Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-09-06
2003-11-11
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S421100, C029S897000, C310S154120
Reexamination Certificate
active
06643911
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-278262 filed on Sep. 13, 2000 and Japanese Patent Application No. 2001-224989 filed on Jul. 25, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a yoke of an electric motor.
2. Description of Related Art
As shown in
FIGS. 12A and 12B
, a previously proposed electric motor typically includes a yoke
91
, a pair of magnets
92
, a rotatable shaft
93
, and an armature
94
.
The yoke
91
is formed as a flat cylindrical container. That is, the yoke
91
has two opposite flat portions
91
a
in parallel with each other in a cross section perpendicular to a central axis of the yoke
91
(i.e., a cross section shown in FIG.
12
B), curved portions
91
b
that connect both the flat portions
91
a
together. Each of the magnets
92
is fixed on the inside of the corresponding curved portion
91
b
of the yoke
91
. The rotatable shaft
93
is held in the inside of the yoke
91
such that the rotatable shaft
93
can rotate. Furthermore, the armature
94
is coaxially fixed on the rotatable shaft
93
such that the surface of the armature
94
faces the magnets
92
.
As shown in
FIGS. 13A
to
16
B, for example, such a yoke
91
can be fabricated by a method in which a metal plate is drawn into a predetermined form by performing a multistage-drawing process.
In other words, as shown in
FIGS. 13A and 13B
, the metal plate (not shown) is drawn into a first-drawing product
95
in the shape of an elliptically cylindrical container at an initial drawing stage of the multistage-drawing process. Here, the term “elliptically cylindrical container” represents a cylindrical tube with a bottom in which an opening (i.e., an inner side) thereof has an elliptical cross section in a radial direction perpendicular to an axis of the product
95
. In addition, such an elliptical cross section will be simply described as “an elliptical shape” in the following description. As shown in
FIGS. 14A and 14B
, subsequently, the first-drawing product
95
is further drawn into a re-drawing product
96
at a middle drawing stage of the multistage-drawing process. The re-drawing product
96
has a depth greater than that of the first-drawing product
95
. Also, an external periphery of the elliptical shape of the re-drawing product
96
is smaller than that of the first-drawing product
95
.
Next, as shown in
FIGS. 15A and 15B
, the re-drawing product
96
is further drawn into a second re-drawing product
97
at a late stage of the multistage-drawing process. The second re-drawing product
97
has a depth greater than or equal to that of the re-drawing product
96
. Also, an external periphery of the elliptical shape of the second re-drawing product
97
is smaller than that of the re-drawing product
96
. As shown in
FIGS. 16A
and
16
B, furthermore, the second re-drawing product
97
is drawn into the yoke
91
at a final stage of the multistage-drawing process. In this case, an arc segment
97
a
with a small amount of curvature of the elliptical shape of the second re-drawing product
97
is provided as the flat portion
91
a
while another arc segment
97
b
with a large amount of curvature is provided as the curved portion
91
b.
However, as described above, the conventional method of manufacturing the yoke
91
by sequentially deforming into the desired shape thereof has the following disadvantages. In each stage of the multistage-drawing process, a thickness T
95
a
-T
97
a
of each arc segment
95
a
-
97
a
having a small amount of curvature of the elliptical shape is thinner than a thickness T
95
b
-T
97
b
of the corresponding arc segment
95
b
-
97
b
having a large amount of curvature. Thus, a thickness T
91
a
of the flat portion
91
a
of the yoke
91
is thinner than a bard thickness T
91
b
of the curved portion
91
b.
Therefore, the thickness T
91
b
of the curved portion
91
b
on which the magnet
92
is fixed is more than necessary even though it is a small effective part as a magnetic circuit, so that the amount of wasted part of the material increases. In addition, the thickness T
91
a
is decreased even though the flat portion
91
a
is the most effective portion as a magnetic circuit. As a result, the amount of magnetic reluctance increases while the output of the motor (i.e., a torque performance of the motor) decreases.
SUMMARY OF THE INVENTION
The present invention addresses the above disadvantages, and it is an objective of the present invention to provide a method of manufacturing a yoke by performing a multistage-drawing process, which allows a reduction in the amount of a material waste of the yoke while contributing a high torque performance of an electric motor.
To achieve the objective of the present invention, there is provided a method of manufacturing a yoke by drawing a plate material into a final product as the yoke by performing a multistage-drawing process, where the final product as the yoke is in the shape of a cylindrical container having a couple of flat portions parallel to each other in a cross section perpendicular to a central axis of the final product and a couple of curved portions that connect the flat portions together. The method includes steps of drawing the plate material into a first-drawing product in the shape of an elliptically cylindrical container at an initial drawing stage, the first-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section; drawing the first-drawing product obtained at the initial drawing stage into a re-drawing product in the shape of a generally circular cylindrical container at a middle drawing stage; drawing the re-drawing product obtained at the middle drawing stage into a late-drawing product in the shape of an elliptically cylindrical container at a late drawing stage, the late-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section, the major axis and the minor axis of the elliptical cross section of the late drawing product obtained at the late drawing stage being reversed with respect to the major axis and the minor axis of the elliptical cross section of the first-drawing product obtained at the initial drawing stage, the late-drawing product having a couple of opposed smaller-curvature arc segments of a smaller curvature and a couple of opposed larger-curvature arc segments of a larger curvature; and drawing the late-drawing product obtained at the late drawing stage into the final product as the yoke at a final stage, such that the smaller-curvature arc segments of the late-drawing product are converted to the flat portions of the final product, respectively, and the larger-curvature arc segments of the late-drawing product are converted to the curved portions of the final product, respectively.
To achieve the objective of the present invention, there is also provided a method of manufacturing a yoke by drawing a plate material into a final product as the yoke by performing a multistage-drawing process, where the final product is in the shape of a cylindrical container. The method includes steps of clamping the plate material between a first presser plate and a first die and drawing the plate material into a first-drawing product using a first punch at an initial drawing stage, one of the first presser plate and the first die having two or more protrusions opposed to the other of the first presser plate and the first die, the first-drawing product being formed in the shape of an elliptically cylindrical container at the initial drawing stage, the first-drawing product including an elliptical cross section having a major axial length measured along a major axis of the
Kanno Kazunobu
Kuribayashi Tsuyoshi
Muroi Shinsuke
Nakamura Kouichi
Shimoda Taketo
Arbes Carl J.
Asmo Co. Ltd.
Phan Tim
Posz & Bethards, PLC
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