Patent Publication Number: US-2007102230-A1

Title: Electric power steering apparatus for automobile having belt-type transmission device

Description:
RELATED APPLICATION  
      This application claims priority to Korean Patent Application No. 2005-105633 filed on Nov. 4, 2005.  
     FIELD OF THE INVENTION  
      The present invention relates to an electric power steering apparatus for an automobile having a belt-type transmission device. More particularly, the present invention relates to an electric power steering apparatus for an automobile having a belt-type transmission device, as well as a self-aligning bearing positioned on a motor shaft for easy belt mounting and a simple assembly process of the transmission device.  
     BACKGROUND OF THE INVENTION  
      As generally known in the art, power steering apparatuses for automobiles include a hydraulic power steering apparatus utilizing hydraulic pressure of a hydraulic pump, which has been used since its initial introduction, and an electric power steering apparatus utilizing an electric motor, use of which has been gradually universalized since the 1990&#39;s.  
      In the existing hydraulic power steering apparatus, a hydraulic pump, which is a power source for supplying steering power, is driven by an engine, which causes the hydraulic pump to continuously consume energy regardless of whether or not the steering wheel is being rotated. In the electric power steering apparatus, when steering torque is generated by rotation of a steering wheel, a motor supplies steering power in proportion to the generated steering torque. Therefore, in terms of energy efficiency, the electric power steering apparatus is more advantageous than the hydraulic power steering apparatus.  
       FIG. 1  illustrates the construction of a conventional electric power steering apparatus.  
      As shown in  FIG. 1 , a conventional electric power steering apparatus for an automobile includes a steering system  100 , which includes elements leading from a steering wheel  101  to both wheels  108 , and a steering power mechanism  120  for supplying steering power to the steering system  100 .  
      The steering system  100  includes a steering shaft  102  having an upper end connected to the steering wheel  101  and a lower end connected to a pinion shaft  104  via a pair of universal joints  103 , so that the steering shaft  102  rotates together with the steering wheel  101 . The pinion shaft  104  is connected to a rack bar  109  via a rack-pinion mechanism  105 . Both ends of the rack bar  109  are connected to the wheels  108  of the automobile via tie rods  106  and knuckle arms  107 .  
      The rack-pinion mechanism  105  includes a pinion gear  111  formed on the lower end of the pinion shaft  104  and a rack gear  112  formed on one side of the outer peripheral surface of the rack bar  109  to engage with the pinion gear  111 . The rack-pinion mechanism  105  converts the rotational motion of the pinion shaft  104  into a linear motion of the rack bar  109 . Particularly, when the driver operates the steering wheel  101 , the pinion shaft  104  rotates accordingly. The rotation of the pinion shaft  104  causes the rack bar  109  to move linearly in the shaft direction. The linear motion of the rack bar  109  is transmitted to and thereby operates the wheels  108  via the tie rods  106  and the knuckle arms  107 .  
      The steering power mechanism  120  includes a torque sensor  121  for sensing steering torque applied to the steering wheel  101  by the driver and outputting an electric signal in proportion to the sensed steering torque, an ECU (electronic control unit)  123  for generating a control signal based on the electric signal from the torque sensor  121 , a motor  130  for generating steering power based on the control signal from the ECU  123 , and a belt-type transmission device  140  for transmitting the steering power from the motor  130  to the rack bar  109  via a belt.  
      The electric power steering apparatus is operated as follows: when the driving wheel  101  is rotated, driving torque is generated and transmitted to the rack bar  109  via the rack-pinion mechanism  105 . In addition, the generated steering torque causes the motor  130  to generate steering power, which is transmitted to the rack bar  109  via the belt-type transmission device  140  and a ball screw unit  150 . As such, the steering torque generated by the steering system  100  is combined with the steering power generated by the motor  130 , so that the rack bar  109  is moved in the shaft direction.  
       FIG. 2  is a partial sectional view showing a conventional electric power steering apparatus for an automobile.  
      As shown in  FIG. 2 , the conventional electric power steering apparatus for an automobile includes a rack bar  109  extending in the transverse direction of the automobile and having a rack gear positioned on one side of the outer peripheral surface thereof; a pinion shaft  104  having a pinion gear positioned on its lower end to engage with the rack gear; a ball screw unit  150  having a ball nut  205  adapted to engage with a ball screw  203  via a ball  201 ; a belt-transmission device  140  for connecting the ball nut  205  to a motor shaft  221 ; and a motor  130 .  
