Externally serviceable ball screw having internal return means

The specification discloses a ball screw having an internal return which is selectively removable externally of the ball nut for servicing. The return is held in place by a spring clip operatively fitted into a dove-tail slot built into the exterior of the ball nut. The provision of the dove-tail slot provides for easy removal and replacement of the internal return, external insertion of the ball bearings when the ball screw must be reassembled for any reason, and, if desired, for shimming the internal return to keep the return at the proper height when the ball screw is rebuilt.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to ball screws of the type having internal 
return means, and more particularly to an externally servicable ball screw 
having internal return means. 
2. Description of the Prior Art 
The advantages of mechanical ball screws as converters of rotary to linear 
motion with extreme accuracy and low friction has been known in the art 
for many years. The first type of ball screw to be developed was one 
having an external return tube whereby the balls interposed between the 
ball screw and the ball nut travelled outside the actual ball nut to make 
their return from trailing edge of the ball nut back to the leading edge 
of the nut for continuous recirculation. It was found, however, that 
becuase of the additional space required by the external return tubes, 
many applications could not adapt to the use of such ball screws. 
Therefore, a ball screw having an internal return means was developed 
which satisfactorily fit in many smaller spaces. However, it was found 
that both type of these ball screws have a serious problem as they age and 
become worn. As the balls and/or ball races become worn, a looseness of 
fit develops and the ball screws lose their accuracy, although they retain 
their low friction. 
Due to the expense of the ball screw, it has been found desirable to 
rebuild ball screws to the extent possible, rather than replacing them. 
Such rebuilding can involve something as simple as placing slightly larger 
ball bearings in the ball races, to re-turning the ball screw and/or ball 
nut. 
Whichever method of rebuilding is used, a serious problem exists in 
reassembling the ball screw after it has been disassembled for rebuilding. 
This involves holding the balls in the races of the ball nut while 
screwing the ball screw into position. This becomes a problem because, 
whether the external or internal return means are used, balls cannot be 
placed into the ball race of the ball nut from the outside of the ball 
screw assembly. This means that the tracks, must be greased, the balls 
must be placed in the tracks, and the ball screw carefully rotated in the 
ball nut to prevent the balls from falling out. It can be understood that 
this reassembly can be a rather long and laborious process, and may cost 
more than the actual rebuilding of the ball screw. 
SUMMARY OF THE INVENTION 
In order to solve the problems of servicing ball screw units I have 
provided a ball screw having internal return means, and have provided that 
such internal return means are selectively removable externally of the 
ball nut for servicing, and yet are positioned so that when the ball 
bearings must be placed back into the ball screw, the ball bearings may be 
inserted from outside the ball nut, and the internal return means be may 
replaced and held in place by an easily removable clip which fits in a 
dove-tail slot milled into the exterior of the ball nut. The provision of 
the dove-tail slot and clip provides for easy removal and replacement of 
the internal return means, and also for shimming the same to keep the 
return means at the proper height for operation of the ball screw 
regardless of the wear on the track portion of the internal return means. 
Therefore, it is one of the objects of the present invention to make a more 
easily servicable ball screw. 
It is a further object of the present invention to provide a better method 
of reinserting the ball bearings in the ball race after the rebuilding of 
a ball screw. 
It is a further object of the present invention to make rebuilding and 
servicing of ball screws less time consuming and less costly. 
It is a further object of the present invention to make a ball screw 
wherein the balls may be inserted into the ball races from a position 
external to the ball nut. 
It is a still further object of the present invention to provide for an 
externally servicable ball screw of the foregoing nature having internal 
return means. 
Further objects and advantages of the present invention will become 
apparent from the following description and appended claims, reference 
being made to the accompanying drawings forming a part of the 
specification, wherein like reference characters designate corresponding 
parts in the several views.

