JP2007239782A - Fixing structure for ball screw nut and electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing structure of a ball screw nut capable of securing smooth operation by restraining deformation of a ball screw nut. <P>SOLUTION: The ball screw nut 12 is screwed into an inner periphery of a hollow motor shaft 8 through a screw part 20 formed on an outer periphery. Thus, stress applied to fixing of the ball screw nut 12 with respect to the motor shaft 8 almost uniformly acts around an outer periphery of the ball screw nut 12 having the screw part 20. Therefore, deformation of the ball screw nut 12 can be restrained, distortion of a rolling passage accompanied by deformation can be prevented, and smooth rolling of balls can be secured. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention relates to a ball screw nut fixing structure and an electric power steering apparatus.

  In recent years, electric power steering devices (EPS) using a motor as a drive source have been widely adopted as vehicle power steering devices. Such an EPS has a rack shaft that can be reciprocated in accordance with a steering operation, and a hollow shaft that is arranged coaxially with the rack shaft and that is rotated by a motor drive. There is a so-called rack assist type that applies an assisting force to the steering system by converting the rotation into a reciprocating movement of the rack shaft by a ball screw mechanism (see, for example, Patent Document 1).

  That is, as shown in FIG. 3, in such a rack assist type EPS, the ball screw mechanism 40 includes a screw portion 42 formed on the outer periphery of the rack shaft 41 and a ball screw nut provided on the inner periphery of the hollow shaft 43. 44 and a plurality of balls 45 interposed between the screw portion 42 and the ball screw nut 44.

More specifically, in many cases, the ball screw nut 44 is fitted in a fixing groove 46 formed on the inner periphery of the hollow shaft 43, and the fastening force of the lock nut 47 screwed into the fixing groove 46 is used. By being clamped between the lock nut 47 and the side wall 46a of the fixing groove 46, the hollow shaft is fixed integrally. Each ball 45 is disposed in a rolling path formed by the screw portions 42 and the screw grooves 48 and 49 on the inner periphery of the ball screw nut 44 facing each other. Accordingly, the rack shaft 41 is configured to move in the axial direction with respect to the hollow shaft 43 by rolling in the rolling path.
JP 2003-335249 A

  However, in the configuration in which the ball screw nut is sandwiched between the lock nut and the side wall of the fixing groove as described above by the fastening force of the lock nut, the fastening force is received at both axial ends of the ball screw nut. An axial compressive stress acts on the screw nut. For this reason, deformation is likely to occur in the ball screw nut, and there is a risk that smooth rolling of each ball may be hindered by the accompanying distortion of the rolling path. In EPS, such a worsening of the movement of each ball may surface as a steering feeling such as generation of abnormal noise during steering or a so-called catching feeling.

  The present invention has been made to solve the above-described problems, and its object is to fix a ball screw nut that can prevent deformation of the ball screw nut and ensure a smoother operation, and electric power. The object is to provide a steering device.

  In order to solve the above-mentioned problems, the invention according to claim 1 is a ball screw nut fixing structure for fixing a ball screw nut of a ball screw mechanism to the inner periphery of a hollow shaft, and the ball screw nut has an outer periphery. The gist is to be screwed to the inner periphery of the hollow shaft through a screw part formed in

  According to the above configuration, the stress applied to the fixing of the ball screw nut to the hollow shaft acts substantially evenly over the outer periphery of the ball screw nut in which the screw portion is formed. Therefore, unlike the conventional case, the axial compressive stress does not act as in the case of the fixing structure that clamps both ends of the ball screw nut in the axial direction, thereby suppressing the deformation of the ball screw nut and accompanying the deformation. It is possible to prevent distortion of the rolling path and ensure smooth rolling of each ball. As a result, the ball screw mechanism can be operated more smoothly.

The gist of the invention described in claim 2 is that a detent means for restricting relative rotation between the ball screw nut and the hollow shaft is provided.
That is, when the hollow shaft rotates and the ball screw mechanism operates, a stress that rotates the ball screw nut to the side opposite to the hollow shaft acts on the ball screw nut. Therefore, according to the above configuration, the ball screw nut and the hollow shaft can be prevented from rotating relative to each other by the action of such stress, and the ball screw nut can be more securely fixed to the hollow shaft.

