U.S. patent application number 09/015402 was filed with the patent office on 2001-06-07 for ball screw mechanism.
This patent application is currently assigned to SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC. Invention is credited to KURAMOCHI, MICHIHIRO, TAKAHASHI, NOBUMITSU.
Application Number | 20010002555 09/015402 |
Document ID | / |
Family ID | 26366419 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010002555 |
Kind Code |
A1 |
KURAMOCHI, MICHIHIRO ; et
al. |
June 7, 2001 |
BALL SCREW MECHANISM
Abstract
In order to prevent balls 3 from, when the balls 3 are
discharged from a nut 102 in accordance with relative axial
movement of the nut 102 and a screw shaft 1, contacting with thread
grooves 102a and 1a of the nut 102 and the screw shaft 1 at three
points, a counterbore 102c is formed around the thread groove 102a
of the nut 102 in the vicinity of a discharge port for the balls 3
in the nut 102. When the balls 3 are contacted with the thread
grooves 102a and 1a, therefore, a four-point contact is always
maintained. Consequently, reaction forces which are applied from
the thread grooves 102a and 1a to the balls 3 balance with each
other, whereby stable holding of the balls 3 is ensured.
Inventors: |
KURAMOCHI, MICHIHIRO; (NEW
HAVEN, CT) ; TAKAHASHI, NOBUMITSU; (GUNMA,
JP) |
Correspondence
Address: |
SUGHRUE MION ZINN MACPEAK & SEAS
2100 PENNSYLVANIA AVE NW
WASHINGTON
DC
20037
|
Assignee: |
SUGHRUE, MION, ZINN, MACPEAK &
SEAS, PLLC
|
Family ID: |
26366419 |
Appl. No.: |
09/015402 |
Filed: |
January 29, 1998 |
Current U.S.
Class: |
74/459.5 |
Current CPC
Class: |
F16H 25/2214 20130101;
F16H 2025/2242 20130101; Y10T 74/19958 20150115 |
Class at
Publication: |
74/459.5 |
International
Class: |
F16H 025/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 29, 1997 |
JP |
HEI 9-28337 |
Nov 21, 1997 |
JP |
HEI 9-336579 |
Claims
What is claimed is:
1. A ball screw mechanism comprising: a screw shaft having a thread
groove formed on its outer peripheral surface; a nut member movable
relative to said screw shaft in an axial direction thereof and
having a thread groove which is formed on its inner peripheral
surface and is opposed to said thread groove of said screw shaft; a
number of balls rotatingly movable along a trackway formed by said
opposed thread grooves of said screw shaft and said nut member; a
ball returning member for returning said balls discharged from a
discharge port of said trackway to an inlet port of said trackway
so as to form a ball circulation passage with said trackway, in
which said balls are respectively contacted at two points with said
thread grooves when said balls are moved within said trackway; and
a recess formed around said thread groove of said nut member in the
vicinity of said discharge port for preventing said balls from,
when said balls are discharged from said trackway in accordance
with the relative axial movement between said nut member and said
screw shaft, contacting with said thread grooves at three
points.
2. The ball screw mechanism according to claim 1, in which said
recess has a curvature larger than a radius of said ball.
3. The ball screw mechanism according to claim 1, in which said
ball returning member comprises: a through hole elongating in the
axial direction of said nut member; and end caps mounted to both
end faces of said nut member and each having a curved path through
which said through hole communicates with said trackway.
4. The ball screw mechanism according to claim 3, in which said nut
member further comprises: a step portion formed in the vicinity of
said discharge port and disposed at a position opposite to said
recess with respect to said ball.
5. The ball screw mechanism according to claim 3, in which said
ball are subjected to a pre-load between said opposed thread
grooves of said screw shaft and said nut member, and said pre-load
is canceled when said balls pass on said recess.
6. The ball screw mechanism according to claim 1, in which said nut
member is a cylindrical member having a flat portion and two holes
which formed in said flat portion so as to open and communicate
with parts of said thread groove of said nut member; said ball
returning member comprises a ball circulation tube, one end of
which is communicated with said discharge port and other end of
which is communicated with said inlet port; and said ball
circulation tube is fixed to said flat portion of said nut
member.
