Patent Publication Number: US-2020284334-A1

Title: Rack bar blank material, rack bar, rack bar blank material manufacturing method, and rack bar manufacturing method

Description:
TECHNICAL FIELD 
     The present invention relates to a rack bar blank material, a rack bar, a rack bar blank material manufacturing method and a rack bar manufacturing method. 
     BACKGROUND ART 
     In a known rack bar as a rack bar for use in a rack-and-pinion steering system, a solid shaft material is used, and a plurality of rack teeth are formed on the solid shaft material through cutting or the like. Additionally, a so-called hollow rack bar is also known whose weight is reduced by use of a hollow shaft material. 
     A hollow rack bar is generally manufactured as below. Firstly, an axial end side of a hollow shaft material is drawn to be formed smaller in diameter than the other axial end side, and a flat collapsed portion having a flat planar shape is provided at part of the formed small-diameter portion. Then, a tooth die is fixed in abutment with an outer surface of the flat collapsed portion, and a mandrel is press fitted in an interior of the flat collapsed portion. The mandrels whose sizes increase gradually are press fitted sequentially one by one, and then, a shape of the tooth die is transferred to the flat collapsed portion as a result of such a press-fit replacement of the mandrels repeatedly, whereby a plurality of rack teeth are formed on the outer surface of the flat collapsed portion (for example, refer to Patent Document 1: JP-A-2016-30271). 
     In the rack bar manufacturing method of the related art, the individual portions of the rack bar shaft material are finished through grinding after the rack teeth are formed on the outer surface of the flat collapsed portion, and then, a screw groove for a ball screw is formed on an outer surface of a large-diameter portion on the rack bar shaft material. The screw groove is formed by, for example, cutting, during which the rack bar shaft material is rotated with both axial end portions of the shaft material supported rotatably. Thus, the cutting accuracy of the screw groove is affected by the coaxiality of both the end portions of the shaft material and the straightness of the overall shaft material. To cope with this, in the rack bar manufacturing method of the related art, the relevant portions of the shaft material are finished through grinding before the screw groove is formed. 
     In the rack bar manufacturing method of the related art, however, outside diameters of the end portion on the small-diameter portion side and the end portion on the large-diameter portion side which are supported rotatably differ from each other. This makes it difficult to cut both the end portions simultaneously. Thus, the large-diameter portion including the end portion on the large-diameter portion side and the small-diameter portion including the end portion on the small-diameter portion side are cut separately, and this leaves a problem with a reduction in the number of manufacturing steps. Additionally, there still remains room for improvement in the coaxiality of both the end portions and the straightness of the overall shaft material. 
     One or more embodiments provide a rack bar improved working accuracy and a simple manufacturing process. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view of an example of a rack bar blank material for use for describing an embodiment of the invention. 
         FIG. 2  is a sectional view of the rack bar blank material shown in  FIG. 1 . 
         FIG. 3A  is a cross-sectional view taken along a line IIIA-IIIA in  FIG. 2 . 
         FIG. 3B  is a cross-sectional view taken along a line in  FIG. 2 . 
         FIG. 3C  is a cross-sectional view taken along a line IIIC-IIIC in  FIG. 2 . 
         FIG. 4  is a front view of an example of a rack bar manufactured using the rack bar blank material shown in  FIG. 1 . 
         FIG. 5A  is a schematic drawing of a step of a manufacturing method of the rack bar blank material shown in  FIG. 1 . 
         FIG. 5B  is a schematic drawing of another step of the manufacturing method of the rack bar blank material shown in  FIG. 1 . 
         FIG. 5C  is a schematic drawing of a further step of the manufacturing method of the rack bar blank material shown in  FIG. 1 . 
         FIG. 5D  is a schematic drawing of a step of the manufacturing method of the rack bar blank material shown in  FIG. 1 . 
         FIG. 5E  is a schematic drawing of another step of the manufacturing method of the rack bar blank material shown in  FIG. 1 . 
         FIG. 5F  is a schematic drawing of a further step of the manufacturing method of the rack bar blank material shown in  FIG. 1 . 
         FIG. 5G  is a schematic drawing of a step of the manufacturing method of the rack bar blank material shown in  FIG. 1 . 
         FIG. 5H  is a schematic drawing of another step of the manufacturing method of the rack bar blank material shown in  FIG. 1 . 
         FIG. 6  is a schematic drawing of an example of an outer diameter grinding performed in  FIG. 5H . 
         FIG. 7  is a schematic drawing of another example of an outer diameter grinding performed in  FIG. 5H . 
