JP3856606B2 - Electric power steering device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a configuration in which the rotation of a steering assist motor is converted into a linear motion and transmitted to a steering shaft such as a rack shaft in a rack and pinion type steering device, and the steering shaft is moved in the axial length direction to assist steering. The present invention relates to an electric power steering apparatus.
[0002]
[Prior art]
An electric power steering apparatus configured to drive a steering assist motor based on a detection result of a steering torque applied to a steering wheel for steering and transmit the rotational force of the motor to the steering apparatus to assist steering, Compared to a hydraulic power steering device that uses a hydraulic actuator as a source of steering assist force, it has the advantage that the assist force characteristics can be easily controlled according to the running state, such as the vehicle speed and the steering frequency, In recent years, the application range tends to expand.
[0003]
Various types of steering devices for automobiles have been put into practical use, and one of them is a rack and pinion type steering device. This is because a rack shaft formed by forming rack teeth over a predetermined length in the axial direction extends in the left-right direction of the vehicle body, and both ends of the rack shaft are steered through tie rods (steering wheels ( In general, it is connected to a steering wheel via a column shaft, and a pinion shaft that meshes with the rack teeth is connected to the steering wheel. Through the rack shaft, and the rack shaft is moved in the axial length direction to perform steering.
[0004]
When such a rack and pinion type steering device is configured as an electric power steering device, a motion conversion mechanism that converts rotation (rotational motion) of a steering assist motor into movement (linear motion) in the axial direction of the rack shaft. is required. Further, this motion conversion mechanism is preferably accompanied by a predetermined deceleration (power increase) in the process of the motion conversion in order to compensate for the shortage of the output of the motor for assisting steering and to reduce the size of the motor to improve the onboard performance. Conventionally, a ball screw mechanism has been widely adopted as a highly efficient motion conversion mechanism capable of meeting such requirements.
[0005]
An electric power steering apparatus having such a ball screw mechanism forms a male screw rail in a part of the outer periphery of the rack shaft while avoiding the rack tooth formation area, while the rack is provided inside the housing that supports the rack shaft. A cylindrical nut member is supported so as to be rotatable coaxially with the shaft, and a female screw rail formed on the inner periphery of the nut member and a male screw rail on the outer periphery of the rack shaft roll along these. The rotation of the nut member caused by transmission from the steering assist motor is transmitted to the rack shaft via a plurality of balls rolling along the screw rail, The rack shaft is moved in the axial direction, and the steering performed as described above is assisted in accordance with this movement.
[0006]
[Problems to be solved by the invention]
FIG. 4 is an enlarged cross-sectional view of a threaded portion of the ball screw mechanism. Figure 1 is a rack shaft, 2 is the nut member, the ball screw mechanism, as described above, is equal to the inner circumference of the external thread rail 10 and the nut member 2 that is formed with an equal lead to the outer periphery of the rack shaft 1 The female thread rail 20 formed with the leads is screwed through a large number of balls 3, 3. When the nut member 2 is rotated by this configuration, the balls 3, 3... Move along the male screw rail 10 and the female screw rail 20 while moving separately, and one of the nut members 2 is moved by a circulator (not shown). The rack shaft 1 is circulated by returning from the side to the other side. At this time, the rack shaft 1 is pressed in the axial direction by the action force from each of the balls 3, 3,.
[0007]
The male thread rail 10 and the female thread rail 20 have respective centers at positions overlapped in the axial direction on the fitting circumference of the rack shaft 1 and the nut member 2 as shown in the figure. Are provided with a cross-sectional shape formed by combining two circular arcs having a curvature larger than the outer diameter of each of the above-described outer diameters, and the balls 3, 3... Are respectively arc-shaped inner surfaces of the male thread rail 10 and the female thread rail 20. On the other hand, it is desirable to interpose so as to face each other with a slight gap close to zero.
[0008]
However, in order to realize the above-described interposition of the balls 3, 3..., The shape of each ball 3, 3. When high accuracy is required for the concentricity of these parts and the assembly is performed in a state where these precisions are insufficient, the contact surface pressure of some of the balls 3, 3. The smooth rolling of the balls 3, 3... Is hindered, and there is a possibility that the motion conversion with high efficiency may not be performed.
