Title: Ball screw and wheel steering device having the same
Abstract: A ball screw mechanism comprises a screw shaft, a nut and balls. The nut has a cylindrical portion in the axial middle on the outer peripheral surface thereof that becomes a fitting surface for fitting in a sleeve, and small diameter step portions in the opposite axial sides on the outer peripheral surface thereof that become non-fitting surfaces not fitting in the sleeve. A portion of a wheel steering shaft is provided with the screw shaft, and a plurality of balls are disposed in a rolling way defined between a thread groove formed in the outer periphery of the screw shaft and a thread groove formed in the inner periphery of the nut.
Patent Number: 6,991,062 Issued on 01/31/2006 to Yoshida, et al.
| Inventors:
|
Yoshida; Isamu (Iwata, JP);
Kazuno; Keisuke (Iwata, JP);
Yoshioka; Morihisa (Iwata, JP)
|
| Assignee:
|
NTN Corporation (Osaka-fu, JP)
|
| Appl. No.:
|
065076 |
| Filed:
|
September 13, 2002 |
Foreign Application Priority Data
| Sep 13, 2002[JP] | 2001-295640 |
| Current U.S. Class: |
180/444; 74/499 |
| Current Intern'l Class: |
B62D 5/04 (20060101) |
| Field of Search: |
180/444
74/499,459,388.PS,424.71,892.3,892.5,893.2,893.3,893.7,894.5,216.3,893.6
|
References Cited [Referenced By]
U.S. Patent Documents
| 5590732 | Jan., 1997 | Sugino et al.
| |
| 5738181 | Apr., 1998 | Kato.
| |
| 6112610 | Sep., 2000 | Welling.
| |
| 6186268 | Feb., 2001 | Onodera et al.
| |
| 6378646 | Apr., 2002 | Bugosh.
| |
| 6464034 | Oct., 2002 | Toda et al.
| |
| Foreign Patent Documents |
| 08-133098 | May., 1996 | JP.
| |
| 11-082666 | Mar., 1999 | JP.
| |
| 2000/-296780 | Oct., 2000 | JP.
| |
| 2000/-326857 | Nov., 2000 | JP.
| |
Primary Examiner: Boehler; Anne Marie
Attorney, Agent or Firm: J. C. Patents
Claims
What is claimed is:
1. A ball screw comprising a screw shaft having a thread groove in the outer
periphery thereof, a nut having a thread groove in the inner periphery thereof
opposed to said screw shaft, and a plurality of balls disposed in a rolling way
defined between said thread groove in said screw shaft and said thread groove in
said nut, wherein a fitting surface engaging a support member with a direct contact
for supporting said nut fitted therein is formed on the outer peripheral surface
of the nut in the axial middle region of said rolling way and non-fitting surfaces
that do not contact said support member are formed on the outer peripheral surface
of the nut on the opposite axial sides of said rolling way, wherein the axial middle
region includes a center point of the nut, wherein the fitting surface and the
non-fitting surfaces of the nut are smoothly joined as a smooth convex surface,
wherein said non-fitting surfaces extend over a distance of at least one lead
axially inward from the two opposite axial ends of said rolling way.
2. A ball screw as set forth in claim 1, wherein said nut has a substantially
cylindrical shape, and the nut is smaller in outer diameter at the opposite axial
sides formed with said non-fitting surfaces than at the axial middle thereof.
3. A ball screw as set forth in claim 1, wherein the outer peripheral surface
of the nut in the opposite axial sides is gradually reduced toward the axial ends
of the nut.
4. A ball screw as set forth in claim 1, wherein the fitting surface is tightly-fitting
the support member supporting said nut fitted therein.
