Abstract:
An apparatus ( 10 ) for turning steerable wheels of a vehicle comprises a housing ( 12 ). A rack bar ( 54 ) is movable longitudinally relative to the housing ( 12 ) for turning the steerable wheels. A pinion gear ( 68 ) is located within the housing ( 12 ). Teeth ( 76 ) of the pinion gear ( 68 ) are in meshing engagement with teeth of the rack bar ( 54 ). The apparatus ( 10 ) also comprises a hydraulic motor ( 60 ) for moving the rack bar ( 54 ) relative to the housing ( 12 ). A valve assembly ( 98 ), responsive to rotation of a steering wheel ( 94 ) for directing fluid to the hydraulic motor ( 60 ), has an actuated position and an unactuated position. The apparatus ( 10 ) further comprises a mechanism ( 156 ) for dampening longitudinal oscillations of the rack bar ( 54 ). The mechanism ( 156 ) comprises a yoke bearing ( 158 ) which contacts the rack bar ( 54 ) with a variable pressure that is dependent upon a velocity of rack bar ( 54 ) relative to the housing ( 12 ).

Description:
TECHNICAL FIELD 
     The present invention relates to rack and pinion steering gears and, more particularly, to hydraulic power-assisted rack and pinion steering gears. 
     BACKGROUND OF THE INVENTION 
     A known rack and pinion steering gear includes a pinion gear that is rotatably mounted in a housing and is connectable with a steering wheel of a vehicle. A rack bar extends through the housing and has opposite end portions that are connectable with steerable vehicle wheels. The rack bar is moves longitudinally relative to the housing for turning the steerable wheels of the vehicle. Gear teeth formed on the rack bar are disposed in meshing engagement with gear teeth on the pinion gear. A hydraulic motor, when actuated, assists in moving the rack bar longitudinally relative to the housing. A yoke assembly is disposed in the housing to support and guide movement of the rack bar relative to the housing. The yoke assembly includes a yoke bearing having an arcuate surface across which the rack bar moves. A spring biases the yoke bearing against the rack bar. 
     When the hydraulic motor is actuated to move the rack bar to a desired position relative to the housing, hydraulic fluid exerts a force in a first direction on a piston that is attached to the rack bar. The force causes the rack bar to move in the first direction toward the desired position. An inertial force of the rack bar, when the rack bar is moving toward the desired position relative to the housing, tends to cause the rack bar to overshoot the desired position. When the rack bar overshoots the desired position, the hydraulic motor is actuated to apply a force in a second direction, opposite the first direction, to move the rack bar back toward the desired position. Again, the rack bar tends to overshoot the desired position and the hydraulic motor is again actuated to move the rack bar in the first direction. As a result, the rack bar continues to oscillate past the desired position. Oscillation of the rack bar past the desired position may produce a rattling noise or a small twitch in the steering wheel that may be felt by the driver. This is especially true when the vehicle contacts a discontinuity in the road surface, such as a pothole, that causes a reaction in the vehicle suspension that removes the resistance to rack movement. When the resistance to rack movement is removed, the velocity of rack movement increases and the magnitude of the overshoot increases. 
     A rack and pinion steering gear that dampens the movement of the rack bar to eliminate or reduce the overshoot that result from movement of the rack bar toward the desired position is desirable. By dampening movement of the rack bar, the rattling noise and the twitch may be eliminated. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus for turning steerable wheels of a vehicle in response to rotation of a steering wheel. The apparatus comprises a housing. A rack bar is movable longitudinally relative to the housing for turning the steerable wheels. The rack bar has a portion that extends through the housing and opposite ends that are connectable to the steerable wheels. The portion that extends through the housing includes teeth. A pinion gear is located within the housing and includes teeth. Teeth of the pinion gear are in meshing engagement with teeth of the rack bar. The apparatus also comprises a hydraulic motor for, when actuated, moving the rack bar relative to the housing. A valve assembly is responsive to rotation of the steering wheel for directing fluid to the hydraulic motor. The valve assembly has an actuated position for actuating the hydraulic motor and an unactuated position for discontinuing operation of the hydraulic motor. The apparatus further comprises a mechanism for dampening longitudinal oscillations of the rack bar. The mechanism comprises a yoke bearing which contacts the rack bar with a variable pressure that is dependent upon a velocity of rack bar movement relative to the housing. 
