U.S. patent application number 10/737811 was filed with the patent office on 2004-07-01 for ball screw mechanism.
This patent application is currently assigned to NSK LTD.. Invention is credited to Nakazawa, Takeshi, Ohkubo, Tsutomu.
Application Number | 20040123692 10/737811 |
Document ID | / |
Family ID | 17897331 |
Filed Date | 2004-07-01 |
United States Patent
Application |
20040123692 |
Kind Code |
A1 |
Ohkubo, Tsutomu ; et
al. |
July 1, 2004 |
Ball screw mechanism
Abstract
Between two mutually adjoining load balls 5, there is interposed
a retaining piece 10 having two concave surfaces 11 which are
respectively opposed to the two adjoining load balls 5. The
retaining piece 10 has an outside diameter dimension equal to or
more than 0.5 times the outside diameter dimension of the load ball
5. Further, the retaining piece 10 has an outside diameter
dimension which, when the retaining piece 10 passes through a screw
groove circulation passage and a tube circulation passage, prevents
the retaining piece 10 from touching the inner walls of these screw
groove circulation passage and tube circulation passage, or has an
outside diameter dimension which is equal to or less than 0.9 times
the outside diameter dimension of the load ball. Accordingly, it is
possible to provide a ball screw mechanism which can prevent a
retaining piece from interfering with the inner walls of
circulation passages to circulate load balls stably to thereby
prevent variations in torque as well as prevent the wear of the
retaining piece to thereby enhance the durability thereof.
Inventors: |
Ohkubo, Tsutomu; (Gunma,
JP) ; Nakazawa, Takeshi; (Gunma, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3202
US
|
Assignee: |
NSK LTD.
|
Family ID: |
17897331 |
Appl. No.: |
10/737811 |
Filed: |
December 18, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10737811 |
Dec 18, 2003 |
|
|
|
10254672 |
Sep 26, 2002 |
|
|
|
10254672 |
Sep 26, 2002 |
|
|
|
09693980 |
Oct 23, 2000 |
|
|
|
Current U.S.
Class: |
74/424.88 ;
384/49 |
Current CPC
Class: |
F16H 25/2214 20130101;
F16C 33/3713 20130101; Y10T 74/19777 20150115; F16C 33/3706
20130101; Y10T 74/19767 20150115; Y10T 74/19749 20150115; F16H
25/2238 20130101 |
Class at
Publication: |
074/424.88 ;
384/049 |
International
Class: |
F16C 029/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 1999 |
JP |
P.HEI. 11-301473 |
Claims
What is claimed is:
1. A ball screw mechanism comprising: a screw shaft having a
spiral-shaped screw groove on its outer peripheral surface; a nut
member having a spiral-shaped screw groove on its inner peripheral
surface mutually facing said spiral-shaped screw groove of said
screw shaft; a plurality of load balls disposed in a spiral-shaped
screw groove circulation passage formed by said mutually facing
screw grooves and also in a return circulation passage which is
formed in said nut member and is continuously connected to said
spiral-shaped screw groove circulation passage, wherein said load
balls taken out from said screw groove circulation passage can be
returned again into said screw groove circulation passage through
said return circulation passage; a plurality of retaining pieces
each interposed between every two mutually adjoining ones of said
load balls, said retaining piece having two concave surfaces
respectively facing said two load balls and also an outside
diameter dimension in the range from 0.5 to 0.9 times the outside
diameter dimension of said load ball.
2. The ball screw mechanism according to claim 1, wherein a radius
of curvature of a bent portion in said return circulation passage
satisfies a following equation R.apprxeq.(BCD-Dw)/2 where R defines
the radius curvature, BCD expresses a load ball pitch circle
diameter, and Dw expresses a load ball outside diameter.
3. The ball screw mechanism according to claim 1, wherein an
diameter-direction outer peripheral surface of at least one of said
retaining pieces is formed into a concave shape.
