Ball nut with tangential no load-guided return path

A ball nut (60) has internal return paths (62) for a pair of parallel helical tracks (64). The return paths (62) are formed by grooves (66) machined into the body of the ball nut (60) from one end of the helical tracks (64) to the other. Return guide covers (68) and return guide inserts (69) are inserted into the ball groove tracks (64) on each end of the tracks. These covers (68) and inserts (69) divert the ball bearings through the grooves (66) and into the opposite end of the circuit formed by the helical tracks (64). The ball bearings (82) are deflected by the return guide insert (69) smoothly in a tangential path from the helical track (64). Thus, no abrupt change in direction for the ball bearings (82) while they are under load is encountered.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to an improved ball nut for use in 
combination with ball screws to give a positioning assembly. More 
particularly, it relates to such a ball nut in which abrupt changes in 
direction in a return path for ball bearings in the ball nut are avoided. 
Most especially, it relates to such a ball nut with an internal return 
path. 
2. Description of the Prior Art 
In conventional ball screw assemblies, a ball nut travels along a lead 
screw. Ball bearings are positioned between one or more sets of helical 
threads on an inside surface of the ball nut and corresponding helical 
threads on the lead screw. The ball nut and lead screw threads form a race 
for the ball bearings positioned between the threads. The ball bearings 
move in rolling contact in the threads as the lead screw and the ball nut 
are rotated with respect to one another. As the lead screw and the ball 
nut rotate with respect to one another, the ball bearings travel along the 
helical threads on the inside surface of the ball nut, are removed at one 
end of those helical threads by a pick-off member and are delivered by a 
return path to an opposite end of the helical threads, so that the ball 
bearings are constantly recirculated in the race as the lead screw and 
ball nut continue to rotate with respect to each other. Ball screw 
assemblies having both external and internal return paths are known in the 
art. 
Examples of prior art ball screw assemblies are shown in the following 
issued U.S. Pat. Nos.: 3,902,377, issued Sept. 2, 1975 to Lemor; 
4,074,585, issued Feb. 21, 1978 to Richaud et al.; 4,074,587, issued Feb. 
21, 1978; U.S. Pat. No. 4,138,902, issued Feb. 13, 1979 and 4,186,620, 
issued Feb. 5, 1980, all to Brusasco; U.S. Pat. No. 4,211,125, issued Jul. 
8, 1980 to Benton; U.S. Pat. No. 4,258,584, issued Mar. 31, 1981 to 
Haegele et al.; U.S. Pat. No. 4,274,297, issued Jun. 23, 1981 to Blaurock 
et al.; U.S. Pat. No. 4,357,838, issued Nov. 9, 1982 to Blaurock et al.; 
U.S. Pat. No. 4,439,011, issued Mar. 27, 1984 to Machmerth; U.S. Pat. No. 
4,660,431, issued Apr. 28, 1987 to Heine and U.S. Pat. No. 4,896,552, 
issued Jan. 30, 1990 to Virga. As disclosed in these issued patents, it is 
known in such ball screw assemblies to provide a pick-off for the ball 
bearings at the entrance to the return path which is tangential to the 
helical threads forming the race between the ball nut and the lead screw. 
However, the prior art structures for providing tangential pick-offs are 
complex and expensive. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a ball nut and a 
ball screw assembly having a tangential pick-off in a simplified 
structure. 
It is a further object of the invention to provide such a ball nut and a 
ball screw assembly in which the balls are picked off under a no-load 
condition. 
The attainment of these and related objects may be achieved through use of 
the novel ball nut and ball screw assembly herein disclosed. A ball nut in 
accordance with this invention has a body with an axial bore extending 
through the body. At least one substantially helical path track on the 
axial bore receives a plurality of ball bearings for engaging threads of a 
lead screw. At least one return path for the plurality of ball bearings 
connects ends of the substantially helical path track. At least one 
pick-off for the ball bearings connects the at least one substantially 
helical path track and the at least one return path. The at least one 
pick-off engages the at least one substantially helical path track tangent 
to the at least one substantially helical path track. The pick-off 
comprises a return guide insert engaging the at least one substantially 
helical path track and the return path includes a return guide cover 
engaging the return guide insert. The return guide insert and the return 
path guide each have mating curved passages to define a smooth curve path 
from the at least one substantially helical path track to the at least one 
return path. 
A ball nut assembly in accordance with the invention includes the ball nut 
and a lead screw in the axial bore. A plurality of ball bearings are 
between the substantially helical path track and the lead screw. 
The attainment of the foregoing and related objects, advantages and 
features of the invention should be more readily apparent to those skilled 
in the art, after review of the following more detailed description of the 
invention, taken together with the drawings, in which:

DETAILED DESCRIPTION OF THE INVENTION 
Turning now to the drawings, more particularly to FIGS. 1 and 2, there is 
shown a ball nut 60 having internal return paths 62 for a pair of parallel 
helical tracks 64 (see FIG. 3). The return paths 62 are formed by grooves 
66 machined into the body of the ball nut 60 from one end of the helical 
tracks 64 to the other. Return guide covers 68 and return guide inserts 69 
are inserted into the ball groove tracks 64 on each end of the tracks. 
These covers 68 and inserts 69 divert the ball bearings through the 
grooves 66 and into the opposite end of the circuit formed by the helical 
tracks 64. The outside diameter of the ball nut 60 is covered by a 
friction fit sleeve 70 to retain the ball bearings in the grooves 66. 
