Abstract:
A steering system of a vehicle is provided that includes a steering gear assembly including a tubular housing coupled to the vehicle. A pre-load compliant mounting bracket is coupled to the tubular housing and mounted to the vehicle for laterally placing the steering gear assembly into a pre-loaded state. A compliant mounting bracket is coupled to the tubular housing and mounted to the vehicle for limiting the steering gear assembly from laterally displacing relative to the vehicle beyond a predetermined distance. The pre-load compliant mounting bracket is compliant to the lateral displacement of the steering gear assembly for producing an understeer condition when a steering force above a predetermined steering force threshold is applied to the vehicle and is non-compliant when a steering force below the predetermined steering force threshold is applied to the vehicle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       REFERENCE TO A SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX  
       [0003]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0004]     1. Field of the Invention  
         [0005]     This invention relates in general to steering systems, and in particular, to a compliant steering gear assembly.  
         [0006]     2. Description of the Related Art  
         [0007]     Steering systems in motor vehicles typically comprise rack and pinion steering systems. The rack and pinion steering systems consist of a pinion shaft, pinion teeth, a rack including rack teeth, and a housing. The housing houses a hydraulic valve section, an extended rack section, and houses the meshing of the pinion teeth and the rack teeth. The housing is commonly supported by one or more mounting brackets. The mounting brackets are secured to the housing and are bolted to a vehicle frame or a unibody structure. Rubber grommets or bushings disposed between the mounting bracket and the vehicle frame are typically used to dampen noise and vibrations from the road.  
         [0008]     The mounting brackets typically are either rigid to prevent movement of the steering gear assembly relative to the vehicle frame or resilient to accommodate movement of the steering gear assembly relative to the vehicle frame. Typically when a vehicle is cornering at low lateral acceleration forces, it is desirable to have the steering gear assembly rigid so that the steerable gears are immediately responsive to the driver&#39;s demands. At increased lateral acceleration forces, lateral movement of the steering gear assembly (i.e., a resilient mounting of the steering gear relative to the vehicle frame) is desirable in order to create an understeer condition. That is, when lateral movement of the steering gear assembly occurs, the steering wheel must be rotated an additional number of degrees before the driver&#39;s intended steering affect is produced. The steering system will respond with a smaller steering angle than if the steering gear assembly were rigidly mounted to the vehicle. The understeer condition is more desirable as the vehicle approaches adhesion limits than an oversteer condition at the adhesion limits.  
         [0009]     Thus, it is desirable to have a steering system which maintains a rigidly mounted steering gear assembly when low lateral acceleration forces are exerted on the vehicle and changes to a compliant steering gear assembly when high lateral acceleration forces are exerted on the vehicle.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The present invention has the advantage of providing a rigid steering system which is immediately responsive to steering and guidance inputs as intended by the driver of a vehicle when lateral acceleration forces exerted on the vehicle are below a predetermined threshold and providing a compliant steering system when lateral acceleration forces exerted on the vehicle are greater than the predetermined threshold.  
         [0011]     In one aspect of the invention, a steering system of a vehicle is provided that includes a steering gear assembly including a tubular housing coupled to the vehicle. A pre-load compliant mounting bracket is coupled to the tubular housing and mounted to the vehicle for laterally placing the steering gear assembly into a pre-loaded state. A compliant mounting bracket is coupled to the tubular housing and mounted to the vehicle for limiting the steering gear assembly from laterally displacing relative to the vehicle beyond a predetermined distance. The pre-load compliant mounting bracket is compliant to the lateral displacement of the steering gear assembly for producing an understeer condition when a steering force above a predetermined steering force threshold is applied to the vehicle and is non-compliant when a steering force below the predetermined steering force threshold is applied to the vehicle.  
         [0012]     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a perspective view of a tube and bracket assembly for a rack and pinion steering system of a vehicle according to a first preferred embodiment of the present invention.  
         [0014]      FIG. 2  is a cross section, taken along line  2 - 2  in  FIG. 1 , of a pre-load compliant mounting bracket and tubular housing.  
         [0015]      FIG. 3  is a cross section of a pre-load compliant mounting bracket and tubular housing similar to  FIG. 2 , but illustrating a second preferred embodiment of the present invention.  
         [0016]      FIG. 4  is a cross section of a pre-load compliant mounting bracket and tubular housing similar to  FIG. 2 , but illustrating a third preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]      FIG. 1  shows a steering gear assembly  10  for a rack and pinion steering system, indicated generally at  11 , mounted to a frame  27  of a vehicle. The rack and pinion steering system may be an electrical, hydraulic or a manual steering system. The steering gear assembly  10  includes a pinion gear housing  12  that houses an input shaft  14 . A first end of the input shaft  14  is coupled to a steering wheel (not shown) through conventional steering linkages (not shown) and receives rotational input from a driver of the vehicle. The input shaft  14  has a pinion gear  16  formed on a second end of the shaft  14 .  
