Patent Publication Number: US-2022227411-A1

Title: Worm gear hub

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This patent application is a divisional application of, and claims priority to, U.S. patent application Ser. No. 15/799,551, filed Oct. 31, 2017, which is a divisional of, and claims priority to, U.S. patent application Ser. No. 14/449,737, filed Aug. 1, 2014, now U.S. Pat. No. 9,868,459, issued Jan. 16, 2018, which is a continuation-in-part of, and claims priority to, U.S. patent application Ser. No. 12/818,675, filed Jun. 18, 2010, which claims priority to U.S. Provisional Patent Application Ser. No. 61/218,801, filed Jun. 19, 2009, the disclosure of each above-referenced application incorporated herein by reference in their respective entirety. 
    
    
     FIELD OF THE INVENTION 
     The subject invention relates to a worm gear hub and more particularly a worm gear hub assembly suitable for use in electric power steering units and systems. 
     BACKGROUND 
     In an Electric Power Steering (EPS) unit an electric motor drives a worm shaft and worm gear to provide assist torque to the turning of a steering shaft. This reduces the effort required to steer a vehicle. Currently worm gears used in these systems have been made using a solid steel puck. Each puck is then machined with a knurl on the perimeter. The puck then is the base or hub of the worm gear assembly. 
     The knurled surface is bead blasted to prep for a silane solution treatment that prepares the metal for bonding. A ring of plastic, made by a spin cast method, is placed on the metal. After the plastic is pressed on, the worm gear assembly is heated to cause the plastic to melt into the knurl surface of the hub and bond to the steel. This is followed by an annealing cycle to stress relieve the plastic. The hub assembly is pressed onto a shaft and teeth are hobbed (or cut) into the plastic ring to complete the gear assembly. As use in an electric power steering application or other application, the knurl to plastic bond transfers assist torque from the worm shaft, through the worm gear assembly, to the steering shaft. The process of making one gear hub assembly can be found in U.S. Pat. No. 6,638,390. 
     Machining of gear hubs to create the knurled surface with which to bond the plastic is expensive, as are powdered metal hubs. 
     Accordingly, it is desirable to provide a worm gear hub and worm gear hub assembly capable of transferring torque between a worm shaft and a steering shaft without the prior disadvantages. 
     SUMMARY OF THE INVENTION 
     According to one exemplary embodiment of the disclosure, an electric power steering system includes a steering shaft connected to a handwheel at one end and a rack and pinion steering mechanism at an opposite end. Also included is a steering assist unit comprising an electric motor operated by a controller and driving a worm and a worm gear interposed between said worm and said steering shaft, said worm having worm teeth and said worm gear fitted on said steering shaft. The worm gear also includes a disk having a first face axially disposed opposite a second face. Also included is a first plurality of individual lugs formed on said first face circumferentially adjacent an outer circumferential edge of said disk, wherein each lug of said first plurality of individual lugs having a first inner circumferential edge formed on said first face along a first retaining diameter, said first retaining diameter generally less than a disk diameter, circumferentially adjacent lugs of said first plurality of individual lugs having a first circumferential spacing therebetween. Further included is a ring overlaying a portion of said disk including said first plurality of individual lugs, said ring having an outer diameter, said outer diameter generally greater than said disk diameter. Yet further included is a plurality of gear teeth on an outer edge surface of said ring for meshing with said worm teeth. 
