Machine element or mechanism – Gearing – Teeth
Reexamination Certificate
1998-01-29
2001-09-04
Bucci, David A. (Department: 3682)
Machine element or mechanism
Gearing
Teeth
C074S089100
Reexamination Certificate
active
06282972
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a ball screw mechanism comprising a screw shaft and a nut member.
A ball screw mechanism is known as a mechanism which converts rotational motion to linear motion. As an example of such a ball screw mechanism, the whole configuration of a circulation tube type ball screw mechanism will be described with reference to FIG.
1
.
FIG. 1
is an axial section view of a ball screw mechanism. In the figure, a screw shaft
1
which is partly shown is a shaft member which has in the outer periphery a spiral groove (thread groove)
1
a
having a section shape similar to a Gothic arch as described later. A nut
2
serving as a nut member is a cylindrical member which has in the inner periphery
2
e
a spiral groove (thread groove)
2
a
corresponding to the spiral groove (thread groove)
1
a
of the screw shaft
1
, and a spiral ridge
2
d
defined between the adjacent spiral grooves
2
a
. Although not illustrated, two through holes elongate from the upper face
2
c
of the nut
2
to the spiral groove (thread groove)
2
a
. Ends of a ball circulation tube
4
having a U-like shape as indicated by a phantom line are fittingly inserted into the through holes, respectively.
The screw shaft
1
is passed through the inside of the inner periphery
2
e
of the nut
2
so that the spiral groove (thread groove)
1
a
opposes to the spiral groove (thread groove)
2
a
of the nut. A number of balls
3
are rotatably housed in a trackway defined by the two opposed thread grooves.
When the nut
2
and the screw shaft
1
perform relative spiral movement, the balls
3
repeat circulation in which the balls rotatingly move along the trackway formed between by the spiral grooves (thread grooves)
1
a
and
2
a
, are scooped up from the trackway while being guided by a tongue portion (not shown) formed at one end of the ball circulation tube
4
, to be directed into the ball circulation tube
4
, pass through the tube, and then return to the trackway via the other end of the nut.
In addition to this,
FIG. 6
shows an example of an end cap type ball screw device of the prior art. In the conventional example, a screw shaft
1
having a spiral groove (thread groove)
1
a
in the outer peripheral surface is threadedly engaged with a cylindrical ball screw nut
10
having in the inner peripheral surface a spiral groove (thread groove)
2
a
opposing to the spiral groove (thread groove)
1
a
of the screw shaft, via balls
3
which rotatingly move in the mutually opposing spiral grooves (thread grooves)
1
a
and
2
a
. The ball screw nut
10
comprises two kinds of members, i.e., a nut member
402
, and disk-like ball circulation members (so called end caps)
11
which are detachably joined to the end faces of the nut member
402
. A ball return passage
12
which consists of a through hole elongating in the axial direction is disposed in a thick portion of the nut member
402
. In each of the end faces of the ball circulation member
11
where the member is joined to the nut member
402
, disposed is a curved path
13
through which the spiral grooves (thread grooves)
1
a
and
2
a
communicate with the ball return passage
12
.
When the screw shaft
1
and the ball screw nut
10
are relatively rotated, the balls
3
rotatingly advance in the two opposing screw grooves
1
a
and
2
a
of the screw shaft
1
and the ball screw nut
10
so as to repeat circulation in which the balls pass through the curved paths
13
disposed in the ball circulation members at the ends, and the ball return passage
12
disposed in the nut member
402
, to return to the original position.
Since the balls which rotate in accordance with the rotation of the screw shaft move along the trackway, continuation of the relative spiral movement of the nut (or the nut member) and the screw shaft causes the balls to be discharged from the nut in due course of time. When the nut (or the nut member) is to be moved by a considerably long distance, therefore, any ball screw mechanism must be provided with a circulation unit such as a ball circulation tube which returns balls discharged from one end of the nut (or the nut member) to the other end of the nut. However, the provision of such a circulation unit produces a problem peculiar to a ball screw mechanism.
Before the discussion of the problem, the relationship between balls and thread grooves is first described.
FIG. 2
is an enlarged section view showing the vicinity of a thread groove of the ball screw mechanism of
FIG. 1
, along a direction perpendicular to the thread groove. Referring to the figure, a ball
3
is disposed between the thread groove
1
a
of the screw shaft
1
and the thread groove
2
a
of the nut
2
.
As apparent from
FIG. 2
, the sections of the thread grooves
1
a
and
2
a
are not parts of a perfect circle, and have a shape which is a so-called Gothic arch and each of which is configured by combining two arcs (called flanks) with each other. Specifically, the sections of the thread grooves
1
a
and
2
a
constitute a shape in which arcs of a radius of curvature Rc are arranged in a laterally symmetrical manner. When the radius of the ball
3
is indicated by R, the relationship of Rc >R is held.
In view of the above-mentioned relationship between the radius R of the ball
3
and the radius of curvature Rc, the thread grooves
1
a
and
2
a
and the ball
3
are contacted with each other at four points, that is the points N
1
, N
2
, S
1
, and S
2
in the FIG.
2
. According to this configuration, a controlled pre-load can be easily applied to the balls, so that a back lash can be eliminated. When the pre-load is applied, the reaction forces produced at the four points balance with each other.
Hereinafter, the problem peculiar to a ball screw mechanism will be described with reference to the drawings.
FIG. 3
is an enlarged view of an end portion of the nut
2
of the end cap type ball screw mechanism as seen in the axial direction, and
FIG. 4
is a view of the nut
2
of
FIG. 3
as seen in the direction of the arrow IV. As apparent from comparison of
FIG. 2
with
FIGS. 3 and 4
, since the thread groove
2
a
has a lead angle &thgr; and the thread groove is cut by the end face
2
b
of the nut
2
which is perpendicular to the axis, the thread groove
2
a
has an opening shape which elongates in the peripheral direction.
In
FIGS. 3 and 4
, the ball
3
which rotates in the thread groove
2
a
is indicated by phantom lines. The relative spiral movement of the nut and the screw shaft (not shown) causes the ball
3
to rotate so that the center of the ball
3
moves in the sequence of the positions C, B, and A, and the ball is finally discharged from the nut
2
. The line N
1
TR
indicates the locus of the contact point N
1
between the ball
3
and the thread groove
2
a
, and the line N
2
TR
indicates the locus of the contact point N
2
between the ball
3
and the thread groove
2
a.
Until the center of the ball
3
reaches the position C, the ball is contacted with the thread groove
2
a
of the nut
2
at the two points, and also with the thread groove of the screw shaft (not shown) at two points. In other words, the contact relationship between the ball and the thread grooves is in the normal state shown in FIG.
2
.
At the timing when the center of the ball
3
passes the position C, however, the line N
2
TR
is interrupted at the point N
2
-C as shown in FIG.
4
. By contrast, the line NLTR further elongates to continue to the point N
1
-A.
FIG. 5
is a section view similar to that of FIG.
2
and showing the state in which the center of the ball
3
is at the position B of FIG.
4
. As apparent from
FIG. 5
, the flank (arc) portion of the thread groove
2
a
which is in the upper and right side of the figure does not exist. The flank which exists in the normal state is shown by a phantom line. In other words, during a period when the center of the ball moves from the position C to the position A, the ball is contacted with the thread grooves
1
a
and
2
a
at three points.
In such a
Kuramochi Michihiro
Takahashi Nobumitsu
Bucci David A.
Hansen Colby
NSK Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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