U.S. patent application number 09/818568 was filed with the patent office on 2002-01-10 for ball screw mechanism and electrically powered steering device utilizing the same.
Invention is credited to Ikeda, Yoshinori, Tateishi, Koji, Yoshida, Isamu, Yoshioka, Morihisa.
Application Number | 20020003059 09/818568 |
Document ID | / |
Family ID | 18614577 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020003059 |
Kind Code |
A1 |
Yoshioka, Morihisa ; et
al. |
January 10, 2002 |
Ball screw mechanism and electrically powered steering device
utilizing the same
Abstract
A ball screw mechanism (20) compact in size, employing a
minimized number of component parts and having a high load capacity
includes a rotary nut (22) having an internally threaded helical
groove (26) and a mounting hole (30) both defined therein, and a
simplified bridge member (24) mounted in the rotary nut (22). The
bridge member (24) has a plurality of connecting grooves (28)
defined on an inner surface thereof each operable to communicate
neighboring convolutions of the internally threaded helical groove
(26). The bridge member (24) has its opposite side edges formed
with respective guide walls (36) protruding in a direction radially
outwardly of the rotary nut (22) that are crimped to allow the
bridge member (24) to be fixedly retained within the mounting hole
(30).
Inventors: |
Yoshioka, Morihisa;
(Iwata-shi, JP) ; Tateishi, Koji; (Iwata-shi,
JP) ; Yoshida, Isamu; (Iwata-shi, JP) ; Ikeda,
Yoshinori; (Iwata-shi, JP) |
Correspondence
Address: |
SUGHRUE, MION.ZINN, MACPEAK & SEAS
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037
US
|
Family ID: |
18614577 |
Appl. No.: |
09/818568 |
Filed: |
March 28, 2001 |
Current U.S.
Class: |
180/444 |
Current CPC
Class: |
B62D 5/0448 20130101;
F16H 25/2223 20130101; F16H 2025/2242 20130101; Y10T 74/19753
20150115 |
Class at
Publication: |
180/444 |
International
Class: |
B62D 005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 2000 |
JP |
2000-100356 |
Claims
What is claimed is:
1. A ball screw mechanism which comprises: a ball screw shaft; a
rotary nut having an internally threaded helical groove defined on
an inner peripheral surface thereof in face-to-face relation with
the ball screw mechanism; a plurality of balls rollingly
accommodated in series within a ball rolling passage defined
between the ball screw shaft and the internally threaded helical
groove for transmitting a force from one of the rotary nut and the
ball screw shaft to the other thereof; and at least one bridge
member having at least one groove defined therein and mounted in
the rotary nut with the groove communicating neighboring
convolutions of the internally threaded helical groove, said bridge
member being fixedly connected with the rotary nut by means of a
plastic deformation.
2. The ball screw mechanism as claimed in claim 1, wherein the
bridge member includes an arm engageable with the internally
threaded helical groove of the rotary nut for positioning the
bridge member axially relative to the rotary nut.
3. The ball screw mechanism as claimed in claim 1 or 2, wherein the
rotary nut has at least one mounting hole defined therein for
accommodating the bridge member; wherein the bridge member has
opposite side edges in a direction circumferentially of the rotary
nut, said opposite side edges of the bridge member being formed
with respective guide walls upstanding therefrom in a direction
radially of the rotary nut; and wherein said guide walls being
crimped to engage respective inner side faces of the mounting hole
defined in the rotary nut.
4. A ball screw mechanism which comprises: a ball screw shaft; a
rotary nut having an internally threaded helical groove defined on
an inner peripheral surface thereof in face-to-face relation with
the ball screw mechanism, said rotary nut having at least one
mounting hole defined therein; a plurality of balls rollingly
accommodated in series within a ball rolling passage defined
between the ball screw shaft and the internally threaded helical
groove for transmitting a force from one of the rotary nut and the
ball screw shaft to the other thereof, at least one bridge member
accommodated within the mounting hole and having at least one
groove defined therein and mounted in the rotary nut with the
groove communicating neighboring convolutions of the internally
threaded helical groove; and a separate fixing member mounted on
the bridge member and including guide walls upstanding therefrom in
a direction radially of the rotary nut along opposite side edges of
the rotary nut with respect to a circumferential direction thereof,
said guide walls being resiliently engaged with opposite inner side
faces of the mounting hole defined in the rotary nut to thereby
allow the bridge member to be fixedly retained within the mounting
hole.
5. The ball screw mechanism as claimed in claim 4, wherein the
fixing member is prepared from a steel plate by means of a press
work and is mounted on the bridge member by means of crimping the
bridge member
6. The ball screw mechanism as claimed in claim 4, wherein the
fixing member is prepared from a steel plate by means of a press
work and is mounted on the bridge member by utilization of a
resiliency of the fixing member.
7. A ball screw mechanism which comprises: a ball screw shaft; a
rotary nut having an internally threaded helical groove defined on
an inner peripheral surface thereof in face-to-face relation with
the ball screw mechanism, said rotary nut having at least one
mounting hole defined therein, said mounting hole having opposite
side faces formed with respective engagement steps defined therein;
a plurality of balls rollingly accommodated in series within a ball
rolling passage defined between the ball screw shaft and the
internally threaded helical groove for transmitting a force from
one of the rotary nut and the ball screw shaft to the other
thereof, at least one bridge member accommodated within the
mounting hole and having at least one groove defined therein and
mounted in the rotary nut with the groove communicating neighboring
convolutions of the internally threaded helical groove, said bridge
member being formed with engagement grooves aligned and cooperable
with the respective engagement steps; and a fixing member made up
of a wire and engaged in part with the engagement steps and in part
within the engagement grooves to thereby allow the bridge member to
be fixedly retained within the mounting hole.
