Abstract:
A rack and pinion steering apparatus ( 10 ) comprises a housing ( 12 ). Helical teeth ( 60 ) of a pinion gear ( 54 ) meshingly engage teeth of a rack bar ( 42 ) and, during rotation of the pinion gear ( 54 ), result in an axial force. A shaft ( 86 ) connects with the pinion gear ( 54 ). Rotation of the shaft ( 86 ) effects rotation of the pinion gear ( 54 ). An assembly is interposed between the shaft ( 86 ) and the housing ( 12 ). The assembly comprises a first snap ring ( 142 ) that is attached to the shaft ( 86 ) and a second snap ring ( 144 ) that is attached to the housing ( 12 ). Interposed between the first and second snap rings ( 142  and  144 ) are only a fluid tight seal ( 146 ) for blocking fluid leakage from the housing ( 12 ) and a bushing ( 108 ) for enabling rotation of the shaft ( 86 ) relative to the housing ( 12 ) and for engaging the first snap ring ( 142 ) to block axial displacement of the pinion gear ( 54 ) due to the axial force acting on the pinion gear ( 54 ).

Description:
TECHNICAL FIELD 
     The present invention relates to a rack and pinion steering gear. More particularly, the present invention relates to a rack and pinion steering gear with a pinion gear having helical teeth. 
     BACKGROUND OF THE INVENTION 
     A known rack and pinion steering gear includes a housing. A rack bar extends longitudinally through the housing and has opposite ends that are connectable with the steerable wheels of the vehicle. A portion of the rack bar extending through the housing includes teeth. As the rack bar is moved linearly relative to the housing, the steerable wheels of the vehicle are turned. 
     The known rack and pinion steering gear also includes a pinion gear. The pinion gear includes a plurality of helical teeth. The pinion gear is located within the housing so that the helical teeth of the pinion gear are in meshing engagement with the teeth of the rack bar. The pinion gear is rotatable relative to the housing. 
     An input shaft extends partially into the housing. A first end of the input shaft is connectable with the steering wheel of the vehicle. A second end of the input shaft is connected with the pinion gear, generally through a torsion bar. 
     A roller bearing is interposed between the housing and the input shaft. The roller bearing enables rotation of the input shaft relative to the housing. Rotation of the input shaft effects rotation of the pinion gear by either directly rotating the pinion gear or by actuating a power assist motor which moves the rack bar and thus causes rotation of the pinion gear. 
     Rotation of the pinion gear results in a force that is directed along an axis of the input shaft. A pinion nut resists movement of the pinion gear that may result from the axial force. However, if the pinion nut loosens, the axial force may move the pinion gear axially relative to the rack bar and detrimentally affect the meshing engagement of the rack bar and pinion gear. 
     To prevent disengagement of the pinion gear with the rack bar in the event of a loose pinion nut, the known rack and pinion steering gear includes a retaining ring and a capture washer. The retaining ring is seated in a groove in the input shaft and is thus axially fixed to the input shaft. The capture washer is located adjacent the roller bearing and is interposed between the roller bearing and the retaining ring. When an axial force is applied to the input shaft, the input shaft moves axially until the retaining ring contacts the capture washer. The capture washer blocks further axial movement and thus, blocks further axial displacement of the pinion gear. 
