Abstract:
A fluid power assist rack and pinion steering system ( 12 ) for a vehicle having steerable wheels comprises a rack ( 66 ) connected with steering linkage of the vehicle for, upon movement of the rack, moving the steering linkage to effect turning of the steerable wheels. A rotatable pinion ( 64 ) is in meshing engagement with the rack ( 66 ). The steering system ( 12 ) also includes a control valve ( 22 ) comprising a valve core ( 40 ) rotatable relative to a valve sleeve ( 42 ). A first part ( 56 ) connects the valve sleeve ( 42 ) for rotation with the pinion ( 64 ). A mechanism ( 38 ) resists relative rotation between the valve core ( 40 ) and the valve sleeve ( 42 ) as vehicle speed increases by placing axial force on the valve sleeve. A second part ( 170 ) on the valve sleeve ( 42 ) resists axial movement of the valve sleeve relative to the valve core ( 40 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to a vehicle power steering system, and more specifically to a hydraulic vehicle power steering system in which the resistance to actuation of a power steering control valve increases with increasing vehicle speed.  
           [0003]    2. Description of Related Art  
           [0004]    A vehicle power steering system including a rotary control valve is shown in U.S. Pat. No. 5,293,954. The valve has an inner valve member that is coaxial with and rotatable relative to an outer valve member or sleeve. To effect actuation of the power steering motor to turn steerable vehicle wheels, the inner valve member is rotated relative to the outer valve member. A fluid pressure reaction chamber is provided to regulate the torque required to rotate the inner valve member relative to the outer valve member. The fluid pressure in the reaction chamber increases as vehicle speed increases to increase the resistance felt by an operator of the vehicle to rotation of the inner valve member relative to the outer valve member.  
           [0005]    The valve sleeve is assembled into the control valve by sliding it axially until a hitch pin, press fitted in a pinion, moves into an axially extending slot in the end of the valve sleeve. The engagement of the hitch pin in the slot couples the valve sleeve for rotation with the pinion in a follow-up manner.  
           [0006]    Changes in the fluid pressure in the reaction chamber affect the forces acting on the valve sleeve. These forces can tend to urge the valve sleeve to move axially in its housing, relative to the hitch pin and the pinion. Upon such movement, seals that are disposed between the valve sleeve and the housing can be forced out of their grooves and into the annular space between the valve sleeve and the housing. This can adversely affect the seals.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is a fluid power assist rack and pinion steering system for a vehicle having steerable wheels. The steering system comprises a rack connected with steering linkage of the vehicle for, upon movement of the rack, moving the steering linkage to effect turning of the steerable wheels. A rotatable pinion is in meshing engagement with the rack. The steering system also includes a control valve comprising a valve core rotatable relative to a valve sleeve. A first part connects the valve sleeve for rotation with the pinion. A mechanism resists relative rotation between the valve core and the valve sleeve as vehicle speed increases by placing axial force on the valve sleeve. A second part on the valve sleeve resists axial movement of the valve sleeve relative to the valve core. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The foregoing and other features of the present invention will become apparent to one skilled in the art upon reading the following description with reference to the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 is a schematic view of a portion of a vehicle fluid power assist rack and pinion steering system including a power steering control valve in accordance with the present invention;  
         [0010]    [0010]FIG. 2 is an enlarged sectional view of a portion of the steering system of FIG. 1; and  
         [0011]    [0011]FIG. 3 is a further enlarged view of a portion of the steering system of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    The present invention relates to a vehicle power steering system, and more specifically to a hydraulic vehicle power steering system in which the resistance to actuation of a power steering control valve increases with increasing vehicle speed. As representative of the present invention, FIG. 1 illustrates a vehicle fluid power assist rack and pinion steering system  12 .  
         [0013]    The steering system  12  is of the type shown in U.S. Pat. No. 5,293,954 and is operable to turn steerable vehicle wheels (not shown) upon rotation of a steering wheel  18  by an operator of the vehicle. Rotation of the steering wheel  18  actuates a hydraulic power steering directional control valve  22  to port hydraulic fluid from an engine driven pump  24  and supply conduit  26  to either one of a pair of motor conduits  28  and  30 . The high pressure fluid conducted from the supply conduit  26  through one of the motor conduits  28  or  30  effects operation of a power steering motor  31  to turn the steerable vehicle wheels in one or another direction.  
         [0014]    Simultaneously, fluid is conducted from the motor  31  to a reservoir  32  through the other one of the motor conduits  28  or  30 , the control valve  22 , return conduits  34  and  36 , and a speed responsive control unit shown schematically at  38 .  
