Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2003-03-04
2004-08-03
Lerner, Avraham (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C180S428000
Reexamination Certificate
active
06769507
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electric power steering apparatus using an electric motor as a source for generating a steering assist force.
Controlling the steering of a vehicle is carried out by transmitting the rotation operation of the steering wheel disposed inside a vehicle interior to a steering mechanism disposed outside the vehicle interior in order to steer the steering control wheels to be steered (generally the front wheels of the vehicle).
FIG. 1
is a sectional view showing a conventional electric power steering apparatus, and
FIG. 2
is a sectional view showing the reduction mechanism portion thereof
As shown in
FIG. 1
, the conventional electric power steering apparatus of a vehicle for example comprises a first steering shaft
101
connected to a steering wheel
100
, a second steering shaft
103
whose upper end portion is coaxially connected to the lower end portion of the first steering shaft
101
via a torsion bar
102
and whose lower end portion is connected to a steering mechanism linked to wheels, a torque sensor
104
for detecting torque applied to the first steering shaft
101
depending on torsion generated in the torsion bar
102
by the rotation of the steering wheel
100
, a steering assist motor
105
driven on the basis of the detection result by the torque sensor
104
, and a reduction mechanism having a worm
106
and a worm wheel
107
and connected to the output shaft of the motor
105
to reduce the rotation speed of the output shaft and to transmit the rotation to the second steering shaft
103
. In this electric power steering apparatus having the above-mentioned configuration, the operation of the steering mechanism depending on the rotation of the steering wheel
100
is assisted by the rotation of the motor
105
, whereby the steering effort of the driver can be reduced.
As shown in
FIG. 2
, the worm
106
constituting the reduction mechanism is supported in the fitting holes of a housing
110
via a pair of rolling bearings
108
a
and
108
b
. The second steering shaft
103
provided with the worm wheel
107
is supported in the fitting holes of the housing
110
via a pair of rolling bearings
109
a
and
109
b
as shown in
FIG. 1
, and is prevented from moving in the radial and axial directions thereof.
In the case when such reduction mechanism is used, the worm
106
, the worm wheel
107
, the rolling bearings
108
a
,
108
b
,
109
a
and
109
b
, the second steering shaft
103
and the housing
110
, machined so that the distance between the rotation centers of the worm
106
and the worm wheel
107
coincides with the distance between the centers of the fitting holes for the rolling bearings
108
a
and
108
b
and the centers of the fitting holes for the rolling bearings
109
a
and
109
b
within an allowable range, are assembled selectively in order to reduce the amount of the backlash at the meshing portion of the worm
106
and the worm wheel
107
. However, this assembly work takes a long time. In addition, the amount of the backlash increases when the amount of the wear at the teeth of the worm
106
and the worm wheel
107
increases. Countermeasures for these problems have been demanded.
In addition, another electric power steering apparatus is known. This apparatus is configured to reduce the amount of the backlash at the meshing portion of the worm
106
and the worm wheel
107
by providing rubber rings between the outer peripheral surfaces of the bearings
108
a
and
108
b
for supporting the worm
106
connected to the output shaft of the motor
105
and the fitting holes of the housing
110
, and by energizing the worm
106
toward the worm wheel
107
(hereafter referred to as a first radial direction Y) by virtue of the elastic restoring forces of the rubber rings. One of the pair of fitting holes into which the bearings
108
a
and
108
b
are fitted is made eccentric with respect to the other.
However, the tooth lead of the worm wheel
107
of the reduction mechanism is inclined in its rotation direction with respect to its rotation center line. For this reason, when rotation torque is applied from the worm
106
to the worm wheel
107
, that is, when steering is assisted by the rotation of the motor
105
, the worm
106
is pressed relatively strongly so as to be moved in its radial direction (hereafter referred to as a second radial direction X) along the tooth lead of the worm wheel
107
. In the case of the conventional electric power steering apparatus configured to reduce the amount of the backlash by using the rubber rings as described above, the elastic restoring force (spring constant) of the rubber ring is constant along the entire circumference thereof. This causes problems described below.
When the elastic restoring force of the rubber ring is relatively small, the rotation torque applied to the meshing portion of the worm
106
and the worm wheel
107
(the torque applied to the steering shaft) can be made relatively small. However, the amount of the movement of the worm
106
in the second radial direction X becomes relatively large, and the worm
106
is in danger of being damaged.
In addition, when the elastic restoring force of the rubber ring is relatively large, the amount of the movement of the worm
106
in the second radial direction X can be made very small. However, the rotation torque applied to the meshing portion of the worm
106
and the worm wheel
107
becomes relatively large, and the rotation performance of the worm
106
becomes worse. This worsens the return of the steering wheel and also worsens the feeling of the steering wheel when steering is neutral.
Furthermore, as shown in
FIG. 2
, both end portions of the worm
106
constituting the reduction mechanism are supported in the fitting holes of the housing
110
via the pair of rolling bearings
108
a
and
108
b
. A screw ring
111
screwed into a screw hole formed so as to be connected to the fitting hole of the housing
110
is made contact with the outer ring of one of the rolling bearings, that is,
108
a
. By rotating the screw ring
111
, the outer ring and the inner ring of the rolling bearing
108
a
are moved relatively in its axial direction, thereby eliminating thrust gaps at the pair of rolling bearings
108
a
and
108
b
, and preventing the worm
106
from rattling in its axial direction. Moreover, the second steering shaft
103
, on which the worm wheel
107
is provided, is supported in the fitting holes of the housing
110
via the pair of rolling bearings
109
a
and
109
b
, thereby being prevented from moving in its radial and axial directions.
However, when preventing the worm
106
from rattling in its axial direction by using the screw ring
111
, it is necessary to use error absorption means for absorbing an adjustment error owing to the rotation operation of the screw ring
111
on the side of the screw ring
111
in order to securely eliminate the thrust gaps at the pair of rolling bearings
108
a
and
108
b
. Besides, since the outer ring of the rolling bearing
108
a
is directly pressed by the rotation operation of the screw ring
111
, the rolling bearing
108
a
is in danger of being preliminary pressurized more than necessary owing to a slight rotation error of the screw ring
111
. In this case, the rotation torque of the worm
106
increases, thereby increasing the frictional resistance of the entire steering assist mechanism. This worsens the return of the steering wheel and also worsens the feeling of the steering wheel when steering is neutral. Countermeasures for this problem have been demanded.
In addition, as means for reducing the amount of the backlash at the meshing portion of the worm
106
and the worm wheel
107
, a configuration is designed, set and manufactured so that a minus gap, i.e., a minus tolerance, is generated in the radial direction at the meshing portion of the worm
106
and the worm wheel
107
by making the distance between the centers of the fitting holes for the rolling bearings
108
a
and
108
b
and the centers o
Iino Takeo
Ito Yoshio
Kobayashi Masanori
Murakami Tetsuya
Otsuka Tatsuo
Darby & Darby
Koyo Seiko Co. Ltd.
Lerner Avraham
LandOfFree
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