JP2000255437A - Motor-driven power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide compactness in the diameter direction, an increase in load capacity, manhour reduction, cost reduction, enhanced durability, looseness prevention, and enhanced reliability. SOLUTION: A ball screw mechanism 23 incorporated into a motor-driven power system device is formed of a ball screw shaft 10, a rotary nut 11, and a plurality of torque-transmitting balls 13 placed on a ball rolling passage 12 formed between the ball screw shaft 10 and the rotary nut 11. Through holes 15 are formed in the rotary nut 11 to circulate the torque-transmitting balls 13. A sintered alloy is used to form end caps 14a, 14b mounted on both the ends of the rotary nut 11 and having reverse passages 17a, 17b for reversing the torque-transmitting balls 13 in the through holes 15 of the rotary nut 11.

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 apparatus for an automobile, and more particularly to a rack and pinion type electric power steering apparatus in which the output of an electric motor is transmitted to a coaxial rack section via a ball screw.

[0002]

2. Description of the Related Art There are various types of electric power steering devices that obtain power for assisting a steering force of an automobile from an electric motor. 5
JP-A-9-50864 (basic configuration), JP-A-9-14
No. 2315 (using a ball screw mechanism of a frame circulation type), Japanese Patent Application Laid-Open No. 54-47236 (using a ball screw mechanism of a tube circulation (circulator) type), and Japanese Patent Application Laid-Open No. 6-201013 (end cap type made of resin and Resin resin circulation pipe system). There are various types of ball screws for transmitting the output of the electric motor to the rack.

[0003] Japanese Patent Application Laid-Open No. 9-142315 discloses
As shown in FIG. 7A, a ball screw nut 110 is provided around a shaft 104 having an outer screw groove 104a formed therein. An inner screw groove 110b formed on the inner periphery of the ball screw nut 110 is The rolling path of the interposed ball 112 is formed facing the screw groove 104a. The inner screw groove 1 adjacent to the ball screw nut 110
A top 110d, which is a passage connecting the 10b with each other, is arranged. Here, the ball screw nut 110 is the shaft 1
When the shaft 104 rotates relative to the shaft 04, the shaft 104 moves in the axial direction with respect to the ball screw nut 110 due to the reaction force from the thread groove. The ball 112 rolls along the thread groove and comes out of the rolling path with the rotation of the ball screw nut 110 and the movement of the shaft 104, so it is necessary to refill the ball in accordance with the amount of rotation. By providing the top 110d, the ball 112 that has rolled along the thread groove can be returned to the thread groove via the top 110d.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 54-47236 discloses a ball group in the range of the axial length LA or LB of the ball nut 2, that is, as shown in FIGS. One of the ball groups in the row rolls on the thread groove formed on the ball screw shaft 1 within a range corresponding to the axial length interposed between the ball screw grooves. The two rows of balls 3A and 3B that are guided and circulated by the two rows of circulators 6A and 6B provided on the ball nut 2 are combined with the thread groove 2a of the ball nut 2 and the thread grooves 1a and 1b of the ball screw shaft 1. And the diameter of each ball of one of the ball groups 3A is formed to be appropriately larger than the diameter of the other ball group. Screw groove 2a and ball screw shaft 1
Large-diameter portion d rolling contact with the _first screw groove 1a, rolling between the small-diameter of the thread groove 2a of the ball group 3B ball nut 2, a thread groove 1b of the small-diameter portion d ₂ of the ball screw shaft 1 It is set to touch. Thus, large-diameter ball group 3A guided by the circulator 6A is a screw groove 2a of the ball nut 2, the screw groove 1a of the large diameter portion d ₁ of the ball screw shaft 1
And a zero clearance. When the steering wheel of the automobile turns the handle to the right or left to rotate the ball screw shaft 1, the ball nut 2 moves to the right or left via each screw groove and the ball groups 3A and 3B.

Japanese Patent Application Laid-Open No. 6-201013 discloses that end caps are attached to both ends of a nut to return a ball to a circulation pipe, and that the end cap and the circulation pipe are made of resin.