         FIG. 8  is a schematic diagram of an example of a manufacturing method of the rack bar shown in  FIG. 4 . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
       FIG. 1  shows an example of a rack bar blank material for use for describing an embodiment of the invention, and  FIGS. 2 and 3A to 3C  show a section and cross sections of the rack bar blank material shown in  FIG. 1 . 
     A rack bar blank material  10  shown in  FIG. 1  is a primarily processed material of a rack bar to be incorporated in, for example, a rack-and-pinion steering system. The rack bar blank material  10  is formed of a hollow shaft material of a metallic material such as steel, for example. The rack bar blank material  10  has, on an axial end side thereof, a rack portion  11  and an end portion  12  which is provided closer to the axial end side of the shaft material than the rack portion  11  and has a shaft portion  13  on the other axial end side. 
     The rack portion  11  has a flat collapsed portion  14  extending in an axial direction and a plurality of rack teeth  15  provided on an outer circumferential surface of the flat collapsed portion  14 . The rack portion  11  meshes with a pinion via these rack teeth  15 . In this embodiment, the rack teeth  15  have a constant pitch and provide a constant gear ratio (CGR). However, the pitch may vary to thereby provide a variable gear ratio (VGR). 
     The shaft portion  13  is supported by a housing of the steering system so as to move in the axial direction. An axial direction acting element may be provided on the shaft portion  13  in addition to the rack portion  11 . 
     A heat treatment including at least hardening is applied to the rack portion  11  and the shaft portion  13  except an intermediate portion  16  defined between the rack portion  11  and the shaft portion  13 . 
     As shown in  FIGS. 3A to 3C , an outside diameter Da of the end portion  12  of the rack bar blank material  10  is larger than a diameter Db of a minimum circle C embracing the rack portion  11  in a cross section taken perpendicular to the axial direction (Da&gt;Db) and is equal to an outside diameter Dc of the shaft portion  13  (Da=Dc). 
       FIG. 4  shows an example of a rack bar manufactured by use of the rack bar blank material  10 . 
     A rack bar  20  shown in  FIG. 4  has the rack portion  11  formed in the stage where the rack bar blank material  10  is manufactured on an axial end side and has a screw groove  21  for a ball screw as another axial direction acting element, and the screw groove  21  is formed on an outer circumferential surface of the shaft portion  13 . 
     Although its illustration is omitted, a female thread is formed individually on the end portion  12  on the rack portion  11  side and an end portion  17  of the shaft portion  13  side, and a ball joint which is coupled with a tie-rod of the steering system is connected to the female thread. These female threads may be formed in the state where the rack bar blank material  10  is manufactured. 
       FIGS. 5A to 5H  shows an example of a manufacturing method of the rack bar blank material  10 . 
     &lt;Pre-Forming Step&gt; 
     As shown in  FIG. 5A , a hollow shaft material  30  is used to manufacture the rack bar blank material  10 . The shaft material  30  has a cylindrical shape whose outside diameter and inside diameter are constant over a full length of the shaft material  30  in an axial direction thereof. 
     As shown in  FIG. 5B , a small-diameter portion  31  is formed at a portion on an axial end side of the shaft member  30  through rolling, drawing such as swaging, cutting or the like, whereby an end portion  12  which is relatively large in diameter is formed at a portion lying closer to the end side than the small-diameter portion  31 . The end portion  12  keeps the original diameter of the shaft material  30  and has the same outside diameter as that of a shaft portion  13  on the other axial end side of the shaft material  30 . 
     &lt;Teeth Forming Step&gt; 
     Next, as shown in  FIG. 5C , a circumferential portion of the small-diameter portion  31  of the shaft material  30  is collapsed to be flat through pressing, whereby a flat collapsed portion  14  extending in an axial direction of the shaft material  30  is formed. Thereafter, as required, a forming treatment is applied to the shaft material  30  in which a phosphate layer is formed on a surface of the shaft material  30 . Then, a plurality of rack teeth  15  are formed on the flat collapsed portion  14 . 
     The plurality of rack teeth  15  are formed as below. As shown in  FIG. 5D , a tooth die  32  is fixed in such a state that the tooth die  32  is in abutment with an outer surface of the flat collapsed portion  14 , and a mandrel  33  is press fitted in an interior of the flat collapsed portion  14  by a push rod  34  through an opening at an end of the end portion  12 . Then, the mandrel  33  press fitted is then pushed back by a push rod  35  to thereby be discharged from the shaft material  30 . 