It is to be understood that the present invention is not limited in its 
application to the details of construction and arrangement of parts 
illustrated in the accompanying drawings, since the invention is capable 
of other embodiments, and of being practiced or carried out in various 
ways within the scope of the claims. Also, it is to be understood that the 
phraseology and terminology employed herein is for the purpose of 
description, and not of limitation. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 there is shown an example of the oldest type of 
ball screw, that being an external return ball screw assembly generally 
designated by the number 20, consisting of a ball screw, generally 
designated by the numeral 21 and having a helical ball race 22 formed 
between the threads 23. The pitch and depth of the ball race will depend 
upon the particular application to which the ball screw is to be put. To 
provide for movement between the ball screw 21 and the ball nut 25 there 
is provided on the ball nut 25 a plurality of complimentary ball races 26 
equal in pitch to the ball screw 21. 
As is well known in the art, to provide for continuous operation of the 
ball screw, the ball bearings 24 must continuously move from the beginning 
of one circuit, in this case consisting of three turns of the ball race 
22, to the end of the same circuit, and then through the external return 
tube 27, to continuously circulate. The external return tubes 27 are held 
in place by clips 28. A flange 29 aids in retaining the ball screw in 
place. 
Referring now to FIG. 2, a more recent development in the ball screw art 
has been the provision of a ball screw having internal return means 34 
provided in place of the external returns tubes 27. The internal return 
means 34 cannot be removed externally of the ball nut 25 because they are 
held in place from the interior of said ball nut, and must be removed from 
the inside. An epoxy coating 35 is necessary to seal the internal return 
means 34 because they are not sealed from the elements to the same degree 
as the external return means 27. The rest of the construction of the 
internal return ball screw assembly 33 is substantially identical to the 
external return ball screw assembly 20, having a ball screw 21, a helical 
ball race 22 separated by threads 23, and ball bearings 24 (not shown) 
interposed between the ball nut 25 and the ball screw 21. 
In both of the prior art ball screws, as discussed above, when the ball 
screw becomes worn and it is necessary to service the same, the ball screw 
21 is screwed out of the ball nut 25, and the ball bearings 24 are 
removed. The ball screw 21 and/or ball nut 25 are serviced, as needed, and 
the same or larger ball bearings 24 must then be placed between the ball 
screw and the ball nut. At the present time, this can only be done by 
greasing the ball race 22 of the ball nut 25, placing the ball bearings 
around the perimeter of the interior of the ball nut, and attempting to 
screw in the ball screw 21 without disloding the balls. This is required 
because the internal return means 34 are not removable externally of the 
ball nut 25 because of the flanges or other holding means (not shown) 
which prevent their removal. Such flanges are needed to insure that the 
internal return is not forced out through the return opening 38 destroying 
the epoxy seal 35. 
Also the forces exerted by the ball bearings on the ball nut and the 
internal return are too great to depend upon the epoxy seal holding the 
internal return in place, and thus the internal retaining means are 
needed. However, as previously discussed, this prevents loading the ball 
bearings externally during reassembly, and also prevents any motion on the 
part of the internal return means 34, it being fixed in place. Also, it 
has been found desirable to provide for a "rocking" motion of the return 
to prevent problems from arising should a piece of debris happen to be 
present in the ball race as one of the ball bearings 24 is attempting to 
traverse the return race 39 in the internal return insert 36 (FIG. 8). For 
ease of understanding, I have designated the externally removable internal 
return means of the present invention as the "internal return insert 36" 
to distinguish it from the fixed internal return means 34 of the prior 
art. However, the combination of the internal return insert 36, the return 
opening 38, and the spring clip 41, with or without the shim 45, still 
will be referred to as constituting the "internal return means" of the 
present invention. If the internal return insert 36 can "float", the ball 
will not become jammed between the debris and the return race 39. However, 
this floating or rocking motion is not possible with the conventional 
construction. 
Another problem also arises in ball screws having fixed internal return 
means, and that is the necessity of supplying new internal return means to 
compensate for wear. Since the relative height between the return race 39 
and the relevant portion of the ball race 22 is fixed, once the return 
race 39 becomes worn, in many rebuilding applications there is nothing to 
be done except to discard the internal return means 34. This results in a 
great deal of waste and expense, and may improved construction provides 
not only for external loading of the ball bearings, and a rocking motion 
of the internal return insert 36, but also for a height adjustment of the 
internal return insert 36, thus, solving many longstanding problems in the 
art. 