         [0018]     The steering gear assembly  10  further includes a tubular housing  18  that houses a rack  20 . The rack  20  includes rack gear teeth  22  that mesh with the pinion gear  16  within the pinion gear housing  12  for laterally displacing the rack  20  when the input shaft  14  is rotated. The opposite ends of the rack  20  each include a ball and socket joint (not shown) for pivotably connecting tie rods (not shown) to the rack  20 . The tie rods are connected to steerable wheels (not shown) for steering the vehicle via the driver&#39;s manual input commands to the steering wheel.  
         [0019]     The steering gear assembly  10  includes a compliant mounting bracket  24  and a pre-load compliant mounting bracket  26  for securing the steering gear assembly  10  to a portion of the vehicle frame  27 , which may be a frame a cross member or a portion of a unibody structure. Preferably, the compliant mounting bracket  24  includes a semi-circular portion  30  for receiving the tubular housing  18  and apertures  32  for receiving fastening members  36  for mounting the tubular housing  18  to the vehicle frame  27 . Alternatively, the compliant mounting bracket  24  may utilize shapes other than semi-circular.  
         [0020]     Rubber mounting grommets  34  are disposed between the compliant mounting bracket  24  and the vehicle frame  27  at the locations of the apertures  32 . The rubber grommets  34  allow lateral movement of the steering gear assembly  10  relative to the vehicle frame  27 . The rubber grommets  34  have a predetermined modulus of elasticity that will allow them to elastically deform to an extent that the steering gear assembly  10  can move laterally to a predetermined maximum distance. In alternative embodiments, the rubber grommets  34  are produced from an alternative material which still allows the steering gear assembly  10  to laterally move this predetermined maximum distance when fastened to the vehicle frame  27 . Other mounting methods may be used to attach the compliant mounting bracket  24  to the vehicle frame including using spike mounts (i.e., studs) that are integrally formed or affixed to the attaching vehicle member. The spike mounts extend through apertures are securely fastened by a nut or similar fastening device. Various types of attachments in addition to those described herein may be utilized to attach the compliant mounting bracket to the vehicle without deviating from the scope of the invention.  
         [0021]     The pre-load compliant mounting bracket  26  includes a spring-like bushing  40  that is contoured to an exterior surface  38  of the tubular housing  18 . In the preferred embodiment, the spring-like bushing  40  has a generally cylindrical shape and is preferably made of a resilient material such as rubber which provides a predetermined spring rate. Alternatively, other shapes may be utilized in addition to other suitable resilient materials utilized in place of rubber such as metal or plastics. A corresponding cylindrical plate  41  is formed about an outer cylindrical surface  50  of the bushing  40  for encasing all or at least a portion of the bushing  40 . The cylindrical plate  41  includes apertures  42  for receiving fastening members  46  that secure the cylindrical plate  41  around the bushing  40  and to the vehicle frame  27 . The cylindrical plate  41  is preferably made of metal, although other materials such as plastic or composites may be utilized.  
         [0022]     The pre-load compliant mounting bracket  26  and tubular housing  18  cooperatively form a latch system  49  (shown in  FIG. 2 ).  FIG. 2  shows a cross section of the pre-load compliant mounting bracket  26  and the tubular housing  18  illustrating the latch system  49  according to the first preferred embodiment. The bushing  40  of the pre-load compliant mounting bracket  26  includes an inner generally cylindrical surface  48  and the outer circumference surface  50 . The inner surface  48  receives and conforms to the exterior surface  38  of the tubular housing  18 .  
         [0023]     The tubular housing  18  includes a detent  52  integrally formed on the exterior surface  38  of the tubular housing  18 . In this embodiment, the detent  52  is a raised surface area that extends circumferentially about the tubular housing  18 . Alternatively, the raised surface may include individual bumped surface areas as opposed to a continuous raised surface area. In addition, the detent  52  may be separately formed from the tubular housing  18  and may thereafter be welded, press-fit, or attached to the tubular housing  18  by other means.  
         [0024]     The bushing  40  includes a recessed portion  54  that extends circumferentially about the inner surface  48  of the bushing  40 . The recessed portion  54  mates, in a complementary fashion, with the detent  52 . The detent  52  and the recessed portion  54  form the latch system  49 , which maintains alignment between the pre-load compliant mounting bracket  26  and the tubular housing  18  until a steering force exerted on the vehicle (i.e., steering gear assembly  10 ) reaches a predetermined limit.  