The gist of the invention described in claim 3 is that the axial end portion of the ball screw nut is provided with a spigot fitting portion that comes into contact with the inner periphery of the hollow shaft.
That is, when the ball screw nut is screwed to the inner periphery of the hollow shaft, it is difficult to make the ball screw nut and the motor shaft coaxial with high accuracy by the looseness of the screwed portion. In this regard, according to the above configuration, the radial movement and the shaft runout of the ball screw nut are regulated by the spigot fitting portion provided at the axial end portion of the ball screw nut. Therefore, high coaxiality can be easily ensured with a simple configuration.

  In addition, although it is sufficient to provide such a spigot fitting part in at least one of the axial direction edge parts of a ball screw nut, a higher coaxial degree can be ensured by providing in the axial direction both ends. The shape is not particularly limited, but when considering the function of restricting the radial movement and shaft runout of the ball screw nut, the outer periphery of the spigot fitting portion is the entire inner periphery of the hollow shaft. It is desirable to have a shape that makes contact over the circumference, that is, an annular cross section or a shape close to this.

  According to a fourth aspect of the present invention, the threaded portion that is threadedly engaged with the threaded portion is formed at the bottom of a fixing groove that is recessed in the inner periphery of the hollow shaft, and a lock nut is formed on the bottom. A gist is that a second screwing portion to be screwed is formed, and the ball screw nut is pressed in an axial direction by a fastening force of the lock nut screwed to the second screwing portion. To do.

  According to the above configuration, when the lock nut is pressed in the axial direction, the frictional force between the screw portion of the ball screw nut and the screwed portion on the hollow shaft side is increased. And the relative rotation between the hollow shaft and the hollow shaft is restricted. Therefore, the ball screw nut 1 can be more firmly fixed to the hollow shaft, and the axial pressing force is received by the screw thread of the screw portion, which may cause deformation of the ball screw nut. Absent. Note that, as described above, the lock nut itself has a function as an anti-rotation means, but in consideration of the rotation of the ball screw nut when the lock nut is screwed (so-called “accompaniment”), It is more effective to use the anti-rotation means together.

  The invention according to claim 5 is an electric power for converting the rotation of the motor into a reciprocating movement of the rack shaft by the ball screw mechanism having the ball screw nut fixing structure according to any one of claims 1 to 4. The gist is that it is a steering device.

  According to the said structure, the smooth operation | movement of a ball screw mechanism is ensured and high silence and favorable steering feeling can be implement | achieved.

  According to the present invention, the object of the present invention is to provide a ball screw nut fixing structure and an electric power steering device capable of ensuring a smoother operation by suppressing deformation of the ball screw nut.

Hereinafter, an embodiment in which the present invention is embodied in a rack assist type electric power steering apparatus (EPS) provided with a ball screw mechanism will be described with reference to the drawings.
As shown in FIG. 1, the EPS 1 includes a pinion shaft 2 that rotates by a steering operation, and a rack that changes the steering angle of a steered wheel (not shown) by reciprocating in the axial direction according to the rotation of the pinion shaft 2. In addition to the shaft 3, a motor 4 as a drive source and a ball screw mechanism 5 as conversion means for converting the rotation of the motor 4 into reciprocating movement of the rack shaft 3 are provided.

  More specifically, the EPS 1 includes a housing 6 formed in a substantially cylindrical shape, and the rack shaft 3 is provided along the axial direction of the housing 6. The rack shaft 3 is supported by a rack guide 7 and a slide bearing (not shown) provided in the housing 6 so that the rack shaft 3 can reciprocate along the axial direction. In the housing 6, the pinion shaft 2 is rotatably supported in a state of intersecting with the rack shaft 3, and the rack shaft 3 is engaged with the pinion shaft 2 by being pressed by the rack guide 7. Yes. A steering shaft is connected to one end of the pinion shaft 2, and a steering wheel is fixed to the tip of the steering shaft (not shown). Then, the pinion shaft 2 rotates in accordance with the steering operation, and the rotation is converted into the reciprocating motion of the rack shaft 3, whereby the steering angle of the steered wheels (not shown) is changed.