      The pinion shaft  104  is connected to the driving wheel via a driving shaft. The rack bar  109  has a screw formed on one side of the outer peripheral surface thereof with a predetermined length and is contained in a rack housing  223 . The rack housing  223  includes a first housing  225  adjacent to the rack gear and a second housing  227  adjacent to the motor.  
      The belt-type transmission device  140  includes a belt  229  for connecting the motor shaft  221  and the ball nut  205 . The belt-type transmission device  140  transmits steering power from the motor  130  to the rack bar  109  via the ball nut  205  in proportion to steering torque applied to the steering wheel.  
      If the motor shaft  221 , the ball nut  205 , and the rack bar  109  are not parallel to one another, the belt-type transmission device  140  cannot fully transmit power from the motor  130  to the rack bar  109 . In addition, if the belt  229  is not tightly coupled to the motor shaft  229  and the ball nut  205 , power transmission is insufficient.  
      In order to position the motor shaft  221  and the ball nut  205  parallel to each other and tightly connect the belt  229  to them, the motor shaft  221  and the ball nut  205  may be allowed to move relative to each other.  
      In particular, the motor  221  and the ball nut  205  are initially placed adjacent to each other, in order to couple the belt  229  to them, and, after the coupling, they are placed far from each other to tension the belt  229 .  
      However, this approach makes the belt-type transmission device complicated and increases the number of components. As a result, the assembly process becomes complicated and the manufacturing cost increases.  
     SUMMARY OF THE INVENTION  
      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 an electric power steering apparatus for an automobile having a belt-type transmission device, as well as a self-aligning bearing positioned on a motor shaft, to simplify the structure of the belt-type transmission device, reduce the number of components, and decrease the manufacturing cost.  
      In order to accomplish this object, there is provided an electric power steering apparatus for an automobile including a pinion shaft connected to a steering wheel of the automobile; a rack bar having a ball screw formed on an outer peripheral surface thereof; a motor for generating steering power; a motor pulley positioned on a motor shaft of the motor; a motor pulley housing for enclosing the motor shaft and the motor pulley; a self-aligning bearing positioned between an outer peripheral surface of an end of the motor shaft and an inner peripheral surface of the motor pulley housing; a ball nut adapted to engage with the ball crew via a ball; a ball nut pulley positioned on an outer peripheral surface of the ball nut; and a belt for connecting the motor pulley and the ball nut pulley to each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      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:  
       FIG. 1  shows the construction of a conventional electric power steering apparatus for an automobile;  
       FIG. 2  is a partial sectional view showing a conventional electric power steering apparatus for an automobile;  
       FIG. 3  is a sectional view showing a belt-type transmission device according to a first embodiment of the present invention;  
       FIGS. 4   a,    4   b,  and  4   c  briefly show a series of steps for connecting a belt to a driving shaft provided with a self-aligning bearing, respectively;  
       FIG. 5  shows the construction of an electric power steering apparatus for an automobile according to a second embodiment of the present invention; and  
       FIG. 6  is a partial sectional view showing an electric power steering apparatus for an automobile according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. In the following description and drawings, the same reference numerals are used to designate the same or similar components, and so repetition of the description on the same or similar components will be omitted.  
       FIG. 3  is a sectional view showing a belt-type transmission device according to a first embodiment of the present invention.  
      As shown in  FIG. 3 , the belt-type transmission device according to a first embodiment of the present invention includes a transmission device housing  301 ; a rotatable driving shaft  303  contained in the transmission device housing  301 ; a self-aligning bearing  305  positioned between the outer peripheral surface of an end of the driving shaft  303  and the inner peripheral surface of the transmission device housing  301 ; a rotatable driven shaft  307  spaced a predetermined distance from the driving shaft  303 ; and a belt  229  for connecting the driving shaft  303  and the driven shaft  307 .  
      The transmission device housing  301  encloses various components of the belt-type transmission device, including the driving shaft  303  and the driven shaft  307 , and protects them from external impact or dust to ensure proper driving.  
      The transmission device housing  301  may be divided into at least two sub-housings, which enclose the driving shaft  303  and the driven shaft  307 , respectively, for convenient fabrication and assembly of the belt-type transmission device. Preferably, the transmission device housing  301  is made of a metallic material having predetermined rigidity.  