     In a further aspect of the invention, the apparatus comprises a housing. A rack bar is movable longitudinally relative to the housing for turning the steerable wheels. The rack bar has a portion that extends through the housing and opposite ends that are connectable to the steerable wheels. The portion that extends through the housing includes teeth. A pinion gear is located within the housing and includes teeth. Teeth of the pinion gear are in meshing engagement with teeth of the rack bar. The apparatus also comprises a hydraulic motor for, when actuated, moving the rack bar from an initial position relative to the housing to a desired position relative to the housing in response to rotation of the steering wheel and a yoke assembly for supporting and guiding the rack bar relative to the housing. The yoke assembly includes a pressure chamber and a yoke bearing. The yoke bearing contacts the rack bar on a side of the rack bar opposite the pinion gear. Friction between the yoke bearing and the rack bar increases in response to an increase in fluid pressure in the pressure chamber. The apparatus further comprises a fluid source for supplying fluid to the pressure chamber of the yoke assembly. The fluid source increases fluid pressure in the pressure chamber in response to movement of the rack bar relative to the housing toward the desired position. 
     In yet a further aspect of the invention, the apparatus comprises a housing. A rack bar is movable longitudinally relative to the housing for turning the steerable wheels. The rack bar has a portion that extends through the housing and opposite ends that are connectable to the steerable wheels. The portion that extends through the housing includes teeth. A pinion gear is located within the housing and includes teeth. Teeth of the pinion gear are in meshing engagement with teeth of the rack bar. The apparatus also comprises a hydraulic motor for, when actuated, moving the rack bar relative to the housing. A valve assembly is responsive to rotation of the steering wheel for directing fluid to the hydraulic motor. The valve assembly has an actuated position for actuating the hydraulic motor and an unactuated position for discontinuing operation of the hydraulic motor. The apparatus further includes a yoke assembly for supporting and guiding the rack bar relative to the housing. The yoke assembly includes a pressure chamber and a yoke bearing. The pressure chamber receives fluid from the valve assembly. The yoke bearing contacts the rack bar on a side of the rack bar opposite the pinion gear. Friction between the yoke bearing and the rack bar increases as fluid pressure in the pressure chamber increases. Fluid pressure in the pressure chamber increases in response to movement of the valve assembly from the actuated position toward the unactuated position. 
    
    
     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 cross-sectional view of a rack and pinion steering gear constructed in accordance with the present invention; 
     FIG. 1A is an enlarged portion of the rack and pinion steering gear of FIG. 1; 
     FIG. 2 is a view taken approximately along line  2 — 2  in FIG. 1; 
     FIG. 3 is a cross-sectional view of a rack and pinion steering gear constructed in accordance with a second embodiment of the present invention; and 
     FIG. 3A is an enlarged portion of the rack and pinion steering gear of FIG.  3 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     FIG. 1 is a sectional 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  has a first tubular portion  14  that extends along axis A. The first tubular portion  14  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 through the first tubular portion  14 . Four radially extending openings extend from the inner surface  20  to the outer surface  22  of the first tubular portion  14 . The openings include a fluid inlet opening  26 , a fluid outlet opening  28 , and first and second motor openings  30  and  32 , respectively. 
     The housing  12  also includes a second tubular portion  34 . The second tubular portion  34  extends perpendicular to the first tubular portion  14  along axis B. As shown in FIG. 1A, the second tubular portion  34  has first and second ends  36  and  38 , respectively, and inner and outer surfaces  40  and  42 , respectively. The second end  38  of the second tubular portion  34  unites with the first tubular portion  14  near the second axial end  18  of the first tubular portion  14 . The inner surface  40  of the second tubular portion  34  defines a yoke bore  44 . The yoke bore  44  mates with the passage  24  of the first tubular portion  14  near the second axial end  18  of the first tubular portion  14 . The inner surface  40  of the second tubular portion  34 , adjacent the first end  36 , is threaded, illustrated schematically in FIGS. 1 and 1A. 
     A thickened wall portion  46  of the housing  12  is formed where the first tubular portion  14  mates with the second tubular portion  34 . The thickened wall portion  46  is located between the fluid outlet opening  28  of the first tubular portion  14  and the second tubular portion  34  of the housing  12 . A fluid passage  48  extends through the thickened wall portion  46  and connects the passage  24  of the first tubular portion  14  to the yoke bore  44  of the second tubular portion  34 . Preferably, the fluid passage  48  is cast into the thickened wall portion  46  of the housing  12 . As shown in FIGS. 1 and 1A, the fluid passage  48  has first and second ends  50  and  52 , respectively. The first end  50  of the fluid passage  48  terminates in the passage  24  of the first tubular portion  14 . The second end  52  of the fluid passage  48  terminates in the yoke bore  44 . 
