Metal deforming – By use of closed-die and coacting work-forcer – Forcing work into or within closed die; e.g. – forging
Patent
1998-06-22
1999-11-30
Hail, III, Joseph J.
Metal deforming
By use of closed-die and coacting work-forcer
Forcing work into or within closed die; e.g., forging
72337, B21D 2200
Patent
active
059922052
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the manufacture of automobile steering rack bars, and particularly but not exclusively to the manufacture of variable ratio rack bars. The process and the design of manufacturing equipment are disclosed, as are related rack designs having particular functional advantages compared to those of the prior art.
BACKGROUND ART
Rack bars normally comprise a round bar with teeth cut or formed at a "toothed end", the remaining non-toothed region referred to as a "shank end". Variable ratio rack bars such as described in U.S. Pat. No. 3,753,378 incorporate teeth of varying cross-section and varying skew angle with respect to the rack bar longitudinal axis, as well as varying tooth pitch. The pinion which engages the rack is usually helical and is installed in the steering gear at an oblique angle to the normal to the rack bar longitudinal axis, henceforth termed the "pinion installation angle". For variable ratio rack bars the teeth at the toothed end of the rack bar are usually symmetrically disposed about a mid point, this "on-centre region" corresponding to the pinion meshing position when the vehicle is steered straight ahead.
The geometry of such teeth makes variable ratio rack bars very difficult to machine by known rack manufacturing processes, and precision forging is therefore often resorted to, notwithstanding the high precision that is required for satisfactory pinion meshing.
The optimum geometry of the teeth of such rack bars varies widely according to specific vehicle requirements, and whether the steering gear is of the power or manual type. When viewed in cross-section normal to the longitudinal axis of the rack bar, the teeth are frequently positioned towards the periphery of the bar so that the bending strength at the toothed end of the rack bar is minimally reduced as compared to that at the shank end.
U.S. Pat. No. 4,116,085 describes a rack bar having at its toothed end a cross section of triangular or "Y" section which is particularly suited for variable ratio racks. Such a rack may be formed in forging dies such as those described in U.S. Pat. No. 4,571,982 and U.S. Pat. No. 5,862,701, and is supported in the steering gear in a V shaped rack pad. The above-referenced United States patents are being incorporated by reference. This arrangement provide resistance to rack roll under the action of tooth meshing forces as described in U.S. Pat. No. 4,116,085. An additional advantage of the above Y section, and of the related manufacturing process, is that the cross-sectional area of the Y section (including the mean height of the teeth in the toothed region) may be made to match that of the round rack bar blank from which the rack is forged, thereby saving material and enabling the construction of a die in which there is no flash. This die construction enables very high forming pressures to be achieved, so that precise filling of the tooth cavities of the die may be achieved even if forging is conducted at relatively low temperatures, the latter which is also conducive to the avoidance of scaling. The Y section of the rack is of larger diameter over corners so that its strength in bending at the toothed end approximately matches that at the shank end. A suitable temperature has been found to be around 700.degree. C. (often referred to as "warm forging") in contrast to a temperature of 1100.degree. C. typically used in conventional hot forging.
This use of a Y section design has been found to be highly desirable in variable ratio power steering gears where a large change of steering ratio is appropriate to the vehicle handling characteristics and, as a result, the skew angle of some of the teeth is large. The use of a Y section serves to stabilise the rack bar under the influence of rolling moments caused by the pinion/rack tooth contact forces, in particular the lateral component of these forces caused by the presence of large skew angles. However, in certain circumstances, such as when the skew angles are relatively small, for example during the introductory phase of va
REFERENCES:
patent: 3314278 (1967-04-01), Bergman
patent: 3842646 (1974-10-01), Kuhn
patent: 4587828 (1986-05-01), Matsumoto et al.
patent: 5862701 (1999-01-01), Bishop et al.
Bishop Steering Pty Limited
Hail III Joseph J.
Tolan Ed
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