4. A ball screw mechanism comprising: a screw shaft having a
spiral-shaped screw groove on its inner peripheral surface mutually
facing said spiral-shaped screw groove of screw shaft; a nut member
having a spiral-shaped screw groove on its on its inner peripheral
surface mutually facing said spiral-shaped screw groove of the
screw shaft; a plurality of load balls disposed in a spiral-shaped
screw groove circulation passage formed by said mutually facing
screw grooves and also in a return circulation passage which is
formed in said nut member and is continuously connected to said
spiral-shaped screw groove circulation passage, wherein said load
balls taken out from said screw groove circulation passage can be
returned again into said screw groove circulation passage through
said return circulation passage, wherein, between every two
mutually adjoining ones of said load balls, there is interposed a
retaining piece having two concave surfaces respectively facing
said two load balls, said retaining piece has an outside diameter
dimension equal to or more than 0.5 times the outside diameter
dimension of said load ball, and said retaining piece has an
outside diameter dimension which, when said retaining piece passes
through said screw groove circulation passage and said tube
circulation passage, prevents said retaining piece from touching
the inner walls of said screw groove circulation passage and said
return circulation passage, wherein a radius of curvature of a bent
portion in said return circulation passage satisfies a following
equation R.apprxeq.(BCD-Dw)/2 where R defines the radius curvature,
BCD expresses a load ball pitch circle diameter, and Dw expresses a
load ball outside diameter.
5. The ball screw mechanism of claim 1, wherein said plurality of
load balls and said plurality of retaining pieces comprise such a
number so as to satisfy the following equations: S1>0; and
S2<0.8.times.ds, wherein, when all the load balls and all the
retaining pieces are gathered to one side so as to form a
continuous chain, S1 is a clearance formed between a load ball
positioned at a head of the chain and a tail retaining piece
positioned at a tail of the chain, S2 is a clearance between the
head load ball and a tail load ball when the tail retaining piece
is removed, and ds is the diameter of the retaining pieces.
6. The ball screw mechanism of claim 1, wherein at least one of
said retaining pieces is shaped as a sphere and has recesses formed
therein, wherein said recesses are opposite one another.
7. The ball screw mechanism of claim 1, wherein at least one of
said retaining pieces is X-shaped in cross section.
8. The ball screw mechanism of claim 1, wherein at least one of
said retaining pieces is shaped as a cylinder and has recesses
formed therein, wherein said recesses are opposite one another.
9. The ball screw mechanism of claim 1, wherein at least one of
said retaining pieces, in cross section, further comprises rounded
corners.
10. The ball screw mechanism of claim 4, wherein said plurality of
load balls and said plurality of retaining pieces comprise such a
number so as to satisfy the following equations: S1>0; and
S2<0.8.times.ds, wherein, when all the load balls and all the
retaining pieces are gathered to one side so as to form a
continuous chain, S1 is a clearance formed between a load ball
positioned at a head of the chain and a tail retaining piece
positioned at a tail of the chain, S2 is a clearance between the
head load ball and a tail load ball when the tail retaining piece
is removed, and ds is the diameter of the retaining piece.
11. The ball screw mechanism of claim 4, wherein said retaining
piece is shaped as a sphere, and said two concave surfaces are
opposite one another.
12. The ball screw mechanism of claim 4, wherein said retaining
piece is X-shaped in cross section.
13. The ball screw mechanism of claim 4, wherein said retaining
piece is shaped as a cylinder and said two concave surfaces are
opposite one another.
14. The ball screw mechanism of claim 4, wherein said retaining
piece, in cross section, comprises rounded corners.
15. A ball screw mechanism comprising: a screw shaft having a
spiral-shaped screw groove on its outer peripheral surface; a nut
member having a spiral-shaped screw groove on its inner peripheral
surface mutually facing said spiral-shaped screw groove of said
screw shaft; a plurality of load balls disposed in a spiral-shaped
screw groove circulation passage formed by said mutually facing
screw grooves and also in a return circulation passage which is
formed in said nut member and is continuously connected to said
spiral-shaped screw groove circulation passage, wherein said load
balls taken out from said screw groove circulation passage can be
returned again into said screw groove circulation passage through
said return circulation passage; a plurality of retaining pieces
each interposed between every two mutually adjoining ones of said
load balls, said retaining piece having two concave surfaces
respectively facing said two load balls and also an outside
diameter dimension in the range from 0.5 to 0.9 times the outside
diameter dimension of said load ball, wherein an outer peripheral
portion of each of the retaining pieces is projected in a radial
direction thereof.