  In the present embodiment, a brushless motor having a hollow cylindrical motor shaft 8 is employed as the motor 4. The motor 4 has a stator 9 fixed to the inner periphery of the housing 6 and a motor shaft. The rack shaft 3 is inserted into the rack 8 so as to be coaxial with the rack shaft 3. The motor shaft 8 is rotatably supported by the housing 6 by virtue of pivotal support at both ends near the bearings 10a and 10b.

  On the other hand, in the present embodiment, the ball screw mechanism 5 includes a screw portion 11 formed on the outer periphery of the rack shaft 3, a ball screw nut 12 provided on the inner periphery of the motor shaft 8, and the screw portion 11 and the ball screw. It is composed of a plurality of balls 13 interposed between the nuts 12. Specifically, in the present embodiment, the ball screw nut 12 is formed in a substantially cylindrical shape, and screw grooves 15 and 16 are threaded on the inner periphery of the ball screw nut 12 and the outer periphery of the screw portion 11, respectively. It is engraved. Each ball 13 is arranged to roll in a spiral rolling path formed by the opposing screw grooves 15 and 16.

  That is, the screw portion 11 and the ball screw nut 12 are screwed together via these balls 13, and each ball 13 rotates relative to the rack shaft 3 and the ball screw nut 12, that is, the ball screw nut 12. As the motor shaft 8 is fixed, it rolls in the rolling path while receiving the load. In the ball screw mechanism 5 of the present embodiment, a so-called deflector system is employed in which a plurality of top members (not shown) disposed on the ball screw nut 12 communicate with each other between two adjacent rolling paths. The balls 13 that have rolled in the rolling path circulate infinitely in the rolling path by passing through these top members. As the balls 13 roll, the relative position in the axial direction of the rack shaft 3 and the ball screw nut 12 is displaced, so that the rotation of the motor shaft 8 is converted into the reciprocating motion of the rack shaft 3. Assist power is given to the system.

(Ball screw nut fixing structure)
Next, the fixing structure of the ball screw nut in the EPS of this embodiment will be described.
In this embodiment, the ball screw nut 12 is screwed so as to be coaxial with the motor shaft 8 on the inner periphery of the motor shaft 8 that is a hollow shaft. More specifically, as shown in FIG. 2, in this embodiment, a screw portion 20 having a helical thread is formed on the outer periphery of a ball screw nut 12 having a substantially cylindrical shape. An annular fixing groove 21 is formed in the inner periphery of one end of the motor shaft 8 (the right end in the figure), and the ball screw nut 12 is formed in the bottom 21a of the fixing groove 21. A threaded portion 22 is formed in which a spiral thread groove corresponding to the threaded portion 20 is threaded. The ball screw nut 12 is screwed onto the inner periphery of the motor shaft 8 by screwing the screw portion 20 and the screw portion 22 on the motor shaft 8 side.

  Further, in the present embodiment, the second screwing portion is located in the fixing groove 21 on the motor shaft 8 side on the opening side (right side in the figure) of the screwing portion 22 to which the ball screw nut 12 is screwed. 23 is formed, and a lock nut 24 having a substantially cylindrical shape is screwed to the second screwing portion 23. The ball screw nut 12 is pressed in the axial direction by the fastening force of the lock nut 24 when one end on the opening side of the ball screw nut 12 abuts on one end of the lock nut 24. In the present embodiment, the inner side end (the left side in the figure) of the ball screw nut 12 is not in contact with the side wall 21 b of the fixing groove 21.

  Furthermore, in this embodiment, a first spigot fitting portion 25 and a second spigot fitting portion 26 that are in contact with the inner periphery of the motor shaft 8 are formed at both ends in the axial direction of the ball screw nut 12.

  Specifically, a cylindrical first spigot fitting portion 25 extending in the axial direction of the ball screw nut 12 is formed at an inner side end portion (left side in the figure) of the ball screw nut 12. The outer periphery of the first spigot fitting portion 25 is an annular fitting groove 27 provided adjacent to the fixing groove 21 on the inner side of the motor shaft 8 than the fixing groove 21 to which the ball screw nut 12 is screwed. It is in contact with the inner periphery. On the other hand, an annular second spigot fitting portion 26 protruding in the radial direction of the ball screw nut 12 is formed at the opening side end portion (right side in the figure) of the ball screw nut 12. The second spigot fitting portion 26 is in contact with a non-thread groove forming portion between the screwed portion 22 and the second screwed portion 23 in the fixed groove 21 facing each other. In the present embodiment, the fixing groove 21 includes a first groove 28 in which a screwing portion 22 into which the ball screw nut 12 is screwed and a second screwing portion 23 in which the lock nut 24 is screwed. The second groove 29 has a groove depth deeper than that of the first groove 28. A non-thread groove forming portion with which the second spigot fitting portion 26 abuts is formed in the second groove 29.