      The driving shaft  303  is a cylindrical rotation shaft having a predetermined diameter and is adapted to transmit inputted rotational force to the driven shaft  307 . The driving shaft  303  has various actuators, including a motor, connected to the input side to rotate the driving shaft  303 .  
      A bearing  309  is positioned between the outer peripheral surface of an output-side end  304  of the driving shaft  303  and the inner peripheral surface of the transmission device housing  301  to rotatably support the driving shaft  303 . The bearing  309  may be a ball bearing.  
      The driving shaft  303  has a tapered output-side end  304  so that, after the belt  229  is connected to the driving shaft  303 , the driving shaft  303  can be easily inserted into the bearing  309 . This simplifies the assembly process.  
      The driving shaft  303  has a driving pulley  311  formed thereon in such a manner that it encloses the outer peripheral surface of the driving shaft  303 . Therefore, the belt  229  can move while making contact with the outer peripheral surface of the driving pulley  311  and transmit the rotational force of the driving shaft  303  to the driven shaft  307 . The driving pulley  311  may be integral with the driving shaft  303 . The driving pulley  311  and the belt  229  may have protrusions formed on the outer and inner peripheral surfaces thereof, respectively, to fasten the belt  229  to the driving pulley  311 .  
      The self-aligning bearing  305  is positioned between the outer peripheral surface of the driving shaft  303  and the inner peripheral surface of the transmission device housing  301  while being spaced a predetermined distance from the driving pulley  311 . The self-aligning bearing  305  includes an outer wheel  313  and an inner wheel  315 .  
      The outer wheel  313  encloses the outer peripheral surface of the inner wheel  315  and is attached to the inner peripheral surface of the transmission device housing  301  to fix the position of the self-aligning bearing  305 . The inner peripheral surface of the outer wheel  313  is a spherical surface, as in the case of the inner peripheral surface of the inner wheel  315 .  
      The outer peripheral surface of the inner wheel  315  is a spherical surface. Therefore, the inner wheel  315  of the self-aligning bearing  305  can freely rotate in any direction while making contact with the inner peripheral surface of the outer wheel  313 . In contrast, the inner wheel of a conventional bearing can solely perform circular motion about a central shaft of the bearing.  
      The structure and function of the self-aligning bearing are widely known in the art, and further description thereof will be omitted herein.  
      The driven shaft  307  is adapted to receive rotational force from the driving shaft  303  and output it. Particularly, the driven shaft  307  is spaced a predetermined distance from the driving shaft  303  and receives rotational force from the driving shaft  303  via the belt  229 .  
      The driven shaft  307  has a driven pulley  317  formed thereon while enclosing the outer peripheral surface of the driven shaft  307 , so that the belt  229 , which is connected to the driving shaft  303 , can move while making contact with the outer peripheral surface of the driven pulley  317  and transmit rotational force to the driven shaft  307 . The driven pulley  317  and the belt  229  may have protrusions formed on the outer and inner peripheral surfaces thereof, respectively, to fasten the belt  229  to the driven pulley  317 .  
      At least one belt  229  connects the driving shaft  303  and the driven shaft  307  to each other and transmits rotational force from the driving shaft  303  to the driven shaft  307 . The belt  229  may be made of any of various materials, including metal and plastic. The belt  229  may have protrusions formed on the inner peripheral surface thereof, which engage with the protrusions formed on the driving shaft  303  or the driven shaft  307 .  
       FIGS. 4   a,    4   b,  and  4   c  briefly show a series of steps for connecting the belt to the driving shaft, which is provided with the self-aligning bearing, respectively.  
      As shown in  FIGS. 4   a,    4   b,  and  4   c,  when the belt  229  is connected to the driving shaft  303  provided with the self-aligning bearing  305 , the inner wheel  315  of the self-aligning bearing  305  can freely rotate while the outer wheel  313  remains stationary. Therefore, the driving shaft  303  can be slanted in the direction of the driven shaft  307 . After the belt  229  is connected to the driving shaft  303 , the driving shaft  303  can return to a position parallel to the driven shaft  307 .  
       FIG. 5  shows the construction of an electric power steering apparatus for an automobile according to a second embodiment of the present invention, and  FIG. 6  is a partial sectional view of the electric power steering apparatus for an automobile according to the second embodiment of the present invention.  
