Hollow rack shaft and method of manufacturing the same

Metal deforming – Process – Tube making or reshaping

Reexamination Certificate

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C029S893340, C074S457000

Reexamination Certificate

active

06442992

ABSTRACT:

The present invention is based on Japanese Patent Applications No. 2000-033684 and No. 2000-063974 filed in Japan, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hollow rack shaft and, more particularly, to a hollow rack shaft for a steering system used in automobiles.
2. Description of Related Art
As the steering system for automobiles, a rack and pinion steering system is most frequently adopted. According to this rack and pinion steering system, when a steering wheel is turned by a driver, the rotation of this steering wheel is transmitted to a pinion. The rotation of the pinion is then transmitted to a rack shaft in mesh with the pinion, with the rotary motion being converted into a lateral (in the longitudinal direction of the rack shaft) motion. Because the rack shaft is connected to a steering rod, the direction of the front wheels is changed by the lateral motion of the rack shaft. The steering mechanism of this type is a known art and therefore will not be further explained.
The conventional rack shaft has been manufactured by gear cutting on a rod-like material. Recently, however, there has been the tendency for a rack shaft to be light-weightized in order to improve automotive fuel efficiency. In Japanese Patent Laid-Open Nos. Hei 06-246379, Sho 58-218339, and Hei 11-180318, methods of manufacturing the rack shaft have been disclosed.
According to the method of manufacturing the rack shaft disclosed in Japanese Patent Laid-Open No. Hei 6-246379, a tubular workpiece is inserted into a first split die, and the tublar workpiece is pressed by a primary press-forming process. Then the primary formed material is inserted into a second split die having the teeth on a part of the inner surface correspondingly to the rack teeth. Finally a semi-circular mandrel is pressed in from one end to thereby form the rack teeth.
The method of manufacturing the rack shaft disclosed in Japanese Patent Publication No. Sho 58-218339 involves inserting a mandrel into the tubular workpiece, and under the state that the tubular workpiece is surrounded by a stationary die, a second die having teeth corresponding to the rack teeth is pressed to the outside of the tubular workpiece, to thereby form rack teeth.
The method of manufacturing disclosed in Japanese Patent Laid-Open No. Hei 11-180318 is a technique invented by Okubo, one of inventors of the present invention. According to this method of manufacturing, an approximately rectangular plate is bent into a gutter shape along a longitudinal center. The gutter-shaped member has a flat bottom formed at the central part, and on both ends is provided a semi-circular shape with leg portions. On the flat bottom-shaped central part the rack teeth are formed through a plastic working process. Thereafter, the leg portions of the central part and the leg portions of both ends are bent into the form of a tube to thereby form a rack shaft.
In the technique of the first two patent documents described above, the tubular workpiece is used as a raw material, while in the latter technique a plate is used in place of the tubular workpiece. Therefore, not only is the invention advantageous in material cost, transportation cost and storage cost, but it has the merit which can achieve a substantial weight reduction effect without adding material to a portion other than the rack teeth in order to secure the thickness of the bottom section of the rack teeth.
The rack shaft is required to be compact and to withstand a great deal of load. This requirement is achievable by increasing the width of the rack teeth without increasing the whole body of the rack shaft.
In prior art methods, however, a tubular workpiece is used. Therefore, it is substantially impossible to manufacture the rack shaft in such a manner that the tooth width will exceed the size of the chord which crosses the outer periphery of the tubular workpiece (tube). That is, in the method disclosed in Japanese Patent Laid-Open No. Hei 6-246379, when the tubular workpiece is forced into the die corresponding to the rack teeth by the use of a mandrel, a small-diameter mandrel is pressed into a narrow tubular workpiece. Therefore, the diameter of the mandrel is limited in size and accordingly a rack having a large tooth width cannot be formed.
In the method disclosed in Japanese Laid Open Publication No. Sho 58-218339 (Examined Japanese Patent Publication No. Hei 4-28582), the die having teeth corresponding to the rack teeth is pressed on the tube to thereby forge the rack teeth. At this time, since the workpiece is upset in the direction of the center axis, it is almost impossible to increase the tooth width.
These two methods of manufacture have such a drawback that the inner side (the inner surface of the hollow section) of the rack teeth is flat, and therefore the tooth bottom section will decrease in thickness and strength compared with the other portion. Besides, these methods also have the drawback that it becomes necessary to add excess material to the other portion in case the metal thickness of the tooth bottom is to be assured, as a result a sufficient weight reducing effect cannot be obtained.
The above-described problem can be solved to some extent by the method of manufacture disclosed in Japanese Patent Laid-Open No. Hei 11-180318 described above, which, however, is still insufficient for the following reason.
Referring to
FIGS. 1-5
, in the latter method of manufacturing the hollow rack shaft, a gutter-shaped workpiece
103
(
FIGS. 3
a
and
3
b
), having a flat central portion and a semi-circular portion with legs on both ends, is used. A rack portion
101
(
FIG. 4
a
) is formed in the flat bottom section of the workpiece
103
. To form the rack portion
101
, an upper die
151
, having a complementary shape (the tooth
106
) of the rack tooth form in the lower part, and a lower die
152
, having the inner side shape (the tooth
108
) of the rack tooth form in the upper part, are used. Between these dies, the flat bottom section of the workpiece
103
is placed (
FIGS. 3
a
and
3
b
). The workpiece is then press-formed by letting the upper die
151
down. On the sides of the upper die
151
and the lower die
152
, pressure plates
153
,
154
,
155
, and
156
, to support workpiece
103
from the sides, are provided, as shown in
FIG. 3
b.
As shown in
FIG. 2
a
and
FIG. 2
b
, the width B of the lower surface
107
of the tooth bottom section of the rack
101
in the hollow section
102
of the hollow rack shaft is the length of the chord geometrically determined by the radius R of the hollow section
102
, tooth bottom position X, and plate thickness t of the rack tooth bottom. In a conventional rack and pinion steering system, the width A of a portion
108
to form the teeth of the lower die
152
has been designed to be the same size as the width B. In this type, the position X of the booth bottom requires to be more than a given amount in order to maintain the bending strength of the hollow rack shaft
100
; therefore it is impossible to lower the position X (or X-t) to the vicinity of the center of the hollow portion
102
. Therefore the width B must be made substantially smaller than the diameter (
2
R) of the hollow portion
102
, and accordingly the rack tooth width could not be increased.
Furthermore, it has been understood that when the lower die
152
stated above is used, the effective tooth width C of the rack teeth
105
(the tooth width in the pitch surface of the rack teeth
105
) is worked narrower than intended. A reason for the above-described drawback will be explained below.
FIG. 5
is a view for explaining where the material of the workpiece
103
in the region of the flat bottom section before the forming of the rack teeth
105
transfers after the forming of the rack teeth
105
. At the center of the rack teeth
105
, the material within the region A
7
has moved to the region a
7
, while the material within the region A
8
, to the region a
8
. In the meantim

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