Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2000-01-28
2002-10-15
Zeender, F. (Department: 3627)
Motor vehicles
Steering gear
With electric power assist
C091S380000, C074S089340
Reexamination Certificate
active
06464034
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an electrically powered steering device for an automotive vehicle and, more particularly, to the electrically powered steering device employing a ball screw mechanism for transmitting a drive output from an electric motor to a steering shaft to selectively advance and retract the latter.
2. Description of the Prior Art
The electrically powered steering device is an instrument to assist the steering force of a steering wheel by means of an electrically driven motor and is currently available in various types. One of the types employed is of a design wherein a retractable steering shaft coupled with a steering mechanism for vehicle wheels is imparted an axially shifting force that is transmitted thereto from the steering wheel through a motion translating mechanism such as a rack-and-pinion mechanism for translating a rotary motion of the steering wheel into the axially shifting motion and, also, an axially shifting force that is transmitted thereto from an output of the electrically driven motor through a ball screw mechanism. The ball screw mechanism is currently available in various types according to the manner in which a series of balls are circulated, including a bridge type, a tube circulating type and an end-cap type. The electrically powered steering devices of a rack-and-pinion type employing these ball screw mechanisms are disclosed in, for example, the Japanese Laid-open Patent Publication No. 59-50864 disclosing a basic structure, the Japanese Laid-open Patent Publication No. 9-142315 disclosing the use of the bridge type ball screw mechanism, the Japanese Laid-open Patent Publication No. 54-47236 disclosing the tube (circulator) type ball screw mechanism, and the Japanese Laid-open Patent Publication No. 6-20 1013 disclosing the use of a resin-made end cap type and a resin-made circulating pipe type.
FIG. 23A
illustrates the prior art ball screw mechanism of the bridge type such as disclosed in the Japanese Laid-open Patent Publication No. 9-1423 15. As shown therein, a ball screw shaft
1
having an externally threaded groove
4
formed thereon has a ball screw nut
5
mounted therearound. The ball screw nut
5
has an inner peripheral surface formed with an internally threaded groove
5
b
in alignment with the externally threaded groove
4
to thereby define a ball guide passage for a series of balls
7
. The ball screw nut
5
is arranged with a bridge member
5
d
that defines a connecting passage for connecting neighboring internally threaded grooves
5
b
. When the ball screw nut
5
rotates about and relative to the ball screw shaft
1
, the ball screw shaft
1
is axially moved relative to the ball screw nut
5
by the effect of a reactive force from the threaded grooves. Since the balls
7
successively depart from the ball guide passage as they rollingly advance along and within the threaded grooves incident to rotation of the ball screw nut
5
and axial movement of the ball screw shaft
1
, a supplement of the balls would be required depending on the amount of rotation. In view of this, the provision has been made of the bridge member
5
b
so that the balls
7
successively advanced along and within the threaded grooves can be returned to the threaded grooves through the connecting passage defined in the bridge member
5
d.
On the other hand, shown in
FIGS. 23A and 23B
is an example of the prior art bridge type ball screw mechanisms that is disclosed in the Japanese Laid-open Patent Publication No. 54-47236. As shown in
FIGS. 23A and 23B
, of the groups of balls lined up in a region LA or LB in an axial direction of the ball screw nut
2
, that is, the balls of either one of two rows of the balls rollingly move within threaded grooves formed in the ball screw shaft
1
over a distance corresponding to an axial length and interposed between the ball screw grooves. The two rows of balls
3
A and
3
B guided by respective rows of circulators
6
A and
6
B so as to circulate are interposed between the internally threaded groove
2
a
in the ball screw nut
2
and externally threaded grooves
1
a
and
1
b
in the ball screw shaft
1
while the balls of one row
3
A are so designed as to have a diameter appropriately larger than that of the balls of the other row
3
B, so that during the straight run of the automotive vehicle the balls of the row
3
A can roll in part within the internally threaded groove
2
a
in the ball screw nut
2
and in part within the internally threaded groove
1
a
in the ball screw shaft
1
which has a large diameter portion d
2
while the ball of the row
3
B can roll in part within the internally threaded groove
2
a
in the ball screw nut
2
and in part within the externally threaded groove
1
b
in the ball screw shaft
1
which have a reduced diameter portion d
1
. Accordingly, the row of the balls
3
A of the larger diameter that are guided by the circulator
6
A are interposed between the internally threaded groove
2
a
in the ball nut
2
and the externally threaded groove
1
a
in the large diameter portion d
2
of the ball screw shaft
1
with no gap formed therebetween. In this design, when the steering wheel is turned rightwards (clockwise) or leftwards (counterclockwise) during the steering of the automotive vehicle, accompanied by rotation of the ball screw shaft
1
, the ball screw nut
2
can be moved rightwards or leftwards through the threaded grooved and the rows of the balls
3
A and
3
B.
One example of the prior art end cap type ball screw mechanism is disclosed in the Japanese Laid-open Patent Publication No. 6-201013. According to this patent publication, an end cap is mounted on each of opposite ends of the ball screw nut to allow the balls to be returned to the circulating pipe and the end caps and the circulating pipe are both made of a synthetic resin.
FIG. 24
illustrates another type of the prior art end cap type ball screw mechanism. As shown therein, a rotary nut
71
has a substantially intermediate portion thereof formed with a radially outwardly protruding flange
72
. A rolling bearing
73
is mounted externally on the rotary nut
71
so that the rotary nut
71
can be rotatably supported by and within a housing. A rotor
74
of an electrically driven motor for driving the rotary nut
71
about the longitudinal axis thereof is also mounted on the rotary nut
71
. The radially outwardly protruding flange
72
is utilized for positioning the rolling bearing
73
and the rotor
74
relative to the rotary nut
71
.
The rolling bearing
73
is of a so-called inner-race rotating type in which the inner race
73
a
is rotatable together with the rotary nut
71
and, for this purpose, the rotary nut
71
is press-fitted into the inner race
73
a
with a predetermined interference present between an inner peripheral surface of the inner race
73
a
and an outer peripheral surface of the rotary nut
71
. On the other hand, since the rotor
74
cannot be press-fitted over the rotary nut
71
in a manner similar to the rolling bearing, a portion of the outer peripheral surface of the rotary nut
71
that is connected with the rotor
74
is knurled to provide a knurled surface area
75
for slipless engagement with the rotor
74
. While the knurled surface area
75
can be formed by any known rolling process, an annular groove
76
is necessarily formed in the rotary nut
71
on each side of the knurled surface area
75
for the convenience of the rolling process. Also, a portion of the outer peripheral surface of the rotary nut
71
on one side of the knurled surface area
75
adjacent the end thereof is formed as a cylindrical guide portion
77
for guiding the rotor
74
onto the knurled surface area
75
before the rotor
74
is mounted on the knurled surface area
75
in a slipless fashion. The cylindrical guide portion
77
has a diameter slightly smaller than the outer diameter of the knurled surface area
75
of the rotary nut
71
so as to facilitate mounting of the rotor
74
onto the rotary
Tateishi Koji
Toda Masaaki
Yamauchi Kiyoshige
Yoshida Isamu
Yoshioka Morihisa
NTN Corporation
Sughrue & Mion, PLLC
Zeender F.
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