     SUMMARY OF THE INVENTION 
     The present invention is a rack and pinion steering apparatus for turning steerable wheels of a vehicle in response to rotation of a steering wheel. The apparatus comprises a housing. A rack bar extends through the housing and is movable relative to the housing for turning the steerable wheels of the vehicle. The rack bar has a plurality of teeth. A pinion gear is located within the housing and is rotatable relative to the housing. The pinion gear includes a plurality of helical teeth that meshingly engage teeth of the rack bar. Engagement of the helical teeth of the pinion gear with the teeth of the rack bar during rotation of the pinion gear in a first direction results in an axial force acting on the pinion gear. A shaft is connectable with the steering wheel of the vehicle and is rotatable relative to the housing. At least a portion of the shaft extends into the housing and connects with the pinion gear. Rotation of the shaft effects rotation of the pinion gear. An assembly is interposed between the shaft and the housing. The assembly includes a first snap ring that is attached to the portion of the shaft that extends into the housing. A second snap ring is attached to the housing. Interposed between the first and second snap rings are only a fluid tight seal for blocking fluid leakage from the housing and a bushing for enabling rotation of the shaft relative to the housing. The bushing includes a radially extending surface for engaging the first snap ring to block axial displacement of the pinion gear due to the axial force acting on the pinion gear. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which: 
     FIG. 1 is a sectional elevation view of a rack and pinion steering gear constructed in accordance with the present invention and showing a first snap ring axially spaced from a powdered metal bushing; 
     FIG. 1A is an enlarged view of a portion of FIG. 1; 
     FIG. 2 is an enlarged view of the section of FIG. 1 labeled FIG. 2; 
     FIG. 3 is a sectional elevation view of the rack and pinion steering gear of FIG. 1 showing a first snap ring contacting a powdered metal bushing; and 
     FIG. 4 is a plan view of the powdered metal bushing for the rack and pinion steering gear of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The figures that are referred to hereinafter illustrate a rack and pinion steering gear  10  having a pinion nut  68  that has loosened. Although the loosening of a pinion nut of a rack and pinion steering gear is a rare occurrence, the rack and pinion steering gear  10  constructed in accordance with the present invention includes provisions for maintaining the integrity of the steering gear should the pinion nut loosen. 
     FIG. 1 is a sectional elevation view of a rack and pinion steering gear  10  constructed in accordance with the present invention. The rack and pinion steering gear  10  of FIG. 1 is a hydraulic power-assisted rack and pinion steering gear. 
     The rack and pinion steering gear  10  includes a housing  12 . The housing  12  includes a first tubular portion  14  that extends axially along axis A. The first tubular portion  14  of the housing  12  has first and second axial ends  16  and  18 , respectively, and cylindrical inner and outer surfaces  20  and  22 , respectively. The inner surface  20  of the first tubular portion  14  of the housing  12  defines a passage  24  that extends axially through the first tubular portion  14 . 
     A first opening  26  of the passage  24  is located on the first axial end  16  of the first tubular portion  14  and is defined by the inner surface  20  of the first tubular portion  14  adjacent the first axial end  16 . A groove  28  extends into the inner surface  20  of the first tubular portion  14  of the housing  12  adjacent the first axial end  16  of the housing  12 . The groove  28  extends around the entire circumference of the inner surface  20  of the first tubular portion  14  of the housing  12 . 
     A second opening  30  of the passage  24  is located at the second axial end  18  of the first tubular portion  14  of the housing  12 . The second opening  30  is defined by the inner surface  20  of the first tubular portion  14  adjacent the second axial end  18 . A radially inwardly extending, annular wall  32  extends from the inner surface  20  of the first tubular portion  14  near the second axial end  18 . 
     The housing  12  also includes a second tubular portion  34 . The second tubular portion  34  of the housing  12  extends perpendicular to the first tubular portion  14  and unites with the first tubular portion near the second axial end  18  of the first tubular portion. The second tubular portion  34  has inner and outer surfaces  36  and  38 , respectively. The inner surface  36  defines a yoke bore  40 . The yoke bore  40  of the second tubular portion  34  mates with the passage  24  of the first tubular portion  14  near the second axial end  18  of the first tubular portion. 
     A longitudinally extending rack bar  42  extends through the housing  12  in a direction perpendicular to axis A. The rack bar  42  has a generally circular cross-sectional shape. The rack bar  42  includes a plurality of helical teeth. Opposite end portions (not shown) of the rack bar  42  are connected with the steerable wheels (not shown) of the vehicle (not shown). Linear movement of the rack bar  42  results in the turning of the steerable wheels of the vehicle. 