8. The ball screw mechanism as claimed in any one of claims 1 to 7,
wherein the bridge member is made of a sintered alloy.
9. The ball screw mechanism as claimed in any one of claims 1 to 7,
wherein the bridge member has a plurality of connecting grooves
each for communicating neighboring convolutions of the internally
threaded helical grooves.
10. An electrically powered steering device which comprises: a
housing; a steering shaft accommodated in the housing and drivingly
coupled with a steering mechanism for steering wheels; a motion
translating mechanism for converting a rotational force of a
steering wheel into a force required to displace the steering shaft
in a direction axially thereof; a ball screw mechanism including a
ball screw shaft defined by a portion of the steering shaft and a
rotary nut operatively mounted on the ball screw shaft, said ball
screw mechanism being of a structure as defined in any one of
claims 1 to 9; and an electric motor for driving the rotary nut
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a ball screw mechanism and
an electrically powered steering device utilizing such ball screw
mechanism.
[0003] 2. Description of the Prior Art
[0004] 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.
[0005] 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 13C, 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.
[0006] 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.
[0007] 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.
[0008] (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).
[0009] (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.
[0010] 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.
[0011] 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 57A having a plurality of
connecting passages 58 defined therein as shown in FIGS. 14 and 15.
The bridge member 57A 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 68a at a free edge thereof such that when the bridge
member 57A is mounted in position within a mounting hole 56A
defined in the rotary nut 53, the separation preventive projections
68a integral with the respective guide walls 68 are resiliently
engaged against associated steps 69 formed on opposite side faces
of the mounting hole 56A. By this engagement, the bridge member 57A
is retained firmly within the mounting hole 56A in the rotary nut
53.
[0012] However, it has been found that where the guide walls 68
having the separation preventive projections 68a are integrally
formed with the bridge member 57A, 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
[0013] 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.
[0014] The present invention has another object to facilitate
formation by molding of the bridge member by simplifying the
structure of the bridge member.
[0015] An additional object of the present invention is to provide
an electrically powered steering device employing the ball screw
mechanism of the type referred to above for transmitting an output
of an electric motor, which mechanism is compact in size and easy
to manufacture and having a high load capacity so that the device
as a whole can be assembled compact in size.
[0016] In order to accomplish these objects of the present
invention, in a first aspect of the present invention there is
provided a ball screw mechanism which includes a ball screw shaft;
a rotary nut having an internally threaded helical groove defined
on an inner peripheral surface thereof in face-to-face relation
with the ball screw mechanism; a plurality of balls rollingly
accommodated in series within a ball rolling passage defined
between the ball screw shaft and the internally threaded helical
groove for transmitting a force from one of the rotary nut and the
ball screw shaft to the other thereof, and at least one bridge
member having at least one groove defined therein and mounted in
the rotary nut with the groove communicating neighboring
convolutions of the internally threaded helical groove. The bridge
member is fixedly connected with the rotary nut by means of a
plastic deformation.
[0017] According to this structure of the present invention, the
bridge member is fixedly integrated with the rotary nut by the
utilization of the plastic deformation. Accordingly, a simplified
structure can be employed to integrate the bridge member with the
rotary nut. Specifically, since the bridge member is allowed to
undergo plastic deformation, the bridge member need not be provided
with complicatedly shaped elements for engagement and can therefore
be simplified in shape. For this reason, molding can easily be
performed where the bridge member is to be formed of a sintered
metal by the use of an injection molding technique.
[0018] Also, where the bridge member is formed with a plurality of
connecting grooves, the pitch of the internally threaded helical
groove can be reduced such that without the length of the rotary
nut being increased, the number of the balls to be circulated can
be increased to thereby increase the load bearing capacity. Because
of this, together with the advantage of a reduced outer diameter of
the rotary nut as is the case with that used in the conventional
bridge type ball screw mechanism, it can have a compact size and an
increased load capacity. Moreover, by forming the plural connecting
grooves in one bridge member, the number of the bridge members to
be employed can advantageously be reduced, resulting in a minimized
number of component parts and ease to assembly and, accordingly the
cost of manufacture can advantageously be reduced. In addition, the
use of the plural connecting grooves in the single bridge member
facilitates increase of the preciseness.
[0019] In the practice of the present invention, the bridge member
may include an arm engageable with the internally threaded helical
groove of the rotary nut for positioning the bridge member axially
relative to the rotary nut.
[0020] By allowing the arm integral with the bridge member to
engage in the internally threaded helical groove formed in the
rotary nut as a ball rolling face, the bridge member can be highly
accurately positioned relative to the rotary nut.
[0021] Also, in the practice of the present invention, the rotary
nut may have at least one mounting hole defined therein for
accommodating the bridge member and, on the other hand, side edges
of the bridge member that are opposite to each other in a direction
circumferentially of the rotary nut may be formed with respective
guide walls upstanding therefrom in a direction radially of the
rotary nut. In this structure, the guide walls are crimped to
engage respective inner side faces of the mounting hole defined in
the rotary nut to thereby allow the bridge member to be fixedly
retained within the mounting hole.