5. A wheel steering device having a ball screw comprising a screw shaft portion
formed as a portion of a steering shaft for steering a wheel and having a thread
groove formed therein, a nut having a thread groove in the inner periphery thereof
opposed to the screw shaft portion and rotated by a motor, and a plurality of balls
disposed in a rolling way defined between said screw shaft portion and said nut,
wherein a fitting surface engaging a support member with a direct contact for
supporting said nut fitted therein is formed on the outer peripheral surface of
the nut in the axial middle region of said rolling way and non-fitting surfaces
that do not contact said support member are formed on the outer peripheral surface
of the nut on the opposite axial sides of said rolling way, wherein the axial middle
region includes a center point of the nut, wherein the fitting surface and the
non-fitting surfaces of the nut are smoothly joined as a smooth convex surface,
wherein said non-fitting surfaces extend over a distance of at least one lead
axially inward from the two opposite axial ends of said rolling way.
6. A wheel steering device as set forth in claim 5, wherein said nut has a substantially
cylindrical shape, and the nut is smaller in outer diameter at the opposite axial
sides formed with said non-fitting surfaces than at the axial middle thereof.
7. A wheel steering device as set forth in claim 5, wherein the outer peripheral
surface of the nut in the opposite axial sides is gradually reduced toward the
axial ends of the nut.
8. A wheel steering device as set for the in claim 5, wherein said nut is fitted,
with a tight fit, in a rotating member that rotates said nut by the motor.
9. A wheel steering device as set forth in claim 5, wherein the fitting surface
is tightly-fitting the support member supporting said nut fitted therein.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the priority benefit of Japanese application serial
no. 2001-295640, filed on Sep. 27, 2001.
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to a ball screw used, for example, in an automobile
wheel steering device, and also relates to a wheel steering device having the same.
2. Prior Art
Devices that use ball screws for the steering of wheels of an automobile
are known. One such device is an electrically operated power steering device that
supplements the steering force on the steering wheel of an automobile by an electric motor.
Described in Japanese Patent Application Laid-Open under No. H09-142315
is an electrically operated power steering device using a ball screw. This electrically
operated power steering device is so designed that the thread groove diameter of
the nut constituting the ball screw is gradually increased as the nut extends axially
outward. Therefore, when the axis of the screw shaft deviates or bends with respect
to the nut, distortion of the screw shaft and the nut is prevented, ensuring smooth
operation of the ball screw.
Further, described in Japanese Patent Application Laid-Open under No. H11-268658
is an electrically operated power steering device using another ball screw. This
electrically operated power steering device is concerned with balls constituting
a ball screw, and is designed so that the balls disposed in the axial middle region
are lager in diameter than those disposed in the opposite axial ends. Therefore,
when a deflection or axis deviation occurs in the screw shaft, the distortion of
the screw shaft and the nut is prevented, ensuring smooth operation of the ball screw.
With the arrangements of the prior art described above, the following problems
are left. In the construction described in Japanese Patent Application Laid-Open
under No. H09-142315, the thread groove of the nut must be machined so that the
groove diameter gradually increases as the nut extends axially outward, requiring
much time for machining, management of the thread groove diameter after machining
not being easy, a fact which forms a cause of interference with an effort toward
cost reduction.
On the other hand, in the construction described in Japanese Patent Application
Laid-Open under No. H11-268658, two kinds of balls must be installed in the middle
and on the opposite sides, so that much time is required in assembly operation
and the two kinds of balls must be managed, a fact which forms a cause of interference
with an effort toward cost reduction. Further, if a mistake is made in incorporating
the two kinds of balls, the prevention of distortion, which is the intended object,
cannot be attained, possibly incurring the shortening of life of the ball screw.
SUMMARY OF INVENTION
With such situation in mind, the invention provides a ball screw that is capable
of preventing the distortion of a screw shaft and nut in the prior art, which is
the intended object, shortening the time taken for machining and assembling, and
reducing costs; and it also provide a wheel steering device having the same.
In order to achieve such object, a ball screw according to an embodiment of the
present invention employs an arrangement comprising a screw shaft having a thread
groove in the outer periphery thereof, a nut having a thread groove in the inner
periphery thereof opposed to the screw shaft, and a plurality of balls disposed
in a rolling way defined between the thread groove in the screw shaft and the thread
groove in the nut, wherein a fitting surface engaging a support member supporting
the nut fitted therein is formed on the outer peripheral surface of the nut in
the axial middle region of the rolling way and non-fitting surfaces that do not
contact the support member are formed on the outer peripheral surface of the nut
on the opposite axial sides of the rolling way.
