Patent Publication Number: US-2018043925-A1

Title: Steering rack and method of manufacturing the same

Description:
BACKGROUND OF THE INVENTION 
     Some steering racks are provided with a plurality of steering rack teeth that are either machined or warm/hot formed into their desired shape. The current processes to provide the plurality of steering rack teeth may result in dimensional issues that include form and finish errors due to tool wear and dulling, machine setup variation, and heat treat distortion. Additionally, the forming processes may result in high residual stresses that may cause distortion of the plurality of steering rack teeth. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present disclosure, a method of manufacturing a steering rack is provided. The method includes cutting a plurality of steering rack teeth into a first portion of an elongate member. Each tooth of the plurality of steering rack teeth has a first tooth width and a first tooth height. The method further includes placing the elongate member having the plurality of steering rack teeth into a press. The press has a die and a punch. The die is configured to receive a second portion of the elongate member. The punch is provided with a plurality of tooth cavities that face towards the first portion of the elongate member. 
     According to another embodiment of the present disclosure, a method of manufacturing a steering rack is provided. The method includes providing an elongate member having a first portion disposed opposite the second portion. The method further includes cutting a plurality of steering rack teeth into the first portion of the elongate member. Each tooth of the plurality of steering rack teeth has a first width, a first height, and a first profile. The method still further includes upsetting the plurality of steering rack teeth with a press. The plurality of steering rack teeth are upset such that each tooth of the plurality of steering rack teeth has a second width greater than the first width, a second height less than the first height, and a second profile different from the first profile. 
     According to yet another embodiment of the present disclosure a steering rack is provided. The steering rack includes an elongate member, a plurality of steering rack teeth, and a ramp function. The elongate member has a first portion disposed opposite a second portion. The plurality of steering rack teeth are cut and formed into the first portion. Each tooth of the plurality of steering rack teeth has a substantially constant height across a length of the elongate member. The ramp function is formed into the second portion of the elongate member. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter of the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a partial perspective view of a steering rack; 
         FIG. 2  is a perspective view of a tooth of the plurality of steering rack teeth prior to forming and after forming; 
         FIG. 2 a    is a perspective view of a first portion of an elongate member body of the steering rack; 
         FIG. 3  is a section view of a rack tooth showing a tooth shape prior to forming and after forming; 
         FIG. 4  is a partial perspective view of adjacent teeth of the plurality of steering rack teeth after forming; 
         FIG. 5  is an illustrative view of a press; 
         FIG. 6  is an illustrative view of a punch of the press; 
         FIG. 7  is an illustrative view of a die of the press; and 
         FIG. 8  is a flowchart of a method of manufacturing a steering rack. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to a steering rack and a method of manufacturing a steering rack. 
     Referring now to the Figures, where the present disclosure will be described with reference to specific embodiments, without limiting same, it is to be understood that the disclosed embodiments are merely illustrative of the present disclosure that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present disclosure. 
     Referring to  FIG. 1 , a partial perspective view of a steering rack  10  is shown. The steering rack  10  is formed from an elongate member  20  having an elongate member body  22  extending along an axis between a first elongate member end  24  and a second elongate member end  26 . The elongate member  20  may be but need not be a cylindrical bar. 
     The elongate member body  22  includes a first portion  30  and a second portion  32 . The first portion  30  is disposed opposite the second portion  32 . The first portion  30  includes a plurality of steering rack teeth  40 . The plurality of steering rack teeth  40  are disposed proximate the first elongate member end  24 . The plurality of steering rack teeth  40  are spaced apart from the first elongate member end  24 . The second portion  32  includes a ramp function  42 . 
     Referring to  FIGS. 1-4 , the plurality of steering rack teeth  40  are cut and formed into the first portion  30 . An initial form of the plurality of steering rack teeth  40  are cut into the first portion  30  of the elongate member body  22  by a cutting process such as broaching, milling, or the like. Each tooth of the initial form of the plurality of steering rack teeth  40  includes a first width, Tto, a first height, Ho, a first profile, a bottom thickness, Tb, and a first tooth length (Lo). In at least one embodiment, each tooth of the initial form of the plurality of steering rack teeth  40  are cut at an angle relative to the axis along which the elongate member  20  extends. 
     The first width, Tto, is measured at a tip  50  of the tooth of the initial form of the plurality of steering rack teeth  40 . The first height, Ho, is measured from the tip  50  of the tooth of the initial form of the plurality of steering rack teeth  40  to a root or base  52  of the initial form of the plurality of steering rack teeth  40 . The first profile cut into the plurality of steering rack teeth  40  of the first portion  30  of the elongate member body  22  may include a variable pitch, helix angle, or twist such that the steering rack  10  may have a variable ratio plurality of steering rack teeth or a straight ratio plurality of steering rack teeth. 
