Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2001-11-21
2004-05-18
Lerner, Avraham (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C074S424830, C074S424870
Reexamination Certificate
active
06736235
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 electrically driven 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 currently 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.
FIG. 21
illustrates an application of the conventional end-cap type ball screw mechanism to the electrically powered steering device. A rotary nut
51
has its outer periphery on which a rolling bearing assembly
53
for supporting the rotary nut
51
relative to a housing (not shown) and a rotor
54
of an electrically driven motor for driving the rotary nut
51
are mounted. A flange
52
is utilized for positioning the rolling bearing
53
and the rotor
54
.
Since the rolling bearing
53
is of an inner race rotating type in which an inner race
53
a
is rotatable, the inner race
53
a
of the rolling bearing
53
is press-fitted onto the rotary nut
51
with an inner peripheral surface of the inner race
53
a
held in tight contact with an outer peripheral surface of the rotary nut
51
. On the other hand, since the rotor
54
of the electric motor cannot be press-fitted in a manner similar to the rolling bearing
53
, a portion of the outer peripheral surface of the rotary nut
51
on one side opposite to the rolling bearing
53
is formed with a knurled pattern
55
in the form of, for example, axial serrations so that when the rotor
54
is mounted on the rotary nut
51
, ridges or projections of the knurled pattern
55
can be mechanically interlocked with the inner peripheral surface of the rotor
54
. Thus, the rotor
54
and the rotary nut
51
are firmly coupled together so that a rotational torque can be transmitted from the rotor
54
to the rotary nut
51
without being accompanied by any relative rattling motion therebetween in a direction conforming to the direction of rotation thereof.
In the case of the structure shown in
FIG. 21
, an misalignment between the motor rotor
54
and the rotary nut
51
tends to occur during the assemblage as a result of failure to observe precision to such an extent as to result in increase of the rotational torque of the ball screw mechanism and/or variation in torque.
On the other hand, the ball screw mechanism is available in various types depending on the manner of circulation of balls, one of which is known as a bridge type.
FIG. 22
illustrates an example of the conventional bridge type ball screw mechanism. The ball screw shaft
71
has an outer periphery formed with an externally threaded helical groove
72
whereas the rotary nut
73
has an internally threaded helical groove
74
complemental to the externally threaded helical groove
72
. A plurality of balls
75
are interposed between the externally and internally threaded helical grooves
72
and
74
so that the ball screw shaft
71
can be drivingly coupled with the rotary nut
73
. A cylindrical barrel portion of the rotary nut
73
is formed with a plurality of oval bores
76
extending completely across the thickness of the wall of the cylindrical barrel portion of the rotary nut
73
while depleting respective portions of the internally threaded helical groove
74
, and corresponding oval bridges
77
are engaged in those oval bores
76
. Each of the bridges
77
is a component part in which a connecting groove segment
78
for communicating the neighboring turns of the internally threaded helical groove
74
together is formed. Thus, about one turn of the internally threaded helical groove
74
and the corresponding connecting groove segment
78
altogether define a ball rolling path for the balls
75
. The balls
75
interposed between the externally and internally threaded helical grooves
72
and
74
within the ball rolling path can move along the externally and internally threaded helical grooves
72
and
74
, then guided along the connecting groove segment
78
in the bridge
77
and return to the neighboring internally threaded helical groove
74
after having ridden over a screw thread on the ball screw shaft
71
.
FIG. 23
illustrates in a development elevation the rotary nut
73
, employed in the above described bridge type ball screw mechanism, as viewed from inside the rotary nut
73
. In the bridge type ball screw mechanism, in the internally threaded helical groove
74
, a non-circulating portion
79
that is a space where no ball exist (as shown by a cross-hatched area) is formed between the neighboring bridges
77
between turns of the ball rolling path (show by a hatched area) each corresponding to about one turn of the internally threaded helical groove
74
.
During assemblage of the above described bridge type ball screw mechanism, as shown in
FIG. 24
, after a dummy shaft
80
in place of the ball screw shaft
71
has been set inside the rotary nut
73
so that a free end of the dummy shaft
80
is aligned with the position of the bridge
77
, a number of the balls
75
, for example,
17
balls corresponding to one turn of the circulating path are inserted into the rotary nut
73
and, thereafter, the balls
75
within the rotary nut
73
are successively guided to the circulating path one at a time by the use of pincette.
However, in this type of the ball screw mechanism, the balls
75
tend to enter the non-circulating portion
79
during the assemblage of the ball screw mechanism. If the ball screw mechanism is used in practice with the balls
75
mixed into the non-circulating portion
79
, the balls
75
will detrimentally break the ball rolling path and there is a high risk that the ball screw mechanism would be consequently locked. In view of this, in assembling the above described bridge type ball screw mechanism, and particularly during insertion of the balls
75
, the attendant worker has to take utmost care by carefully watching the work being performed so as to avoid entry of the balls
75
into the non-circulating portion
79
and/or by counting the number of the balls
75
being inserted into the circulating path. However, even though the utmost care is taken during the assemblage, no complete avoidance of the balls
75
being mixed into the non-circulating portion
79
is possible and, therefore, the possibility of the ball screw mechanism being locked cannot be avoided sufficiently.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been devised to substantially eliminate the above discussed problems and is intended to provide an improved electrically powered steering device that is easy to assemble and in which any possible misalignment of the rotor of the electrically driven motor relative to the nut of the ball screw mechanism can be compensated for and, therefore, an undesirable increase of the rotational torque and variation in torque resulting from the misalignment can advantageously be eliminated.
Another important object of the present invention is to provide an improved bridge type ball screw mechanism for use in the electrically powered steering device, which is easy to assemble and in which an undesirable entry of the balls into the non-circulating portion during assemblage can be effectively and securely avoided.
In order to accomplish these objects of t
Ikeda Yoshinori
Shibata Kiyotake
Yoshida Isamu
Lerner Avraham
NTN Corporation
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