[0006]

The one employing the above-described ball circulation type ball screw mechanism has the advantage that the outer diameter of the ball screw nut can be reduced, but a separate piece is fitted to the outer diameter cutout portion of the nut. Since they are mounted together, the number of component parts is large, and since the top which is a circulating component is arranged on the nut outer diameter, it is difficult to fit a motor, a bearing or the like to the ball screw nut outer diameter. . In addition, the above-mentioned frame circulation method
When applied to a ball screw with a limited nut outer diameter, there are the following restrictions. That is, in order to reduce the nut outer diameter and obtain a high load capacity with a small cross-sectional height, it is necessary to accommodate many small-sized balls. In this case, it is advantageous to employ a multi-thread screw capable of increasing the load capacity. However, in the case of a multi-thread screw, the ball return piece 110d must be arranged across the adjacent rolling groove. It is not possible to employ multi-start threads.

On the other hand, tube circulation (circulator)
In the case of adopting the ball screw mechanism of the system, when the diameter of the ball circulating inside the ball screw is the same, the outer diameter of the ball screw nut becomes large, and in a steering device such as an automobile, the problem is that the space of the entire device is enlarged. Remains.

[0008] Further, in Japanese Unexamined Patent Publication No. Hei 6-201013, the circulation pipe is formed by resin molding separately from the nut, so that there is a problem that the number of parts and the number of assembly steps are increased. Due to the resin material, there is a problem that the ball circulation operation is not smooth due to durability such as abrasion and dimensional change due to thermal expansion and contraction. In addition, the tightening torque management of the bolt for fixing the end cap to the nut Is also difficult.

Conventionally, the end cap is fixed to the nut by screwing the bolt to the nut. However, the bolt may be loosened by vibration or the like, and the end cap may be loosened. The play of the end cap hinders the smooth circulation of the ball, and there is a problem that the function of the ball screw is reduced.

An object of the present invention is to solve the above-mentioned problems of the conventional electric power steering apparatus,
Provided is an electric power steering device that can realize compactness in a radial direction, increase in load capacity, reduction in man-hours, cost reduction, improvement in durability, prevention of loosening, improvement in reliability, and the like.

[0011]

An electric power steering apparatus according to a first aspect of the present invention uses a housing, a steering shaft connected to a steering mechanism for steering wheels, and a rotational force from a steering wheel. An electric power including a conversion mechanism for converting a shaft into a force for moving the shaft in a longitudinal direction, a ball screw mechanism provided integrally with the steering shaft, and an electric motor fitted to a rotating nut of the ball screw mechanism. In the steering device, the ball screw mechanism includes a ball screw shaft having a helical ball rolling surface on an outer diameter surface, and a rotating nut having a helical ball rolling surface on the inner diameter surface facing the ball screw shaft. And a plurality of torque transmitting balls for transmitting a force between the rotating nut and the steering shaft, the plurality of torque transmitting balls being arranged in tandem with a ball rolling path formed between the two ball rolling surfaces, The In order to circulate the torque transmitting ball, a through hole is formed in the rotating nut, and an end cap which is mounted on both ends of the rotating nut and inverts the torque transmitting ball into the rotating nut through hole is formed of a sintered alloy. Characterized by being formed by: According to the configuration of claim 1, the through-hole is formed in the thickness of the rotating nut so that the ball circulating mechanism does not protrude from the outer diameter portion of the rotating nut. The diameter of the torque transmission ball can be reduced by turning the torque transmission ball into the through hole of the rotating nut by the end cap, so that the torque transmission ball can be smoothly circulated. Since separate parts such as pipes are not used, the number of parts, the number of assembly steps, the cost, and the like can be reduced. Also,
By forming the end cap with a sintered alloy, even when the ball rolling path has a multi-threaded shape, the inverted portion of the end cap can be mass-produced accurately and inexpensively by injection molding into a mold. By selecting a combination of metal powder materials to be used, strength can be increased and durability can be improved.

The electric power steering apparatus according to a second aspect of the present invention is characterized in that the rotating nut is formed of a sintered alloy. According to the configuration of claim 2, the rotating nut can be mass-produced accurately and inexpensively by injection molding and sintering of the metal powder material into the mold, and by selecting the combination of the metal powder materials to be mixed, High strength and durability can be maintained.

