Patent Publication Number: US-2020277002-A1

Title: Manufacturing method for hollow rack bar and hollow rack bar manufacturing apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a manufacturing method for a hollow rack bar for use in a rack-and-pinion steering system of a motor vehicle or the like and a hollow rack bar manufacturing apparatus. 
     BACKGROUND ART 
     As a rack bar for use in a rack-and-pinion steering system of the like, for example, a hollow rack bar is known in which a rack is formed on a cylindrical solid shaft material, while a so-called hollow rack bar is also known in which a cylindrical hollow shaft material is used for reducing the weight of the rack bar. 
     A shaft material for use for a hollow rack bar generally has a cross section which is substantially constant along an axial direction, and its thickness is set according to the height of rack teeth. A thickness which is set according to the height of rack teeth is excessive for a shaft section excluding a toothed section where a rack is formed. Hence, a hollow rack bar is also known in which a shaft section is formed thinner than a toothed section not only to reduce the weight of the rack bar but also to save on a material used for the rack bar (for example, refer to Patent Document 1). 
     [Patent Document 1] JP-A-5-345231 
     SUMMARY OF THE INVENTION 
     Problem that the Invention is to Solve 
     In the hollow rack bar described in Patent Document  1 , the toothed section is formed relatively thick while the shaft section is formed relatively thin by drawing a shaft material as a base material; however, no specific drawing process is described in Patent Document 1. 
     The invention has been made in view of the situation described above, and an object of the invention is to provide a method and apparatus for manufacturing a hollow rack bar which can reduce the weight thereof and save on a material used therefor. 
     Means for Solving the Problem 
     In an aspect (1), a manufacturing method for a hollow rack bar made of a hollow shaft material and including a toothed section which has a rack on an outer surface and a shaft section which is formed thinner than the toothed section, the manufacturing method includes drawing the hollow shaft material by using a die and a plug, and preforming regions of the hollow shaft material including a toothed section forming region configured to form the toothed section and a shaft section forming region configured to form the shaft section to have thicknesses according to the regions respectively, and forming the rack at the toothed section forming region of the hollow shaft material which is preformed. The plug has a first working section and a second working section having a larger diameter than that of the first working section, as working sections to define inside diameters of the hollow shaft material. In the preforming, when the toothed section forming region passes through the die, the first working section of the plug is disposed inside a working section of the die configured to define an outside diameter of the hollow shaft material, and when the shaft section forming region passes through the die, the second working section of the plug is disposed inside the working section of the die. 
     In an aspect (2), a reduction rate from sectional areas at the toothed section forming region and the shaft section forming region of the hollow shaft material perpendicular to an axial direction of the hollow shaft material before the preforming, to the sectional areas after the preforming, is 10% or larger and 35% or smaller. 
     In an aspect (3), in forming the rack, a mandrel is press fitted in the hollow shaft material in a state that a tooth die is pressed against the toothed section forming region and a material of the toothed section forming region is caused to plastically flow towards the tooth die to form the rack. 
     In an aspect (4), a hollow rack bar manufacturing apparatus, the hollow rack bar made of a hollow shaft material and including a toothed section which has a rack on an outer surface and a shaft section which is formed thinner than the toothed section, the apparatus includes a die having a working section configured to define an outside diameter of the hollow shaft material; 
     a plug having working sections configured to define inside diameters of the hollow shaft material, a driving unit configured to move the plug in an axial direction of the hollow shaft material, and a drawing unit configured to draw the hollow shaft material through between the die and the plug. The plug has a first working section and a second working section having a larger diameter than that of the first working section, as the working sections. When a toothed section forming region of the hollow shaft material configured to form the toothed section passes through the die, the driving unit disposes the first working section of the plug inside the working section of the die configured to define the outside diameter of the hollow shaft material, and when a shaft section forming region of the hollow shaft material configured to form the shaft section passes through the die, the driving unit disposes the second working section of the plug inside the working section of the die. 
