Abstract:
An apparatus for urging components of power steering system into engagement includes a rack including gear teeth, a pinion including gear teeth engaged with the gear teeth of the rack, a yoke bearing contacting the rack, a spring for resiliently contacting the yoke and urging the rack into engagement with the pinion, a source of variable hydraulic pressure communicating with the yoke bearing and applying to the yoke bearing a force that urges the rack into engagement with the pinion, and a valve that opens a connection between the pressure source and the yoke bearing at a first speed in response to a first pressure magnitude at the pressure source and that closes said connection at a second speed slower than the first speed when pressure at the pressure source decreases relative to the first pressure magnitude.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to a power steering system for a motor vehicle, in which a toothed rack is biased toward engagement with a pinion gear. More particularly, the invention pertains to adjusting the magnitude of a force that urges a yoke toward the rack to maintain the teeth on the pinion and rack in mutual engagement. 
         [0002]    A rack and pinion steering assembly includes a rack, which is disposed in meshing engagement with a pinion. A housing encloses the rack and pinion. A yoke presses the rack toward the pinion to maintain meshing engagement between gear teeth on the rack and gear teeth on the pinion. In a power steering system, steering effort of the vehicle operator in displacing the rack is assisted by a hydraulically-actuated double acting piston, secured to the rack, and a hydraulic motor, the source of hydraulic pressure applied to the piston. The power steering system reduces the level of effort required by the vehicle operator to change the position of the steered wheels in response to changes in the angular position of the steering wheel controlled manually by the operator. 
         [0003]    Because the gear teeth on the rack and pinion are helical teeth, the turning force transmitted between the engaged teeth has a component tending to force the rack teeth away from engagement with the pinion teeth. This force urges the rack and yoke to move away from the pinion against the effect of a spring force. In addition, impulse forces transmitted from the road surface to the assembly  10  due to the wheels hitting potholes, rocks or debris, etc., called “road events,” can also move the rack away from pinion in a direction transverse to a longitudinal central axis of the rack. 
         [0004]    Empirical data show that road events not only create forces which are transferred through tie-rods to the rack assembly, but also induce hydraulic pressure events in the hydraulic motor due to movement of the rack assembly in response to road events. These pressure events can be measured using pressure transducers. Normally pressure in the tower hydraulic system is in the range of 15-75 psi., but road events can create brief pressure pulses over 300 psi. lasting about 5 ms. 
         [0005]    It is conventional to rely on a compression spring to maintain a biasing force applied to the yoke and urging the rack to remain engaged with the pinion. In operation, however, the yoke bearing is susceptible to large loads induced by road surface imperfections tending to disengage the rack and the pinion. It is desirable to augment the spring force with hydraulic pressure force that would resist such disengagement. The spring force also increases frictional forces from the yoke to the rack, thereby reducing sudden movement of the rack. 
         [0006]    It is desireable that this force be as low as possible so that it is easier for the driver to maneuver the vehicle. A continuously large force makes the steering gear harder to turn. 
       SUMMARY OF THE INVENTION 
       [0007]    In one embodiment a steering assembly for use in turning wheels of a vehicle includes a rack, a pinion, and a yoke continuously pressed against the rack by a yoke spring. A force, developed by hydraulic pressure and arranged in series with the spring, elastically resists displacement of the yoke away from the rack and urges the rack toward engagement with the rack. It is therefore, an advantage of this arrangement that energy produced by road events, which normally create forces that try to separate the rack teeth from the pinion teeth, is used to produce a pressure force that keep the pinion and rack teeth engaged, thereby reducing noise, vibration and harshness. 
         [0008]    Hydraulic pressure for this purpose supplied to the yoke bore for energizing the yoke assembly, is carried from a location in the steering assembly where fluid flow from a hydraulic pump is directed by a control valve to the hydraulic motor and to a fluid reservoir. Alternatively, hydraulic tower pressure is supplied to the yoke bore from a return line port or supply line port. The hydraulic force (attack and decay) can be tuned by adjusting the supply line inside diameter, the size of an orifice in the supply line, or use of mechanical or electromechanical valves. A hydraulic auto adjustment feature reduces or eliminates yoke sensitivity to mechanical lash set during assembly. 
         [0009]    An apparatus for urging components of power steering system into engagement includes a rack including gear teeth, a pinion including gear teeth engaged with the gear teeth of the rack, a yoke bearing contacting the rack, a spring for resiliently contacting the yoke and urging the rack into engagement with the pinion, a source of variable hydraulic pressure communicating with the yoke bearing and applying to the yoke bearing a force that urges the rack into engagement with the pinion, and a valve that opens a connection between the pressure source and the yoke bearing at a first speed in response to a first pressure magnitude at the pressure source and that closes said connection at a second speed slower than the first speed when pressure at the pressure source decreases relative to the first pressure magnitude. 