[0022] The provision of the guide walls in the bridge member and
the use of the crimping technique to crimp the guide walls to fix
the bridge member relative to the rotary nut effectively
facilitates fixture of the bridge member. In particular, the use of
the crimping technique effectively eliminates the need to form the
separation preventive projections at the free ends of the
respective guide walls, resulting in simplification in shape of the
bridge member. Because of this, where the bridge member is made of
the sintered metal by the use of an injection molding technique,
the molding can easily be accomplished.
[0023] In a second aspect of the present invention, there is
provided a ball screw mechanism which is similar to that according
to the first aspect of the present invention, but differs therefrom
in that in place of the bridge member that is fixed to the rotary
nut by the utilization of the plastic deformation, a separate
fixing member is mounted on the bridge member. This separate fixing
member includes guide walls upstanding therefrom in a direction
radially of the rotary nut along opposite side edges of the rotary
nut with respect to a circumferential direction thereof. These
guide walls are resiliently engaged with opposite inner side faces
of the mounting hole defined in the rotary nut.
[0024] According to this second aspect of the present invention,
the guide walls of the separate fixing member mounted on the bridge
member resiliently engage with the circumferentially spaced inner
side faces of the mounting hole defined in the rotary nut to firmly
secure the bridge member to the rotary nut. The use of the fixing
member separate from, but mounted on the bridge member is effective
to simplify the shape of the bridge member. Because of this, where
the bridge member is made of a sintered metal by the use of an
injection molding technique, the molding thereof can easily be
accomplished.
[0025] Where the fixing member separate from the bridge member is
employed, the separate fixing member may be prepared from a steel
plate by means of a press work and is mounted on the bridge member
by means of crimping the bridge member. Using the separate fixing
member formed from a steel plate by the use of the press work makes
it possible to mass-produce the fixing members at a low cost. Also,
mounting of the fixing member on the bridge member by the use of a
crimping technique facilitates integration of the fixing member
with the bridge member.
[0026] Where the fixing member separate from the bridge member is
employed, the fixing member may be prepared from a steel plate by
means of a press work and is mounted on the bridge member by
utilization of a resiliency of the fixing member.
[0027] Where the separate fixing member is formed from a steel
plate by the use of the press work, it is possible to utilize the
resiliency of the fixing member to mount the fixing member on the
bridge member easily. Mounting of the fixing member by the
utilization of its own resiliency can be accomplished if, for
example, the fixing member is of a generally U-shaped configuration
so that it can be fitted in a fashion sandwiching the bridge
member.
[0028] According to a third aspect of the present invention, there
is provided a ball screw mechanism which is similar to that
according to the second aspect of the present invention, but
differs therefrom in that in place of the separate fixing member
employed in the second aspect, the mounting hole in the rotary nut
has engagement steps defined in side faces of the mounting hole
that are opposite to each other in a circumferential direction of
the rotary nut and, on the other hand, the bridge member is formed
with engagement grooves aligned and cooperable with the respective
engagement steps, and a fixing member made up of a wire is utilized
to engage in part with the engagement steps and in part within the
engagement grooves to thereby allow the bridge member to be fixedly
retained within the mounting hole.
[0029] According to this design, by allowing the fixing member in
the form of a wire to be engaged in part within the engagement
grooves in the bridge member and in part with the engagement steps
in the rotary nut, the bridge member can be fixedly retained by the
rotary nut. Even this design makes it possible to simplify the
shape of the bridge member since the fixing member is separate from
the bridge member. Because of this, where the bridge member is made
of a sintered metal by the use of an injection molding technique,
the molding can be easily accomplished.
[0030] In the structure according to any one of the first to third
aspects of the present invention, the bridge member is preferably
made of a sintered alloy. The use of the sintered alloy as material
for the bridge member makes it possible for the bridge member to be
manufactured by the use of injection molding and sintering
techniques with no need to use any mechanical machining such as a
grinding or a milling, resulting in a good mass-productivity.
Accordingly, inexpensive manufacture is possible. Also, even in any
one of the second and third aspects of the present invention, the
plural connecting grooves may be formed in the single bridge member
as is the case with that in the first mentioned aspect of the
present invention.
[0031] The present invention also provides an electrically powered
steering device which includes a housing; a steering shaft
drivingly coupled with a steering mechanism for steering wheels; a
motion translating mechanism for converting a rotational force of a
steering wheel into a force required to displace the steering shaft
in a direction axially thereof, a ball screw mechanism including a
ball screw shaft defined by a portion of the steering shaft and a
rotary nut operatively mounted on the ball screw shaft; and an
electric motor for driving the rotary nut. The ball screw mechanism
employed in this electrically powered steering device is of the
structure described in connection with any one of the first to
third aspects of the present invention.
[0032] In this electrically powered steering device, the ball screw
mechanism employed therein for transmitting the output of the
electric motor is compact in structure and has a high load capacity
and, therefore, the electrically powered steering device itself can
be assembled compact in size.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] In any event, the present invention will become more clearly
understood from the following description of preferred embodiments
thereof, when taken in conjunction with the accompanying drawings.