     The material of the flat collapsed portion  14  is worked by the mandrel  33  so plied while the mandrel  33  is reciprocated over a full length of the flat collapsed portion  14  and flows plastically towards the tooth die  32 . Mandrels  33  which are gradually increased in diameter are used to be press fitted into the flat collapsed portion  14  repeatedly, causing the material of the flat collapsed portion  14  to bite into the tooth die  32 , whereby the shape of the tooth die  32  is transferred onto the flat collapsed portion  14 , and a plurality of rack teeth  15  are formed on the flat collapsed portion  14 . 
     As the rack portion  11  (the flat collapsed portion  14  and the plurality of rack teeth  15 ) is worked plastically, a bend may be generated in the shaft material  30 , and hence, the bend of the shaft material  30  may be corrected as required after the teeth forming step. 
     &lt;Heat Treatment Step&gt; 
     Next, as shown in  FIG. 5E , to enhance the hardness of the rack portion  11  configured to mesh with a pinion and the shaft portion  13  which is supported movably in a housing of a steering system, hardening is applied to the rack portion  11  and the shaft portion  13 . However, in consideration of a possibility that a bend generated in the shaft material  30  is corrected in a correction step, which will be described later, the intermediate portion  16  between the rack portion  11  and the shaft portion  13  is left not hardened. To heat the rack portion  11  and the shaft portion  13  for hardening, for example, high-frequency induction heating can be made use of, however, the invention is not limited to the high-frequency induction heating. 
     To recover the toughness of the rack portion  11  and the shaft portion  13  to which the hardening is applied, tempering may be applied locally to the rack portion  11  and the shaft portion  13  or may be applied to the whole of the shaft material  30 . To remove an oxide layer generated on the surface of the shaft material  30  as a result of the heat treatment such as hardening being applied to the surface, shot-peening may be applied. This shot-peening may be applied locally only to the rack portion  11  except the shaft portion  13  to which outside diameter grinding is applied in a post-step, for example or may be applied to the whole of the shaft material  30 . 
     &lt;Correction Step&gt; 
     Next, a bend generated in the shaft member  30  by the heat treatment such as hardening is corrected. 
     Since the intermediate portion  16  between the rack portion  11  and the shaft portion  13  is left not hardened in the heat treatment step, the intermediate portion  16  is relatively easy to be bent. As shown in  FIG. 5F , for example, with the intermediate portion  16  and the end portion  17  on the shaft portion  13  side supported, a load is exerted on the rack portion  11 , whereby the intermediate portion  16  is bent as required. This enhances the straightness of the rack portion  11  with respect to the shaft portion  13 , whereby the coaxiality of the end portion  17  on the shaft portion  13  side with the end portion  12  on the rack portion  11  side is also enhanced. 
     Preferably, a connecting portion  18  between the end portion  12  and the rack portion  11  is bent further. Since the connecting portion  18  is also left not hardened, the connecting portion  18  is relatively easy to be bent as with the intermediate portion  16 . As shown in  FIG. 5G , for example, with the connecting portion  18  and the intermediate portion  16  supported, the connecting portion  18  is bend as required by applying a load on the end portion  12 . This enhances further the straightness of the shaft material  30  and the coaxiality of the end portion  17  on the shaft portion  13  side with the end portion  12  on the rack portion  11  side. 
     After the correction step, as required, the plurality of rack teeth  15  are inspected, a tooth rear surface of the rack portion  11  positioned on an opposite side to the side where the plurality of rack teeth  15  are formed is abraded, and the shaft material  30  is inspected magnetically for a flaw. In addition, a female thread is formed on the end portion  12  on the rack portion  11  side and the end portion  17  on the shaft portion  13  side as required. 
     &lt;Grinding Step&gt; 
     Next, as shown in  FIG. 5H , an outer diameter grinding is applied to the end portion  12  on the rack portion  11  side and the shaft portion  13  including the end portion  17  of the shaft material  30  which is corrected to free from a bend. Here, the end portion  12  keeps its diameter which remains the same as that of the shaft material  30  through the pre-forming step to the correction step and has an outside diameter which is the same as that of the shaft portion  13 . When an outer diameter grinding is applied to this end portion  12 , the outer diameter grinding is applied to the end portion  12  and at least part of the shaft portion  13  at the same time. 
       FIGS. 6 and 7  show examples of the outer diameter grinding. 