In this case, the axial middle region of the rolling way refers to, of the entire
axial length of the rolling way defined between the screw shaft and the nut and
in which balls are disposed, the region located in the axial middle, and the opposite
axial sides of the rolling way refer to, of the entire axial length of the rolling
way, the regions located in the opposite axial sides.
Thus, if an arrangement is employed in which a fitting surface engaging a support
member supporting the nut fitted therein is formed on the outer peripheral surface
of the nut in the axial middle region of the rolling way and non-fitting surfaces
that do not contact the support member are formed on the outer peripheral surface
of the nut on the opposite axial sides of the rolling way, then the outer peripheral
surface of the nut in the axial middle region fits in the support member, thereby
suppressing a play between balls, the screw shaft and the nut in the axial middle,
suppressing the backlash of the ball screw, so as to realize a smooth operation,
and on the other hand, since, in the opposite axial sides, the outer peripheral
surface of the nut does not engage the support member, the nut is allowed to elastically
deform in the loading direction when the deviation or bending of the axis of the
screw shaft occurs; thus the occurrence of distortion can be prevented. That is,
with a ball screw comprising a screw shaft having a constant thread groove diameter,
a nut and balls having a single ball diameter, it is possible to prevent the occurrence
of distortion, resulting in the time, and accordingly the cost, required for machining
the ball screw and assembly operation being reduced.
The nut may have a substantially cylindrical shape, and the nut may be smaller
in outer diameter at the opposite axial sides formed with the non-fitting surfaces
than at the axial middle thereof provided with the fitting surface, to thereby
make the nut simple in shape, and further reduce costs.
The outer peripheral surface of the nut in the opposite axial sides may be gradually
reduced toward the axial ends of the nut, to thereby facilitate incorporation of
the nut into the support member adapted to have the nut fitted therein.
The non-fitting surfaces may extend over a distance of at least one lead axially
inward from the opposite axial ends of the rolling way, to thereby effectively
prevent the occurrence of distortion in the ranges at the opposite ends over one
lead that are the regions where distortion is most likely to occur.
According to another embodiment of the invention, a wheel steering device
includes a ball screw comprising a screw shaft portion formed as a portion of a
steering shaft for steering a wheel and having a thread groove formed therein,
a nut having a thread groove in the inner periphery thereof opposed to the screw
shaft portion and rotated by a motor, and a plurality of balls disposed in a rolling
way defined between the screw shaft portion and the nut, wherein a fitting surface
engaging a support member supporting the nut fitted therein is formed on the outer
peripheral surface of the nut in the axial middle region of the rolling way and
non-fitting surfaces that do not contact the support member are formed on the outer
peripheral surface of the nut on the opposite axial sides of the rolling way. Thus,
it is possible to prevent the ball screw from being distorted when subjected to
an excessive load, to provide an improved steering feeling, and to reduce costs.
Again, the nut may have a substantially cylindrical shape, and the nut may
be smaller in outer diameter at the opposite axial sides formed with the non-fitting
surfaces than at the axial middle thereof. The outer peripheral surface of the
nut in the opposite axial sides may be gradually reduced toward the axial ends
of the nut. The non-fitting surfaces may extend over a distance of at least one
lead axially inward from the opposite axial ends of the rolling way.
The nut may be fitted, with a tight fit, in a rotating member that rotates the
nut by the motor, so that the fixing of the nut is further simplified and that
the play between the balls, the screw shaft and the nut that occurs in the axial
middle region of the rolling way can be adjusted by increasing or decreasing the interference.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a cutaway front view of an electrically operated power steering device
according to a first embodiment of the invention;
FIG. 2 is a partial enlarged sectional view of FIG. 1;
FIG. 3 is a cutaway front view of an electrically operated power steering device
according to a second embodiment of the invention; and
FIG. 4 is a partial enlarged sectional view of FIG. 2.