[0009]
Therefore, in the prior art, the highest priority is given to smooth rolling of the balls 3, 3..., And the dimensions and tolerances of the respective parts are set such that the inner surface of the male screw rail 10 or the female screw rail 20 and the balls 3, 3. It is set to ensure a relatively large gap δ with the peripheral surface. The gap δ in FIG. 4 is shown large for convenience of illustration, but the actual gap δ is about several tens of μm.
[0010]
However, in the electric power steering apparatus having such a ball screw mechanism, a large number of balls 3 interposed between the external thread rail 10 on the outer periphery of the rack shaft 1 and the internal thread rail 20 on the inner periphery of the nut member 2 are provided. 3..., For example, move freely within the gap δ due to vibrations propagated during traveling, and collide with both screw rails 10 and 20 and the adjacent balls 3 to generate an annoying collision sound. When the rack shaft 1 and the nut member 2 resonate with the movement of the balls 3, 3..., The driver feels the resonance and noise, and some abnormality occurs. There was a risk of being misidentified. Further, when the moving direction of the rack shaft 1 is reversed, there is a problem that the driver feels the rattling caused between the rack member 1 and the nut member 2 due to the gap δ, and the steering feeling is deteriorated. .
[0011]
The ball screw mechanism as described above is not limited to the rack shaft 1 in the rack and pinion type steering device, and various types of steering devices including a steering shaft that is steered according to movement in the axial length direction are used as an electric power steering device. Is widely used to transmit the rotation of the steering assist motor to the steering shaft, and the above-described problem occurs in this electric power steering apparatus as well.
[0012]
The present invention has been made in view of such circumstances. In the ball screw mechanism used to convert the rotation of the steering assist motor into the movement of the steering shaft in the axial length direction, the present invention is caused by the looseness of the ball. It is an object of the present invention to provide an electric power steering device that can prevent generated vibration and noise and rattling with a simple configuration without requiring high accuracy in each peripheral portion.
[0013]
[Means for Solving the Problems]
An electric power steering apparatus according to the present invention includes a female thread rail formed with a lead equal to the inner periphery of a cylindrical nut member that rotates around an axis by transmission from a steering assist motor, and an outer periphery of a steering shaft. A ball screw mechanism for motion conversion is configured by screwing together a male screw rail formed with a lead equal to a plurality of balls rolling along these, and the steering by the ball screw mechanism. In the electric power steering device for assisting steering by converting rotation of the motor driven in accordance with operation into movement in the axial direction of the steering shaft, the lead of the internal thread rail on the inner periphery of the nut member, and the steering shaft The lead of the external male thread rail is different from that of the outer periphery.
[0014]
Further, the difference between the leads is that the ball should come into contact at two points in different directions between the internal thread rails on the inner periphery of both ends of the nut member and the external thread rails on the outer periphery of the steering shaft. It has been determined.
[0015]
In the present invention, the male thread rail on the outer periphery of the steering shaft and the female thread rail on the inner periphery of the nut member are formed with different leads, and both screw rails are axially extended at the fitting portion between the steering shaft and the nut member. The balls located at different positions in the axial length direction among the plurality of balls interposed between them are in contact with each other at two points in different directions with respect to the corresponding screw rail, These balls prevent the ball from moving between the screw rails, and alleviate the occurrence of vibration, noise, and backlash due to this movement. The ball in the two-point contact state can smoothly roll between the screw rails as the nut member rotates, and the motion is converted without deteriorating the original efficiency.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings illustrating embodiments thereof. FIG. 1 is a partially broken front view showing a configuration of a main part of an electric power steering device (hereinafter referred to as the present device) according to the present invention.
[0017]
Figure 1 is a rack shaft, Yes and movably supported to the inside in the axial direction of the rack housing H ₁ forming the lateral direction extending in-cylinder-shaped body, not shown, the rack housing H ₁ Both ends of the rack shaft 1 projecting on both sides of the rack are connected to left and right wheels (generally front wheels) (not shown) via separate tie rods.
