JP2010132060A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device without a drop in torque transmission efficiency. <P>SOLUTION: The electric power steering device, in which a ball nut 23 is screwed into a ball screw shaft 21 formed in a rack shaft 18, and an electric motor 26 is disposed parallel to the rack shaft, performs steering assist by transmitting the driving force of the electric motor through a reduction gear mechanism 27. In a housing 20 axially movably supporting the rack shaft, the ball nut is rotatably disposed in a position near the shaft end of the rack shaft, and a guide bush 42 for slidably guiding the outer periphery of the rack shaft is also disposed. The guide bush is supported on the inner periphery of the ball nut through a rolling bearing 41. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electric power steering apparatus configured to receive a radial load acting on a rack shaft by using a ball nut.

  In the electric power steering apparatus, for example, as described in Patent Document 1, a ball nut is screwed onto a ball screw shaft formed on a rack shaft via a ball, and an electric motor is arranged in parallel with the ball nut. In addition, a drive gear is connected to the motor shaft of the electric motor, a driven gear is provided on the ball nut, and a motor parallel type electric motor provided with a gear reduction mechanism in which the drive gear and the driven gear are engaged via an intermediate gear. There is a power steering device.

  In this type of electric power steering apparatus, a radial load is applied to the rack shaft by the turning operation of the steered wheels. If this radial load is transmitted to a ball nut that is screwed onto the ball screw shaft, and an unbalanced load acts on the ball nut, this will cause a decrease in the durability of the ball nut. For this purpose, generally, the end side of the rack shaft is slidably guided to the housing via the rack bushing so that the radial load acting on the rack shaft does not apply an uneven load to the ball nut.

  By the way, in order to ensure the mountability to the vehicle, the ball nut may be arranged on the end (end) side of the rack shaft. For example, depending on the vehicle, the engine oil pan or the like may be disposed close to the axis of the rack shaft. In such a vehicle, the diameter of the housing is small in order to avoid interference with the oil pan or the like. It is necessary to dispose a large ball nut portion on the end side of the rack shaft.

  However, if the ball nut is disposed on the end side of the rack shaft, it becomes impossible to secure a space for arranging the rack bush supporting the end side of the rack shaft in the housing. In such a case, a rack bush may be attached to a ball nut disposed on the end side of the rack shaft, and the end side of the rack shaft may be slidably guided to the ball nut via the rack bush.

Conventionally, there is a known one in which a bush is press-fitted and fixed to the inner peripheral portion on the tip side of the ball nut and the rack shaft is supported so as to be linearly slidable (see, for example, Patent Document 2). The bush (40) is stretched between the bush support (17) of the housing and the ball nut (32).
JP 2008-105649 A JP-A-2005-343308

  However, as described in Patent Document 2, if the rack shaft is slidably guided by a bush (rack bush) press-fitted to the ball nut, the rack shaft is caused by a radial load acting on the rack shaft. Abuts against the guide bush, and sliding friction occurs between the rack bush and the rack shaft. For this reason, as shown in FIG. 4, the rotational force F1 of the ball nut 1 is lost by the sliding friction force F3 between the rack bush 2 and the rack shaft 3, and there is a problem that the torque transmission efficiency of the ball screw mechanism is lowered.

  The present invention has been made to solve the above-described conventional problems, and an object of the present invention is to provide an electric power steering device in which torque transmission efficiency does not decrease.

  In order to solve the above problems, the invention according to claim 1 is characterized in that a ball nut is screwed onto a ball screw shaft formed on a rack shaft, an electric motor is disposed in parallel with the rack shaft, and the electric motor An electric power steering device for assisting steering by transmitting a driving force to the ball nut via a gear reduction mechanism, comprising a housing that accommodates the rack shaft so as to be movable in the axial direction. A guide bush that is rotatably disposed at a position near the shaft end of the rack shaft and that slidably guides the outer periphery of the rack shaft is provided, and the guide bush is disposed on the inner periphery of the ball nut via a rolling bearing. It is to support.

  A feature of the invention according to claim 2 is that, in claim 1, the guide bush is made of a material softer than the rolling bearing.

  A feature of the invention according to claim 3 is that, in claim 2, the guide bush is made of a soft metal material.

  According to the first aspect of the invention, since the ball nut is rotatably disposed in the housing at a position close to the end of the rack shaft, for example, an engine oil pan or the like approaches the axis of the rack shaft. Even in a vehicle that is arranged in this manner, it is possible to reliably avoid interference between the housing and the oil pan. In addition, since the guide bush is supported via the rolling bearing on the inner periphery of the ball nut disposed near the shaft end of the rack shaft, the friction generated between the rack shaft and the rack bush is reduced by the rolling friction. It can be. As a result, it is possible to realize an electric power steering device that can reduce friction loss and does not reduce the torque transmission efficiency of the ball screw mechanism.

