Metal working – Method of mechanical manufacture – Gear making
Reexamination Certificate
2002-12-09
2004-08-24
Omgba, Essama (Department: 3726)
Metal working
Method of mechanical manufacture
Gear making
C029S893300, C029S893350, C072S370040, C072S370210, C074S422000, C074S457000
Reexamination Certificate
active
06779271
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a hollow rack shaft, and more particularly to a method for manufacturing a hollow rack shaft for a steering system used in an automobile.
2. Description of the Related Art
In the case of a rack-and-pinion type steering system for an automobile frequently used in an automobile industry, the rotation of a steering wheel operated by a driver is transmitted to a pinion. The rotation of this pinion is transmitted to a rack shaft engaged with the pinion and this is converted into a motion in a lateral direction (a longitudinal direction of the rack). Since the rack shaft is connected to the steering rod, the direction of front wheels is changed by a lateral motion of the rack shaft. Since such a steering mechanism as described above is well known in the art, further description will be eliminated.
The aforesaid rack shaft has been attained by gear cutting on a solid, namely not hollow, material. In recent years, as already described in Japanese Laid-Open Patent No. Hei 6-246379, Japanese Laid-Open Patent No. Sho 58-218339 (Japanese Patent Publication No. Hei 4-028582) and Japanese Laid-Open Patent No. Hei 11-180318, fuel efficiency of an automobile has been improved by making a hollow rack shaft to attain the lightweightization of the rack shaft.
A manufacturing method for forming rack is disclosed in Japanese Laid-Open Patent No. Hei 6-246379, which describes that a tube material is inserted into a first forming split die, a primary forming is carried out, then, the aforesaid primary formed material is inserted into a secondary forming split die having, at a part of its inner surface, teeth corresponding to the rack teeth, a semi-circular mandrel is press fitted at one end of it.
A manufacturing method disclosed in Japanese Laid-Open Patent No. Sho 58-218339 describes that a die having teeth corresponding to the rack teeth is pushed to a tube material to form rack teeth under the state that a mandrel is inserted into the tube material surrounded with fixed die.
The manufacturing method disclosed in Japanese Laid-Open Patent No. Hei 11-180318 is an invention invented by Mr. Ohkubo, one of the inventors of the present invention. The manufacturing method comprises the following first to third steps. At the first step, a plate-like material having a substantial rectangular shape as original one is bent into a gutter shape along a center of longitudinal direction. At this time, the central part of the gutter-shaped member is formed into a flat bottom section, and both the side portions are formed into a semi-circular shape having legs therein. At the second step, the rack teeth are plastically formed at the flat bottom portion. At the third step, the gutter-shaped material is bent such that the legs of the central part and the both sides are abutted to each other, and then a hollow rack shaft is formed.
In the case disclosed in the first two gazettes above, the hollow original workpiece is applied as the material, while, in the case disclosed in the third gazette (the gazette of Japanese Laid-Open Patent No. Hei 11-180318), a flat plate workpiece is applied in place of the hollow workpiece, so that the third manufacturing method has some merits in view of material cost, transportation cost and storing cost. Further, according to the third manufacturing method, it is avoidable to give each portion except a tooth bottom portion an excessive thickness in order to maintain the tooth bottom portion at a required wall thickness. Then, the rack shaft can be sufficiently light in its weight.
SUMMARY OF THE INVENTION
However, it has become apparent that the manufacturing method disclosed in the gazette of Japanese Laid-Open Patent No. Hei 11-180318 has some problems not considered up to now as described below.
Although, in general, almost the deformation applied to the workpiece by a plastic working operation gives a permanent deformation, temporary deformation, i.e. elastic deformation remains in a part of deformation. As the lead applied by the die is removed, the elastic deformation remaining at each minute element in the workpiece is restored and it is transferred to a new balanced state where each of the forces of the other fine elements keeps its balance. Due to this fact, after the workpiece is removed from the die, the entire workpiece does not take a shape coinciding with the die and its shape deviates from that of the die. In particular, in the case that a degree of forming is widely different in reference to each of the locations at the workpiece, this deviation shows a large value.
FIGS. 1
a
to
1
c
show the state when the flat plate workpiece is formed into a gutter-like shape at the first step of the prior art manufacturing method (the final stage of press stroke). In this case,
FIG. 1
a
is a front sectional view for showing a workpiece and a die.
FIG. 1
b
is a sectional view taken along line B—B of
FIG. 1
a
.
FIG. 1
c
is a sectional view taken along line C—C of
FIG. 1
a
. The die set at the first step comprises an upper die set
110
including a right upper die
111
, a left upper die
112
and a central upper die
113
; and a lower die set
120
including a right lower die
121
, a left lower die
122
and a central lower die
123
. As apparent from these figures, a workpiece W is formed by the lower die set
120
and the upper die set
110
into a gutter-like shape having a flat bottomed central portion
2
a
, a rack tooth not-yet formed, and having semi-circular bottomed portions
1
extending from the both sides of the flat bottomed central portion
2
a.
Upon completion of the first step, when the upper die set
110
is lifted up to take out the workpiece W, the workpiece W takes a different shape from that of the lower die set
120
and the upper die set
110
.
FIG. 2
is a sectional view for showing the workpiece W expressed under the state of releasing such a stress as above being exaggerated, as shown in this figure, the workpiece W is warped like a bow with the bottom part of the gutter being placed inside, i.e. the opening of the legs being placed outside.
The reason why such a state occurs is as follows. The original plate workpiece is formed into different shape at each potion, therefore, the degree of deformation given to reach portion differs from others. Namely, the degree of deformation given to the flat bottom portion
2
a
, the semicircular bottom portion
1
and the interface portion
9
of the workpiece are different from each other. When the load applied to the workpiece is unloaded after plastic working, each portion recovers elastically in correspondingly different manner. Hence the workpiece warps like a bow.
Further, a similar problem may also occur at the second step of the prior art manufacturing method.
FIG. 3
is a sectional view for showing the workpiece W and the dies when rack teeth
2
b
are formed at the portion
2
a
(
FIG. 2
) at the second step. The die set at the second step comprises an upper die set
210
including a right upper die
211
, a left upper die
212
and a central upper die
213
; and a lower die set
220
including a right lower die
221
, a left lower die
222
and a central lower die
223
. The upper surface of the central lower die
223
is formed with teeth complementary shape to the rack teeth to be formed. In addition, the lower surface of the central upper die
213
is provided with a corrugated shape corresponding to the rack teeth at the surface.
The rack teeth unformed portion
2
a
of the workpiece W is formed with rack teeth by approaching the central lower die
223
and the central upper die
213
to each other under the state that the semi-circular part
1
of the workpiece W is held between the right upper die
211
, the right lower die
221
and between the left upper die
212
and the left lower die
222
.
Since many teeth are formed at the rack portion
2
b
the degree of forming at each location is different. When the workpiece W is removed from the die upon completion of the second step, the workpiece W sometimes sho
Okubo Kiyoshi
Tsubouchi Akira
Watanabe Yasushi
NSK Ltd.
Omgba Essama
Sidley Austin Brown & Wood LLP
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