[0018]
A pinion housing H ₂ is connected to the middle portion of the rack housing H ₁ so as to cross the shaft center. A pinion shaft 4 is supported inside the pinion housing H _{2 so} as to be rotatable about its axis. The pinion shaft 4 protrudes from the upper portion of the pinion housing H ₂ to an appropriate length, and this protrusion is connected to a steering wheel (not shown) so as to rotate around the axis in accordance with the operation of the steering wheel for steering. There is.
[0019]
A pinion 40 is integrally formed at the lower part of the pinion shaft 4 extending inside the pinion housing H ₂ as indicated by a broken line in the drawing. Further, the rack shaft 1 supported in the rack housing H ₁ is formed with rack teeth 11 over a predetermined length range in the axial direction, and the pinion shaft 4 at the crossing position with the pinion housing H _2. It is meshed with the pinion 40 at the lower part of.
[0020]
And Thus, rotation of the pinion shaft 4 caused by the operation of the steering wheel is converted by the meshing of the pinion 40 and the rack teeth 11 into movement in the axial direction of the rack shaft 1, the rack shaft 1 in the rack housing H ₁ The movement is transmitted to the left and right wheels via the tie rods at both ends thereof, and a rack and pinion type steering mechanism is configured in which these wheels are steered in accordance with the operation of the steering wheel.
[0021]
In the illustrated electric power steering apparatus, the motor 5 that assists the steering performed as described above has a cylindrical shape in which the middle portion of the rack housing H ₁ that surrounds the rack shaft 1 is expanded to an appropriate length. The motor housing H ₃ is configured as a three-phase brushless motor having a stator 51 fixed on the inner peripheral surface of the motor housing H ₃ and a rotor 52 coaxially arranged inside the stator 51. ing.
[0022]
The rotor 52 is configured to hold a magnetic pole 53 facing the inner surface of the stator 51 with a slight gap on the outer periphery of a cylindrical body having an inner diameter larger than the outer diameter of the rack shaft 1. A pair of ball bearings 54 and 55 are rotatably supported around the axis of the motor housing H ₃ , and rotate in both forward and reverse directions in response to energization of the stator 51.
[0023]
Motor 5 for steering assist which is constructed as described above, based on the detection result of the torque sensor (not shown) configured in the interior of the pinion housing H _2, in the direction of the steering torque applied to the steering wheel, the steering torque Drive control is performed to generate a rotational force corresponding to the magnitude. The rotational force generated by the motor 5 by this drive control is converted to an axial force by a ball screw mechanism for motion conversion formed on one side of the rotor 52 and applied to the rack shaft 1, and the inside of the rack housing H ₁ . The steering caused by the movement of the rack shaft 1 is assisted.
[0024]
The ball screw mechanism, one side (support side by a ball bearing 55) ends coaxially press-fitted and fixed to a cylindrical nut member 2 of the rotor 52, the rack housing H ₁ by four-point contact ball bearing 56 The inner periphery of the nut member 2 is supported on the inner periphery of the nut member 2, and is formed on the inner periphery of the nut member 2. The male thread rail 10 formed over the length is screwed through a large number of balls 3, 3... Interposed between them.
[0025]
With the above configuration, the nut member 2 rotates integrally around the axis as the motor 5 rotates, that is, the rotor 52 rotates as the stator 51 is energized, while the rack shaft 1 includes the rack teeth 11. to maintain the engagement state between the pinion 40 in the formation region of, are constrained to rotation about the axis of the inside the rack housing H ₁ by a rotation restraining means (not shown). Therefore, when the nut member 2 is rotated by transmission from the motor 5, the balls 3, 3... Are externally threaded rail 10 formed on the outer periphery of the rack shaft 1 and internally threaded rail 20 formed on the inner periphery of the nut member 2. At this time, the rack shaft 1 is pressed by the axial component of the acting force applied along the outer peripheral male thread rail 10 from each of the balls 3, 3,..., And moves in the axial length direction. This movement assists the steering performed as described above.
[0026]
In the motion conversion by the ball screw mechanism as described above, the movement length of the rack shaft 1 generated per rotation of the nut member 2 is small, and the rotation of the steering assist motor 5 is accompanied by sufficient deceleration (intensification). The movement of the shaft 1 is converted, and this movement conversion is performed with high efficiency by rolling of the balls 3, 3... Interposed between the male screw rail 10 and the female screw rail 20. Therefore, the steering assisting motor 5 can be made compact.
