Machine element or mechanism – Gearing – Directly cooperating gears
Reexamination Certificate
2001-12-13
2003-11-11
Hannon, Thomas R. (Department: 3682)
Machine element or mechanism
Gearing
Directly cooperating gears
C384S051000
Reexamination Certificate
active
06644140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ball screw for use in a feed mechanism of a machine tool and, more particularly, to a ball screw having spacers interposed between respective adjacent pairs of balls.
2. Description of the Prior Art
Ball screws of the aforesaid type for use in a feed mechanism generally have a construction as shown in FIG.
6
. As shown, the ball screw
100
includes a screw shaft
101
having a helical ball rolling groove
102
formed in an outer peripheral surface thereof, a ball nut
103
having a helical ball rolling groove
105
formed in an inner peripheral surface thereof in an opposed relation with the ball rolling groove
102
and ball circulation channels
106
,
108
,
110
provided therein to connect opposite ends of the ball rolling groove
105
, and a plurality of balls
111
provided in a channel defined between the ball rolling grooves
102
and
105
and the ball circulation channels
106
,
108
,
110
. The ball nut
103
includes a cylindrical nut body
104
and end caps
107
,
109
provided at opposite ends of the nut body
104
. The nut body
104
is formed with the ball rolling groove
105
and the ball circulation channel
106
. The end caps
107
and
109
are formed with the ball circulation channels
108
and
110
, respectively.
The ball screw
100
shown in
FIG. 6
is of a so-called end cap type. Other exemplary types include a return tube type and a guide plate type, depending on the arrangement of the ball circulation channels.
In the ball screw
100
having the aforesaid construction, however, the balls
111
have slight variations in revolving speed during the rolling movement thereof due to variations in the profile of the ball rolling grooves
102
,
105
. If an upstream ball
111
revolves at a higher revolving speed than a downstream ball
111
with respect to a traveling direction of the balls
111
, the upstream and downstream balls
111
collide and push against each other, so that a compression force is exerted on a contact between the balls
111
pushing against each other. The compression force acting on the contact between the balls
111
causes the balls
111
sliding contact in a direction such as to prevent the rolling of the balls
111
. This develops a great resistance to prevent the rolling of the balls
111
, thereby varying or significantly increasing the dynamic torque of the ball screw
100
. Further, clogging with the balls
111
may occur.
To solve the aforesaid problem, a ball screw has been proposed in which spacers are interposed between respective adjacent pairs of balls
111
. Examples of the spacers are shown in
FIGS. 7 and 8
.
A spacer
120
shown in
FIG. 7
is a hollow cylindrical member having opposite end faces
121
,
121
of a concave spherical shape. The spacer
120
is interposed between each adjacent pair of balls
111
,
111
with the opposite end faces
121
,
121
in contact with the balls
111
to prevent direct contact between the balls
111
,
111
. The concave spherical end faces
121
,
121
of the spacer
120
entirely contact the balls
111
,
111
, so that a great contact resistance is developed during the rolling of the balls
111
. Therefore, dynamic torques of the balls
111
are increased, resulting in clogging with the balls.
A spacer
130
shown in
FIG. 8
has opposite end faces
131
,
131
of a generally concave spherical shape, which are each entirely formed with a plurality of projections
132
. With this arrangement, areas of contact between balls
111
and the end faces
131
are reduced, so that a contact resistance occurring therebetween is reduced to solve the aforesaid problem.
However, the spacer
120
of FIG.
7
and the spacer
130
of
FIG. 8
suffer from the following problems. There are dimensional variations such as variations in the effective diameter of the channel defined between the ball rolling grooves
102
and
105
, the diameter of the balls
110
and the total length of the ball circulation channels
106
,
108
,
110
due to machining errors. Even if the balls
111
and the spacers
120
(
130
) are tightly inserted in an alternating relation in the channel defined between the ball rolling grooves
102
and
105
and the ball circulation channels
106
,
108
,
110
, a gap inevitably occurs between an initially inserted ball
111
and a finally inserted spacer
120
(
130
) due to the dimensional variations. Therefore, when the screw shaft
101
(or ball nut
103
) is rotated, the dynamic torque of the ball screw
100
is varied or significantly increased, or the spacers
120
(
130
) are inclined, thereby preventing the circulation of the balls
111
.
One conceivable approach to this problem is to prepare several types of spacers
120
(
130
) having different lengths and select a spacer
120
(
130
) having a length appropriate for filling the gap when it is finally inserted in the channel. However, the production, maintenance and assembly of such spacers are troublesome with little practical advantage.
In view of the foregoing, it is an object of the present invention to provide a ball screw having spacers which can assuredly be retained between balls and prevent direct contact between the balls.
SUMMARY OF THE INVENTION
In accordance with the present invention to achieve the aforesaid object, there is provided a ball screw which comprises: a screw shaft having a helical ball rolling groove formed in an outer peripheral surface thereof; a ball nut having a helical ball rolling groove formed in an inner peripheral surface thereof in an opposed relation with the ball rolling groove of the screw shaft, and a ball circulation channel provided therein to connect opposite ends of the ball rolling groove thereof; a plurality of balls provided in a channel defined between the ball rolling grooves and the ball circulation channel; and a plurality of spacers interposed between respective adjacent pairs of balls; wherein the spacers each comprise a base plate having front and back surfaces, and at least three projections provided on each of the front and back surfaces of the base plate, the projections on the front surface being offset from the projections on the back surface; wherein the spacers are each provided perpendicularly to a traveling direction of the balls with the projections on the front and back surfaces resiliently abutting against the balls adjacent thereto.
In the ball screw according to the present invention, the spacers each have the base plate and the at least three projections provided on each of the front and back surfaces of the base plate, and the projections on the front surface are offset from the projections on the back surface. The spacers are provided perpendicularly to the traveling direction of the balls between the balls. When an action force is exerted on an adjacent pair of balls to urge the balls toward each other, the projections on the front and back surfaces of a spacer provided between the balls abut against the balls, and receive reaction forces from the balls. At this time, the reaction forces act on different points on the front and back surfaces of the base plate, because the projections on the front surface are offset from the projections on the back surface. The base plate is planar and, hence, is more liable to be resiliently deformed than the cylindrical member of the conventional spacer. In addition, the reaction forces act on the different points on the front and back surfaces. Therefore, the spacer can easily absorb the reaction forces by the resilient deformation thereof to permit reduction of a spacing between the balls.
Thus, the spacers according to the present invention per se are resiliently deformed to adjust the spacing between the balls. Even if there is a gap between an initially inserted ball and a finally inserted spacer under no load due to dimensional variations such as variations in the effective diameter of the channel defined between the ball rolling grooves, the diameter of the balls and the length of the ball circulation channel
Armstrong Westerman & Hattori, LLP
Hannon Thomas R.
Mori Seiki Co. Ltd.
Van Pelt Bradley J.
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