     For example, a centerless grinding can be used when the outer diameter grinding is applied to the end portion  12  and the shaft portion  13 , and the centerless grinding includes a trough-feed grinding (a through-feed grinding) and an infeed grinding (a stop grinding). 
       FIG. 6  shows schematically an example of the trough-feed grinding, in which the shaft material  30  is supported by a grinding wheel  40 , a control wheel  41  and a support blade  42 . When the grinding wheel  40  and the control wheel  41  are rotated, with a center axis of the control wheel  41  inclined with respect to a center axis of the shaft material  30  and a center axis of the grinding wheel  40 , the shaft material  30  which is held by the grinding wheel  40  and the control wheel  41  on the support blade  42  is fed in the axial direction while being rotated. An overall length G 3  of the grinding wheel  40  is smaller than an overall length L 1  of the shaft material  30 , and an outer circumferential surface of the shaft material  30  which is in contact with the grinding wheel  40  is ground continuously while the shaft material  30  is being fed in the axial direction. In this through-feed grinding, since the overall length L 3  of the grinding wheel  40  is larger than an axial length L 2  of the rack portion  11 , and the grinding wheel  40  has such a length that the grinding wheel  40  extends between the end portion  12  and the intermediate portion  16  between which the rack portion  11  is held, the end portion  12  and part of the shaft portion  13  are ground externally and outer circumferentially at the same time. 
       FIG. 7  shows schematically an example of the infeed grinding, in which the shaft material  30  is supported by a grinding wheel  50 , a control wheel  51  and a support blade  52  in a similar way to that used in the through-feed grinding shown in  FIG. 6 . However, the infeed grinding differs from the through-feed grinding in that an overall length L 4  of the grinding wheel  50  is equal to or larger than the overall length L 1  of the shaft material  30 , a center axis of the control wheel  51  is disposed parallel to the center axis of the shaft material  30  and a center axis of the grinding wheel  50 , and the axial feeding of the shaft material  30  is stopped, and the end portion  12  and the whole of the shaft portion  13  are ground externally and outer circumferentially. 
     The outer diameter grinding applied to the end portion  12  and the shaft portion  13  is not limited to the centerless grinding. For example, an external cylindrical grinding can also be used in which the shaft material is supported at its axis at both ends of the shaft material. For the external cylindrical grinding, either of a traverse grinding in which the shaft material  30  is fed in the axial direction as with the through-feed grinding and a plunge grinding in which the axial feeding of the shaft material  30  is stopped as with the infeed grinding may be used. 
     Since the end portion  12  keeps its diameter equal to the diameter of the shaft material  30  which is the diameter of the material of the rack bar blank material  10  and has the outside diameter equal to that of the shaft portion  13 , when the end portion  12  and at least part of the shaft portion  13  are ground at the same time, the end portion  12  and the shaft portion  13  are brought into contact with the grinding wheel uniformly. This can enhance the coaxiality between the end portion  12  on the rack portion  11  side and the end portion  17  on the shaft portion  13  side of the rack bar blank material  10  which is manufactured through the pre-forming step to the grinding step and the straightness of the whole of the rack bar blank material  10 , thereby making it possible to simplify the manufacturing process. 
     In particular, in this embodiment, the bend generated in the shaft material  30  is corrected in the correction step, whereby the end portion  12  and the shaft portion  13 , which are ground externally and outer circumferentially, are brought into a contact with the grinding wheel more uniformly, and this can enhance further the coaxiality between both the end portions  12 ,  17  and the straightness of the whole of the shaft material  30 . 
     From the view point of enhancing the coaxiality between the end portion  12  on the rack portion  11  side and the end portion  17  on the shaft portion  13  side and the straightness of the whole of the shaft material  30 , of the through-feed grinding and the infeed grinding, the infeed grinding is preferable in which the end portion  12  and the whole of the shaft portion  13  are ground externally and outer circumferentially at the same time. 
       FIG. 8  shows an example of a manufacturing method of a rack bar  20 . 
     A rack bar  20  has the rack portion  11 , which is formed in the stage where the rack bar blank material  10  is formed, on an axial end side and the screw groove  21  of the ball screw as another axial direction acting element on the other axial end side thereof, as described above. The screw groove  21  is formed on the outer circumferential surface of the shaft portion  13  of the rack bar blank material  10  through whirling or the like. 