     A final form of the plurality of steering rack teeth  40  are formed into the first portion  30  of the elongate member body  22  by a cold upsetting process. The cold upsetting process forms the initial form of the plurality of steering rack teeth  40  into the final form of the plurality of steering rack teeth  40 . The cold upsetting process forms the final form of the plurality of steering rack teeth  40  at ambient shop temperatures thereby avoiding heating the elongate member  20  prior to forming. Each tooth of the final form of the plurality of steering rack teeth  40  after forming includes a second width, Ttf, a second height, Hf, a second profile, a bottom thickness, Tb, and a second tooth length (Lf). 
     The second width, Ttf, measured at the tip  50  of the tooth of the final form of the plurality of steering rack teeth is greater than or equal to the first width, Tto, measured at the tip  50  of the initial form of the plurality of steering rack teeth  40 . The second height, Hf, measured from the tip  50  of the tooth of the final form of the plurality of steering rack teeth  40  to the base  52  of the final form of the plurality of steering rack teeth  40  is less than the first height, Ho, measured from the tip  50  of the tooth of the initial form of the plurality of steering rack teeth  40  to the base  52  of the initial form of the plurality of steering rack teeth  40 . For example, the first height, Ho, of the initial form of the plurality of steering rack teeth  40  may be 110% to 130% of the second height, Hf, of the final form of the plurality of steering rack teeth  40 . The second height, Hf, of the final form of the plurality of steering rack teeth  40  is substantially constant across a length of the elongate member  20  that includes the final form of the plurality of steering rack teeth  40 . 
     The second profile formed into the final form of the plurality of steering rack teeth  40  of the first portion  30  of the elongate member body  22  may be different from the first profile cut into the initial form of the plurality of steering rack teeth  40 . The cold upsetting process may twist some teeth of the plurality of steering rack teeth  40  to achieve a final pitch and/or helix angle. The profile of the plurality of steering rack teeth  40  determines whether the steering rack  10  includes straight ratio or variable ratio steering rack teeth. 
     The bottom thickness, Tb, of the initial form of the plurality of steering rack teeth  40  is substantially similar to or the same as the bottom thickness, Tb, of the final form of the plurality of steering rack teeth  40 . As such, the bottom thickness of each tooth of the plurality of steering rack teeth  40  remains substantially constant during the cold upsetting process. 
     The volume of the first configuration of each tooth must be sufficiently large that material can flow along the tooth flank profiles (established by the tooth forming die) in a manner such that the teeth widen while accurate tooth flank surfaces are formed. Consequently, it may be necessary to add up to 30% extra volume to the cut teeth above and beyond the volume of material contained in theoretically accurate teeth that would be machined into a circular rack blank. In other words, the cut tooth forms must include extra material to flow transversely in the die cavity while being adequately and accurately shaped by the die surfaces. This extra material benefits the strength and function of the rack teeth by enabling expansion the face width of the teeth during the forming process. 
     Referring to  FIGS. 1-4 , the ramp function  42  of the second portion  32  of the elongate member body  22  of the elongate member  20  is formed as a semi-circular surface form disposed opposite the plurality of steering rack teeth  40 . In at least one embodiment, the ramp function  42  may have other geometric features or shapes that enhance supporting of the steering rack  10 . The ramp function  42  is formed into the second portion  32  during the cold upsetting process. The ramp function  42  configures the second portion  32  of the elongate member body  22  of the elongate member  20  as a steering rack bearing contact surface. 
     The ramp function  42  of the second portion  32  of the elongate member body  22  may have an arcuate profile. The ramp function  42  provides a maximum thickness of the of the steering rack  10  having the final form of the plurality of steering rack teeth proximate a center tooth  60  of the plurality of steering rack teeth  40 . The maximum thickness of the steering rack  10  is measured from a pitch line  64  of the plurality of steering rack teeth  40  at the center tooth  60  or a tooth space location of the final form of the plurality the steering rack teeth  40  to a point  62  on the ramp function  42  opposite the tip of the center tooth  60  of the final form of the plurality of steering rack teeth  40 . A thickness of the steering rack  10  decreases in a direction that extends away from the center tooth  60  towards either the first elongate member end  24  or the second elongate member end  26  due to the shape of the ramp function  42 . 
     The ramp function  42  controls the tooth clearances or tooth interference of the final form of the plurality of steering rack teeth  40  when the steering rack  10  is installed with other components of a steering assembly. The ramp function  42  controls the tooth clearances of the final form of the plurality of steering rack teeth  40  without crowning of the final form of the plurality of steering rack teeth  40  because the final form of the plurality of the steering rack teeth  40  may be designed to have an uncompensated or flat pitch plane across a length of the steering rack  10 . 
