Abstract:
A rack-driven auxiliary power steering apparatus prevents a ball nut and a nut pulley from idling due to a slip thereof and increases durability when the drive belt is operated by driving of a motor, and minimizes vibrations and noise transferred through the rack bar, the ball nut, and the nut pulley when the rack bar is slid while the nut pulley and the ball nut are rotating, thereby providing a comfortable steering feeling to the driver.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from and the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2013-0022027, filed on Feb. 28, 2013, and Korean Patent Application No. 10-2013-0027655, filed on Mar. 15, 2013, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the invention 
         [0003]    The present invention relates to an electric power steering apparatus of the ball screw type, and more particularly to an electric power steering apparatus of the ball screw type which prevents the load from being concentrated on the ball located at the outermost of the ball nut within the inner peripheral screw groove with the ball nut bended, thereby preventing the durability of the ball nut and the ball from being deteriorated. 
         [0004]    2. Description of the Prior Art 
         [0005]    A general electric power steering apparatus includes a steering system extending from a steering wheel to two wheels, and an auxiliary power mechanism for supplying auxiliary steering power to the steering system. 
         [0006]    The auxiliary power mechanism includes a torque sensor for detecting a steering torque applied to a steering wheel by the driver and outputting an electric signal proportional to the detected steering torque, an electronic control unit (ECU) for generating a control signal based on the electric signal transferred from the torque sensor, a motor for generating auxiliary steering power based on the control signal transferred from the electronic control unit, and a belt type power transmission unit for transmitting auxiliary power generated by the motor to a rack bar via a belt. 
         [0007]    An electric power steering apparatus of a ball screw type as the auxiliary power mechanism may include a return tube type, a deflector type, an end cap type and so on. 
         [0008]      FIG. 1  is a sectional view schematically illustrating the electric power steering apparatus of the ball screw type according to the related art, and  FIG. 2  is a sectional view schematically illustrating a portion of the electric power steering apparatus of the ball screw type according to the related art. 
         [0009]    As illustrated in the drawings, the rack-driven auxiliary power steering apparatus according to the related art includes a driving unit  120  for generating a power to be controlled by an electronic control unit (ECU) which determines the degree of the steering of a steering wheel  113  by a torque sensor  111  mounted to a top end of a steering column  110 , a rack bar  130  for making a tie rod  133  connected to a wheel moved according to the degree of the steering of the steering wheel  113 , and a driven unit  140  for transforming the receiving rotating force of the driving unit  120  to an axially moving force and then transmitting it to the rack bar  130 . 
         [0010]    Here, the driving unit  120  includes an electric motor  121  controlled by an electronic control unit (ECU), a driving pulley  123  coupled to a shaft of the electric motor  121 , and a drive belt  125  wound on a driving pulley  123 . 
         [0011]    The driven unit  140  also includes the ball nut  141  supporting the rack bar  130  within a rack housing surrounding the rack bar  130 , and the driven pulley  143  coupled to an outer peripheral surface of the ball nut  141 . 
         [0012]    The driving pulley  123  connected to the shaft of the electric motor  121  and the driven pulley  143  connected to the ball nut  141  are disposed parallel to each other, the driving belt  125  is mounted on the driving pulley  123  and the driven pulley  143  to transmit a rotating force of the electric motor  121  to the rack bar  130 , and the rack bar  130  is moved leftward and rightward by an operation of the ball nut  141  to generate an auxiliary steering force. 
         [0013]    An inner peripheral screw groove  201  having a gothic arc-shaped cross-section is spirally formed on an inner peripheral surface of the ball nut  141 , an outer peripheral screw groove  203  having a gothic arc-shaped cross-section is spirally formed on an outer peripheral surface of the rack bar  130 , a ball is inserted between the inner peripheral screw groove  201  and the outer peripheral screw groove  203 . While contacting the inner peripheral screw groove  201  and the outer peripheral screw groove  203 , the ball  210  moves along the inner peripheral screw groove  201  and the outer peripheral screw groove  203 , rolls from one side to the opposite side, and circulates through the ball circulation passage  205  formed between the end cap  220  and the ball nut  141  coupled to two sides of the inner peripheral surface of the ball nut  141  respectively. 