The electric power steering apparatus according to a third aspect of the present invention is characterized in that a counterbore portion of the end cap is swaged to prevent a fixing bolt fixed to the rotating nut from rotating. According to the configuration of claim 3,
After screwing and tightening the fixing bolt that fixes the end cap to the rotating nut, the counterbore part can be crimped with a crimping tool to prevent rotation, and the vibration of the fixing bolt after screwing Can be prevented with a simple configuration to improve reliability.

The electric power steering apparatus according to a fourth aspect of the present invention is characterized in that a ball rolling path of the ball screw mechanism has a multi-start thread shape. According to the configuration of the fourth aspect, it is possible to arrange a large number of torque transmitting balls by reducing the diameter of the torque transmitting balls, and increase the load capacity while reducing the radial dimension to achieve compactness. be able to.

[0015]

FIG. 5A is a schematic sectional view of an electric power steering apparatus to which the present invention is applied. This device includes a conversion mechanism 22 for converting a rotational force from a handle (not shown) into a force for moving a steering shaft 21 in a longitudinal direction, a ball screw shaft 10, a rotating nut 11, a torque transmitting ball 13, and the like. , And an electric motor 24 for rotating the rotating nut 11.
Although omitted in FIG. 5A, a rack is formed on the steering shaft 21, and the conversion mechanism 22 is configured by the rack and a pinion that meshes with the rack. Further, the ball screw mechanism 23 has a plurality of torque transmitting balls 13 interposed between the ball screw shaft 10 and the rotating nut 11, and the outer diameter of the rotating nut 11 is provided with the rotor portion 24 a of the electric motor 24 and the rotating nut 11. A rolling bearing 26a rotatably supported by the housing 25 is fitted. The other rolling bearing 26
b is fitted to the sleeve 27 fixed to the rotating nut 11. The stator portion 24b of the electric motor 24 is fixed to the housing 25 corresponding to the rotor portion 24a. Both ends of the steering shaft 21 are supported by support members 28 and 29 integrated with the housing 25 so as to be movable in the longitudinal direction.

FIG. 1 is a schematic sectional view of a ball screw mechanism according to the present invention. In FIG. 1, a ball screw mechanism 23 according to the present invention includes a ball having a helical ball rolling surface 10b formed on an outer diameter surface 10a in the form of a multi-thread screw (FIG. 1 illustrates the case of a double-thread screw). A rotary nut 11 having a screw shaft 10 and a spiral ball rolling surface 11b correspondingly formed in a multi-threaded shape on an inner diameter surface 11a, and formed between these ball rolling surfaces 10b, 11b. And a plurality of torque transmitting balls 13 arranged in series with the ball rolling path 12. And the torque transmitting ball 1
3 is a reversing path 17 formed in the end cap 14a at the left end of the rotary nut 11 in FIG.
a, and rolls through the axially extending through-hole 15 formed in the rotary nut 11 rightward in the figure to reach the right end cap 14b, via the reversing path 17b formed in the end cap 14b. Then, it is returned to the ball rolling path 12 formed between the ball screw shaft 10 and the rotary nut 11 and circulates sequentially. The through holes 15 are formed in a thickness corresponding to the number of the multi-threaded screws in the circumferentially equal position of the rotating nut 11. As described above, in the present invention, the reversing path 1 serving as a ball circulation path communicating both ends of the ball rolling path 12.
7a, 17b and through-hole 15 with the rotating nut 11,
End caps 1 attached to both ends of this rotating nut 11
4a and 14b.

As shown in FIG. 2A, the end caps 14a and 14b are mounted on both ends of the rotating nut 11 and fixed with fixing bolts 16 such as hexagon socket head bolts. Bolt 1 for fixing end caps 14a, 14b
As shown in FIGS. 2 (B) and 3 (A) (B), the mounting portion 6 has a counterbore portion 19, and the head of the fixing bolt 16 is attached to the end caps 14a, 14b. It is sunk so as not to protrude from the end face, and the
(A in FIG. 3 shows an example in which two are provided), and after the fixing bolt 16 is screwed, the convex portion 19a is appropriately tightened by a crimping tool (not shown). And plastically deformed, and as shown in FIG.
a 'is formed.

The reversing paths 17a and 17b formed in the end caps 14a and 14b are formed so as to be open toward the end faces as shown in FIGS. The reversing paths 17a and 17b correspond to the installation positions and the number of the through holes 15 of the rotating nut 11 (FIG. 4A shows an example in which there are two through holes). The ball screw shaft 1 is provided so as to smoothly communicate with the rolling path 12.
Around 0, the ball rolling path 12 is formed in an arc shape or a tangential shape along the spiral direction.