DETAILED DESCRIPTION
A first embodiment of the invention will now be described with reference to FIGS.
1 and 2. FIG. 1 is a cutaway front view of an electrically operated power steering
device that is an example of a wheel steering device having the ball screw of the
invention. FIG. 2 is an enlarged view of a ball screw mechanism for this electrically
operated power steering device. In FIG. 1, a housing
1 has a bracket (not
shown) and is fixed to a car body. A steering shaft
2 extends through the
housing
1 and is connected at its opposite sides to a wheel steering mechanism
(not shown). A steering wheel shaft
5 is installed to extend obliquely upward
from one end of the housing
1, and has a steering wheel connected to the
upper end thereof. The steering wheel shaft
5 is rotatably supported and
its rotation is transmitted as an axial advancing/retracting force to the steering
shaft
2 through a conversion mechanism
6 at the lower end thereof.
The conversion mechanism
6 comprises a rack
7 formed in a longitudinal
portion of the steering shaft
2, and a pinion (not shown) installed on the
lower end of the steering wheel shaft
5, the pinion meshing with the rack
7 within the housing
1. A steering toque detector (not shown) is
installed for the steering wheel shaft
5 so as to detect the steering toque
produced therein.
The housing
1, which is cylindrically formed, is constructed by joining
end members
1b and
1c to the opposite ends of a middle
cylindrical body
1a. Installed in the axial middle of the housing
1 is the stator
9 of an electric motor
8. The stator
9
is composed of a core and a stator coil. The rotor
10 of the electric motor
8 is installed around the inner periphery of the stator
9 with a
gap defined therebetween. The rotor
10 is cylindrically formed of magnetic
material, and is attached to the outer periphery of a sleeve
11 so that
it rotates as a unit with the sleeve
11. The steering shaft
2 is
inserted through the sleeve
11 for axial advance and retraction. The electric
motor
8 is controlled by a motor control circuit (not shown) according to
the detected value given by the steering torque detector.
The sleeve
11 is rotatably supported in the housing
1 through bearings
12 and
13. The sleeve
11 has the nut
14 fitted therein
and rotates as a unit with the nut
14. The steering shaft
2 extends
through the nut
14.
As shown enlarged in FIG. 2, the nut
14 is substantially cylindrical,
and
has a cylindrical portion
14a serving as a fitting surface for fitting
in the sleeve
11, on the outer peripheral surface of the nut in the axial
middle, and small-diameter step portions
14b serving as non-fitting
surfaces that do not fit in the sleeve
11, on the outer peripheral surface
of the nut in the axial opposite sides. The nut
14 is fixed so that its
cylindrical portion
14a closely adheres to the inner periphery of
the sleeve
11.
A portion of the steering shaft
2 is provided with a screw shaft portion
2a, and a plurality of balls
15 are disposed in a rolling
way
19 defined between a thread groove
17 formed in the outer periphery
of the screw shaft portion
2a and a thread groove
18 formed
in the inner periphery of the nut
14. The screw shaft portion
2a,
nut
14 and balls
15 constitute a ball screw mechanism
20.
The operation and function of this electrically operated power steering device
will now be described. When the vehicle is linearly moving with the turning of
the steering wheel stopped, no torque signal is delivered from the steering torque
detector (not shown) of the steering wheel shaft
5, and the electric motor
8 is placed in its rotation stop state by a motor control means (not shown).
Therefore, this electrically operated power steering device is in a state not delivering
an auxiliary steering force. When the steering wheel is rotated, a toque signal
is delivered from the steering torque detector of the steering wheel shaft
5,
and the electric motor
8 rotates the rotor
10 under the control of
the motor control circuit. When the rotor
10 rotates, the nut
14
of the ball screw mechanism
20 rotates together with the rotor
10,
and the steering shaft
2 constituting a portion of the screw shaft portion
2a axially advances and retracts, thereby generating an auxiliary
steering force. At this time, the balls
15 of the ball screw mechanism
20
roll in the rolling way
19 defined between the thread grooves
17
and
18 as the nut
14 is rotated. Thus, the steering force is supplemented
by the electric motor
8.