     A longitudinally extending rack bar  54  extends through the housing  12  in a direction that is perpendicular to both axis A and axis B. The rack bar  54  has a generally circular cross-sectional shape that is defined by a generally cylindrical outer surface  56  (FIG.  1 A). An upper surface  58  of the rack bar  54  includes a plurality of teeth (not shown). Opposite end portions (not shown) of the rack bar  54  are connectable with steerable wheels (not shown) of a vehicle (not shown). Movement of the rack bar  54  in a longitudinal direction relative to the housing  12  results in the turning of the steerable wheels of the vehicle. 
     A hydraulic motor  60 , shown schematically in FIG. 1, is also formed in the housing  12 . The hydraulic motor  60  includes a piston  62 , which is attached to the rack bar  54 . The piston  62  separates two variable volume chambers  64  and  66 , respectively. One chamber  64  or  66  is located on each side of the piston  62 . The hydraulic motor  60  is actuated when a differential pressure arises between the two chambers  64  and  66 . The hydraulic motor  60  discontinues operation when the pressure between the two chambers  64  and  66  equalizes. When the hydraulic motor  60  is actuated, fluid pressure moves the piston  62 . Movement of the piston  62  results in movement of the rack bar  54  in the longitudinal direction relative to the housing  12 . 
     As shown in FIG. 1, a pinion gear  68  includes a gear portion  70 , a first support portion  72 , and a second support portion  74 . The gear portion  70  has a plurality of teeth  76  for meshingly engaging the teeth of the rack bar  54 . The first support portion  72  of the pinion gear  68  forms a first axial end of the pinion gear  68 . The first support portion  72  includes a cylindrical outer surface  78 . An axially extending cavity  80  extends into the first support portion  72 . A hole, shown generally at  82 , extends radially through the first support portion  72  and terminates at the cavity  80 . 
     The second support portion  74  of the pinion gear  68  forms a second axial end of the pinion gear  68 . The second support portion  74  has a cylindrical outer surface  84 . The diameter of the second support portion  74  of the pinion gear  68  is less than the diameter of the first support portion  72 . An end of the cylindrical outer surface  84  of the second support portion  74 , opposite the gear portion  70 , is threaded for receiving a pinion nut  86 . 
     An input shaft  88  includes first and second axial ends  90  and  92 , respectively. The first axial end  90  of the input shaft  88  is connectable with a steering wheel  94  of the vehicle. The second axial end  92  of the input shaft  88  includes a valve core part  96 . 
     The rack and pinion steering gear also includes a valve assembly, shown generally at  98 . The valve assembly  98  includes a valve sleeve part  100  and the valve core part  96 . The valve sleeve part  100  of the valve assembly  98  of the rack and pinion steering gear  10  is tubular. FIG. 2 shows a cross-section of the valve sleeve part  100 . For clarity, FIG. 2 shows the structure of the valve sleeve part  100  in a single plane. The valve sleeve part  100  includes inner and outer surfaces  102  and  104 , respectively, and first and second axial ends  106  and  108 , respectively (FIG.  1 ). An inner surface  102  of the valve sleeve part  100  includes six axially extending grooves  110  (FIG.  2 ). The six axially extending grooves  110  define six lands  112 . Three radially extending passages  114  extend between the inner surface  102  of the valve sleeve part  100  and the outer surface  104  of the valve sleeve part  100 . A port  116  for each passage  114  on the inner surface  102  of the valve sleeve part  100  is centrally located upon a land  112 , equidistant from adjacent grooves  110 . 
     A first set of passages  118  extends radially outwardly through the valve sleeve part  100 . The first set of passages includes three passages  118 . Only one of the passages is shown in FIG.  1 . One passage  118  from the first set of passages  118  is located in each groove  110  that is located immediately counterclockwise of a radially extending passage  114 . 
     A second set of passages  120  extends radially outwardly through the valve sleeve part  100 . The second set of passages  120  includes three passages  120 . Only one of the passages  120  is shown in FIG.  1 . One passage  120  from the second set of passages  120  is located in each groove  110  immediately clockwise of a radially extending passage  114 . 
