Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2001-03-28
2002-08-27
Lerner, Avraham (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C074S424830
Reexamination Certificate
active
06439338
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ball screw mechanism and an electrically powered steering device utilizing such ball screw mechanism.
2. Description of the Prior Art
The electrically powered steering device for an automotive vehicle is a device for assisting a steering force of the steering wheel by means of an electric motor and is available in various types. One of them is a model designed not only to apply an axially shifting force to a retractable steering shaft drivingly coupled with a steering mechanism for wheels by translating a rotation of the steering wheel by means of a motion translating mechanism such as a rack-and-pinion mechanism but also to apply an output of an electric motor as an axially shifting force to the retractable steering shaft through a ball screw mechanism. In order to render the electrically powered steering device to be assembled compact in size and, hence, to the automotive vehicle to be lightweight in its entirety, the ball screw mechanism is desired to have a compact size and a high load capacity.
The ball screw mechanism is also available in various types depending on the mode of circulation of a series of balls, including a bridge type ball screw design. The prior art bridge type ball screw mechanism is shown in
FIGS. 13A
to
13
C, reference to which will now be made. A ball screw shaft
51
has its outer peripheral surface formed with an externally threaded helical groove
52
and has a rotary nut
53
mounted thereon. This rotary nut
53
has an inner peripheral surface with an internally threaded helical groove
54
cooperable with the externally threaded helical groove
52
and is threadingly engaged with the ball screw shaft
51
through a series of balls
55
that are received in part in the externally threaded helical groove
52
and in part in the internally threaded helical groove
54
. A cylindrical wall defining the rotary nut
53
has a plurality of generally elliptical mounting holes
56
each extending completely across the thickness of the cylindrical wall of the rotary nut
53
having its opposite ends aligned respectively with the neighboring convolutions of the externally threaded helical groove
52
. These elliptical mounting holes
56
are closed by a similarly elliptically shaped bridge member
57
that is fixedly plugged therein. The respective bridge member
57
has a concave surface where a connecting groove
58
is defined so that the neighboring convolutions of the internally threaded helical groove
54
are communicated with each other to thereby define a ball rolling passage along which the series of the balls
55
rollingly traverse from one of the convolutions of the internally threaded helical groove
54
to the next adjacent convolution of the same internally threaded helical groove
54
. Thus, the series of the balls
55
movably interposed between the internally and externally threaded helical grooves
52
and
54
can rollingly move along and between the internally and externally threaded helical grooves
52
and
54
and are then guided along the connecting grooves
58
in the bridge members
57
so as to ride over corresponding threads of the ball screw shaft
51
from one convolution of the externally threaded helical groove
54
onto the next adjacent convolution of the externally threaded helical groove
54
.
The bridge type ball screw mechanism has an advantage in that the rotary nut
53
can have a reduced outer diameter, but has a disadvantage in that because of the plural bridge members
57
necessitated the number of component parts forming the ball screw mechanism is large. Also, if an attempt is made to increase the load capacity, the bridge type ball screw mechanism tends to pose the following problems.
While one of the following means for increasing the load capacity is available in the ball screw mechanism, the bridge type ball screw mechanism cannot employ any of those means because of the reason described therein.
(1) To reduce the pitch to make it possible to increase the number of balls to be circulated. Where this means is employed, the use of the balls of a relatively small diameter results in decrease of the load capacity. Because of this, it is necessary to reduce the pitch without the ball diameter being altered. However, with the bridge type ball screw mechanism, it is not possible to reduce the pitch without the ball diameter being altered, because one bridge member
57
is necessitated for a single pitch (the span between the neighboring helical grooves).
(2) To form a groove between the successive leads to thereby form a multi-thread screws so that an effect similar to that afforded when the number of the balls to be circulated is increased. In the case of the multi-thread screw, the lead (the distance of movement per rotation) is limited and, therefore, the lead cannot be reduced. Also, although the multi-thread screw can be employed in an end-cap type ball screw mechanism, the bridge type ball screw mechanism cannot employ the multi-thread screw because as discussed in the previous paragraph one bridge member
57
is necessitated for a single pitch.
As discussed above, where the load capacity is desired to be increased in the bridge type ball screw mechanism, there is no way other than to employ the rotary nut having an increased length so that an increased number of the balls can be employed and, accordingly, while the bridge type ball screw mechanism is advantageous in that the rotary nut of a relatively small outer diameter can be employed, the use of the rotary nut of the increased length hampers compactization of the bridge type ball screw mechanism as a whole. Also, the use of the rotary nut of the increased length requires a corresponding increase of the number of the bridge members
57
used and, therefore, the number of machining processes and the number of component parts tend to increase, resulting in increase of costs required to manufacture the bridge type ball screw mechanism.
To alleviate the foregoing problems, the assignee of the present invention has filed the Japanese patent Application No. 11-313518 and the U.S. patent application Ser. No. 09/704,678, in which it is suggested a bridge member
57
A having a plurality of connecting passages
58
defined therein as shown in
FIGS. 14 and 15
. The bridge member
57
A disclosed therein has opposite side edges spaced in a direction circumferentially of the rotary nut
53
, which edges are formed with respective guide walls
68
upstanding therefrom in a direction radially outwardly of the rotary nut
53
. These guide walls
68
are each formed with a separation preventive projection
68
a
at a free edge thereof such that when the bridge member
57
A is mounted in position within a mounting hole
56
A defined in the rotary nut
53
, the separation preventive projections
68
a
integral with the respective guide walls
68
are resiliently engaged against associated steps
69
formed on opposite side faces of the mounting hole
56
A. By this engagement, the bridge member
57
A is retained firmly within the mounting hole
56
A in the rotary nut
53
.
However, it has been found that where the guide walls
68
having the separation preventive projections
68
a
are integrally formed with the bridge member
57
A, the guide walls
68
are prone to a considerable deformation and a difficulty in manufacture because each of the guide walls
68
is thin walled having a considerably small wall thickness.
SUMMARY OF THE INVENTION
Accordingly, the present invention is intended to provide a compact ball screw mechanism having a minimized number of component parts and also having a high load bearing capacity, wherein the bridge member of a simplified structure can easily, but firmly be fitted to the rotary nut.
The present invention has another object to facilitate formation by molding of the bridge member by simplifying the structure of the bridge member.
An additional object of the present invention is to provide an electrically powered steering device employing the ba
Ikeda Yoshinori
Tateishi Koji
Yoshida Isamu
Yoshioka Morihisa
Lerner Avraham
NTN Corporation
Sughrue & Mion, PLLC
LandOfFree
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