16. A ball screw mechanism comprising: a screw shaft having a
spiral-shaped screw groove on its outer peripheral surface; a nut
member having a spiral-shaped screw groove on its inner peripheral
surface mutually facing said spiral-shaped screw groove of said
screw shaft; a plurality of load balls disposed in a spiral-shaped
screw groove circulation passage formed by said mutually facing
screw grooves and also in a return circulation passage which is
formed in said nut member and is continuously connected to said
spiral-shaped screw groove circulation passage, wherein said load
balls taken out from said screw groove circulation passage can be
returned again into said screw groove circulation passage through
said return circulation passage; a plurality of retaining pieces
each interposed between every two mutually adjoining ones of said
load balls, said retaining piece having two concave surfaces
respectively facing said two load balls and also an outside
diameter dimension in the range from 0.5 to 0.9 times the outside
diameter dimension of said load ball, wherein each of said
retaining pieces is concave in a radial direction thereof.
17. The ball screw mechanism according to claim 1, wherein at least
one of said plurality of retaining pieces has an outside diameter
dimension of 0.9 times the outside diameter dimension of said load
ball.
18. The ball screw mechanism according to claim 15, wherein at
least one of said plurality of retaining pieces has an outside
diameter dimension of 0.5 times the outside diameter dimension of
said load ball.
19. The ball screw mechanism according to claim 16, wherein at
least one of said plurality of retaining pieces has an outside
diameter dimension of either 0.5 or 0.9 times the outside diameter
dimension of said load ball.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a ball screw mechanism in
which, even when a retaining piece is interposed between two
mutually adjoining load balls, decrease in the number of load balls
can be controlled as much as possible to thereby prevent the load
balls from degrading the load capacity and rigidity thereof, can
reduce friction between the load balls and retaining piece to
thereby enhance the circulating performance of the retaining piece,
and can prevent the load balls from butting against each other to
thereby prevent degradation in the operation efficiency thereof,
deterioration in the quality of sounds generated, and the friction
and damage of the load balls.
[0002] Conventionally, in a ball screw mechanism, as shown in FIG.
9, on the outer peripheral surface of a screw shaft 1 and on the
inner peripheral surface of a nut 2, there are respectively formed
spiral-shaped screw grooves 3 and 4 which disposed opposed to each
other and, in a screw groove circulation passage defined by the two
screw grooves 3 and 4, there are disposed a large number of load
balls 5 in such a manner that they are free to roll. In case where
the screw shaft 1 and nut 2 are rotated with respect to each other
to thereby move one of them in the axial direction thereof, the
screw shaft 1 and nut 2 are smoothly moved in a spiral manner with
respect to each other.
[0003] In such ball screw mechanism, the load balls 5 are arranged
densely within the screw grooves 3 and 4 and they roll in the same
direction within the individual screw grooves 3 and 4; and, in the
rolling movements thereof, at contact points between the two
mutually adjoining load balls 5 and 5, the load balls 5 rolling in
the mutually opposite directions are contacted with each other to
thereby interfere with their mutual rolling movements, with the
result that the load balls 5 and 5 are caused to slip at the
contact points. This interferes with the free rolling movements of
the load balls 5, degrades the operation efficiency of the load
balls 5, gives rise to friction and damage in the load balls 5,
causes variations in the torque of the load balls 5, and swells the
noises that are produced by the load balls 5.
[0004] To cope with these problems, as shown in FIG. 10, between
the mutually adjoining load balls 5, there are respectively
interposed spacer balls 6 each having a diameter smaller by several
tens .mu. than the load balls 5. That is, such interposition of the
spacer balls 6 prevents the load balls 5 against slippage, improves
the operation efficiency of the load balls 5, and reduces the
friction and damage of the load balls 5 to thereby prevent
variations in the torque.
[0005] However, in the ball screw mechanism shown in FIG. 10, while
the load balls 5 are, for example, ten in number, the spacer balls
6 are also, for example, ten in number. Therefore, when compared
with the ball screw mechanism shown in FIG. 9, a clearance between
the two adjoining load balls 5 is large and the load balls 5 are
reduced in number down to about one half, which decreases the load
capacity of the ball screw mechanism and also degrades the rigidity
thereof.