7. An end cap type ball screw mechanism comprising: a screw shaft
in which a thread groove is formed in an outer peripheral surface;
a nut member which has in an inner peripheral surface a thread
groove opposing to said thread groove of said screw shaft, and, in
a thick portion, a ball return passage consisting of an axial
through hole; a ball circulation member having a curved path
through which said thread grooves communicate with said ball return
passage, said ball circulation member being joined to end faces of
said nut member; and a number of balls which can circulate with
rotatingly moving in said opposing thread grooves, said ball return
passage, and said curved path, said balls being respectively
contacted at two points with said thread grooves, a pre-load being
applied to said balls, wherein a recess is formed around said
thread groove in the vicinity of a discharge port for said balls in
said nut member, so as to immediately cancel the pre-load.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a ball screw mechanism comprising a
screw shaft and a nut member.
[0002] 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) 1a 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 2e a spiral groove (thread
groove) 2a corresponding to the spiral groove (thread groove) 1a of
the screw shaft 1, and a spiral ridge 2d defined between the
adjacent spiral grooves 2a. Although not illustrated, two through
holes elongate from the upper face 2c of the nut 2 to the spiral
groove (thread groove) 2a. 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.
[0003] The screw shaft 1 is passed through the inside of the inner
periphery 2e of the nut 2 so that the spiral groove (thread groove)
1a opposes to the spiral groove (thread groove) 2a of the nut. A
number of balls 3 are rotatably housed in a trackway defined by the
two opposed thread grooves.
[0004] 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) 1a and 2a, 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.
[0005] 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) 1a
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) 2a opposing to the spiral
groove (thread groove) 1a of the screw shaft, via balls 3 which
rotatingly move in the mutually opposing spiral grooves (thread
grooves) 1a and 2a. 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) 1a and 2a communicate
with the ball return passage 12.
[0006] 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 1a and 2a 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.
[0007] 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.
[0008] 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 1a of the screw shaft 1 and the thread
groove 2a of the nut 2.
[0009] As apparent from FIG. 2, the sections of the thread grooves
1a and 2a 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 1a and 2a 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.
[0010] In view of the above-mentioned relationship between the
radius R of the ball 3 and the radius of curvature Rc, the thread
grooves 1a and 2a and the ball 3 are contacted with each other at
four points, that is the points N1, N2, S1, and S2 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.
[0011] 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 2a has a lead angle .theta. and the thread groove
is cut by the end face 2b of the nut 2 which is perpendicular to
the axis, the thread groove 2a has an opening shape which elongates
in the peripheral direction.
[0012] In FIGS. 3 and 4, the ball 3 which rotates in the thread
groove 2a 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 N1.sub.TR indicates the locus of the
contact point N1 between the ball 3 and the thread groove 2a, and
the line N2.sub.TR indicates the locus of the contact point N2
between the ball 3 and the thread groove 2a.
[0013] Until the center of the ball 3 reaches the position C, the
ball is contacted with the thread groove 2a 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.
[0014] At the timing when the center of the ball 3 passes the
position C, however, the line N2.sub.TR is interrupted at the point
N2-C as shown in FIG. 4. By contrast, the line N1.sub.TR further
elongates to continue to the point N1-A.
[0015] 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 2a 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 1a and 2a
at three points.
[0016] In such a case, the reaction force Fn1 exerted between the
ball 3 and the thread groove 2a of the nut 2 at the point N1
opposes to the reaction force Fs2 exerted between the ball 3 and
the thread groove 1a of the screw shaft 1 at the point S2 to
balance therewith. Because of the above-mentioned non-existence of
the flank, however, no reaction force which opposes to the reaction
force Fs1 exerted between the ball 3 and the thread groove la of
the screw shaft 1 at the point S1 is produced. As a result, the
ball 3 receives a force of FC (the force obtained by subtracting
the friction force between the ball and the thread groove from the
reaction force Fs1) in the direction of Fs1 at the point S1.
[0017] In order to eliminate backlash a pre-load is applied between
the balls and the thread grooves, so that the force FC pushes the
ball 3 in the direction of the force, to thereby pushing the ball 3
to bite the grooves in the direction in which a flank does not
exist. Even in the case where a pre-load is not applied, when a
load is externally applied, a force similar to the force FC is
produced so as to cause the ball 3 to bite the grooves.