  According to the invention of claim 2, since the guide bush is made of a softer material than the rolling bearing, the rack shaft is damaged even if the rack shaft is pressed against the guide bush by a radial load acting on the rack shaft. Without sliding, the rack shaft can be smoothly slid.

  According to the invention of claim 3, since the guide bush is made of a soft metal material, the same effect as that of claim 2 can be obtained, and the durability of the guide bush can be improved.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic view of an electric power steering apparatus 10 according to the present embodiment, and FIG. 2 is a cross-sectional view showing a configuration in which a guide bush 42 is supported on a ball nut 23 provided in the electric power steering apparatus 10. .

  In FIG. 1, an electric power steering apparatus 10 steers a steered wheel 12 by a steering shaft 14 that transmits a steering torque applied to a steering wheel 13 to steer the steered wheel 12, and the steering torque from the steering shaft 14. For example, a steering mechanism 15 composed of a rack and pinion mechanism, and an intermediate shaft 16 as a shaft coupling connecting the steering shaft 14 and the steering mechanism 15 are provided.

  The steering mechanism 15 includes a pinion shaft 17 as an input shaft, a rack shaft 18 as a steered shaft extending in the lateral direction of the automobile (a direction orthogonal to the straight traveling direction), and a housing 20 that houses the pinion shaft 17 and the rack shaft 18. And have. The pinion shaft 17 is rotatably supported by the housing 20, and the rack shaft 18 is supported by the housing 20 so as to be linearly reciprocable. The pinion teeth 17 a of the pinion shaft 17 are meshed with the rack teeth 18 a of the rack shaft 18. Both ends of the rack shaft 18 are coupled to the steered wheels 12 via tie rods and knuckle arms.

  Thereby, when the steering wheel 13 is steered, the steering torque is transmitted to the steering mechanism 15 via the steering shaft 14 and the like, and the pinion shaft 17 is rotated. The rotation of the pinion shaft 17 is converted into linear movement of the rack shaft 18 by the pinion teeth 17a and the rack teeth 18a, whereby the steered wheels 12 are steered.

  As shown in FIG. 2, a ball screw shaft 21 is integrally formed on the rack shaft 18, and a ball nut 23 that is screwed onto the ball screw shaft 21 via a large number of balls 22 is concentric with the rack shaft 18. The housing 20 is rotatably supported via a bearing 35. The ball screw shaft 21, the ball 22, and the ball nut 23 constitute a ball screw mechanism 24 that converts the rotation of the ball nut 23 into the axial motion of the ball screw shaft 21.

  Further, as shown in FIG. 1, the electric power steering apparatus 10 includes a torque sensor 25 that detects steering torque, an electric motor 26 that is controlled based on the output of the torque sensor 25 and assists the steering torque, and an electric motor. A gear reduction mechanism 27 that reduces the rotation of the rotation 26 is provided. The gear reduction mechanism 27 meshes with the drive gear 31 driven by the electric motor 26, the driven gear 32 that is integrally press-fitted into the outer periphery of one end of the ball nut 23, and the drive gear 31 and the driven gear 32. And an intermediate gear 33 that transmits the rotation of 31 to the driven gear 32.

  In order to avoid interference with an engine oil pan or the like, the ball nut 23 is connected to the housing 20 through a bearing 35 (see FIG. 2) at a position near the shaft end of the rack shaft 18 as shown in FIG. The screw shaft 21 (rack shaft 18) is concentrically arranged to be rotatable. For this reason, the housing 20 on the shaft end side of the rack shaft 18 is configured such that a space for arranging a guide bush for guiding the rack shaft 18 cannot be secured.

  As shown in FIG. 2, the driven gear 32 is integrally press-fitted to the outer periphery of one end of the ball nut 23. An outer ring 41a of a rolling bearing 41 such as a ball bearing is fitted and fixed to the inner circumference of the ball nut 23 into which the driven gear 32 is press-fitted. A ring-shaped guide bush 42 is fixedly fitted. The guide bush 42 is made of a relatively soft metal, for example, bronze, and is usually fitted to the outer periphery of the rack shaft 18 with a slight gap.

  Thus, even if the rack shaft 18 abuts against the guide bush 42 due to a radial load acting on the rack shaft 18, the rack shaft 18 can be smoothly guided in the axial direction without damaging the rack shaft 18, and the guide bush 42. Durability can be improved.

