Abstract:
Barrel-shaped pinions and methods for their engagement with racks are provided allowing for a larger margin of inaccuracy between the rack and pinion gear set avoiding the necessity of costly, high-precision manufacturing processes. The generally barrel-shaped pinion has an outer toothed surface extending axially along a portion of the pinion. The toothed outer surface has first and second ends and a middle section. A diameter of the middle section is larger than a diameter of both the first and second ends. Also provided is a dual-pinion rack gear system utilizing a first driven pinion, a second generally barrel-shaped drive pinion and a rack. Teeth of the first driven pinion and teeth of the second drive pinion mesh with teeth of the rack at two separate locations.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to a barrel-shaped pinion and method for its engagement with a rack.  
           [0002]    Pinion-driven electrical steering gears are frequently used in vehicles. Both single-pinion and dual-pinion electrical steering gears are utilized in combination with a rack. The prior art has utilized cylindrically-shaped pinions for both single and dual-pinion arrangements.  
           [0003]    The dual-pinion gear has one pinion that is driven by an electrical motor (“the driven pinion”) and a second pinion that is connected to the steering wheel via a coupling and a column (“the drive pinion”). Both the teeth of the driven pinion and the teeth of the drive pinion are in meshing engagement with the teeth of the rack at different locations along the rack. The driven pinion leads the rack and causes axial movement of the rack. Different package situations sometimes require alignment of each of the dual-pinions at an angle to each other around the circumference of the rack.  
           [0004]    The tolerance for this angle is partly dependent upon the tolerances of the teeth at each rack location which are engaged with the respective teeth of each dual-pinion. Additionally, the tolerance for this angle is dependent upon the axial machine tolerances of the pinion tower housings which hold each pinion. If the pinion tower housings are not machined straight, the angle between the pinions changes. When this occurs, the clearance between the teeth of the rack and the teeth of each pinion within an improperly machined pinion tower housing decreases because the pinion&#39;s centerline is no longer perpendicular to the rack. Therefore, the clearance varies as the pinion rotates due to the uneven alignment along the rack.  
           [0005]    Because of this, the angle of the dual-pinions with respect to each other around the circumference of the rack must be very tight in order to avoid a misaligned pinion/rack gear set. It is important to avoid such misalignment because such can lead to the detrimental effects of pinion torque increase, bad return, worsened peak-to-peak variation and noise vibration harshness problems such as knocking.  
           [0006]    The current manufacturing processes for the tooth system on a rack are broaching, grinding and forming. After these processes a heat treatment process is performed to harden the teeth, and also sometimes to harden the rack. The tension/stress during the heat treatment process often generates torsion and bending of the rack. After the heat treatment process the rack undergoes a straightening process to remove the torsion and bending.  
           [0007]    On a rack with only one tooth system area the above referenced manufacturing processes are practicable without extraordinary efforts or expenses. However on a rack with two tooth system areas, and in particular on a rack with two tooth system areas at an angle to each other around the circumference of the rack sometimes having different teeth ratios, the manufacturing processes need to be extremely accurate. To maintain this accuracy during manufacturing of the pinions and rack tight tolerance ranges are required. This necessitates the use of high-expense, time-consuming, high-manpower manufacturing processes and high-quality measuring equipment.  
           [0008]    A new pinion design is required which allows a larger margin of inaccuracy between the rack and pinion gear set to avoid the necessity of costly, high-precision manufacturing processes.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    It is in general an object of the invention to provide a barrel-shaped pinion and method for its engagement with a rack.  
           [0010]    In one aspect this invention provides a pinion comprising a generally barrel-shaped toothed outer-surface extending axially along a portion of the pinion. The toothed outer surface has first and second ends and a middle section. A diameter of the middle section is larger than a diameter of the first end, and larger than a diameter of the second end.  
           [0011]    In another aspect this invention provides a dual-pinion rack gear system. The system comprises a first driven pinion driven by an electrical motor, a second drive pinion connected to a steering wheel and a rack. The first driven pinion has a toothed outer-surface. The second drive pinion has a generally barrel-shaped toothed outer-surface extending axially along a portion of the second drive pinion. The rack has a toothed outer-surface. The teeth of the first driven pinion mesh with the teeth of the rack at a first location and the teeth of the second drive pinion mesh with the teeth of the rack at a second location.  