However, the embodiments and the drawings are given only for the
purpose of illustration and explanation, and are not to be taken as
limiting the scope of the present invention in any way whatsoever,
which scope is to be determined by the appended claims. In the
accompanying drawings, like reference numerals are used to denote
like parts throughout the several views, and:
[0034] FIG. 1A is a side view of a ball screw mechanism according
to a first preferred embodiment of the present invention;
[0035] FIG. 1B is a longitudinal sectional view of the ball screw
mechanism shown in FIG. 1A;
[0036] FIG. 1C is a perspective view of one of bridge members
employed in the ball screw mechanism shown in FIG. 1A;
[0037] FIG. 2A is a longitudinal sectional view, on an enlarged
scale, showing a rotary nut employed in the ball screw mechanism
shown in FIG. 1A;
[0038] FIG. 2B is a fragmentary end view, with a portion cut out,
showing one of radial halves of the rotary nut of FIG. 2A;
[0039] FIG. 2C is a rear view of the rotary nut of FIG. 2A as
viewed in a direction opposite to that shown in FIG. 2A;
[0040] FIG. 3A is a front elevational view, on an enlarged scale,
showing one of bridge members employed in the ball screw mechanism
according to the first embodiment of the present invention;
[0041] FIG. 3B is a plane view, with a portion cut out, of the
bridge member shown in FIG. 3A;
[0042] FIG. 3C is a rear elevational view of the bridge member as
viewed in a direction opposite to FIG. 3A;
[0043] FIG. 3D is a perspective view of the bridge member shown in
FIG. 3A;
[0044] FIG. 3E is an end view of the bridge member shown as
isolated from the rotary nut;
[0045] FIG. 4A is a longitudinal sectional view, with a portion cut
out, of the rotary nut employed in the ball screw mechanism,
showing a conceptual structure thereof,
[0046] FIG. 4B is an end view, with a portion cut out, of the
rotary nut shown in FIG. 4A;
[0047] FIGS. 5A to 5C are end sectional views ol the rotary nut,
respectively, showing the sequence of mounting of the bridge member
on the rotary nut;
[0048] FIG. 6 is an end sectional view of the rotary nut employed
in the ball screw mechanism according to a second preferred
embodiment of the present invention;
[0049] FIG. 7A is a front elevational view of the bridge member
employed in the ball screw mechanism shown in FIG. 6;
[0050] FIG. 7B is an end view of the bridge member employed in the
ball screw mechanism shown in FIG. 6;
[0051] FIG. 7C is a cross-sectional view along line VII-VII of FIG.
7A;
[0052] FIGS. 8A to 8C are end views of the bridge member showing
different embodiments of the present invention, respectively;
[0053] FIG. 9 is an end sectional view of the rotary nut employed
in the ball screw mechanism according to a further embodiment of
the present invention;
[0054] FIG. 10 is a fragmentary end sectional view, on an enlarged
scale, showing a portion of the rotary nut in the ball screw
mechanism shown in FIG. 9, showing how the bridge member is fixedly
retained within a mounting hole in the rotary nut;
[0055] FIG. 11 is a schematic plane view of a fixing member
employed to fix the bridge member in position within the mounting
hole;
[0056] FIG. 12 is a longitudinal sectional view, with a portion cut
out, showing an electrically powered steering device utilizing the
ball screw mechanism according to the present invention;
[0057] FIG. 13A is a longitudinal side view of the conventional
ball screw mechanism;
[0058] FIG. 13B is a longitudinal sectional view of the
conventional ball screw mechanism shown in FIG. 13A;
[0059] FIG. 13C is a perspective view of one of bridge members
employed in the conventional ball screw mechanism shown in FIG.
13A;
[0060] FIG. 14 is an end view, with a portion cut out, of the
rotary nut employed in the ball screw mechanism proposed by the
assignee of the present invention; and
[0061] FIG. 15 is a perspective view of the bridge member employed
in the rotary nut shown FIG. 14.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0062] A first preferred embodiment of the present invention will
now be described with particular reference to FIGS. 1 to 5. As
shown in FIGS. 1A to 1C, a ball screw mechanism 20 shown therein
includes a ball screw shaft 21, a rotary nut 22 mounted on the ball
screw shaft 21 for movement axially of and relative to the ball
screw shaft 21 and a series of balls 23 interposed between the ball
screw shaft 21 and the rotary nut 22. The rotary nut 22 includes a
substantially cylindrical nut body 22a having two bridge members 24
mounted thereon in a manner as will be described later. The ball
screw shaft 21 has an externally threaded helical groove 25 defined
on an outer peripheral surface thereof. The rotary nut 22 has an
internally threaded helical groove 26 defined in an inner
peripheral surface of the cylindrical nut body 22a in cooperable
relation with the externally threaded helical groove 25 to define a
ball rolling passage 27. The series of the balls 23 are rollingly
accommodated within the ball rolling passage 27. As clearly shown,
the rotary nut 22 has a portion of its outer peripheral surface
knurled to define a surface roughened area 29 that is utilized for
coupling with a rotor of an electric motor.
[0063] Each of the bridge member 24 has a concave surface which
when mounted on the cylindrical nut body 22a faces towards the ball
screw shaft 21 and is formed with a plurality of parallel
connecting grooves 28 defined therein. Specifically, each of those
connecting groove 28 of the bridge member 24 communicates the two
neighboring convolutions of the internally threaded helical groove
26 together and, in the illustrated embodiments, the connecting
grooves 28 communicates the successively neighboring convolutions
of the internally threaded helical groove 26 together to thereby
define single circumferentially extending passage. Accordingly, the
internally threaded helical groove 26 of the rotary nut 22 is
divided into a plurality of circumferentially extending passages
over an axial length of the rotary nut 22 that is encompassed by
the bridge member 24. Each of the parallel connecting grooves 28 in
the bridge member 27 has a depth sufficient to allow each of the
balls 23 to ride over a thread between the neighboring convolutions
of the externally threaded helical groove 25 in the ball screw
shaft 21 while being confined within the respective connecting
passage 28.