         [0015]    Thus, return conduit  36  conducts fluid from the steering valve  22  to the speed responsive control unit  38 . Return conduit  34  conducts fluid from the steering valve  22  and the speed responsive control unit  38  to the reservoir  32 . Conduit  230  conducts fluid from the pump  24  to the speed responsive control unit  38 .  
         [0016]    The control valve  22  includes an inner rotary valve member  40  and an outer rotary valve member or sleeve  42 . The outer valve member  42  encloses the inner valve member  40 . The inner valve member  40  and outer valve member  42  are rotatable relative to (a) each other and (b) a housing  44  about a common central axis  46 .  
         [0017]    The inner valve member  40  is formed on a part of a cylindrical input member or valve stem  50  that is connected with the steering wheel  18 . The outer valve member  42  is connected with a follow-up member or pinion  54  by a diametrically opposed pair of hitch pins  56 . The follow-up member  54  is rotatably supported in the housing  44  by bearings  58  and  60 . The follow-up member  54  has a pinion gear portion  64  that is in meshing engagement with the toothed portion of a rack  66 . The rack  66  is drivingly connected with the power steering motor  31  and steerable vehicle wheels as is well known in the art.  
         [0018]    The inner valve member  40  and the outer valve member  42  are drivingly interconnected through a resilient torsion bar spring  51  (which is only partially visible in FIG. 1), as is well known in the art, and a drive mechanism  55  defined by dogs  57  on an end of the follow-up member  54  and tines  59  on an end of the input member  50 . The dogs  57  and the tines  59  allow limited rotational movement of the input member  50  relative to the follow-up member  54  when the torque in the pinion gear portion  64  required to displace the rack  66  exceeds the torque required to deflect the torsion bar  51 . Hence, the input member  50  can be displaced by a few degrees relative to the follow-up member  54  with the displacement occurring in the torsion bar  51 .  
         [0019]    The outer valve member  42  is fixed against rotation relative to the follow-up member  54  by the hitch pins  56 . Accordingly, the input member  50  and the inner valve member  40  can be rotated slightly with respect to the follow-up member  54  and the outer valve member  42 . The amount of relative rotation, within the limits of the dog and tine drive mechanism  55 , is proportional to the torque in the torsion bar  51  and other elements of the manual steering drive line, such as the follow-up member  54  and the input member  50 . This relative rotation between the input member  50  and the outer valve member  42  is used to control the flow of hydraulic fluid from the pump  24  to the steering motor  31 .  
         [0020]    The pump  24  is a fixed positive displacement pump. The control valve  22  is of the open-center type. Therefore, when the control valve  22  is in an initial or unactuated neutral condition, that is when there is no steering demand, fluid flow from the pump  24  is directed by the control valve  22  to the return conduits  34  and  36  and reservoir  32 . Hence, fluid is circulated at low pressure, by the pump  24  through the valve  22  and back to the reservoir  32 .  
         [0021]    Upon rotation of the steering wheel  18  and rotation of the valve stem  50 , the inner valve member  40 , if there is sufficient resistance to displacement of the rack  66  caused by frictional engagement of the vehicle tires with the ground or road surface, will be rotated about the axis  46  relative to the outer valve member  42 . This relative rotation moves valving edges on the inner valve member  40  relative to valving edges on the sleeve  42 , creates, in a known manner, a demand for higher pressure fluid from the pump  24  and directs the higher pressure fluid from the pump  24  to one of the motor conduits  28  or  30  and directs fluid from the other motor conduit to the reservoir  32 .  
         [0022]    As the power steering motor  31  operates, the rack  66 , which is also the rod for the motor  31 , rotates the pinion  64  and follow-up member  54 . This rotation of the follow-up member  54  together with the torque from the torsion bar  51  rotates the outer valve member  42  relative to the inner valve member  40  tending to return the valve  22  to its open center, neutral position. When the motor  31  is operated to turn the steerable vehicle wheels to an extent corresponding to the extent of rotation of the inner valve member  40 , the feedback of the rotation of the follow-up member  54  caused by movement of the rack  66  rotates the pinion  64  through a distance sufficient to move the outer valve member  42  to its initial position relative to the inner valve member. When this occurs, the fluid pressure in the motor cylinder chambers  72  and  74  falls and equalizes and the motor  31  stops operating.  
         [0023]    Pressurized fluid from the pump  24  is conducted to an annular central groove  80  formed in the outer valve member  42 . Fluid flows to the inside of the valve member  42  through a pair of diametrically opposite passages  82  and  84 . The inner and outer valve members  40  and  42  may have the same construction and cooperate with each other and the torsion bar  51  in the same manner as described in U.S. Pat. No. 4,276,812 issued Jul. 7, 1981 and entitled “Power Steering Valve and Method of Making Same”. However, the inner and outer valve members  40  and  42  could have a different construction if desired.  
