Abstract:
An electronic power steering apparatus for a vehicle which. is capable of (a) improving the structures of a stop ring and a groove for thereby preventing a certain operational gap which is formed when installing the system and (b) providing a torque limiting function when an over load occurs in the motor. A circular groove in a motor shaft is formed with a beveled surface disposed in contact with a similarly beveled or inclined surface on a stop ring inserted into the groove. The stop ring is formed to be expandable in the outer direction and upon expansion in the groove moves in an axial direction due to the engaged beveled surfaces.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic power steering apparatus for a vehicle, and in particular to an electronic power steering apparatus for a vehicle which is capable of providing a support structure of a ball nut engaged with a motor shaft on a rack pinion type gear mechanism having a motor for thereby enhancing a steering force. 
     2. Description of the Background Art 
     Generally, a steering apparatus for a vehicle is directed to changing a running direction of a vehicle and includes an operating mechanism formed of a steering wheel, a steering shaft and a steering column, a gear mechanism for changing an operating direction of the operating mechanism, and a link mechanism for transferring a movement of the gear mechanism to a steering wheel. 
     The gear mechanism includes a worm sector type formed of a worm gear and a sector gear, and a rack pinion type which is operated based on a combination of a rack gear and a pinion gear. The electronic power steering apparatus further includes a motor and a decelerator capable of enhancing a steering force of the gear mechanism. 
     FIG. 1 is a cross sectional view illustrating a conventional rack pinion type electronic power steering apparatus. As shown therein, the conventional rack pinion type electronic power steering apparatus includes a torque sensor  10  for receiving a rotational force from the operating mechanism, a pinion housing  20  which is installed across the torque sensor  10  and has a rack bar  30  therein, a tie rod  40  formed at both ends of the rack bar  30 , and a motor  50  installed at an intermediate portion of the rack bar  30 . 
     The motor  50  includes a motor housing  51  having a certain diameter larger than that of the pinion housing  20 , a stator  52  installed closely to an outer surface of the interior of the same, a rotor  53  rotatably installed in an inner side of the stator  52 , a motor shaft  54  extended toward the inner side of the rotor  53 , and a ball nut  55  provided at one end in the interior of the motor shaft  54 . 
     At this time, a screw groove is formed in an inner side of the ball nut  55  and an outer side of the rack bar  30 , and a ball is disposed therebetween. Therefore, when the ball nut  55  is rotated, the rack bar  30  is moved in the axial direction. 
     One end of the motor shaft  54  is closely contacted with a step formed in the interior of the ball nut  55 , and the other end of the same is fixed by a lock screw  56 , and a stop ring  58  is installed in a rear side of the lock screw  56 . 
     The lock screw  56  is capable of preventing an escape of the ball nut  55  and includes a threaded portion formed in an outer surface of the same for thereby being engaged with a threaded portion formed in the interior of the motor shaft  54 . The stop ring  58  is capable of preventing an escape of the lock screw  56  and is inserted onto the outer portion of the groove  57  formed in the interior of the motor shaft  54 , so that the inner side supports the lock screw  56 . 
     In the conventional motor, the groove in which the stop ring is installed and the cross section of the stop ring are formed in a square shape, and the width of the groove is larger compared to the thickness of the stop ring for a smooth installation. Therefore, there is a certain operational gap therebetween. 
     Therefore, if the lock screw is loosened, since the lock screw is moved in the axial direction by the distance corresponding to the operational gap between the stop ring and the groove, the rotation and reciprocating movement of the ball nut fixed in the interior of the motor shaft becomes free. 
     Namely, since the rotational force transferred from the motor shaft to the ball nut is not properly obtained, a certain problem occurs in the steering force, and a noise occurs because the ball nuts collide with each other. 
     In addition, since a torque limit device is not provided, the motor may be damaged when an overload occurs. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an electronic power steering apparatus for a vehicle which overcomes the problems encountered in the conventional art. 
     It is another object of the present invention to provide an electronic power steering apparatus for a vehicle which is capable of improving the structures of a stop ring and a groove for thereby preventing a certain operational gap which is formed when installing the system and providing a torque limiting function when an over load occurs in the motor. 
     To achieve the above objects, there is provided an electronic power steering apparatus for a vehicle according to the present invention in which a groove is formed in a circular shape, an inner tapered surface is formed in an outer surface, a stop ring is formed to be expandable in the outer direction, and an outer tapered surface is formed in a portion opposite to the inner tapered surface at an outer surface for thereby being closely contacted with the groove based on a movement in the axial direction due to the expansion force of the stop ring wherein the electronic power steering apparatus for a vehicle includes a motor housing provided at an intermediate portion of a pinion housing having a rack bar therein, a ball nut inserted into an inner end portion of the motor shaft which is rotatable in the interior of the motor housing and moves the rack bar, a lock screw engaged to a portion of the motor shaft for thereby closely contacting the ball nut to the inner portion of the motor shaft, a groove formed in a rear portion of the lock screw, and a stop ring inserted into the groove and supporting the rear portion of the lock screw. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become better understood with reference to the accompanying drawings which are given only by way of illustration and thus are not limitative of the present invention, wherein; 
     FIG. 1 is a cross sectional view illustrating a conventional electronic power steering apparatus for a vehicle; 
     FIG. 2 is a cross sectional view illustrating an electronic power steering apparatus for a vehicle; and 
     FIG. 3 is a perspective view illustrating a stop ring according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be explained with reference to the accompanying drawings. 