[0064] The bridge members 24 are positioned on the nut body 22a at
two respective locations spaced axially of the rotary nut 22 and
are circumferentially spaced 180.degree. about the longitudinal
axis of the rotary nut 22. The number of the connecting grooves 28
in one of the bridge members 24 may differ from that in the other
of the bridge members 24. In the illustrated embodiment
particularly shown in FIG. 1, three connecting passages 28 are
shown as formed in one of the bridge members 24 whereas two
connecting passages 28 are shown as formed in the other of the
bridge members 24. It is, however, to be noted that the number of
the bridge members 24 employed in one rotary nut 22 may not be
limited to two such as shown, but may be one or three or more.
[0065] FIGS. 2A to 2C illustrates the details of the rotary nut 22
and FIG. 3 illustrates the details of one of the bridge members 24
with an outer appearance (an outer diametric shape) of the rotary
nut 2 being schematically shown. The cylindrical nut body 22a of
the rotary nut 22 has a bridge mounting hole 30 for each bridge
member 24 defined therein so as to extend completely across the
thickness of a cylindrical wall of the nut body 22a and the
respective bridge member 24 is, after having been inserted into the
hollow of the rotary nut 22 and then radially outwardly into the
mounting hole 30, nested within the mounting hole 30.
[0066] Each of the bridge members 24 has a pair of arms 31 engaged
in the internally threaded helical groove 26 of the rotary nut 22
for positioning the respective bridge member 24 axially relative to
the nut body 22a of the rotary nut 22. These arms 31 are formed
with respective opposite ends of each bridge member 24 so as to
protrude therefrom in respective directions opposite to each other
with respect to the circumference of the rotary nut 22. Respective
portions of the internally threaded helical groove 26 of the rotary
nut 22 in which the arms 31 are engaged define a ball
non-circulating portion.
[0067] Each of the arms 31 has a generally semi-circular cross
sectional shape in conformity with the curvature of the internally
threaded helical groove 26 and is formed with a slit 32 extending
from a free end thereof towards a base end thereof. It is to be
noted that the externally and internally threaded helical grooves
25 and 26 have a cross sectional shape which specifically
represents a Gothic arch. Each of the arms 31 has a rear surface,
opposite to a front surface which is in contact with the internally
threaded helical groove 26, formed with a generally semi-circular
sectioned rear side groove 33 with the associated slit 32 tunneling
from the front surface thereof to the rear side groove 33. Also,
the front surface of each of the arms 31 is formed with a plurality
of longitudinal grooves 34 of a depth not reaching the rear side
groove 33.
[0068] A portion of an outer peripheral surface of each bridge
member 24 adjacent a corresponding side edge thereof with respect
to the circumferential direction of the rotary nut 22 is inwardly
depressed to define a respective setback area 35 (See FIG. 35E),
that is set backwards from the outer peripheral surface of the
bridge member 24, while leaving an adjacent guide wall 36 that
bestrides from the corresponding side edge of the respective bridge
member 24 in a direction radially outwardly of the rotary nut 22.
As shown in FIG. 4B, the mounting hole 30 for each bridge member 24
has opposite outer side edges formed with respective engagement
steps 39 so as to have an increased width W larger than the width
of the corresponding mounting hole 30 as measured in a direction
perpendicular to the longitudinal axis of the rotary nut 24.
[0069] Each of the bridge members 24 is fitted into the respective
mounting hole 30 by first inserting it into the hollow of the nut
body 22a and then urging it radially outwardly so as to be nested
within the mounting hole 30 with the arms 31 engaged in the
internally threaded helical groove 26. As the respective bridge
member 24 is nested within the corresponding mounting hole 30, the
guide walls 36 are plastically deformed so as to be fixed in
position relative to the nut body 22a. This fixing by plastic
deformation is carried out by crimping the guide walls 36 to
respective inner side faces of the associated mounting hole 30 that
are opposite to each other in a direction circumferentially of the
rotary nut 22. More specifically, by causing the guide walls 36
integral with the respective bridge member 24 to be engaged with
the respective engagement steps 39 by crimping respective free ends
ol the guide wall 36, the bridge member 24 is fixed in position
relative to the rotary nut 22.
[0070] FIGS. 5A to 5C illustrate a manner in which the guide walls
36 are crimped. As shown in FIG. 5A, the bridge member 24 is fitted
into the mounting hole 30 defined in the nut body 22a from inside
of the hollow of the rotary nut 22 in the manner described
previously. Starting from this condition, a core 48 for holding the
bridge member 24 is inserted into the hollow of the nut body 22a as
shown in FIG. 5B, followed by pressing of a punch 49 in a direction
radially inwardly of the nut body 22a. The punch 49 has a working
end formed integrally with a pair of outwardly tapering crimp edges
and, accordingly, as the punch 49 is pressed towards the nut body
22a, the tapering crimp edges thereof causes the associated guide
walls 36 to deform outwardly and be then crimped to allow the
crimped guide walls 36 to firmly engage the engagement steps 39 in
the mounting hole 30.