         [0014]    In the rack-driven auxiliary power steering apparatus according to the related art, as there is generally equally formed a center radius of the ball(R, i.e., distance from the axis of the ball nut to a center of the ball) moving along the inner peripheral screw groove  201 , the load is concentrated on the ball located at the outermost of the ball nut with the longitudinal direction within the inner peripheral screw groove with the ball nut bended, thereby causing the durability of the ball nut and the ball to be deteriorated. 
         [0015]    Further, there is a step between the inner peripheral screw groove and an inlet of the end cap (i.e., part where the ball escapes the inner peripheral screw groove and enters the end cap) at a location where they meet with each other, thereby causing noise and vibrations to be generated as soon as the ball moving along the inner peripheral screw groove from one side to the opposite side enters the inlet of the end cap. 
       SUMMARY OF THE INVENTION 
       [0016]    Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a electric power steering apparatus of the ball screw type which prevents the load from being concentrated on the ball located at the outermost of the ball nut within the inner peripheral screw groove with the ball nut bended, thereby preventing the durability of the ball nut and the ball from being deteriorated. 
         [0017]    In order to accomplish this object, there is provided an electric power steering apparatus of a ball screw type comprising: a rack bar having a spiral outer peripheral screw groove on an outer peripheral surface thereof; a ball nut coupled to a rack bar via a ball and having a spiral inner peripheral screw groove corresponding to the outer peripheral screw groove on an outer peripheral surface thereof and a ball circulation passage axially penetrating between the inner peripheral screw groove and the outer peripheral screw groove; and an end cap coupled to two inner peripheral surfaces of the ball nut respectively and having a ball moving passage wherein one side thereof passes through the ball circulation passage and the other side thereof passes through the outer peripheral screw groove and the inner peripheral screw groove, wherein a center radius of the ball of the inner peripheral screw groove gradually increases toward an inlet of the ball moving passage and a distance from a central axis to an outermost of the inner peripheral screw groove gradually increases from the center of the ball nut to two sides thereof. 
         [0018]    The electric power steering apparatus of the ball screw type according to an embodiment of the present invention prevents the load from being concentrated on the ball located at the outermost of the ball nut within the inner peripheral screw groove with the ball nut bended, thereby preventing the durability of the ball and the ball nut from being deteriorated. 
         [0019]    Further, the electric power steering apparatus of the ball screw type according to an embodiment of the present invention connects the inner peripheral screw groove of the ball nut and an inlet of the end cap without the step, which prevents the moving passage of the ball circulating within the ball nut from being suddenly changed, thereby reducing the generation of noise and vibrations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0021]      FIG. 1  is a sectional view schematically illustrating the electric power steering apparatus of the ball screw type according to the related art; 
           [0022]      FIG. 2  is a sectional view schematically illustrating a portion of the electric power steering apparatus of the ball screw type according to the related art; 
           [0023]      FIG. 3  is a perspective view illustrating of a ball nut according to an embodiment of the present invention; 
           [0024]      FIG. 4  is a sectional view and a partially exploded view of  FIG. 3 ; 
           [0025]      FIG. 5  is a drawing viewing the ball nut of  FIG. 4  in the W direction; 
           [0026]      FIG. 6  is a sectional view according to A-A′, B-B′, C-C′ lines of  FIG. 5 ; 
           [0027]      FIG. 7  is a graph illustrating an exemplary embodiment of a radius of the inner peripheral screw groove of the ball nut; 
           [0028]      FIG. 8  is a drawing compared the loads applied to balls in the ball nuts according to the related art to an embodiment of the present invention respectively; 
           [0029]      FIG. 9  is a sectional view of the ball nut according to the other embodiment of the present invention; and 
           [0030]      FIG. 10  is a graph illustrating an exemplary embodiment of a distance from a central axis of the ball nut to an outermost of the inner peripheral screw groove. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, it should be noted that if it is described in the specification that one component is “connected,” “coupled” or “joined” to another component, a third component may be “connected,” “coupled,” and “joined” between the first and second components, although the first component may be directly connected, coupled or joined to the second component. 