     Referring to  FIGS. 1 and 4 , the plurality of steering rack teeth  40  have a substantially constant height, H, or operating position when the steering rack  10  is disposed against its functional opposite, i.e. pinion. The height, H, is measured from the pitch line  64  of the plurality of steering rack teeth  40  to a tip  50  of a tooth of the plurality of steering rack teeth  40 . Adjacent teeth of the plurality of steering rack teeth  40  are spaced apart from each by a desired space width, Wds. 
     Referring to  FIGS. 5-7 , the cold upsetting process may be performed by a press  70 . The press  70  includes a punch  72  and a die  74  oriented relative to the punch  72 . The die  74  is configured to receive the second portion  32  of the elongate member body  22  of the elongate member  20 . The die  74  orients the first portion  30  of the elongate member body  22  of the elongate member  20  having the initial form of the plurality of steering rack teeth  40  towards the punch  72 . 
     The punch  72  is configured to move toward the first portion  30  of the elongate member body  22  of the elongate member  20  having the initial form of the plurality of steering rack teeth  40  to form the final form of the plurality of steering rack teeth  40 . The punch  72  is provided with a plurality of tooth cavities  80 . The plurality of tooth cavities  80  face towards the initial form of the plurality of steering rack teeth  40  cut into the first portion  30  of the elongate member body  22  of the elongate member  20  that is disposed on the die  74 . 
     The plurality of tooth cavities  80  correspond to the final form of the plurality of steering rack teeth  40 . The plurality of tooth cavities  80  are configured to form the initial form of the plurality of steering rack teeth  40  into the final form of the plurality of steering rack teeth  40 . The plurality of tooth cavities  80  are provided with a second profile different from the first profile of the initial form of the plurality of steering rack teeth  40 . 
     The punch  72  is attached to a press ram  90 . The press ram  90  is configured to selectively lower and raise the punch  72  relative to the die  74 . The press ram  90  lowers the punch  72  having the plurality of tooth cavities  80  towards the die  74 . The plurality of tooth cavities is  80  are configured to receive the initial form of the plurality of steering rack teeth  40  to cold upset the initial form of the plurality of steering rack teeth  40  into the final form of the plurality of steering rack teeth  40 . The plurality of tooth cavities  80  of the punch  72  cold coins each tooth of the initial form of the plurality of steering rack teeth  40  such that each tooth flows within each respective tooth cavity to form each tooth into the final form. 
     In at least one embodiment, the plurality of tooth cavities  80  of the punch  72  sculpts corners and edges of each tooth of the plurality of steering rack teeth  40 . The plurality of tooth cavities  80  provides corner or edge relief as the punch  72  cold upsets the initial form of the plurality of steering rack teeth  40  to the final form of the plurality of steering rack teeth  40 . The corner or edge relief forming reduces locally concentrated stresses proximate the tip  50  of each tooth of the final form of the plurality of steering rack teeth  40 . 
     The die  74  receives the second portion  32  of the elongate member body  22  of the elongate member  20 . The die  74  defines a groove  100  that receives the second portion  32  of the elongate member body  22  of the elongate member  20 . The groove  100  may be configured as a semi-circular groove or another advantageous semi-circular surface form or non-circular surface form. The groove  100  is configured to form the ramp function  42  onto or into the second portion  32  of the elongate member body  22  of the elongate member  20  during the cold upsetting process. 
     The steering rack  10  is manufactured by a process illustrated by the flowchart of  FIG. 8 . At block  200 , the elongate member  20  is provided. At block  202 , the initial form of the plurality of steering rack teeth  40  are cut into the first portion  30  of the elongate member body  22  of the elongate member  20 . 
     At block  204 , the elongate member  20  having the initial form of the plurality of steering rack teeth  40  cut into the first portion  30  is placed into a press  70 . The second portion  32  of the elongate member body  22  of the elongate member  20  is placed into the groove  100  of the die  74  of the press  70 . The first portion  30  of the elongate member body  22  of the elongate member  20  faces towards the punch  72  of the press  70 . 
     At block  206 , the punch  72  having the plurality of tooth cavities  80  is lowered towards the first portion  30  of the elongate member body  22  of the elongate member  20  having the initial form of the plurality of steering rack teeth  40 . At block  208 , the initial form of the plurality of steering rack teeth  40  are cold upset within a corresponding cavity of the plurality of tooth cavities  80  of the punch  72  into the final form of the plurality of steering rack teeth  40 . Substantially simultaneously with the cold upsetting of the initial form of the plurality of steering rack teeth  40 , the groove  100  of the die  74  sculpts or forms the ramp function  42  into the second portion  32  of the elongate member body  22  of the elongate member  20 . 
     The punch  72  and the die  74  are able to shape the initial form of the plurality of steering rack teeth  40  into the final form of the plurality steering rack teeth  40  without generating high press loads or die stresses due to the cutting of the initial form of the plurality steering rack teeth  40  prior to pressing. 
     While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure may be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments or combinations of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description.