JP4930771B2 - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain enhancement of durability of a ball screw fitting part and prevention of generation of noise in an electric power steering device. <P>SOLUTION: Output rotation of an electric motor 6 is transmitted to a first end 17a of a ball nut through a speed-reduction mechanism 7, a rotation member 22 and an elastic coupling 23, and is converted to movement in an axial direction X1 of a rack shaft 5. A first bearing 26 rotatably supports the rotation member 22. The elastic coupling 23 cancels a combination error (misalignment in position and angle) of the ball nut 17 and the rotation member 22. A second bearing 30 supports a second end 17b of the ball nut 17 in an axial direction and allows radial movement of the second end 17b. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

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

As a so-called rack assist type electric power steering device, there has been proposed an electric power steering device that converts output rotation of an electric motor for steering assistance into axial movement of a steering shaft including a rack by a ball screw mechanism (patent) Reference 1).
For example, the steering shaft includes a rack shaft support device for pressing the rack shaft against the pinion while supporting the steering shaft so as to be slidable in the axial direction, a ball nut screwed to a screw shaft formed on the steering shaft, Three points are supported by the housing via the rack bush. Usually, the rack shaft support device is disposed at a position opposite to the pinion across the rack shaft. The ball nut is disposed between the rack shaft support device and the rack bush with respect to the position of the steering shaft in the axial direction, and the driven gear of the speed reduction mechanism that reduces the output rotation of the electric motor is integrated with the outer periphery of the ball nut. Is formed.

However, the rack shaft is bent due to variations in manufacturing dimensions of each component and the influence of external input (road surface input) to the rack. That is, each part of the rack shaft supported by the rack shaft support device, the ball nut and the rack bush is not arranged coaxially.
Under such circumstances, since the ball nut is supported rigidly, any one of the above three points is twisted, and an excessive radial load is generated, causing abnormal wear. There is a problem that play occurs in any one of the supporting parts and abnormal noise is generated.

On the other hand, in order to suppress the bending of the screw shaft inside the ball nut, an electric power steering device is proposed in which a pair of bearings functioning as sliding bearings are interposed between the inner periphery of both ends of the ball nut and the screw shaft. (See, for example, Patent Document 2).
JP-A-5-262243 JP-A-2005-280555

The electric power steering device of Patent Document 2 has a problem that loss torque due to sliding resistance increases because the slide bearing slides with the screw shaft. Durability may be reduced by wear of the bearing as a slide bearing.
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 that can improve the durability of a ball screw fitting portion and can prevent occurrence of abnormal noise. To do.

In order to achieve the above object, the present invention has a pinion (3) that rotates together with a steering member (2) and a rack (4) that meshes with the pinion, and a cylindrical housing (9) that is movable in the axial direction (X1). ) Housed in a rack shaft (5), a screw shaft (18) formed on a part of the rack shaft, and a ball nut (17) screwed into the screw shaft via a ball (19) in the housing A rotating member (22) connected to the first end (17a) of the ball nut via a joint (23; 23A) and transmitting a rotational driving force to the ball nut (17); The first bearing (26) that rotatably supports the rotating member and the second end (17b) of the ball nut held by the housing are supported in the axial direction, and the second end can be moved in the radial direction. A second bearing (30) to be allowed, and the joint absorbs a combination error between the rotating member and the first end of the ball nut. Comprise an elastic coupling having a that function, low-friction member (34) is interposed between the second end and the second bearing of the ball nut, the second bearing through the low-friction member The second end of the ball nut is supported in the axial direction .

  In the present invention, the rotational driving force from the rotating member is transmitted to the ball nut through the joint, and the rotation is converted into the axial movement of the rack shaft having the screw shaft to assist the steering. At this time, the joint connecting the first end of the ball nut and the rotating member allows a combination error (eccentricity, declination, etc.) of the ball nut with respect to the rotating member, while the second end of the ball nut. Is supported by the second bearing in a state in which radial movement is permitted.