     The valve core part  96  is tubular and includes inner and outer surfaces  122  and  124 , respectively. FIG. 2 also shows a cross-section of the valve core part  96 . For clarity, FIG. 2 shows the structure of the valve core part  96  in a single plane. The outer surface  124  of the valve core part  96  includes six axially extending grooves  126 . The axially extending grooves  126  define six lands  128 . The valve core part  96  also includes three radially extending passages  1   30  that extend between the outer surface  124  and the inner surface  122  of the valve core part  96 . Each radially extending passage  130  has a port  132  that is located in an axially extending groove  126  of the valve core part  96 , equidistant from adjacent lands  128 . The inner surface  122  of the valve core part  96  defines an axially extending passage  134 . 
     A torsion bar  136  (FIG. 1) includes first and second axial end portions  138  and  140 , respectively, and an intermediate portion  142 . The first axial end portion  138  is cylindrical. A radially extending hole  144  extends through the first axial end portion  138 . The second axial end portion  140  is also generally cylindrical and includes a splined outer surface  146 . The intermediate portion  142  of the torsion bar  136  is elongated and has a cylindrical outer surface  148  (FIG.  2 ). The cylindrical outer surface  148  of the intermediate portion  142  has a diameter that is approximately one-half the diameter of the first and second axial end portions  138  and  140 . 
     In one method of assembling the valve assembly  98 , the second axial end portion  140  of the torsion bar  136  is inserted into the cavity  80  on the first axial end of the first support portion  72  of the pinion gear  68 . The splined outer surface  146  of the second axial end portion  140  of the torsion bar  136  fixes the second axial end portion of the torsion bar relative to the pinion gear  68 . The second axial end  108  of the valve sleeve part  100  is then fixed to the first support portion  72  of the pinion gear  68  with a pin  150  that extends through the radially extending hole  82  in the first support portion  72  of the pinion gear  68 . The input shaft  88  is then disposed between the valve sleeve part  100  and the torsion bar  136 . When properly positioned between the valve sleeve part  100  and the torsion bar  136 , a small, annular passage  152  located within the cavity  80  of the first support portion  72  of the pinion gear  68  extends around the second axial end  92  of the input shaft  88 . The first axial end portion  138  of the torsion bar  136  is then fixed to the input shaft  88  using a pin  154 . 
     The rack and pinion steering gear  10  also includes a yoke assembly  156 . As shown in FIG. 1A, the yoke assembly  156  includes a yoke bearing  158 , a spring  160 , and a yoke plug  162 . The yoke bearing  158  includes a cylindrical side wall  164  and axially opposite first and second surfaces  166  and  168 , respectively. The cylindrical side wall  164  includes a circumferentially extending groove  170 . 
     The first surface  166  of the yoke bearing  158  is generally flat and extends in a plane that is perpendicular to axis B. A recess  172  extends into the first end surface  166  of the yoke bearing  158 . The recess  172  is defined by an arcuate recess surface  174 . Preferably, the arc of the recess surface  174  is partially cylindrical with a radius that is equal to the radius of the outer surface  56  of the rack bar  54 . In one embodiment, the recess surface  174  is a dual radius surface for making line contact with the outer surface  56  of the rack bar  54 . 
     The second end surface  168  of the yoke bearing  158  is generally flat and extends in a plane that is parallel to the plane of the first end surface  166 . A recess  176  extends from the second end surface  168  into the yoke bearing  158 . An opening to the recess  176  is circular and is located in the plane of the second end surface  168 . The circular opening is coaxial with the cylindrical side wall  164  o f t he yoke bearing  158 . A cylindrical side wall  178  and an end wall  180  define the recess  176 . The side w all  178  extends in a direction that is perpendicular to the second end surface  168 . A cylindrical spring guide  182  extends outwardly of the end wall  180  of the recess  176 . The spring guide  182  is centered in the recess  176  and includes a first radially extending surface  184  for supporting a portion of the spring  160 . The spring guide  182  terminates at a radially extending end wall  186 . The end wall  186  is located axially outwardly of the first radial surface  184  and within the recess  176 . 
     The yoke plug  162  is cup-shaped and includes a threaded outer surface  188 , an annular flange  190 , and a generally flat end wall  192 . Although not shown in FIG. 1, a cylindrical spring guide may extend outwardly, along axis B, of the end wall  192  of the yoke plug  162 . The spring  160  of the yoke assembly illustrated in FIG. 1 is a helical compression spring. The spring  160  has a first axial end  194  and an opposite second axial end  196 . The spring  160  also has a known spring constant. 