     A hydraulic motor  44 , shown schematically in FIGS. 1 and 3, is also formed in the housing  12 . The hydraulic motor  44  includes a piston  46 , which is attached to the rack bar  42 , and two chambers  48  and  50 , one on each side of the piston. As hydraulic fluid pressure in one chamber  48  or  50  increases relative to the hydraulic fluid pressure in the other chamber  50  or  48 , the piston  46  is moved. Movement of the piston  46  causes linear movement of the rack bar  42 . 
     The rack and pinion steering gear  10  also includes a pinion assembly  52 . The pinion assembly  52  includes a pinion gear  54 , a first support portion  56 , and a second support portion  58 . The pinion gear  54  has a plurality of helical teeth  60  for meshingly engaging the teeth of the rack bar  42 . 
     The first support portion  56  of the pinion assembly  52  forms the first axial end of the pinion assembly. The first support portion  56  includes a cylindrical outer surface  62 . An axially extending cavity  64  extends into an end of the first support portion  56 . A hole (not shown) extends radially through the first support portion  56  adjacent the cavity  64 . 
     The second support portion  58  of the pinion assembly  52  forms a second axial end of the pinion assembly. The second support portion  58  has a cylindrical outer surface  66 . An axial end of the cylindrical outer surface  66  of the first support portion  58  is threaded for receiving a pinion nut  68 . 
     The rack and pinion steering gear  10  also includes a directional control valve, shown generally at  70 . The directional control valve  70  includes a valve sleeve  72  and a valve core  74 . The directional control valve  70  is controlled by rotation of the steering wheel to direct hydraulic fluid to one of the chambers  48  or  50  of the hydraulic motor  44 . The hydraulic fluid is supplied to the directional control valve  70  by a pump  76  that draws fluid from a reservoir  78 . 
     The valve sleeve design is known in the art and will not be discussed in detail. The valve sleeve  72  is generally tubular and includes first and second axial ends  80  and  82 , respectively. A hole  84  extends radially through second axial end  82  of the valve sleeve  72 . 
     The valve core  74  is also of known design and will not be discussed in detail. The valve core  74  forms a second axial end  88  of an input shaft  86 . The input shaft  86  also includes first and second cylindrical portions  90  and  92 , respectively and a tapered portion  94  that is interposed between the first and second cylindrical portions. The first cylindrical portion  90  forms a first axial end  96  of the input shaft  86 . The first cylindrical portion  90  includes a generally cylindrical outer surface  98 . The tapered portion  94  of the input shaft  86  has a frustoconical outer surface  100 . The first cylindrical portion  90  of the input shaft  86  connects to a narrow end of the tapered portion  94 . The second cylindrical portion  92  of the input shaft  86  extends between a wide end of the tapered portion  94  and the valve core  74 . The second cylindrical portion  92  includes a generally cylindrical outer surface  102 . A groove  104  (FIG. 2) extends radially inwardly into the cylindrical outer surface  102  of the second cylindrical portion  92  of the input shaft  86 . The groove  104  extends circumferentially around the second cylindrical portion  92 . 
     The rack and pinion steering gear  10  also includes a torsion bar  106 . Only a small portion of the torsion bar  106  is shown in FIG.  1 . The torsion bar  106  includes first and second axial ends (not shown). The second axial end is splined. The first axial end of the torsion bar  106  is rotatable relative to the second axial end of the torsion bar which causes the torsion bar to twist. 
     FIG. 4 shows a plan view of a powdered metal bushing  108  of the rack and pinion steering gear  10 . A partial cross-section of the powdered metal bushing  108  is shown in FIG.  2 . The powdered metal bushing  108  is self-lubricating and is formed of a combination of iron, copper, and carbon. The percentage of iron in the powdered metal bushing ranges from 91.1% to 95.4%. The percentage of copper in the powdered metal bushing  108  ranges from 4.0% to 8.0% and the percentage of carbon ranges from 0.6% to 0.9%. The powdered metal bushing may also include small amounts of other materials, such as magnesium sulfite. The powdered metal bushing  108  is formed by pressing, or compressing, powdered metal in a die having the appropriate shape. 