      As shown in  FIGS. 5 and 6 , the electric power steering apparatus for an automobile according to the second embodiment of the present invention includes a steering wheel  101  of the automobile; a pinion shaft  104  connected to the steering wheel  101  and having a pinion gear  111  formed on the lower end thereof; a rack bar  109  connected to both wheels  108  of the automobile and having a rack gear  112  formed on one side thereof, which engages with the pinion gear  111 , and a ball screw  203  formed on the other side thereof; a motor  130  for generating steering power in proportion to steering torque generated by the steering wheel  101 ; a motor pulley  601  positioned on a motor shaft  221 ; a motor pulley housing  603  for enclosing the motor shaft  221  and the motor pulley  601 ; a self-aligning bearing  305  positioned between the outer peripheral surface of an end of the motor  221  and the inner peripheral surface of the motor pulley housing  603 ; a ball nut  205  adapted to engage with the ball screw  203  of the rack bar  109  via a ball  201 ; a ball nut pulley  602  positioned on the outer peripheral surface of the ball nut  205 ; and a belt  229  for connecting the motor pulley  601  and the ball nut pulley  602  to each other.  
      The pinion shaft  104  is connected to the steering wheel  101  via a steering shaft  102  and has a pinion gear  111  formed on one side thereof.  
      Both ends of the rack bar  109  are connected to the wheels  108  of the automobile via tie rods  106  and knuckle arms  107 , respectively, while being contained in a rack housing. The rack bar  109  has a rack gear  112  formed on one side thereof, which engages with the pinion gear  111 , and seals positioned in predetermined positions on the inner sides of both ends thereof, in order to prevent lubricant from leaking.  
      The rack bar  109  receives power from the motor shaft  221  via the belt  229 . The rack bar  109  has a ball screw  203  formed on the outer peripheral surface of the other side thereof while extending a predetermined length as a helical groove.  
      The ball nut  205  engages with the ball screw  203  via the ball  201  while enclosing the outer peripheral surface of the rack bar  109 .  
      The ball nut pulley  602  is formed on the outer peripheral surface of the ball nut  205  and connects the belt  229  to the ball nut  205 . The ball nut pulley  602  may be integral with the ball nut  205 . The ball nut pulley  602  may have protrusions formed on the outer peripheral surface thereof.  
      The motor  130  includes a motor housing  611 , a cylindrical stator (not shown) contained in the motor housing  611 , a rotator (not shown) positioned inside the stator (not shown), and a motor shaft  221  coupled to the rotator (not shown).  
      The motor shaft  221  is positioned parallel to the rack bar  109 . The self-aligning bearing  305  is coupled to one side of the motor shaft  221  so that, when the belt  229  is connected to the motor shaft  221 , the motor shaft  221  can be slanted toward the rack bar  109 . After connecting the belt  229  to the motor shaft  221 , the motor shaft  221  can return to a position parallel to the rack bar  109 .  
      The left end  628  of the motor shaft  221  is tapered and is rotatably supported by a bearing  309 , which is positioned on the motor pulley housing  603 . The bearing  309  may be a ball bearing, but the type is not limited to that herein.  
      The motor pulley  601  is positioned adjacent to the left end  628  of the motor shaft  221 . The belt  229  is connected to the motor pulley  601  and transmits steering power from the motor  130  to the rack bar  109 . The motor pulley  601  may be integral with the motor shaft  221 . The motor pulley  601  may have protrusions formed on the outer peripheral surface thereof.  
      The belt  229  connects the motor pulley  601  and the ball nut pulley  602  to each other to transmit steering power from the motor  130  to the rack bar  109 . If necessary, the belt  229  may have protrusions formed on the inner peripheral surface thereof, which correspond to the protrusions formed on the outer peripheral surface of the motor pulley  601  or the ball nut pulley  602 , so that the belt  229  can be driven while being fastened to the motor pulley  601  or the ball nut pulley  602 .  
      The remaining construction and operation of the electric power steering apparatus according to the second embodiment are the same as those of the first embodiment. Therefore, the same components are given the same reference numerals, and repeated description thereof will be omitted herein.  
      As mentioned above, the electric power steering apparatus for an automobile according to the present invention is advantageous in that a self-aligning bearing is positioned on a motor shaft to simplify the structure of the belt-type transmission device, reduce the number of components, and decrease the manufacturing cost.  
      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.