[0006] In view of the above problems, there is also known a
structure in which, as shown in FIG. 11, between every two mutually
adjoining load balls 5, there is interposed a retaining piece 10
having two concave surfaces 11 respectively facing the two load
balls 5. According to this structure, while the load balls 5 are in
contact with the concave surfaces 11 of the retaining piece 10,
they are allowed to circulate well within the spiral-shaped screw
grooves 3 and 4. Therefore, when compared with the conventional
structure using spacer balls, the spacer, that is, the retaining
piece 10 can be made thinner, which makes it possible to control
reduction in the number of load balls and thus avoid degradation in
the load capacity and rigidity of the ball screw mechanism.
[0007] However, in the ball screw mechanism shown in FIG. 11, since
the relation between the outside dimension of the retaining piece
10 and the outside dimension of the load ball 5 is not always
considered properly, there is a fear that, when the retaining piece
10 passes through the spiral-shaped screw groove circulation
passage and tube circulation passage, it can interfere with the
inner walls of these circulation passages. Therefore, such
interference makes it difficult for the load balls 5 to circulate
stably, thereby giving rise to generation of variations in the
torque, or causing the retaining piece 10 to wear.
SUMMARY OF THE INVENTION
[0008] The present invention aims at eliminating the drawbacks
found in the above-mentioned conventional ball screw mechanisms.
Accordingly, it is an object of the invention to provide a ball
screw mechanism which prevents the retaining piece from interfering
with the inner walls of the circulation passages to thereby allow
the load balls to circulate stably, thereby preventing not only
variations in the torque but also the wear of the retaining piece
for enhancement of the durability of the retaining piece.
[0009] In attaining the above object, according to a first aspect
of the invention, there is provided a ball screw mechanism
structured such that, on the outer peripheral surface of a screw
shaft and on the inner peripheral surface of a nut, there are
respectively formed mutually facing spiral-shaped screw grooves,
there are rollably disposed a large number of load balls in a
spiral-shaped screw groove circulation passage formed by these two
screw grooves, and, to the screw groove circulation passage, there
is continuously connected a return circulation passage through
which load balls taken out from the screw groove circulation
passage can be returned again into the screw groove circulation
passage, wherein, between every two mutually adjoining ones of the
large number of load balls, there is interposed a retaining piece
having two concave surfaces respectively facing the two mutually
adjoining load balls, the retaining piece has an outside diameter
dimension equal to or more than 0.5 times the outside diameter
dimension of the load ball, and the retaining piece has an outside
diameter dimension which, when the retaining piece passes through
the screw groove circulation passage and tube circulation passage,
prevents the retaining piece from touching the inner walls of the
screw groove circulation passage and tube circulation passage.
[0010] Also, according to a second aspect of the invention, there
is provided a ball screw mechanism structured such that, on the
outer peripheral surface of a screw shaft and on the inner
peripheral surface of a nut, there are respectively formed mutually
facing spiral-shaped screw grooves, there are rollably disposed a
large number of load balls in a spiral-shaped screw groove
circulation passage formed by these two screw grooves, and, to the
screw groove circulation passage, there is continuously connected a
return circulation passage through which load balls taken out from
the screw groove circulation passage can be returned again into the
screw groove circulation passage, wherein, between every two
mutually adjoining ones of the large number of load balls, there is
interposed a retaining piece having two concave surfaces
respectively facing the two mutually adjoining load balls, and the
retaining piece has an outside diameter dimension in the range of
0.5 to 0.9 times the outside diameter dimension of the load
ball.
[0011] As described above, according to the invention, as a first
condition, the retaining piece has an outside diameter dimension
equal to or more than 0.5 times the outside diameter dimension of
the load ball. This is basically because, in case where the
retaining piece has an outside diameter dimension less than half
the outside diameter dimension of the load ball, the retaining
piece cannot be lifted up from between the two mutually adjoining
load balls, which disables the retaining piece to fulfil the
expected function thereof.
