JP2006199054A - Motor-driven power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a motor-driven power steering device, preventing abnormal sound in a power transmission system while reducing the size. <P>SOLUTION: A ball screw mechanism 11 adopting an end deflector type circulation system is an ordinary tube type or deflector type. As compared with the so-called end cap type circulation system ball screw mechanism, the space in the ball circulating passage is small and a ball 55 can be tangentially scooped up so that it is possible to restrain irregular contact between the balls 55 in the circulating passage. Further, as compared with the tube type, no projection of the tube is caused and the outside diameter of a nut 45 can be reduced to decrease the inertia of the power transmission system. Further, since the effective number of turns per one circulating passage can be made larger, even if a necessary load capacity is set, the nut length can be kept short to contribute to space saving. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electric power steering apparatus that generates a steering force by transmitting power of an electric motor to a steering mechanism.

  2. Description of the Related Art Conventionally, in a vehicle, a power steering device that uses a vane hydraulic pump as a power source and generates a steering assist force by driving the hydraulic pump with an engine is known. However, this type of power steering device has a large engine drive loss due to the constant drive of the hydraulic pump (several horsepower to about 10 horsepower at the maximum load), so it can be used in light vehicles with small displacement. It is difficult, and it has been inevitable that the fuel consumption will be affected even by a car with a relatively large displacement.

  In order to solve these problems, an electric power steering apparatus using an electric motor as a power source has attracted attention in recent years. The electric power steering device uses an electric motor driven by an in-vehicle battery as a power source, so there is no direct engine drive loss, and the electric motor is started only during steering assist, resulting in a decrease in driving fuel consumption. In addition, the electronic control is extremely easy.

  On the other hand, as a steering mechanism for a passenger car, a rack and pinion type is currently mainstream because of its high rigidity and light weight. As such an electric power steering device for a rack and pinion type steering mechanism, an electric ball screw mechanism is used in addition to a column assist type in which an electric motor is arranged on the side of a column to drive a steering shaft and pinion itself. A ball screw rack assist type that drives the rack shaft has also been developed. In the ball screw rack assist type electric power steering apparatus, since the assist force does not act on the meshing surface between the pinion and the rack, the contact surface pressure between both members, which causes wear and deformation, is relatively small.

In a rack assist type electric power steering apparatus, a large number of circulating balls (steel balls) are formed on a rolling path formed by a male screw groove of a ball screw shaft formed on a rack shaft and a female screw groove formed on a ball nut. The rack shaft is axially moved by rotating the ball nut with an electric motor. In order to transmit power between the electric motor and the ball nut, there is one using a gear train as shown in Patent Document 1.
JP 2003-2219 A

  Here, when the ball screw mechanism is adopted in the rack assist type electric power steering device, it has been found that the operation sound of the ball screw driven by the electric motor may cause discomfort to the driver and passengers. did. Further, since the ball screw mechanism is used by rotating the nut, it is desirable to make it as small as possible while ensuring the necessary load capacity.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an electric power steering device which is aimed at preventing noise and the like in a power transmission system while reducing the size.

The electric power steering apparatus of the present invention is
An electric motor;
A rack shaft coupled to a steering mechanism, and a ball screw mechanism for transmitting power from the electric motor to the rack shaft,
The ball screw mechanism includes a screw shaft connected to or integrated with the rack shaft, a nut disposed around the screw shaft, and a rolling path formed between the screw shaft and the nut. A ball that rolls along,
The nut includes a ball scooping portion that scoops a ball that has rolled along the rolling path in a tangential direction, a ball return passage that is connected to the scooping portion and extends in an axial direction, and is connected to the ball return passage. And a ball return portion for returning the ball to the rolling path.

  According to the electric power steering apparatus of the present invention, the nut scoops the ball rolling along the rolling path in the tangential direction, and the ball connected to the scooping part and extending in the axial direction. It has a return passage and a ball return portion connected to the ball return passage to return the ball to the rolling path, and as such a ball screw mechanism, there is a ball circulation system called a so-called end deflector type. By making the ball screw mechanism, for example, an end deflector type, generation of unpleasant operating noise can be suppressed. In addition, if this method is used, it is possible to reduce the size of the product while ensuring the necessary load capacity.