  In the present embodiment, the motor shaft 8 is formed with a screw hole 30 penetrating the cylindrical wall 8a of the motor shaft 8 in the radial direction at the screwing position of the ball screw nut 12, and the ball screw nut 12 Is provided with a flat portion 31 where no screw thread is formed at a position corresponding to the screw hole 30. Then, the front end of the set screw 32 screwed into the screw hole 30 is brought into contact with the flat portion 31 on the ball screw nut 12 side and tightened, whereby the relative force between the motor shaft 8 and the ball screw nut 12 is determined by the fastening force. Anti-rotation means for restricting rotation is configured.

As described above, according to the present embodiment, the following operational effects can be obtained.
(1) The ball screw nut 12 is screwed to the inner periphery of the hollow motor shaft 8 via a screw portion 20 formed on the outer periphery. For this reason, the stress applied to the fixation of the ball screw nut 12 with respect to the motor shaft 8 acts substantially equally over the outer periphery of the ball screw nut 12 on which the screw portion 20 is formed. Therefore, unlike the prior art, the axial compressive stress does not act as in the case of the fixing structure that clamps both ends of the ball screw nut 12 in the axial direction, thereby suppressing the deformation of the ball screw nut 12 and making the same deformation. Accordingly, it is possible to prevent the rolling path from being distorted and to ensure smooth rolling of each ball. As a result, the ball screw mechanism 5 can be operated more smoothly, thereby realizing high silence and good steering feeling.

  (2) The motor shaft 8 is formed with a screw hole 30 that penetrates the cylindrical wall 8a of the motor shaft 8 in the radial direction, and on the outer periphery of the ball screw nut 12 at a position corresponding to the screw hole 30. The flat part 31 in which the thread is not formed is provided. Then, the front end of the set screw 32 screwed into the screw hole 30 is brought into contact with the flat portion 31 on the ball screw nut 12 side and tightened, whereby the relative force between the motor shaft 8 and the ball screw nut 12 is determined by the fastening force. A detent means for restricting rotation is configured.

  That is, when the motor shaft 8 rotates and the ball screw mechanism 5 operates, a stress that rotates the ball screw nut 12 to the side opposite to the motor shaft 8 acts on the ball screw nut 12. Therefore, according to the above configuration, the ball screw nut 12 and the motor shaft 8 are prevented from rotating relative to each other by the action of such stress, and the ball screw nut 12 can be more securely fixed to the motor shaft 8. it can.

(3) A first spigot fitting portion 25 and a second spigot fitting portion 26 that are in contact with the inner periphery of the motor shaft 8 are formed at both axial ends of the ball screw nut 12.
That is, when the ball screw nut 12 is screwed to the inner periphery of the motor shaft 8, it is difficult to make the ball screw nut 12 and the motor shaft 8 coaxial with high accuracy by the looseness of the screwed portion. In this regard, according to the above configuration, the ball screw nut 12 is moved and shaken in the radial direction by the first spigot fitting portion 25 and the second spigot fitting portion 26 provided at both axial ends of the ball screw nut 12. Is regulated. Therefore, high coaxiality can be easily ensured with a simple configuration.

  (4) The fixing groove 21 is formed with a second screwing portion 23 together with a screwing portion 22 to which the ball screw nut 12 is screwed, and the second screwing portion 23 has a substantially cylindrical shape. A lock nut 24 is screwed. The ball screw nut 12 is pressed in the axial direction by the fastening force of the lock nut 24 when one end on the opening side of the ball screw nut 12 comes into contact with one end of the lock nut 24.