Referring now to FIGS. 3-6, my improved construction can be seen. As 
before, a conventional ball screw 21 having a helical ball race 22 with a 
helical thread 23 is used. I make no changes in the standard ball screw in 
my improved invention. However, the ball nut 25 having flange portion 29 
and body portion 30 has the external area of the body portion 30 modified 
proximate the return openings 38. As can best be seen by FIGS. 4 and 5, a 
dove-tail slot 40 is milled into the outer diameter of the body portion 30 
of the ball nut 25. Thus, the overall height of the return opening 38 is 
reduced from its previous height of W, representing the wall thickness, by 
a distance A, representing the depth of the dove-tail slot, to a total 
height of B. 
Likewise, the height of the internal return insert 36 is reduced to a 
height of B, or less, as will be hereinafter explained. In my 
construction, as in the prior art, there is no means to restrain the 
internal return insert 36 from travelling downwardly and contacting the 
ball nut if the ball races were empty. However, this situation never 
exists, because upon reassembly after whatever rebuilding operations are 
necessary, the ball screw 21 will be placed inside the ball nut 25, and 
ball bearings will be added through the return opening 38 until the ball 
races 22 are completely filled with ball bearings 24, and the internal 
return insert 36 will then be kept from contacting the ball race of the 
ball nut by the ball bearings. 
However, as previously mentioned the real need for retention is to prevent 
the internal return insert 36 from travelling outwardly due to the 
centrifugal force of the ball bearings 24 in the ball race 22. This is 
accomplished by the provision of a spring clip 41, which is compressed 
sufficiently to fit into the dove-tail slot 40, and is held in place by 
abutment against the side wall 42 and the bottom wall 43 of the dove-tail 
slot 40. It is to be understood that the particular size and strength of 
the spring clip 41 will be chosen in relation to the particular 
application to which my improved ball screw is to be put, just as the size 
of the ball bearings 24, the ball nut 25, and the ball screw 21 would be. 
The strength and size of the clip would be chosen such that there is no 
possible way for the clip to be expelled from the dove-tail slot 40 during 
operation. This can be done by determining the centrifugal force expected 
to be put on the internal return insert 36 by the centrifugal force of the 
ball bearings 24, and a then a proper clip can be chosen such that the 
force needed to expel the clip exceeds by the appropriate safety margin 
the forces expected to be put thereon. 
Referring now to FIG. 7, there is shown a modification of my invention 
which is particularly useful when the ball screw has been in use for 
sometime and needs to be rebuilt. Rather than replacing the internal 
return insert 36, to effectively reduce the height of the ball race 22, a 
shim 45 is inserted between the internal return insert 36 and the spring 
clip 41. Of course, the appropriate amount of material will have to be 
removed from the bottom of the internal return insert 36 so that it does 
not interfere with the operation of the ball screw. In this manner, the 
expense of replacement of the internal return means is eliminated. 
If desired, an epoxy coating 35 (shown in FIGS. 5 and 7 for purposes of 
ilustration, but not shown in FIGS. 4 and 6 for ease of understanding the 
invention) can be placed into the dove-tail slot 40 to seal the return 
opening 38 and prevent dust and dirt from getting into the ball screw much 
as was done in the prior art. 
The fit between the return opening 38 and the internal return insert 36 
will depend upon the particular application of the ball screw. If it 
desired to provide for the aforementioned "rocking" motion, there may be a 
somewhat loose fit between the opening 38 and the return insert 36, with 
sufficient difference in the height B of the return opening 38 and the 
internal return means 36 to provide for such motion. If such motion is not 
desired, the height of the internal return insert 36 and the return 
opening 38 may both be equal to B. And, in addition, a press fit may be 
provided to provide additional resistance to any movement of the internal 
return insert 36. 
Thus, by carefully considering the problems present in the art of building 
and repairing ball screws I have developed a novel ball screw which solves 
several longstanding problems in the art.