         [0024]    The control valve  22  may be a “four land” type valve. The inner valve member  40  has a generally square cross-sectional configuration with rounded corners that form the four valving lands that cooperate with the edges of four axially extending grooves formed inside the outer valve member  42  to control the flow of fluid to and from the motor  31 . The ends of one pair of diametrically opposite grooves on the inside of the outer valve member  42  are connected in fluid communication with an annular outer groove  88  connected with the motor conduit  28 . A second pair of diametrically opposite and axially extending grooves on the inside of the outer valve member  42  are connected in fluid communication with an annular outer groove  90  formed in the outer valve member and connected with the motor conduit  30 .  
         [0025]    One end of the torsion bar  51  is connected to the valve stem  50  and the opposite end of the torsion bar is connected to the follow-up member  54 . The torsion bar  51  resiliently deflects when subjected to torque in a vehicle steering activity enabling relative rotation between the inner and outer valve members  40  and  42 , and when free of torque, urges the inner and outer valve members  40  and  42  to their initial positions all as is well known in the art.  
         [0026]    The torque required to actuate the control valve  22  increases as vehicle speed increases. At relatively low vehicle speeds, relative rotation of the inner and outer valve members  40  and  42  is controlled by the spring constant of the torsion bar  51  and a relatively small torque is required to rotate the inner valve member  40  relative to the outer valve member  42  and hence actuate the hydraulic assist motor  31  making the steering feel less manual. At higher vehicle speeds, the control unit  38  causes fluid pressure to act on a slidable, annular force transmitting member  116 . The member  116  is drivingly connected to the input member  50 , a cam assembly  120 , and outer valve member  42  that cooperates with the torsion bar  51  to require a larger torque to rotate the inner valve member  40  relative to the outer valve member  42  making the steering feel more manual.  
         [0027]    The force transmitting member or slider  116  is disposed in the power steering control valve housing  44 . The force transmitting member  116  rotates about its central axis  46  with the inner valve member  40  and the valve stem  50  and is movable axially along the valve stem  50 .  
         [0028]    The force transmitting member  116  is connected with the outer valve member  42  by a cam assembly  120 . The cam assembly  120  includes a plurality of downwardly facing cam surfaces  122  on the force transmitting member  116 , a plurality of upwardly facing cam surfaces  124  on the outer valve member  42 , and a plurality of balls or spherical cam elements  126  located between the cam surfaces  122  and  124 , preferably four of each. However, a greater or lesser number of cam elements  126  and cam surfaces  122  and  124  could be used if desired.  
         [0029]    The force transmitting member  116  is urged axially toward the outer valve member  42  by a spring  130  acting between a collar  232  connected to the valve stem  50  and the slidable force transmitting member  116 . The force applied against the force transmitting member  116  by the spring  130  urges the cam surfaces  122  and  124  against opposite sides of the balls  126  and maintains and centers the balls on the cam surfaces  122  and  124 .  
         [0030]    Annular upper surface  142  and annular lower surface  144  of the force transmitting member  116  cooperate with a cylindrical inner side surface  134  of the housing  44  and the cylindrical outer surface  135  of the valve stem  50  to partially define a chamber  98  and an annular pressure chamber  136  on axially opposite sides of the force transmitting member  116 . A pair of diametrically opposite openings  94  in the inner valve member  40  extend radially inward to an axially extending central passage in the inner valve member  40  in which (a) the torsion bar  51  is located and (b) is used to conduct hydraulic fluid to the chamber  136  through opening  138  extending radially outwardly from the axially extending central passage.  
         [0031]    The pressure chamber  136  is connected to the reservoir  32  by the return conduits  36  and  34  and the speed responsive control unit  38 . From the pressure chamber  136  the fluid is conducted to the speed responsive control unit  38  by the return conduit  36  and from the speed responsive control unit  38  to the reservoir  32  by the return conduit  34 .  
         [0032]    The force transmitting member  116  has a generally fluid tight fit with the inner side surface  134  of the housing  44 . The chamber  98  is connected in fluid communication with the reservoir  32  by return conduit  34 . Any fluid that leaks from the pressure chamber  136  into the chamber  98  is thus conducted back to the reservoir  32 .  
         [0033]    Although the preferred embodiment of the present invention is shown with the spring  130  located in chamber  136 , the spring  130  might not be used. If there is no spring, the length of the steering control valve housing  44  can be reduced by reducing the axial length of the chamber  136 .  
         [0034]    Rotation of the valve stem  50  and inner valve member  40  relative to the housing  44  and outer valve member  42  is resisted by a force that is related to the spring constant of the torsion bar  51  and a combination of the axial force on the force transmitting member  116  by spring  130  and the fluid pressure force applied against the annular surface  142 .  