         [0010]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will become more fully understood from the detailed description given below, the appended claims, and the accompanying drawings, in which: 
           [0012]      FIG. 1  is a side view partially in cross section illustrating a rack and pinion steering assembly to which the present invention may be applied; 
           [0013]      FIG. 2  is a cross section taken at plane  2 - 2  of  FIG. 1  illustrating a first embodiment for dynamically adjusting a force that urges a yoke bearing and rack toward a pinion for maintaining the teeth on the pinion and rack in mutual engagement; 
           [0014]      FIG. 3  is a cross section through the housing assembly showing a second embodiment for dynamically adjusting the magnitude of the force that urges the yoke bearing and rack toward a pinion; 
           [0015]      FIG. 4  is a schematic diagram of an third embodiment for dynamically adjusting the magnitude of the force that urges the yoke bearing and rack toward a pinion; and 
           [0016]      FIG. 5  is a cross section through a valve for regulating communication between the yoke bearing and a pressure source; 
           [0017]      FIG. 6  is a graph showing transient pressure occurrences at the pressure source caused by road events; and 
           [0018]      FIG. 7  is a graph showing the variation over time of hydraulic force on the yoke bearing due to transient pressures caused road events. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    Referring first to  FIGS. 1 and 2 , a rack and pinion steering assembly  10  includes a rack  12 , whose axially opposite ends are connected to vehicle wheels able to be steered by manual operation of a steering wheel by the vehicle operator. A pinion  14  has gear teeth in meshing engagement with teeth formed on the rack  12 . A housing  16  enclosing the rack  12  and pinion  14  includes a cast main housing section  18 . A circular yoke plug  20  having external threads  22 , which engage internal threads  24  on the main section  18  of the housing, closes the upper end of a cylindrical yoke chamber  26  formed in the main housing section  18 . 
         [0020]    A yoke  28 , located in the yoke chamber  26 , contacts a helical coiled compression spring  32 , disposed between the yoke  28  and the yoke plug  20 . Alternatively, spring  32  may be an elastomeric element or a wave spring. The spring  32  presses the yoke  28  firmly against the rack  12  due to an elastic, resilient force produced by the spring. 
         [0021]    The pinion  14  is supported for rotation on a bearing  38 , fitted in the housing  16 . A nut  37 , threaded into the housing  16 , locates the bearing correctly and secures it in the housing. 
         [0022]    A fluid motor  36 , connected to the rack  12 , assists in turning the vehicle wheels. Rotation of the steering wheel of the vehicle actuates a steering control valve (not shown), which directs flow of hydraulic fluid to and from the fluid motor  36  through conduits  40 ,  42  in response to the directional sense of the steering wheel displacement. When the steering wheel is turned from a neutral or straight-ahead position, the control valve directs pressurized fluid from the outlet of a hydraulic pump through one of the conduits  40 ,  42  to one side of a piston located in the hydraulic motor, and it vents fluid through the other conduit from the opposite side of the piston to a pressure reservoir, from which the pump inlet is supplied with fluid. In either case, the pump outlet continually supplies pressurized fluid to the control valve, which is usually located in a tower  44 , formed integrally with the housing  16 . 
         [0023]    The yoke  28  has an arcuate inner surface  46 , which engages an arcuate outer surface  48  on the rack  12 . The yoke spring  32  continually biases the arcuate inner surface  46  on the yoke  28  toward the outer surface  48  of the rack  12 . The yoke  28  is formed with a wall  50 , which engages a circular cylindrical inner surface of the chamber  26  of main housing section  18 . The yoke wall  50  is formed with a circular cylindrical outer surface. 
         [0024]    The yoke plug  20  includes a flat circular inner surface  58 , which faces the end surface of the yoke  28 . A hexagonal socket  60  formed on the outer surface of the plug  20  can be engaged by a wrench, or a similar turning device, to install and remove the plug from the main housing section  18 . 
         [0025]    A hydraulic line  68  connects chamber  26  with a source of pressurized hydraulic fluid from the top or bottom of the control tower  44 . Alternatively, chamber  26  is hydraulically connected through line  68  to the hydraulic lines that carry fluid from a power steering pump outlet to control valve  72 , or fluid from the control valve to a fluid reservoir, from which the pump inlet is supplied with fluid. In either case, pressure of the fluid in line  68  rises and falls in response to road events transmitted from the vehicle tie rods to the rack  12 . 
         [0026]    A control valve  74 , preferably located in line  68 , regulates the time rate of increase and decrease of pressure in chamber  26 , as described below with reference to  FIGS. 5-7 . The connection between line  68  and chamber  26  is sealed against the passage of fluid by an O-ring  76 , seated in yoke  28  and elastically contacting in the inner surface of main housing section  18 , and by an O-ring  78 , seated in plug  20  and elastically contacting the inner surface of the main housing section. 
