Recirculating ball bearing spline joint

A unit composed of a ball spline and a shaft having a polygonal cross section, the ball spline including an outer sleeve formed in its inner peripheral surface with ball-circulating grooves in vertically or horizontally symmetrical disposition, two separated holders each having an angle slightly larger than an angle .alpha. when assembled in the outer sleeve and holding a desired number of balls, the holders assembled in the outer sleeve with being inflected inwardly, and a pair of holder retainers fitted to the opposite opening ends of the outer sleeve; and the shaft formed with four ball-circulating grooves in the opposite sides of its opposite two sides and inserted in the ball spline.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a unit composed of a ball spline and a shaft with 
a polygonal cross section, the ball spline including an outer sleeve 
formed in its inner peripheral surface with ball-circulating grooves in a 
vertically or horizontally symmetric disposition, a pair of holders each 
having a vertically or horizontally symmetric structure and an angle 
slightly larger than an angle .alpha. when assembled in the outer sleeve, 
the holders holding a desired number of balls and assembled in the outer 
sleeve with being inflected inwardly, and a pair of holder retainers 
fitted to the opposite opening ends of the outer sleeve; and the shaft 
formed with four ball-circulating grooves in the opposite sides of its 
opposite two sides and inserted in the ball spline. 
2. Description of the Prior Art 
There are two types in normally used ball splines for different diameter 
spline shafts, one referred to as a casing type having balls directly 
assembled in a body and the other a cage type employing a holder. Cageless 
type ball splines are also known which eliminate the need for any cage so 
as to reduce the number of required parts and thus reduce the cost. 
However, such cageless type ball splines have a tendency of the balls to 
fall out of the body when assembled or disassembled, resulting in 
difficulties to check or repair. Another type ball splines are known which 
include a holder for guiding balls. However, they are disadvantageous in 
their poor ability to smoothly circulate the balls with a large ball 
rolling resistance due to the less radius of gyration of the balls running 
between load and unload regions and also in their difficulties encountered 
when assembled. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is to provide a unit of a ball 
spline and a shaft having a polygonal cross section, the ball spline 
including an outer sleeve formed in its inner peripheral surface with 
ball-circulating grooves in vertically or horizontally symmetrical 
disposition, two separated holders each having an angle slightly larger 
than an angle .alpha. when assembled in the outer sleeve and holding a 
desired number of balls, the holders assembled in the outer sleeve with 
being inflected inwardly, and a pair of holder retainers fitted to the 
opposite opening ends of the outer sleeve; and the shaft formed with four 
ball-circulating grooves in the opposite sides of its opposite two sides 
and inserted in the ball spline. 
A second object of the present invention is to provide a unit of a ball 
spline and a shaft having a polygonal cross section with the features 
noted in the primary object, in which the ball circulating grooves include 
first axially extending parallel ball grooves 2, 2 formed in the inner 
peripheral surface of the outer sleeve and facing each other for receiving 
loaded balls, second axially extending parallel ball grooves spaced 
circumferentially around the inner peripheral surface of the outer sleeve 
a distance L away from the first ball grooves 2, 2 for receiving unloaded 
balls, and escape portions formed in the opposite end portions of the 
outer sleeve for connecting the first and second ball grooves. 
A third object of the present invention is to provide a unit of a ball 
spline and a shaft having a polygonal cross section with the features 
noted in the primary object, in which the holders each has its inner 
surface including a center vertical surface 22, upper and lower vertical 
surfaces 21, 21, and slant surfaces 23 connecting the center vertical 
surface 22 to the upper vertical surface 21 and the center vertical 
surface 22 to the lower vertical surface 21, each of the holders having 
its outer surface including a center curved surface 24 with the same 
curvature as the first ball groove 2 bottom of the outer sleeve 1, upper 
and lower curved surfaces 25, intermediate curved surfaces 26 with the 
same curvature as the land surface 4 of the outer sleeve 1, slant surfaces 
27 connecting the upper curved surface 25 to the intermediate curved 
surface 26 and the lower curved surface 25 to the intermediate curved 
surface 26, and slant surfaces 28 formed in the upper and lower portion of 
the holder, each of the holders formed with endless ball circulating 
recesses 29, 29 in the portions corresponding to the first and second ball 
grooves and the escape portions 5 formed in the inner peripheral surface 
of the outer sleeve 1, and the endless ball circulating recesses 29, 29 
formed with elongated holes 30, 30. 
A fourth object of the present invention is to provide a unit of a ball 
spline and a shaft having a polygonal cross section with the features 
noted in the primary object, in which the shaft has a quadrilateral cross 
section with its four corners cut such that the cut curved or linear small 
sides 101, 101 are spaced away from the center of the shaft a distance 
less than 2/3 times the outer radius of the outer sleeve, and the shaft 
formed with third axially extending parallel ball grooves in the opposite 
sides of the cut opposite two sides 101, 101 such that the lines extending 
through the points 11, 11 and 12, 12 at which balls contact with the first 
groove of the outer sleeve and the third ball grooves 9, 9 intersect at 
right angles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the accompanying drawings, the numeral 1 designates a 
hollow cylindrical outer sleeve having an outer radius R and an inner 
radius about 2/3R. The outer sleeve 1 has its inner peripheral surface a 
formed with two first axially extending parallel ball grooves 2, 2 facing 
each other for receiving a desired number of loaded balls and also with 
second axially extending parallel ball grooves 3, 3 in the opposite sides 
of each of the first ball grooves 2 for receiving unloaded balls which are 
spaced circumferentially around the inner peripheral surface of the outer 
sleeve a distance L away from the first ball groove 2 so as to form 
axially extending parallel lands 4, 4 in the opposite sides of each of the 
first ball grooves 2, 2. The outer sleeve 1 has its inner surface opposite 
end portions formed with escape portions 5, 5 which form ball-circulating 
grooves along with the first and second ball grooves. In the opposite ends 
of the outer sleeve 1 there are formed holder retaining stepped portions 
6, 6 and concaval recesses 7, 7 in which clips are fitted. 