         [0035]    The balls  126  act as driving connections between the force transmitting member  116  and the outer valve member  42 . Upon rotation of the inner valve member  40 , the cam surfaces  122  and  124  in the force transmitting member  116  and outer valve member  42  create axial and tangential forces on the balls  126  with respect to the force transmitting member  116  and the outer valve member  42 . These forces translate into (a) additional torque in the steering column felt by the operator of the vehicle, and (b) resistance to relative rotation of the inner and outer valve members  40  and  42 .  
         [0036]    Relative rotation between the inner valve member  40  and the outer valve member  42  causes the spherical elements  126  to tend to roll on the cam surfaces  122  and  124  and therefore to move the force transmitting member  116  axially away from an end  146  of the outer valve member  42 . Obviously, the force required to move the force transmitting member  116  axially away from the outer valve member  42  varies as a function of the net force urging the force transmitting member  116  toward the outer valve member  42 . Thus, the greater the net force pressing the force transmitting member  116  against the balls  126 , the greater is the force required to rotate the valve stem  50  and inner valve member  40  relative to the outer valve member  42 .  
         [0037]    The speed responsive control unit  38  responds to steering activity and vehicle speed to control the fluid pressure in the chamber  136 . The speed responsive control unit  38  is connected in fluid communication with the chamber  136  in the housing  44  by the return conduit  36 .  
         [0038]    At engine idle and relatively low vehicle speeds, a relatively low fluid pressure is present in the return conduit  36  and in the chamber  136 . At engine idle and low vehicle speeds, the force of the spring  130  and the low fluid pressure in chamber  136  urge the force transmitting member  116  toward the cam elements  126 . Thus, there is little resistance to relative rotation between the valve stem  50  and outer valve member  42  and the steering effort feels less manual.  
         [0039]    At relatively high speeds of the vehicle, the pressure in chamber  136  is at a maximum and there is maximum resistance to relative rotation of the valve stem  50  and outer valve member  42  and less hydraulic assist is provided and the steering feels more manual.  
         [0040]    Changes in the fluid pressure in the chamber  136  affect the axial forces acting on the valve sleeve  42 . When the pressure in the chamber  136  is relatively low, fluid pressure in the control valve  22  can tend to urge the valve sleeve  42  to move axially in its housing  44 , in an upward direction as viewed in the drawings. Upon such movement, seals  160 , such as the one shown in FIG. 2, that seal the annular space between the valve sleeve  42  and the housing  44 , can be forced out of their grooves and into the annular space between the valve sleeve and the housing. This can adversely affect the sealing ability of the seals  16 .  
         [0041]    In accordance with the present invention, the axial movement of the valve sleeve  42  is limited by a snap ring  170  placed on the valve sleeve  42  after the valve sleeve is assembled with the hitch pins  56 . The valve sleeve  42  (FIGS. 2 and 3) is initially assembled into the control valve  22  by sliding it axially until the hitch pins  56 , which are press fitted in the pinion  54 , move into axially extending slots  162  in a lower end portion  164  of the valve sleeve  42 . FIGS. 2 and 3 show one of the pin/slot assemblies. The engagement of the hitch pins  56  in the slots  162  couples the valve sleeve  42  for rotation with the pinion  54  in a follow-up manner. In this initial condition of assembly, however, the valve sleeve  42  is movable axially off the hitch pins  56 , that is, in an upward direction as viewed in FIGS. 2 and 3, in response to the axial forces in the control valve  22 .  
         [0042]    The snap ring  170  is received in a groove  172  in the valve sleeve  42 . The groove  172  extends 360 degrees around the outer circumference of the valve sleeve  42 , in the lower end portion  164  of the valve sleeve. The groove  172  extends through, and is thus discontinuous at, the two slots  162 . The groove  172  is located so that the snap ring  170  engages the two hitch pins  56  when the snap ring is placed in the groove.  
         [0043]    During operation of the steering system  12 , forces that tend to move the valve sleeve  42  in a downward direction as viewed in FIGS. 2 and 3 are counteracted by the engagement of the hitch pins  56  with the closed ends of the slots  162 . Forces that tend to move the valve sleeve  42  in an upward direction as viewed in FIGS. 2 and 3 are counteracted by the engagement of the snap ring  170  with the hitch pins  56 . The snap ring  170  engages the hitch pins  56  to limit axial movement of the valve sleeve  42  in this direction, relative to the pinion  56  and the valve core  40 . This prevents the seals  160  from being extruded into the gap between the valve sleeve  42  and the housing  44 .  
         [0044]    From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications in the invention. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.