         [0027]    Pressurized fluid in chamber  26  applies a pressure force to the annular surface  80  of the yoke  28 , which force adds to the spring force tending to urge rack  12  into engagement with the teeth  54  of the pinion  14 . The magnitude of the force is equal to the product of the pressure times the area to which the pressure is applied. 
         [0028]    Referring now to  FIG. 3 , the yoke is formed in two portions, an axial inner portion  82 , whose surface  46  contacts the rack  12 , and an axially outer portion  84 , which contacts the spring  32 . The conduit  86  formed in the wall of housing  18  and connected to line  68  is sealed against the passage of fluid by an O-ring  88 , seated in the inner yoke portion  82  and elastically contacting in the inner surface of main housing section  18 , and by an O-ring  90 , seated in the outer yoke portion  84  and elastically contacting the inner surface of the main housing section. A third O-ring  91  is seated in the inner yoke portion  82  and elastically contacts the outer yoke portion. An axial duct  92  communicates a space  94  at the inner end of yoke portion  84  to the end of the yoke portion  84 . 
         [0029]    Pressurized fluid in the annular space  96  between yoke portions  82 ,  84  applies a pressure force to the annular surface  98 , which adds to the spring force tending to urge rack  12  into engagement with the teeth  54  of the pinion  14 . Pressurized fluid in the annular space  96  also applies a pressure force to the annular surface  100 , which opposes the spring force tending to urge rack  12  into engagement with the teeth  54  of the pinion  14 . The magnitude of the pressure force on surface  98  can be increased by increasing the area of surface  98  and reducing the diameter of the projection  102  on yoke portion  84 , which extends into yoke portion  82 . The magnitude of the pressure force on surface  98  can be decreased by decreasing the area of surface  98  and increasing the diameter of projection  102 . 
         [0030]      FIG. 4  illustrates a two-piece yoke  102  comprising an axial inner portion  104 , whose surface  46  contacts the rack  12 , and an axially outer portion  106  having a recess  108  containing an elastic member  32 , such as a helical coiled spring, wave spring or elastomeric member. The yoke is formed with an annular recess  114 , which communicates with line  68  through a conduit  116  formed in the wall of housing  18 . The conduit  116  is sealed at opposite axial sides against the passage of fluid by an O-ring  118 , seated in the yoke portion  104  and elastically contacting in the inner surface of main housing section  18 , and by an O-ring  120  seated in the yoke portion  106  and elastically contacting the inner surface of the main housing section. Line  68  extends to the end of yoke portion  106  and communicates with a recess  113  formed in yoke  106  adjacent plug  20  through a radial passage  122  formed through the wall of housing  18 . 
         [0031]    Pressurized fluid in the annular recess  122  applies a pressure force, which adds to the spring force tending to urge rack  12  into engagement with the teeth  54  of the pinion  14 . Pressurized fluid in the recess  114  also applies to the yoke portion  106  a pressure force, which opposes the spring force. However, a pressure force applied to the axial end face of recess  122  is equal to the pressure force tending to oppose the spring force. Pressurized fluid in recess  122 , therefore, applies a pressure force to the annular surface  26  of yoke portion  106 , which force adds to the spring force tending to urge rack  12  into engagement with the teeth of the pinion  14 . This provides additional control to dampen movement. 
         [0032]      FIG. 5  illustrates an example of a valve  74  that regulates the time rate of increase and decrease of pressure in the yoke chamber  26 .  FIG. 6  illustrates transient pressure changes  126  in line  68  upstream from valve  74  caused by road events.  FIG. 7  illustrates the variation over time of the magnitude of pressure forces applied to the yoke when pressure downstream of valve  74  is regulated by the valve. 
         [0033]    Valve  74  includes a cage  130  containing a control element  132  formed with a central orifice  134 , and a compression spring  136 , which urges the control element toward a seated position on the cage, the position shown in  FIG. 5 . Road events produce a transient pressure increase  126  at the inlet  138  of valve  74 . That pressure compresses spring  136 , unseats element  132 , and allows fluid to flow around the outer periphery of element  132  and through orifice  134  to the outlet  140 , from which fluid flows through line  68  to the yoke chamber  26 . 
         [0034]    In  FIG. 7 , valve  74  opens at  142  due to pressure transient  126 , the pressure force on the yoke increases along ramp  144 , and the valve closes at  146 . Thereafter, the yoke pressure force decreases along ramp  148  in accordance with the size of orifice  134  and leakage past the valve  74 , until the next road event  127 , whereupon valve  74  again opens at  150 . The total force on the yoke is the sum of the pressure force and the force produced by spring  32 . 
         [0035]    The dynamic forces on the yoke bearing are applied when the rack is subjected to impulse movement. The increased forces on the yoke bearing also increase the normalized friction to negate or minimize the effects of the positional movement impulse. This also improves the feel of the steering system to the vehicle operator. 
         [0036]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.