JP3525194B2 - Electric power steering device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering apparatus which is mounted on an automobile and assists a force required for steering operation by a rotating force of an electric motor. 2. Description of the Related Art An electric motor for driving a steering assist is driven based on a detection result of a steering torque applied to a steering wheel, so that a force required for steering an automobile is assisted by a rotational force of the electric motor. Power steering devices have been developed. This type of electric power steering apparatus includes a column type in which an electric motor is connected to a column shaft, and a rack axis type in which an electric motor is connected to a rack shaft. FIG. 5 is a partially cutaway side view showing a rack shaft type electric power steering apparatus in which a rack shaft is driven by a conventional electric motor, and FIG. No. 5-131936). In the figure, reference numeral 1 denotes a rack shaft, which is slidably fitted in a rack shaft case 2 disposed in the left-right direction of the vehicle body so as to be movable in the axial direction. Further, a pinion shaft 3 is disposed in a pinion shaft case 4 interposed near one end of the rack shaft case 2 so as to be oblique to the rack shaft 1, and a lower end of the pinion shaft 3 and the rack shaft. The pinion teeth provided on the pinion shaft 3 and the rack teeth provided on the rack shaft 1 are meshed with each other at a portion opposed to the rack shaft 1. The pinion shaft 3 has a torsion bar interposed in the middle, and the upper end thereof is connected to a steering wheel (not shown). The operation of the steering wheel causes the pinion shaft 3 to rotate, and the rack shaft 1 is moved left and right to move the wheel. Steering is performed. [0005] A torque sensor (not shown) is provided around the torsion bar, and a steering torque generated in the torsion bar in accordance with the operation of the steering wheel is detected, and the detected torque is taken into a control unit (not shown). The control unit calculates a target rotation speed of the electric motor 7 and outputs a control signal to the electric motor 7 based on the target rotation speed and a current rotation speed obtained from a tachometer attached to the electric motor 7. The electric motor 7 includes a stator 72, a rotor 73, and a cylindrical outer cylindrical shaft that is provided with the rotor 73 and is concentric with a center line of the rack shaft 1 in a motor case 71 provided at a substantially central portion of the rack shaft case 2. 74, and an inner cylindrical shaft 75, which also has a cylindrical shape, is disposed on the inner side. [0006] The outer cylindrical shaft 74 and the inner cylindrical shaft 75 are spline-coupled to each other.
The inner cylindrical shaft 75 can move relatively in the axial direction, but is configured to rotate integrally in the circumferential direction. The inner cylindrical shaft 75 is connected to the motor case 71 or the rack shaft case 2 via the first ball screw mechanism 76,
The inner cylindrical shaft 75 is connected to the rack shaft 1 via a second ball screw mechanism 77. The first ball screw mechanism 76 and the second ball screw mechanism 77 have opposite screw directions, and the ball screw lead of the first ball screw mechanism 76 is the second ball screw mechanism.
Is set smaller than that of the ball screw of the ball screw mechanism 77. Next, the operation of such a conventional device will be described. When the rack shaft 1 is moved to the left by operating the steering wheel, the outer cylindrical shaft 74 and the inner cylindrical shaft 75 are both rotated by the same amount in the same direction by the rotation of the electric motor 7. When the inner cylindrical shaft 75 rotates, the inner cylindrical shaft 75 moves to the left with respect to the rack shaft case 2 by the first ball screw mechanism 76, and the rack shaft 1 is moved by the second ball screw mechanism 77 to the inner cylindrical shaft 75. Move to the left. As a result, the rack shaft 1 moves to the left with respect to the rack shaft case 2, and the amount of movement is the amount of movement of the inner cylindrical shaft 75 with respect to the rack shaft case 2 and the amount of movement of the rack shaft 1 with respect to the inner cylindrical shaft 75. And the sum of In the conventional apparatus as described above, the ball screw rail is formed on the same axis as the rack shaft, so that the ball nut is rotated to drive the rack shaft. Also produces a rotational moment. The rack shaft is supported in the rack shaft case so as to be movable in the axial direction, and its rotation is only restrained by the engagement between the rack teeth and the pinion teeth. There has been such a problem that an excessive input is applied to the meshing portion between the teeth and the pinion teeth, resulting in poor meshing. SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to prevent a rack shaft from being caused to rotate with the rotation of a ball nut, and to mesh a rack tooth and a pinion tooth. An object of the present invention is to provide an electric power steering apparatus capable of preventing a failure. An