           [0012]    In yet another aspect this invention provides a method for engaging a pinion with a rack. The method comprises initially providing a first pinion having a generally barrel-shaped toothed outer-surface having first and second ends and a middle section. A diameter of the middle section is larger than a diameter of the first end and larger than a diameter of the second end. Next a rack is provided having a toothed outer-surface. Finally the toothed generally barrel-shaped outer-surface of the first pinion is meshed with the toothed outer-surface of the rack at a first location.  
           [0013]    The present invention together with further objects and advantages will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a front view of the prior art for a single cylindrical pinion;  
         [0015]    [0015]FIG. 2 is a perspective view of the prior art for a dual cylindrical pinion-rack system wherein the dual-pinions are aligned parallel to one another on a circumference of the rack at an installation angle of 0° measured from P-P which is perpendicular to the rack;  
         [0016]    [0016]FIG. 2A is a front view of the pinion  34  shown in FIG. 2;  
         [0017]    [0017]FIG. 2B is a front view of the pinion  30  shown in FIG. 2;  
         [0018]    [0018]FIG. 3 is a perspective view of the prior art for a dual cylindrical pinion-rack system wherein the dual-pinions are aligned at an angle to each other on a circumference of the rack;  
         [0019]    [0019]FIG. 3A is a front view of the pinion  86  shown in FIG. 3;  
         [0020]    [0020]FIG. 3B is a front view of the pinion  82  shown in FIG. 3;  
         [0021]    [0021]FIG. 4 is a perspective view of the prior art for a cylindrical pinion aligned at an angle to a rack;  
         [0022]    [0022]FIG. 4A is a sectional view along F-F in FIG. 4;  
         [0023]    [0023]FIG. 4B is a close-up view of the section shown in circle V in FIG. 4A;  
         [0024]    [0024]FIG. 4C is a sectional view along G-G in FIG. 4;  
         [0025]    [0025]FIG. 4D is a close-up view of the section shown in circle T in FIG. 4C;  
         [0026]    [0026]FIG. 5 is a front view of an embodiment of the present invention for a single barrel-shaped pinion;  
         [0027]    [0027]FIG. 6 is a perspective view of an embodiment of the present invention for a dual barrel-shaped pinion-rack system wherein the dual-pinions are aligned parallel to one another;  
         [0028]    [0028]FIG. 6A is a front view of the pinion  190  shown in FIG. 6;  
         [0029]    [0029]FIG. 6B is a front view of the pinion  194  shown in FIG. 6;  
         [0030]    [0030]FIG. 7 is a perspective view of an embodiment of the present invention for a dual barrel-shaped pinion-rack system wherein the dual-pinions are aligned at an angle to each other;  
         [0031]    [0031]FIG. 7A is a front view of the pinion  246  shown in FIG. 7;  
         [0032]    [0032]FIG. 7B is a front view of the pinion  242  shown in FIG. 7;  
         [0033]    [0033]FIG. 8 is a perspective view of an embodiment of the present invention for a barrel-shaped pinion aligned at an angle to a rack;  
         [0034]    [0034]FIG. 8A is a sectional view along J-J in FIG. 8 when the rack is at a first position turned zero degrees around its centerline;  
         [0035]    [0035]FIG. 8B is a close-up view of the section shown in circle R in FIG. 8A;  
         [0036]    [0036]FIG. 8C is a sectional view along K-K in FIG. 8 when the rack is at a second position turned one degree around its centerline;  
         [0037]    [0037]FIG. 8D is a close-up view of the section shown in circle S in FIG. 8C;  
         [0038]    [0038]FIG. 9 is a perspective view of a D-Shaped Rack Design utilizing an embodiment of the present invention;  
         [0039]    [0039]FIG. 10 is a perspective view of a Y-Shaped Rack Design utilizing an embodiment of the present invention;  
         [0040]    [0040]FIG. 10A is another perspective view of the embodiment shown by FIG. 10;  
         [0041]    [0041]FIG. 11 is a flow diagram illustrating one exemplary method in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]    Referring to the drawings, FIGS.  1 - 4  show various forms of the prior art utilizing cylindrical pinion-rack systems. As shown in FIG. 1, a cylindrical pinion  10  includes a generally cylindrically-shaped outer surface  14  extending axially along the pinion  10 . The outer surface  14  has teeth  18  extending in a helical shape around its circumference  22 . The helix angle h may range from 0° to 45° as measured from the centerline C-C of the pinion  10 .  
