment in that the rotor 217 of the electric motor 215 is connected to the direct conversion rod 261 so that the rotor 217 can be moved left and right. Is. Specifically, the ring member 222 is arranged toward the ball bearing 221a on the right side of the rotor 217,
This and the direct-acting conversion rod 261 were connected. In this case as well, the self-touch function is eliminated, but the structure is simplified in that the connecting rods 39, 139 are not required as compared with the first and second embodiments.

Next, a fourth embodiment will be described with reference to FIG. In addition, about the same structure as 1st Example etc., the hundreds place of the code | symbol used in 1st Example etc. is changed and displayed to "3",
Detailed explanation is omitted. In the power steering device 301 of the fourth embodiment, the rack shaft 303 is not inserted into the electric motor 315, but both are arranged in a parallel position, and the tubular rotating body 380 is located at the position where the rotor 217 was in the third embodiment. And a reduction gear 382 is arranged between the rotating body 380 and the electric motor 315. And the rotating body 3
80 is connected to the connecting rod 361 via the ring member 322, and is configured to switch between engagement and disengagement of the friction engagement type clutches 325 and 327 by moving to the left and right. With this configuration, the electric motor 315 can be decelerated and used, and a small motor having a high rotation speed can also be used. In addition, the degree of freedom in arranging the power assist components in the rack axis direction increases.

Next, a fifth embodiment will be described with reference to FIG. The fifth embodiment has only one ball screw mechanism. In addition, about the same structure as 1st Example etc., the hundreds digit of the code used already is changed and displayed, and detailed description is abbreviate | omitted.

Power steering apparatus 40 of the fifth embodiment
1 is an electric motor 415 at the left end of the gear housing 423.
Is arranged, a ball nut 411 is arranged on the right side thereof, and a casing 412 is provided so as to surround the ball nut 411.
And the casing 412 is attached to the rotor 41.
7, and friction engagement clutches 425 and 427 are disposed between the left and right inner walls of the casing 412 and the ball nut 411.

The ball nut 411 is the casing 4
It is rotatably supported by the gear housing 423 via a support sleeve 411a protruding to the right of 12 and a ball bearing 435. Further, the casing 412 and the ball nut 411 are offset by the springs 463 and 465. The left clutch 427 is formed between the ball nut 411 and the reversing mechanism 414 including the ring gear 414a2, the planetary gear 414b, and the sun gear 414c. Therefore, the ball nut 411 has a different rotation direction depending on which clutch 425, 427 is engaged with the electric motor 415.

Which of the clutches 425 and 427 is to be engaged is the ring member 422 arranged at the ball bearing portion supporting the right end of the casing 412, which is similar to the fourth embodiment. It is switched by moving to the left and right via. In this embodiment, the linear conversion rod 461 is, as shown in FIG.
For example, by connecting with a ring 444b loosely fitted in a groove 444a provided in the cam ring 443, the cam ring 443 is moved left and right as the cam ring 443 moves up and down. The principle of raising / lowering the cam ring 443 is exactly the same as that of the first embodiment.

The fifth embodiment also does not have the self-touch function, but only one ball nut is required and the rack shaft 40
There is an advantage that the configuration is simpler than that of the other embodiments, such as one type of the thread groove of No. 3 is sufficient and the gear houn jig 423 is small in size as a whole.

Next, a sixth embodiment will be described with reference to FIG. This sixth embodiment also has only one ball screw mechanism. In addition, about the same structure as 1st Example etc., the hundreds place of a code | symbol is changed and displayed, and detailed description is abbreviate | omitted. In the power steering device 501 of the sixth embodiment, an electric motor 515 is disposed with the rotor shaft 517a facing down in the center of the gear housing 523, and a ball nut 511 is formed with a sleeve 511a protruding rightward. The collared rotating body 590 is spline-fitted to the sleeve 511a. The collared rotary body 590 includes two collars 591 and 592. Then, the collars 591 and 592 of the rotary body 590 with the collar and the rotor shaft 517.
Friction engaging clutches 525 and 527 are formed by the friction member 517b at the tip of a. Further, similarly to the casing 412 of the fifth embodiment, the collared rotary body 590 is configured to be moved left and right via the ring member 522 and the linear motion conversion rod 561, thereby changing the rotation direction of the ball nut 511. It can be driven. According to the sixth embodiment, there is an advantage that the gear housing 523 can be made more compact than the fifth embodiment.

Although some embodiments of the present invention have been described above, the electric power steering apparatus of the present invention is not limited to the above-mentioned embodiments and various modifications can be made without departing from the gist of the present invention. It is possible to change into a mode. For example, the mechanism for converting the rotational movement of the steering input shaft into the linear movement may be configured by a mechanism different from that of the embodiment. As an example, as shown in FIG. 9A, the input shaft 741
It is possible to convert the rotation of the input shaft 741 into a linear movement by penetrating the linear conversion rod 761 through the protrusion 750 protruding from the above and contacting the stoppers 750a and 750b with the protrusion 750. As another example, FIG.
As shown in (B), a pin 850a is further projected from the projection 850 provided on the input shaft 841, and this pin 850a is engaged with the groove 850b of the linear motion conversion rod 861 to convert the rotary motion into the linear motion. It can also be configured to convert. In addition, it goes without saying that a mechanical mechanism for converting various rotary motions into linear motions can be adopted. Further, as the assist mechanism, another mechanism can be adopted instead of the ball screw mechanism.