Therefore, even if the rack shaft is bent, the ball nut can be displaced in the radial direction of the rack shaft so as to follow the rack shaft. Thereby, it can prevent that a twist arises between a ball nut and the screw axis | shaft (rack axis | shaft) which penetrates this, As a result, durability can be improved. Further, it is possible to prevent the generation of noise due to rattling between the ball nut and the screw shaft (rack shaft). On the other hand, the reaction force when the rack shaft is moved in the axial direction is received by the second bearing via the ball nut.
Further, a low friction member is interposed between the second end portion of the ball nut and the second bearing, and the second bearing connects the second end portion of the ball nut via the low friction member. Since it is supported in the axial direction, the ball nut can move smoothly in the radial direction, and the rack shaft can be reliably prevented from being twisted. As the low friction member, for example, a synthetic resin plate made of PTFE or a synthetic resin coating layer can be used. Further, a lubricating oil impregnated member can be used.

  Further, the present invention provides a rack shaft guide device including a guide (12) for guiding the axial movement of the rack shaft and a biasing member (13) for biasing the rack shaft toward the pinion by biasing the guide. (10) and a rack bush (16) supported by the housing and supporting the rack shaft, and the ball nut is disposed between the rack shaft guide device and the rack bush with respect to the axial direction of the rack shaft. It may be arranged (Claim 2). In this case, the rack shaft is supported by the rack bush and the rack shaft guide device on both sides of the ball nut with respect to the axial direction of the rack shaft, and the rack shaft is stably supported without causing twisting. be able to.

The joint may contain a O Dame joint (23A) (claim 3).
In the above description, the alphanumeric characters in parentheses represent reference numerals of corresponding components in the embodiments described later, but the scope of the claims is not limited by these reference numerals.

Preferred embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing a schematic configuration of an electric power steering apparatus of the present invention. Referring to FIG. 1, an electric power steering apparatus 1 includes a steering member 2, a pinion 3 that is connected to the steering member 2 via a steering shaft and an intermediate shaft (not shown), and rotates together with the steering member 2, and the pinion. 3 and a rack shaft 5 as a steering shaft extending in the left-right direction of the vehicle.

  Tie rods 6 are coupled to both ends of the rack shaft 5, and each tie rod 6 is connected to a corresponding steering wheel (not shown) via a corresponding knuckle arm (not shown). When the steering member 2 is operated, this rotation is converted by the pinion 3 and the rack 4 into a linear motion of the rack shaft 5 along the left-right direction of the vehicle. This achieves steering of the steered wheels.

  On the other hand, an electric motor 6 for assisting steering is provided that is controlled based on the steering torque loaded on the steering member 2 and the vehicle speed detected by the vehicle speed sensor. Specifically, the output rotation of the electric motor 6 is transmitted to the ball screw mechanism 8 as a motion conversion mechanism through the speed reduction mechanism 7 and converted into movement in the axial direction X1 of the rack shaft 5 to assist steering. Is done. The electric power steering apparatus 1 is a so-called rack assist type.

The rack shaft 5 is housed and supported in a cylindrical housing 9. A rack shaft guide device 10 that slidably supports the rack shaft 5 in the axial direction X1 while urging the rack shaft 5 toward the pinion 9 is provided at one end 9a of the housing 9.
The rack shaft guide device 10 is supported by a part of the housing 9 so as to protrude in the radial direction of the housing 9, and is supported so as to be able to advance and retract in the depth direction of the support tube 11, and the rack shaft 5 is moved in the axial direction X1. A guide 12 that is slidably guided, a biasing member 13 made of, for example, a compression coil spring that biases the rack shaft 5 toward the pinion 3 by biasing the guide 12, and a biasing force of the biasing member 13 Adjustment bolt 14 screwed into the threaded portion of the inner periphery of the inlet of the support cylinder 11 and a lock nut 15 that fixes the adjustment bolt 14 to the support cylinder 11 are provided.