         [0043]    [0043]FIGS. 2, 2A and  2 B show a dual cylindrical pinion-rack system  26  having a first cylindrical drive pinion  30  connected to a steering wheel (not shown) and a second cylindrical driven pinion  34  driven by an electrical motor (not shown). The dual-pinions  30  and  34  are aligned parallel to one another on a circumference of a rack  58  at an installation angle of 0° measured from P-P which is perpendicular to the rack  58 . The first cylindrical drive pinion  30  includes a generally cylindrically-shaped outer surface  36  extending axially along the drive pinion  30 . The outer surface  36  has teeth  38  extending around its circumference  42 . The teeth  38  have a helix angle h which may range from 0° to 45° as measured from the centerline of the pinion  30 . Similarly the second cylindrical driven pinion  34  includes a generally cylindrically-shaped outer surface  46  extending axially along the driven pinion  34 . The outer surface  46  has teeth  50  extending around its circumference  54 . The teeth  50  have a helix angle h which may range from 0° to 45° as measured from the centerline of the pinion  34 . An elongated rack  58  has first and second outer surfaces  62  and  64  extending axially along the rack  58 . The first and second outer surfaces  62  and  64  of the rack  58  each have teeth  66  and  68  extending axially along the rack  58 . The teeth  66  and  68  of the first and second outer surfaces  62  and  64  in this embodiment may have different teeth-to-rack circumference ratios. The teeth  38  of the first cylindrical drive pinion  30  mesh with the teeth  66  of the first outer surface  62  of the rack  58  at a first location  70 . The teeth  50  of the second cylindrical driven pinion  34  mesh with the teeth  68  of the second outer surface  64  of the rack  58  at a second location  74 .  
         [0044]    [0044]FIGS. 3, 3A and  3 B are identical to FIGS. 2, 2A and  2 B with the exception that they show a dual cylindrical pinion-rack system  78  having a first cylindrical drive pinion  82  angled at degrees with respect to a second cylindrical driven pinion  86  on the circumference of a rack  90 . The angle of degrees requires an even tighter tolerance than required in FIG. 2 at the first and second locations  94  and  98  where the teeth  102  of the first cylindrical drive pinion  82  and the teeth  106  of the second cylindrical driven pinion  86  engage the teeth  110  and  112  of the rack  90 . The teeth  110  and  112  at the first and second locations  94  and  98  must be machined in two steps. The angle tolerance at these first and second locations  94  and  98  must be extremely tight to avoid a negative effect in steering performance. Further, the tolerance range is made even tighter from additional tolerance limitations due to the installation of the housing towers (not shown) which hold the pinions  82  and  86 . These tolerance requirements necessitate complex and expensive machining during manufacture of the rack  90 .  
         [0045]    [0045]FIG. 4 shows a cylindrical pinion  114  aligned at an installation angle i on the circumference of a rack  118 . The pinion  114  has teeth  126  having a helix angle h ranging from 0° to 45° from the pinion&#39;s centerline C-C. FIG. 4A shows a sectional view F-F of the meshing of the teeth  122  of the rack  118  with the teeth  126  of the pinion  114  when the rack  118  is at a first position turned zero degrees (0°) around its centerline RC. FIG. 4B shows the clearance  128  between the teeth  122  of the rack  118  and the teeth  126  of the pinion  114  at the first position shown by circle V in Sectional view F-F. FIG. 4C shows a sectional view G-G of the meshing of the teeth  122  of the rack  118  with the teeth  126  of the pinion  114  when the rack  118  is rotated one degree (1°) around its centerline RC. FIG. 4D shows the clearance  130  between the teeth  122  of the rack  118  and the teeth  126  of the pinion  114  at the Sectional view G-G shown by circle T when the rack is rotated one degree (1°) around its centerline RC.  