[0071] Each of the bridge member 24 may be made of a sintered
alloy. Where the bridge members 24 are made of a sintered alloy,
they can be manufactured by the use of an injection molding
machine, using a metallic powder that is adjusted to a plastic
state. During the injection molding of the bridge members 24, a
mixture of the metallic powder, a plastic material and a binder is
mixed by a kneading machine to provide a kneaded mixture which is
subsequently palletized to provide metallic pellets. The metallic
pellets so formed are then supplied to a hopper of the injection
molding machine and are subsequently injected in a molten state
into a mold assembly to complete each bridge member 24. The
metallic powder is preferably of a kind capable of being carburized
and may include, for example, 0.3% of carbon (C) and 1 to 2% of
nickel (Ni), the balance being iron (Fe).
[0072] The nut body 22a of the rotary nut 22 may also be made of a
sintered alloy which may be or may not be the same as that used for
the bridge members 24.
[0073] In the ball screw mechanism 20 of the structure described
above, since each of the bridge members 24 is formed with the guide
walls 36 so that the guide walls 36 can be subsequently crimped to
allow the respective bridge member 24 to be fixed in position
within the respective mounting hole 30 in the rotary nut 22, firm
fitting of the bridge member 24 can be easily and assuredly
accomplished. The use of the crimping technique for this purpose
eliminates the need to form separation preventive projections at
respective tips of the guide walls 36, allowing the respective
bridge member 24 to be simplified in shape. For this reason, where
each bridge member 24 is to be formed of the sintered metal by the
use of the injection molding technique, the molding thereof can
easily be accomplished. Specifically, where each bridge member 24
is made of the sintered alloy, molding by the use of the injection
molding technique and sintering are sufficient to manufacture it
and no mechanical processing such as grinding or milling is
necessary, resulting in a good productivity. Accordingly,
inexpensive bridge members can be manufactured.
[0074] Since as hereinabove described each of the bridge members 24
is mounted inside the respective mounting hole 30 in the nut body
22a from inside of the hollow of the nut body 22a with the arms 31
consequently engaged in the internally threaded helical groove 26
and the guide walls 36 are subsequently engaged to the outer side
edges of the corresponding mounting hole 30 by the use of a
crimping technique, the bridge member 24 can be firmly interlocked
in position with no possibility of being separated radially
inwardly or outwardly of the corresponding mounting hole 30. Also,
since each of the bridge members 24 is provided with the arms 31
that are engaged in the internally threaded helical groove 26
formed as a ball rolling surface in the rotary nut 22, a highly
accurate positioning is possible. Yet, since the arms 31 are formed
with the respective slits 32, the arms 31 can have a resiliency
with which the arms 31 can conform to the internally threaded
helical groove 26 with no gap formed therebetween.
[0075] The ball screw mechanism according to a second preferred
embodiment of the present invention will now be described with
reference to FIGS. 6 and 7. The ball screw mechanism 20 according
to this embodiment is substantially similar to that according to
the first embodiment shown in FIGS. 1 to 5, except that in place of
the guide walls 36 formed integrally with each of the bridge
members 24 in the previously described embodiment, a separate
fixing member 41 is employed for each of the bridge members 24.
[0076] The fixing member 41 is of a structure including a pair of
guide walls 41b upstanding radially outwardly of the rotary nut 22
adjacent the respective side edges of the respective bridge member
24 that are opposite to each other in the circumferential direction
of the rotary nut 22. These guide walls 41b of the fixing member 41
are adapted to be resiliently engaged to opposite inner side faces
of the corresponding mounting hole 30 formed in the nut body 22a,
to thereby lock the bridge member 24 in the nut body 22a.
[0077] The fixing member 41 for each bridge member 14 is formed by
the use of a press work, i.e., by pressing a steel plate to a
specific shape and includes a generally groove-shaped principal
body 41a having set-down pieces 41ab extending from respective
opposite ends of a bottom piece portion 41aa (See FIG. 7B), and the
guide wall 41b formed therewith and extending from a free end of
the corresponding set-down piece 41ab so as to lie adjacent and
parallel to such corresponding set-down piece 41ab. Each of the
bridge members 24 has an outer diametric surface that is generally
rectangular in shape, and the groove-shaped principal body 41a of
the corresponding fixing member 41 rests on a rectangular portion
24a of the outer diametric surface of the bridge member 24. The
outer diametric surface of each bridge member 24 that is flat is
formed with an engagement groove 43 of a width sufficient to
accommodate the fixing member 41, and the bottom piece portion 41aa
of the groove-shaped principal body 41a of the associated fixing
member 41 is received within such engagement groove 43.
[0078] The fixing member 41 is fixed in position in the respective
bridge member 24 by means of a plurality of crimping portions 42
provided in the bridge member 24. The crimping portions 42 are made
up of a plastic deformed portion that is plastically deformed to
hold down opposite side walls of the bridge member 34 that define
the engagement groove 43 in the bridge member 34 (See FIG. 7C).
[0079] Other structural features of the second embodiment than
those described above are similar to those described in connection
with the previously described embodiment.
[0080] With the structure according to the second embodiment, each
bridge member 24 can be fixed in position within the associated
mounting hole 30 in such a manner that the guide walls 41b of the
fixing member 41, which is a separate member provided for the
bridge member 24, are elastically held in contact with the inner
side faces defining the mounting hole 30 in the rotary nut 22.