electric power steering apparatus according to the present invention has a helical ball screw rail for a ball screw rotated by an electric motor and a rack tooth meshing with a pinion tooth connected to a steering wheel. In a power steering apparatus provided with rack shafts each of which is formed and assisting the operation of the steering wheel with the rotational force of the electric motor, the center line of the ball screw rail is formed eccentrically from the axis of the rack shaft. It is characterized by having been done. According to the present invention, the axis of the ball screw rail formed on the rack shaft is eccentric from the axis of the rack shaft, and the eccentric position is supported by the rack shaft case via the electric motor. By doing so, even if the ball nut rotates, the force acting on the rack shaft with this rotation does not cause the rack shaft to rotate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings showing embodiments thereof. FIG. 1 is a sectional view of an electric power steering apparatus according to the present invention (corresponding to a sectional view taken along line II of FIG. 2), FIG. 2 is a sectional view showing a relationship between a rack shaft and an electric motor, and FIG. FIG. 4 is an enlarged sectional view taken along line IV-IV of FIG. 3, wherein 1 is a rack axis, 2 is a rack axis case, and 3 is a pinion axis. Reference numeral 4 denotes a pinion shaft case. The rack shaft 1 is inserted into a cylindrical rack shaft case 2 fixed to the vehicle body with the longitudinal direction being the left-right direction, and as shown in FIG. The bushings 21 and 21 are slidably supported in the axial direction. Both ends of the bush are protruded outward from both ends of the rack shaft case 2, and both protruding ends are connected via ball joints 11 and 11. The tie rods 12 are connected to one ends of the tie rods 12, 12, respectively. The other ends of the tie rods 12 are connected to the left and right front wheels, and the front wheels are steered left and right by moving the rack shaft 1 in the axial direction. As shown in FIG. 2, the pinion shaft 3 is provided inside a pinion shaft case 4 connected to one end of the rack shaft case 2 so that its axis is oblique to the rack shaft 1. Supported. As shown in FIG. 1, the pinion shaft 3 includes an upper shaft 32 and a lower shaft 33 coaxially connected via a torsion bar 31, and the upper shaft 32 is supported on a pinion shaft case 4 by a rolling bearing 41. The upper end is operatively connected to the steering wheel via a universal joint (not shown). The lower shaft 33 is supported by the lower portion of the pinion shaft case 4 and the rack shaft case 2 over the pinion shaft case 4 and the rack shaft case 2 by ball bearings 42 and 43, respectively. In the middle of the lower shaft 33, pinion teeth 34 are formed over an appropriate length in the axial direction thereof,
The pinion teeth 34 mesh with rack teeth 13 formed near one end of the rack shaft 1 inside the rack shaft case 2. At a position where the rack teeth 13 and the pinion teeth 34 mesh with each other, a rack guide 15 for pressing the rack shaft 1 by the urging force of the pressing spring 14 toward the pinion shaft 3 is provided so that the rack teeth 13 and the pinion teeth 34 are reliably meshed. is there. In FIG.
Is a torque sensor for detecting a steering torque applied to the torsion bar 31, and detects a relative displacement in the circumferential direction generated between the upper shaft 32 and the lower shaft 33 due to the torsion of the torsion bar 31 according to the steering torque. Detect and output as potential. The torque sensor 6 is initially adjusted to output a predetermined reference potential when the torsion bar 31 is not twisted, in other words, when no steering operation is performed. The output signal of the torque sensor 6 is input in time series to a control unit (not shown), and the control unit compares the signal with the reference potential to recognize the direction and magnitude of the steering torque. The electric motor 7 for steering assist as shown in FIG. In FIG. 3, reference numeral 71 denotes a motor case of the electric motor 7, 72 denotes a stator, and 73 denotes a rotor. The motor case 71 is formed in a cylindrical shape whose inner and outer diameters are larger than those of the rack shaft case 2.
The flanges formed on the ends of the rack shaft case 2 and both ends of the motor case 71 are fastened with set screws 71a in the middle of the rack shaft case 2 divided into left and right, so that the rack shaft case 2 It is interposed in a mode that also serves as a part. The stator 7 is provided on the inner peripheral wall of the motor case 71.
2 is fixed, and a cylindrical shaft 74 to which the inside of the stator 72 is concentrically penetrated and the rotor 73 is externally fixed is provided. One end of the cylindrical shaft 74 protruding from one end of the stator 72 faces the inside of the rack shaft case 2 whose diameter has been increased. Here, the rack shaft is provided via a ball bearing 73 a provided between the rack shaft case 2 and the inner peripheral surface. A cylindrical shaft 7 supported by the case 2 and protruding from the other end of the stator 72.