[0012] Also, as a second condition, the retaining piece has an
outside diameter dimension which, when the retaining piece passes
through the screw groove circulation passage and tube circulation
passage, prevents the retaining piece from touching the inner walls
of these screw groove circulation passage and tube circulation
passage, or has an outside diameter dimension which is equal to or
less than 0.9 times the outside diameter dimension of the load
ball. This is basically because, to circulate the load balls
stably, the concave surface holding allowance for the load balls
(the steel ball holding allowance) may be preferably set large,
which increases the outside diameter of the retaining piece
accordingly; but, in case where the concave surface holding
allowance is excessively large, the retaining piece interferes with
the inner walls of the screw groove circulation passage and return
circulation passage having various curvatures, thereby degrading
the operation efficiency of the retaining piece. Under these
circumstances, to balance them with each other, the outside
diameter of the retaining piece is set as large as possible on one
side but, on the other hand, the outside diameter of the retaining
piece is controlled to such a degree that it does not interfere
with the inner walls of the circulation passages.
[0013] As described above, according to the invention, since,
between every two mutually adjoining load balls, there is
interposed a retaining piece having two concave surfaces
respectively facing the two load balls, even in case where a
retaining piece between two mutually adjoining load balls, the
number of load balls can be reduced as many as possible without
degrading the load capacity and rigidity of the ball screw
mechanism; and, friction between the load balls and retaining
pieces can be reduced as much as possible, so that not only the
circulating performance of the retaining piece can be enhanced, but
also the load balls can be prevented from butting each other,
thereby preventing the degraded operation efficiency of the load
balls, generation of noises by the load balls, the deteriorated
quality of sounds generated by the load balls, and the friction and
damage of the load balls.
[0014] In addition, because the relation between the outside
diameter dimension of the retaining piece and the outside diameter
dimension of the load balls is properly considered, the
interference of the retaining pieces with the inner walls of the
circulation passages can be prevented, thereby allowing the load
balls to circulate stably. This can prevent variations in torque as
well as can prevent the wear of the retaining pieces to thereby
enhance the durability thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a typical view of a ball screw mechanism according
to an embodiment of the invention;
[0016] FIG. 2 is a typical view of a state of a retaining piece and
its two adjoining load balls when there exist clearances
respectively between the retaining piece and two load balls;
[0017] FIG. 3 is a typical view the shape of the side surfaces of
the retaining piece;
[0018] FIG. 4(a) is a typical view of a state of the retaining
piece and two load balls when the outside dimension (ds) of the
retaining piece is equal to or more than 0.5 times the outside
dimension (Dw) of the load ball, and FIG. 4(b) is a typical view of
a state of the retaining piece and two load bails when the outside
dimension (ds) of the retaining piece is equal to or less than 0.5
times the outside dimension (Dw) of the load ball;
[0019] FIG. 5 is a typical view of a modification of the above
embodiment of a retaining piece according to the invention;
[0020] FIG. 6 is a graphical representation of the test results of
the torque characteristic of a conventional ball screw mechanism
when the outside diameter dimension of a retaining piece is 0.95
times the outside diameter dimension of a load ball;
[0021] FIG. 7 is a graphical representation of the test results of
the torque characteristic of a ball screw mechanism according to
the invention when the outside diameter dimension of a retaining
piece is 0.9 times the outside diameter dimension of a load
ball;
[0022] FIG. 8(a) is an explanatory view of the principle of a ball
screw mechanism according to an embodiment of the invention, and
FIG. 8(b) is a section view of a retaining piece employed in the
embodiment;
[0023] FIG. 9 is a side view of a first conventional ball screw
mechanism;
[0024] FIG. 10 is a side view of a second conventional ball screw
mechanism; and,
[0025] FIG. 11 is a section view of a third conventional ball screw
mechanism.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Now, description will be given below of the preferred
embodiments of a ball screw mechanism according to the invention
with reference to the accompanying drawings.
[0027] Prior to description of the preferred embodiments of a ball
screw mechanism according to the invention, the number of load
balls, the number of retaining pieces and the clearance relation
between the load balls and retaining pieces will be discussed with
reference to FIG. 8.