In addition, the electric motor and the ball screw in the first to third embodiments are reversed, and one ball screw mechanism arranged between two electric motors rotating in opposite directions is used. The power assist direction with respect to the rack shaft may be switched by engaging with the electric motor.

In addition, the input shaft mechanism is not limited to that of the embodiment, but an input shaft using a torsion bar may be used, and the electric motor is not limited to a device configuration in which it is always slowly rotated, and is not always required. It may be stopped.
Further, as for the cam ring, as shown in FIG. 10A, the vertical first cam surface 1001 and the oblique second cam surface 1 are provided.
003 and a vertical third cam surface 1005 may be formed. In this case, at the beginning of rotation of the handle, the direct-acting conversion rod 1007 quickly moves to the left and right along the second cam surface 1003, and then the direct-acting conversion rod 1
007 comes into contact with the vertical cam surface 1001 or 1005 and is in a holding state. Further, as shown in FIG. 7B, the cam surface 1010 may have a curved cross section. The dotted line in the figure shows the cam surface in the example for comparison.

[0040]

As described in detail above, according to the present invention,
Provides an electric power steering device that has excellent durability and is suitable for delicate control, without applying excessive force to the steering shaft or rack gear, and without causing problems such as noise reversing the direction of power assist. can do.

Further, if the self-touching structure described in claim 2 is adopted, the greater the demand for power assist, the stronger the engagement of the clutch mechanism, and the more accurate power assist is performed without slipping the clutch. can do. And the durability of the clutch portion is also improved.

In addition, if the ball screw mechanism is used as in the third aspect of the present invention, it is possible to carry out highly precise and delicate control while suppressing rattling without bending the rack shaft. It will be easy.

[Brief description of drawings]

FIG. 1 is a sectional view showing a configuration of a main part of a first embodiment.

FIG. 2 is a sectional view of a cam ring portion of the first embodiment.

FIG. 3 is an explanatory diagram showing a relationship between a clutch engagement region and an input torque in the first embodiment.

FIG. 4 is a cross-sectional view showing the configuration of the main part of the second embodiment.

FIG. 5 is a cross-sectional view showing the configuration of the main part of the third embodiment.

FIG. 6 is a cross-sectional view showing the configuration of the main part of the fourth embodiment.

7A and 7B are a cross-sectional view showing a configuration of a main part of a fifth embodiment and a front view of a part thereof.

FIG. 8 is a cross-sectional view showing a configuration of a main part of a sixth embodiment and a perspective view of a part thereof.

FIG. 9 is an explanatory diagram showing a modification of the linear motion conversion mechanism.

FIG. 10 is an explanatory diagram showing a modified example of the cam ring.

[Explanation of symbols]

1, 101, 201, 301, 401, 501 ... Electric power steering device, 3, 103, 203, 3
03,403,503 ... Rack axis, 11, 13, 1
11,113,211,213,311,313,41
1,511 ... Ball nut, 15,115,21
5,315,415,515 ... electric motor, 25,
27, 125, 127, 225, 227, 325, 32
7,425,427,525,527 ... Clutch,
39, 139 ... Connecting rods, 41, 141, 74
1,841 ... Input shaft, 43,143,443 ...
Cam ring, 45 ... Oblique long hole, 47 ... Vertical long hole, 55, 57 ... Spring, 61, 161, 26
1, 361, 461, 561, 761, 861 ... Linear motion conversion rods, 63, 65, 163, 165, 263
265, 363, 365 ... Spring, 67, 16
7 ... Torque detector, 414 ... Inversion mechanism, 590
... Rotating body with a collar.

Claims (3)

[Claims] 1. An electric motor, which is provided on a rack shaft side, and which, when engaged with the electric motor via a clutch mechanism, gives an assisting force to the rack shaft itself according to rotation of a steering shaft. In the electric power steering device including a mechanism, a motor that rotates only in one direction is used as the electric motor, and the clutch mechanism is caused to function as the assist mechanism for left steering with the rotational force of the electric motor. The steering shaft is configured to have a first engaged state and a second engaged state in which the rotational force of the electric motor serves as an assist force for right steering to cause the assist mechanism to function. Converting to linear motion and transmitting the linear motion to the clutch mechanism,
An electric power steering apparatus comprising a mechanical switching mechanism for switching the clutch mechanism between the first engagement state, the second engagement state and the disengagement state. 2. The clutch mechanism is configured integrally with the assist mechanism, and the mechanical switching mechanism moves the assist mechanism directly to the left and right to thereby provide a first and second engagement state by the clutch mechanism. Switching the disengaged state, the assist mechanism and the mechanical switching mechanism, the first,
2. The electric power steering device according to claim 1, wherein, in each of the second engaged states, the reaction force received by the assist mechanism is configured to make the clutch mechanism self-touch. 3. The ball screw mechanism, wherein the assist mechanism is composed of a screw groove formed on the rack shaft and a ball nut engaging with the screw groove. 2. The electric power steering device according to any one of 2.