         [0032]      FIG. 3  is a perspective view illustrating of a ball nut according to an embodiment of the present invention.  FIG. 4  is a sectional view and a partially exploded view of  FIG. 3 .  FIG. 5  is a drawing viewing the ball nut of  FIG. 4  in the W direction.  FIG. 6  is a sectional view according to A-A′, B-B′, C-C′ lines of  FIG. 5 .  FIG. 7  is a graph illustrating an exemplary embodiment of a radius of the inner peripheral screw groove of the ball nut.  FIG. 8  is a drawing compared the loads applied to balls in the ball nuts according to the related art and an embodiment of the present invention respectively.  FIG. 9  is a sectional view of the ball nut according to the other embodiment of the present invention.  FIG. 10  is a graph illustrating an exemplary embodiment of a distance from a central axis of the ball nut to an outermost of the inner peripheral screw groove. 
         [0033]    As illustrated in the drawings, the electric power steering apparatus of the ball screw type according to the present invention includes a rack bar  130  having a spiral outer peripheral screw groove  203  on an outer peripheral surface thereof, a ball nut  300  coupled to a rack bar  130  via a ball  210  and having a spiral inner peripheral screw groove  301  corresponding to the outer peripheral screw groove  203  on an outer peripheral surface thereof and a ball circulation passage  205  axially penetrating between the inner peripheral screw groove  301  and the outer peripheral screw groove  203  and an end cap  320  coupled to two inner peripheral surfaces of the ball nut  300  respectively and having a ball moving passage  310  wherein one side thereof contacts the ball circulation passage  205  and the other side thereof contacts the outer peripheral screw groove  203  and the inner peripheral screw groove  301  wherein a center radius (R 1 ,R 2 ,R 3 ) of the ball of the inner peripheral screw groove  301  gradually increases toward an inlet of the ball moving passage  310 . 
         [0034]    For convenience of explanation,  FIG. 4  illustrates what the end cap  320  is provided only on one side of the ball nut  300 . 
         [0035]    The rack bar  130  coupled to a tie rod  133  coupled to a wheel  131  in  FIG. 1 . The rack bar  130  receives a rotating force generated by a rotation of the steering wheel  113  with a driver to linearly move and steers the wheel  131 . 
         [0036]    A spiral outer peripheral screw groove  203  is formed in some parts of an outer peripheral surface of the rack bar  130  and a cross section of the outer peripheral screw groove  203  is gothic arc-shaped. 
         [0037]    The ball nut  300  is coupled to the rack bar  130  via the ball  210 , the spiral inner peripheral screw groove  301  corresponding to the outer peripheral screw groove  203  is formed on an inner peripheral surface thereof, and the cross section of the inner peripheral screw groove  301  is also gothic arc-shaped. 
         [0038]    There is formed a projection part  303  which is projected radial outwards on an central end of the outer peripheral surface of the ball nut  300 , one side of the outer peripheral surface of the ball nut  300  is coupled to a bearing(not illustrated) supporting the ball nut  300  and the other side thereof is coupled to the driving pulley  123 . The driving pully  123  is coupled to the shaft of the electric motor  121  in  FIG. 1 . The driven pulley  143  and the driving pully  123  is coupled with driving belt  125 . 
         [0039]    The ball circulation passage  205  is formed on the ball nut ( 300 ) axially penetrating between the inner peripheral screw groove and the outer peripheral screw groove. One side and the other side of the ball circulation passage  205  are connected to the end cap  320  respectively. 
         [0040]    A groove  305  with a hollow shape outwards which contains a lubricant is formed on the inner peripheral screw groove  301 . A radial cross-sectional area of the groove  305  gradually decreases toward the inlet of the ball moving passage  310  so that the groove  305  is not formed near the inlet  311  of the ball moving passage  310  of the end cap  320 . 
         [0041]    Further a center radius of the ball R 3  of the inner peripheral screw groove  301  and a center radius of the ball of the ball moving passage  310  of the end cap  320  is equal to each other at a location where the inner peripheral screw groove  301  and an inlet  311  of the ball moving passage  310  of the end cap  320  meet with each other. 
         [0042]    Because the radial cross-sectional area of the groove  305  formed on the inner peripheral screw groove  301  gradually decreases toward the inlet of the ball moving passage  310  of the end cap  320  so that the groove  305  is not formed near the inlet  311  of the ball moving passage  310  of the end cap  320 , the inner peripheral screw groove  301  and the inlet  311  of the ball moving passage  310  is connected to each other without the step. 