     The powdered metal bushing  108  is annular and includes radial inner and outer surfaces  110  and  112 , respectively, as shown in FIG.  2 . The inner surface  110  defines the inner diameter of the powdered metal bushing  108 . The inner diameter is slightly larger than the diameter of the second cylindrical portion  92  of the input shaft  86 . The outer surface  112  of the powdered metal bushing  108  defines the outer diameter of the powdered metal bushing and is coaxial with the inner surface  110 . The outer diameter is slightly larger than an inner diameter of the housing  12 . 
     The powdered metal bushing  108  also includes upper and lower surfaces  114  and  116 , respectively. The upper surface  114  includes first and second radially extending surfaces  118  and  120 , respectively, and an axially extending surface  122 . The first and second radially extending surfaces  118  and  120  extend perpendicular to the inner and outer surfaces  110  and  112  of the powdered metal bushing  108 . The first radially extending surface  118  is annular and extends radially inwardly from the outer surface  112  a distance of approximately eighty percent of the distance between the inner and outer surfaces  110  and  112 . The second radially extending surface  120  extends radially outwardly from the inner surface  110  approximately twenty percent of the distance between the inner and outer surfaces  110  and  112 . The axially extending surface  122  extends perpendicular to the first and second radially extending surfaces  118  and  120  and connects the first radially extending surface to the second radially extending surface. The second radially extending surface  120  and the axially extending surface  122  collectively define a first pocket  124  of the powdered metal bushing  108 . 
     The lower surface  116  of the powdered metal bushing  108  also includes first and second radially extending surfaces  126  and  128 , respectively, and an axially extending surface  130 . The first and second radially extending surfaces  126  and  128  extend perpendicular to the inner and outer surfaces  110  and  112 . The first radially extending surface  126  is annular and extends radially inwardly from the outer surface  112  a distance of approximately eighty percent of the distance between the inner and outer surfaces  110  and  112 . The second radially extending surface  128  extends radially outwardly from the inner surface  110  approximately twenty percent of the distance between the inner and outer surfaces  110  and  112 . The axially extending surface  130  extends perpendicular to the first and second radially extending surfaces  126  and  128  and connects the first radially extending surface to the second radially extending surface. The second radially extending surface  128  and the axially extending surface  130  collectively define a second pocket  132  of the powdered metal bushing  108 . 
     A plurality of holes  134  extends through the powdered metal bushing  108 . In the embodiment illustrated in FIG. 4, six holes  134  extend in an axial direction through the powdered metal bushing  108  from the upper surface  114  to the lower surface  116 . Each hole  134  includes a first opening  136  on the first radially extending surface  118  of the upper surface  114  and a second opening  138  on the first radially extending surface  126  of the lower surface  116 . The holes  134  are spaced an equidistance from a central axis and are equally spaced around the circumference of the powdered metal bushing  108 . 
     The rack and pinion steering gear  10  also includes a bearing  140 , first and second snap rings  142  and  144 , respectively, and a fluid-tight seal  146 . As best shown in FIG. 1A, the bearing  140  includes inner and outer races  148  and  150 , respectively, and a plurality of balls  152  that are interposed between the inner and outer races. The outer race  150  includes first and second end surfaces  154  and  156 , respectively, and inner and outer surfaces  158  and  160 , respectively. The outer surface  160  of the outer race  150  defines an outer diameter of the bearing  140 . The inner surface  158  partially supports the plurality of balls  152 . The inner race  148  also includes first and second end surfaces  162  and  164 , respectively, and inner and outer surfaces  166  and  168 , respectively. The outer surface  168  of the inner race  148  partially supports the plurality of balls  152 . The inner surface  166  of the inner race  148  defines an inner diameter of the bearing  140 . 
