Metal deforming – By three or more coacting relatively movable tools – Concurrently actuated tools
Patent
1984-01-04
1986-02-25
Crane, Daniel C.
Metal deforming
By three or more coacting relatively movable tools
Concurrently actuated tools
72312, 72353, 72406, 72469, 72412, 291592, B21J 718, B21K 176, B21K 712
Patent
active
045719829
DESCRIPTION:
BRIEF SUMMARY
This invention relates to steering rack bars for automobiles and their manufacture.
BACKGROUND ART
In the past, substantially all such racks have been made from a cylindrical bar of steel having cut therein transverse teeth over about one quarter of the length extending from one end. Typically a flat is first machined on the bar to a depth somewhat less than half the radius of the bar, and the teeth have a depth of about half that of the "flat". The remaining depth of section through the bar beneath the teeth is thereby reduced to about two-thirds of its diameter, so reducing its resistance to bending to less than half. Such racks are made of medium carbon steel and have their teeth induction hardened to improve their resistance to wear.
As installed in automobile steering gears, such rack bars are subject to severe bending loads because of forces transmitted from the suspension through the tie rods to the overhung ends of the rack bar where it protrudes through the steering gear housing. Such bending loads reach the same maximum value on the right side of the vehicle as on the left, and hence, as the rack bar is designed to have adequate strength to resist this bending on the toothed end, it follows that it will have double the required strength in the cylindrical end. As the latter comprises about three-quarters of the length of the rack bar, it is evident that such a rack is far heavier than necessary and wastes material.
The above shortcomings of such racks as commonly made may be overcome by employing a rack whose cross section resembles the capital letter "Y", but with the area between the upper limbs filled in and teeth cut therein.
Such a section, the use of which is described in U.K. Pat. No. 1,525,760, resembles a girder and is strong for its weight in bending, its strength being diminished less by the cutting of the teeth than in the case of round rack bars.
In the following text the term "Y form rack" will be used to describe a steering rack of the type just described. However the comparison to the capital letter "Y" should not be held to imply that the underside of the limbs are necessarily flat or that there need to be a stem or tail there between. The lower surfaces may, for instance, be made convex or concave in section and the surface between them may have a smaller discontinuity than implied by the term "Y form".
The lower side of the "Y" limbs of such racks must act as guide surfaces in the same manner as the cylindrical surface of conventional racks, and hence must be smooth and accurately related to the pitch line of the teeth opposite within a tolerance of 0.025 mms or less. Machining of these "Y" faces to such a finish and close relationship to the juxtaposed teeth is difficult by known machining methods.
In another recent development in steering rack bars, the regularly spaced teeth hitherto used are replaced by teeth of irregular form and pitch as described in U.K. Pat. No. 1,356,172, providing a variable steering ratio. Such teeth offer considerable advantages in reducing the parking effort, but cannot readily be produced by any known method such as gear cutting, broaching or grinding.
As neither of the foregoing developments are readily amenable to conventional machining methods, most racks made to date incorporating them have had to be made by highly unsuitable forging methods, as will be described.
Such racks do have the advantage that, in forging, the grain of the steel is caused to flow around the contours of the teeth and transverse to their length, so enhancing the rack tooth fatigue strength, as is well known in the art of gear forging.
U.K. Pat. No. 2,056,894 purports to show how steering racks, including those incorporating variable ratio, may utilize the above-described beneficial effect of forging, and also reinforce such effect by arranging that the grain of the bar of the material from which the rack is made also lies in a direction transverse of the teeth.
In actual fact the beneficial effects of the forging of gear teeth and the rolling of threads, on the fatigu
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Bishop Arthur E.
Roeske Klaus J.
Scott David W.
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