         [0046]    In order to determine the tolerance of the angle between dual-pinions around the circumference of a rack, the tolerances of each pinion tower&#39;s housing and the tolerances of the teeth of the rack at each meshing location must be approximated and combined. To simulate the sum of these tolerances, the clearance between the teeth of the rack and the teeth of the pinion when the rack is rotated one degree (1°) around its centerline has been determined. The rotation of the rack around its centerline RC simulates the machining tolerances of the toothed locations on the circumference of the rack. The testing has shown that the clearance  128  between the teeth  122  of the rack  118  and the teeth  126  of the pinion  114  at the first position, when the rack has been turned zero degrees (0°) around its centerline C, is typically two to three times larger than the clearance  130  between the teeth  122  of the rack  118  and the teeth  126  of the pinion  114  when the rack is rotated one degree (1°) around its centerline C.  
         [0047]    The decrease in clearance as the rack  118  rotates around its centerline C is caused by the teeth  122  of the rack  118  moving closer to the teeth  126  of the pinion  114  due to the cylindrical shape of the pinion&#39;s outer surface  134  and the uneven alignment of the teeth  122  of the rack  118  with the centerline of the pinion  114 . This occurs because the pinion&#39;s outer surface  134  is cylindrically-shaped keeping the teeth  126  of the pinion  114  at a constant position despite the teeth  122  of the rack  118  being forced to move closer to the teeth  126  of the pinion  114  due to the rotation of the rack  118  and the uneven alignment.  
         [0048]    On a rack with two tooth system areas engaged with dual-pinions the decrease in clearance upon rotation of the rack requires the manufacturing processes to be very accurate to keep the tolerances within the range required. Further effecting the tolerance range is an effect referred to as “rack roll” which is caused by the meshing of a helical shaped pinion with a rack. During “rack roll”, the rack turns slightly along its centerline during the rack&#39;s axial movement due to a lack of restraint of the rack. This occurs due to machining tolerances and heat treatment distortion resulting in differing pressure angles, differing helix angles and differing diameters of the rack and pinions. It may occur in single and dual-pinion electric systems, electric hydraulic systems, hydraulic systems and in manual steering gears. Furthermore, in a dual-pinion system the tolerance range is altered by potential variance in the desired location of the teeth of the rack at both the first and second locations due to machining tolerances such as the accuracy of the clamping during manufacturing of the rack. As a result, time-consuming, complicated and expensive manufacturing processes are required.  
         [0049]    The present invention discloses a generally barrel-shaped pinion to alleviate the clearance problem caused by the cylindrically-shaped pinions of the prior art. A generally barrel-shaped pinion is defined as a pinion which has a generally arcuate axially extending outer surface. To further define a generally barrel-shaped pinion, the diameter of the generally barrel-shaped pinion in between the pinion&#39;s two ends is greater than the diameter at either pinion end.  
         [0050]    [0050]FIG. 5 shows a generally barrel-shaped pinion  138 . The barrel-shaped pinion  138  includes a generally barrel-shaped outer surface  142  extending axially along a portion  146  of the pinion  138 . The outer surface  142  has generally barrel-shaped teeth  150  corresponding to or complimenting the shape of the outer surface  142  and extending in a generally helical shape around its circumference  154 . The helix angle h of the teeth  150  may range from 0° to 45° measured from the pinion&#39;s centerline C-C. The teeth  150  and the outer surface  142  define grooves  152  running between each set of teeth  150 . The outer surface  142  has a first end  158 , a second end  162  and a middle section  166 . A diameter  174  of the middle section  166  is larger than both a diameter  178  of the first end  158  and a diameter  182  of the second end  162 . A cross-section  170  of the outer surface  142  may generally resemble the shape of a rounded barrel in that the cross section  170  is largest at the middle section  166  and continuously decreases towards both the first end  158  and second end  162 . In some embodiments both the diameter  178  of the first end  158  and the diameter  182  of the second end  162  may be identical. In other embodiments the diameter  178  may differ from the diameter  182 .  