Furthermore, both end caps 14a, 1
The reversing paths 17a and 17b formed in FIG.
As shown in (B), the ball 13 has an inner diameter slightly larger than the outer diameter of the ball 13 and is formed in a substantially semicircular arc-shaped cross section.
The ball rolling path 12 is the deepest in the communication portion with the through hole 15.
For example, the groove portion is formed in an inclined groove shape by continuously changing the groove depth so as to be the shallowest at the communication portion with.

The reversing paths 17a and 17b are shown in FIG.
As shown in (D), the end face of the rotating nut 11 may be formed so as to communicate with the through hole 15. 4 (A) and 4 (B) and FIGS. 4 (C) and 4 (D) show cases where the reversing paths 17a and 17b are formed over the end face of the rotating nut 11 and the end caps 14a and 14b. However, it may be formed on only one of them.

The through hole 15 and the end caps 14a, 14
As shown in FIGS. 4A and 4B and FIGS. 4C and 4D, in order to align with the reversing paths 17a and 17b formed in FIG. It is preferable to form a positioning projection 18a and a positioning recess 18b to be fitted in the positioning projection 18a, and to fit them to perform positioning.

On the outer diameter surface of the rotating nut 11, a surface 11c is fitted with a rolling bearing 26a for rotatably supporting the rotating nut 11 with respect to the housing 25 shown in FIG.
(See FIG. 2A) and the rotor 2 of the electric motor 24.
A surface 11d (see FIG. 2A) into which the 4a is fitted is formed. As shown in FIG. 2A, a flange 11e forming a shoulder for positioning is protruded from the fitting surface 11c, while the fitting surface 11d is provided on the rotor portion 24a.
A flat knurl is formed to prevent rotational creep of the knurl. The flange 11e has an attachment hole 11f (see FIGS. 2A and 2B) for attachment to the rotor portion 24a of the electric motor 24.
The flange 11e can be formed in a ring shape. However, in order to reduce the weight and size, the flange 11e shown in FIG.
As shown in the figure, the left and right sides are cut in parallel to the outer diameter surface of the rotating nut 11 so as to be flat, and the upper and lower sides are cut in parallel so as not to affect the mounting hole 11f, and the upper and lower surfaces are flat. It is desirable to do.

The ball rolling surface 10b formed on the ball screw shaft 10 is formed by rolling, and the end caps 14a, 14b
The b and the rotating nut 11 are formed by MIM (metal injection mold).

The above-mentioned MIM is for producing a product by adjusting a metal powder to a plastic shape and molding the metal powder with an injection molding machine. In this production method by injection molding, first, a metal powder and a binder made of plastic and wax are kneaded by a kneader, and the kneaded product is granulated into pellets. As the metal powder, a material that can be later carburized and quenched is preferable. For example, it is assumed that carbon (C) is 0.3%, nickel (Ni) is 1 to 2%, and the balance is iron (Fe).

The pellets granulated as described above are supplied to a hopper 32 of an injection molding machine 31 shown in FIG.
It is molded by being pressed into a mold 33 in a heated and molten state. The injection molding machine 31 includes a hydraulic cylinder 35 and a hydraulic motor 36 in a cylinder 34 having a nozzle 34a at the tip.
, And a heater 38 for heating and melting is provided outside.

A mold 33 for injection-molding the rotating nut 11
As shown in FIG. 6 (A), comprises a fixed mold 33a, a movable mold 33b, and a shaft-shaped core 39, and a cavity 40 for molding the rotating nut 11 is formed therebetween. At the tip of the core 39, a helical ridge 39a for simultaneously forming a helical ball rolling surface 11b having a multi-threaded shape is formed. It is pulled out while rotating. FIG.
4A, reference numeral 41 denotes a gate, and reference numeral 42 denotes a runner.

FIG. 6B shows one end cap 1.
This schematically illustrates a mold 43 for injection molding 4a, which comprises a fixed mold 43a and a movable mold 43b, between which a cavity 45 for forming the end cap 14a is formed. The mold 43 includes a reversing path 17 a of the end cap 14 a, a counterbore 19 (not shown), and a convex portion or a concave portion for forming a caulking convex portion 19 a (not shown). 45 are formed at predetermined positions on the inner surface. In FIG. 6B, 46 is a gate, and 47 is a runner. The other end cap 14b is also injection molded with a similar mold,
If both sides have a symmetric shape, they can be shared.