     FIG. 2 is a cross sectional view illustrating an electronic power steering apparatus for a vehicle according to the present invention. As shown therein, the electronic power steering apparatus for a vehicle includes a pinion housing  120  which is installed across a torque sensor  110  and has a rack bar in the interior of the same, and a tie rod  140  extended from both sides of the rack bar  130 . 
     In addition, a motor  150  is installed at an intermediate portion of the rack bar  130 . The motor  150  includes a motor housing  151  connected with the pinion housing  120 , a stator  152  fixed in the interior of the motor housing  151 , a rotor  153  installed in the inside of the stator  152 , a motor shaft  154  longitudinally formed in the direction of the inner portion of the rotor  153  and installed based on an angular contact bearing  151   a,  and a ball nut  155  installed at an end inner portion of the motor shaft  154 . 
     A threaded groove is formed in an inner side of the ball nut  155  and an outer portion of the rack bar  130 , respectively, and the rack bar  130  is moved in the axial direction when the ball nut  155  is rotated based on the balls disposed therebetween. 
     In the ball nut  155 , one end of the motor shaft  154  is closely contacted with a step formed in the interior of the same, and the other end of the same is is fixed by a lock screw  156 . A stop ring  158  is formed in a rear portion of the lock screw  156 . 
     Namely, a threaded portion is formed in an outer surface of the lock screw  156  and is engaged with a threaded portion formed in the interior of the motor shaft  154 , and the stop ring  158  is inserted into a groove  157  formed in the interior of the motor shaft  154 , so that an inner circumferential portion of the same is supported by the lock screw  156 . 
     In the present invention, a friction member  159  is provided between the ball nut  155  and the lock screw  159  for thereby performing a torque limiting function when an over load occurs in the motor  150 . 
     The friction member  159  is installed between the inner step of the motor shaft  154  and one side of the ball nut  155  which is closely contacted with the inner step of the same, so that the friction member  159  slides when an over load occurs in the motor  150  for thereby preventing a damage of the motor  150 . 
     Namely, the friction member  159  is formed of a hallow circular plate for thereby being disposed in an outer portion of the rack bar  130  and is installed between the ball nut  155  and the lock screw  156  for thereby performing a torque limiting function. 
     In addition, in the present invention, the stop ring  158  and the groove  157  are formed for thereby preventing a certain operational gap which is formed when installing the stop ring  158 . 
     The groove  157  is formed in a circular shape in the interior of the motor shaft  154 , and the cross section of the groove  157  is rectangular. The groove  157  is formed in a bevel shape in which there is provided an inner tapered surface  157   a.    
     As shown in FIG. 3, the stop ring  158  is provided to correspond with the groove  157  and is inserted into the groove  157 . The stop ring  158  has a certain thickness smaller than that of the groove  157 . 
     The stop ring  158  includes an opened center portion and is inserted into the interior of the groove  157  and has a diameter larger than that of the groove  157  for thereby being enlarged in the outer direction and has a cut outer portion. 
     In addition, a bevel shape stop ring  158  has an outer surface  158   a  tapered to correspond with the inner tapered surface  157   a.    
     Namely, the force applied to the outer portion of the stop ring  158  is converted into a force in an axial direction by the outer tapered surface  158   a  and the inner tapered surface  157   a , so that the stop ring  158  is closely contacted with the inner lock screw  156  of the groove  157 . 
     In the electronic power steering apparatus for a vehicle according to the present invention, in a state that the ball nut  155  is inserted into the motor shaft  154 , and the friction member  159  is engaged to one surface of the lock screw  156 , the lock screw  156  is inserted into the interior of the motor shaft  154  and is rotated for thereby pressing the friction member  159  and supporting the rear portion of the ball nut  155 . 
     In addition, the stop ring  158  is installed in the rear portion of the lock screw  156  for thereby preventing the movement of the lock screw  156  in the axial direction. 
     At this time, the stop ring  158  is retracted in the inner circumferential surface portion for an expansion in the outer direction and is inserted into the groove  157 , and the outer portion of the stop ring  158  is distanced from the outer side of the groove  157 . 
     The outer tapered surface  158   a  of the stop ring  158  and the inner tapered surface  157   a  of the groove  157  are closely contacted, and the outer tapered surface  158   a  is closely contacted with the inner tapered surface  157   a  by a force which is applied to the outer side of the stop ring  158 . 
     The outer side of the stop ring  158  is closely contacted with the outer side of the groove  157 , and the stop ring  158  is closely contacted with an inner surface of the groove  157 , so that it is possible to prevent a loosening of the lock screw  156  which is closely contacted with the inner surface of the stop ring  158 . 
     When an over load occurs in the motor  150 , the motor shaft  154  and the ball nut  155  slide by the friction member  159  for thereby implementing a torque limiting function which limits the rotational force applied therebetween. 
     As described above, in the electronic power steering apparatus for a vehicle according to the present invention, the bevel type stop ring which is originally constructed to move from the inner side of the bevel type groove to the outer side generates a supporting force in the axial direction by the opposite tapered surfaces for thereby preventing an operational gap of the lock screw connected thereto. In addition, since the friction member having a torque limiting function is disposed between the motor shaft and the ball nut, it is possible to prevent the motor from being damaged due to the over load. 
     As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalences of such meets and bounds are therefore intended to be embraced by the appended claims.