[0018] During the period when the center of the ball 3 moves from
the position C to the position A in FIG. 4, therefore, a state in
which the ball 3 is easily bitten by the thread grooves 1a and 2a
arises and a problem in that maloperation such as torque
variations, jerk or Jamming easily occurs in the operation of the
ball screw mechanism.
SUMMARY OF THE INVENTION
[0019] In view of the problem, it is an object of the invention to
provide a ball screw mechanism in which, although the configuration
is simplified, the reliability is enhanced.
[0020] In order to attain the object, the ball screw mechanism of
the invention comprises:
[0021] a screw shaft having a thread groove formed on its outer
peripheral surface;
[0022] a nut member movable relative to the screw shaft in an axial
direction thereof and having a thread groove which is formed on its
inner peripheral surface and is opposed to the thread groove of the
screw shaft;
[0023] a number of balls rotatingly movable along a trackway formed
by the opposed thread grooves of the screw shaft and the nut
member;
[0024] a ball returning member for returning the balls discharged
from a discharge port of the trackway to an inlet port of the
trackway so as to form a ball circulation passage with the
trackway, in which the balls are respectively contacted at two
points with the thread grooves when the balls are moved within the
trackway; and
[0025] a recess formed around the thread groove of the nut member
in the vicinity of the discharge port for preventing the balls
from, when the balls are discharged from the trackway in accordance
with the relative axial movement between the nut member and the
screw shaft, contacting with the thread grooves at three
points.
[0026] Furthermore, the end cap type ball screw mechanism of the
invention comprises: a screw shaft in which a thread groove is
formed in an outer peripheral surface; a nut member which has in an
inner peripheral surface a thread groove opposing to the thread
groove of the screw shaft, and, in a thick portion, a ball return
passage consisting of an axial through hole; a ball circulation
member having a curved path through which the thread grooves
communicate with the ball return passage, the ball circulation
member being joined to end faces of the nut member; and a number of
balls which can circulate with rotatingly moving in the opposing
thread grooves, the ball return passage, and the curved path, the
balls being respectively contacted at two points with the thread
grooves, a pre-load being applied to the balls, and
[0027] a recess is formed around the thread groove in the vicinity
of a discharge port for the balls in the nut member, so as to
immediately cancel the pre-load.
[0028] In the above-mentioned ball screw mechanism, the recess
preferably has a curvature larger than a radius of the ball.
[0029] In the above-mentioned ball screw mechanism, the ball
returning member may comprise:
[0030] a through hole elongating in the axial direction of the nut
member; and
[0031] end caps mounted to both end faces of the nut member and
each having a curved path through which the through hole
communicates with the trackway.
[0032] In addition, in the above-mentioned ball screw mechanism,
the nut member may further comprise:
[0033] a step portion formed in the vicinity of the discharge port
and disposed at a position opposite to the recess with respect to
the ball.
[0034] Further, it is more preferable to modify the above-mentioned
ball screw mechanism in such a manner that the ball are subjected
to a pre-load between the opposed thread grooves of the screw shaft
and the nut member, and the pre-load is canceled when the balls
pass on the recess.
[0035] Furthermore, in the above-mentioned ball screw
mechanism,
[0036] the nut member may be a cylindrical member having a flat
portion and two holes which formed in the flat portion so as to
open and communicate with parts of the thread groove of the nut
member;
[0037] the ball returning member may comprise a ball circulation
tube, one end of which is communicated with the discharge port and
other end of which is communicated with the inlet port; and
[0038] the ball circulation tube may be fixed to the flat portion
of the nut member.