The rotary nut 11 injection-molded by the mold 33 and the end cap 14 injection-molded by the mold 43.
a is taken out from the molds 33 and 43, respectively, sent to a sintering furnace (not shown), and sintered. After sintering, a through hole 15 and a bolt hole 16a (see FIG. 4C) are formed in the rotary nut 11 by drilling, and a bolt hole 16b (FIG. 4) is formed in the counterbore 19 of the end cap 14a. (See (A)). Thereafter, carburizing and quenching are performed as necessary. The annular positioning projection 18a shown in FIGS. 4A and 4B and FIGS. 4C and 4D and the positioning recess 18b fitted to this are preferably formed by injection molding. Also, the bolt hole 1 of the rotating nut 11
6a is preferably a blind hole rather than being formed through the entire length of the rotating nut 11.

As described above, the present invention proposes to manufacture at least the end caps 14a, 14b and the end caps 14a, 14b of the rotating nut 11 by MIM for the following reasons. That is,
End cap method (reversing paths 17a, 17 in which torque transmitting balls 13 are formed in end caps 14a, 14b)
In the method of reversing and circulating in b), the projection is eliminated from the outer diameter portion of the rotating nut 11 to simplify the structure and achieve compactness in the radial direction, so that the entire electric power steering device can be made compact. . On the other hand, the end cap method has a simple structure, but the reversing direction of the torque transmitting ball 13 is substantially a right angle. Therefore, in order to obtain a smooth circulation operation of the torque transmitting ball 13, the end caps 14a, 14b Reversing paths 17a, 17
It is necessary to improve the accuracy of the shape and dimensions of b. Further, when the ball rolling path 12 is formed in a multi-threaded shape, the shape of the reversing paths 17a and 17b of the end caps 14a and 14b becomes complicated, and it takes a lot of man-hours in machining, which is not suitable for mass production. The same problem arises when the ball rolling surface 11b of the rotating nut 11 has a multi-threaded shape.

On the other hand, although it may be made of resin, the torque transmitting ball 1 may be used due to durability such as wear and the dimensional change due to thermal expansion and contraction.
There is a possibility that the circulating operation of 3 may not be smooth, and it is also difficult to control the tightening torque of the fixing bolt. The present invention makes it possible to manufacture the end caps 14a, 14b and the rotary nut 11 of the ball screw mechanism used for the automobile electric power steering apparatus accurately and at low cost by employing the MIM having both of these advantages. 1
1 is simplified and the rotor portion 2 of the electric motor 24 is simplified.
Since the bearing 4a and the bearing 26a can be fitted, it is possible to achieve compactness in the radial direction and increase in load capacity.

Further, if the end caps 14a, 14b are screwed and fixed to both ends of the rotary nut 11 with fixing bolts 16, the fixing bolts 16 are loosened by vibrations or the like when used in an automobile electric power steering apparatus. Although play may occur on the caps 14a and 14b, in the present invention, a convex portion 19a is formed around the counterbore portion 19 of the end caps 14a and 14b, and the fixing bolt 1
After the screwing of 6, the convex portion 19a is swaged with an appropriate swaging tool to form the rotation preventing portion 19a '. Therefore, the loosening of the fixing bolt 16 can be prevented with a simple configuration, and the torque can be reduced. The smooth circulation of the transmission ball 13 can be maintained, and the function and reliability of the ball screw mechanism 23 can be improved.

[0032]

According to the first aspect of the present invention, the outer diameter of the rotary nut of the ball screw mechanism can be reduced to achieve compactness, and the torque transmitting ball can be smoothly inserted into the through-hole of the rotary nut by the end cap. It can be reversed and circulated, and further, the number of parts can be reduced, the number of assembly steps can be reduced, and cost reduction by mass production can be achieved. In addition, since the end cap is formed of a sintered alloy, even when the ball rolling path has a multi-threaded shape, the entire end cap including the reversing portion and the like can be mass-produced accurately and inexpensively. Strength and durability can be easily maintained.