A cylindrical rack bush 16 that holds the rack shaft 5 and supports the rack shaft 5 is held on the inner periphery of the other end portion 9 b of the housing 9.
The ball screw mechanism 8 surrounds the rack shaft 5 and supports a ball nut 17 as a rotating cylinder that supports the rack shaft 5 so as to be movable in the axial direction X1, a screw shaft 18 formed on a part of the rack shaft 5, and a ball A plurality of balls (rolling elements) 19 interposed between the nut 17 and the screw shaft 18 and engaged with the ball nut 17 and the screw shaft 18 are provided.

In the ball screw mechanism 8, the ball nut 17 and the screw shaft 18 are screwed together via a ball 19. The ball screw mechanism 8 has a general configuration of a so-called internal circulation type or external circulation type in which a ball 19 circulates between an inner thread groove of the ball nut 17 and an outer thread groove of the screw shaft 18.
The electric motor 6 has a motor housing 20 fixed to the housing 9 and a rotating shaft 21. The output rotation of the electric motor 6 is decelerated by the speed reduction mechanism 7 and transmitted to the cylindrical rotating member 22, and further transmitted to the ball nut 17 through an elastic joint 23 as a joint.

The speed reduction mechanism 7 includes a small-diameter drive gear 24 that can rotate integrally with the rotary shaft 21 of the electric motor 6, and a large-diameter driven gear 25 that can rotate integrally with the rotary member 22 and meshes with the drive gear 24. ing.
The rotating member 22 is supported by the housing 9 via a pair of first bearings 26 so as to be rotatable and not movable in the axial direction. The driven gear 25 is fitted on the outer periphery of the rotating member 22 so as to be integrally rotatable between the pair of first bearings 26.

The first bearing 26 includes an outer ring 27 held on the inner periphery of the housing 9, an inner ring 28 fitted to the outer periphery of the rotating member 22 so as to be integrally rotatable, and a rolling element interposed between the outer ring 27 and the inner ring 28. 29.
The ball nut 17 has a first end portion 17a and a second end portion 17b that face each other in the axial direction. The elastic joint 23 is interposed between the first end 17a of the ball nut 17 and the end 22a of the rotating member 22 facing the first end 17a. The elastic joint 23 functions to transmit the rotation of the rotating member 22 to the ball nut 17. Further, the elastic joint 23 is elastic so as to absorb a combination error of the first end portion 17a of the ball nut 17 with respect to the rotating member 22 (that is, allow a predetermined amount of eccentricity and a predetermined amount of deviation angle). It is comprised by the member.

The second end 17 b of the ball nut 17 is supported in the axial direction by the second bearing 30. The second bearing 30 includes a first race ring 31 held in the housing 9, a second race ring 32 facing the first race ring 31, the first race ring 31 and the second race ring. And rolling elements 33 interposed between the two.
A low friction member 34 is interposed between the second race ring 32 and the second end 17 b of the ball nut 17. The second bearing 30 supports the second end portion 17 b of the ball nut 17 in the axial direction via a low friction member 34. As the low friction member 34, for example, an annular synthetic resin plate made of PTFE and held in the second race ring 32 and in sliding contact with the second end portion 17b of the ball nut 17 with low friction can be used.

As the low friction member 34, a synthetic resin coating layer made of PTFE may be used. A lubricating oil impregnated member may be used. Further, the low friction member 34 may be held by the second end 17 b of the ball nut 17 and slidably contact the second race ring 32.
Next, referring to FIG. 2 that is an exploded perspective view and FIG. 3 that is a cross-sectional view, the elastic joint 23 includes an annular main body 44 and a plurality of engagement arms 45 that extend radially from the main body 44. Including. The plurality of engaging arms 45 are arranged at equal intervals in the circumferential direction Y1 of the main body 44. Each engagement arm 45 has a pair of power transmission surfaces 46 facing each other in the circumferential direction Y1. The elastic joint 23 is made of, for example, synthetic rubber or synthetic resin such as polyurethane.