     An annular cutting tool  61  is used in whirling in which a plurality of cutting tips  60  are disposed at constant intervals in a circumferential direction on an inner circumferential portion of the annular cutting tool  61 . The rack bar blank material  10  is inserted through the annular cutting tool  61 , and the end portion  12  on the rack portion  11  side and the end portion  17  on the shaft portion  13  side are supported rotatably by a chuck  62  and a center  63 . The cutting tool  61  is disposed eccentric and inclined with respect to the rack bar blank member  10 . When the cutting tool  61  is rotated, the plurality of cutting tips  60  cut sequentially the outer circumferential surface of the shaft portion  13 , and when the rack bar blank member  10  is rotated and the cutting tool  61  is caused to index in the axial direction of the rack bar blank material  10 , the spiral screw groove  21  is formed on the outer circumferential surface of the shaft portion  13 . 
     Since the coaxiality between the end portion  12  on the rack portion  11  side and the end portion  17  on the shaft portion  13  side of the rack bar blank material  10  and the straightness of the rack bar blank material  10  are enhanced, the run-out of the rack bar blank material  10  which is being rotated with both the end portions  12 ,  17  supported rotatably is prevented. This enhances the forming accuracy of the screw groove  21 , that is, the working accuracy of the rack bar  20 . 
     A direct acting element in the axial direction provided on the shaft portion  13  is not limited to the screw groove  21  of the ball screw and hence may be a rack. A separate hollow or solid shaft material on which a rack is formed in advance is joined to an end face of the shaft portion  13  of the rack bar blank material  10 , whereby a rack is provided on the shaft portion  13 . Then, the separate shaft material and the rack bar blank material  10  can be joined together, for example, through frictional press fitting in which the separate shaft material is pressed against the end face of the shaft portion  13  while rotating the rack bar blank material  10 . Then, since the run-out of the rotating rack bar blank material  10  is suppressed, the coaxiality between the separate shaft material and the rack bar blank material  10  and the straightness of the rack bar are enhanced, that is, the working accuracy of the rack bar is enhanced. 
     Thus, as has been described heretofore, the rack bar blank material disclosed in this description has the rack portion configured to mesh with a pinion in an end side of a hollow shaft material in an axial direction than the rack portion, and an end portion which is provided closer to the end side of the hollow shaft material than the rack portion. The end portion has a diameter which is larger than that of a minimum circle embracing a section of the rack portion which is perpendicular to the axial direction and which is equal to that of a shaft portion at the other end side of the hollow shaft material in the axial direction. 
     In the rack bar blank material disclosed in this description, the rack portion and the shaft portion are hardened except an intermediate portion between the rack portion and the shaft portion. 
     The rack bar disclosed in this description includes an axial direction acting element provided on the shaft portion of the rack bar blank material. 
     In the rack bar disclosed in this description, the direct acting element is a screw groove of a ball screw and is provided on an outer circumferential surface of the shaft portion. 
     The rack bar blank material manufacturing method disclosed in this description includes a pre-forming that forms a small-diameter portion on an end side of a hollow shaft material in an axial direction and an end portion provided closer to the end side of the hollow shaft material in the axial direction than the small-diameter portion and having a diameter which is larger than that of the small-diameter portion and which is equal to that of a shaft portion on the other end side of the hollow shaft material in the axial direction, a tooth forming that forms a rack portion configured to mesh with a pinion on the small-diameter portion, and a grinding that applies an outer diameter grinding to the end portion and the shaft portion, the outer diameter grinding being applied simultaneously to at least part of the shaft portion when the outer diameter grinding is applied to the end portion. 
     The rack bar blank material manufacturing method disclosed in this description includes heat treatment that hardens the rack portion and the shaft portion except an intermediate portion between the rack portion and the shaft portion, after the tooth forming and before the grinding. 
     The rack bar blank material manufacturing method disclosed in this description includes correction that bends the intermediate portion between the rack portion and the shaft portion of a rack bar blank material so as to correct the rack portion and the shaft portion to be straight, before the grinding. 
     In the rack bar blank material manufacturing method disclosed in this description, the correction includes a further bending the connecting portion of the end portion connecting to the rack portion so as to correct the end portion, the rack portion and the shaft portion to be straight in the correction. 
     In the rack bar manufacturing method disclosed in this description, the rack bar manufacturing method includes providing an axial direction acting element on the shaft portion while rotatably supporting the end portion and the shaft portion of the rack bar blank material and rotating the rack bar blank material. 
     In the rack bar manufacturing method disclosed in this description, a screw groove of a ball screw is formed on an outer circumferential surface of the shaft portion as the direct acting element. 
     This application claims priority to Japanese Patent Application No. 2017-202925 filed on Oct. 19, 2017, the entire content of which is incorporated herein by reference.