The first ball grooves are formed such that the lines extending through the 
points 11, 11 and 12, 12 at which balls contact with the first ball groove 
of the outer sleeve and third ball grooves 9, 9 formed in a shaft 10 
intersect at right angles. 
The shaft has a quadrilateral cross section with its four corners cut such 
that the cut curved or linear small sides 101, 101 are spaced away from 
the center of the shaft a distance less than 2/3 times the outer radius R 
of the outer sleeve. The shaft formed with third axially extending 
parallel ball grooves 9, 9 in the opposite sides of the cut opposite two 
small sides 101, 101 such that the lines extending through the points 11, 
11 and 12, 12 at which balls contact with the first groove of the outer 
sleeve and the third ball grooves 9, 9 intersect at right angles. The 
outer sleeve 1 has its inner surface formed with slant surfaces 13 
connecting the surfaces a and the second ball grooves 3 as shown in FIG. 
1. 
The numeral 20 indicates a pair of holders formed of a plastic material 
such as plastic hard rubber, a powder metallurgy such as sintered alloys, 
or die-casted alloy and having a horizontally symmetric structure. Each of 
the holders 20 has its inner surface including a center vertical surface 
22, upper and lower vertical surfaces 21, 21, and wide slant surfaces 23, 
23 connecting the center vertical surface 22 to the upper vertical surface 
21 and the center vertical surface 22 to the lower vertical surface 21. 
The holder 20 is formed such that the angle made by the slant surfaces 23, 
23 is at right angles when assembled in the outer sleeve 1. The holder 20 
has its outer surface including a center curved surface 24 with the same 
curvature as the first ball groove 2 bottom of the outer sleeve 1, upper 
and lower curved surfaces 25, 25, intermediate curved surfaces 26, 26 with 
the same curvature as the land surface 4 of the outer sleeve 1, slant 
surfaces 27, 27 connecting the upper curved surface 25 to the intermediate 
curved surface 26 and the lower curved surface 25 to the intermediate 
curved surface 26, and slant surfaces 28 formed in the upper and lower 
portion of the holder. The holder is formed with endless ball circulating 
recesses 29, 29 in the portions corresponding to the first and second ball 
grooves and the escape portions 5 formed in the inner peripheral surface 
of the outer sleeve 1, and the endless ball circulating recesses 29, 29 
are formed with elongated holes 30, 30. 
The holders 20 and the shaft 10 are assembled in the outer sleeve 1 in the 
following order: First, the balls 8 and 8' are charged into the endless 
grooves 29, 29 of one of the holders 20 along with grease, and the holder 
20 is then assembled in the outer sleeve 1 by outwardly pushing the holder 
20 with pressing the upper and lower slant surfaces 28, 28 against the 
respective slant surfaces 13, 13 of the outer sleeve 1 until the center 
curved surface 24 of the holder 20 comes into contact with the bottom of 
the first ball groove 2 of the outer sleeve 1 so that the upper and lower 
slant surfaces 28, 28 can be fitted against the slant surfaces 13, 13 of 
the outer sleeve 1 to hold the holder 20 in place. The other holder 20 is 
then assembled in the outer sleeve 1 in the same manner as done the one 
holder 20. Thereafter, the holder retainers 40, 40 are fitted in the 
holder retaining stepped portions 6, 6 and then the clips 41, 41 are 
fitted in the recesses 7, 7. After the ball spline is completely 
assembled, the shaft 10 having a polygonal cross section and having four 
third ball grooves is inserted into the assembled ball spline. 
In the unit of the ball spline and the shaft having a polygonal cross 
sectional which has two pairs of first and second ball grooves formed in 
the inner peripheral surface of the outer sleeve, there is a larger 
distance L between loaded and unloaded balls as compared with conventional 
ball splines. This permits to provide an increased radiul of gyration of 
the balls running between load and unload regions, whereby the balls can 
smoothly circulate through the ball circulating grooves with a less ball 
rolling resistance. Since the outer sleeve has upper and lower thick wall 
portions, a spline 50 can be formed in one of these portions without any 
problem in strength. Also, since the holder is separated into two parts 
and made of a plastic material, a powder metallurgy, or a die-casted 
alloy, it is possible to make the holder to have an angle slightly larger 
than an angle .alpha. when assembled in the outer sleeve so that the upper 
and lower slant surfaces 28 can be pressed against the slant surfaces 13 
of the outer sleeve 1 to securely hold the holder in place when the holder 
is pushed to bring the intermediate curved surfaces 26 and the center 
curved surface 24 into contact with the land surfaces 4 and the curved 
surfaces of the first ball grooves 2. In addition, the arrangement of the 
shaft having a quadrilateral cross section with its four corners cut such 
that the cut curved or linear small sides are spaced away from the center 
of the shaft a distance less than 2/3 times the outer radius of the outer 
sleeve and formed with four axially extending parallel ball grooves in the 
opposite sides of the cut opposite two sides is easy to produce and low in 
cost. 
Although the loaded balls and unloaded balls are designated by different 
reference numerals, it is to be understood, of course, that these balls 
are of the same kind.