     Advantageous Effect of the Invention 
     According to the invention, it is possible to provide the method and apparatus for manufacturing the hollow rack bar which can reduce the weight and save on the material of the rack bar. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of an example of a rack bar for describing an embodiment of the invention. 
         FIG. 2  is a schematic drawing showing a manufacturing step of the rack bar in  FIG. 1 . 
         FIG. 3  is a schematic drawing showing another manufacturing step of the rack bar in  FIG. 1 . 
         FIG. 4  is a schematic drawing showing a further manufacturing step of the rack bar in  FIG. 1 . 
         FIG. 5  is a schematic drawing showing a manufacturing step of the rack bar in  FIG. 1 . 
         FIG. 6  is a schematic drawing of an apparatus for use in manufacturing the rack bar in  FIG. 1 . 
         FIG. 7  is a schematic drawing showing an operation of the apparatus in  FIG. 6 . 
         FIG. 8  is a schematic drawing showing another operation of the apparatus in  FIG. 6 . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
       FIG. 1  shows an example of a rack bar for describing an embodiment of the invention. 
     A hollow rack bar  1  is made up of a single cylindrical hollow shaft material  2 , and a rack  3  is formed at part of an outer surface of the shaft material  2 . A joining section  4 , to which a joint where a tie-rod of a steering system is coupled is joined, is provided at each end portion of the shaft material  2  in an axial direction, and a female thread  5  is formed on an inner circumferential surface of the joining section  4 . Hereinafter, the hollow rack bar  1  is divided, in the axial direction, into a section where the rack  3  is formed, the section being referred to as a toothed section  6 , and a section excluding the toothed section  6  and the joining sections  4 , the section being referred to as a shaft section  7 . 
     When a thickness of an arc section  8  of the toothed section  6  excluding the rack  3  is referred to as t 1  and a thickness of the shaft section  7  is referred to as t 2 , then, t 1 &gt;t 2 , and the shaft section  7  is formed thinner than the toothed section  6 . In this embodiment, the joining section  4  is given a thickness t 1  which is the same as that of the toothed section  6 ; however, the joining sections  4  may be formed thinner than the toothed section  6  or may be formed thicker than the toothed section  6 . 
     In this embodiment, the rack  3  has a constant gear ratio (CGR) in which rack teeth are formed at a constant pitch. However, the rack  3  may have a variable gear ratio (VGR) in which rack teeth are formed at a variable pitch. 
       FIGS. 2 to 5  show a series of steps of manufacturing the hollow rack bar  1 . 
     As shown in  FIG. 2 , the shaft material  2  as a base material has a sectional shape which is substantially constant in the axial direction, and an outside diameter D 0  of the shaft material  2  is larger than an outside diameter D (refer to  FIG. 1 ) of the hollow rack bar  1 . 
     &lt;Preforming Step&gt; 
     Next, as shown in  FIG. 3 , the shaft material  2 , which is a base material, is drawn and preformed so that sections of the shaft material  2  are given thicknesses suitable for designed purposes of the individual sections. In this preforming, a toothed section forming region  2   a  configured to form a toothed section  6  is formed to have a thickness t 1 , a shaft section forming region  2   b  configured to form a shaft section  7  is formed to have a thickness t 2 , and a joining section forming region  2   c  configured to form a joining section  4  is formed to have a thickness t 1 . A distal end region  2   d  lying adjacent to one joining section forming region  2   c  is an auxiliary region which is drawn during drawing and is cut off after the preforming is completed. 
     &lt;Rack Forming Step&gt; 
     In this embodiment, the rack  3  is formed through rolling using a tooth die. Specifically, firstly, as shown in  FIG. 4 , part of an outer surface of the toothed section forming region  2   a  of the preformed shaft material  2  is pressed to be collapsed flat, whereby a flat rack forming surface  9 , extending in the axial direction of the shaft material  2 , is formed. 