     The first and second snap rings  142  and  144  are annular rings, each with an opening (not shown) for enabling the expansion and contraction of the respective ring. As shown in FIG. 2, the first snap ring  142  has a rectangular cross-section that includes upper and lower surfaces  170  and  172 , respectively, and inner and outer surfaces  174  and  176 , respectively. The inner surface  174  defines an inner diameter of the first snap ring  142  and the outer surface  176  defines an outer diameter. The second snap ring  144  has a rectangular cross-section that also includes upper and lower surfaces  178  and  180 , respectively, and inner and outer surfaces  182  and  184 , respectively. The inner surface  182  of the second snap ring  144  defines an inner diameter of the second snap ring and the outer surface  184  defines an outer diameter. The inner diameter of the second snap ring  144  is larger than the outer diameter of the first snap ring  142 . 
     As best shown in FIG. 2, the fluid-tight seal  146  includes a main body portion  186  having a rigid, non-compressible, annular support structure  188 . The support structure  188  has an L-shaped cross-section. Upper and lower sealing portions  190  and  192 , respectively, extend axially outwardly from the main body portion  186 . The upper sealing portion  190  is frustoconical and narrows, i.e., extends radially inwardly, as it extends away from the main body portion  186 . The lower sealing portion  192  is generally cylindrical and includes an annular spring  194  for biasing a sealing surface of the lower sealing portion  192  in a radially inward direction. 
     The rack and pinion steering gear  10  also includes a yoke assembly  196  (FIGS.  1  and  3 ). The yoke assembly  196  includes a yoke bearing  198 , a spring  200 , and a yoke plug  202 . The yoke bearing  198  includes a low friction surface for contacting the rack bar  42 . 
     To assemble the directional control valve  70  of the rack and pinion steering gear  10 , the splined second axial end of the torsion bar  106  is inserted into the cavity  64  in the first axial end of the first support portion  56  of the pinion assembly  52  to fix rotationally the second axial end of the torsion bar  106  and the pinion assembly  52 . The hole in the first axial end of the first support portion  56  of the pinion assembly  52  is aligned with the hole  84  in the second axial end  82  of the valve sleeve  72 . A pin  204  is inserted through the holes to fix the valve sleeve  72  to the pinion assembly  52 . The first axial end of the torsion bar  106  is received in the valve core  74  of the input shaft  86  and the input shaft is moved relative to the valve sleeve  72  until the valve core is properly positioned in the valve sleeve. The first axial end of the torsion bar  106  is then fixed to the input shaft  86 . 
     The torsion bar  106  supports the valve core  74  relative to the valve sleeve  72 . The torsion bar  106  allows the relative rotation of the valve sleeve  72  and the valve core  74  and ensures that the valve core remains coaxial with the valve sleeve. 
     The first snap ring  142  is then expanded and inserted into the groove  104  in the second cylindrical portion  92  of the input shaft  86 . When seated in the groove  104 , the inner surface  174  of the first snap ring  142  contacts a bottom of the groove and the outer surface  176  of the first snap ring extends radially outwardly of the outer surface  102  of the second cylindrical portion  92  of the input shaft  86 , as best shown in FIG.  2 . The resiliency of the first snap ring  142  ensures proper seating of the first snap ring in the groove  104 . The assembled directional control valve  70 , including the input shaft  86 , the torsion bar  106 , and the pinion assembly  52 , is hereinafter referred to as the valve assembly. 
     To assemble the rack and pinion steering gear  10 , the rack bar  42  is extending through the housing  12 . The valve assembly is inserted into the first opening  26  in the first tubular portion  14  of the housing  12  and is moved through the passage  24  toward the second opening  30  until teeth  60  of the pinion gear  54  are placed in meshing engagement with teeth of the rack bar  42 . The bearing  140  is then inserted into the second opening  30  of the first tubular portion  14  of the housing  12  and pressed between the housing  12  and the second support portion  58  of the pinion assembly  52 . When pressed against the housing  12 , the first end  154  of the outer race  150  of the bearing  140  rests against the radially inwardly extending, annular wall  32  and the first end  162  of the inner race  148  rests against a second axial end of the pinion gear  54  of the pinion assembly  52 . The pinion nut  68  is then screwed onto the threaded outer surface  66  of the second support portion  58  until the pinion nut is pressed against second end  164  of the inner race  148  of the bearing  140 . 
