Patent Application
20100320025
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.
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.
According to one exemplary embodiment of the present invention, a gear blank is provided. It comprises a metal hub blank having an outer perimeter. A plurality of individual adjacent tabs extends radially from the outer perimeter of the metal hub blank, forming an outer circumferential edge that has a first diameter. A polymeric ring overlaying a portion of the metal hub blank, including the tabs is provided. The polymeric ring has an outer diameter, the outer diameter generally greater than the first diameter.
In another exemplary embodiment of the present invention, an electric power steering system is provided. The electric power steering system comprises a steering shaft connected to handwheel at one end and a rack and pinion steering mechanism at an opposite end. A steering assist unit comprising an electric motor operated by a controller and driving a worm is provided. A worm gear is interposed between the worm and the steering shaft, the worm having worm teeth and the worm gear is fitted on the steering shaft. The worm gear further comprises a metal hub blank having an outer perimeter, a plurality of individual adjacent tabs extending from the outer perimeter of the metal hub blank forming an outer circumferential edge that has a first diameter. A polymeric ring overlaying a portion of the metal hub blank, including the tabs is provided. The polymeric ring has an outer diameter and gear teeth on an outer edge surface of the polymeric ring for meshing with the worm teeth.
In yet another exemplary embodiment of the present invention, a method of making a worm gear is also provided. The method comprises cold forming a metal hub blank having tabs about a perimeter of the blank. Thereafter a polymeric material is injection molded around the tabs to form a polymeric ring. The metal hub blank with the polymeric ring is pressed on a shaft and gear teeth are cut onto an outer surface of the polymeric ring with a hobbing process.
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.
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:
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,
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.
As best seen in
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
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
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
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
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.
This patent application claims priority to U.S. Provisional Patent Application Ser. No. 61/218,801 filed Jun. 19, 2009, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61218801 | Jun 2009 | US |