[0027]
On the other hand, in the ball screw mechanism configured as described above, the balls 3, 3... Interposed between the male screw rail 10 and the female screw rail 20 have gaps between these screw rails 10, 20. It moves within the range, collides with the surrounding parts and generates an unpleasant collision sound, generates a large vibration and noise due to the resonance of the rack shaft 1 and the nut member 2, and is also loose in the movement of the rack shaft 1. However, in the device of the present invention, the following configuration prevents the balls 3, 3,...
[0028]
FIG. 2 is an enlarged cross-sectional view of the threaded portion of the ball screw mechanism in the device of the present invention. In this figure, the external thread rail 10 on the outer periphery of the rack shaft 1 and the internal thread rail 20 on the inner periphery of the nut member 2 are fitted around the rack shaft 1 and the nut member 2 in the same manner as those in the conventional ball nut mechanism shown in FIG. It has a cross-sectional shape formed by combining two arcs each having a center at the position where it is overlapped in the axial direction and having a curvature larger than the outer diameter of the balls 3, 3. In addition, the male thread rail 10 on the outer periphery of the rack shaft 1 is formed with a lead (advancing amount per round) as a, whereas the female thread rail 20 on the inner periphery of the nut member 2 is larger than the lead a. The lead b is formed.
[0029]
Therefore, in the state in which the central portion of the nut member 2 is aligned with the corresponding portion of the rack shaft 1 as shown in the figure, the female screw rail 20 on the inner periphery of the nut member 2 is opposite to the corresponding male screw rail 10 on the outer periphery of the rack shaft 1. Of the balls 3, 3... Interposed between the rails 10, 20 by appropriately setting the difference Δ (= a−b) between the leads a, b. A part of the nut member 2 comes into contact with the arc-shaped inner surfaces of the male screw rail 10 and the female screw rail 20 at two points at points A and B in the vicinity of both ends.
[0030]
At this time, the balls 3, 3... In contact with each other are restrained non-movably between the male screw rail 10 and the female screw rail 20 by the preload applied to the contact points A and B, and the other balls 3 positioned between them. , 3... Are also constrained to effectively prevent the above-described problems caused by these movements, and to improve the quietness of the interior of the vehicle interior by reducing vibration and noise. It is possible to alleviate the possibility that a driver who has experienced the vibration and noise is mistakenly recognized as having an abnormality.
[0031]
Further, the contact of the balls 3, 3... At both ends of the nut member 2 occurs in opposite directions, and the axial relative displacement between the nut member 2 and the rack shaft 1 is constrained by this contact. Therefore, when the nut member 2 rotates in both the forward and reverse directions, this rotation can be transmitted to the rack shaft 1 without backlash, and the above-described motion conversion can be performed smoothly, and the steering feeling is deteriorated due to the backlash feeling. Can be relaxed.
[0032]
In addition, the balls 3, 3... In a two-point contact state at both points A and B at opposite positions are subjected to preloading at the contact points A and B when the nut member 2 rotates as described above. Can be easily rolled around the axis connecting the contact points A and B along the male screw rail 10 and the female screw rail 20, and the motion conversion described above can be performed without impairing the high conversion efficiency inherent in the ball screw mechanism. Can be done.
[0033]
In FIG. 2, the lead b of the female thread rail 20 on the inner periphery of the nut member 2 is larger (b> a) than the lead a of the male thread rail 10 on the outer periphery of the rack shaft 1, but the magnitude relationship between the two is reversed. It may be. Such an embodiment is shown in FIG.
[0034]
In this figure, the lead c of the female thread rail 20 on the inner periphery of the nut member 2 is smaller than the lead a of the male thread rail 10 on the outer periphery of the rack shaft 1 (c <a). , 20 are partly in contact with two opposite sides of the male thread rail 10 and the female thread rail 20 at points A and B, respectively. The preload at these contact points A and B is restrained so as not to move, and the other balls 3, 3... Positioned between them are also restrained.