In this embodiment, the nut
14 fits in the sleeve
11 at the cylindrical
portion
14a in the axial middle and does not fit in the sleeve
11
at the small diameter step portions
14b in the axial opposite sides.
Such arrangement allows the cylindrical portion
14a to be firmly
supported in the sleeve
11, thus suppressing the play between the balls,
the screw shaft and the nut in the axial middle, suppressing the backlash of the
ball screw, so as to realize a smooth operation. Further, since the small diameter
step portions
14b do not engage the sleeve
11, when the deviation
or bending of the axis of the screw shaft occurs, the small diameter step portions
14b are allowed to elastically deform in the loading direction, preventing
the occurrence of distortion of the screw shaft and the nut; therefore, a good
steering feeling is obtained. In this case, the small diameter step portions
14b,
which become the non-fitting surfaces, are formed over a width of 1.5 lead axially
inward from the opposite axial ends of the rolling way
19. Thus, the provision
of the small diameter step portions
14b having a width of 1.5 lead
prevents the occurrence of distortion at the axial opposite ends of the rolling
way
19 where such distortion is most likely to occur. The provision of the
small diameter step portions
14b, which become the non-fitting surfaces,
over a width of at least one lead axially inwardly of the nut
14 from the
axial opposite ends of the rolling way
19 makes it possible to effectively
prevent distortion at the axial opposite ends.
A second embodiment of the invention will now be described with reference to
FIGS.
3 and 4. FIG. 3 is a cutaway front view of an electrically operated power steering
device that is an example of a wheel steering device using a ball screw according
to the invention. FIG. 4 is an enlarged view of the ball screw mechanism of the
electrically operated power steering device. What differs from the first embodiment
shown in FIGS. 1 and 2 is only the shape of the axial opposite sides of the nut
14 and the method of fixing the nut, like parts of the rest being denoted
by like reference characters to omit a detailed description thereof.
As shown enlarged in FIG. 4, the nut
14 is substantially cylindrical and
has a fitting surface
14a in its axial middle for fitting in the
sleeve
11 and taper portions
14c in its axial opposite sides
that become non-fitting surfaces not fitting in the sleeve
11. The taper
portions
14c gradually decrease in diameter toward the axial opposite
ends of the nut. Therefore, incorporation of the nut
14 into the sleeve
11 is facilitated. In addition, in this embodiment, the non-fitting surface
is in the form of a taper surface; however, the non-fitting surface may be in the
form of a crowing-shaped surface gradually decreasing in diameter toward the axial
ends of the nut.
The nut
14 is press-fitted at its cylindrical portion
14a in
the inner periphery of the sleeve
11 to be fixed therein with a tight fit.
If the amount of interference this time is set at some μm to some 10 μm
or so, then the axial middle of the nut is reduced in diameter by press-fitting,
so that the thread groove diameter in the axial middle becomes smaller than the
thread groove diameter in the axial opposite ends. For this reason, the play between
the balls, the screw shaft and the nut is suppressed and so is the backlash of
the ball screw, thus realizing a smooth operation, imparting a suitable amount
of play between the balls, the screw shaft and the nut in the axial opposite sides,
preventing the occurrence of distortion; thus, a good steering feeling is obtained.
Embodiments of the invention have so far been described using electrically
operated power steering devices. However, the ball screw of the invention is also
applicable to other uses than electrically operated power steering devices. For
example, it is suitable for uses where a large moment acts on the screw shaft.
Further, the wheel steering device of the invention is also applicable, besides
electrically operated power steering devices, to a wheel steering device for rear
wheels used in a system (called ARS system (active rear steer)) for electronically
controlling the steering for the rear wheel angle, in an automobile in which the
front wheels are steered by a steering wheel, to monitor the wheel speed of the
front wheels to ensure that the car body has always a target car body slip angle.
*