     Next, the powdered metal bushing  108  is inserted into the first opening  26  of the first tubular portion  14  of the housing  12  and is pressed between the housing and the input shaft  86 . When pressed into the housing  12 , the powdered metal bushing  108  is located a short distance away from the first snap ring  142  toward the first axial end  96  of the input shaft  86 . An interference fit is formed between an outer surface  112  of the powdered metal bushing  108  and the inner surface  20  of the housing  12 . Since the inner diameter of the powdered metal bushing  108  is slightly larger than the outer surface  102  of the second cylindrical portion  92  of the input shaft  86 , a loose fit is formed between the inner surface  110  of the powdered metal bushing  108  and the outer surface of the second cylindrical portion of the input shaft. 
     The fluid-tight seal  146  is then inserted into the first opening  26  of the first tubular portion  14  of the housing  12  and between the inner surface  20  of the housing  12  and the input shaft  86 . The main body portion  186  of the seal  146  contacts the inner surface  20  of the housing  12  and the upper and lower sealing portions  190  and  192  of the seal  146  contact the input shaft  86 . The lower sealing portion  192  is biased against the outer surface  102  of the second cylindrical portion  92  of the input shaft  86 . An inner surface of the upper sealing portion  190  makes line contact with the tapered portion  94  of the input shaft  86 . 
     The second snap ring  144  is then inserted into the groove  28  on the inner surface  20  of the first tubular portion  14  of the housing  12  to secure the powdered metal bushing  108  and the fluid-tight seal  146  in the housing  12 . The second snap ring  144  is compressed and inserted into the first opening  26 . The resiliency of the second snap ring  144  seats the second snap ring into the groove  28  such that an outer diameter of the second snap ring contacts the bottom of the groove. The inner surface  182  of the second snap ring  144  extends radially outwardly from the inner surface  20  of the first tubular portion  14  of the housing  12 . The input shaft  86  is now ready to be connected to the steering wheel of the vehicle. 
     The yoke assembly  196  is then inserted into the yoke bore  40 . The yoke bearing  198  of the yoke assembly  196  contacts the rack bar  42  on a side opposite the pinion gear  54 . The yoke assembly  196  applies a force against the rack bar  42  to maintain a proper lash between the teeth of the rack bar and the helical teeth  60  of the pinion gear  54 . 
     During operation of the rack and pinion steering gear  10 , the input shaft  86  is rotated as the vehicle steering wheel is rotated. If the resistance to turning of the steerable wheels is below a predetermined value, rotation of the input shaft  86  will rotate the pinion gear  54  and will move the rack bar  42  to turn the steerable wheels. If the resistance to turning of the steerable wheels is above the predetermined value, the pinion gear  54  will not rotate with the rotation of the input shaft  86 . As a result, the torsion bar  106  will twist and the valve core  74  will move relative to the valve sleeve  72 . The directional control valve  70  will direct fluid to one of the chambers  48  and  50  of the hydraulic motor  44 . As a result, the hydraulic motor  44  assists in moving the rack bar  42  to turn the steerable wheels. As the rack bar  42  moves, the pinion gear  54  rotates and the twisting, or torsion, is removed from the torsion bar  106 . 
     When the pressure of one chamber  48  or  50  of the hydraulic motor  44  increases relative to the other chamber  50  or  48 , the piston  46  moves until the pressures within the chambers again equalize. During the movement of the piston  46 , the lower pressure chamber decreases in volume. As a result, hydraulic fluid is forced from the chamber. The hydraulic fluid returns to the directional control valve  70  and is then directed to a chamber  206  within the first tubular portion  14  of the housing prior  12  to being returned to the reservoir  80 . 