[0028] Now, FIG. 8(a) is an explanatory view of the principle of a
ball screw mechanism according to an embodiment of the invention,
and FIG. 8(b) is a section view of a retaining piece employed in
the embodiment.
[0029] The numbers of load balls 5 and retaining pieces 10 are
respectively set in the following manner that, as shown in FIG.
8(a), when it is assumed that all the balls 5 and all the retaining
pieces 10 inserted into a spiral-shaped circulation passage formed
by screw grooves 3 and 4 are gathered to one side, a clearance
formed between a load ball (LEAD-B) positioned at the head and a
retaining piece (TAIL-S) positioned at the tail is defined as a
total clearance; and, when it is assumed that the total clearance
(S1) is larger than zero (that is, S1>0) and the tail-positioned
retaining piece (TAIL-S) is removed, a clearance (S2) between the
head load ball (LEAD-B) and the tail retaining piece (TAIL-S) is
smaller than 0.8 times the diameter (ds, see FIG. 8(b)) of the
retaining piece (that is, S2<0.8.times.ds).
[0030] As described above, the distance (S1) of the total clearance
of the circulation passage is set such that S1>0 and, when one
retaining piece (TAIL-S) is removed, the clearance (S2) between the
head load ball (LEAD-B) and the tail retaining piece (TAIL-S) is
set such that S2<0.8.times.ds. Therefore, there is no
possibilities that, because the clearance within the circulation
passage is excessively large, the retaining pieces 10 can be caused
to fall down and that, since the clearance within the circulation
passage is excessively small, the load balls 5 and retaining pieces
10 are rubbed against each other to thereby give rise to the poor
operation of the load balls 5 and retaining pieces 10: that is, the
two kinds of clearances (S1, S2) within the circulation passage are
set properly. Thanks to this, there is no fear that the retaining
pieces 10 can fall down further than an angle of 60.degree., which
makes it possible to maintain the proper operating performance of
the load balls 5 and retaining pieces 10.
[0031] Next, in the present embodiment, the outside dimension and
shape of the retaining piece are defined with above-mentioned
conditions taken into account.
[0032] Now, FIG. 1 is a typical view of a ball screw mechanism
according to the present embodiment of the invention; FIG. 2 is a
typical view of a state of a retaining piece and two load balls
when there exist clearances respectively between the retaining
piece and two load balls; FIG. 3 is a typical view of the shape of
the side surfaces of the retaining piece; FIG. 4(a) is a typical
view of a state of the retaining piece and two load balls when the
outside dimension (ds) of the retaining piece is equal to or larger
than 0.5 times the outside dimension (Dw) of the load ball; FIG.
4(b) is a typical view of a state of the retaining piece and two
load balls when the outside dimension (ds) of the retaining piece
is equal to or smaller than 0.5 times the outside dimension (Dw) of
the load ball; and, FIG. 5 is a typical view of a modification of
the retaining piece according to the second embodiment.
[0033] In the present embodiment, as a first condition, the
retaining piece 10 has an outside dimension which is equal to or
larger than 0.5 times the outside dimension (Dw) of the load ball
5.
[0034] The reason why the first condition is established is as
follows: that is, in a ball screw mechanism with retaining pieces
contained therein, since the total clearance exists in the
above-mentioned manner, all the retaining pieces 10 are not
restricted completely between their respective adjoining load balls
5 but some of the retaining pieces 10, as shown in FIG. 2, are held
in a free state (a state in which there exist clearances
respectively between a retaining and two adjoining load balls)
within the circulation passage; and, to return the retaining piece
10 from the free state to the normal state in which the retaining
piece 10 is tightly held by and between its adjoining load balls 5,
it is necessary that, as shown in FIG. 4(a), the retaining piece 10
has an outside diameter dimension equal to or more than 0.5 times
the outside diameter dimension. In other words, when the load balls
5 and retaining piece 10 are arranged within the circulation
passage in order of load ball 5--retaining piece 10--load ball 5,
if the height of the retaining piece 10 does not exist above a line
which connects together the centers of the two load balls 5, as
shown in FIG. 4(b), there does not operate a force which lifts the
retaining piece 10 up to its normal position, so that the retaining
piece 10 is unable to fulfil its function.