  The outer ring of the bearing 35 is integrally fixed to the inner periphery of the housing 20 by a tightening nut 36, and the ball nut 23 is fixed to the inner ring of the bearing 35 by the tightening nut 37 on the same axis as the ball screw shaft 21 (rack shaft 18). Is rotatably supported. As shown in FIG. 1, an electric motor 26 is installed in the housing 20 in parallel with the rack shaft 18. A drive gear 31 is connected to the motor shaft of the electric motor 26, and the drive gear 31 is engaged with the driven gear 32 via an intermediate gear 33.

  The drive gear 31, the intermediate gear 33, and the driven gear 32 described above constitute a gear reduction mechanism 27 that reduces and transmits the rotation of the electric motor 26 to the ball nut 23.

  Next, the operation of the electric power steering apparatus 10 in the above-described embodiment will be described. When the steering wheel 13 is steered, steering torque is input to the pinion shaft 17. The steering torque input to the pinion shaft 17 is detected by the torque sensor 25, and the electric motor 26 is rotationally controlled based on the detected steering torque to generate a steering assist torque. The steering assist torque by the electric motor 26 is transmitted to the ball nut 23 through the drive gear 31, the intermediate gear 33 and the driven gear 32, and the rack shaft 18 is axially integrated with the ball screw shaft 21 by the rotation of the ball nut 23. Moved to. As the rack shaft 18 moves in the axial direction, the steered wheels 12 are steered via the tie rods and knuckle arms, and the steering force of the steering wheel 13 by the driver is reduced.

  By the way, as the steered wheels 12 are steered, a radial load acts on the rack shaft 18, and this radial load is transmitted to the ball nut 23 via the rack bush 42 and the rolling bearing 41, and the radial load is caused by the ball nut 23. Is accepted. At this time, since only the rolling friction force F2 as shown in FIG. 3 is generated between the outer periphery of the rack shaft 18 and the inner periphery of the rack bush 42, the torque transmission efficiency of the ball screw mechanism 24 is not lowered. . That is, since the rolling bearing 41 is arranged between the ball nut 23 and the rack bush 42, the frictional force generated between the rack shaft 18 and the rack bush 42 is changed from the sliding friction F3 shown in FIG. 4 to FIG. It is possible to change to the rolling friction F2 shown in the drawing, whereby the friction loss is reduced, and the torque transmission by the ball screw mechanism 24 can be maintained with high efficiency.

  According to the electric power steering apparatus 10 in the above-described embodiment, since the ball nut 23 is rotatably disposed on the housing 20 at a position near the shaft end of the rack shaft 18, for example, an oil pan of an engine or the like Even in a vehicle arranged close to the axis of the rack shaft 18, interference between the housing and the oil pan can be avoided accurately.

  In addition, since the guide bush 42 is supported on the inner periphery of the ball nut 23 disposed near the shaft end of the rack shaft 18 via the rolling bearing 41, it occurs between the rack shaft 18 and the rack bush 42. The friction force to be made can be rolling friction. As a result, it is possible to realize the electric power steering apparatus 10 that can reduce friction loss and suppress a decrease in torque transmission efficiency of the ball screw mechanism 24.

  In the above-described embodiment, the example in which the guide bush 42 is made of a soft metal has been described. However, the guide bush 42 can be made of resin.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention described in the claims. Is possible.

It is the schematic which shows the electric power steering apparatus provided with the ball screw mechanism which concerns on embodiment of this invention. It is sectional drawing which shows the structure which supported the guide bush on the ball nut. FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 2. It is sectional drawing corresponding to FIG. 3 which shows a comparative example with this Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Electric power steering apparatus, 18 ... Rack shaft, 20 ... Housing, 21 ... Ball screw shaft, 23 ... Ball nut, 24 ... Ball screw mechanism, 26 ... Electric motor, 27 ... Gear reduction mechanism, 41 ... Rolling bearing, 42 …guide bush.

Claims (3)

A ball nut is screwed onto a ball screw shaft formed on the rack shaft, an electric motor is arranged in parallel with the rack shaft, and the driving force of the electric motor is transmitted to the ball nut via a gear reduction mechanism to assist steering. An electric power steering device that
A housing for accommodating the rack shaft so as to be movable in the axial direction is provided, and the ball nut is rotatably disposed in the housing at a position near the shaft end of the rack shaft so that the outer periphery of the rack shaft can be slid. An electric power steering apparatus comprising a guide bush for guiding and supporting the guide bush on the inner periphery of the ball nut via a rolling bearing.   2. The electric power steering apparatus according to claim 1, wherein the guide bush is made of a softer material than the rolling bearing. 3. The electric power steering apparatus according to claim 2, wherein the guide bush is made of a soft metal material.

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