Abstract:
A method and apparatus for reducing rattle in a column mounted electric power steering system. A method of reducing audible noise in a steering system, generated by road impact energy and transmitted through a transmission medium, which includes a column mounted electric power steering system, including interrupting the transmission medium; and absorbing the road impact energy through the steering system. A column assist isolation system includes a sleeve disposed at a housing of an electric power steering system, a first bushing disposed at a first end of the sleeve, and a second bushing disposed at a second end of the sleeve. Both the first bushing and the second bushing include a compliant material.

Description:
BACKGROUND 
     Certain motor vehicles contain column-type electric power steering systems. The electric power steering system provides torque assist to the steering shaft of a vehicle via an electric motor and a worm/worm gear reduction mechanism. The worm/worm gear reduction mechanism is interposed between the output shaft and the motor to obtain an appropriate steering speed as well as sufficient steering assistance in the course of transmission of the rotational force from the motor to the output shaft. 
     When the motor vehicle is traveling over a less-than-smooth road surface, a certain amount of road impact energy is created at the points at which the steerable wheels contact the road surface. The road impact energy travels through the tires, wheels and linkage, through the rack and pinion gear, and through the column mounted electric power steering system. The road impact energy causes high bi-directional acceleration of system components, which creates audible noise, commonly known as rattle. While not indicative of a defect in the system, such rattle oftentimes proves to be an annoyance to the occupants of the motor vehicle. 
     SUMMARY 
     A method and apparatus for reducing rattle and increasing road feel at a steerable wheel in a vehicle. When road impact energy is transmitted through a transmission medium, which includes a column mounted electric power steering system, an audible noise may be generated throughout a steering system of a vehicle. Interrupting the transmission medium and absorbing the road impact energy through the steering system reduces the audible noise in the steering system and increases the road feel at the steerable wheel of the vehicle. 
     A column assist isolation system includes a sleeve disposed at a housing of an electric power steering system, a first bushing disposed at a first end of the sleeve, and a second bushing disposed at a second end of the sleeve. Both the first bushing and the second bushing include compliant materials. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
     FIG. 1 is a schematic view of a steering system of a vehicle; 
     FIG. 2 is an exploded view of an EPS system with column assist isolation system; and 
     FIG. 3 is an exploded view of an alternative embodiment EPS system with column assist isolation system. 
    
    
     DETAILED DESCRIPTION 
     A column assist isolation system can be mounted on any type of column mounted electric power steering (EPS) system. Column mounted EPS systems generally includes shafts, gears, and other components that connect the steerable wheels to the steerable device of a vehicle. These components, also known as a transmission medium, have certain material and engagement characteristics that allow vibration to cause audible noise and to be transmitted throughout the steering system and heard by the occupants of the vehicle. The column assist isolation system absorbs, reflects, and dissipates vibration at the transmission medium that occurs due to the road impact energy. Absorbing, reflecting, and dissipating vibration at the transmission medium reduces the noise throughout the transmission medium. In addition, the column assist isolation system enhances the steering feel at the steering device of a vehicle in which the system is installed. Moreover, the functionality of the column assist isolation system is the same regardless of the type of column mounted EPS system. For this reason, the column assist isolation system can be applied by one of ordinary skill in the art to any column mounted EPS system. 
     Before describing the column assist isolation system, a steering system is described so that the column assist isolation system can be placed in context. Referring to FIG. 1, one type of column mounted EPS system incorporable into a motor vehicle is shown generally at  10  and is hereinafter referred to as “steering system  10 ”. Steering system  10  alters the direction of travel of a motor vehicle (not shown) in response to an input from an operator of the motor vehicle, a sensor disposed in the motor vehicle, or a combination thereof. Such a response ultimately changes the angle of the steerable wheels of the motor vehicle while the motor vehicle is traveling. By changing the angle of the steerable wheels while the motor vehicle is traveling, a change in the direction of travel can be effectuated. 
     Steering system  10  comprises a steering device  14 , which is accessible by an operator (not shown) of the motor vehicle and is disposed on a steering shaft  17 . The rotational force of steering device  14  is transmitted to steering shaft  17 , and is detected by a torque sensor  30  disposed thereon. Torque sensor  30  measures the torque exerted on steering shaft  17  in axial directions and transmits a signal  31  to a controller  32 . Controller  32  then transmits an output signal  33  to a motor  34  to initiate the operation of motor  34 . Motor  34  is in operable communication with a worm shaft  36  so that motor  34  drives worm shaft  36 . 
     Worm shaft  36  axially rotates, which causes worm  38  (shown on FIG. 2) to rotate. Worm  38  meshes with worm gear  26 , which rotates when worm  38  rotates. Worm gear  26  provides turning assistance to steering shaft  17 . As steering shaft  17  axially rotates, intermediate shaft  52  disposed in mechanical communication with a coupling joint  58  rotates a pinion gear (not shown) located under a gear housing  60 . Coupling joint  58  may be a universal joint, as shown, or a torsional member. Rotation of the pinion gear (not shown), which is located in a housing  60 , moves a rack  62 , which moves tie rods  64  (only one shown). When tie rods  64  move, they turn steering knuckles  66  (only one shown), which turn steerable wheels  12  (only one shown). 