     The yoke assembly  156  also includes two fluid-tight seals  198  and  200 . The seals  198  and  200  are preferably O-rings. A first seal  198  is designed to seal between the cylindrical side wall  164  of the yoke bearing  158  and the inner surf ace  40  of the second tubular portion  34  of the housing  12 . A second seal  200  is designed to seal between the flange  190  of the yoke plug  162  and the first end  36  of the second tubular portion  34  of the housing  12 . 
     According to one method of assembling the rack and pinion steering gear  10 , the rack bar  54  is extended longitudinally through the housing  12  so that teeth of the rack bar are located within the housing. The assembled valve assembly  98  is then inserted into the passage  24  of the first tubular portion  14  of the housing  12 . The valve assembly  98  is placed in the first tubular portion  14  so that teeth  76  of the gear portion  70  of the pinion gear  68 , which is attached to the assembled valve assembly  98 , meshingly engage teeth of the rack bar  54  and so that the input shaft  88 , which is also attached to the assembled valve assembly  98 , extends axially outwardly of the opening on the first axial end  16  of the first tubular portion  14 . 
     As shown in FIG. 1, the rack and pinion steering gear  10  includes three bearing assemblies. A first bearing assembly  202  is located adjacent the opening at the first axial end  16  of the first tubular portion  14  of the housing  12 . The first bearing assembly  202  extends between the housing  12  and the input shaft  88  and enables rotation of the input shaft relative to the housing. A retaining ring  204  holds the first bearing assembly  202  in the first tubular portion  14  of the housing  12 . 
     A second bearing assembly  206  is located in the passage  24  of the first tubular portion  14  between the fluid outlet opening  28  and the yoke bore  44 . The second bearing assembly  206  extends between the housing  12  and the first support portion  72  of the pinion gear  68  and enables rotation of the pinion gear relative to the housing. 
     A third bearing assembly  208  is located in the passage  24  of the first tubular portion  14  between the yoke bore  44  and the second axial end  18  of the first tubular portion. The third bearing assembly  208  extends between the housing  12  and the second support portion  74  of the pinion gear  68  and enables rotation of the pinion gear relative to the housing. The third bearing assembly  208  is held in the housing  12  and relative to the pinion gear  68  by a pinion nut  86  that is screwed onto the threads of the second support portion  74 . 
     The first seal  198  is then inserted into the groove  170  (FIG. 1A) in the side wall  164  of the yoke bearing  158  and the yoke bearing is inserted into the yoke bore  44  of the second tubular portion  34  of the housing  12 . When properly inserted, the recess surface  174  of the yoke bearing  158  will contact the outer surface  56  of the rack bar  54  in a location opposite the teeth  76  of the gear portion  70  of the pinion gear  68 . The spring  160  is then placed on the spring guide  182  (FIG. 1A) of the yoke bearing  158  so that the first axial end  194  of the spring  160  contacts the first radially extending surface  184  of the spring guide  182 . The second seal  200  (FIG. 1A) is placed around the outer surface  188  of the yoke plug  162  and the yoke plug is screwed into the first end  36  of the second tubular portion  34  of the housing  12 . When the yoke plug  162  is screwed into the housing  12 , the second seal  200  creates a fluid-tight seal between the annular flange  190  of the yoke plug  162  and the first end  36  of the second tubular portion  34  of the housing  12 . The end wall  192  of the yoke plug  162  contacts the second axial end  196  of the spring  160  and compresses the spring. The yoke plug  162  may be screwed into the housing  12  a distance necessary to compress the spring  160  a predetermined amount. 
     When the rack and pinion steering gear  10  is assembled, four annular channels are formed between the valve sleeve part  100  and the inner surface  20  of the first tubular portion  14  of the housing  12 . As shown in FIG. 1, the four annular channels include an annular inlet channel  210 , an annular outlet channel  212 , and an annular first and second motor channels  214  and  216 , respectively. Fluid-tight seals  218 , four of which are shown in FIG. 1, seal each of the four channels  210 - 216  from adjacent channels  210 - 216 . 
