Machine element or mechanism – Gearing – Directly cooperating gears
Reexamination Certificate
2001-08-17
2004-01-13
Bucci, David A. (Department: 3682)
Machine element or mechanism
Gearing
Directly cooperating gears
Reexamination Certificate
active
06675669
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-252602, filed Aug. 23, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ball screw apparatus used in a feed mechanism for various equipments or the like.
2. Description of the Related Art
A ball screw apparatus is a mechanical element which can convert rotation movement to linear movement. The ball screw apparatus comprises a screw shaft formed on its outer circumferential face with a spiral ball screw groove, and a nut member fitted on the outer circumferential face of this screw shaft. A ball screw groove opposed to the ball screw groove of the screw shaft is formed on an inner circumferential face of the nut member.
A spiral ball rolling contact way is constituted by the ball screw groove of the screw shaft and the ball screw groove of the nut member opposed to each other. One portion and another portion of the ball rolling contact way which are spaced from each other are connected to each other by a connecting way provided on the nut member. An endless ball circulating way is constituted by the ball rolling contact way and the connecting way. Many balls are received in the ball circulating way in an endless continuous manner.
The nut member is rotatable relative to the screw shaft. For example, when the screw shaft is rotated relative to the nut member, the respective balls positioned between the screw shaft and the nut member circulate in the ball circulating way in an endless manner while they are rotating. Thus, the balls are circulated in the circulating way in the endless manner according to rotation of the screw shaft, so that the nut member can be moved accurately and smoothly relative to the screw shaft in an axial direction of the screw shaft.
FIG. 7
is a cross-sectional view schematically showing a conventional ball screw apparatus
100
of a tube type. A nut member
2
is fitted on an outer circumferential face of a screw shaft
1
. A spiral ball screw groove
1
a
is formed on an outer circumferential face of the screw shaft
1
. A ball screw groove
2
a
is formed on an outer circumferential face of the nut member
2
. The ball screw groove
2
a
is opposed to the ball screw groove
1
a
of the screw shaft
1
. These ball screw grooves
1
a
and
2
a
constitute a spiral ball circulating way
3
.
Through holes
4
a
and
4
b
are formed on a circumferential wall of the nut member
2
. These through holes
4
a
and
4
b
are connected to each other by a tube
5
. The tube
5
is disposed on an outer circumferential portion of the nut member
2
. Both end portions of the tube
5
are inserted into the through holes
4
a
and
4
b
. A connection way
6
is constituted by the through holes
4
a
,
4
b
and the tube
5
. The connecting way
6
connects a portion and another portion of the ball rolling contact way
3
spaced from each other.
Thus, both ends of the ball rolling contact way
3
communicates with each other through the connecting way
6
so that an endless ball circulating way
7
is constituted. Many balls
8
are received in the ball circulating way
7
. These balls
8
are arranged in the circulating way
7
in an endless manner. Each ball
8
is formed from material selected from steel, ceramic, plastic or the like. The balls
8
are circulated in an endless manner while they are rolling in the circulating way
7
as the screw shaft
1
is rotated.
When the nut member
2
is rotated relative to the screw shaft
1
, for example, in a direction indicated with arrow R shown in
FIG. 7
, each ball
8
is rolled in a direction of arrow R along the screw groove
1
a
. The moving speed of the ball
8
is slower than the rotating speed of the nut member
2
. Therefore, each ball
8
moves relatively in a direction opposed to the arrow R towards the screw groove
2
a
of the nut member
2
through one through hole
4
a
from the tube
5
.
The conventional ball screw apparatus
100
has a corner portion
10
at a connecting portion between the rolling contact way
3
and the connecting way
6
, namely a boundary portion between the screw groove
2
a
and the connecting way
6
, as shown in FIG.
7
. For this reason, in the conventional ball screw apparatus
100
, there is a case that a smooth circulating movement of the ball
8
circulating in the circulating way
7
is prevented by the corner portion
10
, which results in deterioration in operability or durability of the ball screw apparatus
100
.
When an external load acts on the ball screw apparatus
100
, such a state occurs that a distance between the screw grooves
1
a
and
2
a
is reduced or shortened. The reason that the distance between the screw grooves
1
a
and
2
a
is because of Hertz's contact between one screw groove
1
a
and the ball
8
, Hertz's contact between the other screw groove
2
a
and the ball
8
, and the elastic deformation of the ball
8
itself. In the specification, the state where the distance between the screw grooves
1
a
and
2
a
is shortened is referred to as “elastic approach”.
In the state where the elastic approach occurs, the ball
8
enters from the tube
5
which is a non-loaded zone in between the screw grooves
1
a
and
2
a
where is a loaded zone. Here, when the elastic approach occurs, the ball
8
can not enter in between the screw grooves
1
a
and
2
a
by its own force. For this reason, a proceeding ball
8
enters in between the screw grooves
1
a
and
2
a
due to pushing of the following ball
8
.
Conventionally, the ball screw groove
2
a
is processed continuously over the entire inner circumference of the nut member
2
on the basis of a diameter of circle passing through the center of each ball
8
. For this reason, when the above-mentioned external load acts on the ball screw apparatus
100
, the screw grooves
1
a
and
2
a
mutually approach to each other over the entire inner circumference of the nut member
2
by the same distance. As a result, as soon as the ball
8
has entered in between the screw grooves
1
a
and
2
a
from the tube
5
, the ball
8
is compressed rapidly.
For example, in a ball screw apparatus of a diameter of 100 mm, a lead of 25 mm, and a ball diameter of 19.05 mm, when a load is applied thereon, the amount of elastic approach between screw grooves reaches 50 &mgr;m in some cases. That is, when the ball enters from a non-loaded zone to a loaded zone, compression of 50 &mgr;m occurs. For this reason, when a load is significantly large, there may occur such a problem as described below.
(1) Stress is concentrated on the corner portion
10
between the screw groove
2
a
and the end of the tube
5
so that flaking occurs thereat.
(2) The ball
8
is easily jammed at the vicinity of the corner portion
10
, so that the ball
8
is prevented from circulating smoothly.
(3) When the ball
8
enters from the tube
5
into the screw groove
2
a
, a preceding ball
8
is pushed by the succeeding ball
8
so that the ball
8
is injured or worn. Powders or particles generated by the wear or abrasion cause lowering of durability of the ball screw apparatus.
BRIEF SUMMARY OF THE INVENTION
The present invention is to provide a screw ball apparatus which allows smooth circulation of balls and which can be improved in durability.
A ball screw apparatus of the present invention comprises:
a screw shaft formed on its outer cirmuferential face with a spiral ball screw groove;
a nut member fitted on an outer circumference of the screw shaft and formed on an inner circumferential face with a ball screw groove opposed to the ball screw groove of the screw shaft;
a ball rolling contact way constituted by the respective ball screw grooves opposed to each other;
a connecting way provided in the nut member for causing one portion and another portion of the ball rolling contact way to communicate with each other;
an endless circulating way constituted by the conne
Iehisa Toshiyuki
Maruyama Daisuke
Miyaguchi Kazuo
Sekiya Noriaki
Bucci David A.
NSK Ltd.
Smith Julie K.
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