The elastic joint 23 is interposed between the opposing surfaces 53 and 54 of the end portion 22a of the cylindrical rotating member 22 and the first end portion 17a of the cylindrical ball nut 17. Each is provided with an engagement element that engages with the engagement arm 45 of the elastic joint 23.
Specifically, a plurality of first engagement protrusions 55 as the engagement elements are formed to protrude on the facing surface 53 of the end 22 a of the rotating member 22. These first engaging protrusions 55 are arranged at equal intervals in the circumferential direction of the rotating member 22. Similarly, a plurality of second engagement protrusions 56 as the engagement elements are formed on the opposing surface 54 of the first end 17a of the ball nut 17 so as to protrude. These second engaging protrusions 56 are arranged at equal intervals in the circumferential direction of the ball nut 17.

  Each engagement arm 45 of the elastic joint 23 is interposed between the corresponding first engagement protrusion 55 and second engagement protrusion 56. That is, in the assembled state of the elastic joint 23, as shown in FIG. 3, the first and second engaging protrusions 55 and 56 are alternately arranged in the circumferential direction, and the first and second adjacent ones in the circumferential direction are adjacent to each other. A corresponding engagement arm 45 of the elastic joint 23 is sandwiched between the engagement protrusions 55 and 56.

In other words, the first and second engaging protrusions 55 and 56 adjacent in the circumferential direction are meshed with each other with the corresponding engaging arm 45 of the elastic joint 23 sandwiched in the circumferential direction. Thereby, both the rotating member 22 and the ball nut 17 are connected in the rotation direction (circumferential direction) via the elastic joint 23, and torque transmission is enabled between them.
Further, the first and second engagement protrusions 55 and 56 include power transmission surfaces 57 and 58 to the corresponding engagement arms 45 of the elastic joint 23, respectively. Further, the engagement arm 45 is sandwiched between the end 22a of the rotating member 22 and the first end 17a of the ball nut 17 with a tight margin in the axial direction. 17 is elastically urged in the axial direction toward the second bearing 30 side.

According to the present embodiment, the output rotation of the electric motor 6 is transmitted to the rotating member 22 via the speed reduction mechanism 7, and the rotational driving force from the rotating member 22 is transmitted to the ball nut 17 via the elastic joint 23. The The rotation of the ball nut 17 is converted into the axial movement of the rack shaft 5 having the screw shaft 18 to assist steering.
At this time, the eccentricity and declination of the ball nut 17 with respect to the rotating member 22 are allowed by the elastic joint 23 that connects the first end 17 a of the ball nut 17 and the rotating member 22. On the other hand, the second end portion 17b of the ball nut is supported by the second bearing 30 in a state in which radial movement is permitted.

Therefore, even if the rack shaft 5 is bent due to a manufacturing error or an external force load, the ball nut 17 is displaced in the radial direction of the rack shaft 5 so as to follow the bending of the rack shaft 5. be able to. Therefore, it is possible to prevent twisting between the ball nut 17 and the screw shaft 18 through which the ball nut 17 is inserted. As a result, durability can be improved.
Moreover, it is possible to prevent abnormal noise due to rattling between the ball nut 17 and the screw shaft 18.

Further, the rack shaft 5 is supported by the rack bush 16 and the rack shaft guide device 10 on both sides of the ball nut 17 with respect to the axial direction X1 of the rack shaft 5, and the rack shaft 5 is twisted. And can be supported stably.
Further, since the low friction member 34 is interposed between the second end portion 17b of the ball nut 17 and the second bearing 30, the ball nut 17 can smoothly move in the radial direction. It is possible to reliably prevent the rack shaft 5 from being twisted.

In the above embodiment, the elastic joint 23 is used as a joint for connecting the ball nut 17 and the rotating member 22, but an Oldham joint 23A as shown in FIG. 4 can be used instead.
The Oldham joint 23A includes a joint body 61 having a cylindrical shape so that the rack shaft 5 can be inserted therethrough. The joint body 61 has first and second end portions 62 and 63 that face each other. A first slide groove 64 is formed in the first end portion 62, and a second slide groove 65 is formed in the second end portion 63.