[0039] According to the invention, in order to prevent the balls
from, when the balls are discharged from the nut member in
accordance with the relative axial movement, contacting with the
thread grooves at three points, a recess is formed around the
thread groove of the nut member in the vicinity of a discharge port
for the balls in the nut member. When the balls are contacted with
the thread grooves, therefore, a four-point contact is always
maintained. Consequently, reaction forces which are applied from
the thread grooves to the balls balance with each other, whereby
stable holding of the balls is ensured.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is an axial section view of a circulation tube type
ball screw mechanism of the prior art;
[0041] 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;
[0042] FIG. 3 is an enlarged view of an end portion of a nut 2 of
an end cap type ball screw mechanism- of the prior art, as seen in
the axial direction;
[0043] FIG. 4 is a view of the nut 2 of FIG. 3 as seen in the
direction of the arrow IV;
[0044] FIG. 5 is a section view similar to that of FIG. 2 and
showing the state in which the center of a ball 3 is at the
position B of FIG. 4;
[0045] FIG. 6 is a longitudinal section view of an end cap type
ball screw device of the prior art;
[0046] FIG. 7 is a view showing a nut 102 of an end cap type ball
screw mechanism which is a first embodiment of the invention;
[0047] FIG. 8 is a section view of the nut 102 of FIG. 1 taken
along the line VIII-VIII and as seen in the direction of the
arrows;
[0048] FIG. 9 is an enlarged view of an end portion of a nut 202 of
an end cap type ball screw mechanism which is a second embodiment
of the invention, as seen in the axial direction;
[0049] FIG. 10 is a view of the nut 202 of FIG. 9 as seen in the
direction of the arrow X, in a similar manner as FIG. 7;
[0050] FIG. 11 is a plan view of a circulation tube type ball screw
mechanism which is a third embodiment of the invention;
[0051] FIG. 12 is a section view of the ball screw mechanism of
FIG. 11 taken along the line XII-XII and as seen in the direction
of the arrows;
[0052] FIG. 13 is a three-dimensional imaginary view showing the
vicinity of a thread groove of a nut 2 in a circulation tube type
ball screw mechanism of the prior art as shown in FIG. 1; and
[0053] FIG. 14 is a three-dimensional imaginary view similar to
FIG. 13 and showing a nut 302 in a circulation tube type ball screw
mechanism which is a third embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0054] Hereinafter, embodiments of the invention will be described
with reference to the drawings.
[0055] FIG. 7 is a view similar to FIG. 4 and showing a nut 102 of
an end cap type ball screw mechanism which is a first embodiment of
the invention, and FIG. 8 is a section view of the nut 102 taken
along the line VIII-VIII and as seen in the direction of the
arrows. In both FIGS. 7 and 8, the shape of a tool (having a
semispherical tip end) of a ball end mill (BEM) which is used as a
cutting tool is indicated by phantom lines. FIG. 8 shows a state in
which the ball 3 is placed in a thread groove 102a of the nut
102.
[0056] The nut 102 in the embodiment is different only in the shape
of an end of the thread groove from the nut of the prior art.
Specifically, a cutting work is conducted on the nut 102 (FIG. 3)
by moving the ball end mill (BEM) along the center line (a spiral
curve) of the thread groove 102a (i.e., in a direction which is
inclined by the lead angle .theta.) until the center of curvature
of the tip end of the ball end mill reaches from the position A to
the position C as shown in FIG. 7. As a result of the cutting work,
a counterbore 102c serving as a recess is formed in the vicinity of
the end of the thread groove of the nut 102. The outer diameter of
the cutting edge of the ball end mill (BEM) is larger than that of
the ball 3. In other words, the radius of the counterbore 102c is
made larger than that of the ball.
[0057] From the view point of the working, it is preferable to set
the direction in which the axis of the ball end mill (BEM)
elongates, to be perpendicular to the thread groove 102a. However,
the thread groove 102a is formed in the inner periphery of the
cylindrical nut 102, and hence the nut 102 interferes with the
cutting edge moving in the direction, thereby disabling the
working. To comply with this, the tip end of the ball end mill
(BEM) is obliquely inserted from both the sides of the nut 102
along the axis of the thread groove 102a, with the result that the
counterbore working can be conducted without cutting an extra
portion. The ball end mill (BEM) which is inserted as described
above is indicated by a phantom line in FIG. 8.
[0058] As seen from FIGS. 7 and 8, the lines N2.sub.TR and
N1.sub.TR which are the loci of the contact points between the ball
3 and the nut 102 are terminated at the points N2-C' and N1-C',
respectively. Therefore, the separations of the ball 3 from the nut
102 at these points are simultaneously performed, so that a
three-point contact of the ball 3 and the thread grooves is
avoided. In other words, the pre-load is instantaneously canceled,
whereby a ball rotation failure due to unbalanced reaction forces
can be prevented from occurring.