[0035]
3, the contact point A with the male thread rail 10 is the inner back side of the nut member 2, and the contact point B with the female thread rail 20 is also the end side. 2 is opposite to the positional relationship in FIG. 2, but the balls 3, 3... In contact can roll around the axis connecting the contact points A and B, and the inherent high conversion efficiency of the ball screw mechanism. The motion conversion can be performed smoothly without impairing the above.
[0036]
In the above embodiment, the configuration in which the rotation of the steering assist motor is transmitted to the rack shaft of the rack and pinion type steering device has been described. However, the present invention moves in the axial direction in the same manner as the rack shaft. In other types of steering devices having a steering shaft for steering by means of this, it can be applied to transmit the rotation of the steering assist motor to the steering shaft, and it goes without saying that the same effect can be obtained in these devices. .
[0037]
In the above embodiment, the steering assist motor 5 is coaxially provided on one side of the rack shaft 1, and the rotation of the motor 5 is transmitted to the nut member 2 fixed to one side of the rotor 52. However, the motor 5 is disposed along the outside of the rack housing H ₁ , and the rotation of the motor 5 is transmitted to the nut member 2 inside the rack housing H ₁ through a transmission mechanism such as a gear. It may be configured as described above.
[0038]
Further, in the above embodiments, the configuration in which the rotation of the steering assist motor is transmitted to the rack shaft of the rack and pinion type steering device has been described. However, the present invention moves in the axial direction in the same manner as the rack shaft. In other types of steering devices having a steering shaft for steering by means of this, it can be applied to transmit the rotation of the steering assist motor to the steering shaft, and it goes without saying that the same effect can be obtained in these devices. .
[0039]
【The invention's effect】
As described above in detail, in the electric power steering apparatus according to the present invention, the male screw rail on the outer periphery of the steering shaft and the female screw rail on the inner periphery of the nut member are formed with different leads, and a ball is interposed between them. Since the ball screw mechanism for motion conversion is configured, both screw rails are displaced in the axial direction between the steering shaft and the nut member, and a part of the ball is in relation to the inner and outer screw rails. The occurrence of defects caused by the movement of these balls can be effectively alleviated with a simple configuration without requiring high precision in the surrounding parts, and the interior of the vehicle interior is quiet. In addition, it is possible to improve the steering sensation and to prevent the driver from feeling uneasy. In addition, the ball in a two-point contact state can roll between the screw rails as the nut member rotates, and can perform motion conversion without losing the original efficiency. The invention has an excellent effect.
[Brief description of the drawings]
FIG. 1 is a partially broken front view showing a configuration of a main part of an apparatus according to the present invention.
FIG. 2 is an enlarged cross-sectional view of a threaded portion of a ball screw mechanism in the device of the present invention.
FIG. 3 is an enlarged cross-sectional view showing another embodiment of the threaded portion of the ball screw mechanism in the device of the present invention.
FIG. 4 is an enlarged cross-sectional view of a threaded portion of a ball screw mechanism used in a conventional electric power steering apparatus.
[Explanation of symbols]
1 Rack shaft 2 Nut member 3 Ball 4 Pinion shaft 5 Motor
10 Male thread rail
20 Female thread rail
51 Stator
52 Rotor H ₁ Rack housing H ₂ Pinion housing H ₃ Motor housing

Claims (2)

A female screw rail that is formed with an inner circumference equal lead of the tubular nut member that rotates around the axis through the transmission from the motor for steering assist, which is formed with equal lead to the outer periphery of the steering shaft A male screw rail is screwed through a plurality of balls that roll along these to form a ball screw mechanism for motion conversion, and the ball screw mechanism drives the motor driven in accordance with a steering operation. In the electric power steering device for assisting steering by converting rotation to a movement in the axial direction of the steering shaft, the lead of the internal thread rail on the inner periphery of the nut member is different from the lead of the external screw rail on the outer periphery of the steering shaft. An electric power steering device characterized by that. The difference in the lead is determined so that the ball contacts two points in different directions between the internal thread rail on the inner periphery of both ends of the nut member and the corresponding external thread rail on the outer periphery of the steering shaft. The electric power steering device according to claim 1.

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