  More specifically, a ball screw mechanism that employs an end deflector type circulation system can be accurately formed in a size close to the ball diameter because the circulation path in the nut is a straight line. Compared with a circulation type ball screw mechanism with a curved part, the clearance in the circulation path is small, and the balls can be picked up tangentially and easily, and irregular contact between the balls in the circulation path can be suppressed. It becomes. Further, since the tube does not protrude and the nut outer diameter is reduced as compared with the tube type, the inertia of the power transmission system can be reduced. In addition, since the effective number of windings per circuit can be increased, the nut length can be kept short even if a necessary load capacity is set, which contributes to space saving.

  Hereinafter, an electric power steering apparatus according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of a steering system including an electric power steering apparatus according to the first embodiment.

  In FIG. 1, a steering shaft 3 having a steering wheel 5 attached to one end is rotatably supported with respect to a column 1 fixed to a vehicle body (not shown). The other end of the steering shaft 3 is connected to the intermediate shaft 9 via the universal joint 7. The intermediate shaft 9 is connected to an electric power steering device 11. The electric power steering apparatus 11 can drive a pair of tie rods 15 connected to a steering mechanism (not shown).

  FIG. 2 is a sectional view in the axial direction of the electric power steering apparatus 11 of FIG. FIG. 3 is an enlarged view showing a part III in the configuration of FIG. In FIG. 2, the electric power steering apparatus 11 has a cylindrical housing 21 fixed to a vehicle body (not shown). An input shaft 13 connected to the intermediate shaft 9 (FIG. 1) is inserted into the left portion of the housing 21 from an obliquely upper side. In the housing 21, the rack shaft 23 extends in the horizontal direction. A pinion (not shown) is formed at the lower end of the input shaft 13 and meshes with the rack teeth 23a of the rack shaft 23. The rotation of the input shaft 13 causes the rack shaft 23 to move left and right in the figure. . Both ends of the rack shaft 23 are connected to the tie rod 15 via joints 27. In the present embodiment, by forming the male screw groove 23b in a part of the rack shaft 23, the rack shaft 23 also serves as a screw shaft (integrated), but the rack shaft 23 is replaced with another screw. It may be connected to a shaft.

  In FIG. 3, the electric motor 35 is attached to the housing 21 so that the rack shaft 23 and the axis are parallel to each other. An output shaft 35 a of the electric motor 35 has a drive gear 37 fixed by a key connection (not shown), and is rotatably supported by bearings 41, 41 on both sides of the drive gear 37 with respect to the housing 21. The driven gear 39 meshed with the drive gear 37 is hollow and is inserted through the rack shaft 23, and cylindrical portions 39 a and 39 b are formed at both ends in the axial direction. The cylindrical portions 39 a and 39 b are formed in the housing 21. In contrast, the bearings 43 and 43 are rotatably supported. One of the driven gears 39 (right side in FIG. 3) has a cylindrical portion 39b that is engaged with a female spline portion 63 provided on the inner periphery thereof and a male spline portion 65 formed on the outer periphery of the end portion of the nut 45. The nut 45 is attached so as to be relatively displaceable in the axial direction and integrated in the rotational direction.

  A male screw groove 23 b is formed in a part of the outer peripheral surface of the rack shaft 23. A cylindrical nut 45 is disposed around the male screw groove 23b, and a female screw groove 45a is formed facing the male screw groove 23b. In a spiral space (transfer path) formed by the male screw groove 23b and the female screw groove 45a, a plurality of balls 55 are arranged to roll freely. The nut 45 is rotatably supported by a bearing 47 with respect to the housing 21. The inner ring of the bearing 47 is fixed by an end flange of the nut 45 and a retaining ring 49 attached to the outer peripheral surface of the nut 45, and the outer ring of the bearing 47 is fixed by a retaining ring 50 attached to the inner peripheral surface of the housing 21. Therefore, the nut 45 does not move in the axial direction but can only rotate.