The other end of 4 is supported by the motor case 71 via a ball bearing 73b provided between the motor case 71 and the inner peripheral surface of the motor case 71. At the other end of the cylindrical shaft 74, a ball nut 81 for the ball screw 8, which is formed concentrically and integrally with the cylindrical shaft 74, extends. The outer peripheral surface of the terminal is the inner peripheral surface of the enlarged rack shaft case 2. And is supported by the rack shaft case 2 by a bearing 81a interposed therebetween. A helical ball screw rail 82 for a ball screw is formed on the outer periphery of the rack shaft 1 facing the inner peripheral surface of the ball nut 81. The axis of the region 1d where the ball screw rail 82 is formed is eccentric by a predetermined dimension Δd with respect to the axis of the entire rack shaft 1 as shown in FIG. The eccentric direction is not particularly limited. Accordingly, the axis of the stator 72, the rotor 73 of the electric motor 7, the ball nut 81 of the ball screw 8, and the ball screw rail 82 are eccentric with respect to the axis of the rack shaft 1 by Δd. A gear 7 is provided on the outer peripheral surface of the one end of the cylindrical shaft 74.
3c is fixed, and the motor case 7 is
The rotation speed of the cylindrical shaft 74 is measured by the rotation meter 9. Next, the operation of such an electric power steering device will be described. When a steering wheel (not shown) is rotated, a steering torque is generated in the torsion bar 31, and this torque is detected by the torque sensor 6 and sent to a control unit (not shown). The control unit calculates a target rotation speed of the electric motor 7 based on the input torque detection value, and calculates the electric motor 7 based on the target rotation speed and the rotation speed detected by the tachometer 9.
Drive control. The electric motor 7 rotates its rotor 73 and cylindrical shaft 74 to move the rack shaft 1 in the direction of its axis through the ball nut 81 of the ball screw 8, and from the steering wheel through the torsion bar 31 and the pinion shaft 3. It assists the axial moving force applied to the rack shaft 1. The rotor 73 and the cylindrical shaft 74 are supported by the rack shaft case 2 and the motor case 71 via ball bearings 73a, 73b and 81a, and a ball screw rail 82 of the rack shaft 1 is formed as shown in FIG. As a result of the rotation about the axis of the region 1d, the rotation of the rotor 73 and the cylindrical shaft 74 does not cause the rotation of the rack shaft 1, and prevents the meshing failure between the rack teeth 13 and the pinion teeth 34 from occurring. Is done. As described above, in the present invention, since the axis of the ball screw rail formed in a spiral shape is eccentric with respect to the axis of the rack shaft, The rotational moment load of the rack shaft can be received by the rack shaft support portion (rack guide and rack bush), so that no extra load is applied to the rack teeth and the pinion teeth, and the durability is greatly improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing a coupling structure between a rack shaft and a pinion shaft of an electric power steering device according to the present invention. FIG. 2 is a sectional view showing a coupling structure of a rack shaft, a pinion shaft, and an electric motor of the electric power steering device according to the present invention. FIG. 3 is an enlarged sectional view showing a coupling structure of a rack shaft, a ball screw, and an electric motor of the electric power steering device according to the present invention. FIG. 4 is an enlarged sectional view taken along line IV-IV of FIG. 3; FIG. 5 is a partially broken side view of a conventional electric power steering apparatus. FIG. 6 is a partially enlarged cross-sectional view showing a coupling structure between an electric motor and a rack shaft of a conventional electric power steering device. [Description of Signs] 1 Rack shaft 1d Region where ball screw rail is formed 2 Rack shaft case 3 Pinion shaft 4 Pinion shaft case 6 Torque sensor 7 Electric motor 8 Ball screw 9 Tachometer 31 Torsion bar 71 Motor case 72 Stator 73 Rotor 73a , 73b Ball bearing 74 Cylindrical shaft 81a Ball bearing 82 Ball screw rail

Continuation of the front page (56) References JP-A-1-95966 (JP, A) JP-A-5-147542 (JP, A) JP-A-5-131936 (JP, A) JP-A-4-62279 (JP) (U, U) (Japanese) 6-42526 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) B62D 5/04 F16H 25/20

Claims (1)

(57) [Claims 1] A rack shaft formed with a rack tooth meshing with a pinion tooth connected to a steering wheel and a helical ball screw rail for a ball screw rotated by an electric motor, In a power steering apparatus configured to assist the operation of a steering wheel by the rotational force of the electric motor, a center line of the ball screw rail is formed to be eccentric from an axis of a rack shaft. Power steering device.