  According to the above configuration, when the lock nut 24 is pressed in the axial direction, the frictional force between the screw portion 20 of the ball screw nut 12 and the screwing portion 22 on the motor shaft 8 side becomes large. Thus, the relative rotation between the ball screw nut 12 and the motor shaft 8 is restricted. Accordingly, the ball screw nut 12 can be more firmly fixed to the motor shaft 8, and the axial pressing force is received by each screw thread of the screw portion 20, so that the deformation of the ball screw nut 12 is reduced. Will not be invited. The rotation of the ball screw nut 12 when the lock nut 24 is screwed (so-called “following”) can be prevented by the configuration (2).

In addition, you may change this embodiment as follows.
In the present embodiment, the present invention is embodied in a rack assist type EPS 1 having a hollow motor shaft 8 and the rack shaft 3 and the motor 4 being coaxial. However, the present invention is not limited to this, and if a ball screw mechanism having a ball screw nut provided on the inner periphery of the hollow shaft is used as the conversion means, a rack cross type in which the motor shaft is obliquely arranged with respect to the rack shaft. It may be embodied in other types of EPS such as a parallel type arranged in parallel, or may be embodied in applications other than EPS.

  In the present embodiment, the first spigot fitting portion 25 and the second spigot fitting portion 26 that are in contact with the inner periphery of the motor shaft 8 are formed on both ends of the ball screw nut 12 in the axial direction. On the other hand, it is good also as a structure which provides such an inlay fitting part.

  -Furthermore, in this embodiment, although the 1st spigot fitting part 25 was formed in the cylindrical shape extended in an axial direction, and the 2nd spigot fitting part 26 was formed in the cyclic | annular form protruded from an outer periphery, The shape of the fitting portion may not be such an annular shape in cross section as long as it is a shape that can regulate the radial movement and shaft runout of the ball screw nut 12. However, considering that the spigot fitting portion can be brought into contact with the entire inner periphery of the motor shaft 8, it is desirable to have an annular cross-section or a shape close thereto.

  In the present embodiment, the screw hole 30 that penetrates the cylindrical wall 8 a of the motor shaft 8 in the radial direction, the flat portion 31 provided at a position corresponding to the screw hole 30 on the outer periphery of the ball screw nut 12, and the screw hole 30 Although the anti-rotation means is constituted by the set screw 32 screwed to the anti-rotation means, the anti-rotation means may be embodied by other configurations.

The schematic block diagram of an electric power steering device (EPS). Sectional drawing of the ball screw mechanism of this embodiment. Sectional drawing of the conventional ball screw mechanism.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Electric power steering device (EPS), 3 ... Rack shaft, 4 ... Motor, 5 ... Ball screw mechanism, 8 ... Motor shaft, 12 ... Ball screw nut, 13 ... Ball, 20 ... Screw part, 21 ... Fixed groove, 21a ... bottom part, 22 ... screwing part, 23 ... second screwing part, 24 ... lock nut, 25 ... first spigot fitting part, 26 ... second spigot fitting part, 30 ... screw hole, 31 ... flat 32, set screw.

Claims (5)

A ball screw nut fixing structure for fixing a ball screw nut of a ball screw mechanism to the inner periphery of the hollow shaft,
A ball screw nut fixing structure, wherein the ball screw nut is screwed to the inner periphery of the hollow shaft via a screw part formed on the outer periphery. In the ball screw nut fixing structure according to claim 1,
A structure for fixing a ball screw nut, characterized in that a detent means for restricting relative rotation between the ball screw nut and the hollow shaft is provided. In the ball screw nut fixing structure according to claim 1 or 2,
The ball screw nut fixing structure is characterized in that an inlay fitting portion that comes into contact with an inner periphery of the hollow shaft is provided at an axial end portion of the ball screw nut. In the fixation structure of the ball screw nut according to any one of claims 1 to 3,
A threaded portion that is threadedly engaged with the threaded portion is formed at the bottom of a fixing groove that is recessed in the inner periphery of the hollow shaft, and a second threaded engagement with a lock nut is threaded on the bottom. And the ball screw nut is pressed in the axial direction by a fastening force of the lock nut screwed to the second screwing portion.   An electric power steering device that converts rotation of a motor into reciprocating movement of a rack shaft by a ball screw mechanism having the ball screw nut fixing structure according to any one of claims 1 to 4.

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