[0059] In the above-described embodiment, the counterbore working
is conducted by using the ball end mill so as to avoid a
three-point contact of the ball and the thread grooves, thereby
causing the pre-load to be instantaneously canceled. The
counterbore working must be conducted over a relatively long
distance. A second embodiment which will be described below can
solve this problem.
[0060] FIG. 9 is an enlarged view of an end portion of a nut 202 of
an end cap type ball screw mechanism which is a second embodiment
of the invention, as seen in the axial direction, and FIG. 10 is a
view of the nut 202 of FIG. 9 as seen in the direction of the arrow
X, in a similar manner as FIG. 7. In both FIGS. 9 and 10, the shape
of a tool (having a semispherical tip end) of a ball end mill (BEM)
which is used as a cutting tool is indicated by phantom lines. FIG.
9 also shows a state in which the ball 3 is placed in a thread
groove 202a of the nut 202.
[0061] The nut 202 in the embodiment is different only in the shape
of an end portion from the nut of the above-described embodiment.
Specifically, as seen from FIG. 10, a step portion 202d is formed
in the vicinity of an end portion of the thread groove 202a of the
nut 202. Assuming that the end face of the step portion 202d of the
nut 202 elongates over the whole periphery of the nut, the line
N2.sub.TR is interrupted at the point N2-C' at the timing when the
center of the ball 3 passes the position C, in the same manner as
the mechanism of the prior art. By contrast, the line N1.sub.TR
elongates longer than the line N2.sub.TR to continue to the point
N1-A.
[0062] Therefore, the step portion 202d is disposed in the vicinity
of the outer end (the portion where a flank is not formed in one
side) of an end portion of the thread groove 202a of the nut 202,
whereby the line N2.sub.TR is prolonged (or the line N1.sub.TR is
relatively shortened). Furthermore, the counterbore (recess) 202c
formed by a ball end mill (BEM) is disposed from the position B to
the position C. This configuration can attain effects that the
lines N2.sub.TR and N1.sub.TR which are the loci of the contact
points between the ball 3 and the thread groove 202a of the nut 202
can be terminated at the points N2-C' and N1-C', respectively, and
that it is sufficient for the counterbore formed by the ball end
mill (BEM) to be disposed over a relatively short distance.
[0063] When the distance between the groove bottom of the nut 202
and the contact points N1 and N2 is indicated by E and the lead
angle of the thread groove 202a is indicated by .theta., the height
S of the step portion 202d from the end face of the nut 202 is set
so as to satisfy the relationship of S.gtoreq.Ecos.theta..
[0064] Next, a circulation tube type ball screw mechanism which is
a third embodiment of the invention will be described with
reference to the drawings. FIG. 11 is a plan view of the
circulation tube type ball screw mechanism, and FIG. 12 is a
section view of the ball screw mechanism of FIG. 11 taken along the
line XII-XII and as seen in the direction of the arrows.
[0065] First, the configuration of the vicinity of a circulation
tube of the circulation tube type ball screw mechanism will be
described with reference to FIGS. 11 and 12. Referring to the
figures, a screw shaft 1 is a shaft member which has in the outer
face a spiral groove la having a Gothic arch-like section shape. A
nut 302 is a cylindrical member which has in the inner face a
thread groove 302a corresponding to the spiral groove 1a of the
screw shaft. A flat portion 302e is formed in a part of the outer
periphery, and two holes 302p are formed in the flat portion 302 so
as to open and communicate with parts of the thread groove 302a.
Ends of a U-like ball circulation tube 4 are fittingly inserted
into the holes of the nut 302, respectively. The ball circulation
tube 4 is fixed to the flat portion 302e of the nut 302 by set
screws 5 via a mounting plate 6.
[0066] The screw shaft 1 is passed through the hole of the nut 302
so that the thread groove 1a of the shaft opposes to the spiral
grooves 302a of the nut 302. A number of balls 3 are rotatably
placed in a ball circulation path configured by the ball
circulation tube 4, and also in the thread grooves between the ends
of the ball circulation path. The balls 3 are closely fitted into
the thread groove 302a of the nut 302 and the spiral groove 1a of
the screw shaft 1. The nut 302 and the screw shaft 1 are enabled to
perform relative spiral movement in the axial direction by rotation
of the balls 3. The relative spiral movement of the nut 302 and the
screw shaft 1 causes the balls 3 to repeat circulation in which the
balls rotatingly move along the spiral grooves 1a and 302a, are
scooped up from the spiral grooves 1a and 302a while being guided
by a tongue portion 4a formed at an end portion of the ball
circulation tube 4, to be directed into the ball circulation tube
4, pass through the tube, and then return to the spiral grooves 1a
and 302a via the other end of the tube.