Arrangement of the fixing member 41 as a member separate from the
bridge member 24 is effective to further simplify the shape of the
bridge member 24. For this reason, even when the bridge members 24
are molded by the use of an injection molding technique using the
sintered metal, the molding can easily be accomplished. Since
although the fixing member 41 is separate from the bridge member
24, it can be handled as an integral component part with the fixing
member 41 fixed beforehand to the associated bridge member 24,
there should be no problem in increase of the manufacturing steps
during assemblage of the ball screw mechanism 20 and, hence, a good
assemblability can be appreciated.
[0081] It is to be noted that in place of the crimping portions 42
employed in each of the bridge members 24, the fixing member 41 may
be integrated with the respective bridge member 24 by the
utilization of the resiliency possessed by the fixing member 41, an
example of which is shown in each of FIGS. 8A and 8B.
[0082] In the example shown in FIG. 8A, the groove-shaped principal
body 41a of the fixing member 41 is engaged in the rectangular
portion 24a of the bridge member 24 and the resiliency of the
groove shaped principal body 41a is utilized to fix the bridge
member 24 in a sandwiched form. No engagement groove such as the
engagement groove 43 employed in the embodiment of FIG. 7 is
employed in the bridge member 24.
[0083] In the example shown in FIG. 8A, the fixing member 41 may be
formed with a pin-shaped projection 41e as shown in FIG. 8C and, on
the other hand, the bridge member 24 may be correspondingly formed
with an aperture 24e for receiving the projection 41e to connect
the fixing member 41 and the bridge member 24 together. Where the
bridge member 24 is an injection molded product made of the
sintered metal and the bridge member 24 in the form as engaged in
the manner described previously is to be sintered, shrinkage of the
bridge member 24 during the sintering can result in firm fastening
of the projection 41e to thereby lock the bridge member 24 by
radially pressing the projection 41e.
[0084] The example shown in FIG. 8B is substantially similar to the
example shown in FIG. 8A, but differs therefrom in that axially
opposite side edges of the principal body 41a of the fixing member
41 are formed with respective clamp pieces 41b in the form of a
bent piece so that a portion of the bridge member 24 can be
sandwiched between the clamp pieces 41b. With this structure shown
in FIG. 8B, the fixing member 41 can be sandwiched in two
directions perpendicular to each other by the set-down pieces 41ab
of the principal body 41a and the clamp pieces 41d, resulting in a
firm positioning of the fixing member 41 in position. Also, the
clamp pieces 41d are effective to facilitate positioning of the
fixing member 41 in an axial direction relative to the bridge
member 24.
[0085] Other structural features of any one of the examples shown
in FIGS. 8A to 8C than those described above are substantially
similar to those described and shown in connection with the second
embodiment of FIGS. 6 and 7.
[0086] FIGS. 9 to 11 illustrate a third preferred embodiment of the
present invention. The ball screw mechanism 20 according to this
embodiment is substantially similar to that according to the first
embodiment, but differs therefrom in that in place of the structure
in which each of the bridge members 24 is integrally formed with
the guide walls 36, a fixing member 45 in the form of a wire such
as, for example, a steel wire is employed. In order for this
particular fixing member 45 to be utilizable, the opposite inner
side faces of each mounting hole 30 in the rotary nut 22 for
accommodating the associated bridge member 24 are formed with
engagement steps 46 and, on the other hand, the respective bridge
member 24 is formed with engagement grooves 47 facing towards and
cooperable with the engagement steps 46. The fixing member 45 in
the form of the wire is engageable in the engagement steps 46 and
the engagement grooves 47 to fixedly retain the bridge member 24
fixed relative to the rotary nut 22. Respective outer peripheral
portions of the inner side faces of the mounting hole 30 that are
positioned on a radially outward side with respect to the
engagement grooves 47 are spaced from each other a distance greater
than respective inner peripheral portions of the inner side faces
of the mounting hole 30 that are positioned on a radially inward
side with respect to the engagement grooves 47. The fixing member
45 in the form of the wire is of a generally U-shaped configuration
as shown in FIG. 11 and delimited by a pair of arms and a bridge
portion connecting the arms together. In a natural, unstressed
condition, the fixing member 45 assumes such a shape as shown by
the phantom line in FIG. 11 with the arms thereof diverging away
from the bridge portion thereof.
[0087] At the time of assemblage, this fixing member 45 is inserted
into the grooves 47 with the arms thereof deformed as shown by the
solid line in FIG. 11 against its own resiliency. When the bridge
member 24 is inserted into the respective mounting hole 30 from
inside the hollow of the nut body 22a to assume a predetermined
position within such mounting hole 30, the arms of the fixing
member 45 then deformed against the resiliency of the fixing member
45 expand to assume the shape shown by the phantom line in FIG. 11
by the effect of the resiliency of the fixing member 45 and are
therefore brought into engagement with the adjacent engagement
steps 46 while contacting the inner side faces of the mounting hole
30. Upon engagement of the arms of the fixing member 45 with the
engagement steps 46, the bridge member 24 is fixedly retained
within the mounting hole 30 and is hence fixed relative to the nut
body 22a. In this way, even though the use is made of the fixing
member 45 prepared from a wire, each of the bridge member 24 can
have a simplified shape and, where each bridge member 24 is formed
by the use of an injection molding technique using the sintered
metal, the molding can easily be accomplished.
[0088] Other structural features in this third embodiment than
those described above are similar to those shown and described in
connection with the first embodiment.