     In another exemplary embodiment of the disclosure, a method of making a worm gear is provided. The method includes forming a gear blank having a plurality of individual lugs formed about an outer circumferential edge of said blank to facilitate a uniform flow of a material around said plurality of individual lugs. The method also includes molding said material around said plurality of individual lugs to form a ring. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, advantages and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which: 
         FIG. 1  is a schematic diagram of a power steering system in accordance with the invention; 
         FIG. 2  is an elevation view of a gear hub blank in accordance with the invention; 
         FIG. 3  is a cross-sectional view of the gear hub blank of  FIG. 2 , taken along line  3 - 3  of  FIG. 2 ; 
         FIG. 4 . is a pictorial view showing another aspect of the invention; 
         FIG. 5  is a pictorial view, partially in cross-section, taken generally along line  5 - 5  of  FIG. 4 ; 
         FIG. 6  is a pictorial view of a finished worm gear in accordance with one aspect of the invention; 
         FIG. 7  is an elevation view, partially in cross-section showing another aspect of the invention; 
         FIG. 8  is a detail view taken from area  8 - 8  of  FIG. 3 ; 
         FIG. 9  is an pictorial view of a gear hub blank in accordance with an alternative embodiment of the invention; 
         FIG. 10  is a cross-sectional view of the gear hub blank of  FIG. 9  taken along line  10 - 10  of  FIG. 9 ; and 
         FIG. 11  is a pictorial view showing another aspect of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the Figures, where the invention will be described with reference to specific embodiments without limiting same, and in accordance with exemplary embodiments of the present invention,  FIG. 1  shows an electric power steering (EPS) system  10  for a motor vehicle. The EPS system  10  includes a rack-and-pinion type steering mechanism  12  that is comprised of a toothed rack (not shown) and a pinion gear (not shown) located under a gear housing  14 . A steering wheel  16  is coupled to an upper steering shaft  18 . As the steering wheel  16  is turned, the upper steering shaft  18 , which is connected to a lower steering shaft  20  through a universal joint  22 , turns the pinion gear. Rotation of the pinion gear moves the toothed rack, which moves tie rods  24  (only one shown) that, in turn, move steering knuckles  26  (only one shown), which turn wheels  28  (only one shown). 
     EPS assist torque is provided through a steering assist unit  30 , which includes a controller  32  and an electric motor  34 . The controller  32  is powered by a vehicle power supply  36  through a supply line  38 . The controller  32  receives a signal indicative of the vehicle velocity on a signal line  40 . Steering pinion gear angle is measured by a position sensor  42  and fed to the controller  32  through a line  44 . As the steering wheel  16  is turned, a torque sensor  43  senses the torque applied to the steering wheel  16  by a vehicle operator. The torque sensor  43  may include a torsion bar (not shown) and a variable-resistance type of sensor (not shown) that outputs a variable resistance signal to the controller  32  through a line  46  in relation to the amount of twist on the torsion bar. 
     In response to the inputs on lines  40 ,  44  and  46 , the controller  32  sends a command signal through a line  48  to the electric motor  34 . The motor  34 , in turn, supplies an assist torque to the steering system  10  through a worm  50  and a worm gear  52 , in order to provide a steering torque assist to the steering system  10  that supplements the steering force exerted by a vehicle operator. 
       FIGS. 2, 3 and 8  show a gear hub blank  110 . As shown, the gear hub blank  110  includes an outer circumferential edge  111 , having a diameter defined by a plurality of bent tabs (or tangs)  112 , extending radially outward from a center axis “X”. Gear hub blank  110  also includes an inner circumferential edge  114 . A body portion  115  of hub  110  extends between outer circumferential edge  111  and inner circumferential edge  114 . 
     As best seen in  FIGS. 2, 3 and 5 , an opposite inner face  132  and an opposite outer face  133  of gear hub blank  110  are generally non-symmetrical on body portion  115 , which comprises a series of concentric ring corrugations  141 ,  142 ,  143 ,  144 ,  145  and  146  falling into a plurality of planes, and arranged about center axis “ X ”. Among other advantages, concentric ring corrugations  141 - 146  provide added rigidity to gear hub blank  110 . It will be appreciated by a person of skill in the art that the number, size and radial width of corrugations may vary depending on design torque forces and/or the gear hub blank material. 
     In an exemplary embodiment, gear hub blank  110  is a cold formed metal incorporating opposing bent tabs  112  extending from an outer perimeter  151  of gear hub blank  110 , and in a further embodiment is made from SAE grade  1015  steel. Outer perimeter  151  also has a diameter, the outer perimeter  151  diameter being less than the diameter of outer circumferential edge  111 . The gear hub blank  110  can be formed from metal utilizing a variety of stamping, spin forming, flow forming and machining techniques as required for producing the desired geometry. 
     Referring again to  FIGS. 2 and 3 , bent tabs  112  of gear hub blank  110  are shown in detail. In the exemplary embodiment shown in  FIG. 2 , there are eighteen separate bent tabs  112  extending from outer perimeter  151  and ending at outer circumferential edge  111  of gear hub blank  110 . 