     A pump  220  (FIG. 1) draws hydraulic fluid from a reservoir  222  and supplies the hydraulic fluid to the rack and pinion steering gear  10 . Conduit  224  extends between pump  220  and the fluid inlet opening  26  of the housing  12  for carrying fluid from the pump  220  to the rack and pinion steering gear  10 . Conduit  226  extends from the fluid outlet opening  28  of housing  12  to the reservoir  222  for returning hydraulic fluid to the reservoir. The rack and pinion steering gear  10  also includes conduit  228  that extends from the first motor opening  30  to chamber  64  of the hydraulic motor  60  and conduit  230  that extends from the second motor opening  32  to chamber  66 . As shown in FIG. 1, conduit  228  provides fluid communication between the first annular motor channel  214  and chamber  64  of the hydraulic motor  60 . Conduit  230  provides fluid communication between the second annular motor channel  216  and chamber  66  of the hydraulic motor  60 . Fluid flow through conduits  228  and  230  is bi-directional. Thus, when the volume of chamber  64  of the hydraulic motor  60  is increasing, fluid flows through conduit  228  toward the hydraulic motor and through conduit  230  away from the hydraulic motor. When the volume of chamber  64  of the hydraulic motor is decreasing, fluid flows through conduit  230  toward the hydraulic motor  60  and through conduit  228  away from the hydraulic motor. 
     Each conduit  224  and  226  used in the rack and pinion steering gear  10  is formed from a flexible material. Preferably, each conduit  224  and  226  is formed from rubber. By using flexible conduits, noise caused by varying pressure fluid flow through the conduits  224  and  226  is reduced. However, the flexible wall conduits also have pressure variable capacities. When subjected to high pressure, flexible wall conduits swell, or increase slightly in diameter. As a result, the fluid capacity of a respective conduit increases when the conduit is subjected to increased pressure. 
     When the rack and pinion steering gear  10  is mounted in a vehicle, the input shaft  88  is operatively coupled to the steering wheel  94  of the vehicle. Rotation of the steering wheel  94  results in rotation of the input shaft  88 . Since the input shaft  88  is fixed relative to the first axial end portion  138  of the torsion bar  136 , rotation of the input shaft  88  results in rotation of the first axial end portion  138  of the torsion bar. If resistance to the turning of the steerable wheels of the vehicle is above a threshold level, the second axial end portion  140  of the torsion bar  136  will not be rotated by rotation of the first axial end portion  138  of the torsion bar. As a result, rotation of the first axial end portion  138  of the torsion bar  136  relative to the second axial end portion  140  will cause torsion or twisting of the intermediate portion  142  of the torsion bar. Torsion of the intermediate portion  142  of the torsion bar  136  causes the valve core part  96  to move relative to the valve sleeve part  100 . 
     FIG. 2 illustrates the valve assembly  98  in a neutral or unactuated position. In the neutral position, hydraulic fluid flows from the annular inlet channel  210  (FIG. 1) and radially inwardly through the radially extending passages  114  in the valve sleeve part  100 . An equal amount of fluid is directed toward the first and second sets of passages  118  and  120 . The first set of passages  118  directs fluid to the first annular motor channel  214  and the second set of passages . 120  directs fluid to the second annular motor channel  216 . Since an equal amount of fluid is directed toward each channel  214  and  216 , the pressure within chamber  64  of the hydraulic motor  60  remains equal to the pressure within chamber  66  of the hydraulic motor. 
     When the valve assembly  98  is in the neutral position and the pressure in the two chambers  64  and  66  of the hydraulic motor  60  is equal, fluid that flows into the valve assembly through the radially extending passages  114  in the valve sleeve part  100  is directed toward the radially extending passages  130  in the valve core part  96 . This is due to the fluid being incompressible and the chambers  64  and  66  of the hydraulic motor  60 , the first and second motor conduits  228  and  230 , and the first and second annular motor channels  214  and  216  being filled to capacity with fluid. The fluid flows through the radially extending passages  130  in the valve core part  96  and into the passage  134  formed by the inner surface  122  of the valve core part  96 . The fluid then flows through passage  134 , through passage  152  (FIG.  1 ), and into the annular outlet channel  212 . 
     When the valve core part  96  is rotated relative to the valve sleeve part  100 , i.e. the intermediate portion  142  of the torsion bar  136  is twisted, the valve assembly  98  moves out of the neutral position, or is actuated, and fluid is directed toward a respective set of passages  118  or  120 . For example, with reference to FIG. 2, if the input shaft  88  is rotated clockwise relative to the valve sleeve part  100 , land s  128  of the valve core part  96  move adjacent lands  112  of the valve sleeve part  100  and fluid flow toward the first set of passages  118  is restricted. Conversely, fluid flow toward the second set of passages  120  is increased. 