         [0051]    The generally barrel-shaped pinion  138  is preferably manufactured of steel using any one of a number of different processes such as a 3D-hobbing process, a forming process or a grinding process.  
         [0052]    [0052]FIGS. 6, 6A and  6 B show a dual-pinion rack gear system  186  incorporating a generally barrel-shaped pinion  194 . The dual-pinion rack gear system  186  has a first generally cylindrical driven pinion  190  driven by an electrical motor (not shown) and a second generally barrel-shaped drive pinion  194  connected to a steering wheel (not shown). In this embodiment the first generally cylindrical driven pinion  190  and the second generally barrel-shaped drive pinion  194  are arranged parallel to each other along the rack  218 . In other embodiments the first generally cylindrical driven pinion  190  and the second generally barrel-shaped drive pinion  194  may be oriented at an angle to each other along the rack  218 .  
         [0053]    The first generally cylindrical pinion  190  includes a generally cylindrically-shaped outer surface  198  extending axially along the driven pinion  190 . The outer surface  198  has teeth  202  extending in a helical shape around its circumference  206 . The second generally barrel-shaped drive pinion  194  includes a generally barrel-shaped outer surface  208  extending axially along the drive pinion  194 . Teeth  210  extend across the outer surface  208  in a generally helical shape around the pinion&#39;s circumference  214 . For both the teeth  202  of the first generally cylindrical pinion  190  and the teeth  210  of the second generally barrel-shaped drive pinion  194 , the helix angle h measured from each pinion&#39;s centerline C-C may range from 0° to 45°. The elongated rack  218  contains first and second outer surfaces  222  and  224  extending radially and axially across the rack  218 . Teeth  226  and  228  extend axially along the first and second outer surfaces  222  and  224  of the rack  218 . The teeth  226  and  228  may be perpendicular to the centerline of the rack or at varying helix angles measured from the centerline, and may have differing tooth to rack circumference ratios. The teeth  202  of the first cylindrical driven pinion  190  mesh with the teeth  226  of the rack  218  at a first location  230 . At a second location  234  the teeth  210  of the second generally barrel-shaped drive pinion  194  mesh with the teeth  228  of the rack  218 .  
         [0054]    [0054]FIGS. 7, 7A and  7 B show a dual-pinion rack gear system  238  incorporating a generally barrel-shaped pinion  242  aligned at an angle À to a cylindrical driven pinion  246  with respect to the circumference of a rack  250 . The angle À may vary. The pinions  242  and  246  may be aligned in varying angles with respect to each other and may be aligned in similar or opposite directions. Further the pinions  242  and  246  may be aligned at any locations along the circumference of the rack  250 . The teeth  254  of the first cylindrical driven pinion  246  mesh with teeth  258  of the rack  250  at a first location  262 . Likewise at a second location  266  the teeth  270  of the second generally barrel-shaped drive pinion  242  mesh with teeth  260  of the rack  250 . The teeth  254  and  270  of the pinions  246  and  242  extend in a generally helical shape across the circumference of the pinions and may have varying helix angles h ranging from zero (0°) to forty-five (45°) degrees measured from the pinions&#39; centerlines. The teeth  258  of the rack  250  at the first location  262  may be angled with respect to the teeth  260  of the rack at the second location  266  and may have varying tooth to pinion circumference ratios. Further, the first and second locations  262  and  266  may be offset radially and/or axially.  
         [0055]    For clearance comparison purposes between the cylindrical pinions of the prior art and the generally barrel-shaped pinion of the instant invention, FIG. 8 shows a generally barrel-shaped pinion  274  aligned at an installation angle i to a rack  278 . FIG. 8A shows a sectional view J-J of the meshing of the teeth  282  of the rack  278  with the teeth  286  of the pinion  274  when the rack  278  is at a first position turned zero degrees (0°) around its centerline C. FIG. 8B shows the clearance  290  between the teeth  282  of the rack  278  and the teeth  286  of the pinion  274  at the first position shown by circle R in Sectional view J-J. FIG. 8C shows a sectional view K-K of the meshing of the teeth  282  of the rack  278  with the teeth  286  of the pinion  274  when the rack  278  is rotated one degree (1°) around its centerline C. FIG. 8D shows the clearance  294  between the teeth  282  of the rack  278  and the teeth  286  of the pinion  274  at the Sectional view K-K shown by circle S when the rack is rotated one degree (1°) around its centerline C.  