The first slide groove 64 is fitted into a first slider 66 formed of a ridge having a rectangular cross section formed in the end 22 a of the rotating member 22, and the first slide groove 64 and the first slider 66 The first sliding pair 68 is configured.
The second slide groove 65 is fitted to a slider 67 formed of a protrusion having a rectangular cross section formed in the first end portion 17 a of the ball nut 17, and the second slide groove 65 and the second slider 67 are fitted. And constitutes a second slip pair 69.

The sliding direction Z1 of the first sliding pair 68 and the sliding direction Z2 of the second sliding pair 69 are different from each other. For example, the directions are orthogonal to each other.
The first slider 66 has a pair of power transmission surfaces 70 that are parallel to each other, and the first slide groove 64 has a pair of power transmission surfaces 71 that are in sliding contact with the pair of power transmission surfaces 70. Yes. Similarly, the second slider 67 includes a pair of power transmission surfaces 72 that are parallel to each other, and the second slide groove 65 has a pair of power transmission surfaces 73 that are in sliding contact with the pair of power transmission surfaces 72. ing.

Also in the present embodiment, the Oldham coupling 23A allows a predetermined amount of eccentricity of the ball nut 17 with respect to the rotating member 22, and also allows a minute amount of deviation, thereby improving durability and preventing the generation of abnormal noise. The same effect as the embodiment of FIG. 2 can be obtained. In the present embodiment, since the fitting body 61 is constituted by an elastic member, it is possible to improve the above eccentricity tolerance of declination.

  In addition, various modifications can be made within the scope of the claims of the present invention.

It is a mimetic diagram showing a schematic structure of an electric power steering device as one embodiment of the present invention. It is a disassembled perspective view of the principal part containing an elastic joint of the said electric power steering device. It is sectional drawing of an elastic coupling. It is a disassembled perspective view of the Oldham coupling as a principal part of the electric power steering device of another embodiment of the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Electric power steering device, 2 ... Steering member, 3 ... Pinion, 4 ... Rack, 5 ... Rack shaft, 6 ... Electric motor, 7 ... Reduction mechanism, 8 ... Ball screw mechanism, 9 ... Housing, 10 ... Rack shaft guide Device: 12 ... Guide, 13 ... Biasing member, 16 ... Rack bush, 17 ... Ball nut, 17a ... First end, 17b ... Second end, 18 ... Screw shaft, 19 ... Ball, 22 ... Rotation 23, elastic joint (joint), 23A ... Oldham joint, 26 ... first bearing, 30 ... second bearing, 34 ... low friction member

Claims (3)

A pinion that rotates with the steering member;
A rack shaft that has a rack that meshes with the pinion and is accommodated in a cylindrical housing so as to be axially movable;
A screw shaft formed on a part of the rack shaft;
A ball nut screwed into the screw shaft via a ball in the housing;
A rotating member connected to the first end of the ball nut via a joint and transmitting a rotational driving force to the ball nut;
A first bearing held by a housing and rotatably supporting the rotating member;
A second bearing that is held by the housing and supports the second end of the ball nut in the axial direction and allows radial movement of the second end;
The joint includes an elastic joint having a function of absorbing a combination error between the rotating member and the first end of the ball nut ,
A low friction member is interposed between the second end of the ball nut and the second bearing, and the second bearing is axially connected to the second end of the ball nut via the low friction member. An electric power steering device characterized by being supported by The rack shaft guide device according to claim 1, comprising: a guide that guides the axial movement of the rack shaft; and a biasing member that biases the rack shaft toward the pinion by biasing the guide;
A rack bush held by the housing and supporting the rack shaft,
The electric power steering device according to claim 1, wherein the ball nut is disposed between the rack shaft guide device and a rack bush in the axial direction of the rack shaft. 3. The electric power steering apparatus according to claim 1, wherein the joint includes an Oldham joint.

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