     This movement of the valve core part  96  relative to the valve sleeve part  100  also blocks the fluid flow to the radially extending passages  130  of the valve core part  96 , i.e., the return. As a result, fluid pressure increases in conduit  224 , in the annular inlet channel  210 , in the second set of passages  120 , in the second annular motor channel  216 , in conduit  230 , and in chamber  66  of the hydraulic motor  60 . A higher pressure in chamber  66  relative to the pressure in chamber  64  results in a differential pressure that causes the piston  60  to move. When the piston  60  moves, the rack bar  54  moves and the steerable wheels are turned. 
     As the volume of chamber  66  increases, the volume of chamber  64  decreases. Fluid flows out of chamber  64 , through conduit  228 , and into the first annular motor channel  214  (FIG.  1 ). Fluid then flows through the first set of passages  118  (FIG. 2) from the first annular motor channel  214 . The fluid flows through the radially extending passages  130  in the valve core part  96  and into the passage  134  formed by the inner surface  122  of the valve core part  96 . The fluid then flows through passage  134 , through passage  152  (FIG.  1 ), and into the annular outlet channel  212 . 
     During movement of the rack bar  54  relative to the housing  12 , interaction of teeth of the rack bar  54  with teeth  76  of the gear portion  70  of the pinion gear  68  rotates the pinion gear. Rotation of the pinion gear  68  rotates the valve sleeve part  100  relative to the valve core part  96 . As a result, movement of the rack bar  54  rotates the valve assembly  98  back into the neutral position. When the valve assembly  98  is in the neutral position, fluid is again directed from the radially extending passages  114  in the valve sleeve part  100  to the radially extending passages  130  in the valve core part  96  and to the annular outlet channel  212  to be returned to the reservoir  222 . 
     As shown in FIG. 1A, the first end  50  of the fluid passage  48  that extends through the thickened wall portion  46  of the housing  12  terminates in the annular outlet channel  212 . The fluid outlet opening  28  in the housing  12  also connects to the annular outlet channel  212 . When the rack and pinion steering gear  10  is operating and fluid is received in the annular outlet channel  212 , fluid flows through the fluid passage  48  in the thickened wall portion  46  and into a pressure chamber  232  located in the yoke bore  44 . Once the fluid passage  48  and the pressure chamber  232  are filled with fluid, additional fluid received in the annular outlet channel  212  flows through the fluid outlet opening  28  and into conduit  226  to be returned to the reservoir  222 . 
     The amount of fluid flowing through conduit  226  is dependent upon the fluid capacity of conduit  226 . When more fluid is directed toward the annular outlet channel  212  than can flow through conduit  226 , fluid pressure in the annular outlet channel  212 , also known as the return pressure, increases. The return pressure increases when the valve assembly  98  rotates from an actuated position in which fluid is directed toward a particular set of passages  118  or  120  to an unactuated or neutral position. When the valve assembly  98  is in the actuated condition, fluid pressure increases in conduit  224 . As a result, conduit  224  swells and retains a greater capacity of fluid. When the valve assembly  98  returns to the neutral position, the pressure in conduit  224  decreases and an increased amount of fluid is suddenly directed toward the annular outlet channel  212 . If the amount of fluid directed to the annular outlet channel  212  is greater than the capacity of conduit  226 , the fluid pressure in the annular outlet channel  212  increases. 
     The fluid pressure in the annular outlet channel  212  is also related to the velocity of the rack bar  54  toward a desired position. The desired position is the position of the rack bar  54  relative to the housing  12  for removing torsion from the torsion bar  136  and returning the valve assembly  98  to the neutral position. Thus, the velocity of the rack bar  54  is also directly related to the rotational velocity of the valve assembly  98  from an actuated position toward the neutral or unactuated position. If the rack bar  54  moves slowly toward the neutral position, the valve sleeve part  100  moves slowly relative to valve core part  96  when moving toward the neutral position. The slow relative movement between the valve sleeve part  100  and the valve core part  96  results in a gradual increase in fluid flow to the radially extending passages  130  of the valve core part  96  and toward the annular outlet channel  212 . The gradual increase in fluid flow toward the annular outlet channel  212 , slowly relief the pressure in conduit  224  and provides conduit  226  with time to return the fluid to the reservoir  222  without a large pressure increase in the annular outlet channel  212 . As a result, slow movement of the rack bar  54  toward the desired position results in a small increase of fluid pressure in the annular outlet channel  212 . 