[0067] FIG. 13 is a three-dimensional imaginary view showing the
vicinity of the thread groove of the nut 2 in the circulation tube
type ball screw mechanism of the prior art as shown in FIG. 1. In
addition, in FIG. 13, Ns1 indicates a sectional surface defined by
cutting the nut 2 along with a plane containing a line
perpendicular to the rotational axis of the nut 2, Ns2 denotes a
sectional surface defined by cutting the nut along with a vertical
plane, Ns3 designates a sectional surface defined by cutting the
nut 2 along with a horizontal plane, 2h indicates a part of the
outer periphery of the nut 2, and 2a' denotes a part of the
adjacent thread groove 2a.
[0068] Note that in order to facilitate the understanding of the
internal shape of the nut, FIG. 13 and FIG. 14, however, which will
be described later show the thread groove and so on as seen from
the outside with making the other components transparent, and do
not show the shape which is actually seen.
[0069] In the thus configured nut 2, the holes for mounting the
circulation tube are formed in the following manner. First, a
cylindrical hole 2f elongating from the upper face 2c to the thread
groove 2a is vertically formed by a cutting process using an end
mill (not shown) having a cylindrical tip end. Thereafter, a
mounting hole 2g is formed around the cylindrical hole 2f.
Alternatively, the mounting hole 2g may be first formed and the
cylindrical hole 2f may be then formed.
[0070] In a ball screw mechanism using the thus configured nut of
the prior art, the line N2.sub.TR is interrupted at the point N2-C,
but the line N1.sub.TR elongates to continue to the point N1-A.
Namely, in such a configuration, a problem of the three-point
contact between a ball and thread grooves is produced and a ball
rotation failure may occur.
[0071] FIG. 14 is a three-dimensional imaginary view similar to
FIG. 13 and showing the nut 302 in the circulation tube type ball
screw mechanism which is the third embodiment of the invention. In
the thus configured nut 302, the holes for mounting the circulation
tube are formed in the following manner. First, a cylindrical hole
302f elongating from the upper face to the thread groove 302a and
having a semispherical recess 302h at the lower end is vertically
formed by a cutting process using a ball end mill (not shown)
having a semispherical tip end the diameter of which is larger than
the outer diameter of the ball. Thereafter, a mounting hole 302g is
formed around the cylindrical hole 302f. Alternatively, the
mounting hole 302g may be first formed and the cylindrical hole
302f may be then formed.
[0072] As apparent from FIG. 14, since the semispherical recess
302h is formed, the lines N2.sub.TR and N1.sub.TR which are the
loci of the contact points between the ball (not shown) and the nut
302 are terminated at the points N2-C and N1-C, respectively.
Therefore, the separations of the ball from these points of the nut
302 are simultaneously performed, so that a three-point contact of
the ball and the thread grooves is avoided. Therefore, a ball
rotation failure due to unbalanced reaction forces can be prevented
from occurring.
[0073] Although the invention has been described by means of its
embodiments, it should be understood that the invention is not
restricted to these embodiments and can be adequately modified or
improved. In the embodiments, for example, the recess is formed by
a counterbore forming process using a ball end mill. Irrespective
of the working method, the effects of the invention can be attained
as far as the recess is formed.
[0074] According to the invention, in order to prevent balls from,
when the balls are discharged from a nut member in accordance with
relative axial movement of a nut member and a screw shaft,
contacting with thread grooves of the nut member and the screw
shaft at three points, a recess is formed around the thread groove
of the nut member in the vicinity of a discharge port for the balls
in the nut member. When the balls are contacted with the thread
grooves, therefore, a four-point contact is always maintained.
Consequently, reaction forces which are applied from the thread
grooves to the balls balance with each other, whereby stable
holding of the balls is ensured.
[0075] In the end cap type ball screw mechanism of the invention,
since the pre-load is immediately canceled in the vicinity of the
ball discharge port in the nut member, a stable operation which is
free from biting of the balls into the thread grooves can be
obtained.
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