     Next, as shown in  FIG. 5 , a tooth die  10  is pressed against the rack forming surface  9 , and a mandrel  11  is press fitted in the shaft material  2  in that state. A material of the rack forming surface  9  is worked by the plied mandrel  11  as the mandrel  11  is press fitted and then bites into the tooth die  10 . Then, the mandrel  11  is replaced by a gradually larger mandrel which is press fitted instead, and this gradually larger mandrel is then replaced by another gradually larger mandrel which is press fitted instead. Repeating such a replacement of mandrels allows the shape of the tooth die  10  to be transferred onto the rack forming surface  9 , whereby the rack  3  is formed on it. 
     The rack  3  may be formed by cutting using, for example, a broaching machine; however, when the rack  3  is formed through rolling using the tooth die  10 , the material of the shaft material  2  can be saved on, and the degree of freedom in setting a pitch at which teeth are formed is enhanced. 
     Thereafter, a female thread  5  (refer to  FIG. 1 ) is formed on an inner circumferential surface of each of the joining section formed regions  2   c,  whereby the hollow rack bar  1  is manufactured. 
       FIG. 6  shows an apparatus for use in the preforming step. 
     A drawing apparatus  20  includes a die  21  through which the shaft material  2 , which is a base material, is inserted, a plug  22  configured to be accommodated in the shaft material  2 , a driving unit  23  configured to move the plug  22  in an axial direction of the shaft material  2 , a drawing unit  24  configured to draw the shaft material  2  through between the die  21  and the plug  22 , and a control unit  25 . 
     The die  21  has a working section  30  configured to define an outside diameter of the shaft material  2 . An inside diameter of the working section  30  is the same as an outside diameter D of the hollow rack bar  1  and is smaller than an outside diameter D 0  of the shaft material  2 , which is a base material. 
     The plug  22  has a first working section  31  and a second working section  32  as working sections for providing for an inside diameter of the shaft material  2 . The second working section is formed in a larger diameter than that of the first working section and the first working section  31  and the second working section  32  are provided coaxially and adjacent to each other. The plug  22  is accommodated in the second shaft material  2  with the first working section  31 , which is relatively smaller in diameter, oriented in a drawing direction of the shaft material  2  indicated by an arrow A in  FIG. 6 . Then, the plug  22  is caused to move in the axial direction of the shaft material  2  in an interior of the shaft material  2  by the driving unit  23  so that the first working section  31  and the second working section  32  are disposed selectively inside the working section  30  of the die  21 . 
     The driving unit  23  has a support rod  33  which is inserted into the shaft material  2  through an opening at one end portion of the shaft material  2  and an actuator  34  configured to push and retract the support rod  33  in a longitudinal direction of the support rod  33 . The plug  22  is held to a distal end portion of the support rod  33  which is inserted into the shaft material  2 , and the support rod  33  is moved back and forth in the longitudinal direction by the actuator  34 , that is, the support rod  33  is moved back and forth in the axial direction of the shaft material  2 , whereby the plug  22  is also moved back and forth in the axial direction of the shaft material  2 . For example, an appropriate direct acting device such as a hydraulic cylinder device, a ball screw device or the like is used as the actuator  34 . 
     The drawing unit  24  has a chuck  35  configured to clamp the distal end region  2   d  (refer to  FIG. 3 ) of the shaft material  2  and an actuator  36  configured to pull the chuck  35  in the drawing direction of the shaft material  2 . The chuck  35 , clamping the distal end region  2   d  of the shaft material  2 , is pulled in the drawing direction of the shaft material  2  by the actuator  36 , whereby the shaft material  2  is drawn through between the die  21  and the plug  22 . An appropriate direct acting device such as a hydraulic cylinder device, a ball screw device or the like is used as the actuator  36 . 
     The control unit  25  controls the driving unit  23  and the drawing unit  24  as a whole. The control unit  25  identifies a region of the shaft material  2  which passes through the die  21  based on an amount by which the drawing unit  24  pulls the shaft material  2 , drives the driving unit  23  based on the identified region of the shaft material  2 , and disposes one of the first working section  31  and the second working section  32  of the plug  22  inside the working section  30  of the die  21 . 