     In the non-limiting embodiment shown, each bent tab  112  spans an arc “A” that in the embodiment shown is an 18 degree arc. The spacing between adjacent bent tabs  112 , identified as “B” has a 2 degree arc. It will be appreciated that other numbers of bent tabs  112  may be used on gear hub blank  110 . Further, it will be appreciated that the span of arc “A” may be non-uniform or may vary between adjacent bent tabs  112 , as may the spacing between adjacent bent tabs  112 , identified as “B”, depending on size and torque requirements for gear hub blank  110 . Further, as specifically seen in  FIG. 8 , the geometry of gear hub blank  110  includes an angle “C” of about 45 degrees at the inner circumferential edge  114  and extending radially outwardly. This angle extends about half of the thickness “D” of inner circumferential edge, and in an exemplary embodiment is about 1.5 millimeters. This geometry facilitates pressing the gear hub blank  110  onto a shaft, as will be described herein. 
     As shown, bent tabs  112  are generally orthogonal to inner face  132  and outer face  133  of gear hub blank  110  and extend axially from face  133 . Adjacent bent tabs  112  extend in opposing axial directions, such that they are about 180 degrees opposed. Of course, depending on torque requirements, other configurations for bent tabs  112  may be contemplated, including a configuration in which bent tabs simultaneously extend radially from the center “X” of hub blank  110  and also extend at an acute angle from inner face  132  and outer face  133 . For example, it will be appreciated that bent tabs  112  may extend from each of faces  132  and  133  at an angle from about 45 degrees to about 90 degrees—with the example angle of 90 degrees shown. Further, one skilled in the art will recognize that the adjacent bent tabs  112  described and shown herein as alternating in opposite directions may, instead, take on a different geometric order such that pairs may extend in the same direction or pairs extend from the same face, but at differing angles. 
     Referring now to  FIGS. 4 and 5 , a polymeric ring  160  is placed on gear hub blank  110  to form a gear blank  170 . The polymeric ring  160  includes an inner ring face  161 , outer ring face  162 , an outer edge surface  163  and partial edge surfaces  164  and  165 . As seen partial edge surface  164  extends generally orthogonally from opposite inner face  132  of gear hub blank  110 , while partial edge surface  165  extends generally orthogonally from opposite outer face  133  of gear hub blank  110 . Outer edge surface  163  has an outer diameter greater than the diameter of outer circumferential edge  111 , while partial edge surfaces  164  and  165  have an inner diameter less than the diameter of outer perimeter  151 . The bent tabs  112  of gear hub blank  110  are covered by the polymeric ring  160 , and encased therein. In an exemplary embodiment, polymeric ring  160  is injection molded onto gear hub blank  110 , made possible by the bent tabs  112 . The polymeric ring  160  is injection molded in a generally rectangular cross-section, as seen in  FIG. 5 , forming a generally toroid shape, completing the gear blank  170  as seen in  FIG. 4 . 
     Thereafter, gear blank  170  is pressed or welded onto a shaft, and in the example shown, lower steering shaft  20 . The pressing step is followed by a hobbing process that cuts multiple individual gear teeth  180  into the outer edge surface  163  of polymeric ring  160  of gear blank  170 . The result is the worm gear  52 , shown in  FIG. 6  placed within the steering mechanism  12  of  FIG. 7 . As illustrated,  FIG. 7  shows lower steering shaft  20  and torque sensor  43  connected to torsion bar  45 . Worm gear  52 , shown in cross-section, is pressed on lower steering shaft  20  and driven by the worm  50  which is in turn driven by electric motor  34  (shown in  FIG. 1 ). 
     Bent tabs  112  of gear hub blank  110  provide both axial and radial retention of the polymeric material comprising polymeric ring  160  over gear hub blank  110 . In addition, the thickness of the base stock from gear hub blank  110  in bent tabs  112  provides the ability to transfer torque from one shaft to another, once gear teeth  180  have been cut in the gear blank  170 . In the non-limiting embodiment shown, the bent tabs  112  of worm gear  52  carry torsional stiffness between lower shaft  20  and worm  50 , allowing EPS system  10  to reliably perform at a significant cost reduction. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the present application.