According to the second aspect of the present invention, the rotary nut can be mass-produced with high accuracy and low cost, and the required strength and durability can be easily maintained.

According to the third aspect of the invention, it is possible to prevent the loosening of the fixing bolt for fixing the end cap to the rotating nut due to vibration after screwing with a simple configuration, thereby improving reliability.

According to the fourth aspect of the present invention, it is possible to arrange a large number of torque transmitting balls by reducing the diameter of the torque transmitting balls, and to reduce the radial dimension to reduce the load capacity while reducing the size. Can be increased.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a ball screw mechanism according to the present invention.

FIG. 2A is a side view of a ball screw mechanism according to the present invention, and FIG. 2B is a front view of the ball screw mechanism according to the present invention.

3 (A) is a partially enlarged front view of a counterbore portion of an end cap, and FIGS. 3 (B) and 3 (C) are half of a counterbore portion showing a state before and after caulking of a convex portion after bolting. Sectional view.

4A is an end view of the end cap as viewed from the inside, FIG. 4B is an enlarged cross-sectional view of the reversing path, and FIG. 4C is a rotating nut showing the reversing path formed in communication with the through hole. (D) is an enlarged cross-sectional view showing a state of communication between an end of a through hole formed in the rotating nut and a reversing path.

FIG. 5A is a schematic sectional view showing an embodiment of an electric power steering apparatus to which the present invention is applied, and FIG. 5B is a schematic explanatory view of an injection molding machine to which the present invention is applied.

6A is a schematic sectional view of a mold applied to injection molding of a rotary nut in the present invention, and FIG. 6B is a schematic sectional view of a mold applied to injection molding of an end cap in the present invention.

7A is a schematic sectional view of a conventional frame circulation type ball screw mechanism, FIG. 7B is a schematic plan view of a conventional tube circulation type ball screw mechanism, and FIG. 7C is a schematic sectional view thereof.

[Explanation of symbols]

Reference Signs List 10 Ball screw shaft 10a Outer diameter surface 10b Ball rolling surface 11 Rotating nut 11a Inner diameter surface 11b Ball rolling surface 11c Bearing fitting surface 11d Rotor portion fitting surface of electric motor (knurl forming surface) 11e Flange 12 Ball rolling path DESCRIPTION OF SYMBOLS 13 Torque transmission ball 14a, 14b End cap 15 Through hole 16 Fixing bolt 17a, 17b Reversing path 19 Counterbore 19a Convex 19a 'detent 21 Steering shaft 22 Conversion mechanism 23 Ball screw mechanism 24 Electric motor 24a Rotor 24b Stator part 25 Housing 26a, 26b Bearing 33 Mold for injection molding of rotating nut 43 Mold for injection molding of end cap

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Morikuhisa Yoshioka 1578 Higashikaizuka, Iwata-shi, Shizuoka F-term in NTN Corporation (reference) 3D033 CA04 CA05

Claims (4)

[Claims] 1. A steering mechanism connected to a housing, a steering mechanism for steering wheels, a conversion mechanism for converting a rotational force from a steering wheel into a force for moving the steering shaft in a longitudinal direction, and the steering mechanism. In an electric power steering device including a ball screw mechanism provided integrally with a shaft and an electric motor fitted to a rotating nut of the ball screw mechanism,
A ball screw shaft having a helical ball rolling surface on an outer diameter surface, a rotating nut having a helical ball rolling surface on the inner diameter surface opposed to the ball screw shaft; A plurality of torque transmitting balls for transmitting a force between the rotary nut and the steering shaft, the plurality of torque transmitting balls being disposed in tandem with a ball rolling path formed between the ball rolling surfaces; In order to circulate the ball, a through hole was formed in the rotating nut, and an end cap attached to both ends of the rotating nut and for inverting the torque transmitting ball into the rotating nut through hole was formed of a sintered alloy. An electric power steering device characterized by the above-mentioned. 2. The electric power steering apparatus according to claim 1, wherein the rotating nut is formed of a sintered alloy. 3. The electric power steering apparatus according to claim 1, wherein the counterbore portion of the end cap is swaged to prevent a fixing bolt fixed to the rotary nut from rotating. 4. The electric power steering apparatus according to claim 1, wherein a ball rolling path of the ball screw mechanism has a multi-threaded shape.