     Referring now to FIG. 2, steering shaft  17  is shown in greater detail. Steering shaft  17  comprises an upper shaft  16 , which is in operable communication with a lower shaft  18  through a torsion bar  20 . Needle bearing  22  and pin  24  support torsion bar  20  and are located at the connection between torsion bar  20  and upper shaft  16 . A worm gear  26  is in operable communication with lower shaft  18 . A bearing  28  supports lower shaft  18  at worm gear  26 . 
     A worm  38  is connected to a worm shaft  36 . Worm shaft  36  is supported by two bearings  40 ,  42 . On one side of worm shaft  36 , bearing  42  is pressed against worm  38  and axially retained by a nut  44  and a plug  46 . On the opposite side of worm shaft  36 , bearing  40  is pressed against worm  38  and axially supported by a retaining ring  48 . Upon operation of motor  34 , worm shaft  36  is driven and worm  38  and worm gear  26  correspondingly rotate to provide turning assistance to lower shaft  18 . 
     Worm gear  26  may be contained in a housing  50 . Lower shaft  18  extends through housing  50  and from a downside end  70  thereof to maintain operable communication with an intermediate shaft (not shown). A bearing  54  supports housing  50  at lower shaft  18  and is axially secured by a nut  56 . 
     A column assist isolation system  68  is mounted to downside end  70  of housing  50 . Column assist isolation system  68  comprises a sleeve support  72 , which may be either connected to housing  50  or molded as part of housing  50  that extends from downside end  70 . Sleeve support  72  may be cylindrically shaped and comprises a first sleeve support  74  and a second sleeve support  76 . Sleeve  78  is inserted through an opening  75  in first sleeve support  74  and inserted through an opening (not shown) in second sleeve support  76  so that a first end  80  of sleeve  78  is supported by first sleeve support  74  and a second end  82  of sleeve  78  is supported by second sleeve support  76 . 
     First end  80  of sleeve  78  is retained at first sleeve support  74  by a first bushing  84 . First bushing  84  is pressed onto first end  80  of sleeve  78  so that an elongated end  88  of first bushing  84  is pressed between first sleeve support  74  and sleeve  78 . Second end  82  of sleeve  78  is retained at second sleeve support  76  by a second bushing  86 . Second bushing  86  is pressed onto second end  82  of sleeve  78  so that an elongated end  90  of second bushing  86  is pressed between second sleeve support  76  and sleeve  78 . 
     First bushing  84  and second bushing  86  are made of a material that has compliant properties. An example of such material is that the material could contain a certain amount of rubber or elastomer or be made completely from rubber or elastomer. First bushing  84  and second bushing  86  may also be formed from a wire mesh, or a steel wool material. In addition, spring washers would also be appropriate to provide the compliant properties. Moreover, other materials may also be used so long as such materials have compliant properties. Sleeve  78  does not need to be any special material, however, a hard material is preferable. For instance, materials such as wood, plastic, or steel are examples of materials that could be used for sleeve  78 . 
     Referring to FIG. 3, another embodiment of column assist isolation system is shown generally at  168 . First bushing  84  is fixably attached to first end  80  of sleeve  78  and second bushing  86  is fixably attached to second end  82  of sleeve  78 , which together form a bushing/sleeve unit  100 . Downside end  70  of housing  50  comprises a sleeve support  72 , as described above with reference to FIG. 2, but which also comprises an open slot  102  located at first sleeve support  74  and an open slot  104  located at second sleeve support  76 . Open slots  102 ,  104  are sized so as to allow bushing/sleeve unit  100  to be pressed into and retained at first sleeve support  74  and second sleeve support  76 . 
     Referring to both FIGS. 2 and 3, the placement of the sleeve and the bushings at the downside end of the housing effectively reduces the rattle experienced by the steering system. Such a reduction in rattle is generally a function of the road impact energy being absorbed by the steering system. In addition, the column assist isolation system also adds torsional compliance to the EPS assist mechanism. The type of material employed for the bushings allow the bushings to compress slightly when the steering system undergoes torsional stress. The additional torsional compliance at the EPS assist mechanism allows the electric power steering assist mechanism to move slightly and allows the road impact energy to be felt by the driver through the steering shaft, thereby increasing the road feel at the steering device. 
     It will be understood that a person skilled in the art may make modifications to the preferred embodiment shown herein within the scope and intent of the claims. While the present invention has been described as carried out in a specific embodiment thereof, it is not intended to be limited thereby but is intended to cover the invention broadly within the scope and spirit of the claims.