[0089] An exemplary electrically powered steering device utilizing
the ball screw mechanism 20 according to the present invention will
now be described. FIG. 12 illustrates a longitudinal side view,
with a portion cut out, of the exemplary electrically powered
steering device. Referring to this figure, a housing 1 includes a
bracket not shown and is fixed to a vehicle body structure at the
bracket. A steering shaft 2 extends through the housing 1 and has
its opposite ends connected with respective tie rods 3 and 4. The
tie rods 3 and 4 are connected with a steering mechanism (not
shown) for steering wheels.
[0090] A steering rod 5 is provided so as to extend diagonally
upwardly from a portion of the housing 1 adjacent one end thereof
and has an upper end on which a steering wheel is mounted. The
steering rod 5 is rotatably supported and rotation of the steering
rod 5 is transmitted as an axial shifting force to the steering
shaft 2 from a lower end thereof through a motion translating
mechanism 6. The motion translating mechanism 6 includes a rack
gear 7 formed by a longitudinal portion of the steering shaft 2 and
a pinion (not shown) mounted on a lower end of the steering rod 5,
which pinion is meshed with the rack gear 7 within the housing 1. A
steering torque detector (not shown) for detecting a steering
torque is mounted in association with to the steering rod 5.
[0091] The housing 1 is formed in a cylindrical shape by connecting
end members 1b and 1c to respective opposite ends of an
intermediate barrel body 1a. A portion of the housing intermediate
of the length thereof is provided with a stator 9 of an electric
motor 8. The stator 9 includes a core and a stator coil. A rotor 10
of the electric motor 8 is housed within the stator 9 with a gap
intervening therebetween. The rotor 10 is a magnetic body formed
into a cylindrical shape and is fitted on an outer periphery of a
sleeve 11 for rotation together with the sleeve 11. Within this
sleeve 11, the steering shaft 2 is inserted for movement in a
direction axially thereof. The electric motor 8 is controlled by a
motor control circuit (not shown) in dependence on a detected value
of the steering torque detector referred to above.
[0092] One end of the sleeve 11, for example, one end of the sleeve
11 adjacent the steering rod 5 so far shown, is rotatably supported
within the housing 1 by means of a bearing 12. The bearing 12 may
be a single independent bearing or a combination of a plurality of
bearings and is of a type capable of supporting both a radial load
and a thrust load.
[0093] Rotation of the electric motor 8 is transmitted as an
axially shifting force to the steering shaft 2 through the ball
screw mechanism 20. The ball screw mechanism 20 is of a structure
in which an axial portion of the steering shaft 2 serves as the
ball screw shaft 21. The rotary nut 22 of the ball screw mechanism
20 has an outer diametric portion rotatably supported by a bearing
16 within the housing 1 and also has an outer diametric surface
engaged with one end of the rotor 10 of the electric motor 8. One
end of the rotor 10 adjacent the rotary nut 22 protrudes beyond the
sleeve 11, a protruding portion thereof being engaged with the
rotary nut 22. Also, the rotary nut 22 has the surface roughened
area 29 described hereinbefore and shown in FIG. 1A, and the rotor
10 is mounted on the rotary nut 22 with the surface roughened area
29 engaged with an inner peripheral surface of the rotor 22 in a
slipless fashion.
[0094] The bearing 16 may be a single independent bearing or a
combination of a plurality of bearings and is as a whole of a type
capable of supporting both a radial load and a thrust load. This
bearing 16 may be employed. In the form of, for example, a rolling
bearing such as an angular ball bearing or the like, having an
inner race, an outer race and a series of rolling elements
interposed between the inner and outer races.
[0095] The operation and function of the above described
construction will be described. Assuming that the vehicle is driven
straight and the steering wheel is held standstill, no torque
signal is outputted from the steering torque detector (not shown)
for the steering rod 5 and the electric motor 8 is held in a halted
condition by the motor control means (not shown). Accordingly, the
electrically powered steering device is in condition not to provide
a steering assist force.
[0096] When the steering wheel is steered, a torque signal is
outputted from the steering torque detector of the steering rod 5
and, under the control of the motor control circuit, the electric
motor 8 drives the rotor 10. Upon rotation of the rotor 10, the
rotary nut 22 of the ball screw mechanism 20 undergoes rotation
together with the rotor and the steering shaft 2 having a portion
thereof formed as the ball screw shaft 2a is consequently axially
moved to thereby generate the steering assist force.
[0097] At this time, the balls of the ball screw mechanism 20
rollingly move within the rolling passage defined between the
internally and externally threaded helical grooves 25 and 26.
[0098] In this way, the steering force of the steering wheel is
assisted by the electric motor 8.
[0099] Since the ball screw mechanism 20 employed in the electric
power steering device as a principal component parts for
transmitting the motor output is so designed and so configured as
hereinbefore fully described, the ball screw mechanism 20 can be
assembled compact in size and have a high load capacity and,
therefore, the electrically powered steering device utilizing such
ball screw mechanism 20 can also be assembled compact in size.
[0100] Although the present invention has been fully described in
connection with the preferred embodiments thereof with reference to
the accompanying drawings which are used only for the purpose of
illustration, those skilled in the art will readily conceive
numerous changes and modifications within the framework of
obviousness upon the reading of the specification herein presented
of the present invention. Accordingly, such changes and
modifications are, unless they depart from the scope of the present
invention as delivered from the claims annexed hereto, to be
construed as included therein.
* * * * *