         [0043]    Therefore, there is solved the problem that a step between the inner peripheral screw groove and an inlet of the end cap at the location where they meet with each other causes noise and vibrations to be generated as soon as the ball moving along the inner peripheral screw groove from one side to the opposite side enters the inlet of the end cap. 
         [0044]    Meanwhile the center radius (R 1 ,R 2 ,R 3 ) of the ball of the inner peripheral screw groove  301  to the direction of the inlet  311  of the ball moving passage  310  gradually increases toward the inlet  311  of the ball moving passage  310 . 
         [0045]    While the center radius of the ball at the section of the inner peripheral screw groove  301  without some distance from the inlet  311  of the ball moving passage  310  of the end cap  320  is generally equal to R 1  according to the related art, the center radius of the ball at the section of the inner peripheral screw groove  301  with some distance from gradually increases such as R 1 &lt;R 2 &lt;R 3  according to the embodiment as described above. 
         [0046]    The section where the center radius of the ball of the inner peripheral screw groove gradually increases may be equal to the section of angle α° with 15° to 70°. 
         [0047]    What the center radius of the ball of the inner peripheral screw groove  301  to the direction of the inlet  311  of the ball moving passage  310  gradually increases toward the inlet  311  of the ball moving passage  310  can prevent the load from being concentrated on the ball located within the inner peripheral screw groove  301  at the longitudinal outermost of the ball nut  300  with the ball nut  300  bended. 
         [0048]    In other words, while the center radius of the ball is generally equal in case of the ball nut  141  according to the related art referring to  FIG. 8(   a ) so that the load is concentrated on the ball  210 ′ within the inner peripheral screw groove  201  at the longitudinal outermost of the ball nut  141  with the ball nut  141  bended, thereby causing the durability of the ball  201 ′ and the ball nut  141  to be deteriorated. 
         [0049]    However in case of the ball nut  300  according to the embodiment of the present, referring to  FIG. 4 ,  FIG. 5  and  FIG. 8(   b ), the center radius of the ball of the inner peripheral screw groove  301  to the direction of the inlet  311  of the ball moving passage  310  gradually increases toward the inlet  311  of the ball moving passage  310  so that predetermined space can be formed between the ball  210 ′ located within the inner peripheral screw groove  301  at the longitudinal outermost of the ball nut  300 . Therefore the load is not concentrated on only the ball  210 ′ located within the inner peripheral screw groove  301  at the longitudinal outermost of the ball nut  300  with the ball nut  300  bended but distributed on the ball  210 ′ within the inner peripheral screw groove  301  near the longitudinal outermost inner peripheral screw groove  301 , thereby preventing the durability of the ball  201 ′ and the ball nut  300  from being deteriorated. 
         [0050]    Further, a center radius of the ball of the inner peripheral screw groove  301  may linearly(I) or non linearly(II) gradually increase to an inlet  311  of the ball moving passage  301  of the end cap  320 . 
         [0051]    Referring to  FIG. 9 , an electric power steering apparatus of a ball screw type according to the other embodiment of the present invention includes a rack bar  130  having a spiral outer peripheral screw groove  203  on an outer peripheral surface thereof, a ball nut  900  coupled to a rack bar  130  via a ball  310  and having a spiral inner peripheral screw groove  901  corresponding to the outer peripheral screw groove  203  on an outer peripheral surface thereof and a ball circulation passage  205  axially penetrating between the inner and the outer peripheral surfaces, and an end cap  320  coupled to two inner peripheral surfaces of the ball nut  900  respectively and having a ball moving passage  310  wherein one side thereof passes through the ball circulation passage  205  and the other side thereof passes through the outer peripheral screw groove  203  and the inner peripheral screw groove  901 . According to the other embodiment of the present invention the inner peripheral screw groove  901  of the ball nut  900  is that a distance L from a central axis(A) to an outermost thereof may gradually increases from the center(o) of the ball nut  900  to two sides thereof. 
         [0052]    When the distance L 1 , L 2  and L 3  from the central axis (A) of the ball nut  900  to the outermost of the inner peripheral screw groove  901  gradually increases from the center(o) of the ball nut  900  to two sides thereof, the distance from the central axis(A) to the outermost of the inner peripheral screw groove  901  linearly(i) or nonlinearly(ii) gradually increases according to the distance(X) from the center(o) of the ball nut  900  to two sides thereof to the direction of axis. 