     The chamber  206  in the first tubular portion  14  of the housing  12  is located adjacent the second cylindrical portion  92  of the input shaft  86  and is bordered on an upper side, as viewed in FIGS. 1 and 3, by an assembly formed from the first and second snap rings  142  and  144 , the powdered metal bushing  108 , and the fluid-tight seal  146 . During operation of the rack and pinion steering gear  10 , the fluid in the chamber  206  is under pressure and the pressure forces the fluid into the conduit leading to the reservoir  78 . The plurality of holes  134  in the powdered metal bushing  108  allows fluid to pass through the powdered metal bushing to the fluid-tight seal  146 . Thus, the powdered metal bushing  108  does not act as a seal to prevent fluid leakage from the first opening  26  of the first tubular portion  14  of the housing  12 . Additionally, since fluid may pass through the holes  134  of the powdered metal bushing  108 , fluid pressure across the powdered metal bushing is equalized and does not tend to force the powdered metal bushing toward the first opening  26  of the first tubular portion  14  of the housing  12 . 
     Interaction of the helical teeth  60  of the pinion gear  54  and the teeth of the rack bar  42  during rotation of the pinion gear results in a longitudinal force, i.e., into and out of the plane of the paper, for moving the rack bar and an axial force that is directed along axis A. The axial force tends to move the valve assembly in an axial direction relative to the first tubular portion  14  of the housing  12  and relative to the rack bar  42 . The direction of the axial force is dependent upon the slope of the helical teeth  60  on the pinion gear  54 . In the rack and pinion steering gears shown in FIGS. 1 and 3, rotation of the pinion gear  54  in the counterclockwise direction, shown by the arrow R in FIG. 3, results in an axial force that is directed toward a first axial end of the first tubular portion  14  of the housing  12 , shown at arrow F in FIG.  3 . 
     When the pinion nut  68  is tightened against the second end  164  of the inner race  148  of the bearing  140 , the axial force in the direction F is resisted by engagement of the pinion nut with the second end of the inner race of the bearing. However, in the event that the pinion nut  68  is not tightened against the second end  164  of the inner race  148  of the bearing  140 , as is illustrated in FIGS. 1 and 1A, the axial force in the direction F is resisted by engagement of the first snap ring  142  with the lower surface  116  of the powdered metal bushing  108 . The axial force in the direction F causes the valve assembly to move slightly toward the first opening  26 . During this movement, the first snap ring  142  moves into the second pocket  132  of the powdered metal bushing  108  and engages the second radially extending surface  128  of the lower surface  116  of the powdered metal bushing  108 . When the first snap ring  142  is received in the second pocket  132 , the axially extending surface  130  of the lower surface  116  of the powdered metal bushing  108  resists any radial movement of the input shaft  86 . Additionally, the axially extending surface  130  of the lower surface  116  of the powered metal bushing  108  will not radially expand or unravel the first snap ring  142 . 
     FIGS. 1 and 2 illustrate the position of the first snap ring  142  relative to the powdered metal bushing  108  prior to any axial movement of the valve assembly. FIG. 3 illustrates the engagement of the first snap ring  142  and the powdered metal bushing  108 . 
     The second snap ring  144  helps to resist any axial movement of the powdered metal bushing  108  toward the first opening  26  as a result of the axial force. Any potential axial movement of the powdered metal bushing  108  is resisted by the non-compressibility of the support structure  188  of the fluid-tight seal  146  and the second snap ring  144 . 
     When the axial force is removed, the valve assembly moves back to the position shown in FIG.  1 . When the input shaft  86  is turned clockwise, an axial force directed toward the second opening  30  of the first tubular portion  14  of the housing  12  is resisted by engagement of the second axial end of the pinion gear  54  with the first end  162  of the inner race  148  of the bearing  140 . 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.