[0035] Next, in the present embodiment, as a second condition, the
retaining piece 10 has an outside diameter dimension which, when
the retaining piece 10 passes through a screw groove circulation
passage and a tube circulation passage, prevents the retaining
piece 10 from touching the inner walls of these screw groove
circulation passage and tube circulation passage, or has an outside
diameter dimension which is equal to or less than 0.9 times the
outside diameter dimension of the load ball 5.
[0036] (A) As a first reason for the second condition, to secure a
concave surface holding allowance for the load ball 5 (a steel ball
holding allowance, FIG. 3), it is necessary that the outside
diameter of the retaining piece 10 is set as large as possible.
[0037] That is, to circulate the retaining piece 10 stably in a
state where it is held by and between its adjoining load balls 5,
it is necessary that the concave surface holding allowance for the
load ball 5 (the steel ball holding allowance, FIG. 3) is set
large. However, when the concave surface holding allowance (the
steel ball holding allowance) is set large, the outside diameter of
the retaining piece 10 increases.
[0038] (B) As a second reason for the second condition, it is
necessary to prevent the retaining piece 10 from interfering with
the inner walls of the circulation passages which have various
curvatures.
[0039] That is, in the case of a ball screw mechanism of a tube
type, there are present a screw groove circulation passage for
receiving loads and a tube circulation passage (a return
circulation passage) for returning load balls. The screw groove
circulation passage has a given curvature and, at the same time,
the tube circulation passage, as shown in FIG. 1, has given
curvatures respectively in a bent portion 12 and a ball scoop-up
portion 13. And, it is necessary to prevent the retaining piece 10
from interfering with the inner walls of the circulation passages
having various curvatures. For example, as there is shown a
continuous track of the retaining piece 10 in FIG. 1, it is
necessary for the retaining piece 10 to pass through the ball
scoop-up portion 13 without interfering with the inner walls
thereof.
[0040] As described above, to circulate the load balls 5 stably,
the concave surface holding allowance for the load ball 5 (the
steel ball holding allowance, FIG. 3) may be preferably set large
and, in this case, the outside diameter of the retaining piece 10
is set large accordingly. However, in case where the outside
diameter of the retaining piece 10 is excessively large, the
retaining piece 10 interferes with the inner walls of the screw
groove circulation passage and return circulation passage having
various curvatures, which degrades the operation efficiency of the
retaining piece 10. Under these circumstances, to balance them with
each other, the outside diameter of the retaining piece 10 is set
as large as possible on one side but, on the other hand, it is
controlled such that the retaining piece 10 does not to interfere
with the inner walls of the circulation passages. That is, the
second condition is defined such that the retaining piece 10 has an
outside diameter dimension which, when the retaining piece 10
passes through a screw groove circulation passage and a tube
circulation passage, prevents the retaining piece 10 from touching
the inner walls of these screw groove circulation passage and tube
circulation passage, or has an outside diameter dimension which is
equal to or less than 0.9 times the outside diameter dimension of
the load ball 5.
[0041] Next, as shown in FIGS. 1 to 4, referring to the shape of
the retaining piece 10, the retaining piece 10 includes two concave
surfaces 11 respectively facing the two adjoining load balls 5,
while the outer peripheral surface of the retaining piece 10 in the
diameter direction thereof is set flat. Thanks to this shape, the
retaining piece 10 is allowed to pass through the screw groove
circulation passage and tube circulation passage without
interfering with the inner walls of these circulation passages.
[0042] Also, referring to the shape of the retaining piece 10, the
retaining piece 10, as shown in FIG. 5, the diameter-direction
outer peripheral surface thereof may also be formed as a
triangular-shaped concave shape. In this case, there can be
obtained the same effect as the outside diameter of the retaining
piece 10 is reduced, so that while the concave surface holding
allowance (the steel ball holding allowance) is set large, it is
possible to prevent the retaining piece 10 from interfering with
the inner walls of the bent portion 12 and ball scoop-up portion 13
of the tube circulation passage.
[0043] Further, as shown in FIG. 1, the bend R of the bent portion
12 of the tube circulation passage may also be set in the following
manner.