  FIG. 4 is an enlarged sectional view of the end of the ball screw mechanism according to the present embodiment. FIG. 5 is a view of the configuration of FIG. 4 taken along line IV-IV and viewed in the direction of the arrow. FIGS. 6A and 6B are diagrams showing a circulating top, where FIG. 6B is a front view, FIG. 6A is a left side view of FIG. 6B, and FIG. 6C is a plan view of FIG. The following type of ball screw mechanism is called an end deflector type ball screw mechanism.

  As shown in FIGS. 4 and 5, the ball screw mechanism 11 has a male screw groove 23b having a semicircular cross section on the outer peripheral surface and a semicircular cross section on the inner peripheral surface around the screw shaft 23 extending in the axial direction. A nut 45 having a female thread groove 45a is disposed. The female screw groove 45a of the nut 45 and the male screw groove 23b of the screw shaft 23 are opposed to each other to form a spiral passage (rolling passage) therebetween, and a large number of balls 55 as rolling elements are formed in the spiral passage. Is loaded so that it can roll.

  In the ball screw mechanism 11, the nut 45 (or the screw shaft 23) moves linearly through the rolling of the ball 55 by the rotation of the screw shaft 23 (or the nut 45). A ball return passage 45b penetrating in the axial direction is formed in the wall portion of the nut 45, and a notch 45c into which the circulating top 18 shown in FIG. 45b and both screw grooves 23b and 45a are formed in communication.

  The circulating top 18 serving as a ball scooping portion and a ball returning portion is made of, for example, a synthetic resin molded product. As shown in FIG. 6, a tongue portion 18a for scooping up the ball 55 between the screw grooves 23b and 45a, and a ball return passage. 45b and a curved ball circulation groove 18b communicating between the screw grooves 23b and 45a. An infinite circulation passage for the ball 55 is formed by the tongue portion 18a, the ball circulation groove 18b, the space between the screw grooves 23b and 45a, and the ball return passage 45b.

  By fitting the circulation top 18 into the notch 45 c of the nut 45, the circulation top 18 is fitted and fixed to the end of the nut 45 in a state of being restrained in the radial direction and the circumferential direction. As a method of fitting and fixing the circulating top 18 to the notch 45c, there is a simple fitting method, but for example, press fitting, bonding, or both pressing and bonding can be employed.

  In the present embodiment, the tongue 18a of the circulation top 18 is fitted along the male screw groove 23b of the screw shaft 23 and brought into contact with the male screw groove 23b in such a fixed state. The direction is kept from falling out.

  Thus, in this embodiment, when the circulating top 18 is fitted into the notch 45c, the tongue portion 18a of the circulating top 18 is fitted along the male thread groove 23b of the screw shaft 23 and brought into contact with the male thread groove 23b. Thus, the circulating top 18 can be fitted and fixed to the end of the nut 45 while preventing the circulating top 18 from coming off in the axial direction. The screwing operation to the top nut becomes unnecessary, and the machining cost and the assembly cost can be reduced accordingly, so that the manufacturing cost can be reduced. Moreover, the circulating top 18 can be prevented from coming off in the radial direction and the axial direction. Since the circumferential rotation is prevented, the reliability of the apparatus can be improved.

  Further, since there is almost no step due to the contact between the tongue portion 18a and the male screw groove 23b, the ball 55 can be smoothly picked up by the tongue portion 18a, and the tongue portion 18a and the male screw groove 23b are in contact with each other. As a result, the sealing performance is improved, and the effect of retaining the lubricant and preventing foreign matter from entering from the outside can be improved.