     In drawing the shaft material  2 , a lubricant may be applied to an outer circumferential surface and/or an inner circumferential surface of the shaft material  2 . A lubricant applied to the inner circumferential surface of the shaft material  2  may be supplied from the plug  22  through, for example, the support rod  33  as occasion demands or may be applied to the plug  22  in advance. 
       FIGS. 7 and 8  show an operation of the drawing apparatus  20 . 
       FIG. 7  shows a case where the toothed section forming region  2   a  of the shaft material  2  is passing through the die  21 , and of the first working section  31  and the second working section  32  of the plug  22 , the first working section  31  having the relatively small diameter is disposed inside the working section  30  of the die  21 . A gap between the working section  30  and the first working section  31  is the same as the thickness t 1  of the toothed section forming region  2   a  which results after the completion of the preforming step, and the toothed section forming region  2   a  which is drawn through the die  21  is formed to have the outside diameter D and the thickness t 1 . 
       FIG. 8  shows a case where the shaft section forming region  2   b  of the shaft material  2  is passing through the die  21 , and of the first working section  31  and the second working section  32  of the plug  22 , the second working section  32  having the relatively large diameter is disposed inside the working section  30  of the die  21 . A gap between the working section  30  and the second working section  32  is the same as the thickness t 2  of the shaft section forming region  2   b  which results after the completion of the preforming step, and the shaft section forming region  2   b  which is drawn through the die  21  is formed to have the outside diameter D and the thickness t 2 . 
     When the joining section forming region  2   c  of the shaft material  2  passes through the die  21 , the first working section  31  of the plug  22  is disposed inside the working section  30  of the die  21 , and the joining section forming region  2   c  which is drawn through between the first working section  31  and the working section  30  is formed to have the outside diameter D and the thickness t 1 . 
     Thus, as has been described heretofore, the toothed section forming region  2   a  can be formed relatively thick (the thickness t 1 ) while the shaft section forming region  2   b  can be formed relatively thin (the thickness t 2  (t 1 &gt;t 2 )) simply and securely as the shaft material  2  is drawn by moving the plug  22  according to the region of the shaft material  2  which passes through the die  21 . 
     A cross section reduction rate of the regions of the shaft material  2  is preferably 10% or larger and 35% or smaller. When referred to herein, the cross section reduction rate means a reduction rate of the cross section which is expressed by (S 0 −S)/S 0 , where S 0  denotes a cross section of the shaft material  2 , which is a base material, and S denotes a cross section of the preformed shaft material  2 . The shaft material  2  which is a base material prior to preforming typically has an error in outside diameter. Then, the outside diameter accuracy of the preformed shaft material  2  is enhanced by setting the cross section reduction rate at 10% or larger. In addition, preforming is completed through a single drawing while suppressing the occurrence of seizing between the shaft material  2  and the die  21  and/or the plug  22  by setting the cross section reduction rate at 35% or smaller. 
     Heretofore, the plug  22  is described as having the two working sections, which are the first working section  31  and the second working section  32 , as the working section configured to define the inside diameter of the shaft material  2 . However, the plug  22  may have three or more working sections. For example, the plug  22  may have a third working section, which is larger in diameter than the first working section  31  and smaller in diameter than the second working section  32 , between the first working section  31  and the second working section  32 , whereby the thicknesses of the regions of the shaft material  2  can be set in more precise steps. For example, when the joining section forming region  2   c  of the shaft material  2  is passing through the die  21 , the third working section of the plug  22  is disposed inside the working section  30  of the die  21 , whereby the joining section forming region  2   c  where the female thread  5  (refer to  FIG. 1 ) is formed in the later step can be formed thinner than the toothed section forming region  2   a  but thicker than the shaft section forming region  2   b.  It should be noted that the working sections of the plug  22  are aligned in the decreasing or increasing order of outside diameters. 
     This application claims priority to Japanese Patent Application No. 2017-223770 filed on Nov. 21, 2017, the entire content of which is incorporated herein by reference.