[0044] That is, R.apprxeq.(BCD-Dw)/2 where BCD expresses a load
ball pitch circle diameter, Dw expresses a load ball outside
diameter, and (BCD-Dw) expresses a groove bottom diameter. In this
manner, by setting the bend R (that is, a radius curvature) of the
bent portion 12 of the tube circulation passage relatively large,
the retaining piece 10 can be prevented from interfering with the
bent portion 12, which makes it possible to circulate the retaining
piece 10 stably.
[0045] By the way, in case where the distance between the load
balls 5 is set large, when the balls 5 pass through the bent
portion of the tube circulation passage, there is provided the same
tendency as the outside diameter of the retaining piece 10 is set
large. Here, by setting the distance between the load balls 5 to be
equal to or less than 1.3 times the outside diameter of the load
ball 5, the outside diameter of the retaining piece 10 can be set
in the above-mentioned range, so that the above-mentioned effect
can be expected.
[0046] By the way, the present invention is not limited to the
above-mentioned embodiment but there are possible various changes
and modifications.
[0047] [Embodiment]
[0048] Now, FIG. 1 shows an inserted state of a retaining piece 10
at a section thereof substantially perpendicular to the axial
direction thereof when the retaining piece 10 is inserted into a
ball screw mechanism in which a load ball (steel ball) diameter is
3.175 mm, a ball screw axial diameter is .phi.32 mm, and a lead is
5 mm. In this case, the retaining piece 10 has an outside diameter
90% of the outside diameter of the load ball 5, while the distance
between load balls 5 is 115% of the outside diameter of the load
ball 5. As can be seen from FIG. 1, the retaining piece 10 is
prevented from interfering with the inner walls of the screw groove
circulation passage having a given curvature and the inner walls of
the bent portion 12 and ball scoop-up portion 13 of the tube
circulation passage. In the case of this state, test results show
that there is no problem as to the actual torque characteristic and
the durability of the retaining piece.
[0049] Also, FIG. 6 is a graphical representation of the test
results of the torque characteristic of a conventional ball screw
mechanism when the outside diameter dimension of a retaining piece
is 0.95 times the outside diameter dimension of a load ball. In
this case, since the retaining piece 10 interferes with the bent
portion 12 of the tube circulation passage, there is found an
increase (beard) in the torque at a cycle where the retaining piece
10 passes through the bent portion 12.
[0050] Further, FIG. 7 is a graphical representation of the test
results of the torque characteristic of a ball screw mechanism
according to the invention when the outside diameter dimension of a
retaining piece is 0.9 times the outside diameter dimension of a
load ball. In this case, the retaining piece 10 does not interfere
with the bent portion 12 of the tube circulation passage, nor there
is caused an increase (beard) in the torque when the retaining
piece 10 passes through the bent portion 12. That is, it can be
seen that the torque characteristic of the present ball screw
mechanism is improved.
[0051] As has been described heretofore, according to the
invention, since, between every two mutually adjoining load balls,
there is interposed a retaining piece which has two concave
surfaces respectively facing these two load balls, even in case
where a retaining piece is interposed between its two mutually
adjoining load balls, reduction in the number of load balls can be
controlled as much as possible to thereby prevent degradation in
the load capacity and rigidity of the ball screw mechanism; and,
friction between the load balls and retaining pieces can be reduced
as much as possible to thereby enhance the circulation performance
of the retaining pieces. Also, the load balls are prevented from
butting against each other, which can prevent the degraded
operation efficiency of the load balls, generation of noises,
deterioration in the quality of sounds generated, and the friction
and damage of the load balls.
[0052] In addition, because the relation between the outside
diameter dimension of the retaining piece and the outside diameter
dimension of the load balls is properly considered, the
interference of the retaining pieces with the inner walls of the
circulation passages can be prevented to thereby circulate the load
balls stably. This can prevent variations in torque as well as can
prevent the wear of the retaining pieces to thereby enhance the
durability thereof.
[0053] While there has been described in connection with the
preferred embodiment of the invention, it will be obvious to those
skilled in the art that various changes and modifications may be
made therein without departing from the invention, and it is aimed,
therefore, to cover in the appended claim all such changes and
modifications as fall within the true spirit and scope of the
invention.
* * * * *