In a preferred form of the invention, the ball nut 60 has a plurality of 
essentially C shaped slots 72, defining a plurality of rib segments 74 
connected to a backbone spine 76, which connects head and tail sections 78 
and 80 of the ball nut 60. The rib segments 74 supply flexibility to the 
ball nut 60. Because of their C spring shape, they will flex slightly, 
altering the pitch of the portion of the ball groove tracks 64 that is 
contained inside the rib segment 74. Each rib segment 74 acts 
independently, flexing to alter the pitch as the variations of the lead 
screw affect the rolling path of ball bearings 82 circulating within the 
ball groove tracks 64. The ball bearings 82 are selectively sized to 
create an interference fit within the ball circuit when ball screw 84 and 
the ball nut 60 are assembled. Without the flexibility of the rib segments 
74, the ball bearings would jam inside the ball circuit, causing the 
assembly to run rough, if at all. The rib segments 74 respond to the 
potential points of interference, by moving slightly to relieve jamming. 
Conversely, when variations in the ball groove tracks 64 would cause play 
in the circuit, the rib segments 74 flex to cause compression, keeping the 
preload on the ball bearings. Further details on ball screw assemblies 
incorporating C-shaped rib segments are available in a co-pending, 
commonly-assigned, concurrently filed application Ser. No. 07/678,538 by 
Joseph P. Virga and Glenn R. Simpson III, entitled "Flexible Ribbed, 
No-Backlash Ball Nut," the disclosure of which is hereby incorporated by 
reference herein. It should be understood that, while the use of these 
flexible ribs is highly desirable and are incorporated in the presently 
preferred form of the invention, they are not necessary in order to use 
the novel no-load pickoff structure of this invention, described below. 
As shown in FIG. 4, the ball groove tracks 64 have a "gothic arch" 
cross-section shape, formed by the intersection of two radii, which is by 
far the most commonly used track configuration in ball screw design. The 
rib segments 74 take advantage of the gothic arch shape. As the pitch of 
the ball screw 84 and ball nut 60 vary, the rib segments flex, keeping the 
ball bearings 82 in contact on at least two points 86 and 88 in the groove 
track 64 and groove 90 of ball screw 84. As the pitch varies, the rib 
segments 74 flex, allowing the ball bearings 82 to find the path of least 
resistance. 
The area described by the gothic arch shape is always much larger than the 
nominal ball bearing diameter that it is designed to use. This allows for 
great variations in the diameters of the ball bearings 82 that can be used 
with the ball nut 60. Because of the larger size range that can be used in 
the groove track 64 and the groove 90, the correct diameter of ball 
bearings 82 to achieve the desired preload can be easily found without 
reaching the maximum ball bearing diameter that would fit the arches. It 
can be seen that the ball circle diameter will vary as the variations in 
the groove pitch cause the ball bearings to climb and descend the groove 
walls and as the diameter of the selected ball bearings 82 is increased or 
decreased to vary the preload obtained. The gothic arch groove 90 on the 
ball screw 84 remains fixed, because the screw 84 is solid. The pitch and 
grooves 64 inside the rib segments 74 flex. As in the case of the flexible 
ribs, the use of the gothic arch cross-section shape represents a 
preferred form of assemblies of this invention, but the crosssection shape 
could also be a full radius. 
FIGS. 4 and 5-8 show return path system designs 100 and 102 for a prior art 
ball screw assembly and the ball screw assembly 60 of FIGS. 1-3, 
respectively. In FIG. 4, the return tube 30 has a conventional pickoff 
finger 104 that extends from the ball nut 12 into the helical track 24 
toward the lead screw 14 to engage the ball bearings 26 for deflection 
from the helical track 24 into the return tube 30. The series of ball 
bearings 26 shows that the pick-off finger 104 produces an abrupt change 
of direction for the ball bearings 26 while they are still under load in 
the helical track 24. 
In the return path system 102, the ball bearings 82 are guided from the 
helical track 64 and into the return path 62 by means of return guide 
insert 69 and the return guide cover 68. As is best shown in FIG. 5, the 
ball bearings 82 are guided by the return guide insert 69 smoothly in a 
tangential path under no load from the helical track 64. Thus, no abrupt 
change in direction for the ball bearings 82 while they are under load is 
encountered. The absence of load on the ball bearing 82 just entering the 
return guide insert is shown by the gap 103 between the ball bearing 82 
and the wall of the helical track 64. As is best shown in FIGS. 7 and 8, 
the return guide insert 69 and the return guide cover 68 have tracks 108 
and 110, respectively, which join together to connect the helical track 64 
and the return path 62. 
In practice, it has been found with the ball return system 102 that, for 
smooth operation, the ball bearings 82 should not make a turn or follow a 
path that has a radius of curvature less than twice the radius of the 
balls, whether under load or not. 
FIG. 9 shows another ball nut 120 comparable to the ball nut 60 of FIG. 1, 
but without the slots 72 defining the rib segments 74 in the FIG. 1 
embodiment. The ball nut 120 incorporates return guide covers 122 and 
return guide inserts 124 corresponding to the return guide covers 68 and 
return guide inserts 69 in the FIGS. 1-3 and 5-8 embodiment, to give the 
return path system design of this invention. The ball nut 120 is used for 
heavier loads and larger diameter ball nuts, such as 5/8 inch diameter and 
larger, where jamming of balls 126 in the ball circuit is not a problem. 
Other than as shown and described, the construction and operation of the 
FIG. 9 embodiment of the invention is the same as that of the FIGS. 1--3 
and 5--8 embodiment. 
It should now be readily apparent to those skilled in the art that a novel 
ball nut and ball nut assembly capable of achieving the stated objects of 
the invention has been provided. 
It should further be apparent to those skilled in the art that various 
changes in form and details of the invention as shown and described may be 
made. It is intended that such changes be included within the spirit and 
scope of the claims appended hereto.