         [0053]    If the distance L 1 , L 2  and L 3  from the central axis (A) of the ball nut  900  to the outermost of the inner peripheral screw groove  901  gradually increases from the center(o) of the ball nut to two sides thereof, a space formed between the inner peripheral screw groove  901  of the ball nut  900  and the outer peripheral screw groove  203  of the rack bar  130  gradually increases from the center part of the ball nut  900  to two sides thereof. 
         [0054]    What the distance L 1 , L 2  and L 3  from the central axis (A) of the ball nut  900  to the outermost of the inner peripheral screw groove  901  gradually increases from the center(o) of the ball nut to two sides thereof may prevent the load from being concentrated on the ball  210 ′ within the outermost inner peripheral screw groove  901  with the longitude direction of the ball nut  900  because of an effect on a backlash when the ball nut  900  is bended derived from the deflection or bending with a outer force transmitted to the rack bar  130 , thereby causing the durability of the ball  210 ′ and the ball nut  900  to be enhanced. 
         [0055]    In other words, as the distance L from the central axis (A) of the ball nut  900  to the outermost of the inner peripheral screw groove  901  gradually increases from the center(o) of the ball nut  900  to two sides thereof, so does a space between the inner peripheral screw groove  901  of the ball nut  900  and the outer peripheral screw groove  203  of the rack bar  130 . Therefore there exists a lot of spare space where the ball  210  can move with the radial direction, which makes the load distributed on the ball  201 ′ within the inner peripheral screw groove  901  near the ball  210 ′ within the outermost thereof to the longitude direction of the ball nut  900 , thereby causing the durability of the ball  210 ′ and the ball nut  900  to be enhanced. 
         [0056]    Further there exists a lot of spare space where the ball  210  can move between the inner peripheral screw groove  901  of the ball nut  900  and the outer peripheral screw groove  203  of the rack bar  130 , thereby capable of solving the problem that frictional forces of the ball  210 ′ from one side to the opposite side become not uniform although there changes alignment position where the ball nut  900  is coupled to the rack bar  130 . 
         [0057]    Of course an electric power steering apparatus of a ball screw type  901  according to the other embodiment of the present invention is that the inner peripheral screw groove  901  has a groove  305  with a hollow shape outwards which contains a lubricant. The outwards may mean the radial outer side of the ball nut  900 . 
         [0058]    The groove  305  as described above is formed along the inner peripheral screw groove  901  on the lowest part of the inner peripheral screw groove  901  which is the farthest from the central axis(A) therein. 
         [0059]    This embodiment of the present invention may prevent the load from being concentrated on the ball within the outermost inner peripheral screw groove of the ball nut with the ball nut bended, thereby preventing the durability of the ball and the ball nut to be deteriorated. 
         [0060]    Further the inner peripheral screw groove and the inlet of the ball moving passage of the end cap is couple to each without the step, thereby preventing the moving passage of the ball circulating within the ball nut from suddenly changing to reduce the noise and vibrations. 
         [0061]    Further although alignment position where the ball nut  900  is coupled to the rack bar  130  changes, this embodiment of the present invention can prevent frictional forces of the ball  210 ′ from one side to the opposite side from being not uniform. 
         [0062]    Even if it was described above that all of the components of an embodiment of the present invention are coupled as a single unit or coupled to be operated as a single unit, the present invention is not necessarily limited to such an embodiment. That is, among the components, one or more components may be selectively coupled to be operated as one or more units. 
         [0063]    In addition, since terms, such as “including,” “comprising,” and “having” mean that one or more corresponding components may exist unless they are specifically described to the contrary, it shall be construed that one or more other components can be included. All of the terminologies containing one or more technical or scientific terminologies have the same meanings that persons skilled in the art understand ordinarily unless they are not defined otherwise. A term ordinarily used like that defined by a dictionary shall be construed that it has a meaning equal to that in the context of a related description, and shall not be construed in an ideal or excessively formal meaning unless it is clearly defined in the present specification. 
         [0064]    Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Therefore, the embodiments disclosed in the present invention are intended to illustrate the scope of the technical idea of the present invention, and the scope of the present invention is not limited by the embodiment. The scope of the present invention shall be construed on the basis of the accompanying claims in such a manner that all of the technical ideas included within the scope equivalent to the claims belong to the present invention.