         [0025]     Referring now to  FIGS. 1 and 2 , as lateral acceleration forces are exerted on a vehicle, for example, when the vehicle is cornering, steering loads are reacted by the steering gear system  10 . As the vehicle corners at high speeds, it is advantageous for the vehicle to be in an understeer condition. To achieve this understeer condition when high lateral acceleration forces are exerted on the vehicle, the steering rack system  10  is allowed to move laterally relative to the frame  27 . This is achieved by the interaction between the detent  52  of the tubular housing  18  and the recessed portion  54  of the pre-load compliance mount  26 .  
         [0026]     For an understeer condition where the steering force exerted on the steering gear system  10  is less than a predetermined threshold, the detent  52  is held rigid within the recessed portion  54 . The stiffness of the bushing  40 , along with the size and shape of the detent  52  and recess portion  54 , assures that the detent  52  is held rigid within the recessed portion  54  below this steering force threshold. This maintains a rigid steering system. That is, the steering affect which is desired and expected by the driver is immediately executed by the steering system since the steering rack system  10  is held rigid relative to the frame  27 .  
         [0027]     For a vehicle condition where the steering force exerted on the vehicle is greater than a predetermined threshold, the steering force exerted on the steering gear assembly  10  overcomes holding force of the latch system  49  and forces the detent  52  to move out of alignment with the recessed portion  54 . The steering wheel now has to rotate a greater number of degrees before the driver is provided with the steering affect intended, thus creating an understeer condition of the vehicle. As stated earlier, for a vehicle having zero or low steering forces (less than the predetermined threshold) exerted on the vehicle, a non-compliant steering system is maintained. Non-compliant used herein refers to a rigid steering system or at least a reduced compliant steering system where some lateral movement is allowed for reducing factors such as NVH. Under high steering forces (higher than the predetermined threshold), a compliant steering system is maintained.  
         [0028]     The compliant mounting bracket  24  compliments the pre-load compliant mounting bracket  26  by preventing the steering gear assembly  10  from moving more than a predetermined distance relative to the frame  27 . The limits of the predetermined length that the steering gear assembly  10  may laterally move are based on the modulus of elasticity of the compliant mounting bracket  24 . Changing the properties of the material (i.e., hardness) will change the elasticity of the compliant mounting bracket  24 .  
         [0029]      FIG. 3  shows a cross section of a pre-load compliant mounting bracket  56  and the tubular housing  58  illustrating a latch system  59  according to a second preferred embodiment. The pre-load compliant mounting bracket  56  includes a bushing  60  having a detent  62  integrally formed and extending from the inner surface  64  of the bushing  60 . Alternatively, the detent may be formed separately and affixed to the pre-load compliant mounting bracket  56 . The detent  62  is a raised surface area that extends circumferentially about the inner surface  64  of the bushing  60 . Alternatively, the raised surface may include individual bumped surface areas as opposed to a continuous raised surface area.  
         [0030]     The tubular housing  58  includes a recessed portion  68  that extends circumferentially about the exterior surface  38  of the tubular housing  58 . The recessed portion  68  mates, in a complementary fashion, with the detent  62 . The detent  62  and the recessed portion  68  form a latch system  59  which, like the first embodiment, maintains alignment between the pre-load compliant mounting bracket  56  and the tubular housing  58  until a steering force exerted on the steering gear system  10  reaches a predetermined threshold.  
         [0031]     When the lateral acceleration force exerted on the vehicle results in steering forces reaching a predetermined threshold, the bushing  60  deforms to allow the detent  62  of the bushing  60  to move laterally out of the recessed portion  68  of the tubular housing  58 . The understeer condition is produced in response to the detent  62  unlatching from the recessed portion  68 .  
         [0032]      FIG. 4  shows a cross-section of the pre-load compliant mounting bracket  70  and the tubular housing  18  illustrating a latch system  79  according to a third preferred embodiment. The tubular housing  18  is similar to that discussed in  FIG. 2 . The pre-loaded compliance mount  76  includes a bushing  80  having a detent  82 . The detent  82  includes at least one bore  83  formed in the inner surface wall  84  of the bushing  80 . A spring and ball assembly  85  having a predetermined spring-rate is disposed in the bore  83 . When the spring and ball assembly  85  and the recess portion  54  are aligned, the spring force seats a ball  88  within the recessed portion  54 . When the lateral acceleration forces exerted on the vehicle results steering forces greater than the predetermined threshold, the spring force of the spring and ball assembly  85  is overcome, the spring  86  compresses thereby unlatching the ball  88  from the recessed portion  54 . The understeer condition is produced in response to the detent  82  unlatching from the recessed portion  54 .  
         [0033]     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.