  The operation of this embodiment will be described. In FIG. 1, when the driver rotates the steering wheel 5, the rotational force is transmitted to the input shaft 13 (FIG. 2) via the steering shaft 3 and the intermediate shaft 9. In FIG. 2, when the input shaft 13 rotates, the rack teeth 23 a meshing with the pinion (not shown) are pushed in the axial direction of the rack shaft 23, and the rack shaft 23 moves in the axial direction, via the tie rods 15, 15. By driving a steering mechanism (not shown), the wheels are steered.

  Referring to FIG. 3, a torque sensor (not shown) detects the steering torque applied to input shaft 13 at this time, and an ECU (not shown) supplies electric power to electric motor 35 according to the amount. The drive gear 37 rotates together with the output shaft 35a, and the driven gear 39 engaged therewith is rotated at a predetermined reduction ratio. Since the driven gear 39 and the nut 45 are integrated in the rotation direction by the engagement of the female spline portion 63 and the male spline portion 65, the nut 45 is also rotated by the rotation of the driven gear 39, and the rotational motion is It is converted into an axial movement of the rack shaft 23 via the ball 55. An auxiliary steering force can be output using this axial force. Here, as the nut 45 rotates, the ball 55 that has rolled in the spiral path between the female screw groove 45a of the nut 45 and the male screw groove 23b of the screw shaft 23 is disposed at one end of the nut 45 while suppressing noise. It is scooped up by the circulating top 18 and returned to the spiral passage from the circulating top 18 on the other end side through the ball return passage 45b.

  In the electric power steering apparatus according to the present embodiment, the ball screw mechanism 11 adopting the end deflector type circulation system has a ball circulation function as compared with a circulation type ball screw mechanism such as a normal tube type, top type, or end cap type. Since the gap in the path is small and the balls 55 can be picked up tangentially, irregular contact between the balls 55 in the circulation path can be suppressed. Furthermore, since the tube does not protrude and the outer diameter of the nut 45 is reduced as compared with the tube type, the inertia of the power transmission system can be reduced. In addition, since the effective number of windings per circuit can be increased, the nut length can be kept short even if a necessary load capacity is set, which contributes to space saving.

  As described above, the present invention has been described in detail with reference to the embodiments. However, the present invention should not be construed as being limited to the above-described embodiments, and can be appropriately changed and improved without departing from the spirit thereof. Of course there is. For example, in the above-described embodiment, the rack shaft and the motor rotation shaft are arranged in parallel, but the rack shaft and the motor rotation shaft are arranged coaxially, or the rack. The shaft and the rotating shaft of the motor may be obliquely intersected. The present invention is also applicable to a so-called steer-by-wire (SBW) type steering mechanism in which a steering wheel and a rack shaft are not mechanically connected, a rear wheel steering mechanism used in a four-wheel steering (4WS) vehicle, and the like. .

1 is a perspective view of a steering system including an electric power steering apparatus according to an embodiment. FIG. 2 is an axial sectional view of the electric power steering device 11 of FIG. 1. It is a figure which expands and shows the III section in the structure of FIG. It is an edge part expanded sectional view of the ball screw mechanism concerning this Embodiment. It is the figure which cut | disconnected the structure of FIG. 4 by the IV-IV line and looked at the arrow direction. It is a figure which shows the circulation top as a rolling element circulation member, (b) is a front view, (a) is a left view of (b), (c) is a top view of (b).

Explanation of symbols

1 Column 3 Steering shaft 5 Steering wheel 11 Electric power steering device 18 Circulation top 23 Rack shaft (screw shaft)
45 nut

Claims (1)

An electric motor;
A rack shaft coupled to a steering mechanism, and a ball screw mechanism for transmitting power from the electric motor to the rack shaft,
The ball screw mechanism includes a screw shaft connected to or integrated with the rack shaft, a nut disposed around the screw shaft, and a rolling path formed between the screw shaft and the nut. A ball that rolls along,
The nut includes a ball scooping portion that scoops a ball that has rolled along the rolling path in a tangential direction, a ball return passage that is connected to the scooping portion and extends in an axial direction, and is connected to the ball return passage. An electric power steering device having a ball return portion for returning a ball to a rolling path.

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