         [0056]    The clearance results for the generally barrel-shaped pinion are much improved over the previously discussed clearance results for the generally cylindrically-shaped pinions of the prior art. For example in some embodiments testing has established that the clearance  290  between the teeth  282  of the rack  278  and the teeth  286  of the pinion  274  at the first position, when the rack has been turned zero degrees (0°) around its centerline C, is generally identical to the clearance  294  between the teeth  282  of the rack  278  and the teeth  286  of the pinion  274  when the rack  278  is rotated one degree (1°) around its centerline C. This is due to the pinion&#39;s outer surface  298  being generally barrel-shaped reducing the cross section  302  of the pinion  274  as it gets further away from the middle section  306 , while conversely the teeth  282  of the rack  278  are forced to move closer to the teeth  286  of the pinion  274  as the rack  278  rotates.  
         [0057]    The improved clearance results avoid the necessity of high expense manufacturing processes during formation of the rack by reducing the tolerance level required during manufacturing. Small radial angle differences in the teeth of the pinion and rack are thus accommodated for by the generally barrel-shaped pinion. These radial angle differences may occur due to small radial misalignments between the pinion and the rack, slight radial movements of the rack around its centerline because of meshing interference with the pinion, or incorrect machining of the pinion and/or rack. Further, the improved clearance results accommodate for rack roll effects and variance in the desired locations of teeth on the rack at both first and second locations due to machining tolerances.  
         [0058]    [0058]FIG. 9 shows a D-shaped rack design  310  utilizing a barrel-shaped pinion  314  across a circumference of the D-shaped rack  318 . The rack  318  is referred to as being D-shaped because it has a first generally elongated outer surface  322  and a second generally cylindrical outer surface  326  forming a D. As the pinion  314  rotates  330  with respect to its centerline C-C the rack  318  moves axially  338  causing the rack  318  to experience a rack roll effect  342 . However, use of the barrel-shaped pinion  314  accommodates for the rack roll effect  342 .  
         [0059]    [0059]FIGS. 10 and 10A show a Y-shaped rack design  346  utilizing a barrel-shaped pinion  350  across a circumference of the Y-shaped rack  354 . The rack design  346  is referred to as being Y-shaped because it is in the shape of a Y and slides axially in a Y-shaped yoke housing. The Y-shaped yoke housing prevents the Y-shaped rack  354  from experiencing the rack roll effect by restraining the rack  354  from rotating around its centerline. The barrel-shaped pinion  350  is useful in preventing high pinion torques, bad return, and prevents the necessity for heightened steering efforts because potential inequalities in the gear set are compensated by the barrel-shaped pinion  350 .  
         [0060]    As shown in FIG. 11 one method for engaging a pinion with a rack is to first provide a generally barrel-shaped pinion  358 . The pinion preferably comprises a toothed outer-surface having first and second ends and a middle section. A diameter of the middle section is preferably larger than a diameter of the first end and a diameter of the second end. A second pinion having a generally cylindrically-shaped toothed outer surface may be provided  362 . Next a rack is provided  366 . The rack preferably has a toothed outer-surface. Finally the teeth of the generally barrel-shaped outer-surface of the pinion are meshed with the teeth of the outer-surface of the rack at a first location  370 . The teeth of the generally cylindrically-shaped outer surface of the second pinion may also be meshed with the teeth of the outer-surface of the rack at a second location  374 . The generally barrel-shaped pinion may be aligned parallel to the second pinion along the circumference of the rack, or may be aligned at varying angles in any direction with respect to the second pinion along the circumference of the rack.  
         [0061]    It is to be understood that the invention is not to be limited to the exact construction and/or method which has been illustrated and discussed above, but that various changes and/or modifications may be made without departing from the spirit and the scope of the invention.