     Conversely, if the rack bar  54  moves quickly toward the neutral position, the valve sleeve part  100  moves quickly relative to the valve core part  96  when moving toward the neutral position. The quick relative movement results in a quick opening of the radially extending passages  130  of the valve core part  96  and a sudden release of pressure in conduit  224 . As a result of the sudden release of pressure in conduit  224 , a large amount of fluid is suddenly directed toward the annular outlet channel  212 . If the amount of fluid is greater than the capacity of conduit  226 , fluid pressure in the annular outlet channel  212  suddenly increases. The fluid pressure remains at the increased level until conduit  226  returns enough of the fluid to the reservoir  222  to again reduce the fluid pressure in the annular outlet channel  212 . 
     Since the fluid is incompressible, when pressure in the annular outlet channel  212  increases, fluid pressure in the pressure chamber  232  of the yoke bore  44  increases. The fluid pressure in the pressure chamber  232  acts on a working surface of the yoke bearing  158  to force the yoke bearing toward the rack bar  54 . The working surface of the yoke bearing  158  includes surfaces of the yoke bearing that are subjected to fluid pressure and that do not extends parallel to axis B. For example, in the yoke bearing  158  illustrated in FIG. 1, the working surface of the yoke bearing  158  includes the second end surface  168  of the yoke bearing, the end wall  180 , the first radially extending surface  184 , and the spring guide end wall  186 . The load or pressure exerted by the yoke bearing  158  on the rack bar  54  varies with the fluid pressure in the pressure chamber  232 . The force exerted on the rack bar  54  also varies as a function of the working surface of the yoke bearing  158  upon which the fluid pressure acts. Thus, the working surface of the yoke bearing  158  may be designed with a predetermined surface area for providing a predetermined range of forces on the rack bar  54 . 
     Recess surface  174  of the yoke bearing  158  contacts the outer surface  56  of the rack bar  54 . Recess surface  174  of the yoke bearing  158  also has a known coefficient of friction. By varying the load, or pressure, applied on the rack bar  54  by the yoke bearing  158 , i.e., the normal load, the friction between the rack bar  54  and the yoke bearing  158  is varied. 
     The friction between the rack bar  54  and the yoke bearing  158  acts in a direction opposite the inertial force that causes the rack bar to oscillate about the desired position. The friction acts on the rack bar  54  to reduce the acceleration of the rack bar during movement toward the desired position. By reducing the acceleration of the rack bar  54 , the inertial force is decreased and an amount, or magnitude, of overshoot of the desired position may be decreased or eliminated. As a result, the oscillation of the rack bar  54  is dampened. 
     A particular advantage of the present invention is that the load between the rack bar  54  and the yoke bearing  158  varies as a function of the velocity of the rack bar  54 . Slower rack bar  54  velocity, which would tend to result in a smaller overshoot of the desired position, results in a generally low fluid pressure in the pressure chamber  232  and a generally low load or pressure applied by the yoke bearing  158  on the rack bar  54 . Increased rack bar  54  velocity, which would tend to result in a larger overshoot of the desired position, results in a generally high fluid pressure in the pressure chamber  232  and a generally high load of the yoke bearing  158  on the rack bar  54 . Since friction generally increases as the load on the rack bar  54  increases, the amount of dampening generally increases as the rack bar velocity increases. 
     FIG. 3 illustrates a rack and pinion steering gear  10  constructed in accordance with a second embodiment of the present invention. Structures of FIG. 3 that are the same as, or similar to, structures of FIG. 1 will be referred to with the same reference numerals as in FIG.  1 . 
     The rack and pinion steering gear  10  of FIG. 3 is identical to the rack and pinion steering gear  10  of FIG. 1 with two exceptions. First, the housing  12  of FIG. 3 does not include a thickened wall portion  46  having a fluid passage  48 . Instead, the pressure chamber  232  (FIG. 3A) is pressurized by conduit  240 . Conduit  240  branches off of conduit  226  and is subjected to the pressure of fluid in the annular outlet chamber  212 . As an alternate to conduit  240  branching off of conduit  226 , conduit  240  may be connected directly to the annular outlet channel  212  through an additional opening through the first tubular portion  14  of the housing  12 . Second, the yoke assembly  156  of FIG. 3 does not include a spring  160 . Thus, the load applied to the rack bar  54  by the yoke bearing  158  is only the load resulting from the fluid pressure in the pressure chamber  232 . 
     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.